Abstract 337: Development of an immune-oncology agent containing granzyme B to target VEGFR+ ovarian cancer cells and activated tumor vasculature

Granzyme B is a key effector in immune-mediated cell killing by inducing apoptosis through both caspase-dependent and -independent mechanisms. We developed a fusion protein (GrB-Fc-VEGF121) composed of granzyme B, VEGF121 (a naturally-occurring isoform of the VEGF-A family that binds to VEGF receptors R-1 and R-2), and a human IgG heavy chain fragment (for dimerization and to augment in vivo circulation and anti-tumor efficacy). GrB-Fc-VEGF121 was transiently expressed in HEK-293E cells under serum-free conditions purified with a final yield of approximately 40 mg/L. Comparison of GrB-Fc-VEGF121 with its expected molecular weight suggests significant glycosylation when produced in HEK-293E cells. The enzymatic activity of granzyme B in GrB-Fc-VEGF121 was comparable to that of commercially available human granzyme B. GrB-Fc-VEGF121 readily internalized into VEGFR-2+ cells within 2 hours of treatment while untargeted granzyme B was not internalized, demonstrating that internalization was receptor-mediated. Cytotoxicity studies against a panel of tumor and endothelial cells indicated cytotoxicity in the nanomolar range against cell lines expressing high levels of VEGFR-1 or VEGFR-2 while control (receptor-negative) cells demonstrated IC50 levels in the high micromolar range. Treatment of VEGFR-2+ endothelial and tumor cells with GrB-Fc-VEGF121 resulted in robust cell death via apoptosis and/or necrosis. Ex vivo serum stability of GrB-Fc-VEGF121 indicated a gradual protein loss of GrB-Fc-VEGF121, with an overall loss of about 50% over 96 hours. The maximum tolerated dose of GrB-Fc-VEGF121 was not reached when BALB/c mice were treated with a total 475 mg/kg given every other day over 5 doses. Treatment of mice bearing OVCAR8 tumor xenografts with GrB-Fc-VEGF121 (100 mg/kg over 5 doses) resulted in significant growth inhibition of established tumors compared to vehicle controls. Treated tumors showed a significant decrease in the number of CD31+ blood vessels and Ki-67+ proliferating tumor cells compared to controls, when assessed at the end of the study. These studies clearly indicate that GrB-Fc-VEGF121 has significant anti-tumor effects in vivo with virtually no toxicity against normal tissues at doses 4-5 times higher than therapeutic levels. The tumor- and vascular-targeted agent appears to have significant potential as a new class of targeted therapeutic agents with a unique mechanism of action. Research conducted, in part, by the Clayton Foundation for Research.

Citation Format: Khalid A. Mohamedali, Lawrence H. Cheung, Ana Alvarez-Cienfuegos, Walter N. Hittelman, Michael G. Rosenblum. Development of an immune-oncology agent containing granzyme B to target VEGFR+ ovarian cancer cells and activated tumor vasculature [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 337.

Abstract 5216: Optimizing antitumor efficacy of granzyme B human fusion constructs targeting the Fn14 antigen in solid tumors. Pre IND pharmacokinetics, schedule, dose optimization and MTD studies

We previously demonstrated that a human fusion protein construct targeting the Fn14 receptor and containing the cytotoxic granzyme B (GrB) payload (GrB-Fc-IT4) displays impressive in vitro and in vivo cytotoxic effects. Comprehensive mechanism of action studies show that GrB-Fc-IT4 effectively induces cell death in vitro against a broad selection of tumor types with high specificity. The intracellular pathway includes irreversible activation of the caspase cascade and independent mitochondrial depolarization leading to an intense apoptotic cellular damage. This activation occurs quickly and efficiently. Pharmacokinetic studies in mice showed that GrB-Fc-IT4 was cleared bi-exponentially from plasma with a rapid initial clearance (t ½alpha = 0.36 hours) followed by a prolonged terminal-phase plasma half-life (t 1/2beta = 35 hours) similar to that of IgGs. Based on our pharmacokinetic study, we employed a QODX5 therapy schedule and demonstrated impressive antitumor activity against several tumor models including PDX-lung, A459 (NSCLC) and MDA-MB231(TNBC) tumor models. In the present study we compared the in vivo antitumor efficacy of GrB-Fc-IT4 against A549 (NSCLC) tumor cell line, using two different dosing schedules (QODx5 vs QWx5). There was a clear dose and schedule dependence and that treating mice bearing A549 tumors with 32mg/kg/dose using a QOWx5 schedule induced a complete tumor growth inhibition (7/10 tumors regressed) with 3/10 tumors showing no growth up to 50 days after the last dose. Preliminary results suggest that GrB-Fc-IT4 may reduce the formation of spontaneous A549 metastasis into lungs. Further studies are planned to examine the potential of the fusion construct to prevent spontaneous metastatic spread in lung tumor models. Although the fusion construct cross-reacts with the murine Fn14 antigen, maximum tolerated dose studies have confirmed that GrB-Fc-IT4 is a safe product, showing no evidence of toxicity (weight loss) in mice treated with up to 500mg/kg. This suggests that the therapeutic index (TI) for this class of agents is >4. Our data suggest that GrB-Fc-IT4 is a novel class of antitumor agents with a unique mechanism of action. This agent seems to be an exceptionally safe and effective drug and appears to be an ideal candidate for advancing to clinical trials. Research conducted, in part, by Clayton Foundation for Research.

Citation Format: Ana Alvarez De Cienfuegos, Lawrence H. Cheung, Khalid A. Mohamedali, Mihai Gagea, Michael G. Rosenblum. Optimizing antitumor efficacy of granzyme B human fusion constructs targeting the Fn14 antigen in solid tumors. Pre IND pharmacokinetics, schedule, dose optimization and MTD studies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5216.

Abstract 3439: An internalizing anti-CEA construct for targeted delivery of granzyme B results in significant antitumor activity in human lung carcinoma xenografts

Carcinoembyronic antigen (CEA) is a cell surface protein normally found at low levels in adult tissues but over-expressed in numerous solid tumors. CEA is constitutively released from tumor cells and reaches detectable levels in peripheral blood, making CEA targeting particularly challenging. Granzyme B is a key effector in immune-mediated killing of target cells, with cell death through both caspase-dependent and -independent mechanisms. We developed a fusion protein composed of granzyme B and a human scFv targeting the juxta-membrane epitope of CEA. The construct also incorporates a fragment of human IgG heavy chain to cause dimerization and provide increased in vivo circulation and antitumor efficacy. The GrB-Fc-CEA construct was cloned into a mammalian expression vector, transiently expressed by HEK-293E suspension cells, and secreted into the culture media. The final yield after purification and activation was 10-15 mg/L. The enzymatic activity of granzyme B in GrB-Fc-CEA was comparable to that of commercially available human granzyme B. In vitro cytotoxicity against CEA-positive cell lines demonstrated cytotoxicity in the nanomolar range, compared to micromolar range cytotoxicity against control (CEA-negative) cell lines. Importantly, GrB-Fc-CEA cytotoxicity against tumor cells in vitro was unaffected by exogenously-added CEA (200 ng/ml) extracellular domain, suggesting that the construct is not competitively absorbed by soluble CEA released by tumors and present in the circulation of patients. GrB-Fc-CEA internalized rapidly (within one hour) into the target cell lines A-431 and HT-1080/CEA whereas internalization was not observed with untargeted granzyme B, clearly demonstrating receptor-mediated internalization. GrB-Fc-CEA induced mitochondrial depolarization and caspase activation, consistent with the well-described granzyme B mechanism of action. Treatment of female nude mice bearing A-549 tumors with GrB-Fc-CEA (80 mg/kg over 5 doses) resulted in a significant growth inhibition of the established tumors compared to vehicle controls (1200 mm3 vs <40 mm3). Complete tumor regression was observed in seven of ten tumors in mice treated with GrB-Fc-CEA. Mouse weight was unaffected in both treated and control groups for the duration of the study, suggesting no apparent toxicity of the construct. Thus, GrB-Fc-CEA appears to have significant potential in targeting a large category of CEA-expressing solid tumors and is a promising candidate for further pre-clinical development. Research conducted, in part, by the Clayton Foundation for Research.

Citation Format: Lawrence H. Cheung, Khalid A. Mohamedali, Ana Alvarez-Cienfuegos, Walter N. Hittelman, Michael G. Rosenblum. An internalizing anti-CEA construct for targeted delivery of granzyme B results in significant antitumor activity in human lung carcinoma xenografts [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3439.

Therapeutic efficacy and safety of a human fusion construct targeting the TWEAK receptor Fn14 and containing a modified granzyme B

Background

Antibody-drug conjugates are an exceptional and useful therapeutic tool for multiple diseases, particularly for cancer treatment. We previously showed that the fusion of the serine protease granzyme B (GrB), the effector molecule or T and B cells, to a binding domain allows the controlled and effective delivery of the cytotoxic payload into the target cell. The production of these constructs induced the formation of high molecular aggregates with a potential impact on the efficacy and safety of the protein.

Methods

Our laboratory designed a new Fn14 targeted fusion construct designated GrB(C210A)-Fc-IT4 which contains a modified GrB payload for improved protein production and preserved biological activity. We assessed the construct’s enzymatic activity, as well as in vitro cytotoxicity and internalization into target cells. We also assessed pharmacokinetics, efficacy and toxicology parameters in vivo.

Results

GrB(C210A)-Fc-IT4 protein exhibited high affinity and selective cytotoxicity within the nanomolar range when tested against a panel of Fn14-positive human cancer cell lines. The construct rapidly internalized into target cells, activating the caspase cascade and causing mitochondrial membrane depolarization. Pharmacokinetic studies in mice revealed that GrB(C210A)-Fc-IT4 displayed a bi-exponential clearance from plasma with a fast initial clearance (t_1/2α=0.36 hour) followed by a prolonged terminal-phase plasma half-life (t_1/2β=35 hours). Mice bearing MDA-MB-231 orthotopic tumor xenografts treated with vehicle or GrB(C210A)-Fc-IT4 construct (QODx5) demonstrated tumor regression and long-term (>80 days) suppression of tumor growth. Treatment of mice bearing established, subcutaneous A549 lung tumors showed impressive, long-term tumor suppression compared with a control group treated with vehicle alone. Administration of GrB(C210A)-Fc-IT4 (100 mg/kg total dose) was well-tolerated by mice and resulted in significant reduction of tumor burden in a lung cancer patient-derived xenograft model. Toxicity studies revealed no statistically significant changes in aspartate transferase, alanine transferase or lactate dehydrogenase in treated mice. Histopathological analysis of tissues from treated mice did not demonstrate any specific drug-related changes.

Conclusion

GrB(C210A)-Fc-IT4 demonstrated excellent, specific cytotoxicity in vitro and impressive in vivo efficacy with no significant toxicity in normal murine models. These studies show GrB(C210A)-Fc-IT4 is an excellent candidate for further preclinical development.

Abstract 5176: A novel, completely human fusion construct containing active Granzyme B efficiently kills Folate Receptor alpha positive tumor cells

Folate receptor alpha (FRα) is an attractive therapeutic target because it is highly overexpressed on several tumors such as ovarian, breast and lung cancers and demonstrates low or restricted distribution on normal tissues. In addition, FRα has been shown to play a tumor promoting role with its expression associated with poor prognosis of cancer patients. In our laboratory, we have generated two new molecules targeting FRα and containing active granzyme B (GrB) as the cytotoxic payload. We used as binding domains two different scFvs targeting the FRα from antibodies used in clinical trials. These human scFvs (designated mov003 and mov018) were fused to human GrB through an engineered IgG heavy-chain fragment (designated as Fc) which contains a dimerization domain. The constructs were cloned into the pSECTag vector containing an upstream 5 kDa purification tag with an enterokinase (EK) cleavage site for facile removal. The molecules were expressed in transiently-transfected HEK-293E suspension cells over 3 days at 37°C in 5% CO2. The molecules were isolated from culture media and purified by ion-exchange chromatography followed by EK cleavage to remove the free tag. The yields were 12-18 mg/L for both molecules. The assessment of the specific binding of each molecule to the antigen target is ongoing. The cytotoxicity of the constructs against a panel of cell lines expressing various levels of FRα showed a specific cytotoxic effect with an IC50 in the low nanomolar range (5-100 nM), particularly exhibiting very low IC50 values against breast (MDA-MB-231) and ovarian cell lines (IGROV-1). Receptor negative cells demonstrated IC50 levels in the micromolar range. Based on IC50 values, these data demonstrate suggest the fusion constructs targeting FRα are efficiently internalized and effectively deliver the active serine protease to the cytosol, activating the cytotoxic cascade. In vitro stability assays over 96 hours showed that both constructs are highly stable in mouse serum. Immunofluorescence and confocal studies to examine the time course of internalization and the pro-apoptotic events triggered by these agents are under examination and will be presented. Research conducted, in part, by the Clayton Foundation for Research.

