Bonazzoli E, et al. - Researchers assessed the role of oncogenic PIK3CA mutations as a potential mechanism of resistance to afatinib in high-grade-serous-endometrial (USC) and ovarian-cancers (HGSOC) overexpressing HER2/neu using 6 whole-exome-sequenced primary HGSOC/USC cell-lines and three xenografts overexpressing HER2/neu and harboring mutated or wild-type PIK3CA/PIK3R1 genes. Compared to PI3K-mutated cell-lines, a significantly greater sensitivity (lower IC₅₀) was noted for PI3K wild-type cell-lines. When treated with afatinib, a significantly more rapid tumor growth was seen in xenografts of primary cell-line USC-ARK2, transfected with the PIK3CA-H1047R or E545K hotspot-mutations in vivo vs mice harboring ARK2-tumors transfected with wild-type-PIK3CA. In all cell-lines, effective reduction in total and phospho-HER2 proteins was seen with afatinib by western-blot. Overall, oncogenic PI3K mutations could be a major mechanism of resistance to afatinib. In order to block the PIK3CA/AKT/mTOR pathway more efficiently, combinations of c-erb with PIK3CA, AKT or mTOR inhibitors may be needed.