Background: The combination of CDK4/6 inhibitor (CDK4/6i) + endocrine therapy (ET) provides consistent improvement in PFS and response rates compared with single-agent ET as first- or subsequent-line therapy in HR+, HER2− advanced breast cancer (ABC), but the optimal regimen postCDK4/6i progression, including the role of continued CDK 4/6 blockade, is unclear. Methods: TRINITI-1 is a Phase I/II, open-label trial (NCT02732119) of triplet therapy: ribociclib (RIB; CDK4/6i) + everolimus (EVE; mTORi) + exemestane (EXE; ET) in men or postmenopausal women with HR+, HER2− ABC that progressed on prior CDK4/6i and up to 3 lines of therapy (≥ 1 ET and ≤ 1 chemotherapy regimen). Phase I determined RP2D; Phase II assessed efficacy/safety of RIB 300 or 200 mg + EVE 2.5 or 5 mg + EXE 25 mg/day. Here we present the first results in the entire patient population who received this triplet regimen and the correlation of biomarkers with outcomes. Results: As of October 24, 2018, 95 patients were evaluable (ET refractory and postCDK4/6i) in Phases I (n = 17) and II (n = 78). Continuous RIB + EVE + EXE demonstrated clinical benefit at week 24 in 39 patients (41.1%), exceeding the predefined primary end point threshold (> 10%). ORR was 8.4% by investigator assessment, median PFS was 5.7 months, and 1-year PFS was 33%. AEs were consistent with known safety profile of RIB, EVE, and EXE. Most common AEs were neutropenia (all grades, 41.7%; grade 3/4, 31.3%), stomatitis (41.7%; 3.1%), and fatigue (35.4%; 1.0%). No grade 3/4 QTc prolongation was noted. ctDNA genotyping revealed patients with certain tumor alterations, eg ESR1, had shorter median PFS vs wild-type: 3.5 vs 6.9 mo (HR 1.76, 95% CI 1.01–3.05). Additional genomic results, including PIK3CA, will be presented. Conclusions: TRINITI-1 met its primary efficacy end point and is the first trial to demonstrate clinical benefit and tolerability of continuous triplet therapy with ET + mTORi + CDK4/6i in patients with ET-refractory HR+, HER2− ABC postCDK4/6i progression. Tumor genomic profile might impact the clinical outcome with triplet therapy and warrants additional research to guide rational therapy selection. Clinical trial information: NCT02732119