Abstract 4569: The CDK4/6 inhibitor abemaciclib synergizes with PD-L1 blockade to induce an immune inflamed tumor microenvironment through T cell and tumor cell intrinsic effects

The approval of abemaciclib and additional cyclin dependent kinases 4 & 6 (CDK4/6) inhibitors for the treatment of HR+ breast cancer has provided new therapeutic options to patients. As CDK4/6 inhibitors become part of the standard of care, combination strategies leveraging abemaciclib together with immunotherapy may represent an opportunity to extend benefit to more patients and additional cancers. Accordingly, it is important to understand if and how a cell cycle inhibitor can be combined with immunotherapy. To investigate the immune combinatorial potential of abemaciclib, we studied the effects of treatment alone and in combination with PD-L1 blockade in immunocompetent murine syngeneic tumor models, and directly evaluated the tumor cell and immune cell intrinsic immunologic effect of abemaciclib in vitro. In vivo abemaciclib treatment of murine tumors (CT26, EMT6 and MC38) caused a dose-dependent delay in tumor growth and demonstrated the potential to induce complete tumor regression (CR ~10%). Combination with an anti-PD-L1 antibody after abemaciclib pretreatment greatly enhanced the anti-tumor response compared to abemaciclib and anti-PD-L1 monotherapies. Optimal combination therapy resulted in 50-60% CRs of mice in a setting where anti-PD-L1 monotherapy showed little or no efficacy (0% CRs). Mice maintaining CRs after cessation of combination therapy or abemaciclib monotherapy resisted later CT26 rechallenge, demonstrating the ability to generate immunological memory during abemaciclib therapy. Analysis of intra-tumor gene expression showed that abemaciclib monotherapy induced T cell activation and inflammation signatures. Combination therapy substantially enhanced this effect and was additionally associated with DC maturation, antigen presentation, cytokine signaling and helper T cell phenotype. Suppression of cell cycle genes indicative of inhibition of CDK4/6 was also more prominent during the combination therapy. In Jurkat and primary human T cells, treatment with abemaciclib in vitro resulted in a dose-dependent increase in NFAT activity upon TCR stimulation. This correlated with upregulation of both cell surface markers and genes associated with an enhanced T cell activation phenotype, while only modestly affecting T cell expansion. Abemaciclib also amplified expression of antigen presentation and other immune-related genes in human breast cancer cells. Although it was uncertain if agents that inhibit cell proliferation could be combined with immunotherapy, these preclinical results provide a strong rationale for combining abemaciclib with checkpoint immunotherapy to improve the anti-tumor efficacy. The T cell and tumor cell intrinsic effects, synergistic anti-tumor responses and intra-tumor immune activation, justify clinical investigation of this combination. Citation Format: David Schaer, Richard Beckmann, Jack Dempsey, Lysiane Huber, Amelie Forest, Nelusha Amaladas, Ying Cindy Wang, Erik Rasmussen, Darin Chin, Yanxia Li, Andrew Capen, Marianne Deroose, Carmine Carpenito, Xueqian Gong, Kirk Staschke, Linda Chung, Farhana Merzoug, Trent Stewart, Lacey Litchfield, Philip Iversen, Sean Buchanan, Alfonso de Dios, Ruslan Novosiadly, Michael Kalos. The CDK4/6 inhibitor abemaciclib synergizes with PD-L1 blockade to induce an immune inflamed tumor microenvironment through T cell and tumor cell intrinsic effects [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 4569.