Abstract 4421: Targeting myeloid-derived suppressor cells with actinium-225 lintuzumab, a CD33 antibody radioconjugate to enhance antitumor immunity

Introduction: Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of myeloid lineage cells with potent immunosuppressive activity found enriched in cancer patients. Crosstalk between MDSCs and the tumor microenvironment can promote tumor immune evasion and thus there is growing interest in developing therapeutic approaches to intervene in the MDSC suppressive function. Both monocytic-MDSC (M-MDSC) and granulocytic-MDSC (G-MDSC) subpopulations express the cell surface CD33 myeloid marker. For well-defined cell surface markers like CD33, there is considerable interest in the use of radionuclides as therapeutic payloads, particularly α-particle emitters such as actinium-225 (225Ac) since they deliver substantially higher decay energies over a much shorter distance than β-emitters, rendering them more suitable for precise, potent, and efficient target cell killing while minimizing toxicity to surrounding bystander cells. Actimab A, the anti-CD33 antibody lintuzumab armed with the 225Ac radioisotope (CD33 ARC), is currently being evaluated in R/R AML and has demonstrated significant anti-leukemic activity in Phase 1/2 clinical trials. We therefore hypothesized that MDSCs can be directly targeted by the CD33 ARC. Hence, we evaluated the therapeutic potential of the CD33 ARC to deplete MDSCs through preclinical studies in vitro and in vivo with humanized mouse model.

Methods: CD33 ARC was generated by conjugating lintuzumab with p-SCN-Bn-DOTA and subsequently radiolabeled with 225Ac. Primary MDSCs were isolated from cancer patient peripheral blood or healthy donor PBMCs. The specific binding of CD33 ARC to CD33 positive MDSCs and the decreased viability of MDSCs in response to treatment in vitro was characterized by immunophenotyping using flow cytometry. The therapeutic efficacy of CD33 ARC to deplete MDSC in vivo was evaluated in human CD34 reconstituted humanized NOG-EXL mice.

Results: CD33 positive MDSCs (M-MDSCs and G-MDSCs) were identified in human cancer peripheral blood samples, including colorectal and lung. Significantly more MDSCs were found in the peripheral blood of cancer patients in comparison to healthy donors. CD33 ARC treatment of human cancer MDSCs induced a potent dose-dependent reduction in MDSC viability leading to increased depletion of MDSCs. Furthermore, in the humanized NOG-EXL mouse model, CD33 ARC therapy demonstrated in vivo activity of CD33 ARC to deplete human MDSCs.

Conclusions: In this study, we demonstrate CD33 ARC alpha targeted radiotherapy depletes human CD33 positive immune suppressing MDSCs present in multiple cancer types, to enhance antitumor immunity. These findings present a translatable strategy that supports further evaluation of 225Ac lintuzumab as a MDSC targeting agent to improve the efficacy of antitumor therapies.

Citation Format: Amanda Chin, Mary Chen, Jason Li, Megan McCloskey, Caroline Jennings, Le-Cun Xu, Monideepa Roy, Denis Beckford, Helen Kotanides. Targeting myeloid-derived suppressor cells with actinium-225 lintuzumab, a CD33 antibody radioconjugate to enhance antitumor immunity. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4421.

Abstract 5040: Novel HER3 targeting antibody radioconjugates, 225Ac-HER3 ARC and 177Lu-HER3 ARC, exhibit potent antitumor efficacy in HER3-positive solid tumors

Background: HER3 is a unique member of the EGFR family that collaborates with other EGFR receptors to induce tumorigenesis and drug resistance. Moreover, HER3 expression is linked to poor survival for patients with solid tumors. Despite HER3 being a rational cancer therapeutic target, no HER3-directed therapies have been approved for clinical use. However, new therapeutic strategies such as antibody drug conjugates (ADCs) are being investigated. Targeted radiotherapy, including radiolabeled antibodies (ARCs), is unique mechanistically by inducing cell death independent of biologic pathway inhibition, and can be efficacious with less toxicity relative to other therapeutic modalities. Therefore, we hypothesized that the cytotoxic effects of alpha (225Ac) or beta (177Lu) emitting radionuclides combined with the specificity of anti-HER3 antibody targeting is a compelling therapeutic approach for HER3-expressing tumors. Here we evaluated the antitumor effects of 225Ac or 177Lu armed HER3 ARCs across multiple HER3-expressing cancer models such as ovarian, colorectal, prostate, and renal cancer.

Methods: ARCs were prepared by radiolabeling AT-02, an anti-HER3 antibody, with 225Ac or 177Lu using p-SCN-Bn-DOTA to yield 225Ac or 177Lu-HER3 ARC. The binding activity and tumor cell cytotoxicity of HER3 ARCs were assessed by ELISA using human recombinant HER3, flow cytometry on HER3-expressing cells, and colony forming assays. To evaluate the antitumor growth effects of 225Ac-HER3 and 177Lu-HER3 ARCs in vivo, preclinical human tumor xenograft models were developed. Mice bearing HER3-positive tumors were dosed with 225Ac-HER3 ARC (0.2 or 0.4 µCi), or 177Lu-HER3 ARC (200 or 400 µCi) and tumor growth and body weight was monitored.

