Abstract 2113: Caveolae-mediated endocytosis as a novel mechanism of resistance to T-DM1 ADC
Authors: Sung Matthew S. , Tan Xingzhi , Hosselet Christine , Cinque Michael , Upeslacis Erik , Golas Jonathon , Wang Fang , Lu Bingwen , Tylaska Laurie , King Lindsay , Myers Jeremy , Rosfjord Edward , Lucas Judy , Gerber Hans-Peter , Loganzo Frank 
Published: July 2016

Abstract
Trastuzumab emtansine (T-DM1) is an antibody-drug conjugate (ADC) comprised of the HER2-targeting antibody trastuzumab and DM1, a microtubule depolymerizing agent covalently attached via a non-cleavable thioether linker. T-DM1 has demonstrated clinical benefit for patients with metastatic breast cancer, however activity may be limited by inherent or acquired resistance during prolonged treatment periods. The molecular mechanisms that drive clinical resistance to T-DM1, especially in HER2 positive tumors, are not well understood. We used the HER2+ cell line N87 to develop a model of T-DM1 resistance utilizing a cyclical dosing schema in which cells received T-DM1 in an “on-off” routine until a T-DM1 resistant population of N87 cells (N87-TM) was generated. N87-TM cells displayed 103-fold resistance toward T-DM1 treatment compared to the parental N87 cells. The N87-TM cells were cross-resistant to a panel of trastuzumab-ADCs (T-ADCs) with non-cleavable-linked auristatins. N87-TM cells do not have a decrease in HER2 protein levels or an increase in drug efflux pump (e.g. MDR1) protein expression compared to parental N87 cells. Comparative proteomic profiling suggested an enrichment in proteins (e.g. caveolin-1, CAV1) that mediate caveolae formation and endocytosis in the N87-TM cells. Indeed, N87-TM cells internalize ADCs into intracellular CAV1+ puncta and alter their trafficking to the lysosome compared to N87 cells. Intriguingly, T-ADCs utilizing auristatin payloads attached via an enzymatically cleavable linker (i.e. ValCit linker) overcome T-DM1 resistance in N87-TM cells. Importantly, N87-TM cells implanted into athymic mice in vivo formed T-DM1 refractory tumors which remain sensitive to T-ADCs with cleavable-linked auristatin payloads. When comparing antigen positive patient-derived xenograft models that were refractory to T-DM1 yet responded to T-ADCs with ValCit linker-payloads, CAV1 was found to be a predictive protein biomarker identifying T-DM1 refractory tumors. These data implicate caveolae-mediated endocytosis in ADC biology and suggest that alterations in this pathway may impact a tumor's response profile to ADCs with non-cleavable linkers. We also propose CAV1 as a novel protein biomarker whose high tumoral expression predicts a refractory response to the T-DM1 ADC.
Citation Format: Matthew S. Sung, Xingzhi Tan, Christine Hosselet, Michael Cinque, Erik Upeslacis, Jonathon Golas, Fang Wang, Bingwen Lu, Laurie Tylaska, Lindsay King, Jeremy Myers, Edward Rosfjord, Judy Lucas, Hans-Peter Gerber, Frank Loganzo. Caveolae-mediated endocytosis as a novel mechanism of resistance to T-DM1 ADC. [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 2113.