Abstract A138: Loss of the lysosomal transporter SLC46A3 is a mechanism for innate and acquired resistance to non-cleavable maytansine and pyrrolobenzodiazepine-based antibody-drug conjugates

Abstract

Abstract Antibody-drug conjugates (ADCs) combine a monoclonal antibody with a potent cytotoxic drug to preferentially eliminate antigen-positive cells for the treatment of cancer. ADCs bearing non-cleavable warheads such as the pyrrolobenzodiazepine (PBD) SG3376 rely upon lysosomal degradation of the antibody for release of the cytotoxic warhead. Here we sought to identify the mechanism of lysosomal membrane transport following release of these membrane-insoluble warheads. We used a multiplexed RT-PCR screen to compare the expression of 43 lysosomal membrane-specific genes in cell lines with inherent sensitivity or resistance to various SG3376-based ADCs. We discovered that only the expression of SLC46A3 correlated with sensitivity to the ADCs. Functional validation through gene knockout or forced gene expression in vitro revealed that loss or gain of SLC46A3 altered the potency of SG3376- and DM1-ADCs but had no effect on ADCs carrying the non-cleavable warhead MMAF or the cleavable PBD payload SG3249. To explore a role in acquired resistance, we interrogated trastuzumab-DM1 (T-DM1) sensitive and resistant cell line pairs and found that resistant lines had lost expression of SLC46A3. Sensitivity to both SG3376- and DM1-ADCs was restored upon reintroduction of SLC46A3. Furthermore, we examined a patient-derived xenograft (PDX) breast cancer model of acquired T-DM1 resistance and found that expression of SLC46A3 was decreased by 92% compared to the parental, T-DM1 sensitive model. Taken together, our results establish SLC46A3 as a transporter of both SG3376 and DM1 catabolites from the lysosome, and suggest that SLC46A3 may be a predictive response marker for ADCs bearing these non-cleavable warheads. Citation Format: Krista Kinneer, John Meekin, Sandrina Phipps, Binyam Bezabeh, Ryan Fleming, Christine Kiefer, Marlon Rebelatto, Nazzareno Dimasi, Michael J. Wick, Alyssa Moriarty, Megan Groves, Ronald Herbst, Arnaud Tiberghien, Luke Masterson, Philip Howard, David Tice. Loss of the lysosomal transporter SLC46A3 is a mechanism for innate and acquired resistance to non-cleavable maytansine and pyrrolobenzodiazepine-based antibody-drug conjugates [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr A138.