men and women ≥ 18 years of age

histologically or cytologically confirmed recurrent, metastatic or unresectable hnscc of the oral cavity, oropharynx, hypopharynx, and larynx that that has either progressed during or after platinum-based chemotherapy administered for metastatic disease or has recurred during or within 6 months after the completion of platinum-based neoadjuvant or adjuvant therapy

presence of radiographically measurable disease as defined by recist 1.1

ecog performance status of 0 or 1

prior malignancy (other than hnscc), except for adequately treated basal cell or squamous cell skin cancer, in situ cervical cancer, or other cancer from which the subject has been disease free for ≥ 2 years or which will not limit survival to < 2 years

known central nervous system metastases and/or carcinomatous meningitis

a life-threatening illness, medical condition (including psychiatric conditions) or organ system dysfunction which, in the investigator's opinion, could compromise the subject's safety

significant cardiovascular disease such as uncontrolled or symptomatic arrhythmias, congestive heart failure, or myocardial infarction within 6 months of starting study drug