Abstract 5232: Phosphatidylethanolamine is a marker of tumor vasculature and can be used as a target for optical imaging

New imaging agents are needed that home rapidly to tumors and saturate their target molecules with the unbound residue then clearing rapidly from the blood stream. Such agents should enable rapid detection of tumors. We have found that a phosphatidylethanolamine-binding peptide known as duramycin is an effective targeting moiety for construction of tumor-specific imaging agents. Phosphatidylethanolamine (PE) is one of two major phospholipids that present in the inner leaflet of the cell membranes, the other being phosphatidylserine (PS). We have previously demonstrated that oxidative stress conditions within the tumor microenvironment cause PS to flip to the outer leaflet in tumor endothelial cells. The exposed PS provides a highly specific marker of tumor vasculature. Since PE and PS distribution within the membrane is co-regulated, we hypothesized that PE would also be a marker of tumor vasculature. To target exposed PE, we generated several biotinylated duramycin constructs. Biotinylated duramycin bound to PE which became exposed on endothelial cells after they had been subjected to oxidative stresses. When the biotinylated duramycin was injected intravenously into mice bearing different types of tumors it rapidly localized to tumor vasculature and did not bind blood vessels in normal tissues. Next, we conjugated duramycin conjugated to a near-infrared fluophore (IRDye 800CW) and used it for optical imaging of subcutaneous RM-1 prostate tumors in mice. The duramycin-targeted near infrared probe efficiently labeled tumors and did not accumulate in non-target tissues giving clear detection of the tumors. These results show that PE exposure, like PS exposure, is a marker of tumor vasculature in many types of cancer. Duramycin labeled with detectable probes may have broad application in cancer imaging.

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 5232.

Abstract 1919: Phosphatidylserine on dying tumor cells suppresses dendritic cell activation and inhibits tumor immunity: reversal with PS-targeting antibody

We recently showed that PS on the surface of apoptotic cancer cells can suppress recognition of tumor antigens by the immune system. It appears from work in several laboratories that dendritic cells (DC) that ingest PS-expressing tumor cells fail to mature in response to external maturation signals. In the present study, we investigated whether masking PS on apoptotic tumor cells with anti-PS antibody (2aG4) can reverse the inhibitory effect of PS on DC. Mouse bone marrow derived DC were cocultured with irradiated (PS-expressing) 4T1 breast tumor cells in the presence or absence of 2aG4. We found that: (i) masking tumor cell PS with 2aG4 increases their phagocytosis by DCs; (ii) masking tumor cell PS with 2aG4 allows DCs to mature and express immunostimulatory molecules (CD40, CD80, CD86 and MHC II); (iii) masking PS with 2aG4 decreases the production of anti-inflammatory cytokines (TGFβ and IL-10) and increases the production of inflammatory cytokines (TNFα, CCL5, IL-1β and IL-6), indicating that PS-blockade polarizes DC towards the immunostimulatory type I phenotype; (iv) immunizing mice with 2aG4-treated irradiated 4T1 cells enhances the immunogenicity of the tumor cells, rendering them resistant to rechallenge with syngeneic viable 4T1 tumor cells. These results have important implications for our understanding of the immunosuppressive effects of PS in cancer and could lead to the development of a novel whole cell cancer vaccine strategy in which PS-blocking is used to enhance immunogenicity.

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 1919.

Anti-phospholipid human monoclonal antibodies inhibit CCR5-tropic HIV-1 and induce beta-chemokines

Traditional antibody-mediated neutralization of HIV-1 infection is thought to result from the binding of antibodies to virions, thus preventing virus entry. However, antibodies that broadly neutralize HIV-1 are rare and are not induced by current vaccines. We report that four human anti-phospholipid monoclonal antibodies (mAbs) (PGN632, P1, IS4, and CL1) inhibit HIV-1 CCR5-tropic (R5) primary isolate infection of peripheral blood mononuclear cells (PBMCs) with 80% inhibitory concentrations of <0.02 to ∼10 µg/ml. Anti-phospholipid mAbs inhibited PBMC HIV-1 infection in vitro by mechanisms involving binding to monocytes and triggering the release of MIP-1α and MIP-1β. The release of these β-chemokines explains both the specificity for R5 HIV-1 and the activity of these mAbs in PBMC cultures containing both primary lymphocytes and monocytes.

