Evaluation and comparison of colorimetric, radiometric and high performance liquid chromatographic assays for aminopyrine-N-demethylation by rat liver microsomes

Toxicokinetics of dimethylacetamide (DMAc) in rat isolated perfused liver

Dimethylacetamide (DMAc) is a skin-penetrating solvent able to induce hepatic damage after chronic exposure. Previous research has indicated that metabolism may be saturated at its present TLV/TWA (10 ppm). Biological monitoring of monomethylacetamide (MMAc), the primary metabolite of DMAc, might therefore underestimate exposure to DMAc and related health hazards. We used the recirculating perfusion technique in isolated rat liver to evaluate DMAc metabolism. Medium concentrations starting at about 30, 50, 100 and 275 microM, respectively, were tested. Perfusate samples were taken regularly and analysed for DMAc; pharmacokinetic parameters (extraction ratio and clearance) were calculated for each perfusion. Inlet DMAc concentrations were calculated and concentration groups divided in 16, 36, 70, 160, 225 microM. The extraction ratio of the 16 microM group differed significantly from the other concentration groups tested. DMAc metabolism was saturated at a DMAc concentration of 36 microM. Extraction ratios were unaffected when cimetidine, an inhibitor of cytochrome P450 activity, was added to the perfusion medium or when cimetidine-pretreated animals were used. DMAc clearance was 2.20 ml min-1 at a medium concentration of about 36 microM. Extrapolation of the observed (rat) liver clearance to man showed that airborne concentrations of 18 ppm would, under the presumptions used, lead to saturated metabolism of DMAc; however, saturation at even lower concentrations could not be excluded.

The effects of cimetidine, ranitidine and famotidine on rat hepatic microsomal cytochrome P-450 activities

It has been questioned whether the interaction of H2-antagonists with cytochrome P-450 that is observed in vitro is also relevant for the in vivo situation. Until now the possibility that cytochrome P-450 may function with different modes of action has been neglected in this respect. We studied the effect of cimetidine, ranitidine and famotidine on the monoxygenase, the oxidase and the peroxidase action of cytochrome P-450. Biotransformation catalyzed by the monoxygenase and oxidase action of cytochrome P-450 was affected by cimetidine (probably via its ligand interaction with cytochrome P-450), whereas metabolism by the peroxidase mode of action of cytochrome P-450 was hardly influenced. Ranitidine and famotidine (both pharmacodynamically more potent than cimetidine) only slightly affected cytochrome P-450 activities.

The effect of carbon monoxide on aminopyrine metabolism in the isolated perfused rabbit lung

Carbon monoxide (CO) is a ubiquitous environmental pollutant widely recognized for its ability to inhibit cytochrome P450-mediated metabolism of xenobiotics in vitro. In recent years, the importance of the lung in the metabolic disposition of certain airborne and systemically administered xenobiotics has been demonstrated. The purpose of this investigation was to establish a threshold for the CO-induced inhibition of cytochrome P450-mediated activity in the isolated perfused rabbit lung and to determine if hemoglobin would alter the carbon monoxide-cytochrome P450 interaction. On the basis of its half-life and the stoichiometry of its metabolism, aminopyrine was shown to be a good substrate for monitoring mixed function oxidase activity in the intact rabbit lung. First-order rate constants for aminopyrine metabolism were significantly lower in isolated rabbit lungs perfused with either artificial medium (39%) or whole blood (67%) and ventilated with a 7.5% CO/20% O2 mixture for 2.5 hr than in the respective control lungs ventilated with breathing air. The threshold level (7.5% CO) for this inhibition is the same in lungs perfused with artificial medium and in whole blood-perfused lungs and is well above environmentally relevant levels of exposure.

Inhibition of diazepam metabolism in microsomal- and perfused liver preparations of the rat by desmethyldiazepam, N-methyloxazepam and oxazepam

Hydroxylated metabolites of diazepam can be conjugated and are therefore generally thought not to affect the metabolism of diazepam. Liver microsomes, obtained from phenobarbital-pretreated rats, showed an inhibition of diazepam (10(-5) M) metabolism by desmethyldiazepam as well as by N-methyloxazepam or oxazepam (5 X 10(-5) M). In a single-pass perfusion of the rat liver an inhibition of diazepam disposition by exogenously administered desmethyldiazepam and by hydroxylated diazepam metabolites was also demonstrated. No oxazepam glucuronides were found after oxazepam infusion. However, infusion with N-methyloxazepam resulted in large amounts of oxazepam-glucuronides. The results indicate that administration of N-demethylated as well as hydroxylated metabolites may result in inhibition of the metabolism of their precursor. If hydroxylated metabolites are formed in situ they become more easily conjugated in comparison with administered hydroxylated metabolites and are therefore less effective as inhibitor.

Effect of multiple administration of orphenadrine or mono-N-desmethylorphenadrine on cytochrome P-450 catalyzed reactions in the rat

Multiple administration (i.p.) of orphenadrine or its mono-N-demethylated metabolite, tofenacine (day 1, 20 mg/kg; day 2-5, 30 mg/kg) results in a considerable induction (50%) of the total cytochrome P-450 content. In addition, approximately 6% of the total amount of cytochrome P-450 was found to be blocked by a metabolic intermediate, formed from orphenadrine or tofenacine. Induction is apparent in enhancing the in vitro N-demethylation of aminopyrine and ethylmorphine and the p-hydroxylation of aniline. Pretreatment induced orphenadrine metabolism in vitro. The metabolism of tofenacine, however, was reduced. Probably this is due to a specific inhibition caused by the irreversible interaction of the metabolic intermediate with cytochrome P-450. In vivo, no induction of the aminopyrine metabolism (30 mg/kg, i.v.) is apparent, i.e., no change in the clearance was observed after pretreatment. This is probably due to the presence of relatively high, inhibitory concentrations of tofenacine (in the vicinity of cytochrome P-450). These results show that during chronic administration of orphenadrine or tofenacine, the in vivo disposition of concomitantly ingested compounds is determined by the influence of induction, high substrate and/or metabolite levels and complexation of cytochrome P-450. Moreover, based on these results an hypothesis is put forward in order to explain the phenomenon of product inhibition, which has been suggested to occur in man under chronic orphenadrine dosing conditions.

A simple high-performance liquid chromatographic pre-column technique for investigation of drug metabolism in biological fluids

A simple reversed-phase high-performance liquid chromatographic pre-column technique for investigation of drug metabolism is described. This rapid method circumvents the "classical" sample purification via extraction by direct purification and enrichment of the sample on the pre-column. Almost 100% recovery of a drug (aminopyrine) and its metabolites from biological fluids is achieved. This is in strong contrast to "classical" sample preparation which allows a recovery of 30-100% depending on the polarity of the investigated compound. The procedure described has been successfully applied to the investigation of the metabolic pattern of aminopyrine in rat plasma and cell incubation media.