Abstract B034: Preclinical immune antitumor activity of myeloid-targeting E7046 and Treg depleting E7777

Purpose: To assess the combined activity of E7046 and E7777 against multiple murine syngeneic tumor models. Background: Both regulatory T cells (Tregs)and myeloid cells are significant constituents in the tumor microenvironment in many types of cancer, where they help to maintain an immunosuppressive milieu that inhibits cytotoxic T cell function, which favors tumor persistence. Prostaglandin E2 (PGE2) engages the EP4 receptor on monocytes to direct myeloid cell activities away from antigen presentation and toward immunosuppression (wound healing). PGE2-driven myeloid responses have been reported to enhance Treg immunosuppressive activity, while Tregs with high expression of the FoxP3 transcription factor have been reported to up-regulate cyclooxygenase expression and PGE2 secretion. E7046 is a highly selective small molecule that potently competes with PGE2 binding to EP4. Single agent E7046 slows the growth of multiple syngeneic murine tumor types, in a mechanism dependent upon the presence of T cells. Denileukin diftitox (ONTAK®), an IL-2/diphtheria toxin fusion protein, was originally approved by the US FDA for treating patients with relapsed/refractory cutaneous T cell lymphoma. E7777 is a new version of denileukin diftitox developed using an improved manufacturing process. We and others have shown that denileukin diftitox selectively depletes Tregs. Neither E7046 nor E7777 affects tumor cell viability in vitro. Here we assessed their combined activity against syngeneic tumor models and the mechanisms driving their activity. Methods: Tumors were implanted subcutaneously in groups of 5-10 female BALB/c mice. When tumors were > 50 mm3, mice were treated with E7046 (150 mg/kg, QD x 21, p.o.) and/or E7777 (2.5 or 3 μg/head, Q7D x 2 or 3, i.v). Tumors, spleens, and tumor-draining lymph nodes were excised and analyzed by flow cytometry for immune cell composition and function. Results: As single agents, both E7046 and E7777 delayed the growth of established tumors. When combined, their anti-tumor activities were significantly enhanced, with up to 20% of animals rendered stably tumor-free. Activity was significantly reduced if animals were treated with antibody to deplete CD8+ T cells. In the tumors, the combination treatment dramatically increased the ratio of CD8 T cells to CD4+CD25+Foxp3+ Tregs, and also decreased the frequency of immunosuppressive myeloid cells. Importantly, the CD8+ T cells in the treated tumors, but not in the control tumors, were foun