Results: The pharmacological binding properties of HER3 antibody radiolabeled with 225Ac or 177Lu were similar to that of unmodified antibody as demonstrated by HER3 binding ELISA and flow cytometry. HER3 ARCs induced cytotoxicity and inhibited colony formation of HER3-positive tumor cell lines. Significant in vivo human tumor xenograft growth inhibition was observed in response to 225Ac or 177Lu HER3 ARCs compared to control groups (unmodified AT02 or IgG ARCs) in the models studied. No significant loss of body weight was observed in mice treated with HER3 ARCs suggesting that all treatments were well tolerated.

Conclusions: In this study, both 225Ac-HER3 ARC and 177Lu-HER3 ARC demonstrated significant antitumor activity against HER3-expressing tumors in a dose-dependent manner. The HER3 targeted radiotherapy approach that we have undertaken could potentially overcome the limitations of current solid tumor therapies in resistance settings and warrants further evaluation in patients with HER3-expressing tumors.

Citation Format: Denis Beckford-Vera, Megan McCloskey, Jason Li, Caroline Jennings, Le-Cun Xu, Debbie Lewis, Patrik Brodin, Amanda Chin, Monideepa Roy, Mary Chen, Helen Kotanides. Novel HER3 targeting antibody radioconjugates, 225Ac-HER3 ARC and 177Lu-HER3 ARC, exhibit potent antitumor efficacy in HER3-positive solid tumors. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5040.

Abstract 609: Anti-HER3 radioimmunotherapy enhances the anti-tumor effects of CD47 blockade in solid tumors

Background: Cancer immunotherapy strategies targeting blockade of the CD47-SIRPα immunosuppressive signal have made significant progress in recent years. However, monotherapies have not shown meaningful clinical responses in solid tumors. Therefore, therapeutic combinations are being explored to improve patient outcomes. CD47 is a macrophage checkpoint inhibitor that acts as a “don’t eat me” signal on cancer cells to evade innate immune detection and destruction. Targeted radiation to cancer cells will upregulate calreticulin (CRT), a pro-phagocytic “eat me” signal. We therefore hypothesize that we can enhance the efficacy of anti-CD47 antibodies by combining them with appropriate targeted antibody radioconjugates (ARC). In this experiment we chose to study an anti-HER3 radioconjugate, as HER3 is overexpressed in a variety of cancers including breast, ovarian, lung, gastric and prostate and is associated with poor clinical prognosis. Additionally, upregulation of HER3 is implicated in the acquired resistance against HER1 or HER2 targeted therapies. Here, we demonstrate enhanced therapeutic efficacy of a novel Actinium-225 (225Ac) armed HER3 specific targeting ARC (225Ac-HER3-ARC) and a CD47 blocking antibody (anti-CD47) combination in preclinical solid tumor models.

Methods: The anti-HER3 antibody (AT-02) was radiolabeled with 225Ac. 225Ac-HER3-ARC biological activity was evaluated using human recombinant HER3 and receptor positive tumor cell lines. 225Ac-HER3-ARC mediated CRT upregulation and cytotoxicity was evaluated using flow cytometry and MTS assay, respectively. The benefits of the 225Ac-HER3-ARC and anti-CD47 combination to enhance macrophage phagocytosis was evaluated by flow cytometry. We further evaluated the therapeutic benefits of the 225Ac-HER3-ARC and CD47 combination in human HER3+ tumor xenograft mouse model.

Results: The 225Ac-HER3-ARC retains similar binding properties to native antibody and demonstrates specific cytotoxicity on tumor cells. CRT was upregulated by 225Ac-HER3-ARC in HER3+ cells. Furthermore, the combination of 225Ac-HER3-ARC and anti-CD47 enhances in vitro macrophage mediated tumor cell phagocytosis compared to each agent alone. Importantly, the in vivo 225Ac-HER3-ARC and CD47 antibody combination shows enhanced antitumor effect with reduced toxicity and improved survival benefit in a human preclinical solid tumor model compared to anti-CD47 agent alone.

Conclusions: We demonstrate enhanced efficacy of the 225Ac-HER3-ARC and CD47 blocking antibody combination in vitro and in a preclinical solid tumor animal model. This approach is an encouraging strategy to potentially improve antitumor responses in patients with HER3+ tumors. Consequently, the findings obtained in this study along with the need to develop better therapies for patients with HER3+ tumors support the further preclinical development of HER3-ARC.

Citation Format: Denis Beckford-Vera, Jason Li, Caroline Jennings, Megan McCloskey, Amanda Chin, Qing Liang, Jesse Hwang, Monideepa Roy, Mary Chen, Helen Kotanides. Anti-HER3 radioimmunotherapy enhances the anti-tumor effects of CD47 blockade in solid tumors [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 609.

