Abstract LB-215: Combination studies using PI3K isoform-selective inhibitors in PTEN-deficient cancer cells

A large and growing body of evidence indicates that dysregulation of phosphoinositide 3-kinase (PI3K) pathway signaling activity is necessary for cancer cell survival, most commonly as a result of genetic defects causing a deficiency of the tumor suppressor PTEN or activating mutations or amplification of PIK3CA (PI3Kα). In the context of PTEN-deficient cancer cells, it appears that in most cases PI3Kβ (PIK3CB) and not PI3Kα is required for PI3K survival signaling. GSK2636771 is a potent, orally bioavailable, selective ATP-competitive inhibitor of PI3Kβ with single digit nanomolar potency. It possesses >10-fold selectivity over PI3Kδ and >1000-fold selectivity over other lipid/atypical kinases including PI3Kα and mTOR. In this study, western blot analysis was performed to assess phosphorylation changes to AKT, ERK, and S6 proteins in 21 tumor cell lines of various histologies and genetic backgrounds treated with GSK2636771. PTEN deficiency (lack of PTEN protein expression) was the best marker to predict response to PI3Kβ inhibition based on reduction of phospho-AKT levels. Some of these “responsive” cells also harbored other genetic alterations such as amplified and/or mutated (V600E) BRAF, NF1 mutations, or EGFR amplification. For example, inhibition of PI3Kβ resulted in partial reduction of phospho-AKT and phospho-S6 levels in PTEN-deficient A2058, MDA-MB-468, and LNCAP cells, whereas PI3Kα inhibition by BYL719 treatment had no effect on AKT, ERK, and S6 phosphorylation. In PTEN-deficient, PIK3CA (E542K) mutant CAL-51 cells, the opposite was observed: GSK2636771 had no effect on phospho levels of AKT, ERK, and S6, but BYL719 treatment caused reductions of phospho-AKT and phospho-S6. Interestingly, when both isoforms were inhibited, the effects on phospho-AKT and phospho-S6 levels were more pronounced in all four cell lines, suggesting that these compounds may synergize to inhibit PI3K signaling. Combination treatment of both inhibitors had modest effects on cell growth in LNCAP cells, although it was observed that 6-day GSK2636771 treatment pulsed with BYL719 for 2 days in LNCAP provided similar antiproliferative effects as co-treatment of both molecules for 6 days. This may suggest that pulsing BYL719 with constant GSK2636771 treatment may be effective at inhibiting tumor growth in vivo while minimizing the on target toxicity that has been observed with sustained PI3Kα inhibition. Citation Format: Christina Blackwell, M. Phillip De Young. Combination studies using PI3K isoform-selective inhibitors in PTEN-deficient cancer cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-215.