A Study of Xevinapant With Cisplatin and Radiation Therapy After Surgery in People With Head and Neck Cancer

Inclusion Criteria: * Age ≥ 18 on the day of signing of the consent form * ECOG 0-1 * Able to swallow liquids or has an adequately functioning feeding tube, gastrostomy, or jejunostomy placed * Squamous cell carcinoma of the head and neck (excluding lip) \* o Eligible primary tumor sites will include the maxillary sinus, oral cavity, HPV-negative oropharynx, larynx, and hypopharynx. * Gross total resection of known disease at the time of surgery within 10 weeks of registration. All efforts will be made to begin treatment within 6 weeks of surgery. * At least one of the following criteria * Close surgical margin (\<5mm) AND ≥2 additional intermediate risk factors (T3 or T4, multiple lymph nodes, LVI, PNI) * Positive margins not eligible for re-resection (defined as \<1mm) * Extranodal extension * Evidence of early gross recurrence on radiation planning scans after definitive intent surgical resection Patients with evidence of gross locoregional disease at the time of radiation are strongly advised to have biopsy confirmation. This requirement may be waived by the PI or the co-PIs. If a biopsy is not performed, patients must meet one of the other entry criteria (close surgical margin and 2 or more additional intermediate risk factors; positive margins not eligible for re-resection; extranodal extension). * Adequate hematologic, renal, and hepatic function as indicated by: * Absolute neutrophil count ≥ 1 500 cells/μL * Platelets ≥ 100 000 cells/μL * Hemoglobi…