Background: AMEERA-1 (NCT03284957) investigates amcenestrant, an oral SERD, as monotherapy and combined with targeted therapies in ER+/HER2– mBC. Here we report data from dose escalation (Part C) and dose expansion (Part D) of amcenestrant + palbo. Methods: Patients (pts) were postmenopausal women with ER+/HER2– mBC and ≥ 6 mos prior advanced endocrine therapy (ET) or adjuvant (adj) ET resistance (relapse on adj ET started ≥ 24 mos ago or < 12 mos after completing adj ET). Prior chemotherapy (≤ 1) for advanced disease was allowed; targeted therapies were not except ≤ 1 CDK4/6i in Part C. Part C assessed dose-limiting toxicities (DLTs) and aimed to establish the recommended phase 2 dose (RP2D) for amcenestrant (200 or 400 mg once daily [QD], in 28-day cycles) in combination with palbo (125 mg QD for 21 days on/ 7 days off). Safety (treatment-emergent adverse events [TEAEs] and lab abnormalities per CTCAE v4.03) and pharmacokinetics (PK) were evaluated. Antitumor activity at the RP2D for amcenestrant + palbo was evaluated in a subset of Part C pts and Part D, according to RECIST v1.1, determined locally by investigators. Results: Feb 8, 2021 data cutoff. In Part C (n = 15; 200 mg: 9; 400 mg: 6), no DLTs occurred and amcenestrant 200 mg QD was selected as the RP2D with palbo, based on PK and safety data. In the pooled safety population at the RP2D (n = 39; Part C: 9; Part D: 30), median (range) age was 59 y (33–86) with ECOG PS 0 (74.4%) or 1 (25.6%) and 2 (1–6) organs involved. Immediate prior therapy was neo/adj (41.0%, all ET resistant) or advanced (59.0%, range 1–4 lines). Median (range) exposure was 32 wks (1–66) with 59.0% pts on ongoing therapy. No amcenestrant dose reductions occurred; 25.6% had ≥ 1 palbo dose reduction. Most common non-hematological TEAEs related to amcenestrant were Grade 1–2 nausea and fatigue (17.9% each), asthenia and hot flush (10.3% each); to palbo were fatigue (30.8%), nausea (25.6%), asthenia and dysgeusia (10.3% each). Two pts discontinued due to AEs. The majority (94.9%) had neutrophil count decrease (53.8% Grade ≥ 3). Preliminary antitumor activity after at least 6 cycles of therapy (unless early treatment discontinuation) is reported in the table below. Conclusions: In pts with ER+/HER2– mBC, safety at the RP2D of amcenestrant + palbo was favorable, with no safety signals of bradycardia or eye disorders. Preliminary antitumor activity was observed (ORR: 31.4% and CBR: 74.3%). Clinical trial information: NCT03284957.

*Response-evaluable pts with no prior mTORi or CKD4/6i ^†Including 1 pt with PR to be confirmed at next assessment