Beta-alethine (BA) is a small molecule drug that has demonstrated impressive anti-tumor immune responses against melanoma, particularly when combine with low dose anti-PD1. Here we report on the dose of BA that optimally enhances anti-PD1, the efficacy of BA combined with various immune checkpoint inhibitors (ICIs) in a murine bladder cancer and we propose a potential mechanism of action of BA to explain its synergistic effect. We tested 3 different doses of BA that were logs apart (30 mg/kg, 30 ug/kg, and 600 ng/kg) either alone or in combination with low dose (10 ug per mouse for a total of 3 doses 3 days apart) anti-PD1 in the DBA mouse Cloudman melanoma mouse model. The lowest dose of BA (600 ng/kg) in combination with anti-PD1 resulted in cures in 33% of the mice when treatment was initiated when tumors were of substantial size (average size of 30 mm2). Previous work starting with smaller tumor sizes (average 20 mm2) resulted in an 80% cure rate when a low dose of BA was used in combination with low dose anti-PD1. Analysis of blood from treated animals demonstrated that the combination led to decreased expression of multiple checkpoint molecules on both T cells and myeloid cells and to a decrease in tumor promoting inflammatory macrophages. We further demonstrated efficacy of combining BA with other ICI therapies in a s.c. MBT2 murine bladder cancer study in C3H mice. In these studies, we saw that BA was synergistic with a-PD1 at all doses tested (30 mg/kg, 30 ug/kg and 600 ng/kg weekly s.c.). The effect was seen even when only 4 weekly doses of BA were given. Interestingly, tumor growth increased when BA was discontinued, a phenomenon observed in previous compassionate use veterinary studies. Synergy with OX40 and CTLA4, were also seen, but further studies of dose and schedule are required for optimization. Our results demonstrate that a low dose of BA is optimal to enhance the efficacy of anti-PD1 and that this observed combinatorial effect is conserved in at least two different tumor types in different mouse strains. Based on analysis of T and myeloid cells in treated mice, we hypothesize that BA works via inducing pleiotropic changes that result in downregulation of multiple immune checkpoint molecules on T cells and on myeloid cells making it less feasible for these cells to use other pathways of immune suppression as compared to single agent anti-PD1 therapy and thus increase potential for cures.

Citation Format: Floyd Taub, Suzin Wright, Amanda Guth. Enhancement of checkpoint inhibitor therapy by beta-alethine [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-145.