Immunological and genomic correlates of response to anti-PD1 checkpoint therapy in mismatch proficient and deficient patients with metastasized castration resistant prostate cancer.

Abstract

248 Background: Response to anti-PD1/PDL1 checkpoint therapy has been witnessed in only a minority of patients with castration-resistant prostate cancer (CRPC). Microsatellite instability (MSI) is the only biomarker predictive of response; additional immune and genomic correlates are needed to improve the proportion of patients that may benefit from checkpoint immunotherapy. Methods: CRPC patients were treated with anti-PD1 checkpoint immunotherapy in two basket-studies, depending on PDL1-positivity ( > 1%) or presence of MSI, and consented to prospective immune-oncology biomarker study. Genomic and immune correlates of response were studied in both MSI as microsatellite stable CRPC patients, and included PDL1 expression , whole genome sequencing on a baseline fresh metastatic biopsy, tumor microenvironment (immuno)profiling using multi-color immunohistochemistry, computational prediction of neoantigens, T-cell receptor sequencing and MHC-tetramer staining. Response was evaluated according to PCWG3 criteria and treatment was continued until radiological progression with lack of clinical benefit or toxicity. Results: At present 11 CRPC patients were included in the program and treated with nivolumab (n = 4) and pembrolizumab (n = 7). Treatment is currently ongoing in 7/11 (64%) of patients, with treatment discontinuation due to immune-related colitis (n = 2) and disease progression (n = 2). Biochemical PSA ( > 50%) responses were seen in both MSI and PDL1 positive patients. Median progression-free survival has not yet been reached. Translational studies on genomic and immune correlates of response will be presented in detail. Conclusions: Encouraging responses to anti-PD1 checkpoint immunotherapy were seen in CRPC patients selected for PDL1-positivity and MSI. An integrative biomarker suite will likely be needed to predict response to anti-PD1 therapy in CRPC.