Lau WM1, Teng E1, Huang KK1,2, Tan JW1, Das K2, Zang Z2, Chia T1, Teh M1, Kono K1,4, Yong WP1,5, Shabbir A4, Tay A4, Phua NS4, Tan P2,6, Chan SL6, So JBY4.
1 Cancer Science Institute of Singapore, National University of Singapore, Singapore.
2 Cancer and Stem Cell Biology Program, Duke-NUS Medical School, Singapore.
3 Department of Pathology, National University of Singapore, Singapore.
4 Department of Surgery, National University of Singapore, National University Hospital, Singapore.
5 Department of Haematology-Oncology, National Cancer Institute of Singapore, National University Hospital, Singapore.
6 Cancer Science Institute of Singapore, National University of Singapore, Singapore.
Preclinical models of diffuse-type gastric cancer (DGC) that reliably predict clinical activity of novel compounds are lacking. To overcome the problem of poor tumor cellularity in DGC, we used cells from malignant ascites to establish DGC patient-derived xenograft (PDX) models that recapitulate the primary cancer. Cells in PDX model GAGA6 with FGFR2 amplification were sensitive to AZD4547, a potent FGFR inhibitor that is being clinically evaluated for FGFR-aberrant cancer types. Intermittent in vivo treatment of GAGA6 tumors with AZD4547 gave rise to PDX tumors with acquired resistance to AZD4547, GAGA6-R. Surprisingly, there were no mutations in the FGFR2 gene in GAGA6-R, negating gatekeeper mutations as a mechanism of drug resistance. Phosphorylation of FGFR2 and downstream signaling molecules AKT/PKB and MAPK/ERK remained inhibited by AZD4547. Further analysis of signaling pathways identified AKT-independent phosphorylation and inhibition of GSK3β as a mechanism of drug resistance in GAGA6-R cells. Treatment of GAGA6-R cells with protein kinase C (PKC) inhibitor H7 in combination with AZD4547 led to dephosphorylation and activation of GSK3β with concomitant downregulation of MCL-1 and BCL-XL. Combined treatment with AZD4547 and H7 in vitro synergistically enhanced cell death in GAGA6-R but not GAGA6 cells. Furthermore, midostaurin, a multikinase inhibitor with PKC-inhibiting activity, in part reversed resistance of GAGA6-R tumor to AZD4547 in vivo. Our results suggest that upon challenge with FGFR inhibitors, FGFR2-amplified tumors that are highly dependent on FGFR2 signaling for survival rapidly develop resistance by switching to a PKC-mediated inhibition of GSK3β to gain a survival advantage. ©2017 AACR.