Citation Format: Ana Alvarez de Cienfuegos, Laurence H. Cheung, Khalid A. Mohamedali, Walter N. Hittelman, Michael G. Rosenblum. A novel, completely human fusion construct containing active Granzyme B efficiently kills Folate Receptor alpha positive tumor cells [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5176.

Abstract 5194: Development of a cytotoxic fusion protein targeting VEGF receptors with improved cytotoxicity, stability and pharmacokinetics

The serine protease granzyme B (GrB), a key effector in immune mediated cell killing is capable of inducing apoptosis through both caspase-dependent and caspase-independent multiple-cascade mechanisms. VEGF121 is a naturally-occurring isoform of the VEGF-A family and binds to VEGF receptors R-1 and R-2, which are over-expressed on tumor cells as well as the endothelium of tumor vasculature but not normal vasculature. We have previously reported the expression of a fusion protein composed of Granzyme B and VEGF121 (GrB/VEGF121) and its subsequent in vitro and in vivo characterization. We hypothesized that incorporation of an IgG heavy chain Fc-fragment, resulting in a relatively higher molecular weight (133 kDa vs 80 kDa), would improve in vivo circulation and efficacy. Accordingly, the VEGF inter-chain dimerization domains were mutated so that dimerization occurred only through the Fc domain. Transient expression of GrB-Fc-VEGF121 in HEK-293E cells under serum-free conditions, followed by purification by immobilized metal affinity chromatography, resulted in yields in the range of 20-25 mg/L. Comparison of GrB-Fc-VEGF121 with its expected molecular weight suggests significant glycosylation when produced in HEK293E cells. The enzymatic activity of GrB in GrB-Fc-VEGF121 was comparable to that of commercially available human GrB. Cytotoxicity studies against a panel of tumor and endothelial cells indicated improved cytotoxicity against cell lines expressing high levels of VEGFR-1 or VEGFR-2, unlike GrB/VEGF121, which targeted VEGFR-2+ cells more efficiently. Control (receptor-negative) cells demonstrated IC50 levels in the high micromolar range. Comparison of ex vivo stability of GrB-Fc-VEGF121 to GrB/VEGF121 over 96 hours indicated GrB/VEGF121 signal reduced by approximately 50% over the first 24 hours of incubation, but minimal loss of protein was observed thereafter for the duration of the study. On the other hand, the protein loss of GrB-Fc-VEGF121 was more gradual, while the overall loss of protein over 96 hours was similar to that of GrB/VEGF121. Thus, GrB-Fc-VEGF121 is very stable in mouse serum and further pharmacokinetic analysis is warranted to determine its stability in vivo. In vivo pharmacokinetics and efficacy against various tumor models are currently ongoing and will be presented. In summary, the GrB-Fc-VEGF121 produced in HEK293E cells is active and has significant potential as a targeted therapeutic. Research conducted, in part, by the Clayton Foundation for Research.

Citation Format: Khalid A. Mohamedali, Lawrence H. Cheung, Ana A. Alvarez-Cienfuegos, Michael G. Rosenblum. Development of a cytotoxic fusion protein targeting VEGF receptors with improved cytotoxicity, stability and pharmacokinetics [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5194.

Abstract 2891: Targeting breast cancer stem cells with anti-EMP2 immunotoxins delivering granzyme b

Despite new therapies, breast cancer remains one of the leading causes of cancer-related death in women worldwide. Part of the issue remains the inability of these therapies to target cancer stem cells (CSCs). These cells show unique resistance to chemotherapy and have enhanced capacity for self-renewal. Recently, our group has shown that Epithelial membrane protein-2, or EMP2, is expressed on CSCs defined by increased aldehyde dehydrogenase activity (ALDH+). Clinical data supports the role of EMP2 in advanced breast cancer as increased expression of the protein is observed in metastatic lesions compared to primary tumors and its expression is enhanced in chemotherapy resistant tumors. In parallel, EMP2 expression correlated with enhanced mammosphere formation, an accepted surrogate for cells with enriched stem cell capacity. We have recently developed a novel IgG1 monoclonal antibody to EMP2, which has been shown to reduce tumor load. As the antibody-antigen complex rapidly internalizes, we hypothesized that its anti-tumor effects could be enhanced through the creation of a fusion protein. Granzyme B (GrB) is a serine protease that plays a critical role in the body's defense against viral infection and tumor development by initiating the apoptotic cascade via both caspase-dependent and -independent mechanisms. We developed novel fusion proteins composed of the anti-EMP2 backbone as the targeting moiety and granzyme B as the cytotoxic payload. Our initial design comprised of the active GrB fused to the IgG heavy chain (Fc) domain containing 2 single-chain antibodies against EMP2, a format that results in a longer plasma retention time. This new construct showed high and specific affinity for EMP2 (10−10M) using both flow cytometry and ELISA based formats. Moreover, treatment with these agents induced cell death to EMP2 positive cells, including those characterized by high ALDEFLUOR activity. Preclinical assessment of the anti-EMP2 immunotoxins showed no acute toxicity and acceptable plasma retention. Preliminary results using a 4T1 model system produced a significant reduction in tumor burden. Our results suggest that continued development of these agents may be successful at targeting advanced disease.

Research supported, in part, by NCI R21 CA234642 and the Clayton Foundation for Research.

Citation Format: Khalid A. Mohamedali, Kok Su-yin, Lawrence H. Cheung, Naina Dhawan, Michael G. Rosenblum, Madhuri Wadehra. Targeting breast cancer stem cells with anti-EMP2 immunotoxins delivering granzyme b [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2891.

Development of a human immuno-oncology therapeutic agent targeting HER2: targeted delivery of granzyme B

Background

Immunotherapeutic approaches designed to augment T and B cell mediated killing of tumor cells has met with clinical success in recent years suggesting tremendous potential for treatment in a broad spectrum of tumor types. After complex recognition of target cells by T and B cells, delivery of the serine protease granzyme B (GrB) to tumor cells comprises the cytotoxic insult resulting in a well-characterized, multimodal apoptotic cascade.

Methods

We designed a recombinant fusion construct, GrB-Fc-4D5, composed of a humanized anti-HER2 scFv fused to active GrB for recognition of tumor cells and internal delivery of GrB, simulating T and B cell therapy. We assessed the construct’s antigen-binding specificity and GrB enzymatic activity, as well as in vitro cytotoxicity and internalization into target and control cells. We also assessed pharmacokinetic and toxicology parameters in vivo.

Results

GrB-Fc-4D5 was highly cytotoxic to Her2 positive cells such as SKBR3, MCF7 and MDA-MB-231 with IC₅₀ values of 56, 99 and 27 nM, respectively, and against a panel of HER2+ cell lines regardless of endogenous expression levels of the PI-9 inhibitor. Contemporaneous studies with Kadcyla demonstrated similar levels of in vitro activity against virtually all cells tested. GrB-Fc-4D5 internalized rapidly into target SKOV3 cells within 1 h of exposure rapidly delivering GrB to the cytoplasmic compartment. In keeping with its relatively high molecular weight (160 kDa), the construct demonstrated a terminal-phase serum half-life in mice of 39.2 h. Toxicity studies conducted on BALB/c mice demonstrated no statistically significant changes in SGPT, SGOT or serum LDH. Histopathologic analysis of tissues from treated mice demonstrated no drug-related changes in any tissues examined.

Conclusion

GrB-Fc-4D5 shows excellent, specific cytotoxicity and demonstrates no significant toxicity in normal, antigen-negative murine models. This construct constitutes a novel approach against HER2-expressing tumors and is an excellent candidate for further development.

Abstract 4182: Impacting early events in metastatic spread: Targeted therapeutics to disrupt formation of the pre-metastatic niche and development of lung metastases

There is now growing interest in understanding the early events which lead to the successful formation of solid tumor metastatic spread because this information may lead to early interventional approaches. Bone marrow derived cells (BMDCs) of various lineages are gradually being identified as central to the ongoing growth of the primary tumor as well as the successful development of metastasis. Upon distant stimulation from the primary tumor, BMDCs migrate from the bone marrow to organ sites and self-assemble into a pre-metastatic niche that hosts migrating tumor cells from the primary site. Thus, disruption of this niche or selective targeting the migratory tumor cells may inhibit metastatic spread. Using GFP+ BMDC's from genetically engineered mice, we characterized the role that recruitment of BMDCs may play in breast cancer metastasis. We assessed the BMDC recruitment profile in lethally irradiated female nu/nu mice transplanted with GFP+ BMDCs from donor mice, followed by orthotopic placement of MDA-MB-231/luc cells or injected with MDA-MB-231/luc conditioned media for 30 days. Flow cytometry results show a gradual increase in the recruitment of CD11b+VEGFR-1+ cells in all the tissues examined from tumor-bearing mice, particularly lungs and lymph nodes. Recruitment of these cells to the liver in mice treated only with MDA-MB-231/luc conditioned media was also apparent and at higher levels than to livers of mice with the orthotopically-placed tumors. Significant recruitment of CD11b+VEGFR-2+ BMDCs was observed to liver, lung and lymph nodes. Interestingly, MDA-MB-231/luc conditioned media appeared to recruit this subset of cells more strongly to liver and lung tissues than did signaling from orthotopic tumors, while spleen and lymph node showed less recruitment of CD11b+VEGFR-2+ BMDCs. Treatment with GrB/VEGF fusion construct - a novel granzyme B-based pro-apoptotic fusion protein which specifically targets cells harboring VEGFRs - indicated that GrB/VEGF normalized recruitment of VEGFR-1+ or VEFGR-2+ cells to lungs when assessed two weeks after the final treatment. GrB/VEGF treatment also increased recruitment of F4/80+ macrophages to the lung, including those with the CD11b+Gr-1- signature, which were otherwise significantly reduced. GrB/VEGF also impacted the BMDC profile of several cell lineages in the primary tumor. Studies are ongoing to understand the impact of GrB/VEGF systemic administration on lung metastasis. Identifying key players in modulating the formation of the pre-metastatic niche and the early development of metastasis will be critical in designing targeted therapeutic approaches to inhibit the metastatic process. Research conducted, in part, by the Clayton Foundation for Research.

Citation Format: Khalid A. Mohamedali, Lawrence H. Cheung, Michael G. Rosenblum. Impacting early events in metastatic spread: Targeted therapeutics to disrupt formation of the pre-metastatic niche and development of lung metastases [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4182.

Abstract 2779: Molecular mechanistic and in vivo efficacy studies of Fn14-targeted fusion constructs containing human granzyme B

The past 15 years have seen impressive preclinical and clinical efforts to develop smart therapeutics with enhanced specificity for tumor cells and with much reduced “off-target” or nonspecific toxicity to normal tissues. For the antibody-drug conjugate approach, these constructs are composed of a targeting antigen binding domain, which delivers the cytotoxic domain into the cell. The majority of the cytotoxic payloads used in these constructs are categorized as either microtubule inhibitors or DNA-damaging drugs. We have developed a fusion protein, GrB-Fc-IT4 (MRT-101), that contains a humanized scFv binding domain targeting the cell surface receptor Fn14, an antigen highly expressed in a variety of solid tumors, and containing the human serine protease granzyme B (GrB) as the cytotoxic payload. The construct includes the IgG hinge Fc domain linker for efficient dimerization and an overall high molecular weight thereby designed to provide a prolonged serum half-life (~40 h). This unique format mimics human immune effector cell function and induces target cell death through activation of a variety of well described pro-apoptotic cascade signals. Western blot studies on human TNBC cells (MDA-MD-231) have shown that intact MRT101 is translocated into the cytosol in less than 1 h after exposure and is detectable in the cytosol for at least 8 h. The free GrB component is also detected by Western blot 4 h after treatment and persists for up to 8 h. Both of these agents trigger apoptotic cascades through activation of various caspases and induction of mitochondrial damage. Studies demonstrating cytochrome C release and mitochondrial depolarization are ongoing and will be reported. Incubation with the lysosomotropic agent chloroquine did not alter the IC50 of MRT-101, suggesting that the fusion protein is not appreciably held in the endosomal compartment. In vivo studies have shown that MRT-101 is well tolerated in BALB/c mice after intravenous administration of 5 doses at 20 mg/kg/dose. This dose level showed no evidence of toxicity in any of the major organs such as liver and kidneys. In vivo efficacy studies conducted on NSG-NOD scid mice demonstrated significant tumor growth inhibition of established orthotopic breast tumors (MDA-MB-231), with no tumor growth for up to 30 days after implantation. Treatment of nude mice bearing lung PDX tumors showed a 60% tumor growth inhibition when compared to the vehicle control group. These results, in combination with previous in vitro and in vivo studies, demonstrate that the completely human MRT-101 construct is a selective, highly potent, non-toxic and effective antigen-driven drug with significant potential for the treatment of Fn-14 positive tumors that acts through a new and unique mechanism of action. Research supported by Mirata Biopharma LLC and conducted, in part, by the Clayton Foundation for Research.

