Abstract A193: Fully human bispecific antibodies induce potent anti-tumor effects against prostate tumors in mice

Despite multiple new treatment options for patients with castrate-resistant prostate cancer (CRPC), this is still an area of un-met need. Prostate specific membrane antigen (PSMA) is highly expressed in prostate cancer and expression is maintained in CRPC, making it an attractive target for late stage prostate cancer. We have developed a novel full-length human IgG bispecific antibody that binds to human and monkey PSMA and CD3 and results in T cell activation via the CD3 complex in the presence of PSMA-expressing tumors. In cell-based assays, our PSMAxCD3 bispecific resulted in target cell-dependent activation and cytokine release by human T cells, and efficient redirected T cell lysis of prostate tumor cells. Of note, in the absence of PSMA-expressing tumor cells, no T cell activation was seen. Our bispecific antibody also resulted in redirected killing by cynomolgus monkey T cells, enabling pre-clinical toxicity studies. To examine bispecific anti-tumor efficacy in vivo, we developed xenogenic and syngeneic tumor models. In studies with human tumor cells, NOD SCID IL2R gamma deficient (NSG) mice were co-implanted with human peripheral blood mononuclear (PBMC) cells plus human prostate tumor cells (22Rv1 or C4-2), and the mice treated immediately with PSMAxCD3 i.p. Significant inhibition of tumor growth was observed against both human prostate tumors with dose levels as low as 0.004mg/kg. Importantly, PSMAxCD3 bispecific antibodies could also suppress growth of established C4-2 tumors when PBMCs were transferred i.p.. Our PSMAxCD3 bispecific was also tested in mice engrafted with human hematopoietic CD34+ stem cells. Newborn SIRPα BALB/c-Rag2null IL2rγnull (BRG) pups were engrafted with hCD34+ fetal liver cells as hematopoietic progenitor cells, which gave rise to human T, B, and NK cells, as well as granulocytes, monocytes, and DCs. Once engraftment was achieved (3 months later), mice were given C4-2 tumor cells subcutaneously. Eight days after tumor implantation, mice were treated with PSMAxCD3 (0.4mg/kg) followed by twice weekly doses. The PSMAxCD3 bispecific reduced tumor growth in this model. To examine the effects of our lead-candidate bispecifics in an immune-competent model, we engineered mice to express both human CD3 and PSMA in the corresponding murine loci. T cells from these mice were efficiently activated by antibodies to human CD3, and the expression of human PSMA allowed assessment of effects on normal tissues that may express PSMA. T cell activation, proliferation and cytokine production in response to our bispecific antibodies were examined in the absence of tumor as well as the absence of target. To assess anti-tumor efficacy in this model, a murine prostate cancer line expressing human PSMA was generated and tumors cells were implanted s.c. into these mice. Our bispecific antibodies significantly suppressed tumor growth when mice were treated at the time of implantation and also when treatment was delayed until tumors were established. Moreover, T cells were still present at normal levels in the spleen of mice treated with our bispecific at the end of the experiment and the treatment was well-tolerated. Taken together, these studies show potent anti-tumor activity of our PSMAxCD3 bispecific in several different tumor models and also demonstrate that treatment was well-tolerated in mice expressing human PSMA and human CD3. Citation Format: Alison Crawford, Kristin Vazzana, Jeffrey VanValkenburgh, Lauric Haber, Jennifer Principio, Cagan Gurer, Kara Olson, Eric Smith, Gavin Thurston, Jessica R. Kirshner. Fully human bispecific antibodies induce potent anti-tumor effects against prostate tumors in mice. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr A193.