CHK1 is a serine/threonine kinase that is activated in response to single strand breaks (SSBs) in DNA caused by either direct DNA damage (e.g. by genotoxic chemotherapeutic agents) or replication stress. Activation of CHK1 initiates a signaling cascade culminating in cell cycle arrest leading to DNA repair, senescence or death. Inhibition of CHK1 abrogates cell cycle arrest, inhibits DNA repair and enhances tumor cell death following DNA damage by a range of chemotherapeutic agents. Cells lacking intact G1 checkpoints through inactivation of p53 are particularly dependent on S and G2/M checkpoints and therefore expected to be more sensitive to genotoxic treatment in the presence of a CHK1 inhibitor, whereas normal cells with functional G1 checkpoints would be predicted to undergo less cell death. Thus CHK1 is a validated therapeutic target for cancer treatment.

In a collaborative program with Sareum Ltd (Cambridge, UK) we used structure-based design to progress fragment hits to potent inhibitors of CHK1, including our recently published inhibitor SAR-020106. Guided by both biochemical and molecular pharmacological studies, we identified a lead series with pM to nM activity against the CHK1 enzyme in vitro and high selectivity vs CHK2 (200- to >500-fold) and CDK1. The lead series compounds have both good in vitro ADME and in vivo pharmacokinetic (PK) properties.

From this lead series we identified CCT244747 as a potent and selective CHK1 inhibitor with oral efficacy. Specifically, CCT244747 is an 8nM inhibitor of CHK1 with >1000-fold selectivity versus CDK1 and CHK2 and excellent selectivity in a kinase panel screen of 120 kinases (only 8/120 with > 80% inhibition at 10μM). CCT244747 shows sub-micromolar activity in a cell-based checkpoint abrogation assay and potentiates the cytotoxicity of a selection of genotoxic chemotherapeutics including gemcitabine in colon, pancreatic and lung human tumor cell lines. CCT244747 has oral bioavailability of 61% in mice with a plasma half-life of approximately 1 hour. Further PK/PD evaluation demonstrated biomarker modulation in human tumor xenografts (pSer296 CHK1) at 6 and 24 hours post CCT244747 administration, in combination with gemcitabine. Efficacy studies of CCT244747 in combination with both irinotecan and gemcitabine, showed significant oral antitumor activity. For example, in the HT29 human colon tumor xenograft model the average percentage treated/control (%T/C) tumor weights were 15.4% for gemcitabine +CCT244747 combined, versus 62.5% and 88.4%, for gemcitabine and CCT244747 alone, respectively as measured on day 18 of therapy.

There is now also evidence that CHK1 inhibitors may have single agent activity in certain cancers. In particular, CHK1 was recently identified in an RNAi screen as a therapeutic target in MYCN amplified neuroblastoma and we have shown significant efficacy of CCT244747 in a MYCN-driven transgenic mouse model of neuroblastoma (TH-MYCN) using MRI tumor volume measurements and explant tumor weights as measures of outcome. In summary, we have identified CCT244747 as a novel, potent and selective inhibitor of CHK1, which demonstrates biomarker modulation and efficacy both alone in neuroblastoma and in combination with genotoxic chemotherapeutic agents in common solid tumors, when delivered by the oral route.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A228.