Tadalafil and Pembrolizumab in Recurrent or Metastatic Head and Neck Cancer

Selected Inclusion Criteria: * Patients (at least 18 years of age) must have recurrent or metastatic squamous cell carcinoma of the head and neck. * Presence of measurable disease. * Life expectancy of greater than 12 weeks * Patients must have normal organ and marrow function Selected Exclusion Criteria: * Prior therapy with an PD-1 or PD-L1 inhibitor in the recurrent or metastatic setting * Uncontrolled central nervous system metastases (stable metastases permitted) * Active autoimmune disease * Chemotherapy ≤28 days prior to first administration of study treatment and/or monoclonal antibody ≤8 weeks prior to first administration of study treatment. * Prior daily use of tadalafil or other long-acting PDE5 inhibitors for one month or greater within 3 months of trial enrollment * Current use of all other long-acting PDE5 inhibitors. * Known severe hypersensitivity to tadalafil or any of the excipients of this product * Current treatment with nitrates * Current systemic treatment with a potent cytochrome P450 3A4 (CYP3A4) inhibitor such as ketoconazole or ritonavir. * Current treatment with guanylate cyclase (GC) stimulators such as riociguat. * History of hypotension and/or blindness and/or sensorineural hearing loss during prior treatment with tadalafil or other PDE-5 inhibitors * History of known hereditary degenerative retinal disorders, including retinitis pigmentosa * Prior history of non-arteritic anterior ischemic optic neuropathy * Pregnant or breastfeeding; a nega…