RecruitingPhase 2Drug trialA Phase II Study of ACR-368 and Low Dose Gemcitabine in R/M HNSCCThe purpose of the study is to determine the activity and safety of ACR-368 (prexasertib) in combination with gemcitabine in participants with Head and Neck Squamous Cell Carcinoma (HNSCC). Participants will receive the study drugs ACR-368 and a low dose of gemcitabine once every 2 weeks in 4-week cycles and will continue on treatment unless the disease deteriorates.