Abstract P2-17-01: The PI3K inhibitor GDC-0032 is selectively potent against PIK3CA mutant breast cancer cell lines and tumors

Abstract

Abstract Mutations in the phosphoinositide-3 kinase alpha isoform (PIK3CA) are frequent in breast cancer and activate the PI3K signaling pathway. We discovered GDC-0032, a selective, potent, orally bioavailable inhibitor of PI3Ka with a Ki = 0.2nM, and with reduced inhibitory activity against PI3Kβ. This selectivity profile, and excellent pharmacokinetic and pharmaceutical properties, allowed for greater efficacy in vivo at the maximum tolerated dose relative to a pan Class I PI3K inhibitor in PIK3CA mutant xenografts. Notably, GDC-0032 preferentially inhibited PIK3CA mutant cells relative to cells with wild-type PI3K. GDC-0032 potently inhibits signal transduction downstream of PI3K and induces apoptosis at low concentrations in breast cancer cell lines and xenograft models that harbor PIK3CA mutations. The mutant-bias of GDC-0032 is linked to unique properties of GDC-0032, including cellular potency against the mutant isoform and reduction of receptor tyrosine kinase (RTK) signaling. Endocrine therapies such as letrozole are commonly used treatment options for metastatic Hormone Receptor positive (HR+) breast cancer but many patients ultimately relapse. Due to the importance of PI3K in breast cancer, PI3K inhibitors such as GDC-0032 are attractive for combination with endocrine therapies. GDC-0032 was evaluated in breast cancer lines and models in combination with letrozole, and assayed for cellular viability, modulation of PI3K pathway, modulation of ER pathway markers, and apoptosis induction. The combination of GDC-0032 and letrozole decreased cellular viability and increased apoptosis relative to either single agent. We observed cross-talk between the PI3K and ER pathways that suggest a mechanism of action for the combination. In a secreted factor screen we found that multiple soluble factors render breast cancer cells non-responsive to letrozole. It was discovered that many of these factors signal through the PI3K pathway and GDC-0032 in combination with letrozole was able to overcome the growth inhibition caused by the soluble factor. We also established letrozole resistant cell lines that grow independently of any estrogen source. These letrozole resistant lines have elevated PI3K pathway signaling and are still sensitive to GDC-0032. Taken together, these data provide rationale for evaluating GDC-0032 in combination with endocrine therapies for ER+ breast cancer treatment in the clinic. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P2-17-01.