Background: BET proteins are epigenetic readers that regulate expression of critical oncoproteins involved in the pathophysiology of hematologic malignancies, including MF. INCB057643 is a small-molecule BET inhibitor evaluated as monotherapy and in combination with ruxolitinib (RUX) in pts with advanced malignancies in 2 previous phase 1/2 clinical trials. Methods: This ongoing phase 1, 3+3 dose-escalation/expansion study (NCT04279847) evaluates safety and tolerability of INCB057643 (4 mg once daily [qd]; escalation up to 12 mg qd) in pts aged ≥18 years as (1) monotherapy (part 1) in R/R MF, myelodysplastic syndromes (MDS), or MDS/myeloproliferative neoplasm (MPN) overlap syndromes (MDS/MPN) or (2) added to RUX (part 2) in pts with MF and suboptimal response to RUX. The primary endpoint is safety and tolerability, including identification of dose-limiting toxicities (DLTs). Results: 13 pts have been treated in part 1 (4 mg, n=6; 8 mg, n=4; 10 mg, n=1; 12 mg, n=2), and 3 pts received 4 mg + RUX in part 2 (overall age range, 50–79 years; men, n=9; study treatment duration range, 15–314 days). 12 pts had MF, and 4 had MDS/MPN. All 6 pts in the 4-mg cohort discontinued treatment (3 for progressive disease [PD]; MF, n=2; MDS/MPN, n=1); 1 pt with MF in the 12-mg cohort discontinued for thrombocytopenia. The other 9 pts remain on treatment. Thrombocytopenia was the most common treatment-emergent adverse event (TEAE; n=9; Table) and the only TEAE leading to discontinuation (n=3). Grade ≥3 TEAEs occurring in ≥1 pt were thrombocytopenia (n=4), anemia (n=3), and hypokalemia (n=2). There were 8 serious AEs across 4 pts, with only COVID-19 occurring in >1 pt (n=2); all but one (pneumonia) were considered unrelated to study treatment. There were 2 DLTs (thrombocytopenia [MDS/MPN pt] and hyperbilirubinemia [MF pt]; both 12-mg cohort) and 2 deaths (both 4-mg cohort due to PD [MF, n=1; MDS/MPN, n=1]). Conclusions: Treatment with INCB057643 monotherapy (4 and 8 mg qd) and in combination (4 mg qd) with RUX was generally well tolerated in this pt population. The 12-mg qd monotherapy dose was not tolerated and caused 2 DLTs. There were no treatment-related fatal events. Dose finding in part 1 is ongoing with 10 mg qd, after which a recommended phase 2 dose will be declared. Combination dose escalation is also ongoing. Preliminary efficacy including spleen size and symptoms will be available for presentation. Clinical trial information: NCT04279847.