Targeting mutant p53 protein and the tumor vasculature: an effective combination therapy for advanced breast tumors

Breast cancer progression depends upon the elaboration of a vasculature sufficient for the nourishment of the developing tumor. Breast tumor cells frequently contain a mutant form of p53 (mtp53), a protein which promotes their survival. The aim of this study was to determine whether combination therapy targeting mtp53 and anionic phospholipids (AP) on tumor blood vessels might be an effective therapeutic strategy for suppressing advanced breast cancer. We examined the therapeutic effects, singly, or in combination, of p53 reactivation and induction of massive apoptosis (PRIMA-1), which reactivates mtp53 and induces tumor cell apoptosis, and 2aG4, a monoclonal antibody that disrupts tumor vasculature by targeting AP on the surface of tumor endothelial cells and causes antibody-dependent destruction of tumor blood vessels, leading to ischemia and tumor cell death. Xenografts from two tumor cell lines containing mtp53, BT-474 and HCC-1428, were grown in nude mice to provide models of advanced breast tumors. After treatment with PRIMA-1 and/or 2aG4, regressing tumors were analyzed for vascular endothelial growth factor (VEGF) expression, blood vessel loss, and apoptotic markers. Individual drug treatment led to partial suppression of breast cancer progression. In contrast, combined treatment with PRIMA-1 and 2aG4 was extremely effective in suppressing tumor growth in both models and completely eradicated approximately 30% of tumors in the BT-474 model. Importantly, no toxic effects were observed in any treatment group. Mechanistic studies determined that PRIMA-1 reactivated mtp53 and also exposed AP on the surface of tumor cells as determined by enhanced 2aG4 binding. Combination treatment led to significant induction of tumor cell apoptosis, loss of VEGF expression, as well as destruction of tumor blood vessels. Furthermore, combination treatment severely disrupted tumor blood vessel perfusion in both tumor models. The observed in vitro PRIMA-1-induced exposure of tumor epithelial cell AP might provide a target for 2aG4 and contribute to the increased effectiveness of such combination therapy in vivo. We conclude that the combined targeting of mtp53 and the tumor vasculature is a novel effective strategy for combating advanced breast tumors.

Organ-specific lymphangiectasia, arrested lymphatic sprouting, and maturation defects resulting from gene-targeting of the PI3K regulatory isoforms p85alpha, p55alpha, and p50alpha

The phosphoinositide 3-kinase (PI3K) family has multiple vascular functions, but the specific regulatory isoform supporting lymphangiogenesis remains unidentified. Here, we report that deletion of the Pik3r1 gene, encoding the regulatory subunits p85alpha, p55alpha, and p50alpha impairs lymphatic sprouting and maturation, and causes abnormal lymphatic morphology, without major impact on blood vessels. Pik3r1 deletion had the most severe consequences among gut and diaphragm lymphatics, which share the retroperitoneal anlage, initially suggesting that the Pik3r1 role in this vasculature is anlage-dependent. However, whereas lymphatic sprouting toward the diaphragm was arrested, lymphatics invaded the gut, where remodeling and valve formation were impaired. Thus, cell-origin fails to explain the phenotype. Only the gut showed lymphangiectasia, lymphatic up-regulation of the transforming growth factor-beta co-receptor endoglin, and reduced levels of mature vascular endothelial growth factor-C protein. Our data suggest that Pik3r1 isoforms are required for distinct steps of embryonic lymphangiogenesis in different organ microenvironments, whereas they are largely dispensable for hemangiogenesis.

Antiphosphatidylserine antibody combined with irradiation damages tumor blood vessels and induces tumor immunity in a rat model of glioblastoma

PURPOSE

The vascular targeting antibody bavituximab is being combined with chemotherapy in clinical trials in cancer patients. Bavituximab targets the membrane phospholipid, phosphatidylserine, complexed with beta2-glycoprotein I. Phosphatidylserine is normally intracellular but becomes exposed on the luminal surface of vascular endothelium in tumors. Phosphatidylserine exposure on tumor vessels is increased by chemotherapy and irradiation. Here, we determined whether treatment with the murine equivalent of bavituximab, 2aG4, combined with irradiation can suppress tumor growth in a rat model of glioblastoma.

