4-Alkoxy-2,6-diaminopyrimidine derivatives: inhibitors of cyclin dependent kinases 1 and 2

The cyclin dependent kinase (cdk) inhibitor NU6027, 4-cyclohexylmethoxy-5-nitroso-pyrimidine-2,6-diamine (IC50 vs cdk1/cyclinB1=2.9±0.1 μM and IC50 vs cdk2/cyclinA3=2.2±0.6 μM), was used as the basis for the design of a series of 4-alkoxy-2,6-diamino-5-nitrosopyrimidine derivatives. The synthesis and evaluation of 21 compounds as potential inhibitors of cyclin-dependent kinases 1 and 2 is described and the structure–activity relationships relating to NU6027 have been probed. Simple alkoxy- or cycloalkoxy-groups at the O4-position were tolerated, with the 4-(2-methylbutoxy)-derivative (IC50 vs cdk1/cyclinB1=12±2 μM and cdk2/cyclinA3=13±4 μM) retaining significant activity. Substitutions at the N6 position were not tolerated. Replacement of the 5-nitroso substituent with ketone, oxime and semicarbazone groups essentially abolished activity. However, the derivative bearing an isosteric 5-formyl group, 2,6-diamino-4-cyclohexylmethoxy-pyrimidine-5-carbaldehyde, showed modest activity (IC50 vs cdk1/cyclinB1=35±3 μM and cdk2/cyclinA3=43±3 μM). The X-ray crystal structure of the 5-formyl compound bound to cdk2 has been determined to 2.3 Å resolution. The intramolecular H-bond deduced from the structure with NU6027 bound to cdk2 is not evident in the structure with the corresponding formyl compound. Thus the parent compound, 4-cyclohexylmethoxy-5-nitrosopyrimidine-2,6-diamine (NU6027), remains the optimal basis for future structure–activity studies for cyclin-dependent kinase inhibitors in this series.