Abstract 3306: Targeting HER3 receptor positive cancers with a novel anti-HER3 antibody radioconjugate (ARC)

Background: HER3 overexpression is reported to be associated with poor survival in breast, ovarian, lung, gastric and prostate cancer. In addition, upregulation of HER3 in response to HER1 or HER2 targeted therapies, is implicated in the acquired resistance against these therapies. Therefore, effective targeting of HER3 can potentially overcome resistance and enhance therapeutic efficacy. Although a number of anti-HER3 antibodies have failed clinical testing with the development focus being shifted to other approaches such as antibody drug conjugates and bispecific antibodies, there are currently no approved HER3-targeted therapies. Here we describe a novel approach that can enhance therapeutic efficacy in HER3+ cancer patients by conjugating an anti-HER3 antibody with the alpha-emitting cytotoxic radioisotope Actinium-225 (225Ac) to create an anti-HER3 antibody radiation conjugate (225Ac-HER3-ARC). Alpha emitting radioisotopes like 225Ac can cause double-strand DNA breaks for which there is no known resistance mechanism. Due to the cytotoxic properties of the radioisotope, lower levels of antibody may be needed, resulting in reduced incidence or less severe toxicities. We hypothesize that targeting HER3 in solid tumors with an ARC will result in tumor specific cell killing especially in a setting where HER-targeting agents are not a viable option. We developed a novel 225Ac-HER3-ARC and evaluated its efficacy in HER3+ in vitro and in vivo tumor models.

Methods: AT-02, an anti-HER3 antibody, was conjugated with p-SCN-Bn-DOTA and radiolabeled with 225Ac. 225Ac-HER3-ARC specific binding to HER3 was assessed by ELISA using human recombinant HER3 and by flow cytometry on HER3+ cells. The cytotoxic effect of HER3 ARC was evaluated in a panel of HER3 expressing cells. We further evaluated the maximum tolerated dose and therapeutic efficacy of the ARC in nude mice bearing human HER3+ xenograft tumors.

Results: In this study we successfully radiolabeled anti-HER3 with 225Ac. 225Ac-HER3-ARC showed similar binding properties to those of the native antibody by ELISA (HER3-ARC: EC50 = 0.0017 µg/ml, HER3 EC50 = 0.0022 µg/ml) and flow cytometry. Treatment with ARC was cytotoxic to HER3+ cells in a dose-dependent manner (EC50 = 54 kBq/ml). 225Ac-HER3-ARC showed potent in vivo efficacy in preclinical solid tumor xenograft models that was correlated with the in vitro cytotoxicity findings. Treatment with 225Ac-HER3-ARC (7.4 - 22.2 kBq, 200 - 600 nCi) led to complete responses and significantly prolonged survival compared to control groups (p < 0.0001).

Conclusions: Our findings demonstrate that targeting HER3 with a novel 225Ac-HER3-ARC results in potent tumor cell cytotoxicity and complete anti-tumor response in HER3 tumor xenograft model. This approach provides a promising therapeutic strategy for HER3 positive tumors and warrants further assessment.

Citation Format: Denis Beckford-Vera, Jason Li, Megan McCloskey, Caroline Jennings, Amanda Chin, Qing Liang, Jesse Hwang, Monideepa Roy, Mary Chen, Helen Kotanides. Targeting HER3 receptor positive cancers with a novel anti-HER3 antibody radioconjugate (ARC) [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 3306.

589 Enhancement of the anti-tumor effects of CD47 blockade in solid tumors by combination with targeted radioimmunotherapy

Background

One mechanism that tumors use to escape immunosurveillance is the overexpression of CD47, which inhibits the macrophage mediated phagocytosis pathway. Although blockade of the CD47-SIRPα axis is a promising approach to enhance tumor targeted phagocytosis, anti-CD47 monotherapies have not shown meaningful responses in clinical studies of solid tumors. Combination cancer therapies aim to increase the probability of response in settings of resistance by combining drugs with different mechanisms of action. Antibody radioconjugates (ARCs) specifically target and deliver therapeutic radiation directly to cancer cells. We rationalized that the immunogenic and cytotoxic properties of ARCs will upregulate calreticulin (CRT), a pro-phagocytic signal, thereby synergizing with CD47 blocking therapies to enhance phagocytosis and antitumor activity. Here for the first time, we demonstrate the combination benefit of a HER2 specific targeting ARC and a CD47 blocking antibody to enhance therapeutic efficacy in preclinical solid tumor models.

Methods

The anti-HER2 antibody trastuzumab was conjugated with p-SCN-DOTA and radiolabeled with Ac-225 or Lu-177. The biological activity of both radioconjugates was evaluated using human recombinant HER2 and receptor positive tumor cell lines. The cytotoxic effect of radioconjugates and the ability to upregulate CRT was evaluated using XTT assay and flow cytometry, respectively, in a panel of HER2 expressing cells. To evaluate the synergy of anti-HER2 ARC and CD47 antibody combination in vitro, a flow cytometry macrophage phagocytosis assay was developed. We further evaluated the antitumor synergy in vivo between anti-HER2 ARC and CD47 antibody in human HER2 positive tumor xenograft mouse model.