EXPERIMENTAL DESIGN

F98 glioma cells were injected into the brains of syngeneic rats where they grow initially as a solid tumor and then infiltrate throughout the brain. Rats with established tumors were treated with 10 Gy whole brain irradiation and 2aG4.

RESULTS

Combination treatment doubled the median survival time of the rats, and 13% of animals were rendered disease free. Neither treatment given individually was as effective. We identified two mechanisms. First, irradiation induced phosphatidylserine exposure on tumor blood vessels and enhanced antibody-mediated destruction of tumor vasculature by monocytes/macrophages. Second, the antibody treatment induced immunity to F98 tumor cells, which are normally weakly immunogenic. Surviving rats were immune to rechallenge with F98 tumor cells. In vitro, 2aG4 enhanced the ability of dendritic cells (DCs) to generate F98-specific cytotoxic T cells. Phosphatidylserine exposure, which is induced on tumor cells by irradiation, likely suppresses tumor antigen presentation, and 2aG4 blocks this tolerogenic effect.

CONCLUSION

Bavituximab combined with radiotherapy holds promise as a vascular targeting and immune enhancement strategy for the treatment of human glioblastoma.

Prevention of ischemic brain injury by treatment with the membrane penetrating apoptosis inhibitor, TAT-BH4

In acute thromboembolic stroke, neurological damage is due to ischemia-induced apoptotic death of neuronal cells and the surrounding vascular network. Here, we demonstrate that the BH4 domain of the anti-apoptotic protein, Bcl-x(L), attached to the membrane transport peptide, TAT, reduces stroke injury after intracerebroventricular infusion into immature rats subjected to carotid artery ligation and additional exposure to hypoxia. The injected TAT-BH4 entered neuron bodies, maintained brain architecture, protected neuronal and endothelial cells from apoptosis and promoted neuronal stem cell recruitment. In vitro, TAT-BH4 enhanced the survival of endothelial cells exposed to H(2)O(2), increased neuronal differentiation, and induced axonal remodelling of adult neuronal stem cells. These findings indicate that TAT-BH4 administration protects against acute hypoxia/ischemia injury in the brain by preventing endothelial and neuron cell apoptosis and by inducing neuronal plasticity.

Targeting inside-out phosphatidylserine as a therapeutic strategy for viral diseases

Targeting exposed anionic phospholipids on a spectrum of virus-infected cells can protect against lethal virus infections in vivo.

Coincident exposure of phosphatidylethanolamine and anionic phospholipids on the surface of irradiated cells

The major anionic phospholipid, phosphatidylserine (PS), and the neutral phospholipid, phosphatidylethanolamine (PE), are largely confined to the inner leaflet of the plasma membrane bilayer in mammalian cells under normal conditions. This asymmetry is lost when cells undergo apoptosis, become activated, or are exposed to irradiation, reactive oxygen species or certain drugs. It is not known whether exposure of anionic phospholipids (APLs) and PE occurs simultaneously or in the same region of the plasma membrane. Here we examined the coincidence of exposure of APLs and PE on the surface of bovine aortic endothelial cells and NS0 myeloma cells after irradiation. The cells were irradiated (5 Gy) and stained for APLs and PE using liposomes coated with either an Fab' fragment of a PS-binding antibody (bavituximab) or a PE-binding peptide (duramycin). Using live cell imaging and flow cytometry, we showed that irradiation leads to synchronous externalization of APLs and PE. The time course of appearance of APLs and PE on the cell surface was the same and the two phospholipid types remained colocalized over time. Distinct patches double positive for APLs and PE were visible. Larger areas of APLs and PE appeared to have detached from the cytoskeleton to form membrane blebs which protruded and drifted on the cell surface. We conclude that APLs and PE coincidently appear on the external leaflet of the plasma membrane of cells after irradiation. Probably, this is because PE and the major APL, PS, share common regulatory mechanisms of translocation.

