Background: RIBO, an oral, selective inhibitor of CDK4/6 (CDK4/6i), is approved for use in combination with endocrine therapy (ET) in women with hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2–) ABC in multiple countries worldwide. Here we report interim safety and efficacy results from CompLEEment-1, a phase 3b trial evaluating RIBO+LET in an expanded patient (pt) population and the largest CDK4/6i trial in ABC to date. Methods: Pts with HR+, HER2– ABC, ≤1 line of prior chemotherapy (CT), and no prior ET for ABC received RIBO+LET. Treatment regimens and study endpoints have been reported previously (De Laurentiis, et al. ASCO 2018. Poster 1056). Results: Overall, 3,246 pts, who received ≥1 dose of study treatment, were evaluated (cut-off date, August 8, 2018). Median duration of RIBO exposure was 8.1 months (min, 0.0; max, 22.4). Demographic and baseline characteristics indicated a diverse population including men (1.2%), premenopausal women (22.2%), and patients aged ≥70 years (19.5%). Pts were well represented in terms of age, race, and disease history; 5.9% of pts received prior CT for ABC. The only non-hematologic any-cause grade ≥3 AEs ≥5% were increased alanine (7.3%) and aspartate (5.3%) aminotransferase. Treatment-related AEs (any grade) led to discontinuation in 11.4% of pts. Of the 51 (1.6%) on-treatment deaths, 26 were due to study indication and 25 to other reasons. The median time to progression was not estimable (NE) (95% confidence interval [CI], 17.1-NE). Overall response rate was 20.5% (95% CI, 19.1%-21.9%) and clinical benefit rate was 66.1% (95% CI, 64.4%-67.7%). Consistent mean change from baseline in Functional Assessment of Cancer Therapy – Breast Cancer questionnaire scores indicated that pts maintained their quality of life throughout treatment. Conclusions: This interim analysis demonstrates the safety, tolerability, and efficacy of RIBO+LET in a large, diverse cohort of pts with HR+, HER2– ABC who had not previously received ET for ABC. Safety results were consistent with those observed in RIBO pivotal studies and no new safety signals were observed. Clinical trial information: NCT02941926