| E.1 Medical condition or disease under investigation | 
| E.1.1 | Medical condition(s) being investigated | 
| Resected squamous cell carcinoma of the head and neck |  | 
| E.1.1.1 | Medical condition in easily understood language | 
| Resected squamous cell carcinoma of the head and neck |  | 
| E.1.1.2 | Therapeutic area | Diseases [C] - Cancer [C04] | 
| MedDRA Classification | 
| E.1.2 Medical condition or disease under investigation | 
| 
| E.1.2 | Version | 21.0 |  
| E.1.2 | Level | PT |  
| E.1.2 | Classification code | 10060121 |  
| E.1.2 | Term | Squamous cell carcinoma of head and neck |  
| E.1.2 | System Organ Class | 10029104 - Neoplasms benign, malignant and unspecified (incl cysts and polyps) |  | 
| E.1.3 | Condition being studied is a rare disease | No | 
| E.2 Objective of the trial | 
| E.2.1 | Main objective of the trial | 
| The purpose of this study is to demonstrate improvement in Disease-Free Survival (DFS) with xevinapant compared to placebo when added to RT irrespective of subsequent anticancer therapy. |  | 
| E.2.2 | Secondary objectives of the trial | 
| - To demonstrate improvement in OS with xevinapant compared to placebo when added to RT followed by subsequent anticancer therapy - To evaluate time to subsequent cancer treatments in participants treated with xevinapant compared to placebo when added to RT
 - To evaluate the safety and tolerability of xevinapant compared to placebo when added to RT
 - To evaluate tolerability of xevinapant compared to placebo when added to RT followed by xevinapant monotherapy in terms of patient-reported head and neck pain, swallowing, and speech as measured by the EORTC H&N35 sub scales, patient-reported fatigue, physical function, and global health status as measured by the EORTC QLQ C-30 sub scales and EQ-5D-5L VAS
 |  | 
| E.2.3 | Trial contains a sub-study | No | 
| E.3 | Principal inclusion criteria | 
| • Participants with Eastern Cooperative Oncology Group Performance Status (ECOG PS) 0-2 and able to tolerate standard of care IMRT treatment according to Investigator assessment
 • Participants with histologically confirmed squamous cell carcinoma with one of the following primary sites: oral cavity, oropharynx, hypopharynx or larynx. Participants have received surgery with curative intent on these sites in the past 4 to 10 weeks before start of treatment
 (Cycle 1 Day 1)
 • Oropharynx (OPC) participants must have known human papillomavirus (HPV) status as determined by p16 expression using immunohistochemistry ICH)
 • Participants with no residual disease by computed tomography (CT) or magnetic resonance imaging (MRI) and have a high risk of relapse with 1 or 2 of the following criteria, confirmed by local histopathology:
 • nodal extra-capsular extension (ECE) and positive resection margins (R1 or close margin less than or equal to (<=) 1 millimeter (mm)
 • Are unfit to receive high-dose cisplatin by meeting one or more of the following criteria: estimated glomerular filtration rate (eGFR) < 60
 milliliter per minute per 1.73 meter square (mL/min /1.73 m^2); History of hearing impairment, defined as Grade >= 2 audiometric hearing loss or tinnitus Grade >=2. An audiogram is not required if one of the other criteria meets unfitness to receive high-dose cisplatin; Peripheral neuropathy > = Grade 2 and if >= 70 years, unfit according to G8 questionnaire (Score <= 14) or ineligible for cisplatin treatment due
 to age limit according to national guidelines.
 • Participants with adequate hematologic, renal and hepatic function as defined in the protocol
 • Other protocol-defined inclusion criteria could apply
 |  | 
| E.4 | Principal exclusion criteria | 
| • Any condition, including any uncontrolled disease state other than SCCHN that in the Investigator’s opinion constitutes an inappropriate risk or a contraindication for participation in the study or that could interfere with the study objectives, conduct, or evaluation • Participant with incomplete surgery
 • Primary tumor of nasopharyngeal, paranasal sinuses, nasal cavity, salivary, thyroid or parathyroid gland, skin or unknown primary site
 • Prior definitive, neoadjuvant, concurrent or adjuvant (C)RT to the head and neck region which may jeopardize the primary tumor irradiation plan,or any other prior SCCHN systemic treatment, including investigational agents
 • Participation in any interventional clinical study within 28 days prior to screening or during participation in this study
 • Known contraindication to undergoing positron emission tomography with 18F-FDG-PET-CT scans, or both contrast-enhanced MRI and contrast enhanced CT scans