Vascular imaging of solid tumors in rats with a radioactive arsenic-labeled antibody that binds exposed phosphatidylserine

PURPOSE

We recently reported that anionic phospholipids, principally phosphatidylserine, become exposed on the external surface of vascular endothelial cells in tumors, probably in response to oxidative stresses present in the tumor microenvironment. In the present study, we tested the hypothesis that a chimeric monoclonal antibody that binds phosphatidylserine could be labeled with radioactive arsenic isotopes and used for molecular imaging of solid tumors in rats.

EXPERIMENTAL DESIGN

Bavituximab was labeled with (74)As (beta(+), T(1/2) 17.8 days) or (77)As (beta(-), T(1/2) 1.6 days) using a novel procedure. The radionuclides of arsenic were selected because their long half-lives are consistent with the long biological half lives of antibodies in vivo and because their chemistry permits stable attachment to antibodies. The radiolabeled antibodies were tested for the ability to image subcutaneous Dunning prostate R3227-AT1 tumors in rats.

RESULTS

Clear images of the tumors were obtained using planar gamma-scintigraphy and positron emission tomography. Biodistribution studies confirmed the specific localization of bavituximab to the tumors. The tumor-to-liver ratio 72 h after injection was 22 for bavituximab compared with 1.5 for an isotype-matched control chimeric antibody of irrelevant specificity. Immunohistochemical studies showed that the bavituximab was labeling the tumor vascular endothelium.

CONCLUSIONS

These results show that radioarsenic-labeled bavituximab has potential as a new tool for imaging the vasculature of solid tumors.

Radiation-enhanced vascular targeting of human lung cancers in mice with a monoclonal antibody that binds anionic phospholipids

PURPOSE

New treatment strategies aimed at damaging tumor vasculature could potentially improve tumor response to radiation therapy. We recently showed that anionic phospholipids, principally phosphatidylserine, are specifically exposed on the luminal surface of tumor blood vessels. Here we tested the hypothesis that radiation therapy can increase phosphatidylserine exposure on lung tumor vasculature, thereby enhancing the antitumor properties of the anti-phosphatidylserine antibody 2aG4.

EXPERIMENTAL DESIGN

The therapeutic efficacy of radiation therapy plus 2aG4 was tested in nude mice bearing radiation-resistant A549 human lung tumors. Radiation-induced phosphatidylserine exposure on endothelial cells and A549 tumor cells was analyzed by immunofluoresence staining. The mechanism of the enhanced antitumor effect was examined by histology and antibody-dependent cell-mediated cytotoxicity experiments.

RESULTS

Focal irradiation of A549 human lung cancer xenografts increased the percentage of tumor vessels with exposed phosphatidylserine from 4% to 26%. Treatment of mice bearing A549 tumors with 2aG4 plus focal radiation therapy inhibited tumor growth by 80% and was superior to radiation therapy or 2aG4 alone (P < 0.01). Combination therapy reduced blood vessel density and enhanced monocyte infiltration into the tumor mass beyond that observed with individual treatments. In vitro, 2aG4 enhanced the ability of macrophages to kill endothelial cells with exposed phosphatidylserine in an Fc'-dependent manner.

CONCLUSION

These results suggest that 2aG4 enhances the antitumor effects of radiation therapy by increasing antibody-dependent cell-mediated cytotoxicity toward tumor vessels with externalized phosphatidylserine. Bavituximab, a chimeric version of 2aG4 in clinical trials, has the potential to enhance the therapeutic efficacy of radiation therapy in lung cancer patients.

