Inhibition of human liver microsomal epoxide hydrolase by valproate and valpromide: In vitro/in vivo correlation

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Abstract
On the basis of drug interactions with carbamazepine epoxide, it has been hypothesized that valproic acid and valpromide are inhibitors of epoxide hydrolase, but the role of epoxide hydrolase in these interactions has not been clearly established. In this study, therapeutic concentrations of valproic acid (< 1 mmol/L) and valpromide (< 10 .mu.mol/L) inhibited hydrolysis of carbamazepine epoxide and styrene oxide in human liver microsomes and in preparations of purified human liver microsomal epoxide hydrolase. Valpromide (KI = 5 .mu.mol/L) was 100 times more potent than valproic acid (KI = 550 .mu.mol/L) as an inhibitor of carbamazepine epoxide hydrolysis in microsomes. After administration of carbamazepine epoxide to voluneers, the transdihydrodiol formation clearance was decreased 20% by valproic acid (blood concentration .apprxeq. 113 .mu.mol/L) and 67% by valpromide (blood concentration (< 10 .mu.mol/L). For both valproic acid and valpromide, a striking similarity exists between in vitro and in vivo inhibitory potencies. Valproic acid and valpromide are the first drugs known to inhibit microsomal epoxide hydrolase, an important detoxification enzyme, at therapeutic concentrations.