Abstract LB-212: HER2-overexpressing breast cancer xenografts amplify FGFR signaling upon acquisition of resistance to dual blockade with trastuzumab and lapatinib
Authors: Garrett Joan T. , Becker Jennifer , Estrada Mónica Valeria , Arteaga Carlos L. 
Published: May 2015

Abstract
We sought to discover mechanisms of acquired resistance to the combination of the HER2 inhibitors trastuzumab and lapatinib in HER2 gene amplified BT474 xenografts. The combination of lapatinib and trastuzumab completely eliminated BT474 tumors established in nude mice after &lt;3 weeks of treatment. However, some tumors recurred as early as 9 weeks after the combination was stopped. Three recurrent tumors did not respond to retreatment with the combination of lapatinib and trastuzumab. More than 90% of tumors derived from these recurrent tumors did not respond to lapatinib/trastuzumab or the combination of trastuzumab and pertuzumab. Both the parental drug-sensitive xenografts and the lapatinib/trastuzumab-resistant (LTR) tumors exhibited 5 to 6-fold HER2 gene amplification as measured by FISH using probes for HER2 and chromosome 17. The LTR cancers maintained levels of P-HER2, P-AKT and P-ERK in the presence of lapatinib and trastuzumab as measured by IHC of tumor sections whereas treatment of parental BT474 xenografts reduced P-HER2 levels. LTR tumors exhibited markedly reduced in situ levels of lapatinib compared to BT474 sensitive tumors (2.157 µg lapatinib/gm tissue versus 4.701 µg lapatinib/gm tissue, p=0.02) as assessed by liquid chromatography-mass spectrometry (LC-MS). Quantitative site-specific mass spectrometric analysis of tyrosine phosphorylation from tumor lysates revealed enriched P-Y877 HER2 in LTR compared to parental BT474 xenografts. We next performed next generation sequencing to characterize base substitutions, short insertions and deletions (indels), copy number alterations and selected fusions across 287 cancer-related genes. There were no mutations in HER2 but we observed amplification of FGF3, FGF4, and FGF19 genes in LTR xenografts. Consistent with increased autocrine activation of FGFR, we observed markedly elevated levels of P-FGFR in LTR xenografts compared to parental xenografts as assessed by IHC. We speculate that ligand-induced FGFR signaling may induce resistance to dual HER2 blockade by 1) a tumor cell autonomous mechanism resulting in transactivation of HER2 and, 2) an alteration in the tumor microenvironment that reduces lapatinib uptake and/or retention by the LTR tumors. In conclusion, we show amplification of FGFR signaling upon acquired resistance to combinations of HER2-targeted agents. Studies with FGFR tyrosine kinase inhibitors to reverse drug resistance are in progress.
Citation Format: Joan T. Garrett, Jennifer Becker, Mónica Valeria Estrada, Carlos L. Arteaga. HER2-overexpressing breast cancer xenografts amplify FGFR signaling upon acquisition of resistance to dual blockade with trastuzumab and lapatinib. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-212. doi:10.1158/1538-7445.AM2014-LB-212