Background: Prostate stem cell antigen (PSCA) is expressed in >80% of metastatic prostate cancers. BPX-601 is an autologous PSCA-directed chimeric antigen receptor (CAR)-T cell immunotherapy engineered to express a rimiducid-inducible MyD88/CD40 costimulation switch to enhance T cell potency and persistence. Safety and activity of BPX-601 with rimiducid in mCRPC and pancreatic cancer is being assessed in an ongoing, phase 1/2 clinical trial (NCT02744287). Results from the first two prostate cancer cohorts are reported. Methods: Eligible mCRPC patients (pts) had progressed on ≥ 2 prior therapies including an androgen receptor antagonist and taxane. Using a 3+3 design, pts received lymphodepleting chemotherapy followed by a single-dose of 5 x 10⁶ BPX-601 cells/kg and single or weekly doses of 0.4 mg/kg rimiducid infused over 2 hours beginning 7 days following cell infusion. Primary objective of phase 1 is to determine safety, tolerability and MTD or RP2D. Secondary objectives include characterization of clinical efficacy, PK of rimiducid and long-term safety. Biomarkers indicative of GoCAR-T cell expansion in blood, immune cell activity, and infiltration to tumor are being monitored. Results: As of Sept 2022, 8 pts received BPX-601 5x10⁶ cells/kg; 3 and 5 pts received a single or weekly (range: 1-30) doses of rimiducid. Most common ≥ grade 3 adverse events were myelosuppression, attributed to lymphodepletion. All patients developed cytokine release syndrome (6 G1, 2 G3). Immune-effector cell associated neurotoxicity syndrome occurred and resolved in 2 pts (1 G1, 1 G4). One pt experienced a DLT of neutropenic sepsis (G5) with possible hemophagocytic lymphohistiocytosis (eg, IL-18, ferritin, M-CSF, fractalkine, MIP-1β and IL-1RA levels were elevated). Of 7 evaluable pts, PSA50 response was observed in 3 pts at Day 28. Preliminary RECIST-based results demonstrated partial response in 1, stable disease in 3, 1 progressive disease; at data cut off 2 had not reached imaging timepoint. One patient continues on study with SD after >9 months, with persistent evidence of rimiducid responsiveness. Peripheral blood BPX-601 cells expanded to an average of 3466 vector copies / μg DNA +/- 3109 during the first week with continued re-expansion to an average of 30234 copies/ug DNA +/- 67031) following rimiducid treatment. Serum IFN-γ, GM-CSF and IL-6 rapidly increased over 24 hours following rimiducid treatment (mean IFN-γ increase 26.9-fold ± 14.2) and subsequently diminished over 2 days. Conclusions: BPX-601, a PSCA-directed GoCAR-T cell product, has preliminary evidence of biologic activity with toxicity characteristic of previously reported CAR-T studies. Markers of rimiducid-induced GoCAR-T cell activation and proliferation were observed. Exploration of escalating weekly rimiducid doses > 0.4 mg/kg and BPX-601 cell doses is planned. Clinical trial information: NCT02744287.