Comparison of Pyrrolobenzodiazepine Dimer Bis-imine versus Mono-imine: DNA Interstrand Cross-linking, Cytotoxicity, Antibody-Drug Conjugate Efficacy and Toxicity

Antibody-drug conjugates (ADC) delivering pyrrolobenzodiazepine (PBD) DNA cross-linkers are currently being evaluated in clinical trials, with encouraging results in Hodgkin and non-Hodgkin lymphomas. The first example of an ADC delivering a PBD DNA cross-linker (loncastuximab tesirine) has been recently approved by the FDA for the treatment of relapsed and refractory diffuse large B-cell lymphoma. There has also been considerable interest in mono-alkylating PBD analogs. We conducted a head-to-head comparison of a conventional PBD bis-imine and a novel PBD mono-imine. Key Mitsunobu chemistry allowed clean and convenient access to the mono-imine class. Extensive DNA-binding studies revealed that the mono-imine mediated a type of DNA interaction that is described as "pseudo cross-linking," as well as alkylation. The PBD mono-imine ADC demonstrated robust antitumor activity in mice bearing human tumor xenografts at doses 3-fold higher than those that were efficacious for the PBD bis-imine ADC. A single-dose toxicology study in rats demonstrated that the MTD of the PBD mono-alkylator ADC was approximately 3-fold higher than that of the ADC bearing a bis-imine payload, suggesting a comparable therapeutic index for this molecule. However, although both ADCs caused myelosuppression, renal toxicity was observed only for the bis-imine, indicating possible differences in toxicologic profiles that could influence tolerability and therapeutic index. These data show that mono-amine PBDs have physicochemical and pharmacotoxicologic properties distinct from their cross-linking analogs and support their potential utility as a novel class of ADC payload.

Abstract 1765: Discovery and first disclosure of AZD8205, a B7-H4-targeted antibody-drug conjugate utilizing a novel topoisomerase I linker-warhead

The cell-surface glycoprotein B7-H4 is overexpressed in a range of solid tumors including breast cancer, ovarian serous carcinoma, endometrial carcinoma, and cholangiocarcinoma, yet has limited expression in normal tissue, making it an attractive target for an antibody-drug conjugate (ADC). This presentation describes for the first time the development of AZD8205, a B7-H4 targeted ADC incorporating a novel topoisomerase 1 inhibitor (TOP1i) linker-warhead, AZ’0133 which was designed to exploit the full potential of B7-H4 as an ADC target. Initially, we investigated a series of more than 35 TOP1i compounds as warheads and achieved activity in a clinically relevant nM range. We further optimized the conjugation site and chemistry to reduce the potential for aggregation while maintaining potency, overcoming major synthetic challenges to deliver a robust synthetic route amenable to scale-up. Finally, with a series of optimized linker-warheads, we explored the impact of linker-warhead design on ADC hydrophobicity, stability, efficacy, pharmacokinetics and tolerability culminating in the development of AZD8205. The primary mechanism of action of AZD8205 is intracellular delivery of the TOP1i warhead to B7-H4 positive cells, leading to DNA damage and apoptotic cell death. AZD8205 drove bystander killing of target negative cells in mixed cultures in vitro, which is further supported by robust antitumor activity observed in in vivo studies with patient-derived xenograft (PDX) tumors with heterogeneous target expression, representing multiple tumor indications. In a study of 26 human TNBC PDX tumors, a single IV administration of 3.5 mg/kg AZD8205 provided an overall response rate of 69% (tumor regression of 30% or greater from baseline) and complete responses observed in 9/26 (36%) of models. To understand the biology underlying antitumor response, we conducted a multiparametric analysis including genomics, proteomics and computational pathology and found that deeper antitumor activity was observed in models with elevated B7-H4 expression as well as in models with defects in DNA damage repair (DDR). To further exploit the DNA damage elicited by the TOP1i warhead, we examined combinations of AZD8205 with small molecules, including a novel PARP1 selective inhibitor, in a BRCA wild type MDA-MB-468 model. These data suggest that AZD8205 is a promising therapeutic candidate for the treatment of B7-H4 positive solid tumors. A first in human phase 1 study in patients with advanced solid tumors is currently ongoing (NCT05123482).

Citation Format: Krista Kinneer, Niall J. Dickinson, Luke Masterson, Thais Cailleau, Ian Hutchinson, Balakumar Vijayakrishnan, Nazzareno Dimasi, R. James Christie, Mary McFarlane, Kathryn Ball, Arthur Lewis, Sofia Koch, Lee Brown, Yue Huang, Anton I. Rosenbaum, Jiaqi Yuan, Si Mou, Noel R. Monks, Jon Chesebrough, Ravinder Tammali, Judith Anderton, Darrin Sabol, Frances Anne Tosto, Philipp Wortmann, Zachary A. Cooper, Pauline Ryan, John Hood, Carlos Fernandez Teruel, Carlos Serra Traynor, Andy Pike, Michael Davies, Elisabetta Leo, Kimberly Cook, Nadia Luheshi, Philip W. Howard, Puja Sapra. Discovery and first disclosure of AZD8205, a B7-H4-targeted antibody-drug conjugate utilizing a novel topoisomerase I linker-warhead [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 1765.

Abstract 5544: Newcastle Disease Virus in human derived tumors: Understanding immune recruitment and activation via virally delivered IL-12

Oncolytic viruses (OV) are defined by their intrinsic or engineered tropism for tumor cells. Understanding the drivers of susceptibility to infection as well as the downstream intracellular signaling and immune activation profile is critical in identifying the patients that will respond to OV therapy. Newcastle Disease Virus (NDV) is a negative stranded RNA avian paramyxovirus, here modified to encode human GM-CSF or IL-12. Patient derived xenografts (PDX) can more accurately reflect the genetic, epigenetic, and architectural heterogeneity of human tumors and were therefore selected to model the in vivo activity of NDV:GM-CSF. Eighty-nine independent models derived from patients with Bladder, Colorectal, Head and Neck, Kidney, Liver, Lung, Ovarian, Pancreatic, and Breast tumors were studied. Mice were dosed intravenously with NDV:GM-CSF, monitored for pharmacodynamic and pharmacokinetic changes 48 hours after the first dose, and followed for tumor growth inhibition. Objective response was observed in 68% of treated mice, with responses observed in all indications. Using total stranded RNAseq, we were able to assess changes in human tumor, murine immune infiltrate, and viral RNA. Results showed that NDV:GM-CSF treatment leads to increases in transcripts of inflammatory chemokines and cytokines such as RANTES and CXCL9, with induction directly correlated to level of viral replication. Additionally, we saw increases in NKp46 and IL-12R, markers of NK cells. To assess if IL-12 would augment immune activation, we treated surgically resected human tumors with NDV:IL-12. NDV:IL-12 was capable of infecting tumor cells, generating IL-12 protein, and driving a strong IFNγ response. Here we show that NDV can infect a wide variety of human tumors across multiple indications and drive a potent immune activation which can be modulated by transgenes. This data provides insight into what types of tumors could be treated with NDV:GM-CSF or NDV:IL-12.

Citation Format: Nicholas M. Durham, Kelly McGlinchey, Shannon Burke, Todd Creasy, Nicola Rath, Noel Monks, Ravinder Tammali, Kevin Schifferli, Nick Holoweckyj, Susie Hayes, Emma Jones, Elizabeth J. Kelly, Danielle Carroll, James Harper, Katie Streicher. Newcastle Disease Virus in human derived tumors: Understanding immune recruitment and activation via virally delivered IL-12 [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 5544.

Aldose Reductase Inhibitor, Fidarestat Prevents High-fat Diet-induced Intestinal Polyps in ApcMin/+ Mice

BACKGROUND

Recent epidemiological and experimental studies have shown that obesity is a major risk factor for Colorectal Cancer (CRC). Regular intake of high fat-containing diet can promote obesity and metabolic syndrome by increasing the insulin resistance and inflammatory response which contribute to carcinogenesis. Previously, we have shown that inhibition of polyol pathway enzyme aldose reductase (AR) prevents carcinogens- and inflammatory growth factorsinduced CRC. However, the effect of AR inhibition on a high-fat diet (HFD)-induced formation of intestinal polyps in Apc-deficient Min (multiple intestinal neoplasia; ApcMin/+) mice is not known.

METHODS

We examined the effect of AR inhibitor, fidarestat on the HFD-induced formation of preneoplastic intestinal polyps in ApcMin/+ mice which is an excellent model of colon cancer.

RESULTS

APC^Min/+ mice fed for 12 weeks of HFD caused a significant increase in the formation of polyps in the small and large intestines and fidarestat given along with the HFD prevented the number of intestinal polyps. Fidarestat also decreased the size of the polyps in the intestines of HFDtreated APC Min mice. Further, the expression levels of beta-catenin, PCNA, PKC-β2, P-AKT, Pp65, COX-2, and iNOS in the small and large intestines of HFD-treated mice significantly increased, and AR inhibitor prevented it.

CONCLUSION

Our results thus suggest that fidarestat could be used as a potential chemopreventive drug for intestinal cancers due to APC gene mutations.

