Early chemo-induced TME alterations as a predictor of response to PD-1 blockade in AVPC: Single-cell data from C3NIRA.

5065 Background: Aggressive variant prostate cancer (AVPC) criteria enrich for androgen-indifferent disease that is highly aggressive and has limited therapeutic options. Single-agent immune checkpoint inhibitors have not demonstrated activity in prostate cancer patients. The C-COLA study (NCT03263650) showed that Cabazitaxel/Carboplatin (CabCarb) elicited inflammatory signaling in responsive patients with AVPC. Based on these findings, we hypothesized that chemotherapy-induced inflammation in the tumor microenvironment (TME) may prime tumors for immunotherapeutic approaches. Methods: The C3NIRA trial (NCT04592237) employed one cycle of CabCarb, followed by the addition of the PD-1 blocker Cetrelimab, from cycle 2 for up to 6 cycles of induction treatment in patients with AVPC. Among 120 enrolled men, 20 discontinued due to toxicities and 40 due to progression during induction. Sixty patients completed induction and were subsequently randomized to Niraparib maintenance with or without Cetrelimab. The 30 randomized to Niraparib with Cetrelimab had significantly longer progression free (PFS) and overall survivals than the 30 randomized to Niraparib alone. To investigate whether treatment-induced alterations in the TME could predict immunotherapy response, biopsies were obtained at baseline, and after cycles 1, 3 and 9 for single-cell RNA sequencing (scRNAseq). Results: scRNAseq analysis of tumor biopsies showed a high proportion of GZMK⁺ CD8⁺ T cells and CD8⁺ T effector memory clusters, and a low proportion in FOXP3 + CD4 + T regulatory cells, in the TME of responders (PFS≥10months) after cycle 1 of CabCarb, which was not seen in progressors. Similarly, SPP1⁺ macrophages scoring high immunosuppressive signatures were reduced in responders after CabCarb but not in progressors. Consistent with these suppressive and effector cell changes, LAG3 + TIGIT + CD8⁺ T exhausted cells gradually accumulated in progressors from baseline through CabCarb and Cetrelimab treatment, while remaining unchanged in responders over the same interval. Conclusions: One cycle of CabCarb induction altered the TME in a subset of AVPC tumors, with reduction of suppressive myeloid and T cell populations and expansion of anti-tumor effector cells associated with clinical responses. These early TME changes may predict benefit from the addition of PD-1 blockade and help refine patient selection for immune checkpoint inhibition. Ongoing analyses are identifying additional therapeutic vulnerabilities in the TME that can be targeted to further improve outcomes in men with AVPC. Clinical trial information: NCT04592237 .