A C-Kit Targeting Antibody-Drug Conjugate Is Efficiently Metabolized And Activated Inside Cancer Cell Lines And Xenograft Tumors

Abstract We have developed an antibody-drug conjugate (ADC) targeting the c-Kit receptor tyrosine kinase, which is selectively expressed on the cell surface of various tumors including gastrointestinal tumors (GIST), small cell lung cancer (SCLC) and acute myelogenous leukemia (AML). The ADC consists of a humanized c-Kit-binding monoclonal antibody (c-Kit-Ab) covalently attached to a maytansinoid tubulin inhibitor, DM1, via a non-cleavable linker, SMCC. Like other ADCs, the c-Kit ADC is a pro-drug that is activated upon metabolism inside cells. Its target-mediated internalization and lysosomal degradation generates active maytansinoid metabolites that bind and inhibit microtubule dynamics. To understand the activation of the ADC, we studied the kinetics and efficiency of its metabolism in cancer cells and xenograft tumor models. Using radiolabeled ADC and c-Kit-Ab, we followed the formation of metabolites in SCLC and AML cells and in SCLC tumors, as well as in engineered cell lines that express various mutant forms of c-Kit. We found that the ADC and c-Kit-Ab are efficiently metabolized in c-Kit-positive cancer cells irrespective of the mutational status or the activation state of c-Kit. The degradation half-life of c-Kit-Ab is 3-fold lower than that observed for T-DM1 and SGN35, indicating a faster rate of activation. Moreover, cell surface repopulation with c-Kit is highly efficient after ADC-mediated internalization, allowing multiple rounds of ADC recruitment and activation. Interestingly, cell surface repopulation of c-Kit is more rapid for cells with a c-Kit mutation than those with wild-type c-Kit, leading to generation of greater levels of active metabolites in cells with mutant c-Kit. This may lead to better targeting of the c-Kit-mutated cancer cells over normal cells with wild-type c-Kit, which may improve the safety of the c-Kit ADC. Consistent with the in vitro findings, the ADC was found to undergo efficient metabolism in tumors in vivo. While tumor localization of the ADC over a non-targeting conjugate is similar to that observed with T-DM1 and SAR3419, its tumor-specific formation of active metabolites is 3-fold greater. Thus, the c-Kit-targeting ADC is particularly efficient at achieving high levels of active metabolites inside c-Kit-positive tumor cells, consistent with its robust efficacy in mouse xenograft models. Citation Format: Erica Hong, Qifeng Qiu, Rui Wu, Alan Wilhelm, Kathleen Whiteman, Jan Pinkas, Hans Erickson, Tinya Abrams, Siew Schleyer. A c-Kit targeting antibody-drug conjugate is efficiently metabolized and activated inside cancer cell lines and xenograft tumors. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5325. doi:10.1158/1538-7445.AM2015-5325