Enrollment challenged by availability of other treatment options

The goal of this clinical study is to learn about the bispecific antibody, acasunlimab (also known as GEN1046) in combination with the cancer drug pembrolizumab for treatment of participants with incurable endometrial cancer (cancer of the womb). The main questions the study aims to answer are:

• How well acasunlimab in combination with pembrolizumab works against endometrial cancer
• What are the potential side effects participants may experience when they are treated with acasunlimab in combination with pembrolizumab

Participants will receive both acasunlimab and pembrolizumab. All participants will receive active drug; no one will receive placebo. participants will participate in 1 of 2 cohorts. A participant will receive study treatment up to a maximum of 24 months. The study duration (including screening, treatment, and follow-up) for each participant will be about 39 months.

This is an open-label multicenter study in participants with advanced (unresectable and/or metastatic) endometrial cancer to evaluate the safety and clinical activity of acasunlimab (GEN1046) in combination with immunotherapy.

The trial consists of two cohorts:

• Cohort A (cohort closed)
• Cohort B

The study will enroll approximately 80 participants in Cohort A and B (approximately 40 participants in each cohort).

Inclusion Criteria:

• Have a histologically confirmed diagnosis of advanced (unresectable, recurrent, and/or metastatic) endometrial carcinoma that is incurable and for which prior standard first-line treatment has failed.
• Prior to Cycle 1 Day 1 (C1D1), documentation of tumor dMMR/MSI-H status must be available based on local testing.
• Must have progressed on or after at least 1 (but no more than 2) prior line(s) of a systemic chemotherapy regimen for unresectable and/or metastatic endometrial cancer of which at least 1 regimen of platinum-based treatment unless participant is ineligible for or intolerant to platinum.
• Cohort A only: Must be treatment naive for CPIs including PD-1 or PD-L1 inhibitors and other immune CPIs (eg, anti-CTLA-4, anti-LAG3, anti-TIGIT).
• Cohort B only: Must have received and progressed on or after prior treatment with a PD-1/PD-L1 inhibitor alone or in combination. Moreover, the participant's duration of CPI containing treatment and best overall response (BOR) is known, and participant has received a minimum of 2 cycles of CPI.

Exclusion Criteria:

• Histological diagnosis of carcinosarcoma, malignant mixed Műllerian tumor, endometrial leiomyosarcoma, or endometrial stromal sarcomas.

• Ongoing or active infection requiring intravenous treatment with anti-infective therapy, or any ongoing systemic inflammatory condition requiring further diagnostic work-up or management during screening.

• Any prior treatment with any type of antitumor vaccine, or autologous cell immunotherapy.

• Radiotherapy within 14 days prior to first dose of acasunlimab. Note: palliative radiotherapy will be allowed for local pain control under certain conditions.

• Treatment with an anticancer agent, including investigational vaccines within 28 days before or 5 times t1/2, whichever is shorter, prior to the planned first dose of trial treatment or is currently enrolled in an interventional trial.

• Prior treatment with live, attenuated vaccines within 30 days prior to initiation of trial treatment.

• Received granulocyte colony-stimulating factor (G-CSF) or granulocyte-macrophage colony-stimulating factor (GM-CSF) support within 4 weeks before the planned first dose of trial treatment.

• Cohort A only: Prior exposure to immune CPIs other than anti-PD-1/anti-PD-L1 (eg, anti-CTLA-4, anti-LAG3, anti-TIGIT) or agents directed at costimulatory T-cell receptors (eg, 4-1BB, OX40)

• Cohort B only:

  • Known history of Grade 3 or higher immune-related adverse events (irAEs) that led to treatment discontinuation of a prior immunotherapy treatment
  • Exposure to any of the following prior therapies/treatments within the specified timeframes:
  • Prior exposure to immune CPIs other than anti-PD-1/anti-PD-L1 (eg, anti-CTLA-4, anti-LAG3, anti-TIGIT) or agents directed at costimulatory T-cell receptors (eg, 4-1BB, OX40)
  • PD-1/PD-L1 antibody within 28 days before the planned first dose of trial treatment

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.