This study evaluates navtemadlin as maintenance treatment for patients with advanced or recurrent endometrial cancer (EC) who have achieved complete response or partial response on chemotherapy.

The study will be conducted in 2 parts. Part 1 will evaluate safety and efficacy of two different doses of navtemadlin alongside an observational control arm to determine the Phase 3 navtemadlin dose. Part 2 will evaluate the efficacy and safety of navtemadlin Phase 3 dose compared to placebo.

Inclusion Criteria:

• ECOG 0-1
• Histologically or cytologically confirmed diagnosis of endometrial cancer documented as TP53WT
• Subjects with advanced or recurrent disease must have completed a single line of up to 6 cycles of taxane-platinum based chemo and achieved a CR or PR per RECIST V1.1
• Adequate hematologic, hepatic and renal function (within 14 days)

Exclusion Criteria:

• Has any sarcomas or small-cell carcinomas with neuroendocrine differentiation
• Prior immune therapy, cytokine therapy, or any investigational therapy (within 28 days)
• Indwelling surgical drains
• Grade 2 or higher QTc prolongation
• History of major organ transplant
• History of bleeding diathesis; major hemorrhage or intracranial hemorrhage (within 24 weeks)