Abstract A004: MEDI3726 (ADCT-401), a novel antibody-drug conjugate targeting PSMA, has potent in vivo antitumor activity in prostate cancer patient-derived xenograft models

Prostate specific membrane antigen (PSMA), a type II membrane glycoprotein, is highly expressed in nearly all prostate cancers, with the highest expression in metastatic castration-resistant prostate cancer (mCRPC). The prevalence, increased surface expression in prostate tumors, and constitutive internalization make PSMA an attractive target for an antibody-drug conjugate (ADC) approach in anticancer treatment of patients with mCRPC.

MEDI3726 (previously known as ADCT-401) is an ADC consisting of an engineered version of anti-PSMA antibody J591, site-specifically conjugated with DNA cross-linking pyrrolobenzodiazepine (PBD) dimer SG3249 for targeting prostatic cancer cells. Using prostate cancer cell line models, we have previously shown that MEDI3726 specifically binds to the extracellular domain of PSMA and, once internalized, releases the PBD dimer to cross link DNA and achieve potent in vitro and in vivo cytotoxicity.

Here we investigated the in vivo activity of MEDI3726 in a series of LuCaP prostate cancer patient-derived xenograft (PDX) models. The selected LuCaP models had varying PSMA expression and heterogeneous genetic and phenotypic backgrounds. In agreement with the earlier cell line xenograft data, dose-dependent antitumor activity was observed in PSMA-positive PDX models with durable tumor regressions in models with high PSMA expression. In the PSMA-negative LuCaP 35CR PDX model, MEDI3726 did not have significant antitumor activity, thus highlighting target-mediated in vivo activity. Increased phosphorylation of histone H2AX was observed in xenografts dosed with MEDI3726, confirming DNA damage induced by interstrand cross-linking PBD dimer as the mechanism of antitumor activity of MEDI3726.

In summary, MEDI3726 demonstrated potent and specific in vivo antitumor activity, concurrent with DNA damage, in clinically relevant prostate cancer PDX models. MEDI3726 is being evaluated in phase 1 clinical trial as an anticancer treatment in patients with metastatic castrate-resistant prostate cancer (NCT02991911).

Citation Format: Song Cho, Francesca Zammarchi, Noel R. Monks, Kapil Vashisht, Ravinder Tammali, Kevin Schifferli, Patrick Strout, Wanda King, Karma Dacosta, Ryan Fleming, David G. Williams, Karin Havenith, Mary Jane Masson Hinrichs, Simon Chivers, Nazzareno Dimasi, Phil W. Howard, John A. Hartley, Steve Coats, Ronald Herbst, Patrick H. van Berkel, David A. Tice. MEDI3726 (ADCT-401), a novel antibody-drug conjugate targeting PSMA, has potent in vivo antitumor activity in prostate cancer patient-derived xenograft models [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr A004.

Abstract LB-296: Discovery and development of MEDI7247, a novel Pyrrolobenzodiazepine (PBD)-based antibody drug conjugate targeting ASCT2, for treating hematological and solid cancers

The sodium dependent neutral amino acid transporter, ASCT2/SLC1A5 is frequently overexpressed in cancer to sustain “glutamine addiction” of cancer cells and to meet their increased demand for energy and nutrients. High expression of ASCT2 is often associated with poor disease prognosis. Immuno-histochemistry (IHC) data from 600+ tumor samples suggest ASCT2 expression in hematologic and solid cancers, including MM, AML, and DLBCL (≥ 95% positive in respective tumor samples), as well as squamous cell carcinoma of the head and neck (SCCHN), non-small cell lung cancer (NSCLC), small cell lung cancer (SCLC), prostate, pancreatic, and colorectal cancer (66% to 95% positivity in respective tumors). Broad expression across various cancers and restricted expression in normal tissues warrant ASCT2 as an attractive candidate for an antibody drug conjugate. MEDI7247 is a novel investigational antibody-drug conjugate (ADC) comprising an anti-ASCT2 human monoclonal antibody site-specifically conjugated to pyrrolobenzodiazepine (PBD) dimer via a protease-cleavable linker, with a drug to antibody ratio (DAR) of 2. MEDI7247 specifically binds to cell surface ASCT2 while exhibiting no affinity to the other members of the family, including ASCT1. Following binding, MEDI7247 is internalized and trafficked to the lysosomes to subsequently release the PBD warhead. PBD enters the nucleus where it induces DNA damage and results in tumor cell death. MEDI7247 shows potent in vitro cytotoxicity in several human cancer cell lines expressing variable levels of ASCT2. Dose dependent in vivo efficacy of MEDI7247 was determined in solid tumor xenografts and disseminated hematological malignancy models. MEDI7247 is cross-reactive with cynomolgous monkey ASCT2 and shows similar binding and killing with cyno ASCT2 expressing cells. MEDI7247 demonstrated acceptable safety profile in toxicity studies with non-human primates to support first in human trials. Based on its combined efficacy and safety, MEDI7247 is in clinical development for the treatment of ASCT2 positive cancers.

Citation Format: Nabendu Pore, M Jack Borrok, Marlon Rebellato, MaryJane Hinrichs, Kevin P. Schifferli, Noel R. Monks, Ravinder Tammali, Ronald Herbst, Steven R. Coats, David A. Tice. Discovery and development of MEDI7247, a novel Pyrrolobenzodiazepine (PBD)-based antibody drug conjugate targeting ASCT2, for treating hematological and solid cancers [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 LB-296.

Abstract LB-295: MEDI7247, a novel pyrrolobenzodiazepine ADC targeting ASCT2 with potent in vivo activity across a spectrum of hematological malignancies

MEDI7247 is a first in class ADC consisting of a human anti-ASCT2 monoclonal antibody site specifically conjugated to DNA cross-linking pyrrolobenzodiazepine (PBD) dimers. ASCT2 (SLC1A5) is a multi-pass, Na+-dependent neutral amino acid transporter that mediates the uptake of amino acids required for tumor growth and progression. ASCT2 is highly overexpressed in many hematologic cancers, most notably Multiple Myeloma (MM - 100% positive), Acute Myeloid Leukemia (AML - 100% positive) and Diffuse Large B cell lymphoma (DLBCL - 95% positive). ASCT2 expression is low in normal tissues. MEDI7247 (Q1Wx4) demonstrated a significant survival advantage in 3 disseminated AML cell line models, TF1α(ASCT2-High), MOLM-13(ASCT2-low) and M.V.411(ASCT2-High), when compared to the untreated control at the lowest dose levels examined: 0.05, 0.1 and 0.1 mg/kg, respectively. Further exemplifying the activity of MEDI7247, both the TF1α and MOLM-13 models did not reach 50% survival by the end of the study, with 80% survival at >200 days for TF1α and 70% survival at >180 days for MOLM-13. Similarly, a single dose of MEDI7247 in the TF1α model resulted in a 60% survival at >200 days at 0.05 mg/kg. MEDI7247 was also tested in a disseminated AML PDX(ASCT2-low) model at 0.05, 0.1 and 0.4 mg/kg. A significant improvement in survival was observed at both 0.1 and 0.4 mg/kg with the higher dose level extending survival by >80 days. MEDI7247 activity was further confirmed by monitoring peripheral blood CD33+ve cells, which initially receded, with the timing of reappearance preempting survival. Multiple Myeloma is another indication that exhibits a high level of ASCT2 expression. MEDI7247 (Q1Wx4) efficacy was examined in 3 disseminated MM cell line models, NCI-H929(ASCT2-High), MM.1S(ASCT2-medium) and OPM2(ASCT2-Medium), with a significant improvement in survival from control at the lowest dose levels examined: 0.1, 0.1 and 0.05 mg/kg, respectively. The activity of MEDI7247 (Q1Wx4) was also examined in the subcutaneous DLBCL model KARPAS 422(ASCT2-High). Tumor regressions were observed at all dose levels tested (0.1, 0.2, 0.3 and 0.4 mg/kg), with the higher two dose levels resulting in complete tumor regression without regrowth beyond 150 days. Additionally, MEDI7247 (Q1Wx4) is efficacious against the disseminated 697(ASCT2-Low) (Acute Lymphoblastic Leukemia - ALL) and RAJI(ASCT2-High) (Burkitt's lymphoma) models. A significant survival advantage was seen in both tumor models at the lowest dose examined of 0.05 mg/kg. In conclusion, MEDI7247 demonstrates antitumor efficacy across all tumor indications tested and varying levels of ASCT2 expression. These data support the use of MEDI7247 in ASCT2 positive hematological malignancies. MEDI7247 is currently in Phase 1 clinical trials.

Citation Format: Noel R. Monks, Kevin P. Schifferli, Ravinder Tammali, M. Jack Borrok, Steven R. Coats, Ronald Herbst, David A. Tice, Nabendu Pore. MEDI7247, a novel pyrrolobenzodiazepine ADC targeting ASCT2 with potent in vivo activity across a spectrum of hematological malignancies [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 LB-295.