ANTIBODY-MEDIATED TARGETING OF “INSIDE-OUT” ANIONIC PHOSPHOLIPIDS IN VIRAL DISEASE (47.21)

The anionic phospholipid phosphatidylserine (PS) is found exclusively in the inner leaflet of the plasma membrane of resting mammalian cells. We hypothesized that certain events that occur during virus replication (eg cell activation or membrane rearrangement) would trigger the exposure of anionic phospholipids on the outer surface of virus- infected cells and subsequently on the enveloped viruses that bud out of these virus- infected cells. We further hypothesized that these exposed anionic phospholipids would serve as targets for anti-viral therapy. We demonstrate here that anionic phospholipids become exposed on the enveloped Pichinde Virus (a model virus for Lassa Fever virus, a potential bioterrorism agent) and on Pichinde virus-infected cells. To detect anionic phospholipids, we used a chimeric monoclonal antibody, bavituximab, that binds anionic phospholipids in a B2-glycoprotein I dependent manner. We show that bavituximab treatment is able to cure overt disease in guinea pigs lethally infected with Pichinde virus. Bavituximab treatment reduced the amounts of virus in multiple tissues and caused direct clearance of virus from the blood. Direct clearance of free virus and antibody-dependent cellular cytotoxicity of virus-infected cells appear to be the major mechanisms that contribute to the anti-viral effect of bavituximab. Bavituximab-treated survivors were immune to reinfection. Furthermore, the murine version of bavituximab, 3G4, shows therapeutic efficacy in a lethal murine model for human cytomegalovirus. Our study demonstrates the promise of anionic phospholipids as targets for new broad-spectrum anti-viral drugs.

TAT-BH4 counteracts Abeta toxicity on capillary endothelium

Oxidative stress is one of the factor contributing to blood brain barrier degeneration. This phenomenon is observed during pathological conditions such as Alzheimer's disease or cerebral amyloid angiopathy in which brain haemorrhages are very frequent. Both diseases are characterized by beta amyloid peptide deposition either in neurons or in vessels. Oxidative stress leads to impairment of mitochondrial functions and apoptotic cell death subsequent to caspases activation. In this paper we demonstrate that BH4 domain of Bcl-xl administrated to endothelial cells as the conjugated form with TAT peptide, reverts Abeta-induced apoptotic cell death by activating a survival programme which is Akt/endothelial nitric oxide synthase dependent.

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

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

Plasma protein beta-2-glycoprotein 1 mediates interaction between the anti-tumor monoclonal antibody 3G4 and anionic phospholipids on endothelial cells

A promising target on tumor vasculature is phosphatidylserine (PS), an anionic phospholipid that resides exclusively on the inner leaflet of the plasma membrane of resting mammalian cells. We have shown previously that PS becomes exposed on the surface of endothelial cells (EC) in solid tumors. To target PS on tumor vasculature, the murine monoclonal antibody 3G4 was developed. 3G4 localizes to tumor vasculature, inhibits tumor growth, and enhances anti-tumor chemotherapies without toxicity in mice. A chimeric version of 3G4 is in clinical trials. In this study, we investigated the basis for the interaction between 3G4 and EC with surface-exposed PS. We demonstrate that antibody binding to PS is dependent on plasma protein beta-2-glycoprotein 1 (beta2GP1). beta2GP1 is a 50-kDa glycoprotein that binds weakly to anionic phospholipids under physiological conditions. We show that 3G4 enhances binding of beta2GP1 to EC induced to expose PS. We also show that divalent 3G4-beta2GP1 complexes are required for enhanced binding, since 3G4 Fab' fragments do not bind EC with exposed PS. Finally, we demonstrate that an artificial dimeric beta2GP1 construct binds to EC with exposed PS in the absence of 3G4, confirming that antibody binding is mediated by dimerization of beta2GP1. Together, these data indicate that 3G4 targets tumor EC by increasing the avidity of beta2GP1 for anionic phospholipids through formation of multivalent 3G4-beta2GP1 complexes.