Citation Format: Ana Alvarez de Cienfuegos, Lawrence A. Cheung, Khalid A. Mohamedali, Landon J. Inge, Timothy Whitsett, Jeffrey A. Winkles, Louis DePalatis, Linda Paradiso, Michael G. Rosenblum. Molecular mechanistic and in vivo efficacy studies of Fn14-targeted fusion constructs containing human granzyme B [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2779.

Abstract 1646: Highly cytotoxic, completely human constructs targeting HER2 and containing the immuno-oncology payload granzyme B

Recent immunotherapeutic approaches designed to augment T- and B-cell mediated killing of tumor cells has met with clinical success suggesting that immuno-oncology (IO) approaches have tremendous potential for treatment in a broad spectrum of tumor types. After complex recognition of target cells by T and B cells, delivery of the serine protease granzyme B (GrB) to tumor cells comprises the cytotoxic insult resulting in a well-characterized, multimodal apoptotic cascade. We designed a recombinant fusion construct composed of a human anti-Her2 scFv fused to active GrB for recognition and delivery of GrB to tumor cells simulating T and B-cell therapy. The GrB-Fc-4D5 dimeric construct (mw 160 kDa) was generated and expressed in stably-transfected CHO-S cells at ~60 mg/L and purified to homogeneity. The enzymatic activity of the fusion construct was similar to commercially-available GrB and the affinity of the construct for purified Her2 extracellular domain (ECD) was 0.328 nM, comparable to that of Herceptin (0.150 nM). The GrB-Fc-4D5 construct was highly cytotoxic to Her2-positive cells such as SKBR3, MCF7 and MDA-MB-231 with IC50 values of 56, 99 and 27 nM respectively. Using immunofluorescence, the fusion construct internalized rapidly into target (SKBR3 or SKOV3) cells within 1 h of exposure, rapidly delivering GrB to the cytoplasmic compartment in a similar manner to that of immune T and B-cell targeting but without the action of the transmembrane pore-forming agent perforin. Against a large panel of various tumor types, GrB-Fc-4D5 was highly cytotoxic to virtually all cells regardless of natural expression levels of the nominal endogenous GrB inhibitor PI-9. Contemporaneous studies with Kadcyla demonstrate similar levels of in vitro activity against virtually all HER2-positive cells tested. GrB-Fc-4D5 demonstrated activity against both log-phase and confluent tumor cells. In keeping with its relatively high molecular weight (~160 kDa), the construct demonstrated a terminal-phase half-life of 13.6 hrs. In vivo efficacy studies are currently ongoing in several orthotopic xenograft models. Also ongoing are pre-IND toxicology, histopathology and clinical chemistry studies. Research conducted, in part, by the Clayton Foundation for Research.

Citation Format: Lawrence H. Cheung, Yunli Zhao, Khalid A. Mohamedali, Ana Alvarez-Cienfuegos, Walter N. Hittelman, Michael G. Rosenblum. Highly cytotoxic, completely human constructs targeting HER2 and containing the immuno-oncology payload granzyme B [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1646. doi:10.1158/1538-7445.AM2017-1646

Abstract 1628: Early events in metastatic spread: new approaches using targeted therapeutics to disrupt formation of the pre-metastatic niche and development of lung metastases

Solid tumors release soluble factors causing migration of a subset of normal bone marrow derived cells (BMDCs) - primarily VEGFR-1+ hematopoietic progenitor cells and VEGFR-2+ circulating endothelial progenitor cells - from the bone marrow to organ sites. These normal BMDCs self-assemble into a pre-metastatic niche- a micro-environment eventually hosting migratory tumor cells from the primary site. Disruption of this niche or selective targeting the migratory tumor cells may inhibit metastatic spread. The significance and specific characteristics of the BMDCs is still unclear. We have previously observed migration of VEGFR-1+ BMDCs to the lung and lymph nodes, and VEGFR-2+ BMDCs to liver, lung and lymph nodes. Using GFP+ BMDC’s from genetically engineered mice, we characterized the role that recruitment of BMDCs may play in breast cancer metastasis. We assessed the BMDC recruitment profile in lethally irradiated female nu/nu mice transplanted with GFP+ BMDCs from donor mice, followed by orthotopic placement of MDA-MB-231/luc cells or injected with MDA-MB-231/luc conditioned media for 30 days. Flow cytometry results show a gradual increase in the recruitment of CD11b+VEGFR-1+ cells in all the tissues examined from tumor-bearing mice. Recruitment of these cells to the liver in mice treated only with MDA-MB-231/Luc conditioned media was also apparent, but the increase was not as high as in livers of mice with the orthotopically-placed tumors. Recruitment of CD11b+VEGFR-2+ BMDCs was also observed but only in liver and lung. Interestingly, conditioned media seemed to recruit this subset of cells more strongly to these two tissues than signaling from orthotopic tumors. Spleen and lymph node showed minimal recruitment of CD11b+VEGFR-2+ BMDCs. We have previously developed the GrB/VEGF fusion construct- a novel pro-apoptotic fusion protein which specifically targets cells harboring VEGFR2 and have utilized this agent to specifically target BMDCs which are VEGFR2 +. Our preliminary data indicate that treatment with GrB/VEGF does not significantly alter the recruitment of VEGFR-1+ or VEFGR-2+ cells to lungs, when assessed two weeks after the final treatment. We observed increased recruitment of F4/80+ macrophages to the lung. On the other hand, CD11b+Gr-1- BMDCs were significantly reduced following GrB/VEGF treatment, although the F4/80+ subpopulation carrying this signature increased. Studies are ongoing to determine whether systemic administration of this agent can disrupt the formation of the niche and the eventual establishment of metastatic tumors. Understanding the role of BMDCs in metastatic spread and the formation of the pre-metastatic niche and their role in the early development of metastasis will be critical in designing targeted therapeutic approaches to inhibit the metastatic process. Research sponsored, in part, by the Clayton Foundation for Research.

Citation Format: Khalid A. Mohamedali, Lawrence H. Cheung, Michael G. Rosenblum. Early events in metastatic spread: new approaches using targeted therapeutics to disrupt formation of the pre-metastatic niche and development of lung metastases [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1628. doi:10.1158/1538-7445.AM2017-1628

Abstract 5615: Genomic differential expression analysis of fusion proteins incorporating the pro-apoptotic molecule Granzyme B reveals new potential targets for treatment of breast cancer

Granzyme B (GrB) is a member of the serine protease family of enzymes that play a critical role in the body’s defense against viral infection and tumor development. Cytotoxic T lymphocyctes (CTL) and natural killer (NK) cells directly deliver granzymes to target cells, which induces apoptosis through both caspase-dependent and caspase-independent multiple-cascade mechanisms. Anti-tumor efficacy studies have suggested that the targeted delivery of human Granzyme B to tumor cells has a significant potential for cancer treatment. We had previously developed a novel fusion construct composed of the VEGF121 growth factor and human GrB. The GrB/VEGF121 construct was found to be highly cytotoxic to vascular endothelial cells expressing the R2 (KDR) receptor for VEGF. In this study, we examined the mechanism of GrB/VEGF121-induced cytotoxicity against cells in culture at the genomic level. The TNBC cell line MDA-MB-231 was treated with an IC50 dose of GrB/VEGF121 for 24 hours; the cells were then harvested and the effect of GrB/VEGF121 on intracellular events was examined by extraction of mRNA followed by microarray analysis. Gene-level differential expression analysis revealed that a total of twenty genes were upregulated by over 3-fold, while 25 were downregulated 3-fold or more. These included genes involved in signal transduction, stress response, cell cycle control, hypoxia and metastasis. Particularly impacted were the Nuclear Receptors Meta pathway, NRF2 pathway, nuclear receptors in lipid metabolism and toxicity, matrix metalloproteinases, and the notch signaling pathway. Validated data will be reported following complete analysis of gene level differential expression and alternative splicing. Our data suggests that GrB/VEGF121 induces expression of genes known to be induced by VEGF alone, as well as molecules previously not associated with either Granzyme B or VEGF.This data suggests a previously unsuspected impact of the serine protease GrB on various molecular pathways within the cell and may lead to a new understanding of how these agents operate at the molecular level. Research conducted, in part, by the Clayton Foundation for Research.

Citation Format: Khalid A. Mohamedali, Lawrence H. Cheung, Ana Alvarez-Cienfuegos, Michael G. Rosenblum. Genomic differential expression analysis of fusion proteins incorporating the pro-apoptotic molecule Granzyme B reveals new potential targets for treatment of breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5615. doi:10.1158/1538-7445.AM2017-5615

Abstract 575: Human serine protease therapeutics- potent constructs targeting Fn14 in multiple tumor types

The cytokine TWEAK, a TNF superfamily member, and its cognate receptor, Fn14, have emerged as potentially valuable targets for cancer therapy since their expression has been frequently identified as independent negative prognostic indicators in a variety of tumor types. The serine protease Granzyme B (GrB) is a highly cytotoxic component of human immune effector cells and induces multiple, intense pro-apoptotic signals when delivered to the cytoplasm of target cells. This human protein has been well-studied, operating through multimodal pathways which are both caspase-dependent and caspase-independent. Our laboratory has designed series of new Fn14 targeted fusion constructs containing an engineered GrB payload. The GrB-Fc-IT4 containing either WTCYS or EACYS mutants of GrB, and a human scFv targeting Fn14 were expressed in HEK293E and suspension CHO cells, harvested under serum-free conditions and purified to homogeneity. Both constructs display high-level protein production (>50 mg/L), enhanced serum stability and impressive cytotoxic activity (IC50 < 20 nM) against Fn14 positive cells. The specific activity of the GrB moiety, as assessed by cleavage of a synthetic chromogenic GrB substrate, was comparable to that of human GrB for the WTCYS while the EACYS variant showed a 50% reduction in specific activity. On the other hand, in vitro stability studies showed that EACYS was more stable after 48h incubation in human serum, retaining over 60% of its initial specific activity. Against a panel of 20 human cell lines expressing Fn14, both constructs showed high affinity and selective cytotoxicity within the nanomolar range (IC50 ranged from 4 to 284 nM) and were two to over a hundred times more potent than free GrB. Mechanistic studies demonstrated that GrB-Fc-IT4 constructs activated caspase cascades and cytochrome C related pro-apoptotic mechanisms in keeping with the known intracellular functions of GrB in target cells. Pharmacokinetic studies in mice revealed that GrB-Fc-IT4 fusion proteins exhibited a bi-exponential clearance from plasma with a rapid initial clearance (t ½ α = 0.36 hours) followed by a prolonged terminal-phase plasma half-life (t 1/2 β = 35 hours). Toxicity studies in mice demonstrated that the MTD is above 100 mg/kg total dose. Murine orthotropic tumor model efficacy and bio-distribution studies are ongoing and will be presented. Research conducted, in part, by the Clayton Foundation for Research.

Citation Format: Ana Alvarez-Cienfuegos, Lawrence H. Cheung, Khalid A. Mohamedali, Yunli Zhao, Kathryn E. Ruisaard, Jeffrey A. Winkles, Michael G. Rosenblum. Human serine protease therapeutics- potent constructs targeting Fn14 in multiple tumor types. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 575.

Abstract 1203: Targeting activated tumor vasculature with human Granzyme B variants engineered for improved stability and activity

Angiogenesis is a critical process in numerous diseases, and targeting tumor neovasculature has therapeutic value in controlling tumor progression and metastatic spread. Vascular endothelial growth factor-A (VEGF-A) and its receptors VEGFR-1 and VEGFR-2 have been implicated as central mediators of normal angiogenesis and tumor neovascularization. VEGF121 is a naturally-occurring VEGF-A splice variant that binds to these receptors, which are over-expressed on the endothelium of activated tumor vasculature or neovasculature but not normal vasculature. Granzyme B (GrB) plays a critical role in the body's defense against viral infection and tumor development by initiating the apoptotic cascade through both caspase-dependent and -independent mechanisms. The fusion protein GrB/VEGF121 expressed in mammalian cells and purified to homogeniety was lethal to tumor and endothelial cells in vitro in a manner that correlated closely to total VEGFR-2 expression. IC50 levels were found to be in the nanomolar range. GrB/VEGF121 internalized rapidly into VEGFR-2 expressing cells while the internalization into VEGFR-1 expressing cells was significantly reduced. We engineered GrB variants for improved protein production and for resistance to serpin B9 (PI-9), a natural inhibitor of Granzyme B found at varying levels in cells and plasma, and compared these GrB/VEGF121 constructs directly to unmodified GrB/VEGF121. Variants took into account the impact of the mutation on substrate specificity, and the ability of the mutated GrB to cleave its various cellular substrates. As expected, mutation of the active site 195serine to alanine (S195A) resulted in a complete loss of GrB enzymatic activity as well as in vitro cytotoxicity against VEGFR-1+ and VEGFR-2+ cells. Enzymatic activity in PBS of the K27E, R28A (“EA-GV”) double mutant was identical to that of GrB/VEGF121. In 50% human serum, however, the enzymatic activity of GrB/VEGF121 steadily declined to less than 10% over 24 hours while the enzymatic activity of EA-GV remained over 40% during that time period. This construct retained higher cytotoxic activity after pre-incubation in serum for 4h, compared to GrB/VEGF121. C-GrB/VEGF121, without an unpaired cysteine, resulted in reduced aggregation and a vastly improved yield with otherwise similar serum stability and in vitro cytotoxicity as unmodified GrB/VEGF121. The pharmacokinetic profile for C-GrB/VEGF121 indicated a t1/2α of 6.6 min and t1/2β of 33.3 h. A limited profile for EA-GV indicates a longer terminal-phase half-life for this construct compared to C-GrB/VEGF121. Our studies suggest that these novel GrB variants may significantly improve in vivo half-life. Further studies are underway to determine whether these modifications impact production of soluble material and in vivo therapeutic efficacy against established tumor xenograft models. Research conducted, in part, by the Clayton Foundation for Research.

