A novel CD3xPSMA bispecific antibody for efficient T cell mediated killing of prostate tumor cells with minimal cytokine release.

Abstract

324 Background: Castration resistant prostate cancer (CRPC) remains an incurable disease and new therapeutics are urgently needed. Prostate specific membrane antigen (PSMA) is expressed on the surface of prostate cancer cells and expression increases with disease progression. Therapies directed against PSMA such as radiolabeled antibodies and T cell redirecting therapies including chimeric antigen receptor T cells (CAR-Ts) and T-cell engaging bispecific antibodies (T-BsAbs) have shown promising efficacy in clinical trials but also induce significant toxicity. In particular CAR-Ts and T-BsAbs potently kill tumor cells but induce cytokine release-related toxicities. Novel anti-CD3 engaging domains may be required to create T-BsAbs with a broader therapeutic window. We have developed fully human CD3xPSMA bispecific antibodies that efficiently eliminate prostate tumor cells while minimizing cytokine release. Methods: Antibodies targeting CD3 and PSMA were generated in transgenic rats that produce human antibodies (UniRat, OmniFlic) followed by repertoire deep sequencing of lymph nodes isolated from immunized animals and high-throughput gene assembly/expression. CD3xPSMA T-BsAbs were assembled and evaluated for T cell activation and ability to eliminate PSMA+ tumor cells in vitro. Results: Primary human T cells were activated only in the presence of both bispecific CD3xPSMA antibodies and PSMA (either plate-bound or on the surface of tumor cells). Potent and selective cytotoxicity against PSMA+ prostate tumor cells was observed in co-cultures of primary human T cells and tumor cells treated with CD3xPSMA bispecific antibodies. Strikingly, CD3xPSMA bispecifics containing a novel low affinity anti-CD3 domain produced similar levels of tumor cell cytotoxicity compared to CD3xPSMA bispecifics containing a traditional high affinity anti-CD3 domain, but with reduced cytokine production. Conclusions: We have created novel CD3xPSMA bispecific antibodies that mediate T-cell killing of PSMA+ tumor cells with minimal production of cytokines. Such T-BsAbs may improve safety, efficacy, and opportunities for combination therapy to treat CRPC.