Background: In the phase 3 SOLAR-1 study, ALP + fulvestrant (FUL) improved PFS in pts with HR+, HER2– ABC with a PIK3CA mutation overall and in the small group of pts with prior cyclin-dependent kinase 4/6 inhibitor (CDKi) use. We report interim data from the BYLieve study in pts with PIK3CA-mutated ABC and prior CDKi exposure. Methods: BYLieve is an ongoing, phase 2, open-label, non-comparative study of ALP 300 mg QD + ET in men and women with PIK3CA-mutated HR+, HER2– ABC whose disease progressed on/after CDKi + ET. Pts are permitted ≤2 prior anticancer therapies and ≤1 prior chemotherapy regimen for ABC. Pts with prior CDKi and AI (FUL cohort) receive ALP and FUL 500 mg Q28d + C1d15 IM. Pts with prior CDKi and FUL (LET cohort) receive ALP and letrozole (LET) 2.5 mg PO QD. In this preplanned interim analysis, conducted after ≥20 pts in FUL had ≥6 mo of follow-up, descriptive data are reported for preliminary safety and efficacy in the FUL and LET cohorts. Results: At data cutoff, 64 and 36 pts were enrolled in the FUL and LET cohorts, respectively; 39 pts (FUL, n = 21; LET, n = 18) have safety and efficacy data with ≥6 mo follow-up and are reported here. Data on 100 pts enrolled at the time of data cutoff will be presented. In the 39 pts with ≥6 mo follow-up, median ALP duration was 5.3 and 5.5 mo in FUL and LET, respectively; median duration of FUL and LET was 5.6 mo. Median relative ALP dose intensity was 93% (FUL) and 87% (LET). Most common grade ≥3 adverse events were hyperglycemia (38.1% (FUL) and 27.8% (LET)) and rash (4.8% (FUL) and 27.8% (LET)). Only 2 pts (5%; 1 pt per cohort) discontinued due to an AE. In pts with centrally confirmed PIK3CA mutation (n = 20 (FUL); n = 17 (LET)), ORR was 20% (FUL) and 18% (LET), CBR was 40% (FUL) and 35% (LET). Efficacy and safety data for the 100 enrolled pts will be presented at the meeting. Conclusions: Pending further readout of the ongoing BYLieve trial, safety and tolerability of ALP and hormonal therapy in pts with prior CDKi are consistent with those of SOLAR-1; discontinuation due to toxicity was rare. NCT03056755. Clinical trial information: NCT02437318