Abstract 628: Pan-Bcl-2 inhibitor, GX15-070 (Obatoclax), decreases human T regulatory lymphocytes while preserving effector T Lymphocytes: A rationale for its use in combination immunotherapy

Abstract

Abstract Bcl-2 inhibitors are currently being evaluated in clinical studies for treatment of patients with solid tumors and hematopoietic malignancies. In this study we explored the potential for combining the pan-Bcl-2 inhibitor GX15-070 (GX15; obatoclax) with immunotherapeutic modalities. We evaluated the in vitro effects of GX15 on human T-cell subsets obtained from PBMCs in terms of activation, memory, and suppressive function. Our results indicated that in healthy-donor PBMCs, mature-activated T cells were more resistant to GX15 than early-activated T cells, and that GX15 preserved memory but not non-memory T-cell populations. Furthermore, GX15 increased the apoptosis of regulatory T cells (Tregs), profoundly down-regulated FOXP3 and CTLA-4 in a dose-dependent manner, and decreased their suppressive function. Treating PBMCs obtained from ovarian cancer patients with GX15 also resulted in increased CD8+:Treg and CD4+:Treg ratios. These results support preclinical studies in which mice vaccinated before treatment with GX15 showed the greatest reduction in metastatic lung tumors as a result of increased apoptotic resistance of mature CD8+ T cells and decreased Treg function brought about by GX15. Taken together, these findings suggest that when a Bcl-2 inhibitor is combined with active immunotherapy in humans, such as the use of a vaccine or immune checkpoint inhibitor, immunotherapy should precede administration of the Bcl-2 inhibitor to allow T cells to become mature, and thus resistant to the cytotoxic effects of the Bcl-2 inhibitor. Citation Format: Peter S. Kim, Caroline Jochems, Italia Grenga, Renee N. Donahue, Kwong Y. Tsang, James L. Gulley, Jeffrey Schlom, Benedetto Farsaci. Pan-Bcl-2 inhibitor, GX15-070 (Obatoclax), decreases human T regulatory lymphocytes while preserving effector T Lymphocytes: A rationale for its use in combination immunotherapy. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 628. doi:10.1158/1538-7445.AM2014-628