Combination of a monoclonal anti-phosphatidylserine antibody with gemcitabine strongly inhibits the growth and metastasis of orthotopic pancreatic tumors in mice

Pancreatic cancer continues to have a dismal prognosis and novel therapy is needed. In this study, we evaluate a promising new target for therapy, phosphatidylserine (PS). PS is an anionic phospholipid located normally on the inner leaflet of the plasma membrane in mammalian cells. In the tumor microenvironment, PS becomes externalized on vascular endothelium. The monoclonal antibody 3G4 binds PS and promotes an inflammatory response against tumor blood vessels, resulting in reduction of tumor growth. Mice with orthotopic pancreatic tumors were treated with 3G4, gemcitabine or a combination of both drugs. Tumor burden including pancreas weight and metastatic lesions (liver, lymph node and peritoneal) were reduced 3- to 5-fold by the combination therapy as compared with 1.5- to 2-fold with 3G4 and gemcitabine alone, respectively. Treatment of tumor-bearing animals with the combination therapy increased macrophage infiltration into the tumor mass 10-fold and reduced microvessel density in the tumor by 2.5-fold compared with tumors from untreated animals. Gemcitabine alone and 3G4 alone were less effective than the combination of the 2 agents together. The additive therapeutic effect of both agents appears to be because chemotherapy increases PS exposure on tumor vascular endothelium and amplifies the target for attack by 3G4. In conclusion, 3G4 enhanced the anti-tumor and anti-metastatic activity of gemcitabine without contributing to toxicity.

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

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

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

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

A new method for radiochemical separation of arsenic from irradiated germanium oxide

Radioarsenic labelled radiopharmaceuticals could be a valuable asset to Positron Emission Tomography (PET). In particular, the long half-lives of (72)As (T(1/2)=26 h) and (74)As (T(1/2)=17.8 d) allow to investigate slow physiological or metabolical processes, like the enrichment and distribution of antibodies in tumor tissue. This work describes the direct production of no-carrier-added (nca) arsenic isotopes *As, with *=71, 72, 73, 74 or 77, the reaction to [*As]AsI(3) and its radiochemical separation from the irradiated solid germanium oxide via polystyrene-based solid-phase extraction. The germanium oxide target, irradiated at a cyclotron or a nuclear reactor, is dissolved in concentrated HF and Ge is separated almost quantitatively (99.97%) as [GeF(6)](2-). [*As]AsI(3) is formed by addition of potassium iodide. The radiochemical separation yield for arsenic is >90%. [*As]AsI(3) is a versatile radioarsenic labelling synthon.

Antitumor effects of a monoclonal antibody that binds anionic phospholipids on the surface of tumor blood vessels in mice

PURPOSE

We recently reported that anionic phospholipids, principally phosphatidylserine, become exposed on the external surface of viable vascular endothelial cells in tumors, possibly in response to oxidative stresses present in the tumor microenvironment. In the present study, we tested the hypothesis that a monoclonal antibody directed against anionic phospholipids might exert antitumor effects by causing vascular damage in tumors.

EXPERIMENTAL DESIGN

A new mouse immunoglobulin G3 monoclonal antibody, 3G4, was raised that binds anionic phospholipids in the presence of serum or beta2-glycoprotein I. The antibody was tested for its ability to localize to tumor vessels and exert antitumor effects in mice.

RESULTS

3G4 recognized anionic phospholipids on the external membrane of H(2)O(2)-treated endothelial cells and in vitro. It localized specifically to tumor vascular endothelium and to necrotic tumor cells after injection into severe combined immunodeficient mice bearing orthotopic MDA-MB-435 tumors. Treatment with 3G4 retarded the growth of four different tumors in mice. It reduced the growth of established orthotopic MDA-MB-231 and MDA-MB-435 human breast tumors in mice by 75% and 65% respectively, large L540 human Hodgkin's tumors by 50%, and small syngeneic Meth A fibrosarcomas by 90%. Histologic examination revealed vascular damage, a reduction in vascular density, and a reduction in tumor plasma volume. Treatment with 3G4 induced the binding of monocytes to tumor endothelium and infiltration of macrophages into MDA-MB-435 and MDA-MB-231 tumors. No toxicity to the mice was observed.

CONCLUSIONS

3G4 localizes specifically to complexes of anionic phospholipids and serum proteins on the surface of vascular endothelial cells in tumors in mice. This results in damage to tumor vasculature and suppression of tumor growth.

