Background: The phase III MONALEESA-7 study (NCT02278120) is the first dedicated trial of endocrine therapy (ET) ± a cyclin-dependent kinase 4/6 (CDK4/6) inhibitor in premenopausal patients (pts) with hormone receptor–positive (HR+)/HER2− ABC. The study met its primary endpoint of significantly longer progression-free survival (PFS) with ribociclib (RIB; a CDK4/6 inhibitor) + ET vs placebo (PBO) + ET (median, 23.8 vs 13.0 mo; HR, 0.55; P< 0.0001; Tripathy D, et al. Lancet Oncol. 2018). Methods: Premenopausal pts (N=672) with HR+/HER2− ABC were treated with RIB or PBO + goserelin and either a nonsteroidal aromatase inhibitor (NSAI; letrozole or anastrozole) or tamoxifen. This is the 2nd of 3 protocol-specified OS analyses (scheduled to occur after ≈ 189 deaths [75% of the planned total events]). OS was evaluated by Kaplan-Meier methods, and statistical comparison was made by 1-sided stratified log-rank test, with a protocol-defined Lan-DeMets (O’Brien-Fleming) stopping boundary of p < 0.01018 for superior efficacy. Results: The data cutoff for this prespecified interim analysis was Nov 30, 2018, and the median follow-up was 34.6 mo (min, 28.0 mo). At cutoff, 173 pts were continuing study treatment (RIB, n=116; PBO, n=57), and OS was evaluated after 192 deaths (RIB, n=83; PBO, n=109). RIB + ET demonstrated a significantly longer OS than PBO + ET (median, not reached vs 40.9 mo [95% CI, 37.80 mo-not evaluable]; HR, 0.712 [95% CI, 0.54-0.95]; p = 0.00973). The result crossed the prespecified stopping boundary for superior efficacy. Estimated OS rates with RIB + ET vs PBO + ET at 42 mo were 70.2% vs 46.0%, respectively. In pts who received an NSAI (n=495), RIB + ET demonstrated a consistent OS improvement vs PBO + ET (HR, 0.699 [95% CI, 0.50-0.98]). Posttreatment therapy use was balanced between treatment arms (RIB, 68.9%; PBO, 73.2%). Conclusions: RIB + ET demonstrated a clinically and statistically significant longer OS than ET alone in premenopausal pts with HR+/HER2− ABC. This is the first time that a CDK4/6 inhibitor or any targeted agent + ET has demonstrated significantly longer OS vs ET alone as initial endocrine-based therapy. Clinical trial information: NCT02278120