Clinical data demonstrate an enhanced radiation sensitivity of HPV+ HNSCC, a feature also observed on the cellular level in HPV+ HNSCC cell lines. For the latter we could show that the underlying mechanism is a defect in DNA double-strand break repair associated with a profound and sustained arrest in G2 (Rieckmann 2013). Specific inhibitors of central components of the DNA damage response (DDR), such as PARP1, Wee1 and Chk1 are being tested in clinical trials in HNSCC and the intrinsic DNA repair defect of HPV+ HNSCC cells may render these tumors especially susceptible for further radiosensitization.
Mechanistic proof of efficacy of the various inhibitors was performed using Western blot, immunofluorescence microscopy and assessment of cell cycle distribution. DDR-Inhibitors: PARP – Olaparib; Chk1 – PF00477736, LY2603618, Prexasertib; Wee1 – AZD-1775. Standard therapeutics: Cetuximab, cisplatin. Radiosensitization was assessed using colony formation assay. HPV+ HNSCC cells: UT-SCC-45, 93-VU-147T, UD-SCC-2, UM-SCC-47, UPCI-SCC-154.
While the inhibition of EGFR by cetuximab is being extensively tested for HPV+ HNSCC in phase 3 clinical trials, on the cellular level cetuximab completely failed to exert a meaningful cytotoxic effect or radiosensitization of any of the 5 HPV+ HNSCC strains tested (Güster 2014). In contrast, the inhibition of Chk1 interfered with the radiation-induced G2-arrest and resulted in radiosensitization in all HPV+ HNSCC cell lines, as well as the targeting of DNA repair processes through the inhibition of PARP1 (Busch 2013, Güster 2014, Busch 2017). Targeting Wee1 resulted in an accumulation of the HPV+ cells in the S-phase rather than in the intended release from the radiation-induced G2-arrest and it induced a compensatory activation of Chk1. The combined inhibition of Wee1 and Chk1, however, was already effective using massively reduced doses of both inhibitors and resulted in efficient radiosensitization (Busch 2017). In all cases the radiosensitizing effect was far stronger in the HPV+ HNSCC strains than in normal human fibroblasts used as a surrogate of p53-proficient normal tissue cells.
While the inhibition of EGFR fails to confer radiosensitization of HPV+ HNSCC on the cellular level, the inhibition of the DNA damage response was found to be effective. Our data strongly suggest that these targeting approaches further interfere with the ability of HPV+ HNSCC cells to cope with radiation-induced DNA damage and may represent viable options for the deintensification of therapy. The verification of these results in independent patient derived xenograft models as a next step towards a clinical use is currently being planned.
Rieckmann et al. 2013. HNSCC cell lines positive for HPV and p16 possess higher cellular radiosensitivity due to an impaired DSB repair capacity. Radiother Oncol. May;107(2):242-6
Güster et al. 2014. The inhibition of PARP but not EGFR results in the radiosensitization of HPV/p16-positive HNSCC cell lines. Radiother Oncol. Dec;113(3):345-51
Busch et al. 2013. HPV-positive HNSCC cell lines but not primary human fibroblasts are radiosensitized by the inhibition of Chk1. Radiother Oncol. Sep;108(3):495-9
Busch et al. 2017. G2-checkpoint targeting and radiosensitization of HPV/p16-positive HNSCC cells through the inhibition of Chk1 and Wee1. Radiother Oncol. Feb;122(2):260-266