Abstract LB-298: MEDI7247: A first in class antibody drug conjugate targeting ASCT2 in a range of solid tumors

MEDI7247 is a first in class ADC consisting of a human anti-ASCT2 monoclonal antibody, site specifically conjugated to highly cytotoxic DNA cross-linking pyrrolobenzodiazepine (PBD) dimers (DAR of 2) via a protease-cleavable linker. ASCT2 (SLC1A5) is a multi-pass, Na+-dependent, neutral amino acid transporter that mediates the uptake of amino acids required for tumor growth and progression. ASCT2 over-expression has been identified on numerous solid tumor indications including pancreatic (94% Positive), small cell (SCLC) and non-small cell (NSCLC) lung cancer (65% positive), colorectal (73% positive), squamous cell carcinoma of the head and neck (SCCHN - 94% positive) and prostate (65% positive). MEDI7247 binds to ASCT2 facilitating internalization and ultimately cell death. We have evaluated the in vivo anti-tumor activity of MEDI7247 in both cell line-derived xenografts (CDX) and patient-derived xenografts (PDX). Cell line-derived xenografts were utilized to examine the anti-tumor activity in colorectal (WiDR), small cell lung cancer (H69) and squamous head and neck (FADU) tumors. Expression of ASCT2 was determined to be high in WiDR and low in both H69 and FADU. Regardless of ASCT2 expression level MEDI7247 controlled tumor growth in each of these models. MEDI7247 was further examined in numerous PDX models of pancreatic, NSCLC and colorectal tumors. Activity was observed across all the PDX models tested. Two pancreatic cancer PDX with both low and high ASCT2 expression levels showed anti-tumor responses to MEDI7247 at all dose levels tested (0.1, 0.2, 0.3 and 0.4 mg/kg). In both models, tumor regressions were observed with durable responses observed beyond 100 days. Flow cytometry analyses reveal high ASCT2 expression of cancer stem cells (CSCs) in Patient Derived Xenografts(PDX). Additionally, MEDI7247 ablates CSCs in vivo in PDX tumors, which may yield opportunity for more durable clinical responses. In summary, preclinical in vivo evaluation of MEDI7247 demonstrates target dependent anti-tumor activity across all the solid tumor indications evaluated and may be important in tumor initiating cell populations. MEDI7247 is currently in Phase I clinical trials.

Citation Format: Kevin P. Schifferli, Noel R. Monks, Ravinder Tammali, M Jack Borrok, Matthew G. Flynn, Elaine M. Hurt, Steven R. Coats, Ronald Herbst, David A. Tice, Nabendu Pore. MEDI7247: A first in class antibody drug conjugate targeting ASCT2 in a range of solid tumors [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 LB-298.

Abstract 821: B7-H3 is a potential antibody drug conjugate target for the treatment of prostate cancer

B7-H3 is a member of the B7 family of immune-regulatory ligands and has been reported to be overexpressed in a broad range of human cancers. In order to understand more comprehensively the expression of B7-H3 on both normal tissues and on tumors, we developed an immunohistochemistry (IHC) assay for the detection of human B7-H3. Using this, we confirmed a restricted normal tissue expression pattern and demonstrated expression of B7-H3 in a range of solid tumors. In particular, a high proportion of prostate cancer samples showed strong homogenous B7-H3 expression. This low normal tissue expression and upregulation in cancer makes B7-H3 an attractive target for an antibody drug conjugate (ADC). To evaluate this approach, antibodies to B7-H3 were isolated by screening a phage display library for binding to B7-H3. These antibodies specifically bind to and are internalised by B7-H3 expressing cancer cells. ADCs were generated by site-specific conjugation of either a DNA-damaging pyrrolobenzodiazepine dimer or a microtubule-disrupting tubulysin payload. These ADCs selectively inhibited the growth of B7-H3 expressing cell lines including androgen-insensitive prostate cancer cell lines such as LNCAP-AI in vitro. Furthermore, these ADCs caused durable tumour regression in human tumor xenograft models in vivo. Together, these data suggest that an ADC targeting B7-H3 may have potential to be used for the treatment of a range of solid tumors including prostate cancer.

Citation Format: Gareth C. Davies, Kelli Ryan, Alison J. Smith, Chris Lloyd, Ravinder Tammali, Noel Monks, Arthur Lewis, Robert W. Wilkinson. B7-H3 is a potential antibody drug conjugate target for the treatment of prostate cancer [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 821.

Abstract 76: Synthetic lethal targeting of BRCA mutant tumors with antibody linked pyrrolobenzodiazepine dimers

Pyrrolbenzodiazepine dimers (PBDs) are amongst the most potent DNA alkylating agents, with activity against a broad spectrum of tumors. PBDs form cross-links within the minor groove of DNA causing double strand breaks (DSB). DNA repair genes such as BRCA1 and BRCA2 play important roles in homologous recombination repair (HRR) of DSB. Cells defective in BRCA1 or BRCA2 are known to be sensitive to DNA interstrand crosslinks. Accordingly, it is possible that PBD-based ADCs will have enhanced killing of cells (synthetic lethality) in which HR processes are defective by inactivation of BRCA1 or BRCA2 genes in breast, ovarian and other cancers. To determine anti-tumor activity of PBD dimers, we have used MEDI0641, PBD-dimer conjugated to anti-5T4 antibody, against BRCA wild type and mutant xenograft tumor models. MEDI0641 was >3-fold more potent in BRCA1 or BRCA2 mutant models than in wild-type xenografts. Similar observations were seen in 25 patient-derived xenograft (PDX) models (19 breast and 6 ovarian) bearing mutations in BRCA1 or BRCA2 (blinded to 5T4 expression) treated with MEDI0641. Out of a total of 25 PDX models, 17 models had tumor regression with a single administration of MEDI0641 at 0.3 mg/kg (response rate = 68%), and 14 models showed response to 0.1 mg/kg of MEDI0641 (response rate = 56%). In BRCA wild-type PDX models, a higher dose of 1 mg/kg was required to achieve full anti-tumor efficacy. Retrospective analysis of 5T4 expression in PDX tumors demonstrated no correlation between efficacy and target expression in BRCA mutant PDX models. To further delineate the role of BRCA1/2 mutations in determining sensitivity to PBD, we used siRNA knock-down of both BRCA1 and BRCA2 in the DNA repair wild type HeLa cells. Knockdown of BRCA genes sensitized Hela cells to PBD payload and MEDI0641 in vitro. Anti-tumor activity of MEDI0641 was further examined in isogenic BRCA2 knockout xenograft models. Genetic deletion of BRCA2 markedly increased anti-tumor activity of MEDI0641. In conclusion, PBD based ADCs may have improved therapeutic window in cancer patients with somatic BRCA mutations.

Citation Format: Haihong Zhong, Ravinder Tammali, Cui Chen, Christine Fazenbaker, Kennedy Maureen, Noel Monks, Jay Harper, Ronald Herbst, Dave Tice. Synthetic lethal targeting of BRCA mutant tumors with antibody linked pyrrolobenzodiazepine dimers [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 76. doi:10.1158/1538-7445.AM2017-76

A Potent HER3 Monoclonal Antibody That Blocks Both Ligand-Dependent and -Independent Activities: Differential Impacts of PTEN Status on Tumor Response

HER3/ERBB3 is a kinase-deficient member of the EGFR family receptor tyrosine kinases (RTK) that is broadly expressed and activated in human cancers. HER3 is a compelling cancer target due to its important role in activation of the oncogenic PI3K/AKT pathway. It has also been demonstrated to confer tumor resistance to a variety of cancer therapies, especially targeted drugs against EGFR and HER2. HER3 can be activated by its ligand (heregulin/HRG), which induces HER3 heterodimerization with EGFR, HER2, or other RTKs. Alternatively, HER3 can be activated in a ligand-independent manner through heterodimerization with HER2 in HER2-amplified cells. We developed a fully human mAb against HER3 (KTN3379) that efficiently suppressed HER3 activity in both ligand-dependent and independent settings. Correspondingly, KTN3379 inhibited tumor growth in divergent tumor models driven by either ligand-dependent or independent mechanisms in vitro and in vivo Most intriguingly, while investigating the mechanistic underpinnings of tumor response to KTN3379, we discovered an interesting dichotomy in that PTEN loss, a frequently occurring oncogenic lesion in a broad range of cancer types, substantially blunted the tumor response in HER2-amplified cancer, but not in the ligand-driven cancer. To our knowledge, this represents the first study ascertaining the impact of PTEN loss on the antitumor efficacy of a HER3 mAb. KTN3379 is currently undergoing a phase Ib clinical trial in patients with advanced solid tumors. Our current study may help us optimize patient selection schemes for KTN3379 to maximize its clinical benefits. Mol Cancer Ther; 15(4); 689-701. ©2016 AACR.