 • Known allergy to Xevinapant (Debio 1143) or any excipient known to be present in Xevinapant (Debio 1143) or in the placebo formulation
 • Participants with recurrent or metastatic disease
 • Other protocol-defined exclusion criteria could apply
 |  | 
| E.5 End points | 
| E.5.1 | Primary end point(s) | 
| Disease-Free Survival (DFS) 
 |  | 
| E.5.1.1 | Timepoint(s) of evaluation of this end point | 
| Time from the date of randomization to the date of first record of disease progression or death, assessed up to 5 years 
 |  | 
| E.5.2 | Secondary end point(s) | 
| 1. Overall Survival (OS) 2. Time to Subsequent Cancer Treatments
 3. Occurrence of  Adverse Events (AEs) and Treatment-related AEs
 4. Change from Baseline in European Organization for Research and Treatment of Cancer Quality of Life Head and Neck Module (EORTC QLQ-HN35) Score
 5. Change from Baseline in European Organization for research and Treatment of Cancer Quality of Life Core Questionnaire (EORTC QLQ-C30) Score
 6. Change from Baseline in EuroQOL 5 Dimension 5 Level Health-Related Quality of Life Measure Visual Analog Scale Score (EQ-5D-5L VAS)
 |  | 
| E.5.2.1 | Timepoint(s) of evaluation of this end point | 
| 1. Time from randomization to death from any cause, assessed up to 5 years 2. Time from randomization to the start of first subsequent cancer treatment, assessed up to 5 years
 3. Time from randomization until end of study (assessed up to 5 years)
 4. Baseline, Cycle 4 Day 1 [at Week 10 (each cycle is 3 weeks)] and End of treatment (at Week 20)
 5. Baseline, Cycle 4 Day 1 [at Week 10 (each cycle is 3 weeks)] and End of treatment (at Week 20)
 6. Baseline, Cycle 4 Day 1 [at Week 10 (each cycle is 3 weeks)] and End of treatment (at Week 20)
 |  | 
| E.6 and E.7 Scope of the trial | 
| E.6 | Scope of the trial | 
| E.6.1 | Diagnosis | No | 
| E.6.2 | Prophylaxis | No | 
| E.6.3 | Therapy | No | 
| E.6.4 | Safety | Yes | 
| E.6.5 | Efficacy | Yes | 
| E.6.6 | Pharmacokinetic | Yes | 
| E.6.7 | Pharmacodynamic | No | 
| E.6.8 | Bioequivalence | No | 
| E.6.9 | Dose response | No | 
| E.6.10 | Pharmacogenetic | No | 
| E.6.11 | Pharmacogenomic | Yes | 
| E.6.12 | Pharmacoeconomic | No | 
| E.6.13 | Others | No | 
| E.7 | Trial type and phase | 
| E.7.1 | Human pharmacology (Phase I) | No | 
| E.7.1.1 | First administration to humans | No | 
| E.7.1.2 | Bioequivalence study | No | 
| E.7.1.3 | Other | No | 
| E.7.1.3.1 | Other trial type description |  | 
| E.7.2 | Therapeutic exploratory (Phase II) | No | 
| E.7.3 | Therapeutic confirmatory (Phase III) | Yes | 
| E.7.4 | Therapeutic use (Phase IV) | No | 
| E.8 Design of the trial | 
| E.8.1 | Controlled | Yes | 
| E.8.1.1 | Randomised | Yes | 
| E.8.1.2 | Open | No | 
| E.8.1.3 | Single blind | No | 
| E.8.1.4 | Double blind | Yes | 
| E.8.1.5 | Parallel group | Yes | 
| E.8.1.6 | Cross over | No | 
| E.8.1.7 | Other | No | 
| E.8.2 | Comparator of controlled trial | 
| E.8.2.1 | Other medicinal product(s) | No | 
| E.8.2.2 | Placebo | Yes | 
| E.8.2.3 | Other | No | 
| E.8.2.4 | Number of treatment arms in the trial | 2 | 
| E.8.3 | The trial involves single site in the Member State concerned | No | 
| E.8.4 | The trial involves multiple sites in the Member State concerned | Yes | 
| E.8.4.1 | Number of sites anticipated in Member State concerned | 5 | 
| E.8.5 | The trial involves multiple Member States | Yes | 
| E.8.5.1 | Number of sites anticipated in the EEA | 110 | 
| E.8.6 Trial involving sites outside the EEA | 
| E.8.6.1 | Trial being conducted both within and outside the EEA | Yes | 
| E.8.6.2 | Trial being conducted completely outside of the EEA | No | 
| E.8.6.3 | If E.8.6.1 or E.8.6.2 are Yes, specify the regions in which trial sites are planned | 
| Argentina |  
| Switzerland |  
| Taiwan |  
| Brazil |  
| Canada |  
| China |  
| Georgia |  
| India |  
| Israel |  
| Japan |  
| Korea, Republic of |  
| Mexico |  
| Turkey |  
| United Kingdom |  
| United States |  
| Austria |  
| Belgium |  
| Czechia |  
| Denmark |  
| Finland |  
| France |  
| Germany |  
| Greece |  
| Italy |  
| Netherlands |  
| Norway |  
| Poland |  
| Portugal |  
| Romania |  
| Spain |  
| Sweden |  | 
| E.8.7 | Trial has a data monitoring committee | Yes | 
| E.8.8 | Definition of the end of the trial and justification where it is not the last
                        visit of the last subject undergoing the trial |  | 
| E.8.9 Initial estimate of the duration of the trial | 
| E.8.9.1 | In the Member State concerned years | 7 | 
| E.8.9.1 | In the Member State concerned months | 6 | 
| E.8.9.1 | In the Member State concerned days |  | 
| E.8.9.2 | In all countries concerned by the trial years | 7 | 
| E.8.9.2 | In all countries concerned by the trial months | 6 |