Protective Effect of Anti-Phosphatidylserine Antibody in a Guinea Pig Model of Advanced Hemorrhagic Arenavirus Infection

Objective

Host derived markers on virally infected cells or virions may provide targets for the generation of antiviral agents. Recently, we identified phosphatidylserine (PS) as a host marker of virions and virally-infected cells.

Methods and Materials

Under normal physiological conditions, PS is maintained on the inner leaflet of the plasma membrane facing the cytosol. Following viral infection, activation or pre-apoptotic changes cause PS to become externalized. We have previously shown that bavituximab, a chimeric human-mouse antibody that binds PS complexed with β2-glycoprotein I (β2GP1), protected rodents against lethal Pichinde virus and cytomegalovirus infections.

Results

Here, we determined the antiviral activity of a fully human monoclonal antibody, PGN632, that directly binds to PS. Treatment with PGN632 protected 20% of guinea pigs with advanced infections of the hemorrhagic arenavirus, Pichinde, from death. Combining PGN632 with ribavirin improved the antiviral activity of both agents, such that the combination rescued 50% of animals from death.

Conclusion

The major mechanisms of action of PGN632 appear to be opsonization of virus and antibody-dependent cellular cytotoxicity of virally-infected cells. PS-targeting agents may have utility in the treatment of viral diseases.

Identification of a Monoclonal Antibody That Attenuates Antiphospholipid Syndrome-Related Pregnancy Complications and Thrombosis

In the antiphospholipid syndrome (APS), patients produce antiphospholipid antibodies (aPL) that promote thrombosis and adverse pregnancy outcomes. Current therapy with anticoagulation is only partially effective and associated with multiple complications. We previously discovered that aPL recognition of cell surface β2-glycoprotein I (β2-GPI) initiates apolipoprotein E receptor 2 (apoER2)-dependent signaling in endothelial cells and in placental trophoblasts that ultimately promotes thrombosis and fetal loss, respectively. Here we sought to identify a monoclonal antibody (mAb) to β2-GPI that negates aPL-induced processes in cell culture and APS disease endpoints in mice. In a screen measuring endothelial NO synthase (eNOS) activity in cultured endothelial cells, we found that whereas aPL inhibit eNOS, the mAb 1N11 does not, and instead 1N11 prevents aPL action. Coimmunoprecipitation studies revealed that 1N11 decreases pathogenic antibody binding to β2-GPI, and it blocks aPL-induced complex formation between β2-GPI and apoER2. 1N11 also prevents aPL antagonism of endothelial cell migration, and in mice it reverses the impairment in reendothelialization caused by aPL, which underlies the non-thrombotic vascular occlusion provoked by disease-causing antibodies. In addition, aPL inhibition of trophoblast proliferation and migration is negated by 1N11, and the more than 6-fold increase in fetal resorption caused by aPL in pregnant mice is prevented by 1N11. Furthermore, the promotion of thrombosis by aPL is negated by 1N11. Thus, 1N11 has been identified as an mAb that attenuates APS-related pregnancy complications and thrombosis in mice. 1N11 may provide an efficacious, mechanism-based therapy to combat the often devastating conditions suffered by APS patients.

Granulocyte-Colony Stimulating Factor Receptor, Tissue Factor, and VEGF-R Bound VEGF in Human Breast Cancer In Loco

BACKGROUND

Doxorubicin and docetaxel-based chemotherapy regimens used in breast cancer patients are associated with high risk of febrile neutropenia (FN). Granulocyte colony-stimulating factors (G-CSF) are recommended for both treating and preventing chemotherapy-induced neutropenia. Increased thrombosis incidence in G-CSF treated patients was reported; however, the underlying mechanisms remain unclear. The principal activator of blood coagulation in cancer is tissue factor (TF). It additionally contributes to cancer progression and stimulates angiogenesis. The main proangiogenic factor is vascular endothelial growth factor (VEGF).

OBJECTIVES

The aim of the study was to evaluate granulocyte-colony stimulating factor receptor (G-CSFR), tissue factor (TF) expression and vascular endothelial growth factor receptor (VEGF-R) bound VEGF in human breast cancer in loco.

MATERIAL AND METHODS

G-CSFR, TF and VEGFR bound VEGF (VEGF: VEGFR) were assessed in 28 breast cancer tissue samples. Immunohistochemical (IHC) methodologies according to ABC technique and double staining IHC procedure were employed utilizing antibodies against G-CSFR, TF and VEGF associated with VEGFR (VEGF: VEGFR).

