Abstract 1708: Effective and reversible control of anti-tumor activity in vivo with a drug-regulated CAR T cell platform (DARIC)
Authors: Leung Wai-Hang , Certo Michael , Horton Holly , Gay Joel , VandenBerg Tracy , Jarjour Jordan , Astrakhan Alexander 
Published: July 2017

Abstract
Redirecting T cells against tumors by introducing antigen-specific chimeric antigen receptors (CAR) has shown promising clinical results as a potential treatment strategy for certain cancers. However, traditional CARs are constitutively active, resulting in the persistent loss of all target cells (including off -tumor, on-target activity against normal tissues that express the target antigen) and enhanced potential of excessive T cell activation to drive cytokine release syndrome. While “off switches” based on suicide cassettes or other depleting cell approaches are in development, such systems by definition result in the elimination of the therapeutic cells. Here we have developed a novel drug-regulated CAR-based antigen targeting approach termed Dimerizing Agent Regulated Immune-receptor Complex (DARIC) that aims to: i) minimize the long-term toxicity of CAR T treatment; ii) allow the targeting of previously inaccessible antigens; and iii) be amenable to multiplex antigen targeting. The DARIC platform separates the antigen recognition and signaling functions of a CAR into two distinct polypeptides that are further engineered to contain the FKPB12 and FRB small-molecule regulated dimerization domains. In the absence of the dimerizing drug (e.g. rapamycin or the non-immunosuppressive rapalog AP21967) the DARIC system lacks signaling activity. However, the addition of dimerizing agent drives the interaction of the two DARIC subunits, fully restoring CAR function. Using CD19 as a model system, we show that treatment of CD19-DARIC+ T cells with rapamycin or AP21967 results in equivalent cytotoxicity, cytokine production and proliferation compared to a standard CD19-targeting CAR. Importantly, CD19-DARIC T cells were activated by picomolar levels of rapamycin and exhibited a higher antigen sensitivity than standard CD19-CAR T cells in vitro. In an aggressive CD19+Nalm-6 xenograft tumor mouse model, CD19-DARIC T cells did not exhibit anti-tumor activity in the absence of dimerizing agent. However, CD19-DARIC treated mice that received either low-dose rapamycin or AP21967 showed an equivalent level of tumor control compared to standard CD19-CAR treated animals. This activity was dependent on the presence of the dimerizing drug, as cessation of drug treatment resulted in the loss of CD19-DARIC T cell activity and the expansion of Nalm-6 tumors cells in the DARIC T cell treated mice, consistent with the ability to switch off CD19-DARIC T cells in vivo by withdrawing drug. Taken together, these results highlight the potential of the DARIC platform to facilitate the regulation of CAR T cell function both in vitro and in vivo.
Citation Format: Wai-Hang Leung, Michael Certo, Holly Horton, Joel Gay, Tracy VandenBerg, Jordan Jarjour, Alexander Astrakhan. Effective and reversible control of anti-tumor activity in vivo with a drug-regulated CAR T cell platform (DARIC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1708. doi:10.1158/1538-7445.AM2017-1708