Abstract 2960: Potentin vivoactivity of site-specific indolino-benzodiazepine antibody-drug conjugates (ADCs) generated via engineered cysteine conjugation

ADCs are widely studied for cancer therapy, with numerous agents in preclinical and clinical development embodying a wide array of targets, linker chemistries, and cytotoxic effector classes. A fourth element of ADC design that has received much attention recently is the site of conjugation of the cytotoxic molecule to the antibody. Historically, lysine- or interchain cysteine-directed conjugation has been used, but site-specific chemistries have become increasingly popular. Our previous evaluation of site-specific and lysine-linked ADCs utilizing a tubulin-acting maytansinoid effector molecule found the lysine-linked version was more active in vivo (Yoder et al., AACR 2015 #645). Here we present a comparison of engineered cysteine site-specific and lysine-linked ADCs utilizing the previously described indolino-benzodiazepine (henceforth referred to as IGN) effector IGN-P1 (Miller et al., AACR 2015 #652) which is designed to undergo proteolytic cleavage upon cell uptake to release a potently cytotoxic DNA alkylator. We show that HC-S442C mutants of human IgG1 can be conjugated via maleimide chemistry to IGN-P1 to give stable, potent, and homogeneous ADCs with drug to antibody ratio (DAR) of 2. The in vitro potency of engineered-cysteine IGN-P1 ADCs is largely dependent on the DAR of the ADC, although some difference is observed between HC-S442C and other cysteine mutants used for conjugation. Pharmacokinetic study of C442 maleimide conjugates suggests that the chemical linkage between effector and antibody is stable upon administration in mice. Further, and in contrast to our previous observations utilizing maytansinoid ADCs, the site-specific and Lys-linked IGN-P1 ADCs showed comparable efficacy in vivo on a molar drug basis. This effect was observed across two different antibodies targeting two different cell surface antigens. These results suggest that, in certain cases, site-specific conjugation chemistry can offer comparable activity to heterogeneous conjugation at well-tolerated doses. Citation Format: Nicholas C. Yoder, Chen Bai, Alan Wilhelm, Erin K. Maloney, Olga Ab, Emily E. Reid, Manami Shizuka, Daniel Tavares, Rassol Laleau, Xiuxia Sun, Megan E. Bogalhas, Lintao Wang, Jan Pinkas, Michael L. Miller, Ravi Chari, Thomas A. Keating. Potent in vivo activity of site-specific indolino-benzodiazepine antibody-drug conjugates (ADCs) generated via engineered cysteine conjugation. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2960.