Abstract 30: Medi3622, a monoclonal antibody to ADAM17, inhibits tumor growth by inhibiting EGFR- and non-EGFR-mediated pathways

ADAM17 is the primary sheddase for HER pathway ligands. We report the discovery of a potent and specific ADAM17 inhibitory antibody, MEDI3622, which induces tumor regression or stasis in many EGFR-dependent tumor models. The inhibitory activity of MEDI3622 correlated with EGFR activity both in a series of tumor models across several indications as well as in a focused set of head and neck patient derived xenograft models. Cynomolgus monkey and rat PK/PD assays showed MEDI3622 inhibited TNFα shedding. Toxicity observed in cynomolgus monkey and rat was similar to EGFR inhibitor-induced rash. However, the antitumor activity of MEDI3622 was superior to that of EGFR/HER pathway inhibitors in OE21 head and neck and COLO205 colorectal xenograft models suggesting additional activity outside of the EGFR pathway. Combination of MEDI3622 and cetuximab in the OE21 model was additive and eradicated tumors. Proteomics analysis revealed novel ADAM17 substrates which function outside of the HER pathways and may contribute towards the antitumor activity of the monoclonal antibody.

Citation Format: Darrin Sabol, Jonathan RiosDoria, Jon Chesebrough, David Stewart, Kevin Schifferli, Raymond Rothstein, Ching Ching Leow, Jenny Heidbrink-Thompson, Li Cheng, Qun Du, Linda Xu, Xiaofang Jin, Ravinder Tammali, Chanshou Gao, Jay Friedman, Brandy Wilkinson, Melissa Damschroder, Andrew Pierce, MunMun Patnaik, Rong Zeng, Yuling Wu, Susan Spitz, Gabriel Robbie, Lorin Roskos, Robert Hollingsworth, David Tice, Emil Michelotti. Medi3622, a monoclonal antibody to ADAM17, inhibits tumor growth by inhibiting EGFR- and non-EGFR-mediated pathways. [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 30. doi:10.1158/1538-7445.AM2015-30

A Monoclonal Antibody to ADAM17 Inhibits Tumor Growth by Inhibiting EGFR and Non-EGFR-Mediated Pathways

ADAM17 is the primary sheddase for HER pathway ligands. We report the discovery of a potent and specific ADAM17 inhibitory antibody, MEDI3622, which induces tumor regression or stasis in many EGFR-dependent tumor models. The inhibitory activity of MEDI3622 correlated with EGFR activity both in a series of tumor models across several indications as well in as a focused set of head and neck patient-derived xenograft models. The antitumor activity of MEDI3622 was superior to that of EGFR/HER pathway inhibitors in the OE21 esophageal model and the COLO205 colorectal model suggesting additional activity outside of the EGFR pathway. Combination of MEDI3622 and cetuximab in the OE21 model was additive and eradicated tumors. Proteomics analysis revealed novel ADAM17 substrates that function outside of the HER pathways and may contribute toward the antitumor activity of the monoclonal antibody.

Abstract A112: Combined targeting of HER2 and HER3 inhibits tumor growth in both trastuzumab-sensitive and trastuzumab-resistant breast cancer models

HER3 (ERBB3) is a tumor driver in divergent cancer types with high HRG levels or heightened EGFR or HER2 kinase activity via growth signal coupling to the PI3K-AKT pathway. HER3 lacks intrinsic tyrosine kinase activity but is capable of signaling after heterodimerizing with tyrosine kinase ERBB family members EGFR or HER2. Heterodimer formation is driven either by the binding of heregulin (NRG1/HRG) to HER3 (ligand dependent - LD), or alternatively, by overexpression of EGFR or HER2 (ligand-independent - LI). Preclinical evaluation of MEDI3379, a human IgG1 modified monoclonal anti-HER3 antibody that antagonizes both LD and LI signaling, demonstrated tumor growth inhibition in the HER2-expressing MDA-MB-361 breast cancer xenograft model. Anti-tumor activity of MEDI3379 in this model was increased in combination with the HER2-targeting drug trastuzumab to a greater extent than MEDI3379 when combined with either lapatinib or pertuzumab. Combined inhibition of HER2 and HER3 led to strong reduction in pHER3. Unexpectedly, co-administration of MEDI3379 together with trastuzumab in MDA-MB-361 tumor-bearing mice but not in naïve mice resulted in reduced serum levels of trastuzumab. A control antibody combined with trastuzumab in MDA-MB-361 tumor bearing mice did not have this effect. Furthermore, in the trastuzumab-resistant JIMT-1 breast cancer xenograft model which has high HRG and pHER3 levels MEDI3379 but not pertuzumab resensitized JIMT-1 tumors to trastuzumab resulting in synergistic anti-tumor activity and a reduction of pHER3. In vivo resistance to this regimen appeared with prolonged treatment of JIMT-1 tumors and was accompanied by a decrease in homogenous expression of HER2. In conclusion, the use of MEDI3379 in combination with trastuzumab in HER2-amplifed breast cancer cell lines has demonstrated additional tumor growth inhibition over single agent trastuzumab and overcoming trastuzumab-resistance mechanisms. This combination warrants further evaluation in preclinical and clinical studies.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):A112.

Citation Format: Philipp Steiner, Leslie Wetzel, Kevin Schifferli, Raymond Rothstein, Ravinder Tammali, Marlon Rebelatto, Zhan Xiao, Andrew Pierce, Robert Hollingsworth. Combined targeting of HER2 and HER3 inhibits tumor growth in both trastuzumab-sensitive and trastuzumab-resistant breast cancer models. [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 A112.

Aldose reductase inhibition prevents colon cancer growth by restoring phosphatase and tensin homolog through modulation of miR-21 and FOXO3a

AIMS

We have shown earlier that inhibition of aldose reductase (AR), an oxidative stress-response protein, prevents colon cancer cell growth in vitro and in vivo. Changes in microribonucleic acid (miR) expression can contribute to cancer by modulating the functional expression of critical genes involved in cancer growth and metastasis. However, the molecular mechanisms by which AR regulates miR expression and their dependent mitogenic effects in cancer cells are not known. Therefore, we investigated how AR regulates growth factor-induced expression of miRs and growth of colon cancer cells.

RESULTS

Inhibition of AR significantly downregulated growth factor-induced miR-21 expression in human colon cancer cells, HT29, SW480, and Caco-2. Further, AR inhibition also increased phosphatase and tensin homolog (PTEN) (a direct target of miR-21) and forkhead box O3A (FOXO3a) in colon cancer cells. Our results obtained with HT29 cells ablated with FOXO3a siRNA showed increased activator protein-1 (AP-1) activation and miR-21 expression, indicating that FOXO3a represses miR-21 via AP-1 inactivation. Inhibition of AR also prevented the epidermal growth factor-induced phosphorylation of phosphatidylinositol 3-kinase (PI3K), serine/threonine kinase (AKT), c-Jun, c-Fos, PTEN, and FOXO3a, and deoxyribonucleic acid (DNA)-binding activity of AP-1. More importantly, in human colon adenocarcinoma xenograft tissues, miR-21 expression was lower, and PTEN and FOXO3a levels were significantly higher in AR inhibitor-treated mice compared to controls.

INNOVATION

These findings demonstrate a novel role of AR in the regulation of miR-21 and its target PTEN in growth factor-induced colon cancer cell growth.

CONCLUSIONS

Collectively, these results show a novel role of AR in mediation of growth factor-induced colon cancer growth by modulating miR-21, PTEN, and FOXO3a expression through reactive oxygen species (ROS)/PI3K/AKT/AP-1.

Inhibition of aldose reductase prevents colon cancer metastasis

Colon cancer is the third most common cause of cancer and is the second leading cause of cancer deaths in the USA. Although inhibition of aldose reductase (AR) is known to prevent human colon cancer cell growth in nude mice xenografts, the role of AR in the regulation of cancer metastasis is not known. We now demonstrate the mechanisms by which AR regulates colon cancer metastasis in vitro and in vivo. Inhibition of AR prevented the epidermal growth factor (EGF) or fibroblast growth factor (FGF)-induced migration and invasion of human colon cancer (HT29; KM20) cells by >70% and also inhibited (>80%) the adhesion of the cancer cells to endothelial cells. Treatment of endothelial cells with AR inhibitors significantly (∼85%) downregulated the EGF or FGF-induced expression of Inter-Cellular Adhesion Molecule-1, Vascular cell adhesion molecule-1 and vascular endothelial-cadherin. Furthermore, liver metastasis of green fluorescent protein-labeled KM20 cells injected into the spleen of athymic nude mice was significantly (>65%) prevented by AR inhibitor, fidarestat or ARsiRNA delivered systemically into the mice. Similar results were observed with HT29 cells. AR inhibition or ablation also prevented (70-90%) the increase in the levels of matrix metalloproteinase-2, cyclin D1, CD31, CD34 and the activation of nuclear factor-kappa-binding protein in metastatic liver. Thus, our results indicate that AR regulates cancer cell adhesion, invasion and migration events which initiate metastasis and therefore, AR inhibition could be a novel therapeutic approach for the prevention of colon cancer metastasis.

