Selective inhibition of drug oxidation after simultaneous administration of two probe drugs, antipyrine and tolbutamide.

The effects of sulphaphenazole, cimetidine and primaquine on the disposition of antipyrine and tolbutamide in healthy volunteers have been investigated. The model substrates were administered simultaneously in order more clearly to define any selective effects of the potential inhibitors. Sulphaphenazole produced a significant increase in the half-life of tolbutamide (7.10 to 21.50 h) and a corresponding decrease in its clearance (0.260 to 0.084 ml.min-1.kg-1). Clearance to hydroxytolbutamide (OHTOL) and carboxytolbutamide (COOHTOL) was also significantly decreased. In contrast, sulphaphenazole had no effect on the disposition of antipyrine. Administration of cimetidine did not significantly alter the disposition of either model drug. However, a 1.6-times higher dose of cimetidine did increase the half lives both of tolbutamide and antipyrine (6.21 to 9.04 h and 14.2 to 19.2 h, respectively) and decrease their clearance (0.226 to 0.148 and 0.50 to 0.31 ml.min-1 kg-1, respectively). Clearance to OHTOL and hydroxymethylantipyrine (HMA) was reduced. A single dose of primaquine had no demonstrable effect on tolbutamide disposition whereas the half-life of antipyrine was increased (12.1 to 15.0 h) and its clearance decreased (0.63 to 0.38 ml.min-1.kg-1). The partial clearance to HMA, 4-hydroxyantipyrine (OHA) and norantipyrine (NORA) was also significantly reduced. The two main inferences are first, that tolbutamide and antipyrine are metabolised by different forms of cytochrome P-450, and second that a battery of model substrates is needed to investigate the inhibitory effects of a drug in man.