Background: Pembrolizumab as part of first-line treatment for patients (pts) with R/M HNSCC has improved survival. However, in order to further improve outcomes in this population investigation of rational combinations targeting different mechanisms that cancers exploit to evade the immune system is required. ICOS, a member of the CD28/B7 immunoglobulin receptor superfamily, provides a co-stimulatory signal augmenting T-cell proliferation, cytokine production, cytotoxic function and survival. GSK609 is a humanized IgG4 antibody selected for its potent agonist activity and non-depleting properties. The rationale for targeting ICOS with GSK609 plus PD-1 blockade with PE is supported by preclinical and clinical evidence (Rischin, et al. Annals of Oncol 2019;30[Supplement_5]:v454–5). INDUCE-3 trial (NCT04128696) will explore if the addition of GSK609 to PE improves outcomes of pts with R/M HNSCC. Methods: INDUCE-3 uses a 2-in-1 adaptive design that has the option to seamlessly expand from an initial Phase 2 to a Phase 3 study. Pts (n = 600) will be stratified by PD-L1 status and HPV status (oropharynx only) then randomly assigned in a 1:1 ratio to receive GSK609 plus PE or PL plus PE, every 3 weeks until progression, unacceptable toxicity, or up to 35 cycles. GSK609 plus PE will be assessed for superiority versus PL plus PE in overall survival (OS) and progression-free survival (PFS) per RECISTv1.1 as dual primary endpoints; secondary endpoints include PFS per immune-based RECIST; milestone OS; safety and tolerability; time to deterioration in patient-reported physical function and pain. Efficacy and patient-reported outcome endpoints will be assessed in the PD-L1 combined positive score (CPS) ≥1 and ≥20 populations. Key eligibility criteria are aged ≥18 years; locally incurable R/M HNSCC of the oral cavity, oropharynx, hypopharynx, or larynx; no prior systemic therapy in the R/M setting; PD-L1 CPS ≥1 by central testing; measurable disease per RECIST v1.1 and ECOG PS 0/1. Recruitment is ongoing in countries across the globe. Funding: Study is funded by GlaxoSmithKline and in collaboration with Merck Sharp & Dohme Corp., a subsidiary of Merck & Co., Inc., Kenilworth, NJ, USA. Clinical trial information: NCT04128696.