Mechanisms of Response and Resistance to PSMA×CD3 Bispecifics in CD34⁺ Humanized Mice

Prostate cancer is considered immunologically “cold,” with low mutational burden, tumor-infiltrating immune cells, and PD-L1 levels, culminating in poor response to immune checkpoint therapies. CD3 bispecific redirection antibodies can elicit T cell–mediated cytotoxicity and hold promise for immune cell recruitment into prostate tumors. CD3 redirection antibodies in solid tumors are still in the early phases of clinical development, and it is not yet understood whether these potential therapies will achieve the high response rates observed in hematologic malignancies or result in durable T-cell responses. In this study, we demonstrated that treatment with a prostate-specific membrane antigen (PSMA)–targeted CD3 redirector resulted in efficacy against LnCaP.AR human prostate xenografts in CD34⁺ cord blood–humanized mice. Efficacy correlated with T-cell infiltration into tumors with an activated phenotype and also increased PD-L1 expression. Engineered overexpression of PD-L1 in LNCaP.AR tumors resulted in resistance to PSMA×CD3 bispecific antibody treatment, whereas sensitivity was restored in combination with anti–PD-1 antibody pembrolizumab. PSMA×CD3 and anti–PD-1 combination treatment resulted in complete tumor responses in approximately 20% of mice and elicited immune responses that delayed growth of rechallenged tumors. In a second prostate model, patient-derived LuCaP 86.2 xenografts, PSMA×CD3 monotherapy treatment resulted in complete responses in 25% of mice. When PSMA×CD3–treated responder mice were rechallenged with LuCaP 86.2 tumors, partial control of tumor regrowth was associated with the expansion of effector memory T cells. These studies show that PSMA×CD3 treatment elicits antitumor memory T-cell responses and that combination with PD-1 blockade can enhance these effects in tumors with immune-suppressive tumor microenvironments.