5-Fluorouracil Response and Optimization STudy (The FROST Trial)

Inclusion Criteria: * Histologically or cytologically confirmed: * RM-HNSCC of the oral cavity, oropharynx, larynx, or hypopharynx, OR * p16+ (HPV-related) level 2-3 neck node and unknown primary site, OR * Second primary HNSCC in a previously radiated field not amenable to curative-intent surgery and/or re-radiation. * Measurable disease per RECIST 1.1. * Previously treated with platinum-based chemotherapy, RM disease within 6 months of definitive cisplatin + radiation therapy (DCisRT) or post-operative adjuvant cisplatin + radiation therapy (POACisRT) OR progressive disease on or after or intolerance to platinum agent given for RM disease. * Previously treated with PD-1 inhibitor, RM disease within 6 months of PD-1 inhibitor given as part of curative-intent therapy OR progressive disease on or after PD-1 inhibitor given for RM disease OR intolerance to prior PD-1 inhibitor in the curative or metastatic setting. * At least 18 years of age * ECOG performance status ≤ 2 * Adequate bone marrow and organ function as defined below: * Absolute neutrophil count ≥ 1.0 K/cumm * Platelets ≥ 100 K/cumm * Hemoglobin ≥ 8.0 g/dL * Total bilirubin ≤ 1.5 x IULN (for subjects with Gilbert's disease ≤ 3 x IULN) * AST(SGOT)/ALT(SGPT)/Alkaline Phosphatase (ALP) ≤ 3.0 x IULN. For subjects with documented bone metastasis, ALP ≤ 5.0 x IULN. * Serum creatinine \<3 mg/dL or creatinine clearance \> 30 mL/min by Cockcroft- Gault. * The effects of 5-FU on the developing human fetus…