Abstract
Major risk factors of head and neck squamous cell carcinoma (HNSCC) are tobacco use and human papillomavirus (HPV). HPV E6 oncoprotein leads to p53 degradation, whereas HPV-negative cancers are frequently associated with TP53 mutations. Peposertib is a potent and selective, orally administered small-molecule inhibitor of the catalytic subunit of the DNA-dependent kinase (DNA-PKcs), a key regulator of nonhomologous end joining (NHEJ). NHEJ inhibition along with irradiation (IR)-induced DNA double-strand breaks has the potential to increase antitumor treatment efficacy. In this study, we investigated the responses of a panel of HNSCC models with distinct HPV and p53 status to treatments with IR, DNA-PKcs inhibition, and their combination in vitro and in vivo. IR-induced DNA damage combined with peposertib administration shortly before IR results in decreased cell viability and proliferation and causes DNA repair delay in all studied HNSCC cell lines. However, our data confirm that the actual cell fate upon this treatment is determined by cellular p53 and/or HPV status. Cells lacking functional p53 due to its degradation by HPV or due to a loss-of-function mutation are arrested in the G2/M phase of the cell cycle and eliminated by apoptosis, whereas p53-proficient HNSCC cell lines preferentially undergo senescence. This is also recapitulated in vivo, where HPV+ UD-SCC-2 xenografts display stronger and more durable responses to the combined treatment as compared with p53 wild-type UM-SCC-74A tumors. In conclusion, DNA-PKcs inhibitor peposertib should be further studied as a potential radiosensitizer for HNSCCs, taking into consideration the genetic background and the HPV status of a particular tumor.