Results

The anti-HER2 ARCs have similar binding properties to native antibody and demonstrate specific cytotoxicity. Importantly, we observe ARC-mediated CRT upregulation in HER2 expressing cells. Furthermore, the combination of HER2 targeting ARC and CD47 blocking antibody enhances in vitro macrophage mediated tumor cell phagocytosis compared to each agent alone. Remarkably, the in vivo anti-HER2 ARC and CD47 antibody combination shows enhanced therapeutic effect with reduced toxicity and improved survival benefit in a human preclinical solid tumor model.

Conclusions

Here for the first time, we demonstrate enhanced therapeutic efficacy between an anti-HER2 ARC and CD47 blocking antibody combination in a preclinical solid tumor model. The finding suggests that ARC mediated upregulation of CRT potentiates the pro-phagocytic signal and synergizes with the anti-CD47 mode of action thereby enhancing antitumor immune response. This combination mechanism provides a very promising strategy to improve therapeutic responses in patients harboring solid tumors and warrants further preclinical evaluation.

Ethics Approval

All animal experiments were approved by IACUC.

590 Anti-CD33 actinium-225 targeted radioimmunotherapy enhances the biologic activity of anti-CD47 antibody immunotherapy in preclinical models of acute myeloid leukemia

Background

Actimab-A, the anti-CD33 antibody lintuzumab armed with the radioisotope Actinium-225 (Ac-225), has demonstrated single agent antileukemic effects in patients with relapsed or refractory acute myeloid leukemia (AML). Up-regulation of CD47, a macrophage checkpoint that suppresses phagocytosis, is one mechanism by which myeloid malignancies such as AML can evade targeting by the innate immune response. Therapeutic blocking antibodies against this pathway have shown early clinical promise. We hypothesized that Actimab-A will enhance phagocytosis in AML cells by specifically upregulating calreticulin (CRT), a pro-phagocytic signal. Moreover, we hypothesized that combination of the anti-CD33 antibody radioconjugate (ARC) and CD47 blocking antibody could act in synergy to enhance therapeutic outcomes in AML compared to single agent. In this study, we examined, for the first time, the potential mechanistic benefit of combining the anti-CD33 ARC armed with Ac-225 or Lutetium-177 (Lu-177) and a CD47 blocking antibody, using in vitro and in vivo human AML preclinical models.

Methods

Lintuzumab was conjugated with p-SCN-DOTA and radiolabeled with Ac-225 or Lu-177. The biological activity of both radioconjugates was examined using human recombinant CD33 and receptor positive AML cells. The cytotoxic effect of radioconjugates and the ability to upregulate CRT was evaluated using XTT assay and flow cytometry, respectively, in a panel of CD33 expressing cells. To assess the therapeutic combination of anti-CD33 ARC and CD47 antibody in vitro, a flow cytometry macrophage phagocytosis assay was used. We further evaluated the therapeutic efficacy in vivo of anti-CD33 ARC and CD47 antibody combination in human AML mouse model.

Results

The anti-CD33 ARCs have similar binding properties to native antibody and demonstrate specific cell cytotoxicity. We show ARC-mediated upregulation of cell surface CRT in a panel of CD33 expressing AML cells. Furthermore, the in vitro combination of CD33 targeting ARC and CD47 blocking antibody enhances macrophage mediated phagocytosis of AML cells compared to each monotherapy. Interestingly, the in vivo anti-CD33 ARC and CD47 antibody combination demonstrates a significant increase in survival and reduces toxicity in a human AML preclinical model.

Conclusions

Our findings suggest a novel synergistic mechanism whereby the CD33 ARC targeted radiation induces upregulation of CRT, thereby potentiating a pro-phagocytic innate immune response in combination with anti-CD47 blocking antibody. More importantly, clinical translation of this approach could enhance therapeutic efficacy in AML and warrants further preclinical exploration.

Ethics Approval

All animal studies were approved by IACUC.

Characterization of 7A5: A Human CD137 (4-1BB) Receptor Binding Monoclonal Antibody with Differential Agonist Properties That Promotes Antitumor Immunity

The CD137 receptor plays a key role in mediating immune response by promoting T cell proliferation, survival, and memory. Effective agonism of CD137 has the potential to reinvigorate potent antitumor immunity either alone or in combination with other immune-checkpoint therapies. In this study, we describe the discovery and characterization of a unique CD137 agonist, 7A5, a fully human IgG1 Fc effector-null monoclonal antibody. The biological properties of 7A5 were investigated through in vitro and in vivo studies. 7A5 binds CD137, and the binding epitope overlaps with the CD137L binding site based on structure. 7A5 engages CD137 receptor and activates NF-κB cell signaling independent of cross-linking or Fc effector function. In addition, T cell activation measured by cytokine IFNγ production is induced by 7A5 in peripheral blood mononuclear cell costimulation assay. Human tumor xenograft mouse models reconstituted with human immune cells were used to determine antitumor activity in vivo. Monotherapy with 7A5 inhibits tumor growth, and this activity is enhanced in combination with a PD-L1 antagonist antibody. Furthermore, the intratumoral immune gene expression signature in response to 7A5 is highly suggestive of enhanced T cell infiltration and activation. Taken together, these results demonstrate 7A5 is a differentiated CD137 agonist antibody with biological properties that warrant its further development as a cancer immunotherapy. GRAPHICAL ABSTRACT: http://mct.aacrjournals.org/content/molcanther/19/4/988/F1.large.jpg.

