DNAPK Inhibition Preferentially Compromises the Repair of Radiation-induced DNA Double-strand Breaks in Chronically Hypoxic Tumor Cells in Xenograft Models

Tumour hypoxia in driving genomic instability and tumour evolution

Intratumour hypoxia is a feature of all heterogenous solid tumours. Increased levels or subregions of tumour hypoxia are associated with an adverse clinical prognosis, particularly when this co-occurs with genomic instability. Experimental evidence points to the acquisition of DNA and chromosomal alterations in proliferating hypoxic cells secondary to inhibition of DNA repair pathways such as homologous recombination, base excision repair and mismatch repair. Cell adaptation and selection in repair-deficient cells give rise to a model whereby novel single-nucleotide mutations, structural variants and copy number alterations coexist with altered mitotic control to drive chromosomal instability and aneuploidy. Whole-genome sequencing studies support the concept that hypoxia is a critical microenvironmental cofactor alongside the driver mutations in MYC, BCL2, TP53 and PTEN in determining clonal and subclonal evolution in multiple tumour types. We propose that the hypoxic tumour microenvironment selects for unstable tumour clones which survive, propagate and metastasize under reduced immune surveillance. These aggressive features of hypoxic tumour cells underpin resistance to local and systemic therapies and unfavourable outcomes for patients with cancer. Possible ways to counter the effects of hypoxia to block tumour evolution and improve treatment outcomes are described.

The cell-line-derived subcutaneous tumor model in preclinical cancer research

Tumor-bearing experimental animals are essential for preclinical cancer drug development. A broad range of tumor models is available, with the simplest and most widely used involving a tumor of mouse or human origin growing beneath the skin of a mouse: the subcutaneous tumor model. Here, we outline the different types of in vivo tumor model, including some of their advantages and disadvantages and how they fit into the drug-development process. We then describe in more detail the subcutaneous tumor model and key steps needed to establish it in the laboratory, namely: choosing the mouse strain and tumor cells; cell culture, preparation and injection of tumor cells; determining tumor volume; mouse welfare; and an appropriate experimental end point. The protocol leads to subcutaneous tumor growth usually within 1-3 weeks of cell injection and is suitable for those with experience in tissue culture and mouse experimentation.

Targeting Non-homologous and Alternative End Joining Repair to Enhance Cancer Radiosensitivity

Many cancer therapies, including radiotherapy, induce DSBs as the major driving mechanism for inducing cancer cell death. Thus, modulating DSB repair has immense potential for radiosensitization, although such interventions must be carefully designed to be tumor selective to ensure that normal tissue toxicities are not also increased. Here, we review mechanisms of error-prone DSB repair through a highly efficient process called end joining. There are two major pathways of end-joining repair: non-homologous end joining (NHEJ) and alternative end joining (a-EJ), both of which can be selectively upregulated in cancer and thus represent attractive therapeutic targets for radiosensitization. These EJ pathways each have therapeutically targetable pioneer factors - DNA-dependent protein kinase catalytic subunit (DNA-PKcs) for NHEJ and DNA Polymerase Theta (Pol θ) for a-EJ. We summarize the current status of therapeutic targeting of NHEJ and a-EJ to enhance the effects of radiotherapy - focusing on challenges that must be overcome and opportunities that require further exploration. By leveraging preclinical insights into mechanisms of altered DSB repair programs in cancer, selective radiosensitization through NHEJ and/or a-EJ targeting remains a highly attractive avenue for ongoing and future clinical investigation.

Radiosensitisation of SCCVII tumours and normal tissues in mice by the DNA-dependent protein kinase inhibitor AZD7648

BACKGROUND AND PURPOSE

Inhibitors of DNA-dependent protein kinase (DNA-PK) are effective radiation sensitisers in preclinical tumours, but little is known about risks of normal tissue radiosensitisation. Here, we evaluate radiosensitisation of head and neck squamous cell carcinoma (HNSCC) cells by DNA-PK inhibitor AZD7648 under oxia and anoxia in vitro, and tumour (SCCVII), oral mucosa and small intestine in mice.

MATERIALS AND METHODS

Radiosensitisation of human (UT-SCC-54C) and murine (SCCVII) HNSCC cells by AZD7648 under oxia and anoxia was evaluated by clonogenic assay. Radiosensitisation of SCCVII tumours in C3H mice by oral AZD7648 (75 mg/kg) was determined by ex vivo clonogenic assay 3.5 days post-irradiation, with evaluation of normal tissue surrogate endpoints using 5-ethynyl-2'-deoxyuridine to facilitate detection of regenerating crypts in the ileum and repopulating S-phase cells in the ileum and oral mucosa of the same animals.

RESULTS

AZD7648 potently radiosensitised both cell lines, with similar sensitiser enhancement ratios for 10% survival (SER₁₀) under oxia and anoxia. AZD7648 diffused rapidly through multicellular layers, suggesting rapid equilibration between plasma and hypoxic zones in tumours. SCCVII tumours were radiosensitised by AZD7648 (SER₁₀ 2.5). AZD7648 also enhanced radiation-induced body weight loss and suppressed regenerating intestinal crypts and repopulating S-phase cells in the ileum and tongue epithelium with SER values similar to SCCVII tumours.

CONCLUSION

AZD7648 is a potent radiation sensitiser of both oxic and anoxic tumour cells, but also markedly radiosensitises stem cells in the small intestine and oral mucosa.