Aldose reductase inhibition prevents hypoxia-induced increase in hypoxia-inducible factor-1alpha (HIF-1alpha) and vascular endothelial growth factor (VEGF) by regulating 26 S proteasome-mediated protein degradation in human colon cancer cells

The development of intratumoral hypoxia, a hallmark of rapidly progressing solid tumors, renders tumor cells resistant to chemotherapy and radiation therapy. We have recently shown that inhibition of aldose reductase (AR), an enzyme that catalyzes the reduction of lipid aldehydes and their glutathione conjugates, prevents human colon cancer cell growth in culture as well as in nude mouse xenografts by inhibiting the NF-κB-dependent activation of oxidative stress-mediated inflammatory and carcinogenic markers. However, the role of AR in mediating hypoxic stress signals is not known. We therefore investigated the molecular mechanisms by which AR inhibition prevents the hypoxia-induced human colon cancer cells growth and invasion. Our results indicate that AR inhibition by the pharmacological inhibitor fidarestat or ablation by AR-specific siRNA prevents hypoxia-induced proliferation of HT29, SW480, and Caco-2 colon cancer cells. Furthermore, hypoxia-induced increase in the level of HIF-1α in colon cancer cells was significantly decreased by AR inhibition. During hypoxic conditions, treatment of HT29 cells with the AR inhibitor fidarestat significantly decreased the expression of vascular endothelial growth factor, a down target of HIF-1α, at both mRNA and protein levels and also prevented the activation of PI3K/AKT, GSK3β, Snail, and lysyl oxidase. Furthermore, inhibition of hypoxia-induced HIF-1α protein accumulation by AR inhibition was abolished in the presence of MG132, a potent inhibitor of the 26 S proteasome. In addition, AR inhibition also prevented the hypoxia-induced inflammatory molecules such as Cox-2 and PGE2 and expression of extracellular matrix proteins such as MMP2, vimentin, uPAR, and lysyl oxidase 2. In conclusion, our results indicate that AR mediates hypoxic signals, leading to tumor progression and invasion.

Abstract 2052: Inhibition of aldose reductase prevents hypoxic stress signaling in human colon cancer cells

Hypoxia, a hallmark of invasive solid tumors and increased angiogenesis, is strongly associated with the progression of malignant phenotypes, poor prognosis, and resistance to chemo and radiation therapies. In a variety of cancers, hypoxic stress-induces inflammatory signaling which is mediated by specific cytokines, chemokines and growth factors. We have recently shown that inhibition of aldose reductase (AR), an enzyme that catalyzes the reduction of lipid aldehydes and their glutathione conjugates, prevents human colon cancer cell growth in culture as well as in nude mice xenografts by inhibiting the NF-kB-dependent activation of oxidative stress-mediated inflammatory and carcinogenic markers. However, the role of AR in mediating hypoxic stress signals is not known. We therefore investigated the molecular mechanisms by which AR inhibition prevents the hypoxia-induced human colon cancer cells growth and invasion. Our results indicate that AR inhibition by pharmacological inhibitor fidarestat or ablation by AR specific siRNA prevents hypoxia-induced proliferation of HT29 colon cancer cells. Further, hypoxia- induced increase in the level of HIF1a in HT29 cells was significantly decreased by AR inhibition. During hypoxic conditions treatment of HT29 cells with AR inhibitor, fidarestat significantly decreased the expression of VEGF, a down target of HIF1a, at both mRNA and protein levels and also prevented the activation of PI3K/AKT and GSK3b. Further, AR inhibition also prevented hypoxia-induced inflammatory molecules such as Cox-2 and PGE2 and expression of extracellular matrix proteins such as MMP2, vimentin, uPAR and LOX2. In addition, AR inhibition also prevented the hypoxia -induced tube formation (angiogenesis) in vascular endothelial cells. In conclusion, our results indicate that AR mediates hypoxia -induced tumor progression, invasion and angiogenesis which can be prevented by AR inhibition or ablation.

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 2052. doi:10.1158/1538-7445.AM2011-2052

Inhibition of aldose reductase prevents angiogenesis in vitro and in vivo

We have recently shown that aldose reductase (AR, EC 1.1.1.21) a nicotinamide adenine dinucleotide phosphate-dependent aldo-keto reductase, known to be involved in oxidative stress-signaling, prevents human colon cancer cell growth in culture as well as in nude mice xenografts. Inhibition of AR also prevents azoxymethane-induced aberrant crypt foci formation in mice. In order to understand the chemopreventive mechanism(s) of AR inhibition in colon cancer, we have investigated the role of AR in the mediation of angiogenic signals in vitro and in vivo models. Our results show that inhibition of AR significantly prevented the VEGF- and FGF -induced proliferation and expression of proliferative marker Ki67 in the human umbilical vein endothelial cells (HUVEC). Further, AR inhibition or ablation with siRNA prevented the VEGF- and FGF -induced invasion and migration in HUVEC. AR inhibition also prevented the VEGF- and FGF- induced secretion/expression of IL-6, MMP2, MMP9, ICAM, and VCAM. The anti-angiogenic feature of AR inhibition in HUVEC was associated with inactivation of PI3 K/AKT and NF-κB (p65) and suppression of VEGF receptor 2 protein levels. Most importantly, matrigel plug model of angiogenesis in rats showed that inhibition of AR prevented infiltration of blood cells, invasion, migration and formation of capillary like structures, and expression of blood vessels markers CD31 and vWF. Thus, our results demonstrate that AR inhibitors could be novel agents to prevent angiogenesis.

Aldose reductase deficiency protects sugar-induced lens opacification in rats

Aldose reductase (AKR1B1), which catalyzes the reduction of glucose to sorbitol and lipid aldehydes to lipid alcohols, has been shown to be involved in secondary diabetic complications including cataractogenesis. Rats have high levels of AKR1B1 in lenses and readily develop diabetic cataracts, whereas mice have very low levels of AKR1B1 in their lenses and are not susceptible to hyperglycemic cataracts. Studies with transgenic mice that over-express AKR1B1 indicate that it is the key protein for the development of diabetic complications including diabetic cataract. However, no such studies were performed in genetically altered AKR1B1 rats. Hence, we developed siRNA-based AKR1B1 knockdown rats (ARKO) using the AKR1B1-siRNA-pSuper vector construct. Genotyping analysis suggested that more than 90% of AKR1B1 was knocked down in the littermates. Interestingly, all the male animals were born dead and only 3 female rats survived. Furthermore, all 3 female animals were not able to give birth to F1 generation. Hence, we could not establish an AKR1B1 rat knockdown colony. However, we examined the effect of AKR1B1 knockdown on sugar-induced lens opacification in ex vivo. Our results indicate that rat lenses obtained from AKR1B1 knockdown rats were resistant to high glucose-induced lens opacification as compared to wild-type (WT) rat lenses. Biochemical analysis of lens homogenates showed that the AKR1B1 activity and sorbitol levels were significantly lower in sugar-treated AKR1B1 knockdown rat lenses as compared to WT rat lenses treated with 50mM glucose. Our results thus confirmed the significance of AKR1B1 in the mediation of sugar-induced lens opacification and indicate the use of AKR1B1 inhibitors in the prevention of cataractogenesis.

Aldose reductase regulates vascular smooth muscle cell proliferation by modulating G1/S phase transition of cell cycle

Abnormal proliferation of vascular smooth muscle cells (VSMC) is a key feature of development of cardiovascular complications, atherosclerosis, and restenosis. Patients with diabetes have higher risk for restenosis after coronary angioplasty than nondiabetic patients due to hyperglycemia-induced release of cytokines such as TNF-alpha. However, the molecular mechanisms regulating VSMC proliferation remain unclear. Herein, we report that inhibition of the polyol pathway enzyme aldose reductase (AR) prevents high glucose (HG)- and/or TNF-alpha-induced VSMC proliferation by accumulating cells at the G1 phase of the cell cycle. Treatment of VSMC with AR inhibitor sorbinil prevented HG- as well as TNF-alpha-induced phosphorylation of retinoblastoma protein and activation of E2F-1. Inhibition of AR also prevented HG- and TNF-alpha-induced phosphorylation of cyclin-dependent kinase (cdk)-2 and expression of G1/S transition regulatory proteins such as cyclin D1, cyclin E, cdk-4, c-myc, and proliferative cell nuclear antigen. More importantly, inhibition of AR prevented the increased expression of E2F-1 and proliferative cell nuclear antigen in diabetic rat aorta. Treatment of VSMC with the most abundant and toxic lipid aldehyde 4-hydroxy-trans-2-nonenal (HNE) or its glutathione conjugate [glutathionyl (GS)-HNE] or AR-catalyzed product of GS-HNE, GS-1,4-dihydroxynonane, resulted in increased E2F-1 expression. Inhibition of AR prevented HNE- or GS-HNE-induced but not GS-1,4-dihydroxynonane-induced up-regulation of E2F-1. Collectively, these results show that AR could regulate HG- and TNF-alpha-induced VSMC proliferation by altering the activation of G1/S-phase proteins such as E2F-1, cdks, and cyclins. Thus, inhibition of AR may be a useful therapeutic approach in preventing vascular complications.