Bispecific Targeting of PD-1 and PD-L1 Enhances T-cell Activation and Antitumor Immunity

The programmed cell death protein 1 receptor (PD-1) and programmed death ligand 1 (PD-L1) coinhibitory pathway suppresses T-cell-mediated immunity. We hypothesized that cotargeting of PD-1 and PD-L1 with a bispecific antibody molecule could provide an alternative therapeutic approach, with enhanced antitumor activity, compared with monospecific PD-1 and PD-L1 antibodies. Here, we describe LY3434172, a bispecific IgG1 mAb with ablated Fc immune effector function that targets both human PD-1 and PD-L1. LY3434172 fully inhibited the major inhibitory receptor-ligand interactions in the PD-1 pathway. LY3434172 enhanced functional activation of T cells in vitro compared with the parent anti-PD-1 and anti-PD-L1 antibody combination or respective monotherapies. In mouse tumor models reconstituted with human immune cells, LY3434172 therapy induced dramatic and potent antitumor activity compared with each parent antibody or their combination. Collectively, these results demonstrated the enhanced immunomodulatory (immune blockade) properties of LY3434172, which improved antitumor immune response in preclinical studies, thus supporting its evaluation as a novel bispecific cancer immunotherapy.

A human monoclonal antibody targeting the stem cell factor receptor (c-Kit) blocks tumor cell signaling and inhibits tumor growth

Stem cell factor receptor (c-Kit) exerts multiple biological effects on target cells upon binding its ligand stem cell factor (SCF). Aberrant activation of c-Kit results in dysregulated signaling and is implicated in the pathogenesis of numerous cancers. The development of more specific and effective c-Kit therapies is warranted given its essential role in tumorigenesis. In this study, we describe the biological properties of CK6, a fully human IgG1 monoclonal antibody against the extracellular region of human c-Kit. CK6 specifically binds c-Kit receptor with high affinity (EC 50 = 0.06 nM) and strongly blocks its interaction with SCF (IC 50 = 0.41 nM) in solid phase assays. Flow cytometry shows CK6 binding to c-Kit on the cell surface of human small cell lung carcinoma (SCLC), melanoma, and leukemia tumor cell lines. Furthermore, exposure to CK6 inhibits SCF stimulation of c-Kit tyrosine kinase activity and downstream signaling pathways such as mitogen-activated protein kinase (MAPK) and protein kinase B (AKT), in addition to reducing tumor cell line growth in vitro. CK6 treatment significantly decreases human xenograft tumor growth in NCI-H526 SCLC (T/C% = 57) and Malme-3M melanoma (T/C% = 58) models in vivo. The combination of CK6 with standard of care chemotherapy agents, cisplatin and etoposide for SCLC or dacarbazine for melanoma, more potently reduces tumor growth (SCLC T/C% = 24, melanoma T/C% = 38) compared with CK6 or chemotherapy alone. In summary, our results demonstrate that CK6 is a c-Kit antagonist antibody with tumor growth neutralizing properties and are highly suggestive of potential therapeutic application in treating human malignancies harboring c-Kit receptor.

Abstract C159: A human monoclonal antibody targeting the stem cell factor receptor (c-Kit) blocks tumor cell signaling and inhibits tumor growth

Stem cell factor receptor also known as c-Kit is a receptor tyrosine kinase that mediates cell growth, survival, and differentiation signals in response to its ligand stem cell factor (SCF). Aberrant c-Kit expression and/or activation through mutations or autocrine/paracrine signaling mechanisms occur in various malignancies and promote tumor development. Specific therapeutic targeting of c-Kit in cancer is warranted given its cancer role. In this study, we characterize the biological properties of CK6, a fully human IgG1 monoclonal antibody against the extracellular region of human c-Kit. CK6 specifically binds human c-Kit receptor with high affinity (EC50= 0.06nM) and strongly blocks its interaction with SCF (IC50= 0.41nM) in solid phase assays. Flow cytometry shows CK6 binding to the cell surface of small cell lung carcinoma (SCLC), melanoma, leukemia, and other human c-Kit expressing tumor cell lines. Furthermore, exposure to CK6 inhibits SCF stimulation of c-Kit tyrosine kinase activity and downstream signaling pathways in these tumor cell lines. Reduced levels of phosphorylated c-Kit, mitogen-activated protein kinase (MAPK) and protein kinase B/Akt were observed. Given these findings, we evaluated the antitumor growth efficacy of CK6 in several human xenograft tumor models in vivo. CK6 monotherapy treatment significantly suppressed tumor growth of NCI-H526 SCLC (T/C%= 50) and Malme-3M Melanoma (T/C%= 58) xenograft models. The combination of CK6 with standard of care (SOC) chemotherapy agents, cisplatin and etoposide for SCLC or dacarbazine for melanoma, led to enhanced tumor growth inhibition (SCLC T/C%= 12; melanoma T/C%= 38) compared to CK6 monotherapy or SOC alone. In summary, our results demonstrate that CK6 is a c-Kit antagonist antibody with tumor growth neutralizing properties and are highly suggestive of potential therapeutic application in treating human cancers harboring c-Kit receptor.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C159.

