Aberrant control of cyclin-dependent kinases (CDKs) is a central feature of the molecular pathology of cancer. Iterative structure-based design was used to optimize the ATP- competitive inhibition of CDK1 and CDK2 by O(6)-cyclohexylmethylguanines, resulting in O(6)-cyclohexylmethyl-2-(4'- sulfamoylanilino)purine. The new inhibitor is 1,000-fold more potent than the parent compound (K(i) values for CDK1 = 9 nM and CDK2 = 6 nM versus 5,000 nM and 12,000 nM, respectively, for O(6)-cyclohexylmethylguanine). The increased potency arises primarily from the formation of two additional hydrogen bonds between the inhibitor and Asp 86 of CDK2, which facilitate optimum hydrophobic packing of the anilino group with the specificity surface of CDK2. Cellular studies with O(6)-cyclohexylmethyl-2-(4'- sulfamoylanilino) purine demonstrated inhibition of MCF-7 cell growth and target protein phosphorylation, consistent with CDK1 and CDK2 inhibition. The work represents the first successful iterative synthesis of a potent CDK inhibitor based on the structure of fully activated CDK2-cyclin A. Furthermore, the potency of O(6)-cyclohexylmethyl-2-(4'- sulfamoylanilino)purine was both predicted and fully rationalized on the basis of protein-ligand interactions.
The adenosine 5'-triphosphate (ATP) competitive cyclin-dependent kinase inhibitor O(6)-cyclohexylmethylguanine (NU2058, 1) has been employed as the lead in a structure-based drug discovery program resulting in the discovery of the potent CDK1 and -2 inhibitor NU6102 (3, IC(50) = 9.5 nM and 5.4 nM vs CDK1/cyclinB and CDK2/cyclinA3, respectively). The SAR for this series have been explored further by the synthesis and evaluation of 45 N(2)-substituted analogues of NU2058. These studies have confirmed the requirement for the hydrogen bonding N(2)-NH group and the requirement for an aromatic N(2)-substituent to confer potency in the series. Additional potency is conferred by the presence of a group capable of donating a hydrogen bond at the 4'-position, for example, the 4'-hydroxy derivative (25, IC(50) = 94 nM and 69 nM vs CDK1/cyclinB and CDK2/cyclinA3, respectively), 4'-monomethylsulfonamide derivative (28, IC(50) = 9 nM and 7.0 nM vs CDK1/cyclinB and CDK2/cyclinA3, respectively), and 4'-carboxamide derivative (34, IC(50) = 67 nM and 64 nM vs CDK1/cyclinB and CDK2/cyclinA3, respectively). X-ray crystal structures have been obtained for key compounds and have been used to explain the observed trends in activity.
A diverse range of chromen-2-one, chromen-4-one and pyrimidoisoquinolin-4-one derivatives was synthesized and evaluated for inhibitory activity against the DNA repair enzyme DNA-dependent protein kinase (DNA-PK), with a view to elucidating structure-activity relationships for potency and kinase selectivity. DNA-PK inhibitory activity varied widely over the series of compounds evaluated (IC(50) values ranged from 0.19 to >10 microM), with excellent activity being observed for the 7,8-benzochromen-4-one and pyrimido[2,1-a]isoquinolin-4-one templates. By contrast, inhibitors based on the benzochromen-2-one (coumarin) or 2-aryl-7,8-benzochromen-4-one (flavone) scaffolds were less potent. Crucially, these studies revealed a very constrained structure-activity relationship at the 2-position of the benzopyranone and pyrimido[2,1-a]isoquinolin-4-one pharmacophore, with only a 2-morpholino or 2-(2'-methylmorpholino) group being tolerated at this position. More detailed biological studies conducted with the most potent inhibitor NU7163 (48; IC(50) = 0.19 microM) demonstrated ATP-competitive DNA-PK inhibition, with a K(i) value of 24 nM, and 48 exhibited selectivity for DNA-PK compared with the related enzymes ATM, ATR, mTOR, and PI 3-K (p110alpha). Compound 48 sensitized the HeLa human tumor cell line to the cytotoxic effects of ionizing radiation in vitro, a dose modification factor of 2.3 at 10% survival being observed with an inhibitor concentration of 5 microM. This study identified these structural classes as novel DNA-PK inhibitors and delineated initial structure-activity relationships against DNA-PK.
Structure-activity relationships for inhibition of DNA-dependent protein kinase (DNA-PK) have been defined for substituted chromen-4-ones. For the 2-amino-substituted benzo[h]chromen-4-ones, a morpholine substituent at this position was essential for activity. Small libraries of 6- and 7-alkoxy-substituted chromen-4-ones showed that a number of 7-alkoxy-substituted chromenones displayed improved activity. Focused libraries incorporating 6-, 7-, and 8-aryl and heteroaryl substituents were prepared. In these cases, 6- and 7-substitution was disfavored, whereas 8-substitution was largely tolerated. Surprisingly, two compounds, 2-N-morpholino-8-dibenzofuranyl-chromen-4-one (NU7427, 32{38}) and the 2-N-morpholino-8-dibenzothiophenyl-chromen-4-one (NU7441, 32{26}) were excellent inhibitors (IC50 vs DNA-PK = 40 and 13 nM, respectively). The ring-saturated analogue 2-N-morpholino-8-(6',7',8',9'-tetrahydrodibenzothiophene)chromen-4-one, 36, retained potent activity (IC50 vs DNA-PK = 23 nM). The dibenzothiophene 32{38} sensitized HeLa cells to ionizing radiation in vitro, with dose modification factors of 2.5 at 10% survival being observed at 0.5 microM. The cytotoxicity of the topoisomerase II inhibitor etoposide was also potentiated.
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