Synthetic lethal targeting of mitotic checkpoints in HPV-negative head and neck cancer

Head and neck squamous cell carcinomas (HNSCC) affect more than 800,000 people annually worldwide, causing over 15,000 deaths in the US. Among HNSCC cancers, human papillomavirus (HPV)-negative HNSCC has the worst outcome, motivating efforts to improve therapy for this disease. The most common mutational events in HPV-negative HNSCC are loss of function alterations in TP53 (>85%) and CDKN2A (>57%). Loss of these tumor suppressors significantly impairs G1/S checkpoints, causing reliance on other cell cycle checkpoints to repair ongoing replication damage. To identify cell-cycle related signaling vulnerabilities of HNSCC cells, we evaluated a panel of clinical agents targeting multiple phases of the cell cycle in a group of p53-deficient HNSCC cell lines. Initial screening identified multiple drugs with single agent activity in reducing cell viability. Subsequent analyses of selected combinations demonstrated potent synergy between the CHK1/2 inhibitor LY2606268 (prexasertib), which eliminates a G2 checkpoint, and the WEE1 inhibitor AZD1775 (adavosertib), which promotes M-phase entry. This combination was more effective in induction of apoptosis, and reduction of anchorage independent growth and clonogenic capacity, than single agents or vehicle. Mechanistically, cells treated with the drug combination had elevated DNA damage reflected by accumulation of ?H2AX foci, accompanied by more significantly reduced activation of CHK1 and its paralog CHK2, and enhanced activation of CDK1. Among cells entering mitosis, the drug combination enhanced rate of mitotic catastrophe, reflected by elimination of arrested M-phase cells that accumulate with single drug treatment. These data suggest potential value of dual inhibition of CHK1 and WEE1 in G1/S-compromised tumors.