Mechanism-based isocoumarin inhibitors for trypsin and blood coagulation serine proteases: new anticoagulants

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Abstract
Trypsin, porcine pancreatic kallikrein, and several blood coagulation enzymes, including bovine thrombin, bovine factor Xa, human factor Xa, human plasma factor XIa, human plasma factor XIIa, and human plasma kallikrein, were inactivated by a number of substituted isocoumarins containing basic functional groups (aminoalkoxy, guanidino, and isothiureidoalkoxy). 3-Alkoxy-4-chloro-7-guanidinoisocoumarins were found to be the most potent inhibitors for the coagulation enzymes tested with kobsb/[I] values in the range of 103-105 M-1 s-1. 4-Chloro-4-isothiureidoalkoxyisocoumarins show high inhibitory potency toward porcine pancreatic kallikrein, human plasma kallikrein, humor factor XIa, human factor XIIa, and trypsin with kobsd/[I] value of the order of 104-105 M-1 s-1 The inhibition of these serine proteases by the substituted isocoumarins are time dependent, and the inactivation of trypsin by 3-alkoxy-4-chloro-7-guanidinoisocoumarins and 7-amino-4-chloro-3-(3-isothiureidopropoxy)isocoumarin occurred concurrently with the loss of the isocoumarin absorbance. The complex formed from inactivation of trypsin by these two types of inhibitors was very stable and retained less than 4% activity in 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid buffer (pH 7.5) after 1 day at 25.degree. C and regained 8-45% activity upon addition of buffered 0.29 M hydroxylamine. Trypsin inactivated by other inhibitors regained full activity upon standing or addition of hydroxylamine. Thrombin inactivated by 3-alkoxy-4-chloro-7-guanidinoisocoumarins was also quite stable and only regained 9-15% activity under similar conditions. These results are consistent with a proposed mechanism, while serine proteases inactivated by aminoalkoxyisocoumarins or isothiureidoalkoxyisocoumarins form acyl enzymes that will deacylate upon standing or addition of hydroxylamine. However, the acyl enzymes fromed from 3-alkoxy-4-chloro-7-guanidinoisocoumarins or 7-amino-4-chloro-3-(3-isothiureidopropoxy)-isocoumarin will decompose further, probably through a quinone imine methide, to give an irreversibly inactivated enzyme by reaction with an active-site nucleophile such as His-57. The quinone imine methide intermedaite may also react with a solvent nucleophile to give an acyl enzyme that can be reactivated by hydroxylamine. The inhibitors 4-chloro-7-guinidino-3-methoxyisocoumarin and 4-chloro-3-ethoxy-7-guanidinoiscoumarin have been tested as anticoagulants in human plasma and were effective at prolonging the prothrombin time. However, they are unstable in plasma (t1/2 = 4-8 min), and their in vivo utility may be limited.