Citation Format: Khalid A. Mohamedali, Lawrence H. Cheung, Michael G. Rosenblum. Targeting activated tumor vasculature with human Granzyme B variants engineered for improved stability and activity. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1203.

Abstract 632: Tumor-targeted fusion constructs containing engineered granzyme B variants with optimized stability and potency

The serine protease Granzyme B (GrB) is highly cytotoxic generating multiple, intense pro-apoptotic signals when delivered to the cytoplasm of cells and operates through both caspase-dependent and caspase-independent mechanisms. Our laboratory has generated fusion constructs of this payload with VEGF, scFvMEL, BlyS, ML3-9, 4D5 and scFvIT4 targeting molecules thus far. The inhibitor serpin B9 (PI-9) is a 42 kDa protein found at varying levels in cells and plasma binding irreversibly to Granzyme B at the latter's catalytic domain serine (195Serine) resulting in inactive Granzyme B. We undertook modification studies of the GrB primary sequence to improve protein production, serum stability and biological activity. Granzyme B variants were engineered using PCR-based mutagenesis and fused to VEGF121 for direct comparision to the unmodified GrB/VEGF121 fusion protein. Variants designed to prevent binding to PI-9 took into account the impact of the mutation on substrate specificity, and the ability of the mutated GrB to cleave its various cellular substrates. As expected, mutation of 195serine to alanine (S195A) resulted in a complete loss of GrB enzymatic activity as well as in vitro cytotoxicity against VEGFR-1+ and VEGFR-2+ cells. The enzymatic activity of the K27E, R28A (“EA”) double mutant was identical to GrB/VEGF121 when the samples were incubated in PBS. In the presence of 50% human serum, however, the enzymatic activity of GrB/VEGF121 steadily declined to less than 10% over 24 hours while the enzymatic activity of EA-GrB/VEGF121 remained over 40% over that time period. The somewhat similar K27L, R28A (“LA”) double mutant appeared to behave intermediate to the wild-type protein and the EA construct. Both EA and LA GrB mutants retain higher cytotoxic activity after pre-incubation in serum for 4h, compared to unmodified GrB/VEGF121. Mutation of a putative self-cleaving domain and a glycosylation site in GrB appeared to improve enzymatic activity in serum compared to GrB/VEGF121. C210A-GrB/VEGF121, constructed to remove additional thiol sites which may cause aggregation, resulted in a vastly improved yield with otherwise similar serum stability and in vitro cytotoxicity as unmodified GrB/VEGF121. Constructs with modification of the 82PKN84 loop region alone or in combination with the EA mutation had either low activity or had no impact on PI-9 inactivation. Finally, a construct with a caspase-3-cleavable amino acid sequence at the N-terminus showed similar cytotoxic activity to the EA and LA constructs after pre-incubation in serum for 4h. Our studies suggest that these novel GrB variants may significantly improve in vivo half-life. Further studies are underway to determine whether these modifications impact production of soluble material and in vivo therapeutic efficacy. Research conducted, in part, by the Clayton Foundation for Research.

Citation Format: Khalid A. Mohamedali, Lawrence H. Cheung, Michael G. Rosenblum. Tumor-targeted fusion constructs containing engineered granzyme B variants with optimized stability and potency. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 632. doi:10.1158/1538-7445.AM2015-632

Abstract 4509: The fully human, proapoptotic fusion protein Granzyme B/VEGF121 targets tumor vasculature and inhibits tumor growth

Angiogenesis is a critical process in numerous diseases, and targeting neovascularization has therapeutic value in several disease states including tumor progression and metastatic spread. Vascular endothelial growth factor-A (VEGF-A) and its receptors play a central role in the control of neovascularization. Within the VEGF-A family, VEGF121 is a naturally-occurring splice variant that binds to receptors (VEGFR-1 and VEGFR-2) that are over-expressed on the endothelium of tumor vasculature but not normal vasculature. Granzyme B (GrB) is a serine protease that plays a critical role in the body's defense against viral infection and tumor development by initiating the apoptotic cascade via both caspase-dependent and -independent mechanisms. We expressed GrB/VEGF121 in HEK-293T cells and harvested the secreted fusion protein under serum-free conditions. Endothelial and tumor cells lines showed varying levels of sensitivity to GrB/VEGF121 that correlated closely to total VEGFR-2 expression. IC50 levels were found to be in the nanomolar range. GrB/VEGF121 internalized rapidly into VEGFR-2 expressing cells while the internalization into VEGFR-1 expressing cells was significantly reduced. Forty percent of VEGFR-2+ cells underwent mitochondrial depolarization within 24 h of exposure to GrB/VEGF121, compared to 13% of controls. Over this same time period, 35% of GrB/VEGF121-treated cells mobilized into early apoptosis, compared to 4% of control cells. In vivo, intravenous administration of GrB/VEGF121 to non-tumor bearing mice at doses up to 40 mg/kg had no observed toxicity. GrB/VEGF121 localized specifically in xenografts of non-VEGFR-expressing PC-3 and MDA-MB-231/Luc cells, primarily in perivascular regions and in other areas of the tumor less well-vascularized, whereas free GrB did not localize to tumor tissue. Treatment of mice bearing established prostate PC-3 tumor xenografts with GrB/VEGF121 at 27 mg/kg resulted in the regression of four of five tumors in this group. Tumors showed a two-fold lower Ki-67-labeling index compared with controls. Treatment of mice bearing orthotopic triple negative breast (MDA-MB-231/Luc) tumors with GrB/VEGF121 resulted in statistically significant differences in tumor volumes by Day 39 when compared to controls (saline vs 10 mg/kg GrB/VEGF121: P < 0.04; saline vs 25 mg/kg GrB/VEGF121: P < 0.03). In both tumor models, the vascular area as a percent of tumor area was significantly lower in tumor samples from mice treated with GrB/VEGF121 vs saline (PC-3: 14-fold, P < 0.0001; MDA-MB-231/Luc: 5-fold, P < 0.01) indicating that GrB/VEGF121 treatment directly impacted tumor vasculature. Our results show that targeted delivery of granzyme B to tumor vascular endothelial cells or to tumor cells activates apoptotic cascades and this completely human construct may have significant therapeutic potential. Research conducted, in part, by the Clayton Foundation for Research.

Citation Format: Khalid A. Mohamedali, Lawrence H. Cheung, Yu Cao, Walter N. Hittelman, Michael G. Rosenblum. The fully human, proapoptotic fusion protein Granzyme B/VEGF121 targets tumor vasculature and inhibits tumor growth. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4509. doi:10.1158/1538-7445.AM2014-4509

Abstract 4512: Construction and characterization of novel, human serine protease granzyme B-based cancer therapeutics targeting the TNFR family member Fn14

The TNF superfamily cytokine TWEAK and its receptor Fn14 have emerged as potentially clinically valuable targets for cancer therapy. The expression of Fn14 is comparatively low in normal tissues but elevated in various human solid tumor types, including brain, breast, lung and melanoma, and has been shown to be a negative prognostic indicator. We generated Granzyme B (GrB) containing, Fn14-targeted constructs using either the extracellular domain (ECD) of the human TWEAK ligand (GrB-TWEAK) or an engineered anti-Fn14 humanized single-chain antibody (GrB-Fc-IT4) as the targeting moieties. Both fusion constructs were expressed in mammalian HEK293T cells and purified to homogeneity from conditioned media. Both GrB-TWEAK and GrB-Fc-IT4 constructs were designed to be relatively large in molecular weight (150 kDa and 160 kDa respectively) to allow prolonged circulation in vivo. BiaCore analysis of GrB-TWEAK and GrB-Fc-IT4 binding to the Fn14 ECD indicated that these proteins bound to Fn14 with Kds of ∼8 nM and ∼18 nM, respectively. Confocal immunofluorescence analysis showed that both proteins specifically and rapidly (within 3 hrs) internalized into Fn14-expressing MDA-MB-231 breast cancer and HT-29 colorectal adenocarcinoma cells. GrB-TWEAK and GrB-Fc-IT4 demonstrated impressive and selective cytotoxicity to a panel of Fn14-expressing human tumor cell lines in the low nanomolar range. The endogenous presence of the GrB proteinase inhibitor 9 (PI-9) had no impact on the cytotoxic effect of either construct. Treatment of cells expressing the multidrug resistance protein MDR1 showed no cross-resistance to the fusion constructs in vitro. Mechanistic studies demonstrated that GrB-TWEAK and GrB-Fc-IT4 activated caspase cascades and cytochrome C-related pro-apoptotic mechanisms consistent with the known intracellular functions of GrB in target cells. Treatment of mice bearing well-established HT-29 xenografts with GrB-TWEAK (at 20 or 40 mg/kg total doses) showed significant tumor growth inhibition compared to controls (P< 0.05). GrB-TWEAK and GrB-Fc-IT4 were administered to mice bearing MDA-MB-231 orthotopic human breast xenograft tumors and both constructs displayed significant tumor growth inhibition. Treatment with GrB-Fc-IT4 displayed a more pronounced and prolonged tumor growth inhibition and longer survival than GrB-TWEAK treatment. IHC analysis demonstrates excellent tumor uptake of both agents. These data suggest that Fc-containing fusions with GrB may have a design advantage with respect to efficacy and may form the basis of novel, highly effective and nontoxic constructs for targeted therapeutic applications. Research conducted, in part, by the Clayton Foundation for Research (MGR) and National Institutes of Health grant R01 NS055126 (JAW).

Citation Format: Hong Zhou, Khalid A. Mohamedali, Yu Cao, Mary Migliorini, Lawrence H. Cheung, Walter N. Hittelman, Jeffrey A. Winkles, Michael G. Rosenblum. Construction and characterization of novel, human serine protease granzyme B-based cancer therapeutics targeting the TNFR family member Fn14. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4512. doi:10.1158/1538-7445.AM2014-4512

Development of human serine protease-based therapeutics targeting Fn14 and identification of Fn14 as a new target overexpressed in TNBC

The cytokine TWEAK and its receptor, Fn14, have emerged as potentially valuable targets for cancer therapy. Granzyme B (GrB)-containing Fn14-targeted constructs were generated containing either the Fn14 ligand TWEAK (GrB-TWEAK) or an anti-Fn14 humanized single-chain antibody (GrB-Fc-IT4) as the targeting moieties. Both constructs showed high affinity and selective cytotoxicity against a panel of Fn14-expressing human tumor cells including triple-negative breast cancer (TNBC) lines. Cellular expression of the GrB inhibitor PI-9 in target cells had no impact on the cytotoxic effect of either construct. Cellular expression of MDR1 showed no cross-resistance to the fusion constructs. GrB-TWEAK and GrB-Fc-IT4 activated intracellular caspase cascades and cytochrome c-related proapoptotic pathways consistent with the known intracellular functions of GrB in target cells. Treatment of mice bearing established HT-29 xenografts with GrB-TWEAK showed significant tumor growth inhibition compared with vehicle alone (P < 0.05). Both GrB-TWEAK and GrB-Fc-IT4 displayed significant tumor growth inhibition when administered to mice bearing orthotopic MDA-MB-231 (TNBC) tumor xenografts. The Cancer Genome Atlas analysis revealed that Fn14 mRNA expression was significantly higher in TNBC and in HER2-positive disease (P < 0.0001) compared with hormone receptor-positive breast cancer, and in basal-like 2 tumors (P = 0.01) compared with other TNBC molecular subtypes. IHC analysis of a 101 patient TNBC tumor microarray showed that 55 of 101 (54%) of tumors stained positive for Fn14, suggesting that this may be an excellent potential target for precision therapeutic approaches. Targeting Fn14 using fully human, GrB-containing fusion constructs may form the basis for a new class of novel, potent, and highly effective constructs for targeted therapeutic applications.

