Abstract IA26: Clinical development of PI3K/AKT pathway inhibitors in breast cancer

Abstract

Abstract Targeting the PI3K/AKT/mTOR pathway has been a focus for drug development since the discovery of PI3K-activating mutations in p110α over 10 years ago. Breast cancer has been a primary indication for development of this class of drugs because of the high prevalence of these mutations, up to 45% in hormone receptor-positive breast cancer, and the synergy observed in combinations with endocrine therapies. An mTOR inhibitor, everolimus, was approved in breast cancer in combination with exemestane in 2012, indicating the synergy between ER and PI3K pathway translates to the clinic. Apitolisib, a dual PI3K pan-isoform and mTOR inhibitor, has discontinued development due to toxicities, despite compelling preclinical activity across multiple tumor types regardless of PI3K and PTEN status. Pictilisib, a pan-isoform PI3K inhibitor, has shown some evidence of activity, but in phase II studies has demonstrated that the activity is limited by dose intensity. Buparlisib, a pan-isoform PI3K inhibitor, has demonstrated efficacy in a large phase III study, but was not deemed sufficiently active relative to the toxicity in HR+ breast cancer to be submitted for regulatory approval. Second-generation PI3K and AKT inhibitors with higher selectivity and mutant specificity are in clinical development and hope to thread a better therapeutic index by dosing higher for more profound inhibition, and/or selectively inhibiting mutated forms of PI3K. Taselisib (GDC-0032) is a beta-sparing and PI3K-mutant selective inhibitor that leads to degradation of only the mutant form of PI3K and is currently in phase III. More recently, taselisib met the primary endpoint of improved response rates in a randomized, neoadjuvant phase II trial, LORELEI. Alpelisib is an alpha-selective PI3K inhibitor that is also in phase III. Recent data with ipatasertib, an AKT inhibitor, has demonstrated better tolerability and efficacy in randomized phase II trials of both prostate cancer and triple-negative breast cancer, where the PI3K/AKT signaling pathway is activated by either loss of PTEN or PI3K mutations, and may offer a better therapeutic index given its unique mechanism of action and place in the PI3K signaling pathway. Overall, the PI3K/AKT pathway is still considered an important pathway to target for certain cancers; however, target selectivity, dosing, and the right combination are critical to find a therapeutic index to provide a good option for patients. Citation Format: Mika Derynck. Clinical development of PI3K/AKT pathway inhibitors in breast cancer [abstract]. In: Proceedings of the AACR Special Conference: Advances in Breast Cancer Research; 2017 Oct 7-10; Hollywood, CA. Philadelphia (PA): AACR; Mol Cancer Res 2018;16(8_Suppl):Abstract nr IA26.