Abstract 3233: Inhibition of aldose reductase prevents lung cancer cell growth in vitro and in vivo

Lung cancer is the second most common cancer and the leading cause of cancer death among both men and women. Although, tobacco smoking is the dominant etiologic factor responsible for the vast majority of lung cancers, other factors such as asbestos, pollutants, polycyclic aromatic hydrocarbons, nutritional and dietary factors which can cause pulmonary inflammation can also drive carcinogenesis by altering cellular and molecular targets and pathways that are crucial to normal tissue homeostasis. However, the molecular signaling events specific to the pulmonary inflammation and their effects on pulmonary cells leading to carcinogenesis are poorly understood. Hence, we investigated the role of polyol pathway enzyme aldose reductase (AR), an oxidative stress response protein that catalyzes the reduction of lipid peroxidation -derived lipid aldehydes and their glutathione conjugates, in the mediation of carcinogenic signals leading to lung cancer. We examined the effect of pharmacological and siRNA -mediated inhibition of AR on in vitro human lung carcinoma cell line A549 growth as well as tumor growth in nude mice xenografts. We have also examined the molecular mechanisms by which AR inhibition prevents carcinogenic signaling events that cause lung cancer cell growth. Treatment of A549 cells with AR inhibitors prevented the activation of NF-kB and expression of NF-kB- dependent inflammatory markers such as iNOS and Cox-2. AR inhibition also prevented the proliferation of cultured human lung cancer cells. Further, AR inhibition significantly arrested the tumor growth in nude mice bearing lung adenocarcinoma xenografts. Similar results were observed in AR-siRNA treated nude mice xenografts. Immunohistochemical analysis of nude mice tumor cross sections indicated that AR inhibition prevents activation of iNOS, Cox-2, AR and NF-kB. Our studies thus strongly suggest that AR may be a critical regulator of signaling pathway(s) that cause lung cancer. Hence, disrupting these signals by AR inhibition could be a novel preventive approach in lung cancer progression.

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 3233.

Abstract 1384: Prevention of angiogenesis by aldose reductase inhibition

We have recently shown that Aldose Reductase (AR) known to be involved in oxidative stress-signaling, initiated by inflammatory markers, prevents human colon cancer cell growth in culture as well as in nude mice xenografts. Inhibition of AR also prevents azoxymethane-induced aberrant crypt foci formation in mice. In order to understand the chemopreventive mechanism of AR inhibition in colon cancer, we have investigated the role of AR in the mediation of angiogenic signals in vitro and in vivo models. Using human umbilical vein endothelial cells (HUVEC), we have investigated the effect of AR inhibition on VEGF- and bFGF- induced tube as well as spheroid formation in in vitro angiogenesis. We have also determined the invasion and migration of HUVEC by inhibition of AR. Further, to determine the in vivo efficacy of AR inhibition on angiogenesis, matrigel plug assay was performed using a rat model. Our results show that inhibition of AR significantly prevented the VEGF- and bFGF -induced proliferation and generation of reactive oxygen species in HUVEC. Further, AR inhibition also prevented the VEGF- and bFGF- induced secretion/expression of IL-6, MMP1, MMP9, ICAM, and VCAM. Matrigel plug model of angiogenesis in rats showed that inhibition of AR prevented the infiltration of blood cells, invasion, migration and formation of capillary like structures, and expression of blood vessels markers CD31 and vWF. Thus, our results demonstrate that AR inhibitors could be novel drugs for cancer chemoprevention by inhibiting angiogenesis.

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 1384.

Abstract 424: Invasion and metastasis of human colon cancer cells is prevented by aldose reductase inhibition

Worldwide colorectal cancer (CRC) is the third most common cause of cancer and is the second leading cause of cancer deaths in the USA. Although therapeutic options for patients with CRC have increased substantially, including earlier diagnosis, improved surgery as well as chemo- and radiation therapies, ∼61% of patients have metastatic disease and of these, 90% die within 5 years of diagnosis. Hence better therapeutic interventions to manage the metastatic process in CRC patients are mandatory. We have recently shown that inhibition of aldose reductase (AR), an enzyme that catalyzes reduction of lipid aldehydes and their glutathione conjugates, prevents human colon cancer cell growth in culture as well as in nude mice xenografts by inhibiting the NF-kB-dependent activation of inflammatory and carcinogenic markers. We have now investigated the effect of AR inhibition in metastatic spread of human CRC cells in nude mice and the molecular mechanisms by which AR inhibition prevents colon cancer cell invasion, migration, adhesion and metastasis. Our results indicate that AR inhibition prevents growth factors-induced migration and invasion of HT-29 CRC cells in a dose-dependent manner. AR inhibition also significantly inhibited adhesion of CRC cells to endothelial cells and inhibited the expression of ICAM-1, VCAM-1 and VE-cadherin. Further, we injected KM20 cells, transfected with a plasmid containing green fluorescent protein (GFP), into the spleen of athymic nude mice to establish liver metastases. The mice were treated with AR inhibitor or siRNA for 30 days. Our results indicate that AR inhibition or ablation prevents in vivo invasion and metastasis of CRC cells by reducing levels of AR, MMP2, cyclin D1, and CD34 and suppresses the activation of NF-kB. Since AR inhibitor fidarestat aggressively prevents human tumor growth and metastasis in nude mice and cellular models, and has already undergone phase-iii clinical studies for diabetic neuropathy, it is an excellent candidate for clinical application in preventing colon cancer tumorigenesis and metastasis.

Funding Support: NIH/NCI CA129383 (SKS).

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 424.

Inhibition of aldose reductase prevents growth factor-induced G1-S phase transition through the AKT/phosphoinositide 3-kinase/E2F-1 pathway in human colon cancer cells

Colon cancer is the leading cause of cancer death in both men and women worldwide. The deregulated cell cycle control or decreased apoptosis of normal epithelial cells leading to uncontrolled proliferation is one of the major features of tumor progression. We have previously shown that aldose reductase (AR), a NADPH-dependent aldo-keto reductase, has been shown to be involved in growth factor-induced proliferation of colon cancer cells. Herein, we report that inhibition of AR prevents epidermal growth factor (EGF)- and basic fibroblast growth factor (bFGF)-induced HT29 cell proliferation by accumulating cells at G(1) phase of cell cycle. Similar results were observed in SW480 and HCT-116 colon cancer cells. Treatment of HT29 cells with AR inhibitor, sorbinil or zopolrestat, prevented the EGF- and bFGF-induced DNA binding activity of E2F-1 and phosphorylation of retinoblastoma protein. Inhibition of AR also prevented EGF- and bFGF-induced phosphorylation of cyclin-dependent kinase (cdk)-2 and expression of G(1)-S transition regulatory proteins such as cyclin D1, cdk4, proliferating cell nuclear antigen, cyclin E, and c-myc. More importantly, inhibition of AR prevented the EGF- and bFGF-induced activation of phosphoinositide 3-kinase/AKT and reactive oxygen species generation in colon cancer cells. Further, inhibition of AR also prevented the tumor growth of human colon cancer cells in nude mouse xenografts. Collectively, these results show that AR mediates EGF- and bFGF-induced colon cancer cell proliferation by activating or expressing G(1)-S phase proteins such as E2F-1, cdks, and cyclins through the reactive oxygen species/phosphoinositide 3-kinase/AKT pathway, indicating the use of AR inhibitors in the prevention of colon carcinogenesis. Mol Cancer Ther; 9(4); 813-24. (c)2010 AACR.

Aldose reductase deficiency in mice prevents azoxymethane-induced colonic preneoplastic aberrant crypt foci formation

Aldose reductase (AR; EC 1.1.1.21), an nicotinamide adenine dinucleotide phosphate-dependent aldo-keto reductase, has been shown to be involved in oxidative stress signaling initiated by inflammatory cytokines, chemokines and growth factors. Recently, we have shown that inhibition of this enzyme prevents the growth of colon cancer cells in vitro as well as in nude mice xenografts. Herein, we investigated the mediation of AR in the formation of colonic preneoplastic aberrant crypt foci (ACF) using azoxymethane (AOM)-induced colon cancer mice model. Male BALB/c mice were administrated with AOM without or with AR inhibitor, sorbinil and at the end of the protocol, all the mice were euthanized and colons were evaluated for ACF formation. Administration of sorbinil significantly lowered the number of AOM-induced ACF. Similarly, AR-null mice administered with AOM demonstrated significant resistance to ACF formation. Furthermore, inhibition of AR or knockout of AR gene in the mice significantly prevented AOM-induced expression of inducible nitric oxide synthase and cyclooxygenase-2 proteins as well as their messenger RNA. AR inhibition or knockdown also significantly decreased the phosphorylation of protein kinase C (PKC) beta2 and nuclear factor kappa binding protein as well as expression of preneoplastic marker proteins such as cyclin D1 and beta-catenin in mice colons. Our results suggest that AR mediates the formation of ACF in AOM-treated mice and thereby inhibition of AR could provide an effective chemopreventive approach for the treatment of colon cancer.

