A Phase 1b, Open-Label Study Of REC-617, A Selective CDK7 Inhibitor, In Patients With Metastatic Or Unresectable RB1-Negative Leiomyosarcoma After Prior Systemic Therapy

Eligibility Criteria A. Disease Characteristics * Histologically confirmed leiomyosarcoma (any site of origin). * RB1-negative tumor, as assessed by immunohistochemistry (IHC) with a cutoff of 0% versus any expression Participants will have measurable diseases by RECIST 1.1. Previously ablated/ radiated/treated areas can only be counted towards measurable disease if there is unequivocal progression after directed therapy. * Participant will have tumor lesion(s) or metastases amenable to biopsy, excluding bone metastases, as confirmed by a radiologist, if appropriate, and as deemed safe by the Investigator. If there is only one target lesion per RECIST 1.1 which is accessible to biopsy, this lesion should be at least 2cm. B. Prior and Current Therapy Requirements * At least one prior line of systemic therapy. C. Participants Characteristics * Age ≥18 years. Because no dosing or adverse event data are currently available on the use of REC-617 in Participants \<18 years of age, children are excluded from this study. * ECOG performance status ≤ 2 (Karnofsky ≥60%,). * Life expectancy of \>3 months, as determined by the investigator. * Ability to swallow and retain oral medication. * Ability to understand and the willingness to sign a written informed consent document and comply with study procedures. D. Organ and Marrow Function Requirements Participants will have adequate organ and marrow functions as defined below: Hemoglobin ≥8.5 g/dL. Absolute neutrophil count ≥1,000/mcL…