A phase I dose escalation and expansion study of the next generation oral SERD AZD9833 in women with ER-positive, HER2-negative advanced breast cancer.

Abstract

1024 Background: AZD9833 is an oral selective estrogen receptor (ER) antagonist and degrader (SERD) that has shown antitumor efficacy in a range of preclinical models of breast cancer. Methods: SERENA-1 (NCT03616587) is an ongoing Phase 1, open-label study in pre- and post-menopausal women, after ≥1 endocrine therapy and ≤2 prior chemotherapies for ER+ HER2- advanced breast cancer (ABC). The primary objective is to determine the safety and tolerability of AZD9833 once daily (QD), with dose-limiting toxicities (DLTs) in 28d defining the maximum tolerated dose. Secondary objectives include pharmacokinetics and anti-tumor response. Pharmacodynamic (PD) analysis includes ER modulation in paired tumor biopsies and ctDNA dynamic changes. Results: At 20 January 2020: 60 patients were treated (median prior therapies 5 (1–9); prior fulvestrant (Fv) 82%; prior CDK4/6i 68%) across five doses; 25 mg QD n=12, 75 mg QD n=12, 150 mg QD n=13, 300 mg QD n=13, 450 mg QD n=10. AZD9833 exposure was dose proportional after multiple doses, with a median terminal t1/2 of 12h. Treatment-related AEs experienced by ≥10% of patients were visual disturbances (53%; 91% G1, 6% G2, 3% G3), bradycardia/sinus bradycardia (45%; 93% G1, 7% G2), nausea (18%; 46% G1, 55% G2), fatigue (13%; 38% G1, 63% G2), dizziness (10%; 83% G1, 17% G3) vomiting (10%; 50% G1, 33% G2, 17% G3), and asthenia (10%; 67% G1, 33% G2). Three patients experienced DLTs: G3 QTcF prolongation (300 mg); G3 vomiting (450 mg); and a combination of G2 visual disturbance, G2 headache and G2 gait disturbance (450 mg). DLTs resolved with dose reduction. No G4 or 5 AEs were reported. Efficacy data are presented in the table below; objective response rate (ORR) and clinical benefit rate (CBR) at 24 weeks. Clinical trial information: NCT03616587 . ER signalling pathway modulation was observed in all dose cohorts. In patients where clinical responses occurred and paired biopsies obtained, 98% reduction in Ki67 was measured. Updated data will be presented. Conclusions: AZD9833 has an encouraging efficacy and dose-dependent safety profile. Evidence of clinical benefit and target engagement was observed at all dose levels in women with ER+ ABC, including patients pre-treated with CDK4/6i and Fv, and those with ESR1 mutations. A Phase 2 study comparing efficacy and safety of three doses AZD9833 vs Fv is planned (NCT04214288). [Table: see text]