RESULTS

Expression of G-CSFR was demonstrated in 20 breast cancer tissue specimens (71%). In 6 cases (21%) the expression was strong (IRS 9-12). Strong expression of TF was observed in all investigated cases (100%). Moreover, expression of VEGF: VEGFR was visualized in cancer cells (IRS 5-8). No presence of G-CSFR, TF or VEGF: VEGFR was detected on healthy breast cells. Double staining IHC studies revealed co-localization of G-CSFR and TF, G-CSFR and VEGF: VEGFR, as well as TF and VEGF: VEGFR on breast cancer cells and ECs.

CONCLUSIONS

The results of the study indicate that GCSFR, TF and VEGF: VEGFR expression as well as their co-expression might influence breast cancer biology, and may increase thromboembolic adverse events incidence.

Antibody-Mediated Blockade of Phosphatidylserine Enhances the Antitumor Effect of Sorafenib in Hepatocellular Carcinomas Xenografts

BACKGROUND

Currently, the only FDA-approved systemic therapy for hepatocellular carcinoma (HCC) is the multi-receptor tyrosine kinase inhibitor, sorafenib, which provides only modest clinical benefit. We recently showed that treatment with a phosphatidylserine (PS) targeting agent suppresses tumor growth by targeting tumor vasculature and reactivating antitumor immunity.

METHODS

We tested the hypothesis that sorafenib increases PS exposure on tumor vasculature, thereby enhancing the antitumor efficacy of PS targeting. We evaluated the efficacy of combining a PS targeting agent (2aG4) with sorafenib in murine xenograft models of human HCC.

RESULTS

Our results demonstrate that combination of 2aG4 and sorafenib had a superior therapeutic effect over single agent therapy. Mechanistic studies showed that sorafenib significantly increased PS exposure on tumor vasculature; the percentage of PS-positive vessels increased from 19 to 52, 23 to 68, and 30 to 55 % in PLC/PRF/5, C3A, and Huh7 tumors, respectively. Combination therapy significantly decreased tumor microvessel density and the level of M2 macrophages, while increasing the apoptotic index of tumor endothelial cells and the frequency of M1 macrophages. Furthermore, we report the findings of a Phase I clinical study of bavituximab, a chimeric version of 2aG4, combined with sorafenib in HCC patients. The Phase I results demonstrate the appropriate dose of bavituximab to be given with sorafenib in future clinical trials.

CONCLUSIONS

Overall, these results strongly support the combination of bavituximab with sorafenib as a promising systemic therapeutic strategy for the treatment for advanced HCC patients.

Tumor-specific targeting by Bavituximab, a phosphatidylserine-targeting monoclonal antibody with vascular targeting and immune modulating properties, in lung cancer xenografts

Bavituximab is a chimeric monoclonal antibody with immune modulating and tumor-associated vascular disrupting properties demonstrated in models of non-small cell lung cancer (NSCLC). The molecular target of Bavituximab, phosphatidylserine (PS), is exposed on the outer leaflet of the membrane bi-layer of malignant vascular endothelial cells and tumor cells to a greater extent than on normal tissues. We evaluated the tumor-targeting properties of Bavituximab for imaging of NSCLC xenografts when radiolabeled with (111)In through conjugation with a bifunctional chelating agent, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). In vitro binding of (111)In-DOTA-Bavituximab to PS was determined by enzyme-linked immunosorbent assay (ELISA). Biodistribution of (111)In-DOTA-Bavituximab was conducted in normal rats, which provided data for dosimetry calculation. Single-photon emission computed tomography/computed tomography (SPECT/CT) imaging was performed in athymic nude rats bearing A549 NSCLC xenografts. At the molar conjugation ratio of 0.54 DOTA per Bavituximab, the PS binding affinity of (111)In-DOTA-Bavituximab was comparable to that of unmodified Bavituximab. Based on the quantitative SPECT/CT imaging data analysis, (111)In-DOTA-Bavituximab demonstrated tumor-specific uptake as measured by the tumor-tomuscle ratio, which peaked at 5.2 at 72 hr post-injection. In contrast, the control antibody only presented a contrast of 1.2 at the same time point.These findings may underlie the diagnostic efficacy and relative low rates of systemic vascular and immune-related toxicities of this immunoconjugate. Future applications of (111)In-DOTA-bavituximab may include prediction of efficacy, indication of tumor immunologic status, or characterization of radiographic findings.

