Abstract 5577: PSMA targeting CAR T cell immunotherapeutic strategy for prostate cancer
Authors: Gonzalez-Valdivieso Juan , Yang Yanping , Vedvyas Yogindra , Alcaina Yago , Jin Moonsoo M. 
Published: June 2022

Abstract
Introduction: Prostate cancer is the third most frequent cause of cancer-related death in men worldwide. Since prostate-specific membrane antigen (PSMA) shows a significant overexpression on prostatic cancer cells, PSMA can be considered a promising target for prostate cancer imaging and treatment. Although chimeric antigen receptor (CAR) T cells have shown great clinical outcomes in hematological malignancies, there is still limited efficacy in solid tumors due to the lack of tumor-specific antigens, low T cell infiltration in the tumors, immune suppressive tumor microenvironment, and on-target off-tumor toxicity in healthy tissues. The antigen recognition module of CAR T cells is usually a single-chain variable fragment (scFv). In contrast, the variable domains of heavy-chain antibodies (VHH) are small, stable, single-domain antibody fragments with high affinity and do not require the supramolecular assembly. Severe fatalities recently halted clinical trials of PSMA CAR T cells, so affinity tuning may be required to avoid CAR T targeting normal tissues. Here, we developed VHH-based CAR T cells targeting PSMA, able to co-express human somatostatin receptor 2 (SSTR2) actioning as a PET reporter for tracking CAR T cell distribution, as an immunotherapeutic strategy for prostate cancer.
Methods: Primary human T cells were isolated and transduced with PSMA CAR lentivirus at 24 and 48 hours after activation with anti-CD3/CD28 Dynabeads. Mouse models of 8505C cell line transduced to overexpress PSMA were utilized to test the efficacy of CAR T cells. Tumor growth was monitored regularly by bioluminescence imaging. PET/CT imaging was performed to monitor T cell localization and biodistribution using ¹⁸F-NOTA-octreotide radiotracer targeting SSTR2 introduced in CAR T cells.
Results: By alanine substitution of CDR3 in the VHH, we identified several low affinity candidates. The affinity of the VHH-based PSMA CAR was determined to be 22 nM by a saturation binding assay using monomeric human PSMA protein. PSMA CAR expression in primary human T cells was higher than 90%. Compared to non-treated mice, PSMA targeting CAR T cells led to tumor reduction and improved survival in animal models. Moreover, high levels of T cell infiltration and localization in tumor tissues were found, based on PET/CT imaging. Considering possible treatment-associated toxicities, which led to 2 patient deaths and the halt of one phase 1 trial of PSMA CAR T cells, we will affinity tune VHH-based CAR to improve the safety profile. Thus, additional studies will be performed to test both activity and safety of affinity variants of PSMA CAR T cells.
Conclusions: We developed PSMA targeting CAR T cells able to induce potent and specific killing of prostate cancer cells. This strategy demonstrated significant therapeutic efficacy in vivo against animal models. Therefore, CAR T cells targeting PSMA may be a promising treatment strategy for patients with prostate cancer.
Citation Format: Juan Gonzalez-Valdivieso, Yanping Yang, Yogindra Vedvyas, Yago Alcaina, Moonsoo M. Jin. PSMA targeting CAR T cell immunotherapeutic strategy for prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5577.