This is a non-randomized open label multi-center study. Patients with high-risk myeloproliferative neoplasms (systemic mastocytosis [SM], advanced symptomatic primary eosinophilic disorder [PED], myelofibrosis [MF], and chronic myelomonocytic leukemia [CMML]) will be treated with SL-401, which will be administered as a brief intravenous infusion for 3 consecutive days initially every 21 days for 4 cycles; every 28 days for cycles 5-7; then every 42 days. Stage 1 will consist of a period in which several doses of SL-401 are evaluated. The Stage 2 portion will enroll up to 30 patients each with one of the two myeloproliferative malignancies: MF and CMML. In entirety, the Stage 2 portion will consist of up to 60 patients who will be treated at a maximum tolerated dose or maximum tested dose in which multiple dose-limiting toxicities are not observed (identified in Stage 1).