Phosphatidylserine-targeted molecular imaging of tumor vasculature by magnetic resonance imaging

Phosphatidylserine (PS), normally restricted to the inner leaflet of the plasma membrane, becomes exposed on the outer surface of viable endothelial cells in tumor vasculature, but not in normal blood vessels. In the present study, we report the use of PGN635, a novel human monoclonal antibody that specifically targets PS, for in vivo molecular MRI of tumor vasculature. The F(ab')2 fragments of PGN635 were conjugated to polyethylene glycol (PEG) coated iron oxide nanoparticles (IO). Targeting specificity of the PS-targeted Nanoprobe, IO-PGN635F(ab')2 was first confirmed by in vitro MRI and histological staining. In vivo longitudinal MRI was then performed before and after i.v. injection of IO-PGN635F(ab')2 into mice bearing 4T1 breast tumors. T2-weighted MR images at 9.4 T revealed inhomogeneous signal loss in tumor as early as 2 h post injection. Furthermore, ionizing radiation induced a significant increase in PS exposure on tumor vascular endothelial cells, resulting in significantly enhanced and sustained tumor contrast (p < 0.05). Spatially heterogeneous MRI contrast correlated well with histological staining of tumor vascular endothelium. Our studies suggest that PS exposed within the lumen of tumor vasculature is a highly specific and useful biomarker for targeted MRI contrast agents.

Highly specific PET imaging of prostate tumors in mice with an iodine-124-labeled antibody fragment that targets phosphatidylserine

Phosphatidylserine (PS) is an attractive target for imaging agents that identify tumors and assess their response to therapy. PS is absent from the surface of most cell types, but becomes exposed on tumor cells and tumor vasculature in response to oxidative stresses in the tumor microenvironment and increases in response to therapy. To image exposed PS, we used a fully human PS-targeting antibody fragment, PGN635 F(ab’)_2, that binds to complexes of PS and β2-glycoprotein I. PGN635 F(ab’)₂ was labeled with the positron-emitting isotope iodine-124 (¹²⁴I) and the resulting probe was injected into nude mice bearing subcutaneous or orthotopic human PC3 prostate tumors. Biodistribution studies showed that ¹²⁴I-PGN635 F(ab’)₂ localized with remarkable specificity to the tumors with little uptake in other organs, including the liver and kidneys. Clear delineation of the tumors was achieved by PET 48 hours after injection. Radiation of the tumors with 15 Gy or systemic treatment of the mice with 10 mg/kg docetaxel increased localization in the tumors. Tumor-to-normal (T/N) ratios were inversely correlated with tumor growth measured over 28 days. These data indicate that ¹²⁴I-PGN635 F(ab’)₂ is a promising new imaging agent for predicting tumor response to therapy.

Phosphatidylserine-targeting antibody induces M1 macrophage polarization and promotes myeloid-derived suppressor cell differentiation

Multiple tumor-derived factors are responsible for the accumulation and expansion of immune-suppressing myeloid-derived suppressor cells (MDSC) and M2-like tumor-associated macrophages (TAM) in tumors. Here, we show that treatment of tumor-bearing mice with docetaxel in combination with the phosphatidylserine-targeting antibody 2aG4 potently suppressed the growth and progression of prostate tumors, depleted M2-like TAMs, and MDSCs, and increased the presence of M1-like TAMs and mature dendritic cells in the tumors. In addition, the antibody markedly altered the cytokine balance in the tumor microenvironment from immunosuppressive to immunostimulatory. In vitro studies confirmed that 2aG4 repolarized TAMs from an M2- to an M1-like phenotype and drove the differentiation of MDSCs into M1-like TAMs and functional dendritic cells. These data suggest that phosphatidylserine is responsible for the expansion of MDSCs and M2-like TAMs in tumors, and that bavituximab, a phosphatidylserine-targeting antibody currently in clinical trials for cancer, could reverse this process and reactivate antitumor immunity.

