Metastatic castration-resistant prostate cancer (mCRPC) is a significant cause of mortality in American men, with over 30,000 deaths annually. Immunotherapy approaches, including vaccines (GVAX, Prost-VAC) and immune checkpoint inhibitors (CTLA-4, PD-1), have shown limited success. Sipuleucel-T, while prolonging survival, represents a need for more effective mCRPC immunotherapies. The lack of response is multifaceted, involving strong immunosuppression in the tumor microenvironment that hinders T cell function. However, tumor-associated antigens like prostate stem cell antigen (PSCA) and prostate-specific membrane antigen (PSMA) offer potential targets for cellular immunotherapy. The success of CAR T cell therapies in hematological malignancies has spurred clinical development for mCRPC. PSCA is highly expressed in prostate cancer, increasing with disease progression, particularly in bone metastases. Preclinical studies demonstrated the safety and efficacy of second-generation PSCA-CAR T cells in eradicating bone metastatic prostate cancer. This first-in-human phase 1 trial aimed to evaluate the safety and bioactivity of PSCA-CAR T cells in patients with mCRPC.

Existing immunotherapeutic strategies for mCRPC, such as vaccine-based therapies (GVAX and Prost-VAC) and immune checkpoint inhibitors targeting CTLA-4 and PD-1, have yielded limited clinical benefits. While sipuleucel-T demonstrates improved survival, the need for more effective immunotherapies remains. The challenges in developing successful immunotherapies for prostate cancer include the strong immunosuppressive tumor microenvironment, which limits T cell infiltration and function. However, the identification of tumor-associated antigens such as PSCA and PSMA provides potential targets for cellular immunotherapies. The remarkable success of CAR T-cell therapies in hematological malignancies has encouraged exploration of their application in solid tumors, including mCRPC. Preclinical data supporting the use of PSCA as a target for CAR T-cell therapy have been established, demonstrating its efficacy in preclinical models of bone metastatic prostate cancer.

This single-center, first-in-human, phase 1 clinical trial (NCT03873805) evaluated the safety and bioactivity of PSCA-directed CAR T cells in patients with mCRPC. The primary endpoints were safety and dose-limiting toxicities (DLTs). Secondary endpoints included CAR T cell expansion and persistence, disease response (PSA decline and RECIST criteria), and survival. Exploratory endpoints encompassed immune cell subset analysis, serum cytokine profiles, and assessment of tumor immune microenvironment. Fifty-eight patients underwent PSCA expression screening via immunohistochemistry; 22 underwent leukapheresis, and 14 received CAR T cell infusions. Three dose levels (DLs) were tested: DL1 (100 million CAR T cells without lymphodepletion), DL2 (lymphodepletion + 100 million CAR T cells), and DL3 (reduced lymphodepletion + 100 million CAR T cells). The PSCA-CAR construct included an anti-PSCA humanized scFv, 4-1BB co-stimulatory domain, and CD3γ cytolytic domain. CAR T cells were manufactured using CD3/CD28 bead stimulation and lentiviral transduction. Treatment response was assessed by PSA levels, RECIST criteria, and imaging. CAR T cell kinetics were evaluated by flow cytometry and qPCR. Toxicity was monitored using Common Terminology Criteria for Adverse Events (CTCAE) v4.0. Additional exploratory analyses included CTC quantification, somatic DNA sequencing, and single-cell RNA sequencing (scRNA-seq) and TCR repertoire analysis in a subset of patients.

No DLTs were observed at DL1. At DL2, a DLT of grade 3 cystitis was encountered, leading to the addition of DL3 with a reduced lymphodepletion regimen. DL3 showed no DLTs. Grade 1 or 2 cytokine release syndrome occurred in 5 of 14 patients. PSA declines (>30%) were observed in 4 of 14 patients, with one patient maintaining a decline beyond 28 days. One patient in DL3 showed radiographic improvement in liver metastases. CAR T cell persistence was limited beyond 28 days post-infusion. Lymphodepletion enhanced CAR T cell expansion and serum cytokine levels, correlating with objective anti-cancer responses. Reduced lymphodepletion in DL3 reduced CRS-related and off-tumor toxicities. One patient in DL2 experienced a >90% PSA decline and radiographic improvement in soft tissue metastases. scRNA-seq revealed increased effector CD8+ T cell subsets in responding patients, with changes in TCR repertoire diversity. Immunofluorescence analysis showed increased T cell infiltration and decreased PD-L1 expression in responding patients. Genomic analysis revealed PTEN loss in three patients, one of whom had the greatest PSA decline.

This phase 1 trial provides initial clinical data supporting PSCA as a viable CAR T cell therapeutic target for mCRPC. The observed biochemical and radiographic responses, along with the modulation of the tumor immune microenvironment, are encouraging. The DLT of cystitis, likely an on-target/off-tumor effect exacerbated by lymphodepletion, was mitigated by dose reduction. The limited CAR T cell persistence highlights a key challenge in solid tumor immunotherapy that needs further investigation, potentially focusing on strategies to extend T cell longevity, such as modifications to the CAR construct, enhanced T cell fitness, or allogeneic cell therapy approaches. Further studies will focus on optimizing the lymphodepletion regimen and exploring combination therapies to enhance the therapeutic efficacy and duration of response.

This phase 1 trial demonstrates the feasibility and safety of PSCA-directed CAR T cell therapy in mCRPC patients, with evidence of anti-tumor activity. The limited persistence of CAR T cells underscores the need for future research to optimize strategies for improving CAR T cell expansion, persistence, and ultimately durable clinical responses. Further studies should focus on dose optimization, lymphodepletion regimen refinement, and combination approaches to enhance treatment efficacy and overcome limitations associated with transient CAR T cell persistence.

The small sample size limits the statistical power of the study and may not fully represent the broader patient population with mCRPC. The slow accrual rate, due to pre-screening requirements and pauses during toxicity assessments, may have introduced bias by excluding patients with more aggressive disease or limited performance status. The relatively short follow-up period restricts the ability to assess long-term efficacy and survival benefits. Finally, the absence of a validated PSCA assay might impact the selection of patients for the trial.