Chemotherapy induced immunogenic cell death alters response to exogenous activation of STING pathway and PD-L1 immune checkpoint blockade in a syngeneic murine model of ovarian cancer.

Poor response to platinum/taxane-based chemotherapy has remained a major hurdle in the management of high grade serous carcinoma of the ovary (HGSC). Recurrent HGSC is often treated with liposomal doxorubicin as a second line chemotherapy. Unfortunately, HGSC patients have not benefited from immunotherapies targeting the PD-1/PD-L1 immune checkpoint axis. In a pre-clinical study evaluating the efficacy of a \"Stimulator of Interferon Genes\" (STING) agonist, we demonstrated the synergistic potential of STING pathway activation in enhancing response to carboplatin chemotherapy and sensitization to PD-1 immune checkpoint blockade (ICB). Since carboplatin and doxorubicin exhibit distinct immunogenic cell death (ICD) inducing potential, we investigated the chemotherapy specific effect in the magnitude of response to exogenous STING pathway activation. Immunocompetent C57/BL6 mice were implanted with ID8-Trp53-/- cells followed by treatment with carboplatin or doxorubicin. Towards rationalized addition of STING agonist with or without PD-L1 blockade, we first determined the expression of 60 known ICD associated genes at an early time point following the initial treatment with carboplatin or doxorubicin with or without STING agonist. Doxorubicin treated tumours showed significantly higher expression of ICD genes, Cxcl10, Cd274, Isg15, Psmb9 and Calr. Expression changes were further amplified following the addition of STING agonist. Significantly higher expression of Cxcl10 and Isg15 were observed in the doxorubicin + STING agonist treated mice compared to carboplatin + STING agonist combination. Interestingly, Ccl5 gene expression was higher in the tumours from carboplatin or carboplatin and STING agonist combination treated mice compared to those treated with doxorubicin. Plasma cytokine analysis showed distinct profiles of CXCL10, CCL5, MCP-1 and IL6 post treatment with each chemotherapy type. Doxorubicin monotherapy treated mice showed significantly longer overall survival compared to their carboplatin counterparts with further increases following addition of either STING agonist or PD-L1 ICB. However, despite the stronger ICD inducing ability of doxorubicin, overall survival of mice treated with carboplatin + STING agonist + PD-L1 ICB was the longest. Findings from our pre-clinical study provide novel insights for rationalized combinations of immune sensitizing agents such as STING pathway activators to improve response of HGSC patients to chemotherapy and ICB in the primary and recurrent settings.