Abstract 3247: Combination therapy using PERK and PD1/PD-L1 inhibitors reprograms tumor associated macrophages and reduces tumor burden

Background: The role of endoplasmic reticulum (ER) stress is profound in increasing malignant activity of neoplastic tumor cells and has been recently demonstrated to also increase immunosuppressive activity of tumor-associated macrophages (TAMs). Immunosuppressive TAM activity is thought to underly resistance to PD-1/PD-L1-targeted immune checkpoint inhibitors used for cancer therapy. Protein kinase RNA-like endoplasmic reticulum kinase (PERK) is a rate-limiting enzyme central to the ER-stress response mechanism that transduces ER-stress signals in cells to regulate protein synthesis, cell death, and cell survival. We hypothesized that inhibition of PERK would improve responses to ICI therapy by reprogramming TAM from immunosuppressive to immunoactivating cells. Methods: To investigate if ER-stress underlies resistance to PD-1/PD-L1 targeted therapies, we utilized ex vivo assays to investigate ER-stress regulation of macrophage phenotype, and in vivo syngeneic murine models of melanoma growth, with HC-5404 (PERKi), a selective and potent first-in-human small molecule PERK inhibitor currently in a phase 1 clinical trial for solid tumors (NCT04834778). Results: Treatment with PERKi sensitized αPD-1/PD-L1 mAb-resistant melanoma tumors (Y1.7/YR1.7) to PD-1/PD-L1 blockade with concomitant increase in tumor infiltrating leukocytes, TH1- reprogrammed TAM, and cytotoxic CD8+ T cells. In addition to tumor microenvironment changes by PERKi, gene expression analysis confirmed reprograming of the tumor microenvironment, marked by a relative increase in TH1-associated genes and downregulation of TH2-associated genes when compared with untreated tumors. We utilized an ex vivo macrophage-splenocyte co-culture assay to depict macrophage dependent T cell suppression prevalent in tumors. This revealed that treating macrophages with PERKi prior to co-culturing with T cells or adding PERKi to the macrophage-T cell co-culture relieved macrophage dependent T cell suppression, illustrating a potential mechanism of therapeutic efficacy. We are now studying how ER-stress is regulated in TAMs and are focusing on identifying components of PERK-dependent cell signaling that increases the tumor promoting behavior of TAMs. Conclusions: The combination therapy targeting PERK and PD-1/PD-L1 signaling increased adaptive immune responses, reprogramed TAMs and reduced tumor growth kinetics. Results from these studies highlight a role for ER-stress signaling in TAMs to maintain an immunosup