Bisubstrate inhibitors for the enzyme catechol O-methyltransferase (COMT):: Dramatic effects of ribose modifications on binding affinity and binding mode

Inhibition of the enzyme catechol O-methyltransferase (COMT) is of significant interest in the therapy of Parkinson's disease. Described herein are structural analogs of the potent bisubstrate inhibitor (-)-1 (IC50 = 9 nm; Table 1) for COMT, with target modifications of the central ribose moiety. Their synthesis involves, as key intermediates, adenosine derivatives, which are transformed to the potential bisubstrate inhibitors by a similar sequence of six steps (Schemes 1-4). The compounds were submitted to an enzymatic assay for determination of their in vitro inhibitory activity against COMT, and the inhibition mechanism with respect to the binding side of the cofactor S-adenosylmethionine (SAM) was analyzed by kinetics measurements (Fig. 3). Both binding affinity and binding mode were exceedingly sensitive towards modifications of the ribose moiety (Table 1). Removal of the 2'-OH group upon changing from (-)-1 to (-)-2 (IC50 = 28 muM) led to a reduction in binding affinity by more than three orders of magnitude. At the same time, competitive inhibition kinetics with respect to the SAM binding site was maintained, thereby supporting a bisubstrate binding mode. Unlike (-)-2, the dideoxyribose inhibitor (-)-3 (IC50 = 3 muM) showed a mixed and the cyclopentane derivative (+)-4 (IC50 = 1 muM) an uncompetitive inhibition mechanism with respect to the SAM binding site. In the complex of the latter, the adenine-substituted cyclopentane ring orients most probably towards the surface of the enzyme into the surrounding solution. The enantiomeric compounds (-)-5 (IC50 = 43 muM) and (+)-5 (IC50 = 141 muM), wherein the ribose had been replaced by a pyrrolidine ring, showed only low binding affinity.