Background: A novel genomically defined subset of higher-risk MDS (HR-MDS) patients with an actionable target characterized by overexpression of RARA has been identified (McKeown 2017). Approximately 30% of HR-MDS patients are RARA-positive (RARA+) by a blood-based biomarker test (Vigil, 2017). Biologically targeted therapy with tamibarotene, an oral selective RARα agonist, has potential to provide clinical benefit for RARA+ HR-MDS patients, irrespective of mutation or cytogenetic risk. Initial clinical data in RARA+ R/R HR-MDS patients treated with tamibarotene showed myeloid differentiation, improved blood counts, and reduced bone marrow blasts, including one patient who achieved marrow complete remission with hematologic improvement (Jurcic 2017). Tamibarotene in combination with azacitidine (aza) showed high complete response rates (CR) with rapid onset of response in RARA+ newly diagnosed (ND) unfit AML patients, including those with low blast count (≤ 30%) AML, and a majority of patients achieved or maintained transfusion independence (de Botton 2020). Tamibarotene/aza was generally well tolerated with no increase in myelosuppression compared to aza alone (de Botton 2020). Historical precedent demonstrating similar clinical outcomes in HR-MDS and AML, particularly low blast count AML (Estey 2021), supports further development of tamibarotene/aza in HR-MDS to improve clinical outcomes of standard of care treatment with hypomethylating agents (HMAs). Methods: This Phase 3, multi-center, randomized, double-blind, placebo-controlled study will compare activity of tamibarotene/aza to placebo/aza in RARA+ patients with ND HR-MDS. The primary objective is to characterize and compare the CR rate of tamibarotene/aza vs placebo/aza, with secondary objectives to characterize ORR, EFS, OS, transfusion independence rate, time to and duration of initial and complete responses, and safety. Approximately 190 patients will be randomized 2:1, providing 90% power to detect the difference in CR rates between the experimental and control arms, with a one-sided alpha of 0.025. Patients must be RARA+ based on the investigational biomarker test, ND with HR-MDS by WHO classification (Arber 2016), classified by IPSS-R risk category as very high, high, or intermediate risk with a blast count > 5% at study entry. Patients suitable for transplant at screening, or with prior treatment for MDS with any HMA, chemotherapy or transplant are excluded. Aza will be administered at 75 mg/m² IV/SC daily on days 1-7 (or 1-5, 8-9) followed by tamibarotene/placebo at 6 mg BID orally days 8-28 of each 28-day cycle. Response will be assessed per modified IWG MDS criteria (Cheson 2006). The SELECT-MDS-1 trial opened in February 2021, recruitment is ongoing, with sites located in North America, Israel, and Europe. Clinical trial information: NCT04797780.