Mephenytoin disposition and serum bile acids as indices of hepatic function in chronic viral hepatitis

The effect of chronic viral hepatitis on liver function may vary from none to hepatic failure. Changes in function are usually the result of impaired hepatocyte function or altered vascular flow and architecture. Conventional liver function tests usually cannot distinguish contributions from these mechanisms or indicate degree of hepatic metabolic dysfunction. An alternative approach is to measure the hepatic metabolism of a highly extracted compound whose oral clearance and systemic bioavailability are dependent on both hepatocyte function and degree of portosystemic shunt.

The stereoselective metabolism of racemic mephenytoin (100 mg oral dose) was investigated in 35 patients with chronic viral hepatitis and compared with 153 healthy subjects. The mephenytoin R/S enantiomeric ratio and cumulative excretion of the 4'-hydroxymephenytoin metabolite in a 0- to 8-hour urine sample were used in addition to serum bile acid levels and pathologic examination of biopsy specimens to assess the severity of hepatic dysfunction and portosystemic shunting.

The patients as a group excreted less 4'-hydroxymephenytoin and had a smaller R/S enantiomeric ratio of mephenytoin. The two measures were discriminatory between the patient groups classified by either serum cholylglycine level or pathologic examination of biopsy specimens. Combination of the two measures of mephenytoin metabolism allowed the patients to be classified into three groups: normal hepatocyte function without portosystemic shunt, normal hepatocyte function with portosystemic shunt, and low hepatocyte function with or without portosystemic shunt.

This study has shown the potential usefulness of mephenytoin metabolism as a sensitive indicator of hepatic pathologic condition with an ability to discriminate between contributory alternative mechanisms.