Background: SY-1425 (tamibarotene) is an orally available, synthetic retinoid approved in Japan for the treatment of relapsed/refractory (R/R) APL. SY-1425 is a more potent and selective retinoic acid receptor alpha (RARα) agonist with improved pharmacologic properties compared to all-trans retinoic acid (ATRA) including increased half-life and lack of metabolism by CYP26A1 resulting in extended relative exposures. SY-1425 binding to RARα relieves pathogenic repression of myeloid differentiation. Super-enhancers associated with RARA and upregulation of RARA expression correlate with increased sensitivity to SY-1425 in vitro and predict for response to SY-1425 with induced differentiation and reduced proliferation in RARA-high PDX AML models, but not in RARA-low models. SY-1425 also induces the RARα target gene DHRS3 in RARA-high AML cell lines. This study is designed to demonstrate pharmacodynamic (PD) and clinical effects of SY-1425 in non-APL AML and MDS patients (pts) positive for the RARA super-enhancer associated biomarker or exploratory RARA pathway biomarker, IRF8. Methods: This study is enrolling pts with R/R AML, R/R higher-risk MDS, newly-diagnosed AML ≥60 yrs unlikely to respond to or tolerate standard therapy, and transfusion dependent lower-risk MDS pts without del 5q who are unlikely to respond to or have failed ESAs. Pts must be biomarker positive based on centralized testing of tumor cells from blood. All pts receive SY-1425 at 6 mg/m²/day PO with continuous twice daily dosing. Primary objectives are to characterize the activity of SY-1425 by ORR in AML and higher-risk MDS pts or transfusion independence in lower-risk MDS pts. Secondary objectives include event-free and relapse-free survival, duration of response, overall survival, hematologic improvement and safety. PD evaluation includes induction of DHRS3 and expression of myeloid differentiation markers. Target enrollment is 80 pts. This trial opened in September 2016. Through a protocol amendment, SY-1425 treatment in combination with azacitidine will also be evaluated. ClinicalTrials.gov identifier: NCT02807558.