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 negative pregnancy test is required within 14 days of
             randomization for all women of childbearing potential.

          -  History of stroke within prior 6 months.

          -  History of acute myocardial infarction within prior 3 months, uncontrolled angina,
             uncontrolled arrhythmia, or uncontrolled congestive heart failure

          -  Left ventricular outflow obstructions, such as aortic stenosis and idiopathic
             hypertrophic subaortic stenosis

          -  Angina requiring treatment with long-acting nitrates

          -  Angina requiring treatment with short-acting nitrates within 90 days of planned
             tadalafil administration

          -  Unstable angina within 90 days of visit 1 (Braunwald 1989)

          -  Positive cardiac stress test without documented evidence of subsequent, effective
             cardiac intervention

          -  History of any of the following coronary conditions within 90 days of planned
             tadalafil administration:

               -  Myocardial Infarction

               -  Coronary artery bypass graft surgery

               -  Percutaneous coronary intervention (for example, angioplasty or stent placement)

               -  Any evidence of heart disease (NYHA ≥ Class II as defined in Protocol Attachment
                  LVHG.3) within 6 months of planned tadalafil administration

          -  Concurrent systemic immunosuppressant therapy (e.g., cyclosporine A, tacrolimus, etc.,
             or chronic administration of >10 mg/day of prednisone or equivalent)

          -  Prior organ transplantation

          -  Known history of human immunodeficiency virus (HIV) or active infection with hepatitis
             C virus (HCV) or hepatitis B virus (HBV).