Phosphatidylserine exposure on the surface of Leishmania amazonensis amastigotes modulates in vivo infection and dendritic cell function

Leishmania amazonensis parasites can cause diverse forms of leishmaniasis in humans and persistent lesions in most inbred strains of mice. In both cases, the infection is characterized by a marked immunosuppression of the host. We previously showed that amastigote forms of the parasite make use of surface-exposed phosphatidylserine (PS) molecules to infect host cells and promote alternative macrophage activation, leading to uncontrolled intracellular proliferation of the parasites. In this study, we demonstrated that treatment of infected mice with a PS-targeting monoclonal antibody ameliorated parasite loads and lesion development, which correlated with increased proliferative responses by lymphocytes. In addition, we observed an enhanced dendritic cell (DC) activation and antigen presentation in vitro. Our data imply that the recognition of PS exposed on the surface of amastigotes plays a role in down-modulating DC functions, in a matter similar to that of apoptotic cell clearance. This study provides new information regarding the mechanism of immune suppression in Leishmania infection.

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

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

IBC's 23rd Antibody Engineering and 10th Antibody Therapeutics Conferences and the Annual Meeting of The Antibody Society: December 2-6, 2012, San Diego, CA

Now in its 23rd and 10th years, respectively, the Antibody Engineering and Antibody Therapeutics conferences are the Annual Meeting of The Antibody Society. The scientific program covers the full spectrum of challenges in antibody research and development from basic science through clinical development. In this preview of the conferences, the chairs provide their thoughts on sessions that will allow participants to track emerging trends in (1) the development of next-generation immunomodulatory antibodies; (2) the complexity of the environment in which antibodies must function; (3) antibody-targeted central nervous system (CNS) therapies that cross the blood brain barrier; (4) the extension of antibody half-life for improved efficacy and pharmacokinetics (PK)/pharmacodynamics (PD); and (5) the application of next generation DNA sequencing to accelerate antibody research. A pre-conference workshop on Sunday, December 2, 2012 will update participants on recent intellectual property (IP) law changes that affect antibody research, including biosimilar legislation, the America Invents Act and recent court cases. Keynote presentations will be given by Andreas Plückthun (University of Zürich), who will speak on engineering receptor ligands with powerful cellular responses; Gregory Friberg (Amgen Inc.), who will provide clinical updates of bispecific antibodies; James D. Marks (University of California, San Francisco), who will discuss a systems approach to generating tumor targeting antibodies; Dario Neri (Swiss Federal Institute of Technology Zürich), who will speak about delivering immune modulators at the sites of disease; William M. Pardridge (University of California, Los Angeles), who will discuss delivery across the blood-brain barrier; and Peter Senter (Seattle Genetics, Inc.), who will present his vision for the future of antibody-drug conjugates. For more information on these meetings or to register to attend, please visit www.IBCLifeSciences.com/AntibodyEng or call 800-390-4078. Members of The Antibody Society and mAbs journal subscribers receive a 20% discount for meeting registration. To obtain this discount, email kdostie@ibcusa.com. mAbs is the official therapeutics journal of The Antibody Society and offers a discounted subscription to Society members for $49.

Phosphatidylserine externalization and membrane blebbing are involved in the nonclassical export of FGF1

The mechanisms of nonclassical export of signal peptide-less proteins remain insufficiently understood. Here, we demonstrate that stress-induced unconventional export of FGF1, a potent and ubiquitously expressed mitogenic and proangiogenic protein, is associated with and dependent on the formation of membrane blebs and localized cell surface exposure of phosphatidylserine (PS). In addition, we found that the differentiation of promonocytic cells results in massive FGF1 release, which also correlates with membrane blebbing and exposure of PS. These findings indicate that the externalization of acidic phospholipids could be used as a pharmacological target to regulate the availability of FGF1 in the organism.

Abstract 4395: Cure of castration-resistant prostate cancer in TRAMP mice by reactivating tumor immunity with a phosphatidylserine-targeting antibody

