Ridaforolimus improves the anti-tumor activity of dual HER2 blockade in uterine serous carcinoma in vivo models with HER2 gene amplification and PIK3CA mutation

• HER2 blockade in combination with mTOR inhibition improves anti-tumor activity in HER2 over-expressing serous endometrial cancer with PIK3CA mutation
• Abrogation of phosphorylated-S6 protein was consistently associated with effective anti-tumor activity in HER2 over-expressing serous endometrial cancers

ObjectiveUterine serous carcinomas (USC) harbor simultaneous HER2 (ERBB2) over-expression and gain of function mutations in PIK3CA. These concurrent alterations may uncouple single agent anti-HER2 therapeutic efficacy making inhibition of the mammalian target of rapamycin (mTOR) a promising option to heighten anti-tumor response.MethodsBoth in vitro and in vivo experiments were conducted to assess proliferation, cell death and anti-tumor activity of ridaforolimus, lapatinib and combination lapatinib, trastuzumab (L/T) and ridaforolimus. With institutional approval, NOD/SCID mice bearing xenografts of non-immortalized, HER2 gene amplified cell lines (ARK1, ARK2) with and without PIK3CA gene mutations were divided into four arm cohorts. Ridaforolimus was administered alone and in combination with L/T. Tumor volumes were assessed and posttreatment analysis was performed.ResultsWe observed dose dependent in vitro abrogation of downstream target proteins including phospho-AKT and phospho-S6. In both in vivo models, single agent ridaforolimus impaired xenograft tumor growth. Combination ridaforolimus and L/T, however, further improved the observed anti-tumor activity only in the ARK1 model with the PIK3CA gene mutation (E542K). The addition of mTOR inhibition to dual HER2 blockade added no additional anti-tumor effects in the ARK2 xenografts. Western blot and immunohistochemical analysis of downstream pathway alterations following in vivo treatment revealed dual HER2 blockade with ridaforolimus was necessary to induce apoptosis, decrease proliferation and abrogate phospho-S6 protein expression in the PIK3CA mutated model.ConclusionsThese pilot data suggest that PIK3CA gene mutation may be an effective biomarker for selecting those HER2 over-expressing USC tumors most likely to benefit from mTOR inhibition.