Citation Format: Maria Lebron, Laura Brennan, Chris Damoci, Marie Prewett, Marina Starodubtseva, Michael Amatulli, Yiwei Zhang, Douglas Burtrum, Paul Balderes, Kris Persaud, David Surguladze, Nick Loizos, Keren Paz, Helen Kotanides. A human monoclonal antibody targeting the stem cell factor receptor (c-Kit) blocks tumor cell signaling and inhibits tumor growth. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C159.

DEGA/AMIGO-2, a leucine-rich repeat family member, differentially expressed in human gastric adenocarcinoma: effects on ploidy, chromosomal stability, cell adhesion/migration and tumorigenicity

We have discovered DEGA, a novel cDNA differentially expressed in human gastric adenocarcinomas. The DEGA gene product contains a signal peptide, five leucine-rich repeat motifs and a single IgG, and transmembrane domain, suggesting its residence on the plasma membrane. Transfection of 293 cells with a DEGA-GFP fusion construct confirmed its cell surface localization. Although the cytosolic portion of the DEGA gene product does not contain known protein domains, approximately one-fifth of these residues are either a serine or a threonine, suggesting that DEGA may play a role in signal transduction. BLAST searches revealed DEGA to be an exact match to AMIGO-2, a recently identified, but functionally uncharacterized protein related to AMIGO, a leucine-rich repeat containing cell adhesion molecule implicated in axon tract development. In this report, we show that DEGA/AMIGO-2 mRNA is differentially expressed in approximately 45% of tumor versus normal tissue from gastric adenocarcinoma patients. Stable expression of a DEGA/AMIGO-2 antisense construct in the gastric adenocarcinoma cell line, AGS, led to altered morphology, increased ploidy, chromosomal instability, decreased cell adhesion/migration, and a nearly complete abrogation of tumorigenicity in nude mice. These findings suggest a potential etiologic role for DEGA/AMIGO-2 in gastric adenocarcinoma.

Cloning, genetic characterization, and chromosomal mapping of the mouse VDUP1 gene

VDUP1 encodes a vitamin D3-inducible gene product that has been shown to be down-regulated in chemically-induced mammary tumors in rats. It has recently been reported to negatively regulate thioredoxin expression and function. We have cloned the mouse VDUP1 gene and characterized its genomic locus. The VDUP1 coding region spans eight exons within a total length of 2.3 kb located on mouse chromosome 3. Consensus sites for polyadenylation were identified 1.3 kb downstream of the gene, defining a long 3' untranslated region. The minimal functional VDUP1 promoter contains TATA and CCAAT boxes and transcription is initiated from two major start sites downstream. A direct repeat element located proximal to the TATA with homology to the USF binding site was identified as a potential regulator of VDUP1gene expression. Expression analysis determined that VDUP1 mRNA was markedly induced in myeloma cells in high density cell culture, but not in sub-confluent cells arrested by serum deprivation. All samples of a panel of mouse immortalized or transformed cell lines were shown to express abundant levels of VDUP1 mRNA.

Vascular endothelial growth factor (VEGF)-driven actin-based motility is mediated by VEGFR2 and requires concerted activation of stress-activated protein kinase 2 (SAPK2/p38) and geldanamycin-sensitive phosphorylation of focal adhesion kinase

In endothelial cells, vascular endothelial growth factor (VEGF) induces an accumulation of stress fibers associated with new actin polymerization and rapid formation of focal adhesions at the ventral surface of the cells. This cytoskeletal reorganization results in an intense motogenic activity. Using porcine endothelial cells expressing one or the other type of the VEGF receptors, VEGFR1 or VEGFR2, or human umbilical vein endothelial cells pretreated with a VEGFR2 neutralizing antibody, we show that VEGFR2 is responsible for VEGF-induced activation of the stress-activated protein kinase-2/p38 (SAPK2/p38), phosphorylation of focal adhesion kinase (FAK), and enhanced migratory activity. Activation of SAPK2/p38 triggered actin polymerization whereas FAK, which was phosphorylated independently of SAPK2/p38, initiated assembly of focal adhesions. Both processes contributed to the formation of stress fibers. Geldanamycin, an inhibitor of HSP90 blocked tyrosine phosphorylation of FAK, assembly of focal adhesions, actin reorganization, and cell migration, all of which were reversed by overexpressing HSP90. We conclude that VEGFR2 mediates the physiological effect of VEGF on cell migration and that two independent pathways downstream of VEGFR2 regulate actin-based motility. One pathway involves SAPK2/p38 and leads to enhanced actin polymerization activity. The other involves HSP90 as a permissive signal transduction factor implicated in FAK phosphorylation and assembly of focal adhesions.

