Phase I Trial of P-PSMA-101 CAR T Cells in Patients with Metastatic Castration-Resistant Prostate Cancer

PURPOSE: Chimeric antigen receptor (CAR)-T cell therapies have potential in solid tumors. A higher proportion of stem cell-like memory T cells (TSCM) in CAR-T products could enhance engraftment, persistence, and prolong immune activity. This phase 1 trial (NCT04249947) evaluated the safety and efficacy of P-PSMA-101, an autologous TSCM-rich bone tropic CAR-T therapy targeting prostate-specific membrane antigen (PSMA), in metastatic castrate-resistant prostate carcinoma (mCRPC) patients. Secondary endpoints included objective response rate, PSA response, radiographic progression-free survival (PFS). PATIENTS AND METHODS: P-PSMA-101 was produced from leukapheresis using the piggyBac® DNA transposon-based platform, which integrates a multi-cistronic transgene encoding an iCasp9 safety switch in addition to the CAR, generating TSCM-rich CAR-T cells. RESULTS: Among 33 treated patients, 18% (n=6) had dose-limiting toxicities (DLTs). Cytokine release syndrome (CRS) occurred in 61% (n=20), with Grade ≥ 3 CRS in 9% (n=3). Activation of the iCasp9-based safety switch was required in 24% (n=8) of cases including one fatal toxicity, and successful resolution in the other seven. P-PSMA-101 yielded ≥50% PSA decline in 21% (n=7) of patients. Among 13 RECIST evaluable patients, one partial response was observed. Stable disease occurred in 61% (n=20), with 21% (n=7) maintaining stability for ≥3 months. Two patients' remissions exceeded 12 months characterized by PSA declines > 90%, corroborated by pharmacokinetic, biomarker, and PSMA-PET imaging data. CONCLUSIONS: Robust expansion of P-PSMA-101 CAR T cells resulted in toxicity but also durable responses in patients with mCRPC. Future trials of CAR T may be informed by the results with this nonviral engineering, TSCM cell-enriched approach.