Study of Narazaciclib (ON 123300) Plus Letrozole in Endometrial Cancer and Other Gynecologic Malignancies

This is a phase 1/2a, open-label, multicenter study to evaluate the safety, tolerability and efficacy of escalating doses of narazaciclib (ON 123300) in combination with letrozole for patients with recurrent metastatic low-grade endometrioid endometrial cancer and other Gynecologic Malignancies. Pharmacokinetics and pharmacodynamics will also be assessed. In Phase 1, eligible patients will be enrolled to escalating dose cohorts. Cohorts will receive escalating doses of oral narazaciclib starting at 160 mg orally, once daily, in combination with letrozole 2.5 mg orally, once daily, in 28-day cycles in a typical 3 + 3 design. The dose of narazaciclib will be increased in 40 mg/day increments from cohort to cohort until the maximum tolerated dose (MTD) and/or the minimal biologically effective dose (MBED) of narazaciclib orally, once daily, in combination with letrozole 2.5 mg orally, once daily, is reached and the RP2D of the combination is established. Three to 6 patients will be enrolled per dose cohort in phase 1. In Phase 2a, narazaciclib and letrozole at the RP2D established in Phase 1 will be administered to approximately 30 eligible patients with documented recurrent metastatic LGEEC for 28-day cycles. Treatment will continue until disease progression, patient withdrawal, or unacceptable drug-related toxicity.