Aldose reductase-regulated tumor necrosis factor-alpha production is essential for high glucose-induced vascular smooth muscle cell growth

Diabetes is associated with increased generation of cytokines and tissue inflammation, but it is unclear how increased cytokine synthesis is causally related to the development of diabetic complications. Here, we report that exposure to high (25 mm) glucose, but not iso-osmotic concentrations of mannitol or 3-methyl glucose, increased TNF-alpha secretion by rat and human aortic smooth muscle cells in culture. The increase in TNF-alpha production was prevented by actinomycin D and cycloheximide, indicating transcriptional activation of TNF-alpha gene. High glucose (HG)-induced TNF-alpha release was specifically inhibited by protein kinase C (PKC)-delta inhibitor (Rottlerin; EMD Biosciences, San Diego, CA), but not PKC-beta2 inhibitor (CGP53353; Tocris Cookson Inc., Ellisville, MO), indicating the possible involvement of PKC-delta in HG signaling. TNF-alpha secretion was also prevented by pretreating cells with aldose reductase (AR) inhibitors, sorbinil or tolrestat and in cells treated with antisense AR mRNA. Inhibition of AR also prevented the increase in TNF-alpha mRNA. Addition of anti-TNF-alpha antibodies or soluble TNF-alpha receptors 1 and 2 to the medium or RNA interference ablation of TNF-alpha attenuated nuclear factor-kappaB activation and prevented HG-stimulated cell growth. These data indicate that AR is required for HG-induced TNF-alpha synthesis and release. In vivo, the release of TNF-alpha by HG leading to autocrine stimulation of TNF-alpha synthesis may be a critical step in the development of the cardiovascular complications of diabetes. Interruption of the autocrine effects of TNF-alpha may be a useful strategy for treating diabetic vasculopathies.

Aldose reductase mediates endotoxin-induced production of nitric oxide and cytotoxicity in murine macrophages

Aldose reductase (AR) is a ubiquitously expressed protein with pleiotrophic roles as an efficient catalyst for the reduction of toxic lipid aldehydes and mediator of hyperglycemia, cytokine, and growth factor-induced redox-sensitive signals that cause secondary diabetic complications. Although AR inhibition has been shown to be protective against oxidative stress signals, the role of AR in regulating nitric oxide (NO) synthesis and NO-mediated apoptosis has not been elucidated to date. We therefore investigated the role of AR in regulating lipopolysaccharide (LPS)-induced NO synthesis and apoptosis in RAW 264.7 macrophages. Inhibition or RNA interference ablation of AR suppressed LPS-stimulated production of NO and overexpression of iNOS mRNA. Inhibition or ablation of AR also prevented the LPS-induced apoptosis, cell cycle arrest, activation of caspase-3, p38-MAPK, JNK, NF-kappaB, and AP1. In addition, AR inhibition prevented the LPS-induced down-regulation of Bcl-xl and up-regulation of Bax and Bak in macrophages. L-Arginine increased and L-NAME decreased the severity of cell death caused by LPS and AR inhibitors prevented it. Furthermore, inhibition of AR prevents cell death caused by HNE and GS-HNE, but not GS-DHN. Our findings for the first time suggest that AR-catalyzed lipid aldehyde-glutathione conjugates regulate the LPS-induced production of inflammatory marker NO and cytotoxicity in RAW 264.7 cells. Inhibition or ablation of AR activity may be a potential therapeutic target in endotoximia and other inflammatory diseases.

Aldose reductase regulates TNF-alpha-induced PGE2 production in human colon cancer cells

Pro-inflammatory cytokines such as TNF-alpha play an important role in the pathophysiology of diseases such as Crohn's and ulcerative colitis which cause increased risk of colorectal cancer. However, the mechanisms underlying colon carcinogenesis are not well understood. Herein we report that inhibition/antisense abolition of polyol pathway enzyme, aldose reductase (AR) inhibited the TNF-alpha-induced synthesis of prostaglandin E2 and the activity of cyclooxygenase (Cox) in human colon cancer cells, Caco-2. Inhibition of AR prevented TNF-alpha-induced activation of PKC and NF-kappaB which resulted in the abrogation of Cox-2 mRNA and protein expression. These results suggest that inhibition of AR could be a novel chemopreventive approach to colon cancer.

Inhibition of aldose reductase prevents lipopolysaccharide-induced inflammatory response in human lens epithelial cells

PURPOSE

Bacterial infections are one of the major causes of human eye disease. Because the bacterial endotoxin lipopolysaccharide (LPS) is known to cause cytotoxicity through oxidative stress and an earlier study has shown that aldose reductase (AR) mediates oxidative stress signals, the purpose of this study was to investigate the anti-inflammatory effects of AR inhibition on LPS-induced activation of NF-kappaB-dependent signals in human lens epithelial cells (HLECs).

METHODS

Growth-arrested HLECs were cultured without or with AR inhibitors or transfected with an AR small interfering (si)RNA. Subsequently, the cells were stimulated with LPS (1-10 mug/mL) for 24 hours. The cell viability was assessed by cell counts and MTT assay, and apoptosis was measured by nucleosomal degradation. Electrophoretic mobility gel shift assays were performed to determine the activation of NF-kappaB and AP1. The levels of nitric oxide, MMP-2, MMP-9, Cox-2, and TNF-alpha were measured by using specific ELISA kits. Western blot analysis was performed to determine the cleavage of poly(ADP-ribose) polymerase (PARP) and the activation of PKC and mitogen-activated protein kinase (MAPK).

RESULTS

Bacterial LPS caused apoptosis of HLECs. Inhibition of AR by two structurally unrelated inhibitors, sorbinil and tolrestat, or ablation by AR siRNA prevented the LPS-induced apoptosis, activation of caspase-3 and cleavage of PARP protein. Inhibition of AR in HLECs also prevented the LPS-induced activation of redox-sensitive transcription factors such as NF-kappaB and AP1 and their downstream signals that lead to expression of Cox-2, MMP-2, MMP-9, and TNF-alpha proteins. In addition, inhibition of AR prevented LPS-induced activation of protein kinases upstream to NF-kappaB activation such as PKC and MAPK in HLECs.

CONCLUSIONS

The results indicate that AR mediates the bacterial endotoxin signaling that could damage HLECs by regulating the signals that activate the redox-sensitive transcription factor NF-kappaB and cause inflammation. Thus, inhibition of AR could be a therapeutic target for Gram-negative bacterial infection-induced visual complications.

Aldose Reductase Regulates Growth Factor-Induced Cyclooxygenase-2 Expression and Prostaglandin E2 Production in Human Colon Cancer Cells

Inhibition of prostaglandin E(2) (PGE(2)) and cyclooxygenase (COX)-2 by nonsteroidal anti-inflammatory drugs reduces the progression of colon cancer. Inhibition of aldose reductase (AR; EC. 1.1.1.21.) by sorbinil or by antisense ablation prevented fibroblast growth factor-induced and platelet-derived growth factor-induced up-regulation of PGE(2) synthesis in human colon cancer cells, Caco-2. AR besides reducing aldo-sugars efficiently reduces toxic lipid aldehydes and their conjugates with glutathione. Inhibition of AR prevented growth factor-induced COX-2 activity, protein, and mRNA and significantly decreased activation of nuclear factor-kappaB and protein kinase C (PKC) and phosphorylation of PKC-beta2 as well as progression of Caco-2 cell growth but had no effect on COX-1 activity. Cell cycle analysis suggests that inhibition of AR prevents growth factor-induced proliferation of Caco-2 cells at S phase. Treatment of Caco-2 cells with the most abundant and toxic lipid aldehyde 4-hydroxy-trans-2-nonenal (HNE) or its glutathione-conjugate [glutathionyl-HNE (GS-HNE)] or AR-catalyzed product of GS-HNE, glutathionyl-1,4-dihydroxynonane (GS-DHN), resulted in increased COX-2 expression and PGE(2) production. Inhibition of AR prevented HNE- or GS-HNE-induced but not GS-DHN-induced up-regulation of COX-2 and PGE(2). More importantly, in vivo studies showed that administration of AR-small interfering RNA (siRNA), but not control siRNA, to nude mice bearing SW480 human colon adenocarcinoma cells completely arrested tumor progression. Collectively, these observations suggest that AR is an obligatory mediator of growth factor-induced up-regulation of COX-2, PGE(2), and growth of Caco-2 cells, indicating that inhibition of AR may be a novel therapeutic approach in preventing the progression of colon cancer.

Aldose reductase mediates the lipopolysaccharide-induced release of inflammatory mediators in RAW264.7 murine macrophages

Abnormal production of inflammatory cytokines and chemokines is a key feature of bacterial endotoxin, lipopolysaccharide (LPS)-induced inflammation, and cytotoxicity; however, the mechanisms regulating production of inflammatory markers remain unclear. Herein, we show that inhibition of the aldehyde-metabolizing enzyme aldose reductase (AR; AKR1B3) modulates NF-kappaB-dependent activation of inflammatory cytokines and chemokines in mouse serum, liver, heart, and spleen. Pharmacological inhibition or small interfering RNA ablation of AR prevented the biosynthesis of tumor necrosis factor-alpha, interleukin 1beta, interleukin-6, macrophage-chemoattractant protein-1, and cyclooxygenase-2 and prostaglandin E(2) in LPS-activated RAW264.7 murine macrophages. The AR inhibition or ablation significantly attenuated LPS-induced activation of protein kinase C (PKC) and phospholipase C (PLC), nuclear translocation of NF-kappaB, and phosphorylation and proteolytic degradation of IkappaBalpha in macrophages. Furthermore, treatment of macrophages with 4-hydroxy-trans-2-nonenal (HNE), and cell-permeable esters of glutathionyl-4-hydroxynonanal (GS-HNE) and glutathionyl-1,4-dihydroxynonane (GS-DHN) activated NF-kappaB and PLC/PKC. Pharmacological inhibition or antisense ablation of AR that catalyzes the reduction of GS-HNE to GS-DHN prevented PLC, PKC, IKKalpha/beta, and NF-kappaB activation caused by HNE and GS-HNE, but not by GS-DHN, suggesting that reduced GS-lipid aldehydes catalyzed by AR propagate LPS-induced production of inflammatory markers. Collectively, these data provide evidence that inhibition of AR may be a significant therapeutic approach in preventing bacterial endotoxin-induced sepsis and tissue damage.

