Background Prostate cancer is unresponsive to current immunotherapies such as checkpoint inhibitors. Overcoming this ‘cold’ environment requires rational combination of therapeutic modalities that can prime anti-tumor immunity. Radiation is a potential candidate as a cornerstone of prostate cancer therapy; however it remains unclear how radiation should be most productively combined with immunotherapies.

Methods We completed an investigator-initiated clinical trial of newly diagnosed men with oligometastatic, hormone-sensitive prostate cancer (mHSPC, NCT03007732). Patients were randomized to receive anti-PD-1 alone x 13 cycles (pembrolizumab, Arm 1, n=12), or anti-PD-1 with intraprostatic injections of the Toll-like receptor 9 (TLR9) agonist SD-101 (Arm 2, n=11). All patients received SBRT to the prostate tumor during cycle 1, and concomitant hormonal therapy with a GnRH agonist and abiraterone. Response was assessed as PSA < nadir + 2 ng/mL at 15 months after ceasing all cancer therapies. For unbiased correlative immune interrogation of these patients, fresh paired biopsies of the primary prostate tumor and PBMCs both before and after starting radiation and immunotherapy were analyzed by multi-omic single-cell genomics to assess changes in immune and non-immune populations with treatment, including T cell repertoire changes.

Results Thirteen of 20 evaluable patients responded by PSA criteria with 3 patients not yet reaching 15 months of post-treatment follow-up. Adverse events included expected IRAEs from pembrolizumab, and self-limited flu-like symptoms from SD-101, with no unexpected AEs. Single-cell analysis of paired tumor biopsies reveals treatment-induced decreases in cytotoxic T cell populations and increases in immunostimulatory LAMP3+ dendritic cells, particularly in patients treated with SD-101 (Arm 2). TLR9 agonism also specifically enhances interferon responses, persistence of pre-existing cytotoxic T cell clonotypes, and enhanced antigen presentation machinery including in tumor and epithelial cells. Shared clonotypes that circulate between tumor and blood are also primarily cytotoxic, with TLR9 agonist treatment favoring mucosal-associated invariant T (MAIT) phenotypes over GZMB+ T cells in these shared cells.

Conclusions The addition of TLR9 agonism with combination of radiation and PD-1 blockade, amplifies remodeling in the T cell and myeloid compartments in prostate tumors, which may guide future immunotherapy strategies.

Acknowledgements This work was supported by Merck, TriSalus Life Sciences, and the Prostate Cancer Foundation.

Trial Registration This randomized clinical trial is registered with Clinicaltrials.gov (NCT03007732).

Ethics Approval This study was approved and overseen by the UCSF Institutional Review Board.