Study of REGN6569 and Cemiplimab in Adult Patients With Advanced Solid Tumor Malignancies

Key Inclusion Criteria: 1. Dose escalation cohorts: Advanced stage (unresectable or metastatic) solid tumor malignancy, confirmed histologically or cytologically as defined in the protocol 2. Dose expansion cohorts: Advanced stage (unresectable or metastatic) head and neck squamous cell carcinoma (HNSCC), confirmed histologically or cytologically. Patients must have evidence of progression on anti-Programmed death-1 (receptor)/Programmed death ligand 1 (PD-1/PD-L1) blockade either as monotherapy or in combination with other therapies, as defined in the protocol 3. Mandatory biopsies: Able and willing to provide tumor tissue at baseline and while on treatment, with at least 1 soft tissue lesion amenable to biopsy by ultrasound or computed tomography (CT)-guided biopsy or under direct visualization as defined in the protocol Key Exclusion Criteria: 1. Has previously received GITR-targeted therapy 2. Has received any previous systemic biologic therapy within 5 half-lives of first dose of study therapy as defined in the protocol 3. Has any condition that requires ongoing/continuous corticosteroid therapy (\>10 mg prednisone/day or anti-inflammatory equivalent) within 14 days prior to the first dose of study therapy 4. Has ongoing or recent (within 5 years) evidence of significant autoimmune disease that required treatment with systemic immunosuppressive treatments as defined in the protocol 5. Has a known history of, or any evidence of, interstitial lung disease, or active, non-infectious pneumonitis in the past 5 years. A history of radiation pneumonitis in the radiation field is permitted as long as pneumonitis resolved ≥6 months prior to first dose of study therapy 6. Has uncontrolled infection with human immunodeficiency virus, hepatitis B or hepatitis C infection, or diagnosis of immunodeficiency 7. Has received a live vaccine within 4 weeks of planned start of study medication. For dose escalation only: Has received a COVID-19 vaccination within 1 week of planned start of study medication or for which the planned COVID-19 vaccinations would not be completed 1 week prior to start of study. 8. Has had prior allogeneic stem cell transplantation or received organ transplants at any time, or autologous stem cell transplantation 9. Has a history of malignancy within 2 years of date of first planned dose on study as defined in the protocol Note: Other protocol-defined Inclusion/ Exclusion criteria apply