Alpelisib (ALP) + fulvestrant (FUL) in patients (pts) with <i>PIK3CA-</i>mutated (mut) hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2–) advanced breast cancer (ABC) previously treated with cyclin-dependent kinase 4/6 inhibitor (CDKi) + aromatase inhibitor (AI): BYLieve study results.

1006 Background: PIK3CA mutations (mut) occur in ~40% of pts with HR+, HER2– ABC and are associated with poor prognosis and resistance to treatment (tx). ALP, a PI3Kα inhibitor, plus FUL demonstrated efficacy in the phase 3 SOLAR-1 trial of HR+, HER2– PIK3CA-mut ABC. Little clinical data and few prospective studies are available to inform tx decisions for pts with HR+, HER2– PIK3CA-mut ABC in the post-CDKi setting. BYLieve is the first trial evaluating ALP + endocrine therapy (ET; FUL or letrozole) in pts with HR+, HER2– PIK3CA-mut ABC who progressed on/after prior therapy, including CDKi. In this ongoing phase 2, open-label, noncomparative study, 112 pts with centrally confirmed PIK3CA mut in tumor tissue are enrolled in each cohort per immediate prior tx of CDKi + AI, CDKi + FUL, or systemic chemo or ET. Enrollment is complete in prior CDKi + AI and CDKi + FUL cohorts and ongoing in prior systemic chemo or ET cohort. We report on the cohort of pts with CDKi + AI as immediate prior tx. Methods: Pts received ALP 300 mg/day + FUL 500 mg Q28D + C1D15. Primary endpoint was proportion of pts alive without disease progression at 6 mo per local assessment; 2-sided 95% confidence intervals (CI) were calculated using Clopper and Pearson (1934) exact method. Evidence of clinically meaningful tx effect was defined as the lower bound of the 95% CI > 30%. Safety was assessed in all patients; AEs presented by preferred term. Results: 127 pts whose immediate prior tx was CDKi + AI were enrolled, of whom 121 had centrally confirmed PIK3CA mut; median follow-up was 11.7 mo. Primary endpoint was met: proportion of pts without disease progression at 6 mo was 50.4% (95% CI, 41.2-59.6). Most frequent all-grade AEs were diarrhea (60%), hyperglycemia (58%), nausea (46%), fatigue (29%), decreased appetite (28%), and rash (28%). Most frequent grade ≥3 AEs included hyperglycemia (28%), rash (9%), and rash maculopapular (9%). Incidence of AEs leading to discontinuation was low; most frequent AEs leading to discontinuation were rash (5 pts, 3.9%), colitis, hyperglycemia, urticaria, and vomiting (2 pts, 1.6% each). Conclusions: With follow-up still ongoing, BYLieve shows in a large number of pts that ALP + FUL demonstrates clinically meaningful efficacy and manageable toxicity post CDKi tx. Building on findings from SOLAR-1, BYLieve further supports use of ALP + FUL for HR+, HER2– PIK3CA-mut ABC. Clinical trial information: NCT03056755 .

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