Incorporation of radiolabelled amino acids into the sulphur-containing metabolites of paracetamol by the hamster.

Various radiolabelled forms of the amino acids cysteine, methionine and serine were co-administered with paracetamol to the hamster to investigate the biosynthesis of the thiomethyl metabolites of paracetamol. T.l.c. and h.p.l.c. were used to determine the presence of sulphur-containing metabolites in the urine. Paracetamol sulphate and the 3-cysteine, mercapturate and thiomethyl derivatives were all detected in urine, along with the 3-methylsulphoxide after hydrolysis. No paracetamol-3-methylsulphone was detected. Both cysteine and methionine acted as sulphur donors via glutathione. The methyl group of the thiomethyl derivatives originated from methionine in a reaction system believed to involve cysteine conjugate-beta-lyase and the thiomethyl shunt.

The bioavailability and metabolism of trilostane in normal subjects, a comparative study using high pressure liquid chromatographic and quantitative cytochemical assays.

High pressure liquid chromatographic (HPLC) analysis of plasma taken over 8 h from ten normal male subjects medicated with 120 mg of trilostane revealed that the drug is rapidly metabolised into at least one metabolite, 17-keto trilostane. Both compounds were detected in the blood stream at concentrations greater than 2 X 10(-7) M within an hour and were cleared from the blood by 6-8 h. Approximately 3 times the concentration of metabolite was detected compared to the parent compound in most samples analysed. There were large subject to subject variations in the handling of drug. Standard curves of pure 17-keto trilostane and trilostane were parallel as assessed by cytochemical bioassay. This assay is based upon the inhibition of 3 beta-hydroxysteroid dehydrogenase activity in unfixed tissue sections of the dioestrous rat ovary. The relative potency of the metabolite compared to trilostane was 1.71 (95% confidence 1.5-2.0) over the dose range 0.15-1.5 microM. Thus, the metabolite may be the major active agent when trilostane is administered for clinical purposes. In a further 4 volunteers, who also received 120 mg trilostane and were sampled over an 8 h period, plasma was analysed independently by HPLC and cytochemical assays. In the majority of cases the bioactivity recorded (relative to a trilostane standard curve) was substantially higher than the molar sum of circulating trilostane and 17-keto-trilostane (as assessed by HPLC). However, if the relative potency of 17-keto-trilostane is taken into consideration, correlation between the two assays was excellent (r = 0.947, n = 18, P less than 0.001). This also suggests that no further active metabolites were present in the plasma samples. The drug profiles seen in the second study were essentially the same as described for the first 10 volunteers. The combination of a bioassay, which detects trilostane-like bioactivity, and HPLC, which reveals the type of metabolism, should aid our understanding of the clinical value of this potentially important drug.

Relative bioavailability of a paracetamol suppository.

The relative bioavailability of a new paracetamol suppository (Panodil) and tablets in doses of 0.5 and 1 g was investigated in eight healthy subjects. The tablets were absorbed faster and higher peak plasma concentrations were obtained than after the suppositories. The bioavailability of the suppositories was approximately 80% of that of the tablets at both dose levels. There was no indication of capacity-limited elimination at either the two doses investigated.

Isolation and identification of paracetamol metabolites.

A comparison has been made of the urinary metabolites of volunteers who had taken therapeutic doses of paracetamol with those of persons who had taken an overdose in an attempt to highlight the metabolic changes associated with massive doses. The main technique for examining urine samples was two-dimensional thin layer chromatography. Other chromatographic techniques were used for the isolation and purification of metabolites. The urinary metabolites after a therapeutic dose of paracetamol were identified as free paracetamol, paracetamol sulphate, 3-hydroxy-paracetamol-3-sulphate, 3-methoxy-paracetamol sulphate, paracetamol glucuronide, 3-methoxy-paracetamol glucuronide, paracetamol 3-cysteine conjugate and paracetamol 3-mercapturate. The same metabolites were also present in urine following overdosage but the proportions were quite different. There was particularly a big increase in the relative amounts of cysteine and mercapturic acid conjugates excreted. No new metabolites were found. The significance of these findings is briefly discussed in relation to the metabolism and toxicology of paracetamol.