Decoupling of the PI3K Pathway via Mutation Necessitates Combinatorial Treatment in HER2+ Breast Cancer

We report here on experimental and theoretical efforts to determine how best to combine drugs that inhibit HER2 and AKT in HER2⁺ breast cancers. We accomplished this by measuring cellular and molecular responses to lapatinib and the AKT inhibitors (AKT_i) GSK690693 and GSK2141795 in a panel of 22 HER2⁺ breast cancer cell lines carrying wild type or mutant PIK3CA. We observed that combinations of lapatinib plus AKT_i were synergistic in HER2⁺/PIK3CA^mut cell lines but not in HER2⁺/PIK3CA^wt cell lines. We measured changes in phospho-protein levels in 15 cell lines after treatment with lapatinib, AKT_i or lapatinib + AKT_i to shed light on the underlying signaling dynamics. This revealed that p-S6RP levels were less well attenuated by lapatinib in HER2⁺/PIK3CA^mut cells compared to HER2⁺/PIK3CA^wt cells and that lapatinib + AKT_i reduced p-S6RP levels to those achieved in HER2⁺/PIK3CA^wt cells with lapatinib alone. We also found that that compensatory up-regulation of p-HER3 and p-HER2 is blunted in PIK3CA^mut cells following lapatinib + AKT_i treatment. Responses of HER2⁺ SKBR3 cells transfected with lentiviruses carrying control or PIK3CA^mut sequences were similar to those observed in HER2⁺/PIK3CA^mut cell lines but not in HER2⁺/PIK3CA^wt cell lines. We used a nonlinear ordinary differential equation model to support the idea that PIK3CA mutations act as downstream activators of AKT that blunt lapatinib inhibition of downstream AKT signaling and that the effects of PIK3CA mutations can be countered by combining lapatinib with an AKT_i. This combination does not confer substantial benefit beyond lapatinib in HER2⁺/PIK3CA^wt cells.