Androgen deprivation therapy (ADT) is initially effective against prostate cancer, but many patients eventually relapse and die from the outgrowth of castration-resistant disease. Here, we tested the hypothesis that tumor immunity against castration-resistant prostate cancer can be elicited by combining ADT with treatment with a monoclonal antibody that binds exposed phosphatidylserine (PS). The rationale was that PS is an immunosuppressive lipid that becomes exposed on tumor blood vessels and malignant cells in prostate tumors responding to ADT, and inhibits immune responses to prostate tumor antigens. To validate this hypothesis, we castrated TRAMP mice after they had developed prostatic adenocarcinomas and treated them with the PS-targeting antibody, mch1N11. We first demonstrated that PS exposure was induced on tumor vascular endothelium and tumor cells by castration. PS exposure on vessels coincided with regions of hypoxia generated in the tumor microenvironment. Next, we demonstrated that castration combined with treatment with mch1N11 inhibited tumor growth and progression. About half of the treated TRAMP mice did not develop castration-resistant tumors, and eventually died of old age. Immunohistochemical studies revealed that the mch1N11 treatment combined with castration generated T-cell immune responses against TRAMP tumor antigens that kept the tumor in check. Extensive disruption of tumor vasculature and abundant infiltration of immune cells, including CD8 positive T lymphocytes, Natural killer (NK) cells and dendritic cells were observed. Splenocytes from mice treated with mch1N11 plus castration were able to kill TRAMP-C2 specifically in vitro, whereas splenocytes from mice in the other groups could not. These results demonstrate that PS, which becomes exposed on prostate cancer cells and tumor vasculature after ADT, inhibits immunity to TRAMP tumor antigens. Treatment with the PS-targeting antibody mch1N11 reactivates tumor immunity, which can permanently hold new tumor development in check.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4395. doi:1538-7445.AM2012-4395

Development of bavituximab, a vascular targeting agent with immune-modulating properties, for lung cancer treatment

Bavituximab is a chimeric monoclonal antibody directed against the membrane phospholipid phosphatidylserine. Phosphatidylserine exposure is increased on endothelial cells and apoptotic cancer cells in solid tumors, allowing tumor-specific targeting of bavituximab. Bavituximab binding results in tumor vessel occlusion and enhanced antitumor immunity. Preclinical investigations have demonstrated efficacy as monotherapy and in combination with other modalities against multiple cancer types. Phase I clinical trials of bavituximab monotherapy and in combination with chemotherapy in adults with refractory solid tumors have been completed. Phase II trials of bavituximab in combination with chemotherapy for the first- and second-line treatment of advanced non-small-cell lung cancer are currently ongoing. This article summarizes the preclinical development and clinical experience with bavituximab in non-small-cell lung cancer.

Phase I safety and pharmacokinetic study of bavituximab, a chimeric phosphatidylserine-targeting monoclonal antibody, in patients with advanced solid tumors

PURPOSE

Bavituximab is a chimeric immunoglobulin G1 phosphatidylserine-targeting monoclonal antibody that triggers vascular disruption and enhances antitumor immune response. This phase I study assessed the safety and pharmacokinetics of bavituximab in patients with advanced solid tumors.

EXPERIMENTAL DESIGN

Patients with refractory advanced solid tumors were enrolled into four sequential dose-escalation cohorts (0.1, 0.3, 1, or 3 mg/kg bavituximab weekly) with two dosing schedules. Patients in the 0.1 and 0.3 mg/kg cohorts received bavituximab on days 0, 28, 35, and 42. Patients in the 1 and 3 mg/kg cohorts were administered bavituximab on days 0, 7, 14, and 21. Safety, pharmacokinetics, and tumor response were assessed.

RESULTS

Twenty-six patients were accrued. No maximum tolerated dose was reached. Six serious adverse events occurred in five patients, including one pulmonary embolism at 3 mg/kg, which was the only dose-limiting toxicity (DLT) in the study. Bavituximab half-life ranged from 37 to 47 hours, with no accumulation seen following administration of multiple doses. Activated partial thromboplastin time was modestly prolonged in vitro at the highest dose tested. As assessed on day 56, a total of 18 patients were evaluable for efficacy, of whom 10 had disease progression and none had an objective response.

CONCLUSIONS

Bavituximab was well tolerated at doses ranging up to 3 mg/kg weekly. Pharmacokinetic studies support a weekly dosing regimen. Additional phase I and II clinical trials are in progress to investigate bavituximab in combination with chemotherapy and other molecularly targeted agents.

