Onvansertib for the Treatment of Recurrent or Refractory Chronic Myelomonocytic Leukemia and Myelodysplastic Syndrome/MPN Overlap Neoplasms

Inclusion Criteria: PRE-REGISTRATION - INCLUSION CRITERIA: * Age \>= 18 years * History of World Health Organization (WHO)-defined diagnosis of proliferative CMML (WBC count \>= 13,000/mm\^3 at time of diagnosis), or MDS/MPN overlap neoplasm with WBC count \>= 13,000/mm3 at time of diagnosis (atypical CML and MDS/MPN-NOS). * NOTE: Hydroxyurea or hypomethylating induced leukopenia does not preclude inclusion. Discussion of WBC \< 13,000/mm\^3 due to treatment at the time of preregistration must be discussed with the Sponsor/Principal Investigator * Relapsed/refractory following treatment with hydroxyurea; or at least 4 cycles of treatment with hypomethylating agents; or who are intolerant of treatment with either therapy. Note: Prior exposure to erythropoiesis stimulating agents is allowed. Hydroxyurea may continue for the first 28 days on study. Continuation of hydroxyurea beyond the first cycle must be discussed with the Sponsor/Principal Investigator * Willing and able to review, understand, and provide written consent before starting any study-specific procedures or therapy * Willing to return to enrolling institution for follow-up (during the active monitoring phase of the study) * Willingness to provide mandatory bone marrow specimens for correlative research * Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0, 1 or 2 * Recovered to grade 1 or baseline or established as sequelae from all toxic effects of previous therapy except alopecia * Platelet …