A Study of ASP-1929 Photoimmunotherapy in Combination With Pembrolizumab in First-line Treatment of Locoregional Recurrent Squamous Cell Carcinoma of the Head and Neck With No Distant Metastases

This is a phase 3 multicenter, randomized study designed to evaluate the efficacy and safety of ASP-1929 photoimmunotherapy (PIT) in combination with pembrolizumab versus pembrolizumab-based standard of care (SOC) as first-line treatment of locoregional recurrent HNSCC in patients with no distant metastases. Patients will be enrolled into the study starting with 3 treatment arms, ASP-1929 PIT at doses of 320 mg/m\^2 or 640 mg/m\^2 in combination with pembrolizumab treatment arms or a SOC treatment arm in which patients may receive pembrolizumab alone or pembrolizumab plus chemotherapy according to physician's choice. Treatment assignment within the SOC treatment arm will be based on the decision of the site investigator. To ensure that the SOC treatment arm is appropriately balanced, a 50% enrollment cap will be imposed on pembrolizumab monotherapy and pembrolizumab plus chemotherapy, so that approximately the same number of patients will have been treated within each category. At the start of the study all patients will be randomized 2:2:1 ratio to ASP-1929 PIT 320 mg/m\^2 plus pembrolizumab versus ASP-1929 PIT 640 mg/m\^2 plus pembrolizumab versus SOC (pembrolizumab alone or pembrolizumab plus chemotherapy according to physician's choice). An interim analysis (IA) will be conducted for this study as part of the dose optimization process. Patient enrollment will continue during the period of preparation and conduct of this IA. Based on the findings from the first interim analysis (IA1), the Sponsor will have the option to close one of the two ASP-1929 PIT treatment arms for enrollment. The final decision on the optimized ASP-1929 PIT dose for this study will be made in agreement with the US Food and Drug Administration (FDA).