The functionalized human serine protease granzyme B/VEGF₁₂₁ targets tumor vasculature and ablates tumor growth

The serine protease granzyme B (GrB) induces apoptosis through both caspase-dependent and -independent multiple-cascade mechanisms. VEGF₁₂₁ binds to both VEGF receptor (VEGFR)-1 and VEGFR-2 receptors. We engineered a unique GrB/VEGF₁₂₁ fusion protein and characterized its properties in vitro and in vivo. Endothelial and tumor cell lines showed varying levels of sensitivity to GrB/VEGF₁₂₁ that correlated closely to total VEGFR-2 expression. GrB/VEGF₁₂₁ localized efficiently into VEGFR-2-expressing cells, whereas the internalization into VEGFR-1-expressing cells was significantly reduced. Treatment of VEGFR-2(+) cells caused mitochondrial depolarization in 48% of cells by 48 hours. Exposure to GrB/VEGF₁₂₁ induced apoptosis in VEGFR-2(+), but not in VEGFR-1(+), cells and rapid caspase activation was observed that could not be inhibited by treatment with a pan-caspase inhibitor. In vivo, GrB/VEGF₁₂₁ localized in perivascular tumor areas adjacent to microvessels and in other areas in the tumor less well vascularized, whereas free GrB did not specifically localize to tumor tissue. Administration (intravenous) of GrB/VEGF₁₂₁ to mice at doses up to 40 mg/kg showed no toxicity. Treatment of mice bearing established PC-3 tumor xenografts with GrB/VEGF₁₂₁ showed significant antitumor effect versus treatment with GrB or saline. Treatment with GrB/VEGF₁₂₁ at 27 mg/kg resulted in the regression of four of five tumors in this group. Tumors showed a two-fold lower Ki-67-labeling index compared with controls. Our results show that targeted delivery of GrB to tumor vascular endothelial cells or to tumor cells activates apoptotic cascades and this completely human construct may have significant therapeutic potential.

Abstract 5479: The fully human pro-apoptotic fusion toxin Granzyme B/VEGF121 targets vasculature and ablates tumor growth

Vascular endothelial growth factor 121 (VEGF121) is a naturally-occurring splice variant that binds to both VEGFR-1 and VEGFR-2 receptors over-expressed on the endothelium of tumor vasculature but not normal vasculature. The serine protease granzyme B (GrB) is capable of inducing apoptosis through both caspase-dependent and caspase-independent multiple-cascade mechanisms. We expressed GrB/VEGF121 in HEK293T cells and harvested the fusion protein under serum-free conditions. The specific activity of the GrB moiety, as assessed by cleavage of a synthetic chromogenic Granzyme B substrate, was comparable to that of human GrB. Cytotoxicity of GrB/VEGF121 indicated specific targeting into PAE/hVEGFR-2 (IC50 ∼ 10 nM), but not PAE/hVEGFR-1 cells (IC50 &gt; 1 μM). The targeting efficacy of GrB/VEGF121 against a panel of cells expressing a range of VEGFR receptor levels demonstrated varying levels of sensitivity to GrB/VEGF121. Internalization of GrB/VEGF121 into PAE/hVEGFR-2 and PAE/hVEGFR-1 cells was assessed by immunofluorescence and confocal microscopy 24 h after treatment. GrB/VEGF121 localized efficiently into PAE/hVEGFR-2 cells while the internalization into PAE/hVEGFR-1 cells was significantly lower. Forty-eight percent of PAE/VEGFR-2 cells underwent mitochondrial depolarization by 48 h, compared to 13% of untreated cells and 14% of cells treated with GrB. A significant degree of mitochondrial depolarization (34%) occurred within the first four hours of treatment. GrB/VEGF121 induced apoptosis in PAE/hVEGFR-2, but not in PAE/hVEGFR-1 cells, as determined by Annexin V staining. Thirty-five percent of GrB/VEGF121-treated PAE/VEGFR-2 cells were found to have mobilized into early apoptosis (Annexin V+ Propidium Iodide−), compared to 4% of control cells. The loss of plasma membrane integrity, as assessed by propidium iodide exclusion, was not observed for these cells over the 24 hour period, indicating that necrosis is not a major mechanism of cell death over the observed period of time. In vivo localization studies in tissues harvested four hours after GrB/VEGF121 or GrB i.v. injection detected GrB/VEGF121 in the tumors by immunofluorescence microscopy. Free GrB did not localize to tumor tissue. GrB/VEGF121 treatment appeared to be well tolerated by mice and significantly delayed tumor growth compared to treatment with saline or GrB alone. There was impressive antitumor efficacy at doses below the estimated Maximum Tolerated Dose (MTD). Our results suggest that targeted delivery of Granzyme B to tumor vascular endothelial cells or to tumor cells may have significant therapeutic potential. Research conducted, in part, by the Clayton Foundation for Research.

Citation Format: Khalid A. Mohamedali, Lawrence H. Cheung, Yu Cao, Walter N. Hittelman, Michael G. Rosenblum. The fully human pro-apoptotic fusion toxin Granzyme B/VEGF121 targets vasculature and ablates tumor growth. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5479. doi:10.1158/1538-7445.AM2013-5479

Abstract 860: In vitro and in vivo studies of a novel, functionalized, completely human serine protease targeting Her2/neu. Activity against Herceptin- and multi-drug resistant tumor cells

A number of recombinant fusion constructs and drug conjugates demonstrate excellent efficacy in preclinical studies but the long-term clinical applicability of these agents may be limited by tolerability issues and the emergence of multi-drug resistance. To address these issues, we employed the human serine protease granzyme B (GrB) as a cytotoxic effector and the humanized scFv designated 4D5 targeting the Her2/neu extracellular domain. A human fusion construct designated GrB/4D5/26 containing the fusogenic peptide 26 was constructed. The GrB/4D5/26 fusion construct showed specific and rapid cellular uptake into Her2/neu positive but not Her2/neu negative cells. We demonstrated efficient, specific delivery of GrB to the cytosol of Her2/neu positive cells. Against a panel of Her2/neu-positive tumor cells, IC50 values of the fusion construct ranged from 10∼100 nM, compared to that of GrB which was in the μM range. Treatment with GrB/4D5/26 resulted in specific cell killing accompanied by activation of caspases, PARP cleavage and cytochrome C release from mitochondria. In contrast to studies from other groups, the cytotoxic effects of the GrB/4D5/26 fusion construct did not require the assistance of the endosomolytic reagent chloroquine. Treatment with GrB/4D5/26 also caused efficient downregulation of the PI3K/Akt and Ras/ERK signaling pathways. Tumor cell lines highly resistant to Lapatinib, Herceptin or chemotherapeutic agents were not cross-resistant to the GrB-based fusion protein. Consecutive intravenous injections of GrB/4D5/26 into nude mice bearing BT474 M1 tumors resulted in significant, long-term tumor growth suppression. Twenty-four hours after i.v. administration, excised tumors displayed an increase in GrB/4D5/26 concentration and apoptotic nuclei compared to saline controls. These studies demonstrate the impressive in vitro and in vivo activity of the the completely human Her2/neu targeted construct and suggest its further evaluation for clinical use. Research conducted, in part, by the Clayton Foundation for Research.

Citation Format: Yu Cao, Khalid A. Mohamedali, John W. Marks, Lawrence H. Cheung, Walter N. Hittelman, Michael G. Rosenblum. In vitro and in vivo studies of a novel, functionalized, completely human serine protease targeting Her2/neu. Activity against Herceptin- and multi-drug resistant tumor cells . [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 860. doi:10.1158/1538-7445.AM2013-860

Construction and characterization of novel, completely human serine protease therapeutics targeting Her2/neu

Immunotoxins containing bacterial or plant toxins have shown promise in cancer-targeted therapy, but their long-term clinical use may be hampered by vascular leak syndrome and immunogenicity of the toxin. We incorporated human granzyme B (GrB) as an effector and generated completely human chimeric fusion proteins containing the humanized anti-Her2/neu single-chain antibody 4D5 (designated GrB/4D5). Introduction of a pH-sensitive fusogenic peptide (designated GrB/4D5/26) resulted in comparatively greater specific cytotoxicity although both constructs showed similar affinity to Her2/neu-positive tumor cells. Compared with GrB/4D5, GrB/4D5/26 showed enhanced and long-lasting cellular uptake and improved delivery of GrB to the cytosol of target cells. Treatment with nanomolar concentrations of GrB/4D5/26 resulted in specific cytotoxicity, induction of apoptosis, and efficient downregulation of PI3K/Akt and Ras/ERK pathways. The endogenous presence of the GrB proteinase inhibitor 9 did not impact the response of cells to the fusion construct. Surprisingly, tumor cells resistant to lapatinib or Herceptin, and cells expressing MDR-1 resistant to chemotherapeutic agents showed no cross-resistance to the GrB-based fusion proteins. Administration (intravenous, tail vein) of GrB/4D5/26 to mice bearing BT474 M1 breast tumors resulted in significant tumor suppression. In addition, tumor tissue excised from GrB/4D5/26-treated mice showed excellent delivery of GrB to tumors and a dramatic induction of apoptosis compared with saline treatment. This study clearly showed that the completely human, functionalized GrB construct can effectively target Her2/neu-expressing cells and displays impressive in vitro and in vivo activity. This construct should be evaluated further for clinical use.

The TWEAK receptor Fn14 is a therapeutic target in melanoma: immunotoxins targeting Fn14 receptor for malignant melanoma treatment

Fn14, the cell surface receptor for TWEAK, is over-expressed in various human solid tumor types and can be a negative prognostic indicator. We detected Fn14 expression in ~60% of the melanoma cell lines we tested, including both B-Raf WT and B-Raf^V600E lines. Tumor tissue microarray analysis indicated that Fn14 expression was low in normal skin but elevated in 173/190 (92%) of primary melanoma specimens and in 86/150 (58%) of melanoma metastases tested. We generated both a chemical conjugate composed of the rGel toxin and the anti-Fn14 antibody ITEM-4 (designated ITEM4-rGel) and a humanized, dimeric single-chain antibody of ITEM-4 fused to rGel (designated hSGZ). Both ITEM4-rGel and hSGZ were highly cytotoxic to a panel of different melanoma cell lines. Mechanistic studies showed that both immunotoxins induced melanoma cell necrosis. Also, these immunotoxins could up-regulate the cellular expression of Fn14 and trigger cell signaling events similar to the Fn14 ligand TWEAK. Finally, treatment of mice bearing human melanoma MDA-MB-435 xenografts with either ITEM4-rGel or hSGZ showed significant tumor growth inhibition compared to controls. We conclude that Fn14 is a novel therapeutic target in melanoma and the hSGZ construct appears to warrant further development as a novel therapeutic agent against Fn14-positive melanoma.

Pharmacodynamics, tissue distribution, toxicity studies and antitumor efficacy of the vascular targeting fusion toxin VEGF121/rGel

As a part of an ongoing assessment of its mechanism of action, we evaluated the in vivo pharmacokinetics, tissue distribution, toxicity and antitumor efficacy of VEGF(121)/rGel, a novel fusion protein. Pharmacokinetic studies showed that VEGF(121)/rGel cleared from the circulation in a biphasic manner with calculated half-lives of 0.3 and 6h for the alpha and beta phases, respectively. Pharmacokinetic evaluation of (64)Cu-DOTA-VEGF(121)/rGel showed relatively high blood retention 30 min after injection (26.6 ± 1.73% ID/g), dropping to 11.8 ± 2.83% and 0.82 ± 0.11% ID/g at 60 and 240 min post injection, respectively. Tissue uptake studies showed that kidneys, liver and tumor had the highest drug concentrations 48 h after administration. The maximum tolerated dose (MTD), based on a QOD×5 i.v. administration schedule, was found to be 18 mg/kg with an LD(50) of 25mg/kg. Treatment of BALB/c mice with VEGF(121)/rGel at doses up to the MTD caused no alterations in hematologic parameters. However, aspartate aminotransferase (AST) and alanine aminotransferase (ALT) parameters increased in a dose-related manner. The no-observable-adverse-effect-level (NOAEL) was determined to be 20% of the MTD (3.6 mg/kg). VEGF(121)/rGel treatment of mice bearing orthotopically-placed MDA-MB-231 breast tumors caused increased vascular permeability of tumor tissue by 53% compared to saline-treated controls. Immunohistochemical analysis showed significant tumor hypoxia and necrosis as a consequence of vascular damage. In summary, VEGF(121)/rGel appears to be an effective therapeutic agent causing focused damage to tumor vasculature with minimal toxic effects to normal organs. This agent appears to be an excellent candidate for further clinical development.

Noninvasive monitoring of orthotopic glioblastoma therapy response using RGD-conjugated iron oxide nanoparticles

Noninvasive imaging techniques have been considered important strategies in the clinic to monitor tumor early response to therapy. In the present study, we applied RGD peptides conjugated to iron oxide nanoparticles (IONP-RGD) as contrast agents in magnetic resonance imaging (MRI) to noninvasively monitor the response of a vascular disrupting agent VEGF(121)/rGel in an orthotopic glioblastoma model. RGD peptides were firstly coupled to IONPs coated with a crosslinked PEGylated amphiphilic triblock copolymer. In vitro binding assays confirmed that cellular uptake of particles was mainly dependent on the interaction between RGD and integrin α(v)β(3) of human umbilical vein endothelial cells (HUVEC). The tumor targeting of IONP-RGD was observed in an orthotopic U87 glioblastoma model. Finally, noninvasive monitoring of the tumor response to VEGF(121)/rGel therapy at early stages of treatment was successfully accomplished using IONP-RGD as a contrast agent for MRI, a superior method over common anatomical approaches which are based on tumor size measurements. This preclinical study can accelerate anticancer drug development and promote clinical translation of nanoprobes.

