Alpelisib (ALP) with fulvestrant (FUL) in patients (pts) with <i>PIK3CA-</i>mutated hormone receptor-positive (HR+), human epidermal growth factor receptor-2-negative (HER2-) advanced breast cancer (ABC): Primary or secondary resistance to prior endocrine therapy (ET) in the SOLAR-1 trial.

1038 Background: A contributor to ETR, phosphatidylinositol 3-kinase (PI3K) pathway hyperactivation can result from mutations to PIK3CA; ~40% of pts with HR+, HER2– ABC exhibit tumors with this mutation. Use of the oral α-specific PI3K inhibitor ALP + FUL significantly improved progression-free survival (PFS) in pts with a PIK3CA mutation (HR 0.65; 95% CI, 0.50-0.85; P&lt;0.001) in SOLAR-1, which included both ET sensitive (ETS) and ETR pts (Table). ETS pts were later excluded by a protocol amendment. ETR was further defined as primary (1R) or secondary (2R) per ESMO criteria in both 1L and 2L pts. This subgroup analysis evaluated pts with a PIK3CA mutation based on tx line and endocrine status. Methods: SOLAR-1 was a phase 3, randomized, double-blind study of ALP 300 mg QD or PBO Q28d + FUL 500 mg Q28d + C1d15 in men and postmenopausal women with HR+, HER2– ABC whose disease progressed on/after an aromatase inhibitor. PFS was estimated by Kaplan-Meier method and median PFS (mPFS) presented by tx arm. A stratified Cox proportional hazards model estimated HR and 2-sided 95% CI. Results: Of 341 pts in the PIK3CA mutant cohort, 39 (11%) were ETS; 302 (89%) were ETR. mPFS in the ALP vs PBO arms was 22.1 vs 19.1 mo (HR 0.87; 95% CI, 0.35-2.17) for ETS pts and 9.4 vs 4.2 mo (HR 0.64; 95% CI, 0.48-0.84) for ETR pts. For ETR pts, mPFS for 1L (n=138) was 9.0 vs 4.7 mo (HR 0.69; 95% CI, 0.46-1.05) and for 2L (n=161) was 10.9 vs 3.7 mo (HR 0.61; 95% CI, 0.42-0.89). Conclusions: In SOLAR-1, mPFS was improved with ALP + FUL vs PBO + FUL across ETR pts in 1L and 2L. Representation of ETS pts was low in SOLAR-1, which included more ETR pts. Analysis of the PI3K pathway in ETS pts is warranted in future studies. Clinical trial information: NCT02437318. [Table: see text]