The efficacy of CHK1 inhibitors is not altered by hypoxia, but is enhanced after reoxygenation.

Inhibitors of CHK1 are in clinical trials for cancer treatment in combination with DNA-damaging agents. Importantly, it was previously suggested that hypoxic cancer cells may be particularly sensitive to CHK1 inhibition. However, this suggestion was based on studies in severe, toxic levels of hypoxia (anoxia). The influence of less severe hypoxia on the efficacy of CHK1 inhibitors, administered either as single agents or in combination with other treatments, remains to be investigated. Here, we have assayed the effects of the CHK1 inhibitors, AZD7762 and UCN-01, during various hypoxic conditions and after reoxygenation in the absence and presence of ionizing radiation. Treatment with CHK1 inhibitors during acute or prolonged hypoxia (< 0.03%, 0.2%, and 1% O2; 3 h or 20-24 h) gave similar effects on cell survival as treatment with these inhibitors during normoxia. CHK1 inhibitors combined with ionizing radiation showed similar radiosensitization in hypoxic and normoxic cells. However, when the inhibitors were administered after reoxygenation following prolonged hypoxia (< 0.03% and 0.2%; 20-24 h), we observed decreased cell survival and stronger induction of the DNA damage marker, γH2AX, in S-phase cells. This was accompanied by enhanced phosphorylation of the single-stranded DNA-binding replication protein A. These results suggest that the cytotoxic effects of CHK1 inhibitors are enhanced after reoxygenation following prolonged hypoxia, most likely due to the increased replication-associated DNA damage. Combining CHK1 inhibitors with other treatments that cause increased reoxygenation, such as fractionated radiotherapy, might therefore be beneficial.