Safety and tolerability of the dual PI3K/mTOR inhibitor LY3023414 in combination with fulvestrant in treatment refractory advanced breast cancer patients.

1064Background: Thephosphatidylinositol 3-kinase (PI3K) /mammalian target of rapamycin (mTOR) pathway is frequently activated in breast cancer. LY3023414 (LY) is an oral ATP- competitive inhibitor that selectively and potently inhibits class I PI3K isoforms, mTOR, and DNA-PK. The recommended phase 2 dose (RP2D) of LY monotherapy was previously established to be 200 mg twice daily (BID). Here we present the safety and preliminary activity data of LY in combination with fulvestrant (F) for breast cancer patients (pts) as part of a multi-cohort Phase 1 study. Methods: Pts with advanced HR+, HER2- breast cancer refractory to standard treatment received 200 mg LY BID + 500 mg F (day 1 and 15, then once monthly). Eligible pts had measurable disease and baseline tumor tissue available. Primary objective was to determine a RP2D. Other objectives included assessment of pharmacokinetics (PK), antitumor activity, and biomarker analysis. Results: 9 pts received LY + F in the breast cancer expansion cohort. All pts ha...