Dual HER2 targeting impedes growth of HER2 gene-amplified uterine papillary serous carcinoma xenografts

Purpose:Uterineserouscarcinoma(USC)isanaggressivesubtypeofendometrialcancerthatcommonly harbors HER2 gene amplification. We investigated the effectiveness of HER2 inhibition using lapatinib and trastuzumab in vitro and in xenografts derived from USC cell lines and USC patient-derived xenografts. Experimental Design: Immunohistochemistry and FISH were performed to assess HER2 expression in 42 primary USC specimens. ARK1, ARK2, and SPEC2 cell lines were treated with trastuzumab or lapatinib. Cohorts of mice harboring xenografts derived from ARK2 and SPEC2 cell lines and EnCa1 and EnCa2 primaryhumanUSCsamplesweretreatedwitheithervehicle,trastuzumab,lapatinib,orthecombinationof trastuzumab and lapatinib. Acute and chronic posttreatment tumor samples were assessed for downstream signaling alterations and examined for apoptosis and proliferation. Results: HER2 gene amplification (24%) correlated significantly with HER2 protein overexpression (55%). All models were impervious to single-agent trastuzumab treatment. Lapatinib decreased in vitro proliferation of all cell lines and in vivo growth of HER2-amplified xenografts (ARK2, EnCa1). In addition, dual therapy with trastuzumab and lapatinib resulted in significant antitumor activity only in ARK2 and EnCa1 tumors. Dual HER2 therapy induced on target alteration of downstream MAPK and PI3K pathway mediators only in HER2-amplified models, and was associated with increased apoptosis and decreased proliferation. Conclusions: Although trastuzumab alone did not impact USC growth, dual anti-HER2 therapy with lapatinib led to improved inhibition of tumor growth in HER2-amplified USC and may be a promising avenue for future investigation. Clin Cancer Res; 20(24); 6517–28. � 2014 AACR.