Abstract 3870: Construction and characterization of completely human serine protease therapeutics targeting Her2/neu

The serine protease granzyme B (GrB) is a powerful enzymatic inducer of apoptosis via a multi-modal intracellular mechanism including direct activation of the caspase cascade, damaging mitochondria resulting in the release of cytochrome C, and damaging nuclear matrix. We developed a novel recombinant immunotoxin linking GrB to a humanized Herceptin-derived scFv (4D5) specific for the Her2/neu cell-surface domain. The GrB/4D5 fusion constructs were efficiently expressed in HEK 293T mammalian cells as active glycosylated molecules and purified by immobilized cobalt affinity chromatography. In vitro studies demonstrated that GrB/4D5 retained the affinity and specificity of the original Herceptin antibody with a Kd of 0.24nM, as well as the biological activity of GrB (BAADT assay, enzymatic activity of 2.08×105U/µM, vs 1.39×105U/µM for free GrB). Target cell-specific activity was observed with IC50 of 10 to 50nM against a panel of Her2/neu positive tumor cells. In addition, the rapid induction of apoptosis in cells was confirmed by Annexin V staining and detection of activated caspase-3 in the cytosol. GrB/4D5 was active against BT474 M1 parental cells as well as Herceptin-resistant cells, despite the fact that these cells were more than 150 fold resistant to Herceptin. Addition of EGF or NRG-1 had no inhibitory effect on the cytotoxicity of the fusion construct against BT474 M1 cells. This is the first report of GrB-based immunotoxins having the potential to overcome Herceptin resistance arising in breast tumor cells. Further in vitro characterization against Her2/neu positive cancer cells expressing MDR or MRP-related proteins and in vivo xenograft model studies are ongoing. Research conducted, in part, by the Clayton Foundation for Research.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3870. doi:1538-7445.AM2012-3870

Abstract 3874: A novel human fusion protein composed of the serine protease Granzyme B and VEGF121 to target the vasculature of solid tumors

Targeting solid tumors and their metastases continue to be viewed as an essential aspect of therapeutic success to achieve long-term tumor control. Targeted therapy is a promising approach in this regard and the utilization of human proteins assures the potential for long-term therapy without immunological restrictions. VEGF121 is a naturally-occurring splice variant that binds to both VEGFR-1 and VEGFR-2 receptors over-expressed on the endothelium of tumor vasculature but not normal vasculature. The serine protease granzyme B (GrB) is capable of inducing intense cellular apoptosis through both caspase-dependent and caspase-independent multiple-cascade mechanisms. We expressed pro-GrB/VEGF121 in HEK293T cells and harvested the fusion protein under serum-free conditions. The purified protein expressed as a homodimer and Western blotting confirmed incorporation of both VEGF121 and GrB components into the construct. Comparison of GrB/VEGF121 with its expected molecular weight by SDS-PAGE suggests minimal (1-2 kDa) glycosylation. The enzymatic activity of GrB in GrB/VEGF121 was comparable to that of human GrB. The cytotoxicity of GrB/VEGF121 confirmed specific targeting and internalization into PAE/hVEGFR-2 (IC50 ∼ 10 nM), but not PAE/hVEGFR-1 cells (IC50 &gt; 300 nM). GrB/VEGF121 induced apoptosis in PAE/VEGFR-2, but not PAE/VEGFR-1 cells, as determined by Annexin V staining. The targeting efficacy of GrB/VEGF121 against a panel of cells expressing a range of VEGFR receptor levels is under study. Studies investigating GrB/VEGF121 signaling events and in vivo efficacy against various tumor models are currently ongoing and will be presented. Research conducted, in part, by the Clayton Foundation for Research.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3874. doi:1538-7445.AM2012-3874

Noncovalent functionalization of carbon nanovectors with an antibody enables targeted drug delivery

Current chemotherapeutics are characterized by efficient tumor cell-killing and severe side effects mostly derived from off-target toxicity. Hence targeted delivery of these drugs to tumor cells is actively sought. We previously demonstrated that poly(ethylene glycol)-functionalized carbon nanovectors are able to sequester paclitaxel, a widely used hydrophobic cancer drug, by simple physisorption and thereby deliver the drug for killing of cancer cells. The cell-killing when these drug-loaded carbon nanoparticles were used was equivalent to when a commercial formulation of paclitaxel was used. Here we show that by further mixing the drug-loaded nanoparticles with Cetuximab, a monoclonal antibody that recognizes the epidermal growth factor receptor (EGFR), paclitaxel is preferentially targeted to EGFR+ tumor cells in vitro. This supports progressing to in vivo studies. Moreover, the construct is unusual in that all three components are assembled through noncovalent interactions. Such noncovalent assembly could enable high-throughput screening of drug/antibody combinations.

Cytotoxicity of VEGF(121)/rGel on vascular endothelial cells resulting in inhibition of angiogenesis is mediated via VEGFR-2

Background

The fusion protein VEGF₁₂₁/rGel composed of the growth factor VEGF₁₂₁ and the plant toxin gelonin targets the tumor neovasculature and exerts impressive anti-vascular effects. We have previously shown that VEGF₁₂₁/rGel is cytotoxic to endothelial cells overexpressing VEGFR-2 but not to endothelial cells overexpressing VEGFR-1. In this study, we examined the basis for the specific toxicity of this construct and assessed its intracellular effects in vitro and in vivo.

Methods

We investigated the binding, cytotoxicity and internalization profile of VEGF₁₂₁/rGel on endothelial cells expressing VEGFR-1 or VEGFR-2, identified its effects on angiogenesis models in vitro and ex vivo, and explored its intracellular effects on a number of molecular pathways using microarray analysis.

Results

Incubation of PAE/VEGFR-2 and PAE/VEGFR-1 cells with ¹²⁵I-VEGF₁₂₁/rGel demonstrated binding specificity that was competed with unlabeled VEGF₁₂₁/rGel but not with unlabeled gelonin. Assessment of the effect of VEGF₁₂₁/rGel on blocking tube formation in vitro revealed a 100-fold difference in IC₅₀ levels between PAE/VEGFR-2 (1 nM) and PAE/VEGFR-1 (100 nM) cells. VEGF₁₂₁/rGel entered PAE/VEGFR-2 cells within one hour of treatment but was not detected in PAE/VEGFR-1 cells up to 24 hours after treatment. In vascularization studies using chicken chorioallantoic membranes, 1 nM VEGF₁₂₁/rGel completely inhibited bFGF-stimulated neovascular growth. The cytotoxic effects of VEGF₁₂₁/rGel were not apoptotic since treated cells were TUNEL-negative with no evidence of PARP cleavage or alteration in the protein levels of select apoptotic markers. Microarray analysis of VEGF₁₂₁/rGel-treated HUVECs revealed the upregulation of a unique "fingerprint" profile of 22 genes that control cell adhesion, apoptosis, transcription regulation, chemotaxis, and inflammatory response.

Conclusions

Taken together, these data confirm the selectivity of VEGF₁₂₁/rGel for VEGFR-2-overexpressing endothelial cells and represent the first analysis of genes governing intoxication of mammalian endothelial cells by a gelonin-based targeted therapeutic agent.

Abstract 1767: Targeting prostate cancer osteoblastic progression with VEGF121/rGel, a single agent targeting osteoblasts, osteoclasts, and tumor neovasculature

Purpose: A hallmark of prostate cancer (PCa) progression is the development of osteoblastic bone metastases, which respond poorly to available therapies. We previously reported that VEGF121/rGel targets osteoclast precursors and tumor neovasculature. Here we tested the hypothesis that targeting non-tumor cells expressing the receptors for VEGF121, namely VEGFR-1 and VEGFR-2, can inhibit tumor progression in a clinically relevant model of osteoblastic PCa.

Experimental Design: We examined the effect of VEGF121/rGel on osteoblast precursors and several PCa cell lines in vitro; on osteoid formation in a mouse calvaria culture assay ex vivo; and on PCa osteoblastic progression in the femurs of mice injected with MDA PCa 118b, a PCa xenograft obtained from a bone metastasis in a patient with castrate-resistant PCa.

Results: VEGF121/rGel was cytotoxic in vitro to osteoblast precursor cells. This cytotoxicity was specific as VEGF121/rGel internalization into osteoblasts was VEGF121 receptor driven. Furthermore, VEGF121/rGel significantly inhibited PCa-induced bone formation in a mouse calvaria culture assay. In vivo, systemic administration of VEGF121/rGel significantly inhibited the osteoblastic progression of PCa cells in the femurs of nude mice. Microcomputed tomography analysis revealed that VEGF121/rGel restored the bone volume fraction of tumor-bearing femurs to values similar to those of the contralateral (non-tumor bearing) femurs. VEGF121/rGel significantly reduced the number of tumor-associated osteoclasts but did not change the numbers of peritumoral osteoblasts. Importantly, VEGF121/rGel-treated mice had significantly less tumor burden than control mice did. Our results thus indicate that VEGF121/rGel inhibits osteoblastic tumor progression by targeting angiogenesis, osteoclastogenesis, and bone formation.

Conclusions: Targeting VEGFR-1- or VEGFR-2-expressing cells is effective in controlling the osteoblastic progression of PCa in bone. These findings provide the basis for an effective multitargeted approach for metastatic PCa. We believe that VEGF121/rGel in combination with tumor cell-targeting therapies (e.g., chemotherapy) constitutes a novel strategy for advanced PCa. Research conducted, in part, by the Clayton Foundation for Research.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1767. doi:10.1158/1538-7445.AM2011-1767

Inhibition of prostate cancer osteoblastic progression with VEGF121/rGel, a single agent targeting osteoblasts, osteoclasts, and tumor neovasculature

PURPOSE

A hallmark of prostate cancer (PCa) progression is the development of osteoblastic bone metastases, which respond poorly to available therapies. We previously reported that VEGF(121)/rGel targets osteoclast precursors and tumor neovasculature. Here we tested the hypothesis that targeting nontumor cells expressing these receptors can inhibit tumor progression in a clinically relevant model of osteoblastic PCa.

EXPERIMENTAL DESIGN

Cells from MDA PCa 118b, a PCa xenograft obtained from a bone metastasis in a patient with castrate-resistant PCa, were injected into the femurs of mice. Osteoblastic progression was monitored following systemic administration of VEGF(121)/rGel.

RESULTS

VEGF(121)/rGel was cytotoxic in vitro to osteoblast precursor cells. This cytotoxicity was specific as VEGF(121)/rGel internalization into osteoblasts was VEGF(121) receptor driven. Furthermore, VEGF(121)/rGel significantly inhibited PCa-induced bone formation in a mouse calvaria culture assay. In vivo, VEGF(121)/rGel significantly inhibited the osteoblastic progression of PCa cells in the femurs of nude mice. Microcomputed tomographic analysis revealed that VEGF(121)/rGel restored the bone volume fraction of tumor-bearing femurs to values similar to those of the contralateral (non-tumor-bearing) femurs. VEGF(121)/rGel significantly reduced the number of tumor-associated osteoclasts but did not change the numbers of peritumoral osteoblasts. Importantly, VEGF(121)/rGel-treated mice had significantly less tumor burden than control mice. Our results thus indicate that VEGF(121)/rGel inhibits osteoblastic tumor progression by targeting angiogenesis, osteoclastogenesis, and bone formation.

CONCLUSIONS

Targeting VEGF receptor (VEGFR)-1- or VEGFR-2-expressing cells is effective in controlling the osteoblastic progression of PCa in bone. These findings provide the basis for an effective multitargeted approach for metastatic PCa.