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

Background

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

Methods

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

Results

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

Conclusions

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

Enhancing the potency of a whole-cell breast cancer vaccine in mice with an antibody-IL-2 immunocytokine that targets exposed phosphatidylserine

Phosphatidylserine (PS), an anionic phospholipid normally restricted to the inner leaflet of the plasma membrane, is immunosuppressive when externalized on the outside of cell membranes. Exposed PS inhibits the maturation and function of dendritic cells (DCs), and induces the production of multiple immunosuppressive mediators. In the present study, we determined whether blocking these effects of PS while simultaneously introducing interleukin-2 (IL-2) could improve the immunogenicity of a whole-cell cancer vaccine. An immunocytokine (2aG4-IL2) was made by genetically linking IL-2 with a PS targeting antibody, 2aG4, that can block the immunosuppressive effects of PS. The 2aG4-IL2/4T1 vaccine was generated by coating the PS exposed on irradiated 4T1 cells with 2aG4-IL2. Tumor growth, spontaneous metastasis, and survival of vaccinated mice challenged with live 4T1 tumor cells were assessed. Eighty percent of mice inoculated with 2aG4-IL2/4T1 vaccine survived free of tumor, as compared with 20% in the 2aG4/4T1 group, 20% in the C44-IL2/4T1 group, and none in the C44/4T1 control group (P=0.001 for 2aG4-IL2/4T1 versus all others groups). The incidence, number of spontaneous lung metastases was significantly lower in the 2aG4-IL2/4T1 vaccinated group than in the other groups. Splenocytes from 2aG4-IL2/4T1 vaccinated mice had significantly higher 4T1 specific cytotoxicity and ability to secrete interferon-gamma (IFNγ) than did splenocytes from mice in the other groups. These results demonstrate that a potent whole-cell vaccine can be created by coating irradiated tumor cells with 2aG4-IL2. Such vaccine could potentially be an effective treatment modality for patients with residual disease or at "high-risk" for recurrence.

Abstract 621: Targeting phosphatidylserine to improve androgen deprivation therapy of prostate cancer

Androgen deprivation therapy (ADT) is effective against androgen-dependent prostate cancer, but patients usually relapse from the outgrowth of androgen-independent disease. Here, we tested the hypothesis that relapse can be prevented by combining ADT with treatment with a monoclonal antibody that binds exposed phosphatidylserine (PS). The rationale behind this hypothesis is that ADT will create hypoxia which induces PS exposure on the tumor vascular endothelial cells, leading to immune cell attack on tumor vasculature, collapse of the vessels and death of hormone-resistant and hormone-sensitive tumor cells through deprivation of oxygen and nutrients. In addition, the antibody should block the immunosuppressive effects of PS and induce reactivation of T-cell immunity. To test this hypothesis, we used ADT and anti-PS antibodies (2aG4 or murine chimeric 1N11) to treat mice bearing hormone-sensitive prostate tumors. The tumor models were: i) immunocompromised mice bearing human LNCaP prostate tumors; ii) transgenic (TRAMP) mice which spontaneously develop adenocarcinoma of the mouse prostate. Castration induced exposure of PS on blood vessels and tumor cells in both models. Vascular PS exposure corresponded with hypoxia, as judged by pimonidazole staining. In the LNCaP model, castration alone delayed tumor growth but all animals relapsed with hormone-independent disease, whereas mice treated with castration plus anti-PS antibody did not relapse. In TRAMP mice castrated at 15-weeks of age, only 54% (7/13) of mice treated with castration alone survived beyond 40-weeks of age, as compared with 92% (12/13) of mice treated with castration and 1N11. No toxicity to the mice was caused by the antibody treatment. Tumor blood flow and perfusion were markedly reduced in mice treated with castration and anti-PS antibody. Histological studies revealed extensive disruption of tumor vasculature and abundant infiltration of immune cells, principally M1 macrophages. These results demonstrate that PS becomes exposed on prostate cancer tumor vasculature and tumor cells in response to hypoxia caused by ADT in the tumor microenvironment. The exposed PS could serve as a target to enhance the efficacy of ADT against prostate cancer.

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 621. doi:10.1158/1538-7445.AM2011-621

Abstract 3643: Phosphatidylserine-targeting antibody combined with Sorafenib has strong anti-tumor activity against human hepatocellular carcinomas in mice

