Comparing and combining xevinapant with ATR and PARP inhibition for the radiosensitization of HPV-negative HNSCC cells

Radiochemotherapy with the apoptosis stimulator xevinapant demonstrated superiority over radiochemotherapy in a randomized phase 2 trial in head and neck squamous cell carcinoma (HNSCC) but the subsequent phase 3 trial failed. Here, we compared the radiosensitization through xevinapant with two other emerging approaches, the inhibition of ATR and PARP through tuvusertib and olaparib, in a panel of four radioresistant HPV-negative HNSCC cell lines (HSC4, UT-SCC-60A, SAS, SAT). Without irradiation the analyses of proliferation and colony formation showed varying sensitivities of the cell lines towards the different agents and their combinations. When combined with radiation in colony formation assays, we observed moderate radiosensitization through xevinapant in individual cell lines but not in general. Both tuvusertib and olaparib induced stronger radiosensitization than xevinapant and combining both agents resulted in especially profound radiosensitization in three out of the four cell lines tested, whereas their combination with xevinapant or the combination of xevinapant with cisplatin was less effective. Assessment of cell death induction via annexin V/DAPI staining failed to generally predict cytotoxicity or radiosensitization of these approaches. Overall, our data are in line with the recent failure of the phase 3 TrilynX trial and suggest further investigation of ATR and PARP inhibition for the curative treatment of HPV-negative HNSCC.