Freshly isolated hepatocytes as a model for studying the toxicity of paracetamol.

The toxicity of paracetamol has been investigated in freshly isolated hamster hepatocytes. Two phases of toxicity have been identified. In phase 1, metabolic activation of paracetamol occurs with depletion of glutathione. In phase 2, there is progressive morphological damage, leading ultimately to cell death. This occurs even in the absence of further exposure to paracetamol. The thiol reductant, dithiothreitol, added at the start of phase 2, prevents and reverses the toxicological damage that would otherwise occur. Thus, it is most likely that paracetamol causes hepatotoxicity through oxidation of SH groups in key enzymes. N-Acetylcysteine, but not methionine, has an effect similar to that of dithiothreitol. This difference is probably due to oxidation of the enzymes involved in the conversion of methionine to cysteine, whereas N-acetylcysteine can still serve as a precursor of glutathione. The glutathione can act both by adduct formation with the metabolite of paracetamol and as a thiol reductant. Species differences in sensitivity to paracetamol toxicity were shown to be due to differences in the rate of oxidation of the drug to its toxic metabolite. Most people are relatively poor activators of paracetamol, but in few subjects the reaction proceeds quite rapidly, rendering such individuals more sensitive to the hepatotoxic effects of the drug.

Induction of mixed function oxidase activity in man by rifapentine (MDL 473), a long-acting rifamycin derivative.

The effects of rifapentine (MDL 473) administration on hepatic mixed function oxidase activity in man have been investigated in six healthy volunteers. Administration of rifapentine (600 mg 48 h-1) for 10 days resulted in a significant reduction in antipyrine half-life (from 13.2 +/- 1.0 h to 7.7 +/- 0.4 h) and a corresponding increase in its total body clearance (from 41.8 +/- 5.5 ml min-1 to 67.4 +/- 5.6 ml min-1). Twelve days after stopping rifapentine administration, these values had largely returned to base-line. 24-Hour excretion of 6 beta-hydroxycortisol was significantly increased, by approximately three-fold, following administration of rifapentine for 10 days. Again, 12 days after stopping drug administration, 6 beta-hydroxycortisol excretion had returned to pretreatment values. Clearance of antipyrine to its three oxidative metabolites was increased by rifapentine administration, although the increase for 3-hydroxymethylantipyrine was not significant. The greatest increase (+140%) was observed for norantipyrine. Twelve days after the last dose of rifapentine, all values had returned to control levels. It is concluded that, like rifampicin, rifapentine is a potent inducer of mixed function oxidase activity in man and that the possibility of clinically significant drug interactions should be anticipated in the therapeutic use of this compound.

Attempts to phenotype human liver samples in vitro for debrisoquine 4-hydroxylase activity.

Twenty-eight samples of human liver have been characterised for cytochrome P-450 content, aldrin epoxidase, debrisoquine 4-hydroxylase and bufuralol 1'-hydroxylase activities. Evidence is presented here and elsewhere that bufuralol 1'-hydroxylase and debrisoquine 4-hydroxylase are activities catalysed by the same form of cytochrome P-450 in man, and that this form is different from that catalysing the epoxidation of aldrin. Attempts to phenotype liver samples in vitro, in the absence of any metabolic data in vivo for debrisoquine 4-hydroxylation status, met with limited success. A combination of enzyme assays will most probably be required in any such phenotyping of human liver samples.

Genetic polymorphism in drug oxidation: in vitro studies of human debrisoquine 4-hydroxylase and bufuralol 1'-hydroxylase activities.

A sensitive, specific assay utilizing fluorescence-HPLC has been developed for determining the 1'-hydroxylation of bufuralol by human liver. The 1'-hydroxylation of the isomers of bufuralol varied threefold, both the Vmax and the Km for the (+) isomer being greater than the corresponding values for the (-) isomer. Debrisoquine was a competitive inhibitor of the 1'-hydroxylation of both isomers and of the racemate of bufuralol. Both isomers and the racemate of bufuralol were competitive inhibitors of debrisoquine 4-hydroxylase activity. The competitive inhibition of debrisoquine and bufuralol of each other's metabolism, together with the similarity in the values for Km and Ki, support the conclusion that the same form of cytochrome P-450 catalyses these two reactions.

Antipyrine elimination as a dynamic test of hepatic functional integrity in obstructive jaundice.

Antipyrine elimination was studied in 29 patients with obstructive jaundice Antipyrine half-lives calculated using plasma concentrations at four and 24 hours ('short antipyrine test') were significantly correlated with those calculated using six time points (p less than 0.001). Mean antipyrine half-life was 28.3 +/- 8 hours (standard error) and was significantly longer than in normal subjects (p less than 0.001). Antipyrine half-life did not correlate with standard biochemical liver function tests, but correlated positively with the postoperative half-time for clearance of endogenous bilirubin (p less than 0.05), and negatively with hepatic cytochrome P-450 content measured in peroperative liver biopsies (p less than 0.05). Of six patients with antipyrine half-life greater than 20 hours, four died, one preoperatively of gastrointestinal haemorrhage and three postoperatively of sepsis. Serial short antipyrine tests were performed in 13 patients before and after biliary drainage. Those with an initial antipyrine half-life greater than 15 hours showed significant changes after drainage, while those with an antipyrine half-life less than 15 hours did not. The test of antipyrine half-life may aid in selecting high risk patients with obstructive jaundice for percutaneous biliary drainage before definitive surgery, and in determining the optimal time for such preliminary biliary decompression.