Will Targeting Chk1 Have a Role in the Future of Cancer Therapy

The development of checkpoint kinase 1 (Chk1) inhibitors as anticancer agents has a long history. In 1982, Arthur Pardee demonstrated that caffeine could abrogate DNA damage-induced G2 arrest and enhance cytotoxicity induced by nitrogen mustards. The G2 arrest provides time for the cell to repair DNA damage, and the addition of caffeine forces the cells to progress into mitosis before the damage is repaired. Caffeine was eventually shown to inhibit the DNA damage response proteins ATR and ATM, but the required concentrations could not be achieved in patients. The downstream targets of ATR and ATM are Chk1 and Chk2, respectively, but subsequent drug development has demonstrated a much more critical role for Chk1 than Chk2 in protecting cells from DNA damage. The first Chk1 inhibitor to enter clinical trials was 7-hydroxystaurosporine (UCN-01), but it had multiple offtarget effects as well as high plasma protein binding, which resulted in variable bioavailability and serious adverse effects when the binding capacity was exceeded. Many Chk1 inhibitors have subsequently been developed, one of which, MK-8776, is the subject of the report by Daud et al, which accompanies this article, in Journal of Clinical Oncology. The original therapeutic strategy required that the tumor cells are first damaged with an anticancer drug such as cisplatin or irinotecan. The resulting damage activates ATR which, in turn, activates Chk1, inhibits CDC25 phosphatases, and prevents activation of CDK1/2, the consequence of which is arrest in the S and G2 phases of the cell cycle (Fig 1). Inhibition of Chk1 reactivates CDK1/2 to abrogate this arrest before the cells can complete repair, driving the damaged S phase cells into G2 and G2 cells through an aberrant mitosis (often called mitotic catastrophe). However, subsequent experiments found variable, if any, increase in cytotoxicity with this strategy, possibly because the concentrations of drug that caused arrest in vitro were themselves cytotoxic. We concluded that inhibition of Chk1 could accelerate the rate of cell death but had limited impact on the overall extent of cell death. However, it is important to note a phase I clinical trial of the Chk1 inhibitor AZD7762 in combination with irinotecan in which one patient showed a dramatic and durable response. This outlier response was tracked to a mutation in the RAD50 gene that impeded its normal participation in irinotecan-induced arrest and repair. Consequently, there may be small subsets of patients who would truly benefit from this drug combination. It is now recognized that Chk1 inhibitors are far more effective at sensitizing cells to antimetabolites such as gemcitabine, hydroxyurea, and cytarabine. For example, we demonstrated up to a 100-fold increase in sensitivity to hydroxyurea on addition of the Chk1 inhibitor MK-8776. The sensitization observed with gemcitabine in vitro was not as pronounced (2to 10-fold), but this has become the preferred combination to test in clinical trials because gemcitabine is already approved for the treatment of many types of tumors. Gemcitabine is a prodrug which, once metabolized, can be incorporated into DNA, but perhaps more relevant here, it irreversibly CHK1 CHK1