Co-targeting PI3K and ras pathways in trastuzumab resistance.

Abstract

e13515 Background: HER2+ breast cancers (BC) account for 20–25% of invasive BC and are associated with an aggressive phenotype and poor patient outcome. The development of trastuzumab and other HER2-targeted therapy dramatically improved outcomes for HER2-positive BC, but most patients with advanced HER2+ BC will eventually become resistant to treatment, underlying the importance of developing alternative or combination treatments. Alterations in the PI3K/mTOR/Akt pathways are cited as contributors to the development of trastuzumab resistance, however targeting these kinases as single agents has yielded less than expected clinical results. This suggests that combinational treatment with other kinase pathway inhibitors may be required, including those targeting the Ras/MAPK pathway, which is typically not mutationally activated in breast cancer. Methods: Trastuzumab resistant HER2+ BC cell lines were subjected to dose responses with the mTOR inhibitor everolimus or the AKT inhibitor MK-2206 alone or in combination with the MEK 1/2 inhibitor GSK212 and changes in EC50s determined by MTT assay. Western blot analysis was performed to assess changes in the mTOR/AKT/ MAPK pathways or apoptotic regulators accompanying single or combination treatments. Results: In 4 of 5 trastuzumab-resistant HER2+ BC cell lines, each lacking activating mutations in the Ras/Raf pathway, combination treatment with everolimus or MK-2206 with GSK212 had significantly greater efficacy than by either inhibitor alone. Furthermore, combinational treatment targeting the mTOR and AKT pathways showed only an additive effect, and was much less effective than targeting both the MEK and PI3K pathways. Western analysis showed that AKT and mTOR inhibition caused a transient increase in ERK 1/2 activity in sensitive cell lines, suggesting that treatment by mTOR or AKT inhibitors activated critical survival pathways in the MAPK signaling that were blocked by GSK212. Conclusions: Treatment with mTOR or AKT inhibitors in combination with MEK inhibitors can act in a synergistic manner with greater efficacy than each inhibitor alone. Moreover, ERK 1/2 activity may serve as a predictive biomarker in trastuzumab-refractory patients treated with mTOR/MEK or AKT/MEK doublet therapy.