Abstract PS12-09: Capivasertib plus cyclin-dependent kinase 4/6 inhibitor and fulvestrant in hormone receptor-positive/human epidermal growth factor receptor 2-negative advanced breast cancer: updated Phase 1b analysis from CAPItello-292

Abstract Background AKT pathway activation is implicated in endocrine therapy (ET) and cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) resistance in patients (pts) with HR+/HER2– advanced breast cancer (ABC). In CAPItello-291, capivasertib (C, a potent inhibitor of all 3 AKT isoforms) + fulvestrant (F) significantly improved PFS versus placebo F in pts with aromatase inhibitor-resistant HR+/HER2– ABC. Simultaneous inhibition of AKT and CDK4/6 pathways may delay CDK4/6i resistance or re-sensitize tumors to ET plus CDK4/6i, leading to improved clinical outcomes. CAPItello-292 (NCT04862663) is an ongoing Phase 1b/3 study examining the efficacy and safety of C + CDK4/6i (palbociclib [P], ribociclib [R]) + F in HR+/HER2– ABC. The recommended Phase 3 dose (RP3D) of C+P+F has been determined (C 400 mg BID, 4 days on/3 days off, P 125 mg, F 500 mg). Here we report ongoing Phase 1b data on C+P+F and C+R+F. Methods: Phase 1b of CAPItello-292 uses a keyboard design (mTPI-2) with the following planned doses: C 200 mg, 320 mg, 400 mg; P 75 mg, 100 mg, 125 mg; R 200 mg, 400 mg, 600 mg; F is fixed at 500 mg every 28 days + loading dose on cycle 1 day 15. Starting doses for C+P+F were C 320 mg BID, 4 days on/3 days off, P 125 mg QD) for 21 days of each 28-day cycle. Starting doses for C+R+F (ongoing) are C 400 mg BID, 4 days on/3 days off, R 400 mg QD for 21 days of each 28-day cycle. Eligible pts have HR+/HER2− ABC and ≥1 prior ET in the advanced setting or disease recurrence within 12 months of completing (neo)adjuvant ET (HER2− defined as immunohistochemistry [IHC] 0, or 1-positive or IHC2-positive/in situ hybridization-negative). Prior use of CDK4/6i, selective estrogen receptor degraders, and chemotherapy is permitted in Phase 1b. Phase 1b primary objectives: assess safety/tolerability; confirm RP3D. Objective response and clinical benefit rates (24 weeks; RECIST v1.1) were also assessed. Results: At data cut-off (Apr 21, 2023), 40 pts (median age: 58.5 years [range 38–82]) who were heavily pre-treated (as follows: 82.5% [33/40] prior CDK4/6i; 47.5% [19/40] prior F; 70.0% [28/40] prior chemotherapy [median 1.5 lines]) were treated with C+P+F. No new dose-limiting toxicities (DLTs) were observed since determination of RP3D. The most frequent adverse events (AEs) occurring in >40% of pts were diarrhea (70.0% [28/40]; 1/28 grade [G] ≥3), neutropenia (55.0% [22/40]; 20/22 G≥3), fatigue, and nausea (both 42.5% [17/40]; all G1/2). Hyperglycemia occurred in 17.5% (7/40) pts (1/7 G3). No treatment-related deaths or new safety risks were identified. At the RP3D, the median (range) duration of exposure to C was 8.6 months (1.7–14.1); 5/8 pts with measurable disease at baseline had confirmed partial response (objective response rate: 62.5%, 95% confidence interval [CI] 24.5–91.5). Two additional pts had stable disease ≥7 weeks as a best objective response. At 24 weeks, the clinical benefit rate was 53.8% (7/13; 95% CI 25.1–80.8).Enrollment into C+R+F is ongoing: as of May 05, 2023, 8 pts had been dosed (6/8 pts had completed cycle 1). Based on clinical and safety review, the safety review committee has endorsed dose escalation to the highest dose level of R; no DLTs have been reported so far, although data are preliminary.Preliminary ctDNA analyses of pts treated with C+P/R+F will be presented. Conclusions: C+P+F was tolerable in heavily pre-treated pts with HR+/HER2– ABC at all dose levels; AEs were as expected, given the known safety profile of the individual treatments. Evidence of clinical activity has been observed in pts treated with the RP3D; data collection is ongoing. Preliminary safety analysis of C+R+F suggests no critical tolerability concerns; more pts and longer follow-up is required to characterize the safety of the combination. Updated data will be presented. Funding: AstraZeneca Capivasertib was discovered by AstraZeneca subsequent to a collaboration with Astex Therapeutics (and its collaboration with the Institute of Cancer Research and Cancer Research Technology Limited) Citation Format: Erika Hamilton, Patrick Neven, Barbara Pistilli, Muralidhar Beeram, Virginia Borges, Mario Campone, José Luiz Miranda Guimarães, Theodoros Foukakis, Annette Raskov Kodahl, Peter Lau, Elgene Lim, Iwona Ługowska, Greg Rychlik, Christopher Gresty, Claire Miller, Roberto Sommavilla, Dhivya Sudhan, Hope Rugo. Capivasertib plus cyclin-dependent kinase 4/6 inhibitor and fulvestrant in hormone receptor-positive/human epidermal growth factor receptor 2-negative advanced breast cancer: updated Phase 1b analysis from CAPItello-292 [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS12-09.