Abstract GS3-08: Alpelisib + fulvestrant for advanced breast cancer: Subgroup analyses from the phase III SOLAR-1 trial

Abstract Background: Hyperactivation of the phosphatidylinositol-3-kinase (PI3K) pathway can occur due to PIK3CA mutations, present in ~40% of patients (pts) with hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2—) ABC. SOLAR-1, a Phase 3 randomized, double-blind trial (NCT02437318), investigated efficacy and safety of ALP (α-specific PI3K inhibitor) + FUL in pts with HR+, HER2— ABC. ALP+FUL met the primary endpoint by significantly extending progression-free survival (PFS) vs placebo (PBO) + FUL in the PIK3CA-mutant cohort (hazard ratio HR 0.65; 95% CI 0.50—0.85; p=0.00065; median 11.0 vs 5.7 months). Here we report overall survival (OS), subgroup data and safety in the PIK3CA-mutant cohort, and PFS by circulating tumor (ct)DNA PIK3CA mutation status in the total population. Methods: Enrollment was open to men/postmenopausal women with PIK3CA-mutant HR+, HER2— ABC and 1 prior line of endocrine therapy. Pts were randomized (1:1) to ALP (300mg/day) + FUL (500mg every 28 days and Cycle 1 Day 15) or PBO+FUL. OS was the key secondary endpoint. PFS was analyzed by PIK3CA mutant status in ctDNA, and in important prognostic subgroups, including line of treatment in ABC and prior cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) use. Safety was analyzed in the PIK3CA-mutant cohort. Results: 341 pts in the PIK3CA-mutant cohort received ALP+FUL (n=169) or PBO+FUL (n=172). Median follow-up from randomization to data cut-off was 20.0 months. At data cut-off, 92 deaths had occurred (52% of the total 178 pts planned for final OS analysis); 40 for ALP+FUL (24%) and 52 for PBO+FUL (30%). OS results were immature at data cut-off (HR 0.73; 95% CI 0.48—1.10; p=0.06; median not estimable vs 26.9 months). There was a 45% risk reduction in PFS for pts with ctDNA PIK3CA mutations (HR 0.55; 95% CI 0.39—0.79; n=186); 20% for pts without (HR 0.80; 95% CI 0.60—1.06; n=363). PFS treatment effect for ALP+FUL vs PBO+FUL was generally consistent across subgroups of interest, with a risk reduction of 29% for pts receiving first-line (1L) treatment (HR 0.71; 95% CI 0.49—1.03; n=177), and 39% for 2L treatment (HR 0.61; 95% CI 0.42—0.89; n=161); 52% in pts with prior CDK4/6i (HR 0.48; 95% CI 0.17—1.36; n=20) and 33% in pts without (HR 0.67; 95% CI 0.51—0.87; n=321). Most frequent all-grade adverse events (AEs; ≥40% in either arm by single preferred term; ALP+FUL vs PBO+FUL) were hyperglycemia (65% vs 9%), diarrhea (54% vs 11%), nausea (46% vs 20%), and rash (40% vs 6%). Grade 3/4 AEs in ≥10% pts in either arm were hyperglycemia (fasting plasma glucose 250mg/dL; 37% for ALP+FUL vs 1% for PBO+FUL) and rash (13% vs 1%); discontinuations of ALP+FUL/PBO+FUL due to AEs were 3% vs 2%. Conclusions: ALP+FUL showed consistent clinically meaningful treatment benefit for pts with ctDNA PIK3CA mutant status, and across pt subgroups, including pts with/without prior treatment for ABC and prior CDK4/6i use. OS data were not yet mature at the data cut-off, but OS appeared numerically longer for ALP+FUL vs PBO+FUL after 52% of events. Key words: advanced breast cancer; PI3K; alpelisib; endocrine therapy Citation Format: Juric D, Ciruelos E, Rubovszky G, Campone M, Loibl S, Rugo HS, Iwata H, Conte P, Mayer IA, Kaufman B, Yamashita T, Lu Y-S, Inoue K, Takahashi M, Pápai Z, Longin A-S, Mills D, Wilke C, Sellami D, Andre F. Alpelisib + fulvestrant for advanced breast cancer: Subgroup analyses from the phase III SOLAR-1 trial abstract. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr GS3-08.