Contribution of aldose reductase to diabetic hyperproliferation of vascular smooth muscle cells

The objective of this study was to determine whether the polyol pathway enzyme aldose reductase mediates diabetes abnormalities in vascular smooth muscle cell (SMC) growth. Aldose reductase inhibitors (tolrestat or sorbinil) or antisense aldose reductase mRNA prevented hyperproliferation of cultured rat aortic SMCs induced by high glucose. Cell cycle progression in the presence of high glucose was blocked by tolrestat, which induced a G0-G1 phase growth arrest. In situ, diabetes increased SMC growth and intimal hyperplasia in balloon-injured carotid arteries of streptozotocin-treated rats, when examined 7 or 14 days after injury. Treatment with tolrestat (15 mg x kg(-1) x day(-1)) diminished intimal hyperplasia and decreased SMC content of the lesion by 25%. Although tolrestat treatment increased immunoreactivity of the lesion with antibodies raised against protein adducts of the lipid peroxidation product 4-hydroxy trans-2-nonenal, no compensatory increase in lesion fibrosis was observed. Collectively, these results suggest that inhibition of aldose reductase prevents glucose-induced stimulation of SMC growth in culture and in situ. Even though inhibition of aldose reductase increases vascular oxidative stress, this approach may be useful in preventing abnormal SMC growth in vessels of diabetic patients.

Regulation of lens aldose reductase activity by nitric oxide

To examine the regulation of aldose reductase (AR) activity by nitric oxide (NO) in human lens epithelial cells (HLEC), cultured rat lens, and normal and diabetic rat lens, we have incubated HLEC or cultured rat lenses with 1 mm of the NO donors S-nitroso-N-acetylpenicillamine (SNAP) or S-nitrosoglutathione (GSNO), and the AR activity and sorbitol content were measured. Non-diabetic and diabetic (treated with streptozotocin 65 mg kg(-1) body wt, i.p.) rats were injected with the nitric oxide synthase (NOS) inhibitor, L-NAME (50 mg kg(-1) body wt day(-1), x 10 days i.p.) or NOS substrate, L-arginine (200 mg kg(-1) body wt day(-1), x 10 days i.p.). In a separate group of rats, a nitroglycerin (NG)-patch that releases 200 ng min(-1) NO was applied to the dorsal neck region. After 10 days of treatment, the lenses were removed and their AR activity and sorbitol content were measured. Incubation of HLEC with SNAP or GSNO reduced AR activity. A similar reduction in AR activity and sorbitol accumulation was observed when diabetic and non-diabetic rat lenses were cultured in the presence of SNAP and GSNO. Total protein-SSG in diabetic lens was lower compared to normal lens. Treatment of diabetic and non-diabetic rats with L-NAME enhanced AR activity and sorbitol accumulation, whereas NG patch and L-arginine significantly decreased AR activity and sorbitol accumulation in diabetic lenses compared to non-diabetic. Increased S-glutathiolation of AR was observed in the presence of SNAP. These results suggest that decreased glutathiolation of cellular proteins in diabetic rat lens compared to non-diabetic lens is related to decreased NO availability in diabetic rats which would decrease GSNO. Restoring the NO levels in diabetic animals increases glutathiolation of cellular proteins, inhibits AR activity and prevents sorbitol accumulation. Exogenous delivery of NO may represent a potentially useful strategy for preventing or delaying diabetic cataractogenesis and the development of other diabetic complications.

Requirement of aldose reductase for the hyperglycemic activation of protein kinase C and formation of diacylglycerol in vascular smooth muscle cells

Activation of protein kinase C (PKC) has been linked to the development of secondary diabetes complications. However, the underlying molecular mechanisms remain unclear. We examined the contribution of aldose reductase, which catalyzes the first, and the rate-limiting, step of the polyol pathway of glucose metabolism, to PKC activation in vascular smooth muscle cells (VSMCs) isolated from rat aorta and exposed to high glucose in culture. Exposure of VSMCs to high glucose (25 mmol/l), but not iso-osmotic mannitol, led to an increase in total membrane-associated PKC activity, which was prevented by the aldose reductase inhibitors tolrestat or sorbinil or by the ablation of aldose reductase by small interfering RNA (siRNA). The VSMCs were found to express low levels of sorbitol dehydrogenase, and treatment with the sorbitol dehydrogenase inhibitor CP-166572 did not prevent high-glucose-induced PKC activation. Stimulation with high glucose caused membrane translocation of conventional (alpha, beta1, beta2, and gamma) and novel (delta and epsilon) isoforms of PKC. Inhibition of aldose reductase prevented membrane translocation of PKC-beta2 and -delta and delayed the activation of PKC-beta1 and -epsilon, whereas membrane translocation of PKC-alpha and -gamma was not affected. Treatment with tolrestat prevented phosphorylation of PKC-beta2 and -delta. High glucose increased the formation of diacylglycerol (DAG) and enhanced phosphorylation of phospholipase C-gamma1 (PLC-gamma1). Inhibition of aldose reductase prevented high glucose-induced DAG formation and phosphorylation of PLC-gamma1 and PLC-beta2 and -delta. Inhibition of phospholipid hydrolysis by D609, but not by the synthetic alkyl-1-lysophospholipid 1-O-octadecyl-2-O-methyl-rac-glycerophosphocholine, or edelfosine, prevented DAG formation. Treatment with sorbinil decreased the levels of reactive oxygen species in high-glucose-stimulated VSMCs. Hence, inhibition of aldose reductase, independent of sorbitol dehydrogenase, appears to be effective in diminishing oxidative stress and hyperglycemic changes in signaling events upstream to the activation of multiple PKC isoforms and PLC-gamma1 and may represent a useful approach for preventing the development of secondary vascular complications of diabetes.

Fermentation of cellulose to acetic acid by Clostridium lentocellum SG6: induction of sporulation and effect of buffering agent on acetic acid production

AIMS

To determine the growth, correlation between sporulation and acetic acid production and effect of buffering agent at high substrate cellulose concentrations of the strain Clostiridium lentocellum SG6.

METHODS AND RESULTS

The strain SG6 was grown in cellulose mineral salt medium containing cellulose (Whatman No. 1 filter paper, Whatmore International Ltd., Maidstone, UK) or cellobiose. The strain fermented cellulose even after several transfers on cellobiose medium. The formation of endospores on third day onwards indicated the lowering of pH in the medium because of the formation of acetic acid. Maintaining the pH 7.2 at higher substrate concentrations resulted in increase of biomass, cellulose fermentation, acetic acid production, etc.

CONCLUSIONS

The strain SG6, with its high fermentation yields and sporulating character can become a potential strain for acetic acid production and also as a probiotic strain in animal nutrition.

SIGNIFICANCE AND IMPACT OF THE STUDY

The direct conversion of cellulosic biomass to acetic acid can eliminate expensive three-step saccharification, fermentation processes. The strain SG6 can ferment cellulose at high substrate concentrations.

Negative regulation of the protection of eIF2alpha phosphorylation activity by a unique acidic domain present at the N-terminus of p67

Eukaryotic initiation factor 2 (eIF2)-associated glycoprotein, p67, has protection of eIF2alpha phosphorylation (POEP) activity, and this activity requires lysine-rich domains I and II of p67. Another unique acidic residue-rich domain is also present at the N-terminus of p67. In this study we analyzed the role of this acidic residue-rich domain in POEP activity. Our data revealed that constitutive expression of a mutant form of p67 (D6/2) in mammalian cells resulted in increased POEP activity, and this activity was partially inhibited when second-site alanine substitutions at the conserved amino acids D251, D262, E364, and E459 were introduced in the D6/2 mutant. In contrast, a similar mutation at the conserved H331 position did not show any effect on POEP activity. Individual alanine substitutions at the above conserved amino acids in wild-type p67 did not show any significant effect on POEP activity except the E459 position where alanine substitution caused approximately 50% increase in POEP activity as compared to the wild type. Although, the levels of endogenous p67 and p67-deglycosylase did not correlate with the POEP activity, we found that the D6/2 mutant of p67 was glycosylated at a higher level in mammalian cells as compared to wild-type p67. The increased POEP activity of the D6/2 mutant also correlated with the higher rate of overall protein synthesis in mammalian cells constitutively expressing this mutant form of p67. Taken together, these data suggest that the acidic residue-rich domain present at the N-terminus of p67 may have a negative role in POEP activity.