Multiplexed PET probes for imaging breast cancer early response to VEGF₁₂₁/rGel treatment

In this study, we applied multiplexed positron emission tomography (PET) probes to monitor glucose metabolism, cellular proliferation, tumor hypoxia and angiogenesis during VEGF₁₂₁/rGel therapy of breast cancer. Two doses of 12 mg/kg VEGF₁₂₁/rGel, administered intraperitoneally, resulted in initial delay of tumor growth, but the growth resumed 4 days after tumor treatment was stopped. The average tumor growth rate expressed as V/V(0), were 1.11 ± 0.07, 1.21 ± 0.10, 1.58 ± 0.36 and 2.64 ± 0.72 at days 1, 3, 7 and 14, respectively. Meanwhile, the VEGF₁₂₁/rGel treatment group showed V/V₀ ratios of 1.04 ± 0.06, 1.05 ± 0.11, 1.09 ± 0.17 and 1.86 ± 0.36 at days 1, 3, 7 and 14, respectively. VEGF₁₂₁/rGel treatment led to significantly decreased uptake of ¹⁸F-FPPRGD2 at day 1 (24.0 ± 8.8%, p < 0.05) and day 3 (36.3 ± 9.2%, p < 0.01), relative to the baseline, which slowly recovered to the baseline at day 14. ¹⁸F-FMISO uptake was increased in the treated tumors at day 1 (23.9 ± 15.7%, p < 0.05) and day 3 (51.4 ± 29.4%, p < 0.01), as compared to the control group. At days 7 and 14, ¹⁸F-FMISO uptake restored to the baseline level. The relative reductions in FLT uptake in treated tumors were approximately 13.0 ± 4.5% at day 1 and 25.0 ± 4.4% (p < 0.01) at day 3. No significant change of ¹⁸F-FDG uptake was observed in VEGF₁₂₁/rGel treated tumors, compared with the control group. The imaging findings were supported by ex vivo analysis of related biomarkers. Overall, longitudinal imaging studies with 4 PET tracers demonstrated the feasibility and usefulness of multiplexed probes for quantitative measurement of antitumor effects of VEGF₁₂₁/rGel at the early stage of treatment. This preclinical study should be helpful in accelerating anticancer drug development and promoting the clinical translation of molecular imaging.

Abstract 5221: Angiogenesis Imaging of Orthotopic Bladder Tumors using a Smart MRI Contrast Agent Containing VEGF121/rGel

Comprehension of the angiogenic vessels from solid tumors and metastatic lesions is a crucial step for the treatment of cancer. In particular, recognition of VEGFR-2 (KDR) as primary proangiogenic factors and their distribution analysis is very important for establishment of the treatment strategy. In this research, we thus formulated the smart imaging probes based on VEGF121/rGel conjugated MnFe2O4 for imaging of tumor neovasculature using an orthotopic bladder tumor xenograft model. VEGF121/rGel was conjugated to MnFe2O4 nanoparticles (MNPs). Their colloidal size was around 45 nm and a high relaxivity coefficient was 423 mM−1s−1. As in vitro test, the targeting efficiency and detection capability of the VEGF121/rGel-MNPs were investigated in both KDR deficient (253JB-V) and KDR-overexpressing (PAE/KDR) cell lines using MRI, respectively. The targeting and internalization of VEGF121/rGel-MNPs into cells was confirmed by electron microscopy. Their phosphorylation ability and cytotoxicity were compared to unconjugated VEGF121/rGel. They demonstrated astonishing targeting capability against KDR-overexpressing PAE/KDR cells, as confirmed by dose-dependent MR images and VEGF121 inhibition tests. The phosphorylation activity of KDR and cytotoxicity of VEGF121/rGel-MNPs were evaluated. The orthotopic tumor mouse model was established by implanting low KDR-expressing 253JB-V cells into the bladder dome. After tail-vein injection of VEGF121/rGel-MNPs, the MR signal enhancement of intratumoral vessels by VEGF121/rGel-MNPs was observed and inhibition test using VEGF121 was also conducted. VEGF121/rGel-MNPs successfully targeted the tumor and provided accurate anatomical details through: i) acquisition of clear neoangiogenic vascular distributions and ii) obvious enhancement of the MR signal in T2*-weighted images. Immunostaining and blocking studies demonstrated the specific targeting ability of VEGF121/rGel-MNPs towards intratumoral angiogenesis. Furthermore, ex vivo MR imaging of tumor tissue, and a CD-31 and rGel fluorescence immunostaining study revealed the targeting ability to VEGFR-2 on the angiogenetic vessels. Consequently, synthesized VEGF121/rGel-MNPs as valuable MR imaging contrast agents can be specifically delivered to tumors and bind to KDR-expressing angiogenic tumor vessels.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5221.

Abstract 711: VEGF121/rGel inhibits prostate cancer-induced osteoblastogenesis by targeting osteoblasts and tumor neovasculature

Prostate cancer metastasis to bone usually produces bone-forming lesions although an osteolytic component is present in most cases. We have previously shown excellent efficacy against prostate cancer growth in bone in an osteolytic model (PC-3) with VEGF121/rGel, a chimeric fusion toxin of VEGF121 and the plant toxin gelonin (rGel) that targets VEGFR-1 and VEGFR-2. In the current study, we demonstrate that VEGF121/rGel systemic administration is also effective in a model of prostate cancer that displays the bone-forming phenotype when growing in the femurs of immunodeficient mice. Both murine osteoblast precursor (MC3T3) cells and primary mouse osteoblasts (PMOs) were shown to express VEGFR-1 but not VEGFR-2. VEGFR-1 mRNA expression was shown to down-regulate during osteoblast (MC3T3) differentiation. In vitro, treatment with VEGF121/rGel showed cytotoxicity against osteoblast precursor cells (IC50 = 15 nM) that was substantially reduced (IC50 &gt; 1000 nM) after MC3T3 had been allowed to undergo differentiation. This suggests that the effect of VEGF121/rGel is specifically mediated by VEGFR-1. Furthermore, immunofluorescence microscopy showed that internalization of VEGF121/rGel into PMOs is VEGF121-receptor driven. In an ex vivo model of osteoblastic disease, 100 nM VEGF121/rGel significantly inhibited prostate cancer-mediated new bone formation in neonatal mouse calvaria. In vivo, systemic (iv, tail vein) administration of VEGF121/rGel significantly inhibited growth of the osteoblastic prostate cancer cell line MDA PCa 118b growing intrafemorally. Only 25% of VEGF121/rGel-treated mice showed the development of osteoblastic lesions compared to 90% of saline-treated mice. Micro-CT (µCT) analysis of isolated femurs demonstrated that intrafemoral growth of MDA PCa118b tumors caused a dramatic increase in bone volume and that treatment with VEGF121/rGel restored the bone volume fraction to normalized levels with no changes to the uninvolved contralateral femurs. H&E results confirmed that the osteoblastic growth of 118b cells was severely impeded. Our results clearly demonstrate that VEGF121/rGel administration may suppress the eventual development of skeletal prostate tumors and may have significant therapeutic effect against prostate cancer-mediated osteoblastic lesions in bone. Research conducted, in part, by the Clayton Foundation for Research.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 711.

Imaging of VEGF receptor in a rat myocardial infarction model using PET

Myocardial infarction (MI) leads to left ventricular (LV) remodeling, which leads to the activation of growth factors such as vascular endothelial growth factor (VEGF). However, the kinetics of a growth factor's receptor expression, such as VEGF, in the living subject has not yet been described. We have developed a PET tracer (64Cu-DOTA-VEGF121 [DOTA is 1,4,7,10-tetraazadodecane-N,N',N'',N'''-tetraacetic acid]) to image VEGF receptor (VEGFR) expression after MI in the living subject.

METHODS

In Sprague-Dawley rats, MI was induced by ligation of the left coronary artery and confirmed by ultrasound (n = 8). To image and study the kinetics of VEGFRs, 64Cu-DOTA-VEGF121 PET scans were performed before MI induction (baseline) and on days 3, 10, 17, and 24 after MI. Sham-operated animals served as controls (n = 3).

RESULTS

Myocardial origin of the 64Cu-DOTA-VEGF121 signal was confirmed by CT coregistration and autoradiography. VEGFR specificity of the 64Cu-DOTA-VEGF121 probe was confirmed by in vivo use of a 64Cu-DOTA-VEGFmutant. Baseline myocardial uptake of 64Cu-DOTA-VEGF121 was minimal (0.30 +/- 0.07 %ID/g [percentage injected dose per gram of tissue]); it increased significantly after MI (day 3, 0.97 +/- 0.05 %ID/g; P < 0.05 vs. baseline) and remained elevated for 2 wk (up to day 17 after MI), after which time it returned to baseline levels.

CONCLUSION

We demonstrate the feasibility of imaging VEGFRs in the myocardium. In summary, we imaged and described the kinetics of 64Cu-DOTA-VEGF121 uptake in a rat model of MI. Studies such as the one presented here will likely play a major role when studying pathophysiology and assessing therapies in different animal models of disease and, potentially, in patients.

Multimodality molecular imaging of glioblastoma growth inhibition with vasculature-targeting fusion toxin VEGF121/rGel

Vascular endothelial growth factor A (VEGF-A) and its receptors, Flt-1/FLT-1 (VEGFR-1) and Flk-1/KDR (VEGFR-2), are key regulators of tumor angiogenesis and tumor growth. The purpose of this study was to determine the antiangiogenic and antitumor efficacies of a vasculature-targeting fusion toxin (VEGF(121)/rGel) composed of the VEGF-A isoform VEGF(121) linked with a G(4)S tether to recombinant plant toxin gelonin (rGel) in an orthotopic glioblastoma mouse model by use of noninvasive in vivo bioluminescence imaging (BLI), MRI, and PET.

METHODS

Tumor-bearing mice were randomized into 2 groups and balanced according to BLI and MRI signals. PET with (64)Cu-1,4,7,10-tetraazacyclododedane-N,N',N'',N'''-tetraacetic acid (DOTA)-VEGF(121)/rGel was performed before VEGF(121)/rGel treatment. (18)F-Fluorothymidine ((18)F-FLT) scans were obtained before and after treatment to evaluate VEGF(121)/rGel therapeutic efficacy. In vivo results were confirmed with ex vivo histologic and immunohistochemical analyses.

RESULTS

Logarithmic transformation of peak BLI tumor signal intensity revealed a strong correlation with MRI tumor volume (r = 0.89, n = 14). PET with (64)Cu-DOTA-VEGF(121)/rGel before treatment revealed a tumor accumulation (mean +/- SD) of 11.8 +/- 2.3 percentage injected dose per gram at 18 h after injection, and the receptor specificity of the tumor accumulation was confirmed by successful blocking of the uptake in the presence of an excess amount of VEGF(121). PET with (18)F-FLT revealed significant a decrease in tumor proliferation in VEGF(121)/rGel-treated mice compared with control mice. Histologic analysis revealed specific tumor neovasculature damage after treatment with 4 doses of VEGF(121)/rGel; this damage was accompanied by a significant decrease in peak BLI tumor signal intensity.

CONCLUSION

The results of this study suggest that future clinical multimodality imaging and therapy with VEGF(121)/rGel may provide an effective means to prospectively identify patients who will benefit from VEGF(121)/rGel therapy and then stratify, personalize, and monitor treatment to obtain optimal survival outcomes.

Overexpression of biologically active VEGF121 fusion proteins in Escherichia coli

Vascular endothelial growth factor-A (VEGF) exists as five different isoforms, which exert their growth stimulatory effects through interaction with the FLK and KDR receptors. The VEGF(121) isoform has been employed as a highly selective carrier of therapeutic agents to target tumor endothelial cells resulting in inhibition of tumor growth and metastasis. VEGF(121) and VEGF(121)/rGel fusion toxin containing hexa-histidine tags were expressed in Escherichia coli AD494 (DE3) pLysS. Media containing glycerol as a primary carbon source increased the specific expression levels of soluble VEGF(121) and VEGF(121)/rGel (mg/L/OD10) by more than two-fold over LB media when grown in a batchtype cultivation in a bioreactor. High cell densities over OD 40 were achieved using a fed-batch method and employing feeding medium containing glycerol and yeast extract. The overall production of the target proteins was improved 18-fold for VEGF(121) (59.2mg/L) and 27-fold for VEGF(121)/rGel (42.5mg/L), respectively, compared to the conventional flask cultivation method (3.3 and 1.6mg/L for VEGF(121) and VEGF(121)/rGel, respectively). The purified VEGF(121) and VEGF(121)/rGel fusion proteins were biologically active as assessed by phosphorylation of KDR receptors and cytotoxicity against KDR expressing cells.

PET of vascular endothelial growth factor receptor expression

For solid tumors and metastatic lesions, tumor vascularity is a critical factor in assessing response to therapy. Here we report the first example, to our knowledge, of (64)Cu-labeled vascular endothelial growth factor 121 (VEGF(121)) for PET of VEGF receptor (VEGFR) expression in vivo.

METHODS

VEGF(121) was conjugated with 1,4,7,10-tetraazadodecane-N,N',N'',N'''-tetraacetic acid (DOTA) and then labeled with (64)Cu for small-animal PET of mice bearing different sized U87MG human glioblastoma xenografts. Blocking experiments and ex vivo histopathology were performed to confirm the in vivo results.

RESULTS

There were 4.3 +/- 0.2 DOTA molecules per VEGF(121), and the VEGFR2 binding affinity of DOTA-VEGF(121) was comparable to VEGF(121). (64)Cu labeling of DOTA-VEGF(121) was achieved in 90 +/- 10 min and the radiolabeling yield was 87.4% +/- 3.2%. The specific activity of (64)Cu-DOTA-VEGF(121) was 3.2 +/- 0.1 GBq/mg with a radiochemical purity of >98%. Small-animal PET revealed rapid, specific, and prominent uptake of (64)Cu-DOTA-VEGF(121) in small U87MG tumors (high VEGFR2 expression) but significantly lower and sporadic uptake in large U87MG tumors (low VEGFR2 expression). No appreciable renal clearance of (64)Cu-DOTA-VEGF(121) was observed, although the kidney uptake was relatively high likely due to VEGFR1 expression. Blocking experiments, immunofluorescence staining, and western blot confirmed the VEGFR specificity of (64)Cu-DOTA-VEGF(121).