A monoclonal antibody that binds anionic phospholipids on tumor blood vessels enhances the antitumor effect of docetaxel on human breast tumors in mice

Anionic phospholipids, principally phosphatidylserine, become exposed on the external surface of viable vascular endothelial cells in tumors, providing an excellent marker for tumor vascular targeting. We recently raised an IgG monoclonal antibody, 3G4, which binds to anionic phospholipids in a beta2-glycoprotein I-dependent manner. It inhibited tumor growth in a variety of rodent tumor models by stimulating antibody-dependent cellular cytotoxicity toward tumor vessels. In the present study, we tested the hypothesis that docetaxel, which is known to have antivascular effects on tumors, might induce exposure of anionic phospholipids on tumor vasculature and, thus, enhance the antitumor activity of 3G4. Treatment of human umbilical vascular endothelial cells with subtoxic concentrations of docetaxel (20 pmol/L) in vitro caused anionic phospholipids to be externalized without inducing apoptosis. Docetaxel treatment of mice increased the percentage of tumor vessels that expose anionic phospholipids from 35% to 60%. No induction of phosphatidylserine was observed on vessels in normal tissues even after systemic treatment with docetaxel. Treatment of mice bearing orthotopic MDA-MB-435 human breast tumors with 3G4 plus docetaxel inhibited tumor growth by 93%. Treatment of mice bearing disseminated MDA-MB-435 tumors with 3G4 plus docetaxel reduced the average number of tumor colonies in the lungs by 93% and half the animals did not develop tumors. In both tumor models, the antitumor effect of the combination was statistically superior (P < 0.01) to that of docetaxel or 3G4 alone. Combination therapy reduced the tumor vessel density and plasma volume in tumors to a greater extent than did the individual drugs. The combination therapy was no more toxic to the mice than was docetaxel alone. These results indicate that, as an adjuvant therapy, 3G4 could enhance the therapeutic efficacy of docetaxel in breast cancer patients.

Effect of rapamycin alone and in combination with antiangiogenesis therapy in an orthotopic model of human pancreatic cancer

PURPOSE

The overall 5-year survival of patients with pancreatic cancer remains <5%. Novel therapeutic strategies are needed. We examined the effect of rapamycin, alone and in combination with antiangiogenesis therapy, on pancreatic cancer in vivo.

EXPERIMENTAL DESIGN

Human pancreatic cancer AsPC-1 cells were orthotopically injected into severe combined immunodeficient/beige mice to evaluate primary tumor growth and liver metastasis after treatment with rapamycin alone or in combination with anti-vascular endothelial growth factor antibody 2C3. Tumor cell proliferation was determined by bromodeoxyuridine incorporation. To detect tumor cell apoptosis, the terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assay was used. Tumor angiogenesis was investigated by using a monoclonal anti-CD31 antibody. All statistical tests were two-sided.

RESULTS

Rapamycin, alone and in combination with 2C3, strongly inhibited primary and metastatic tumor growth in an orthotopic pancreatic cancer animal model. Furthermore, the combination therapy significantly improved the effect on liver metastasis compared with single treatment with either rapamycin (P = 0.0128) or 2C3 (P = 0.0099). Rapamycin alone inhibited pancreatic tumor cell proliferation, induced apoptosis, and decreased tumor angiogenesis. Nevertheless, the combination therapy showed a significant, stronger inhibition of tumor cell proliferation (P = 0.0002 versus rapamycin alone and P < 0.0001 versus 2C3 alone). The induction of apoptosis was significantly higher than in the rapamycin-treated group (P = 0.0039). Additionally, the combination therapy further improved suppression of tumor cell angiogenesis compared with rapamycin treatment (P = 0.029)

CONCLUSIONS

Our studies propose new therapeutic strategies to inhibit both primary and metastatic tumor growth in pancreatic cancer. Considering the fact that liver metastasis is a crucial problem in advanced stages of pancreatic cancer, the combination therapy of rapamycin plus anti-vascular endothelial growth factor antibody 2C3 is a significant advantage compared with single treatment with rapamycin.