Hepatocellular Carcinoma (HCC) is the third leading cause of cancer-related deaths in the world. The incidence of the HCC is increasing in the United States. HCC is refractory to most forms of therapy. Sorafenib is the only chemotherapeutic agent approved for the treatment of advanced HCC. Therefore, there is a huge unmet need for a new systemic therapy for HCC patients. A promising approach would be to combine sorafenib with bavituximab to treat HCC. Bavituximab is a vascular targeting antibody that is in Phase IIb clinical trials in patients with non-small cell lung cancer (NSCLC). Bavituximab targets phosphatidylserine (PS), which is normally intracellular but becomes exposed on the external surface of vascular endothelium in tumors. Bavituximab binds to PS-expressing tumor vascular endothelial cells and causes monocytes and macrophages to attack the vessels by antibody-dependent cellular cytotoxicity (ADCC). This results in death of tumor cells through deprivation of oxygen and nutrients. Chemotherapy and irradiation increase the exposure of PS on tumor vessels and enhance the anti-tumor activity of bavituximab-related antibodies in multiple rodent tumor models. We hypothesized that sorafenib would also induce exposure of PS on HCC vasculature because it interferes with anti-apoptotic signaling by VEGF and other angiogenic growth factors. We found that sorafenib induced PS exposure on human microvascular endothelial cells in vitro and in vivo. Treatment of mice bearing subcutaneous C3A, Huh7 or PLC/PRF/5 HCC tumors with sorafenib increased PS exposure on tumor vessels from 21% to 51% in PLC/PRF/5 tumors, from 22% to 67% in C3A tumors, and from 32% to 52% in Huh7 tumors. Combining a murine version of bavituximab with sorafenib significantly improved the anti-tumor effect in the PLC/PRF/5 xenograft model. The combination inhibited tumor growth by 95%, as compared with 80% for sorafenib alone (p&lt;0.05). Bavituximab in combination with sorafenib has promise as a new systemic therapy for the treatment of advanced HCC patients.

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 3643. doi:10.1158/1538-7445.AM2011-3643

Abstract 3651: Phosphatidylserine-targeting antibody induces differentiation of myeloid-derived suppressor cells into M1-like macrophages

Introduction: The phosphatidylserine (PS)-targeting antibody, bavituximab, is currently in Phase IIb clinical trials in patients with lung cancer. Bavituximab, and its murine counterpart, 2aG4, induce the attack of monocytes and macrophages on PS-expressing tumor vascular endothelium and tumor cells and inhibit the immunosuppressive effects of PS in the tumor microenvironment. Myeloid-derived suppressor cells (MDSC) are one of the major cells responsible for the immunosuppressed state in tumors. In this study, we tested the influence of 2aG4 on the differentiation of MDSC into M1-like tumor associated macrophages (TAMs). Methods: MDSC were isolated from 4T1 tumor bearing mice with anti-Gr1-coated magnetic beads. The purified MDSC were then cultured for 5 days in the presence of 2aG4 or control C44 antibody. For in vivo studies, PC3 tumor-bearing mice were treated with 2aG4 for 30 days and MDSC and TAMs in tumors and spleens were analyzed by FACS and immunohistochemistry. Results: 2aG4 treatment of purified MDSC switched their cytokine production from an immunosuppressive IL-10-dominated response to a pro-inflammatory IL-12- and TNFα -dominated response. The percentage of Gr1+ cells decreased to 8% in the 2aG4-treated cultures (P&lt;0.0001) but only to 50 – 57% in the PBS and C44-treated cultures. Treatment with 2aG4 induced the differentiation of MDSC into M1-like macrophages that expressed lower CD206 and produced more NO than control cultures. Treatment of mice bearing PC3 prostate tumors with 2aG4 decreased the percentage of MDSC from 7% to 4% (P&lt;0.001) in the tumors and from 28% to 20% (P&lt;0.001) in the spleens. The antibody treatment also increased the ratio of M1 to M2 TAMs in PC3 tumors from 0.7% to 1.4% (P&lt;0.001). Conclusion: Taken together, our results suggest that 2aG4 causes the differentiation of MDSCs into macrophages having an M1-like phenotype. 2aG4 treatment decreased IL-10 production and increased IL-12 and TNFα-production. These results suggest that 2aG4 treatment reactivates innate immunity in tumors.

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 3651. doi:10.1158/1538-7445.AM2011-3651

Co-localization of prothrombin fragment F1+2 and VEGF-R2-bound VEGF in human colon cancer

BACKGROUND

Colon cancer (CC) is frequently complicated by thromboembolic episodes. Thrombin plays a role in angiogenesis and among others induces the synthesis of vascular endothelial growth factor (VEGF) and its receptors (VEGFR-1 and VEGFR-2). The aim of this study was to assess the expression of prothrombin fragment F1+2 (F1+2), a byproduct in thrombin generation (indicating the presence of thrombin), in relation to the presence of VEGFR-2-bound VEGF (VEGF:VEGFR-2), as an indicator of VEGFR-2 activation in human CC tissue.

