Effect of tacrine hydrochloride on hepatic drug metabolism

The aim was to assess tacrine hydrochloride (THA) as an inhibitor of rat hepatic oxidative enzymes. A model of hepatic microsome oxidative metabolism was established using antipyrine (AP) incubated with NADPH. AP and its metabolites, 3-hydroxymethyl antipyrine (HMA). 4-hydroxy antipyrine (OHA) and norantipyrine (NORA) were measured by high performance liquid chromatography (HPLC). Aliquots of 200, 400 and 600 microg/ml antipyrine were incubated with the microsomal preparation alone, with 20 microg/ml cimetidine or with 40, 80 or 200 microg/ml THA. Cimetidine inhibited HMA production by 35-38% (P<0.001) and OHA production by 49-52% (P<0.001). Incubation with the 3 concentrations of THA inhibited HMA production by 17%, 24% and 41% (P<0.001) and OHA production by 52%, 55% and 79%, respectively (P<0.001). NORA was identifiable when antipyrine was incubated with NADPH alone, but could not be identified after incubation with either cimetidine or THA. This study has shown that THA causes the inhibition of AP metabolism to HMA, OHA and possibly NORA. We suggest THA is an inhibitor of three different hepatic microsomal cytochrome P-450 enzyme sub-families.