Several CHK inhibitors have been reported in the

Several CHK-1 inhibitors have been reported in the literature. Of these, UCN-01 (7-hydroxystaurosporine) is a potent inhibitor of CHK-1 (=5.6nM), , and has modest selectivity among other kinases. UCN-01 is currently in clinical trials. Other CHK-1 inhibitors reported in the literature include indazole–imidazole analogs, and more selective UCN-01 analogs, such as ICP-1. Recently a series of novel CHK-1 inhibitors were reported, including -aryl-′-pyrazinylurea, 1,3-dihydro-indolone, and furanopyrimidine. Here, we report 4-(amino-alkylamino)-3-benzimidazole-quinolin-one derivatives as potent CHK-1 inhibitors with synergistic effects with DNA-damaging agents in tumor cells. The synthesis of 4-(amino-alkylamino)-3-benzimidazole-quinolinone is shown in . Reaction of ethyl 2-benzimidazole-acetate with 1---methoxy-benzyl (PMB) protected isatoic anhydride in the presence of a base in THF yielded 4-hydroxy-3-benzimidazole-quinolinone in good yields. Conversion of the 4-hydroxy compound to the corresponding bis-triflate was accomplished with triflic anhydride in the presence of pyridine at −5°C. Reaction of the bis-triflate with alkylamines via a SAr reaction was carried out in acetonitrile with Hünig’s base at 80°C. Deprotection of the PMB group and removal of the triflate group on the benzimidazole of the intermediate occurred in one pot under acidic conditions (trifluoroacetic Kinase Inhibitor Library and concentrated HCl; v/v, 7:1) at 90°C to yield 4-alkyl-amino-3-benz-imidazole-quinolinone (4-alkyl ABIQ) compounds in good yields. Unsubstituted 4-aminobenzimidazolequinolone has moderate activity (IC=0.73μM) against CHK-1. A NH group at C-4 is favorable for the potency. Presumably, a hydrogen-bond donor at C-4 or simply a small size hydrogen is important for keeping the two bicycles in a co-planar configuration. Replacement of the C-4 amine group with a hydroxy group reduced the potency by 4-fold, while the 4-diethylamino analog was inactive. The affinity of ABIQ analogs was sensitive to substitutions. The hydrogen bond donor–acceptor–donor (DAD) motif of the quinolinone and benzimidazole interacts with the lower hinge of the ATP-binding pocket. Methylation at N-1 eliminated the hydrogen-bond interaction with the protein and abolished activity. Simple 4-alkylamino groups were tolerated with moderate success (data not shown). Furthermore, an amino-alkylamine, e.g., ()-3-aminoquinuclidine, introduced at the 4-position of product , led to a dramatic increase in potency. To further explore the SAR of the C-4 alkylamine group and the effect of substituents on the A-ring, a set of amino-containing ABIQs was prepared. highlights the inhibitory properties of representative amine-containing alkyl ABIQs. The inhibition of CHK-1 activity was dependent on the nature of aminoalkyl group, especially its stereochemistry. For example, compound was 60-fold less potent than its enantiomer although the effect was less striking for the enantiomeric pair and . To better understand the interactions of these inhibitors with the CHK-1 protein, a co-crystal structure of compound with CHK-1 was obtained. The co-crystal structure showed that the inhibitor formed hydrogen bonds with Glu85 and Cys87 in the hinge region, explaining the general kinase affinity of this ABIQ scaffold. Significantly improved potency for the amino groups at 4-position was derived from the strong interactions of the tertiary-amine of with an acidic patch formed by Glu91 and Glu134, as shown in . This charge–charge interaction also greatly improved selectivity of this series of inhibitors against many other kinases (cdk and cdc data are shown below). Several inhibitors in the literature occupied the same ATP-binding pocket. However, none of the known inhibitors has fully utilized the acidic patch., The ability of the aminoalkyl ABIQ to inhibit CHK-1 activity was also sensitive to the substitution on the A-ring of the quinolinone. Thus, modification of the substitutions on the A-ring provided a series of potent ABIQ-containing CHK-1 inhibitors (IC<1nM). X-ray crystal structure shows a hydrophobic space near C-6. Thus, substitutions at position C-6 are well tolerated. Medium size groups, such as methyl and chloro, enhanced the activity more than 10-fold (compounds – and –). Multiple substitutions on the A-ring diminished the affinity (compound ), while any group at position C-8 (methyl in compound ) was detrimental to the activity.