Combined inhibition of PI3Kβ and mTOR inhibits growth of PTEN null tumours.

Loss of the tumor suppressor PTEN confers a tumor cell dependency on the P13K beta isoform. Achieving maximal inhibition of tumor growth through PI3K pathway inhibition requires sustained inhibition of PI3K signaling; however, efficacy is often limited by suboptimal inhibition or reactivation of the pathway. To select combinations that deliver comprehensive suppression of PI3K signaling in PTEN-null tumors, the PI3K beta inhibitor AZD8186 was combined with inhibitors of kinases implicated in pathway reactivation in an extended cell proliferation assay. Inhibiting PI3K beta and mTOR gave the most effective antiproliferative effects across a panel of PTEN-null tumor cell lines. The combination of AZD8186 and the mTOR inhibitor vistusertib was also effective in vivo controlling growth of PTEN-null tumor models of TNBC, prostate, and renal cancers. In vitro, the combination resulted in increased suppression of pNDRGI, p4EBP1, as well as HMCCS1 with reduced pNDRC1 and p4EBP1 more closely associated with effective suppression of proliferation. In vivo biomarker analysis revealed that the monotherapy and combination treatment consistently reduced similar biomarkers, while combination increased nuclear translocation of the transcription factor FOXO3 and reduction in glucose uptake. These data suggest that combining the PI3K beta inhibitor AZD8186 and vistusertib has potential to be an effective combination treatment for PTEN-null tumors. (C) 2018 AACR.