Abstract 868: The pan-ErbB inhibitor dacomitinib but not the dual EGFR/ErbB2 inhibitor labatinib disrupts membrane localization of the EGFR family of receptor tyrosine kinases
Authors: Jathak Maitreyee K. , Steele Thomas M. , Siddiqui Salma , Mooso Benjamin A. , D'Abronzo Leandro S. , Drake Christiana M. , Ghosh Paramita M. 
Published: August 2018

Abstract
Introduction: Despite ample evidence for the overexpression/overactivation of the epidermal growth factor receptor (EGFR) family, the dual EGFR/ErbB2 inhibitor lapatinib failed in patients with castration-resistant prostate cancer (CRPC). Our preclinical data show that the pan-ErbB inhibitor dacomitinib succeeds where lapatinib fails by decreasing membrane-localized EGFR family protein expression. Experimental Procedures: Nude mice were implanted with CWR22 tumors (human-patient-derived, androgen-dependent) and its CRPC subline 22Rv1 (relapsed CWR22). Mice were castrated or left intact and gavaged daily with 100 mg/kg lapatinib or vehicle. Tumors were analyzed for EGFR/ErbB2/ErbB3/androgen receptor (AR) proteins by both immunohistochemistry in paraffin-embedded tumor sections and Western blots in tumor lysates. Observations were validated in human-derived androgen-dependent and CRPC cell lines treated with lapatinib or dacomitinib by immunoblots and by immunofluorescence. Results: Intratumoral EGFR and ErbB2 (but not ErbB3) increased in intact, lapatinib-treated 22Rv1 tumor-bearing mice compared to placebo-treated ones. Elevated EGFR/ErbB2 was also observed in vitro in lapatinib-treated LNCaP, C4-2, PC-346C cells compared to vehicle-treated cells. EGFR phosphorylation increased from 1/5 intact, lapatinib-treated Rv1 tumors (20%) to 3/5 castrated, lapatinib-treated 22Rv1 tumors (60%). Lapatinib caused an increase in cytoplasmic EGFR and ErbB2 in LNCaP (androgen-dependent) and 22Rv1 (CRPC) cells. Low, physiological doses of dacomitinib overcame lapatinib resistance by suppressing cell viability and this effect was heightened by EGFR family knockdown or when AR activity was simultaneously inhibited. High-magnification microscopy revealed that dacomitinib possibly exerted its effects by decreasing the presence of membrane-localized EGFR, ErbB2 and ErbB3 proteins in comparison to lapatinib. Conclusions: We hypothesize that increased EGFR/ErbB2 heterodimerization is one cause of lapatinib resistance. Activated EGFR family proteins are typically localized in the plasma membrane and dacomitinib, by disrupting this membrane-localization, decreases cell viability. Future studies will attempt to refine the mechanism by which lapatinib and dacomitinib induce changes in RTK localization and how this may impact therapeutic response and prostate tumor progression.
Citation Format: Maitreyee K. Jathak, Thomas M. Steele, Salma Siddiqui, Benjamin A. Mooso, Leandro S. D'Abronzo, Christiana M. Drake, Paramita M. Ghosh. The pan-ErbB inhibitor dacomitinib but not the dual EGFR/ErbB2 inhibitor labatinib disrupts membrane localization of the EGFR family of receptor tyrosine kinases [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 868.