In vitro evaluation of the effect of C-4 substitution on methylation of 7,8-dihydroxycoumarin: metabolic profile and catalytic kinetics.

Daphnetin (7,8-dihydroxycoumarin (7,8-DHC)) and its C-4 derivatives have multiple pharmacological activities, but the poor metabolic stability of these catechols has severely restricted their application in the clinic. Methylation plays important roles in catechol elimination, although thus far the effects of structural modifications on the metabolic selectivity and the catalytic efficacy of human catechol-O-methyltransferase (COMT) remain unclear. This study was aimed at exploring the structure-methylation relationship of daphnetin and its C-4 derivatives, including 4-methyl, 4-phenyl and 4-acetic acid daphnetin. It was achieved by identifying the methylated products generated and by careful characterization of the reaction kinetics. These catechols are selectively metabolized to the corresponding 8-O-methyl conjugates, and this regioselective methylation could be elucidated by flexible docking, in which all the 8-OH groups of these catechols are much closer than the 7-OH groups to catalytic residue LYS144 and methyl donor AdoMet. The results of the kinetic analyses revealed that the Clint values of the compounds could be strongly affected by the C-4 substitutions, which could be partially explained by the electronic effects of the C-4 substituents and the coordination modes of 7,8- dihydroxycoumarins in the active site of COMT. These findings provide helpful guidance for further structural modification of 7,8-DHCs to improve metabolic stability.