Checkpoint abrogation and potentiation of cytotoxic chemotherapeutics with a novel checkpoint kinase 1 inhibitor

Conventional chemotherapeutic agents such as gemcitabine, cisplatin or irinotecan induce DNA damage and activate cell cycle checkpoints. P53 defective tumors lack a functional G1 checkpoint and rely heavily on the S and G2 checkpoints, and the effector kinase Chk1, for protection against this DNA damage. Inhibiting Chk1 potentiates the anti‐tumor effects of these cytotoxic chemotherapeutic agents. Targeting Chk1 is a potential therapeutic opportunity for potentiating the anti‐tumor efficacy of DNA damaging cytotoxic chemotherapeutic drugs without increasing their toxicity to normal cells. Elaboration of a designed kinase directed fragment core utilizing X‐ray structure guided design lead to the identification of a potent pyridone series of Chk1 inhibitors. Further profiling identified V158411 as the lead candidate. X‐ray crystallography identified V158411 as being bound to the ATPase site in the kinase domain of Chk1. V158411 potently inhibited Chk1 and Chk2 with IC50s of 4.4 and 4.5nM respectively. The addition of V158411 to gemcitabine or camptothecin treated cells abrogated the cell cycle checkpoints induced by these agents resulting in the expected modulation of cell cycle proteins and increased apoptosis. V158411 potentiated the cytotoxicity of gemcitabine, cisplatin, SN38 and camptothecin in a variety of p53 deficient but not proficient human tumor cell lines in vitro. V158411 could be formulated in a simple aqueous form suitable for i.v. dosing. In nude mice, V158411 was well tolerated as a single agent (MTD >100mg/kg) and in combination with irinotecan. Intravenous administration to rats and mice resulted in low plasma clearances (20mL/min/kg) and long half‐lives (2.9–3.7h). In tumor bearing animals, V158411 was detected at high concentrations in the tumor (tumor:plasma AUC ratio of 4.7) with a long tumor elimination half life of 22 hours. No pharmacologically relevant in vivo drug‐drug interaction with irinotecan was identified. V158411 potentiated the anti‐tumor activity of gemcitabine and irinotecan in a variety of human tumor xenograft models without additional systemic toxicity. These results demonstrate the potential of combining V158411 with standard of care chemotherapeutic agents to potentiate the therapeutic efficacy of these agents without increasing their toxicity to normal cells. Based on this data, the clinical development of V158411 is currently being actively pursued. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C207.