Acquired antagonistic activity of a bispecific diabody directed against two different epitopes on vascular endothelial growth factor receptor 2

Bispecific antibody (BsAb) technology has been successfully used as a means to construct novel antibody (Ab) molecules with increased avidity for binding, by combining two Ab or their fragments directed against different epitopes within the same antigen. Using two single chain antibodies (scFv) isolated from a phage display library, we have constructed a bispecific diabody directed against two different epitopes on the extracellular domain (ECD) of human vascular endothelial growth factor receptor 2 (VEGFR2), the kinase-insert domain-containing receptor (KDR). Neither of the parent scFv blocks KDR/VEGF interactions or inhibits VEGF-induced receptor activation. The diabody binds to KDR with an affinity that is 1.5- to 3-fold higher than its parent scFv, mainly due to a much slower dissociation rate (k(off)), which is approximately 17- to 26-fold slower than that of the individual scFv. In addition, the diabody binds simultaneously to, and thus cross-links, the two epitopes on the receptor(s). It is rather unexpected that the diabody effectively blocked KDR/VEGF interactions, and inhibited both VEGF-induced activation of the receptor and mitogenesis of human endothelial cells. Taken together, our results suggest that the diabody is most likely to exert its effect through steric hindrance and/or causing major conformational changes of the receptor. This is the first report on the construction of a bispecific diabody with acquired novel antagonistic activity.

Inhibition of vascular endothelial growth factor induced mitogenesis of human endothelial cells by a chimeric anti-kinase insert domain-containing receptor antibody

The kinase insert domain-containing receptor (KDR) is the human vascular endothelial growth factor (VEGF) receptor responsible for the mitogenic and angiogenic effects of VEGF. There is much experimental evidence to suggest that the VEGF/KDR pathway plays an important role in tumor angiogenesis, a process essential for tumor growth and metastasis. Here we produced a chimeric anti-KDR antibody (IgG1), c-p1C11, from a single chain (scFv) antibody isolated from a phage display library. C-p1C11 binds specifically to the extracellular domain of soluble as well as cell-surface expressed KDR. It effectively blocks VEGF-KDR interaction and inhibits VEGF-stimulated activation of KDR and MAP kinases p44/p42 of human endothelial cells. Furthermore, c-p1C11 efficiently neutralizes VEGF-induced mitogenesis of human endothelial cells. Our results suggest that antibodies against KDR have potential clinical applications in the treatment of cancer and other diseases where pathological angiogenesis is involved.

Inhibition of vascular endothelial growth factor-induced receptor activation with anti-kinase insert domain-containing receptor single-chain antibodies from a phage display library

A single-chain antibody phage display library was constructed from spleen cells of mice immunized with a soluble form of a human vascular endothelial growth factor (VEGF) receptor, kinase insert domain-containing receptor (KDR). After two rounds of biopanning, >90% of the clones recovered were specifically reactive to KDR. Subsequent selection identified two clones that blocked VEGF binding to KDR. The clones were expressed in Escherichia coli and purified as soluble single-chain Fv (scFv) antibodies. The affinities of the scFv for binding to KDR were determined by BIAcore analysis (2.1 x 10(-9)-5.9 x 10(-9) M). One scFv, p1C11, was shown to inhibit VEGF-induced KDR phosphorylation and VEGF-stimulated DNA synthesis in human umbilical vein endothelial cells. There is much experimental evidence to suggest that the VEGF/KDR/Flk-1 pathway plays an important role in tumor angiogenesis, a process that is essential for tumor growth and metastasis. The antibodies discussed here, which block VEGF binding to KDR, have potential clinical application in the treatment of cancer and other diseases where pathological angiogenesis is involved.

Monoclonal antibodies targeting the VEGF receptor-2 (Flk1/KDR) as an anti-angiogenic therapeutic strategy

Biological evidence suggests that interference with the function of the angiogenic growth factor receptor VEGFR2 (flk1/KDR) is a particularly promising strategy to inhibit tumor-induced angiogenesis. Proof of concept was established by developing a monoclonal rat anti-mouse VEGFR2 antibody (DC101) and showing that it potently blocked the binding of VEGF to its receptor, inhibited VEGF-induced signaling, and strongly blocked tumor growth in mice through an anti-angiogenic mechanism. Since DC101 does not cross-react with the human VEGFR2 KDR, anti-KDR monoclonal antibodies were generated by standard hybridoma technology and by using phage display library. High affinity antibodies (Kd = 4.9 x 10(-10)-1.1 x 10(-9) M) were found with both approaches. The anti-KDR antibodies compete on an equimolar basis with VEGF for binding to KDR and inhibit with similar potency the VEGF-induced signaling and mitogenesis in human endothelial cells. Although these antibodies cannot be tested for in vivo efficacy in standard murine tumor models because of lack of species cross-reactivity, the similarity of their in vitro properties with those of DC101 suggests that they may be effective in blocking KDR function in vivo.

Interleukin-4-induced STAT6 recognizes and activates a target site in the promoter of the interleukin-4 receptor gene

This study addresses the function of STAT6 in interleukin-4-stimulated gene expression. A specific STAT6 DNA-binding target site has been identified in the promoter of the interleukin-4 receptor gene, and STAT6 is shown to be involved in mediating activation of gene expression via this site. STAT6 can mediate transcription of a heterologous reporter gene construct containing the interleukin-4 receptor STAT6 binding site. In addition, evidence is provided that demonstrates a distinct effect of STAT6 DNA binding specificity on transcriptional regulation since transcription was not stimulated from a competent but different DNA binding site. To confirm the role of STAT6 in gene activation, STAT6 mutant proteins were generated and analyzed for their ability to function in interleukin-4-induced transcription. Although the interleukin-2 gamma chain receptor subunit has been demonstrated to be a component of the interleukin-4 receptor, it is not required for specific gene activation.

