Combination of a Novel EP4 Antagonist E7046 and Radiation Therapy Promotes Anti-tumor Immune Response and Tumor Rejection in Preclinical Tumor Models

We have recently developed a novel and specific EP4 antagonist, E7046, which possesses significant anti-tumor growth activity in multiple preclinical tumor models through modulating myeloid cells, including tumor associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs) (AACR 2015, abstract #275). E7046 is being tested in a phase I trial as a single agent (NCT02540291). The objective of this study was to evaluate the anti-tumor activity of E7046 combined with RT, and to characterize their immunomodulatory effects in preclinical tumor models. Mouse syngeneic tumor models CT-26 and 4T1 were used for pharmacological and mechanistic investigation. GMP grade E7046 was administered to tumor-bearing animals by an oral gavage. RT was given locally to tumor regions using a small animal irradiator. Secondary tumor challenges were conducted by inoculating tumor cells the contralateral to the original tumor site of the animals without further drug treatment. At selected time points, tumors and lymphoid organs were excised for immunophenotyping using flow cytometry. Splenocytic myeloid cells were isolated from tumor-bearing animals that had received either treatments or vehicles, and assayed for their ability to suppress T cell proliferation. In the CT-26 tumor model, treatment of oral E7046 plus a single 9 Gy of RT rendered 9 of 12 animals tumor free, which was significantly better than that of RT alone or E7046 alone. None of the tumor free animals regrew tumors over a subsequent 2 month period that followed the cessation of treatment, and all the nine mice completely rejected tumor rechallenge still without further drug treatment. These results indicated the existence of an effective anti-tumor memory response in those 9 animals cured by the E7046/RT combination. In the 4T1 model, the combination of E7046 and RT also produced significant better tumor growth inhibition activity compared with each treatment alone. Notably, the combination significantly improved survival by inhibiting the subsequent spontaneous lung metastasis of 4T1 tumors, compared with either singe treatment alone, indicating a metastasis controlling benefit of the combination. Combination of E7046 and RT markedly increased infiltration of cytotoxic T cells into the tumors and reduced the immunosuppressive activity of circulating MDSC as measured by the T cell proliferation assay. We have shown here a synergistic or super-additive anti-tumor activity of E7046, an orally available EP4 specific antagonist, and local RT in tumor control and/or tumor rejection through an immune-based mechanism in preclinical tumor models. The findings warrant further investigation of E7046 in combination with RT in clinic.