Vascular targeting agents as cancer therapeutics

Vascular targeting agents (VTAs) for the treatment of cancer are designed to cause a rapid and selective shutdown of the blood vessels of tumors. Unlike antiangiogenic drugs that inhibit the formation of new vessels, VTAs occlude the pre-existing blood vessels of tumors to cause tumor cell death from ischemia and extensive hemorrhagic necrosis. Tumor selectivity is conferred by differences in the pathophysiology of tumor versus normal tissue vessels (e.g., increased proliferation and fragility, and up-regulated proteins). VTAs can kill indirectly the tumor cells that are resistant to conventional antiproliferative cancer therapies, i.e., cells in areas distant from blood vessels where drug penetration is poor, and hypoxia can lead to radiation and drug resistance. VTAs are expected to show the greatest therapeutic benefit as part of combined modality regimens. Preclinical studies have shown VTA-induced enhancement of the effects of conventional chemotherapeutic agents, radiation, hyperthermia, radioimmunotherapy, and antiangiogenic agents. There are broadly two types of VTAs, small molecules and ligand-based, which are grouped together, because they both cause acute vascular shutdown in tumors leading to massive necrosis. The small molecules include the microtubulin destabilizing drugs, combretastatin A-4 disodium phosphate, ZD6126, AVE8062, and Oxi 4503, and the flavonoid, DMXAA. Ligand-based VTAs use antibodies, peptides, or growth factors that bind selectively to tumor versus normal vessels to target tumors with agents that occlude blood vessels. The ligand-based VTAs include fusion proteins (e.g., vascular endothelial growth factor linked to the plant toxin gelonin), immunotoxins (e.g., monoclonal antibodies to endoglin conjugated to ricin A), antibodies linked to cytokines, liposomally encapsulated drugs, and gene therapy approaches. Combretastatin A-4 disodium phosphate, ZD6126, AVE8062, and DMXAA are undergoing clinical evaluation. Phase I monotherapy studies have shown that the agents are tolerated with some demonstration of single agent efficacy. Because efficacy is expected when the agents are used with conventional chemotherapeutic drugs or radiation, the results of Phase II combination studies are eagerly awaited.

Anti-tumor effects and lack of side effects in mice of an immunotoxin directed against human and mouse prostate-specific membrane antigen

BACKGROUND

Prostate-specific membrane antigen (PSMA) is a transmembrane protein that is largely restricted to prostatic epithelial cells in humans and is strongly upregulated on prostatic carcinoma cells. It is also expressed on the endothelium of tumor vasculature in humans, but not on the vasculature of normal tissues. Expression of low levels of PSMA has also been found on non-vascular cells in several normal tissues, most prominently on the brain and kidney in humans. PSMA is an excellent candidate for targeting prostate cancer or targeting tumor vasculature of various solid tumors. The high potential clinical benefit of these agents has prompted the search for an animal model in which to assess the efficacy and safety of anti-PSMA monoclonal antibody (mAb)-based therapies.

METHODS

A rat monoclonal antibody, E6 that recognizes both mouse and human PSMA was generated using conventional hybridoma techniques. The antibody was characterized by enzyme-linked immunosorbent assay (ELISA), Western blot, and immunohistochemistry. An immunotoxin composed of E6, antibody and deglycosylated ricin A-chain (dgA) was prepared chemically. The anti-tumor effects of the immunotoxin were determined in vitro and in mice bearing subcutaneous LnCaP human prostate tumors, which express PSMA on the tumor cell surface.

RESULTS

E6 recognizes the extracellular domain of both human and mouse PSMA in ELISA, immunoblot and by immunohistochemistry. E6 strongly stained the vascular endothelium of tumors from humans but not from mice. E6 stained proximal tubules in mouse and human kidneys, and neurons in the mouse and human hippocampus but, unlike the human, did not detectably stain epithelial cells in mouse prostate or small intestine. An E6-dgA immunoconjugate strongly inhibited the growth of LnCaP tumor xenografts without causing apparent toxicity to the mice. Histological observation indicated that the anti-tumor effects were mediated through direct cytotoxic effects on the tumor cells.

CONCLUSIONS

We have generated and characterized a rat mAb (E6) that reacts specifically with both human and mouse PSMA and have demonstrated that an immunotoxin constructed from E6 is safe and effective against human prostatic carcinoma cells growing subcutaneously in nude mice.