A Study of Pembrolizumab (MK-3475) Plus Platinum and Gemcitabine as First Line Treatment of Recurrent/Metastatic Head and Neck Squamous Cell Carcinoma (PIPER)

Inclusion Criteria: 1. Male/female participants who are at least 18 years of age on the day of signing informed consent with histologically confirmed diagnosis of R/M HNSCC that is considered incurable by local therapies will be enrolled in this study: * Subject may not have had prior systemic therapy administered in the recurrent or metastatic setting. Systemic therapy which was completed more than 6 months prior to signing consent if given as part of multimodal treatment for locally advanced disease is allowed. * The eligible primary tumour locations are oropharynx, oral cavity, hypopharynx, and larynx. * Subject may not have a primary tumour location site of nasopharynx (any histology). 2. A male participant must agree to use a contraception as detailed in Appendix 3: Contraceptive Guidance and Pregnancy Testing of this protocol starting with the first dose of study treatment through the treatment period and for at least 180 days after the last dose of study treatment and refrain from donating sperm during this period. 3. A female participant is eligible to participate if she is not pregnant (see Appendix 3: Contraceptive Guidance and Pregnancy Testing), not breastfeeding, and at least one of the following conditions applies: 1. Not a woman of childbearing potential (WOCBP) as defined in Appendix 3: Contraceptive Guidance and Pregnancy Testing OR 2. A WOCBP who agrees to follow the contraceptive guidance in Appendix 3: Contraceptive Guidance and Pregnancy Testingduring the treatment period and for at least 180 days after the last dose of study treatment. 4. The participant (or legally acceptable representative if applicable) willing and able to provides written informed consent for the trial. The participant may also provide consent for Future Biomedical Research. However, the subject may participate in the main trial without participating in Future Biomedical Research. 5. Have measurable disease based on RECIST 1.1. Lesions situated in a previously irradiated area are considered measurable if progression has been demonstrated in such lesions. 6. Archival or fresh tumor tissues must be available for evaluating relevant biomarkers. Newly obtained core needle or excisional biopsy of a tumor lesion not previously irradiated is preferred to archived tissue. If newly obtained samples cannot be obtained due to inaccessibility or patient safety concern, submission of paraffin block or formalin-fixed, paraffin embedded (FFPE) slides of up to 3 years prior to trial enrolment are acceptable (15 unstained slides of 5 microns in thickness). Refer to Section 6.1.5 for complete information on the tissue sample collection. 7. Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1. Evaluation of ECOG is to be performed within 7 days prior to the first dose of study intervention. 8. Have adequate organ function as defined in the following table (Table 3). Specimens must be collected within 10 days prior to the start of study intervention. * Absolute neutrophil count (ANC) ≥1500/µL * Platelets ≥100 000/µL * Hemoglobin ≥9.0 g/dL or ≥5.6 mmol/La * Creatinine ≤1.5 × ULN * Total bilirubin ≤1.5 ×ULN OR direct bilirubin ≤ULN for participants with total bilirubin levels \>1.5 × ULN * AST (SGOT) and ALT (SGPT) ≤2.5 × ULN (≤5 × ULN for participants with liver metastases) * International normalized ratio (INR) OR prothrombin time (PT), Activated partial thromboplastin time (aPTT) ≤1.5 × ULN unless participant is receiving anticoagulant therapy as long as PT or aPTT is within therapeutic range of intended use of anticoagulants Exclusion Criteria: 1. Has disease that is suitable for local therapy administered with curative intent. 2. Has progressive disease within six months of completion of curatively intended treatment for locoregionally advanced HNSCC. 3. Patient with an expected life expectancy of less than 3 months. 4. A WOCBP who has a positive urine pregnancy test within 72 hours prior to treatment allocation (see Appendix 4). If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required. 5. Has received prior therapy with an anti-PD-1, anti-PD-L1, or antiPDL2 agent or with an agent directed to another stimulatory or co-inhibitory T-cell receptor (eg, CTLA-4, OX40, CD137). 6. Has received prior systemic anti-cancer therapy including investigational agents within 4 weeks \[could consider shorter interval for kinase inhibitors or other short half-life drugs\] prior to start of study treatment. Note: Participants must have recovered from all AEs due to previous therapies to ≤Grade 1 or baseline. Participants with ≤Grade 2 neuropathy may be eligible. Participants with endocrine-related AEs Grade ≤2 requiring treatment or hormone replacement may be eligible Note: If the participant had major surgery, the participant must have recovered adequately from the procedure and/or any complications from the surgery prior to starting study intervention. 7. Has received prior radiotherapy within 2 weeks of start of study intervention. Participants must have recovered from all radiation-related toxicities, not require corticosteroids, and not have had radiation pneumonitis. A 1-week washout is permitted for palliative radiation (≤2 weeks of radiotherapy) to non-CNS disease. 8. Has received a live vaccine or live-attenuated vaccine within 30 days prior to the first dose of study drug. Administration of killed vaccines is allowed. 9. Is currently participating in or has participated in a study of an investigational agent or has used an investigational device within 4 weeks prior to the first dose of study intervention. Note: Participants who have entered the follow-up phase of an investigational study may participate as long as it has been 4 weeks after the last dose of the previous investigational agent. 10. Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy (in dosing exceed