A Phase 3, Randomized, Double-Arm, Open-Label, Controlled Trial of ASP-1929 Photoimmunotherapy Versus Physician's Choice Standard of Care for the Treatment of Locoregional, Recurrent Head and Neck Squamous Cell Carcinoma in Patients Who Have Failed or Progressed On or After at Least Two Lines of Therapy, of Which at Least One Line Must Be Systemic Therapy

• Have a histologically confirmed locoregional persistent, recurrent or second primary squamous cell carcinoma of the head and neck, not amenable to curative treatment

• Have failed or progressed on or after at least 2 lines of therapy for squamous cell carcinoma of the head and neck, one of which must be prior systemic platinum-based chemotherapy

• Have completed prior curative radiation therapy for treatment of their head and neck region

• Have locoregional head and neck tumor site(s) that are all accessible to light illumination

• Have target tumors that are clearly measurable by contrast enhanced CT scan, or MRI scan with gadolinium if CT scan is not adequate or the patient has an allergy to CT contrast media.

• Have a life expectancy of \> 6 months, based on Investigator judgment

• Male participants must agree to use contraception during the treatment period and for at least 6 months after the last ASP-1929 infusion

• Female patients of childbearing potential must not be pregnant or breastfeeding and agree to follow the contraceptive guidance during the treatment period and for at least 6 months after the last dose of trial intervention and must refrain from breastfeeding for at least 2 months after the last ASP-1929 infusion

• Have an Eastern Cooperative Oncology Group (ECOG) score of 0 or 1