Background: 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 had multiple lines of prior systemic therapy (range 3-12), including chemotherapy. Dose limiting toxicity was observed in one pt in the form of grade (Gr) 3 oral mucositis. Common possibly related adverse events included nausea (5 pts), vomiting (4 pts), oral mucositis (4 pts), decreased appetite (3 pts), fatigue (3 pts), mucosal inflammation (2 pts), and paresthesia (2 pts). No obvious impact of LY on F PK or of F on LY PK was observed. Median duration of treatment was 15 weeks (range 3-63). In the 6 pts evaluable for tumor response, there was 1 durable partial response according to RECIST (still on treatment for ≥11 months) and 4 further pts had a decrease in their target lesions for a disease control rate of 56%. The median progression-free survival for this cohort is 4.2 months (90% CI 1.8, NA). Of note, the partial response was observed in a pt harboring an activating PIK3CA mutation (H1047R). Further biomarker analysis is ongoing. Conclusions: The RP2D of LY in combination with F is 200mg BID and may cause tumor regression or stabilization in breast cancer pts. Clinical trial information: NCT01655225