Characterization of the interleukin-4 nuclear activated factor/STAT and its activation independent of the insulin receptor substrate proteins

The activation of a latent DNA binding factor by interleukin-4 (IL-4), the IL-4 nuclear activated factor (IL-4 NAF), occurs within minutes of IL-4 binding to its receptor. Molecular characterization of IL-4NAF by ultraviolet light cross-linking experiments revealed a single protein of 120-130 kDa in contact with the DNA target site. Glycerol gradient sedimentation analysis indicated a molecular mass of IL-4 NAF consistent with a monomer that is capable of binding DNA. The IL-4 NAF target site is a palindromic sequence that is also recognized by the interferon-induced transcription factor, p91/STAT1 alpha. However, IL-4 NAF and p91/STAT1 alpha display distinguishable DNA binding specificities that may generate one level of specificity in the expression of target genes. Previous studies suggested the involvement of the insulin receptor substrate-1 (IRS-1) in the IL-4 signal transduction pathway. Although IRS-1 is involved in the stimulation of mitogenesis, our results demonstrate that activation of IL-4 NAF is independent of IRS-signaling proteins. The results of this study indicate that IL-4 stimulates bifurcating signal pathways that can direct mitogenesis via the IRS-signaling proteins and specific gene expression via the IL-4 NAF.

Requirement of tyrosine phosphorylation for rapid activation of a DNA binding factor by IL-4

Interleukin-4 (IL-4) is an immunoregulatory cytokine produced by activated T lymphocytes to promote the growth and differentiation of cells that participate in immune defense. This study demonstrates the rapid activation of a specific DNA binding factor by IL-4. The IL-4 nuclear-activated factor (IL-4 NAF) appeared within minutes of IL-4 stimulation and recognized a specific DNA sequence found in the promoters of IL-4-responsive genes. Activation of this putative transcription factor required tyrosine phosphorylation, and antibodies specific for phosphotyrosine recognize the IL-4 NAF-DNA complex. Thus, IL-4 appears to transduce a signal to the nucleus through tyrosine phosphorylation of a latent DNA binding factor.

Paraneoplastic cerebellar degeneration. II. Clinical and immunologic findings in 21 patients with Hodgkin's disease

We reviewed clinical findings and serologic data on 18 men and three women with paraneoplastic cerebellar degeneration (PCD) associated with Hodgkin's disease (HD). The patients were 20 to 77 years old (median, 44). The lymphoma preceded neurologic symptoms by 1 to 54 months in 17/21 patients, but stage or activity did not correlate with severity of neurologic disease; six developed PCD while in HD remission. PCD evolved subacutely (over weeks to months) and was pancerebellar in most. Ten had downbeat nystagmus. Thirteen stabilized in a disabled state (wheelchair- or bed-bound), five stabilized ambulatory, and three, who had progressed to a nonambulatory state, recovered. The clinical findings were usually only cerebellar but one patient had an encephalopathy, three long-tract signs, and two sensory neuropathy. Plasmapheresis (seven patients) and corticosteroids or other immunosuppressant medication (eight patients) did not help; one improved dramatically after treatment with clonazepam. Two patients improved spontaneously. Six patients had serum antibodies that reacted specifically with Purkinje cells. The pattern was distinct from that of PCD with gynecologic cancer (anti-Yo) or small-cell lung cancer (anti-Hu). Western blotting failed to identify a discrete Purkinje cell antigen. Seropositive patients did not differ clinically from their seronegative counterparts. HD-associated PCD is more common in men and in a younger age group than in PCD with other malignancies.

Paraneoplastic cerebellar degeneration. I. A clinical analysis of 55 anti-Yo antibody-positive patients

We reviewed the clinical findings in 55 patients with cerebellar degeneration associated with the anti-Yo antibody (an anti-Purkinje cell antibody identified in this study by histochemistry and Western blot). The patients were all women, 26 to 85 years old. Fifty-two of them proved to have malignancies, almost exclusively breast or gynecologic cancers and usually confined to the involved organs and local lymph nodes. One woman had adenocarcinoma of the lung, and in three no malignancy has yet been identified. In 34 of 52 patients with cancer, the neurologic syndrome preceded the diagnosis of cancer and in many led to that diagnosis. Patients subacutely developed a pancerebellar disorder that was substantially disabling in most, with 37 of 48 assessable patients being unable to walk or sit unassisted. Laboratory evaluation revealed lymphocytic pleocytosis in 35 patients, with eventual cerebellar atrophy on imaging studies in seventeen. The disabling neurologic syndrome generally did not respond to treatment, but the cancer was often successfully treated. The presence of the anti-Yo antibody in patients with cerebellar symptoms warrants an aggressive approach to diagnosis and treatment of the underlying cancer, as many are curable at the time neurologic symptoms develop.