MATERIALS AND METHODS

Immunohistochemical ABC and double staining studies were performed using antibodies against F1+2 and VEGF:VEGFR-2 in 59 specimens obtained from CC patients.

RESULTS

Medium and high expression of both F1+2 and VEGF:VEGF2 in association with CC cells and endothelial cells was demonstrated. Moreover, coexpression of F1+2 and VEGF:VEGFR-2 was observed in the cells.

CONCLUSION

The results may suggest a possible functional interaction between thrombin and VEGF-R2 stimulation in human CC in vivo.

Antiphospholipid antibodies promote leukocyte-endothelial cell adhesion and thrombosis in mice by antagonizing eNOS via β2GPI and apoER2

In antiphospholipid syndrome (APS), antiphospholipid antibodies (aPL) binding to β2 glycoprotein I (β2GPI) induce endothelial cell-leukocyte adhesion and thrombus formation via unknown mechanisms. Here we show that in mice both of these processes are caused by the inhibition of eNOS. In studies of cultured human, bovine, and mouse endothelial cells, the promotion of monocyte adhesion by aPL entailed decreased bioavailable NO, and aPL fully antagonized eNOS activation by diverse agonists. Similarly, NO-dependent, acetylcholine-induced increases in carotid vascular conductance were impaired in aPL-treated mice. The inhibition of eNOS was caused by antibody recognition of domain I of β2GPI and β2GPI dimerization, and it was due to attenuated eNOS S1179 phosphorylation mediated by protein phosphatase 2A (PP2A). Furthermore, LDL receptor family member antagonism with receptor-associated protein (RAP) prevented aPL inhibition of eNOS in cell culture, and ApoER2-/- mice were protected from aPL inhibition of eNOS in vivo. Moreover, both aPL-induced increases in leukocyte-endothelial cell adhesion and thrombus formation were absent in eNOS-/- and in ApoER2-/- mice. Thus, aPL-induced leukocyte-endothelial cell adhesion and thrombosis are caused by eNOS antagonism, which is due to impaired S1179 phosphorylation mediated by β2GPI, apoER2, and PP2A. Our results suggest that novel therapies for APS can now be developed targeting these mechanisms.

Abstract 5330: Phosphatidylserine-targeting antibody enhances survival benefit of androgen deprivation therapy in mice bearing syngeneic prostate cancer

Purpose: Androgen deprivation therapy (ADT) remains the cornerstone treatment for advanced prostate cancer despite being principally palliative in nature. New treatment strategies aimed at enhancing tumor immunogenicity and damaging tumor vasculature could potentially improve prostate cancer response to ADT. We recently showed that targeting phosphatidylserine by antibodies resulting in enhanced tumor immunogenicity and tumor vascular damage. Bavituximab, a chimeric version anti-phosphatidylserine antibody, is being tested in clinic trails for different cancers. Here we tested the hypothesis that the fully human anti-phosphatidylserine antibody 1N11 can enhance the antitumor properties of ADT in prostate cancer

Experimental Design: Efficacy of ADT and 1N11 against prostate cancer was assessed in C57BL/6 mice bearing syngeneic RM1 prostate tumor by using castration and mouse chimeric 1N11(mch1N11). Effects on mouse survival were assessed. In vivo localization of 1N11,effects on immune cell infiltration, vasculature density were also estimated by histology.

Results: Our data showed that castration of RM1 tumor bearing mice increased the percentage of tumor vessels with exposed phosphatidylserine from 60% to 93%. Castration, when combined with mouse chimeric 1N11, led to significant survival befit versus individual treatments. The mean survival time was 16 days with control antibody. The prolongation in survival was 15 days with the combination, as compared with 3 days with mch1N11, 5 days with castration alone. Toxicity to the mice was not observed. A dramatic enhancement of tumor infiltration by a variety of immune cells and disruption of vasculature implied the combination to be more immunostimulatory and antivasculitic.

Conclusion: Combined treatment with 1N11 and ADT improves survival of mice bearing prostate cancer. These results provide the foundation for using ADT plus fully human 1N11, or bavituximab, to treat prostate cancer patients.

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 5330.