Background: The prognosis for endometrial cancer (EC) can be favorable when diagnosed in early stages, but prognosis and overall survival are poor in patients with advanced or recurrent EC. First-line standard of care for patients with advanced or recurrent EC is paclitaxel and carboplatin chemotherapy; however, there is a need for more effective and tolerable therapies. In the phase Ib/II trial KEYNOTE-146, which assessed the PD-1 inhibitor pembrolizumab (pembro) in combination with the multikinase inhibitor lenvatinib, an objective response rate (ORR) of 38% was observed (N=108) in patients with previously treated advanced EC. ENGOT-en9/LEAP-001 (NCT03884101) is a randomized, open-label, active-controlled, phase III study investigating pembro + lenvatinib vs chemotherapy in patients with newly diagnosed advanced or recurrent EC. Methods: Patients with newly diagnosed advanced (stage III-IV) or recurrent EC not previously treated with antiangiogenic agents; systemic chemotherapy (unless within a chemoradiation regimen); PD-1, PD-L1, or PD-L2 inhibitors; or other T-cell receptor–targeted therapies will be eligible. Patients will be randomized 1:1 to receive pembro 200 mg every 3 wk (Q3W) + lenvatinib 20 mg daily or paclitaxel 175 mg/m² Q3W + carboplatin AUC 6 Q3W. Randomization will be stratified on the basis of proficient vs deficient mismatch repair (pMMR vs dMMR) status. The pMMR population will be further stratified by prior chemoradiation (yes vs no), measurable disease (yes vs no), and ECOG performance status (0 vs. 1). Patients will continue on treatment for up to 35 cycles of pembro vs 7 cycles of chemotherapy or until initiation of a new anticancer treatment, unacceptable adverse events, or withdrawal of consent. Primary study endpoints are progression-free survival (per RECIST v1.1 by blinded independent central review) and overall survival. Secondary study endpoints are ORR, health-related quality of life, safety and tolerability, and lenvatinib pharmacokinetics. Exploratory endpoints will include disease control rate, clinical benefit rate, and duration of response. Enrollment is ongoing. Clinical trial information: NCT03884101