CONCLUSION

Successful demonstration of the ability of (64)Cu-DOTA-VEGF(121) to visualize VEGFR expression in vivo may allow for clinical translation of this radiopharmaceutical for imaging tumor angiogenesis and guiding antiangiogenic treatment, especially patient selection and treatment monitoring of VEGFR-targeted cancer therapy.

Inhibition of Prostate Tumor Growth and Bone Remodeling by the Vascular Targeting Agent VEGF121/rGel

The pathophysiology of tumor growth following skeletal metastases and the poor response of this type of lesion to therapeutic intervention remains incompletely understood. Vascular endothelial growth factor (VEGF)-A and its receptors play a role in both osteoclastogenesis and tumor growth. Systemic (i.v.) treatment of nude mice bearing intrafemoral prostate (PC-3) tumors with the vascular ablative agent VEGF(121)/recombinant gelonin (rGel) strongly inhibited tumor growth. Fifty percent of treated animals had complete regression of bone tumors with no development of lytic bone lesions. Immunohistochemical analysis showed that VEGF(121)/rGel treatment suppressed tumor-mediated osteoclastogenesis in vivo. In vitro treatment of murine osteoclast precursors, both cell line (RAW264.7) and bone marrow-derived monocytes (BMM), revealed that VEGF(121)/rGel was selectively cytotoxic to osteoclast precursor cells rather than mature osteoclasts. VEGF(121)/rGel cytotoxicity was mediated by Flt-1, which was down-regulated during osteoclast differentiation. Analysis by flow cytometry and reverse transcription-PCR showed that both BMM and RAW264.7 cells display high levels of Flt-1 but low levels of Flk-1. Internalization of VEGF(121)/rGel into osteoclast precursor cells was suppressed by pretreatment with an Flt-1 neutralizing antibody or by placenta growth factor but not with an Flk-1 neutralizing antibody. Thus, VEGF(121)/rGel inhibits osteoclast maturation in vivo and it seems that this process is important in the resulting suppression of skeletal osteolytic lesions. This is a novel and unique mechanism of action for this class of agents and suggests a potentially new approach for treatment or prevention of tumor growth in bone.

The vascular-targeting fusion toxin VEGF121/rGel inhibits the growth of orthotopic human bladder carcinoma tumors

Vascular endothelial growth factor (VEGF) and its receptors (FLT-1 and KDR) are overexpressed by human bladder cancer cells and tumor endothelial cells, respectively. Strategies that target VEGF receptors hold promise as antiangiogenic therapeutic approaches to bladder cancer. A fusion protein of VEGF121 and the plant toxin gelonin (rGel) was constructed, expressed in bacteria, and purified to homogeneity. Cytotoxicity experiments of VEGF121/rGel on the highly metastatic 253J B-V human bladder cancer cell line demonstrated that the VEGF121/rGel does not specifically target these cells, whereas Western blot analysis showed no detectable expression of KDR. Treatment with VEGF121/rGel against orthotopically implanted 253J B-V xenografts in nude mice resulted in a significant suppression of bladder tumor growth (approximately 60% inhibition; P < .05) compared to controls. Immunohistochemistry studies of orthotopic 253J B-V tumors demonstrated that KDR is highly overexpressed in tumor vasculature. Immunofluorescence staining with antibodies to CD-31 (blood vessel endothelium) and rGel demonstrated a dramatic colocalization of the construct on tumor neovasculature. Treated tumors also displayed an increase in terminal deoxynucleotidyl transferase-mediated dUTP-biotin end labeling staining compared to controls. Thus, VEGF121/rGel inhibits the growth of human bladder cancer by cytotoxic effects directed against the tumor vascular supply and has significant potential as a novel antiangiogenic therapeutic against human bladder cancer.

The vascular-ablative agent VEGF(121)/rGel inhibits pulmonary metastases of MDA-MB-231 breast tumors

VEGF(121)/rGel, a fusion protein composed of the growth factor VEGF(121) and the recombinant toxin gelonin (rGel), targets the tumor neovasculature and exerts impressive cytotoxic effects by inhibiting protein synthesis. We evaluated the effect of VEGF(121)/rGel on the growth of metastatic MDA-MB-231 tumor cells in SCID mice. VEGF(121)/rGel treatment reduced surface lung tumor foci by 58% compared to controls (means were 22.4 and 53.3, respectively; P < .05) and the mean area of lung colonies by 50% (210 +/- 37 m(2) vs 415 +/- 10 m(2) for VEGF(121)/rGel and control, respectively; P < .01). In addition, the vascularity of metastatic foci was significantly reduced (198 +/- 37 vs 388 +/- 21 vessels/mm(2) for treated and control, respectively). Approximately 62% of metastatic colonies from the VEGF(121)/rGel-treated group had fewer than 10 vessels per colony compared to 23% in the control group. The VEGF receptor Flk-1 was intensely detected on the metastatic vessels in the control but not in the VEGF(121)/rGel-treated group. Metastatic foci present in lungs had a three-fold lower Ki-67 labeling index compared to control tumors. Thus, the antitumor vascular-ablative effect of VEGF(121)/rGel may be utilized not only for treating primary tumors but also for inhibiting metastatic spread and vascularization of metastases.

Vascular Targeting of Ocular Neovascularization with a Vascular Endothelial Growth Factor121/Gelonin Chimeric Protein

Tumors provide an extremely abnormal microenvironment that stimulates neovascularization from surrounding vessels and causes altered gene expression within vascular cells. Up-regulation of vascular endothelial growth factor (VEGF) receptors has allowed selective destruction of tumor vessels by administration of a chimeric protein consisting of VEGF121 coupled to the toxin gelonin (VEGF/rGel). We sought to determine whether there is sufficient up-regulation of VEGF receptors in endothelial cells participating in ocular neovascularization to permit a similar strategy. After intravenous injection of 45 mg/kg VEGF/rGel, but not uncoupled recombinant gelonin (rGel), there was immunofluorescent staining for rGel within choroidal neovascularization in mice and regression of the neovascularization occurred, demonstrating successful vascular targeting via the systemic circulation. Intraocular injection of 5 ng of VEGF/rGel also caused significant regression of choroidal neovascularization and regression of retinal neovascularization in two models, transgenic mice with expression of VEGF in photoreceptors and mice with ischemic retinopathy, whereas injection of 5 ng of rGel had no effect. These data suggest that the strategy of vascular targeting can be applied to nonmalignant neovascular diseases and could serve as the basis of a new treatment to reduce established ocular neovascularization.

The glycine box: a determinant of specificity for fibroblast growth factor

Acidic fibroblast growth factor (FGF-1), keratinocyte growth factor (FGF-7), and FGF-10 are homologues with distinct specificity. In the presence of heparin, FGF-1 binds and activates in vitro all FGFR subtypes, while FGF-7 exhibits absolute specificity for the IIIb splice variant of FGFR2. FGF-10 exhibits a similar specificity but also binds the FGFR1IIIb isoform. Neither FGF-7 nor FGF-10 will bind to IIIc isoforms of FGFR. Molecular models of FGF, heparin, and the FGFR ectodomain suggested that sequences between beta-strands 10 and 12 of FGF may be important for the interaction of FGF with the heparin-FGFR ectodomain duplex. Site-directed mutants of FGF-7 and FGF-10 were prepared to test whether this domain might underlie failure of FGF-7 and FGF-10 to bind to the FGFRIIIc isoforms. Constructions with substitution of FGF-1 sequences spanning the entire C-terminus encoded in exon 3 or only C-terminal sequences spanning beta-strands 10 through 12 conferred ability on FGF-7 to bind to and activate FGFRIIIc without a significant loss in binding to or activation of FGFR2IIIb. A series of twelve different substitutions of shorter segments of FGF-1 sequences into the C-terminal portion of FGF-7 or FGF-10 revealed that substitution of GSCKRG for GIPVRG or the tri-peptide sequence KKN for NQK just N-terminal to it conferred dual activities on both the FGF-7 and FGF-10 backbones. The results suggest that the combined sequence domain, which we call the FGF glycine box (G-box), is a major determinant for the specificity of the binding of FGF to heparan sulfate-FGFR duplexes.

Probing the functional role of two conserved active site aspartates in mouse adenosine deaminase

Two adjacent aspartates, Asp 295 and Asp 296, playing major roles in the reaction catalyzed by mouse adenosine deaminase (mADA) were altered using site-directed mutagenesis. These mutants were expressed and purified from an ADA-deficient bacterial strain and characterized. Circular dichroism spectroscopy shows the mutants to have unperturbed secondary structure. Their zinc content compares well to that of wild-type enzyme. Changing Asp 295 to a glutamate decreases the kcat but does not alter the Km for adenosine, confirming the importance of this residue in the catalytic process and its minimal role in substrate binding. The crystal structure of the D295E mutant reveals a displacement of the catalytic water from the active site due to the longer glutamate side chain, resulting in the mutant's inability to turn over the substrate. In contrast, Asp 296 mutants exhibit markedly increased Km values, establishing this residue's critical role in substrate binding. The Asp 296->Ala mutation causes a 70-fold increase in the Km for adenosine and retains 0.001% of the wild-type kcat/Km value, whereas the ASP 296->Asn mutant has a 10-fold higher Km and retains 1% of the wild-type kcat/Km value. The structure of the D296A mutant shows that the impaired binding of substrate is caused by the loss of a single hydrogen bond between a carboxylate oxygen and N7 of the purine ring. These results and others discussed below are in agreement with the postulated role of the adjacent aspartates in the catalytic mechanism for mADA.

Site-directed mutagenesis of active site glutamate-217 in mouse adenosine deaminase

Mouse adenosine deaminase (ADA) contains an active site glutamate residue at position-217 that is highly conserved in other adenosine and AMP deaminases. Previous research has suggested that proton donation to N-1 of the adenosine ring occurs prior to catalysis and supports the mechanism as proceeding via formation of a tetrahedral intermediate at C-6 of adenosine. The proposed catalytic mechanism of ADA based on the recent elucidations of the crystal structure of this enzyme with transition- and ground-state analogs hypothesized that Glu217 was involved in this proton donation step [Wilson, D. K., Rudolph, F. B., & Quiocho, F. A. (1991) Science 252, 1278-1284; Wilson, D. K., & Quiocho, F. A. (1993) Biochemistry 32, 1689-1693]. Site-directed mutagenesis of the equivalent glutamate in human ADA resulted in a dramatic loss of enzyme activity [Bhaumik, D., Medin, J., Gathy, K., & Coleman, M. (1993) J. Biol. Chem. 268, 5464-5470]. To further study the importance of this residue, site-directed mutagenesis was used to create mouse ADA mutants. Glu217 was mutated to Asp, Gly, Gln, and Ser, and all mutants were successfully expressed and purified. Circular dichroism and zinc analysis showed no significant changes in secondary structure or zinc content, respectively, compared to the native protein. The mutants showed only a slight variation in Km but dramatically reduced kcat, less than 0.2% of wild-type activity. UV difference and 13C NMR spectra conclusively demonstrated the failure of any of these mutants to hydrate purine riboside, a reaction carried out by the wild-type enzyme that results in formation of an enzyme-inhibitor complex. Surprisingly, Ki values for binding of the inhibitor to the mutants and to wild-type protein are similar, irrespective of whether the inhibitor is hydrated upon binding. These data confirm the importance of Glu217 in catalysis as suggested by the crystal structure of mouse ADA.

The highest levels of purine catabolic enzymes in mice are present in the proximal small intestine

Recent studies on the tissue distribution and developmental regulation of adenosine deaminase (ADA) activity in mice show that very high ADA levels exist in the murine alimentary tract (tongue, esophagus, forestomach, proximal small intestine) and at the fetal-maternal interface. To understand the role of ADA in these tissues, we measured the levels of three other enzymes involved in purine catabolism, purine nucleoside phosphorylase (PNP), guanine deaminase (GDA), and xanthine dehydrogenase (XDH), to see how their levels correlated with ADA activity. Our results show that the highest level of PNP, GDA, and XDH is present in the proximal small intestine. Levels of these purine catabolic enzymes are much lower in the tongue, esophagus, forestomach, and fetal-maternal interface in marked contrast to ADA distribution. We also determined mRNA levels encoding PNP, XDH, and ADA in a variety of tissues. Tissue-specific differences in PNP, XDH, and ADA activity correlated with RNA abundance, indicating that the regulation of gene expression is at the level of mRNA production. Thus, ADA is part of a purine catabolic pathway leading to the production of uric acid that is present at the highest known level in the proximal small intestine. ADA may have additional roles in other tissues.