Safety and Tolerability Study of INCB057643 in Participants With Myelofibrosis and Other Advanced Myeloid Neoplasms

Inclusion Criteria: * Age 18 years and older at the time of signing the informed consent. * Part 1 Monotherapy: Participants with confirmed diagnosis of relapsed or refractory MF (primary, or post-PV and post-ET), MDS, MDS/MPN, or ET who have received at least 1 prior line of therapy; are either refractory, relapsed, or intolerant to the last therapy; and there is no available therapy that would provide clinical benefit in the opinion of the investigator. * a. MF with measurable disease (palpable spleen and symptoms) as defined in the protocol and risk category of intermediate 2 or high according to DIPSS. MF participants must have received a JAK inhibitor(s), such as ruxolitinib. * b. ET participants should have disease refractory to hydroxyurea as defined by the protocol. * Part 2 Combination with ruxolitinib. * a. Primary MF or secondary MFs (post-PV MF and post-ET MF), histologically or cytologically confirmed, with measurable disease (palpable spleen and symptoms) as defined in the protocol, either currently receiving ruxolitinib with suboptimal response or JAKi-naive. * b. Suboptimal response is defined as currently being treated with ruxolitinib monotherapy at a stable dose for ≥ 8 weeks immediately preceding the first dose of study treatment. One dose reduction due to toxicities within 8 weeks prior to Study Day 1 is permitted. * c. JAKi-naive is defined as those participants that have no prior use of any JAK inhibitor, including ruxolitinib, and; * d. …