Background: H3B-6545 inactivates both wild-type and mutant ERα by targeting cysteine 530 and enforcing a unique antagonist conformation. Methods: Women with locally advanced or metastatic HR+ BC are treated (tx) with H3B-6545 administered once daily orally over a 28 day cycle after progression on at least one hormonal therapy and at least one additional therapy/regimen. Dose escalation uses a 3+3 design with the option to backfill previously cleared doses and allows for intrapatient dose escalation. This phase 1 explores the safety, pharmacokinetics and pharmacodynamics of H3B-6545 in women with HR+ BC to identify the recommended Phase 2 dose. Results: As of 10-Dec-2018, 32 pts have been tx with H3B-6545 at doses of 100 to 450 mg/day; 97% had prior tx with a CDK4/6 inhibitor and 56% had received ≥3 lines of prior anti-cancer therapy. No dose-limiting toxicities and only one Grade 3 treatment related adverse event (TRAE) have been observed (lymphocyte count decrease). The most common (≥10%) TRAEs include asymptomatic sinus bradycardia, diarrhea, nausea, fatigue, anemia, decreased appetite, and hot flush. H3B-6545 was rapidly absorbed with a t_max of 2-4 h. Plasma concentration increased with dose from 100 to 450 mg, and was similar on C1D1 and C1D15. Consistent with the H3B-6545 mechanism of action and preclinical data, H3B-6545 inhibits ER target gene expression and shows a 50% decrease in Ki67 levels across all dose levels post-tx. ESR1 (60%) and PIK3CA (34%) mutations were detected in plasma at baseline and changes in mutant allele frequencies show correlation in response to tx. Stable disease was observed in 15 pts (47%) and 34% of pts completed at least 6 months of tx. Partial responses (PRs) were observed in 3 pts: 1 pt (mutant) received 2 prior lines of therapy and 2 pts (1 mutant and 1 wild-type) received >5 prior lines of therapy including fulvestrant and capecitabine; all 3 pts received a prior CDK4/6 inhibitor. Conclusions: H3B-6545 has been well-tolerated up to the 450 mg dose level with early signs of single-agent anti-tumor activity in a post CDK4/6 setting. Dose escalation continues in pts with advanced HR+ BC. Clinical trial information: NCT03250676