Abstract Background: MONALEESA-7 (NCT02278120), the first large randomized phase III clinical trial dedicated to investigating a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) plus ET vs ET + placebo (PBO) in pre- or perimenopausal patients with HR+/HER2− ABC, previously demonstrated a statistically significant improvement in OS with the addition of ribociclib (RIB) to ET vs PBO + ET (median, not reached vs 40.9 months; HR, 0.71 [95% CI, 0.54-0.95]; P = .00973; Im SA, et al. N Engl J Med. 2019). This concluded the protocol-defined final analysis of OS and the patients and investigators were unblinded to their treatment assignment allowing patients on the PBO arm to cross-over to RIB treatment. Longer follow-up allows for more events to further characterize the long-term survival benefits. Here we report an exploratory update of OS after a minimum of ~ four years of follow-up, an additional 20 months since the last report. Methods: Pre- or perimenopausal patients with HR+/HER2− ABC were randomized 1:1 to receive RIB or PBO plus goserelin with either a nonsteroidal aromatase inhibitor (NSAI; letrozole or anastrozole) or tamoxifen. RIB is approved in combination with an NSAI in pre- or perimenopausal patients. Patients who had received a prior CDK4/6i or ET in the advanced setting were excluded. Patients who received ET in the (neo)adjuvant setting or ≤ 1 prior line of chemotherapy for advanced disease were eligible to enroll. Updated OS were evaluated by Cox proportional hazards model and summarized using Kaplan-Meier methods. Additional post-progression endpoints such as progression-free survival 2 (PFS2), time to chemotherapy (CT) and CT-free survival were also evaluated and summarized. Results: The data cutoff for this updated OS analysis was 29 June 2020, and the median follow-up was 53.5 mo (min, 46.9 mo). These updated results with extended follow-up demonstrated an OS benefit with RIB + ET vs PBO + ET (median, 58.7 vs 48.0 mo; HR, 0.76 [95% CI, 0.61-0.96]). In patients receiving an NSAI, a similar OS benefit was observed with RIB + NSAI vs PBO + NSAI (median, 58.7 vs 47.7 mo; HR, 0.80 [95% CI, 0.62-1.04]). The survival benefit shown in subgroup analyses was consistent with the intent-to-treat (ITT) population. PFS2, time to chemotherapy (CT), and CT-free survival for the ITT and NSAI populations are in the Table. Among the patients who discontinued study treatment, 77.3% and 78.1% in the RIB + ET vs PBO + ET arms received a subsequent antineoplastic therapy, respectively, and 12.9% and 26.1% received a subsequent line of CDK4/6i. Additionally there were 15 patients in the PBO arm that crossed over to the RIB arm following unblinding and prior to disease progression. Conclusions: With an extended follow-up of more than 4 years, RIB + ET continued to demonstrate a clinically relevant OS benefit compared with ET alone in pre- or perimenopausal patients with a median OS ~5 years with RIB +ET in HR+/HER2− ABC. A similar benefit with RIB was observed for PFS2, time to CT, and CT-free survival. ITTNSAI cohortRIB + ETn=335PBO + ETn=337RIB + NSAIn=248PBO + NSAIn=247PFS2Events, n (%)177 (52.8)221 (65.6)131 (52.8)159 (64.4)Median, mo44.231.043.630.4HR (95% CI)0.68 (0.56-0.83)0.69 (0.55-0.87)Time to first CTEvents, n (%)144 (43.0)173 (51.3)107 (43.1)129 (52.2)Median, mo50.936.850.936.0ITT HR(95% CI)0.69 (0.56-0.87)0.66 (0.51-0.85)CT-free survivalEvents, n (%)190 (56.7)236 (70.0)139 (56.0)169 (68.4)Median, mo42.426.442.525.9HR (95% CI)0.67 (0.55-0.81)0.64 (0.51-0.81) Citation Format: Debu Tripathy, Seock-Ah Im, Marco Colleoni, Fabio Franke, Aditya Bardia, Nadia Harbeck, Sara Hurvitz, Louis Chow, Joohyuk Sohn, Keun Seok Lee, Saul Campos-Gomez, Rafael Villanueva Vazquez, Kyung Hae Jung, K Govind Babu, Paul Wheatley-Price, Michelino De Laurentiis, Young-Hyuck Im, Sherko Kümmel, Nagi El-Saghir, Mei-Ching Liu, Sharonjeet Kaur, Claudia Gasch, Craig Wang, Yongyu Wang, Arunava Chakravartty, Yen-Shen Lu. Updated overall survival (OS) results from the phase III MONALEESA-7 trial of pre- or perimenopausal patients with hormone receptor positive/human epidermal growth factor receptor 2 negative (HR+/HER2−) advanced breast cancer (ABC) treated with endocrine therapy (ET) ± ribociclib [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PD2-04.