Ribociclib and endocrine therapy as adjuvant treatment in patients with HR+/HER2- early breast cancer: Primary results from the phase III NATALEE trial.

LBA500 Background: RIB + ET has demonstrated significant survival benefits in pre- and postmenopausal pts with HR+/HER2− metastatic BC. To investigate whether RIB + ET also improves outcomes in early BC (EBC), the Phase III NATALEE trial (NCT03701334) evaluated adjuvant RIB + ET in a broad population of pts with stage II or III HR+/HER2− EBC at risk for recurrence, including pts with no nodal involvement (N0). As extended duration of tx is crucial to prolong cell cycle arrest and drive more tumor cells into senescence or death, a 3-y duration of RIB tx at a dose of 400 mg was chosen to improve tolerability while maintaining efficacy. Results from a prespecified interim analysis of invasive disease–free survival (iDFS; primary endpoint) are presented. Methods: Men and pre- or postmenopausal women were randomized 1:1 to RIB (400 mg/day; 3 wk on/1 wk off for 3 y) + ET (letrozole 2.5 mg/day or anastrozole 1 mg/day, for ≥ 5 y) or ET alone. Men and premenopausal women also received goserelin. Eligible pts had an ECOG PS of 0-1 and BC anatomic stage IIA (either N0 with additional risk factors or 1-3 axillary lymph nodes [N1]), stage IIB, or stage III per AJCC (8th ed); prior (neo)adjuvant ET was allowed if initiated ≤ 12 mo before randomization. Stratification factors were menopausal status, disease stage, prior (neo)adjuvant chemotherapy, and geographic region. This prespecified interim analysis of iDFS, defined per STEEP criteria, was planned after ≈ 425 iDFS events (≈ 85% of planned total events). iDFS was evaluated by Kaplan-Meier methods, and statistical comparison was made by a stratified log-rank test, with a protocol-defined Lan-DeMets (O'Brien-Flemming) stopping boundary of a 1-sided P < .0128 for superior efficacy. Results: From 10 Jan 2019 to 20 April 2021, 5101 pts were randomized (RIB+ET, n = 2549; ET alone, n = 2552). As of the data cutoff (11 Jan 2023), median follow-up was 34 mo (min, 21 mo). 3- and 2-y RIB tx was completed by 515 pts (20.2%) and 1449 pts (56.8%), respectively; 3810 (74.7%) remained on study tx (RIB+ET, n = 1984; ET alone, n = 1826). iDFS was evaluated after 426 events (RIB + ET, n = 189; ET alone, n = 237). RIB + ET demonstrated significantly longer iDFS than ET alone (HR, 0.748; 95% CI, 0.618-0.906; P = .0014); 3-y iDFS rates were 90.4% vs 87.1%. iDFS benefit was generally consistent across stratification factors and other subgroups. Secondary endpoints of overall survival, recurrence-free survival, and distant disease–free survival consistently favored RIB. RIB at 400 mg had a favorable safety profile with no new signals. Conclusions: Ribociclib added to standard-of-care ET demonstrated a statistically significant, clinically meaningful improvement in iDFS with a well-tolerated safety profile. The NATALEE results support ribociclib + ET as the treatment of choice in a broad population of pts with stage II or III HR+/HER2− EBC, including pts with N0 disease. Clinical trial information: NCT03701334 .