M. Michiels

A sensitive radioimmunoassay for fentanyl

SummaryAntiserum to fentanyl was obtained in rabbits repeatedly injected with carboxyfentanyl conjugated to bovine serum albumin. Using the antiserum, a highly sensitive radioimmunoassay has been developed, based on the dextran-coated charcoal method. It proved possible to assay the drug directly in plasma, in amounts as small as 30 picogram in 0.5 ml. The antibody was highly specific for fentanyl and no cross-reaction was observed with its major metabolites. This sensitive and specific radioimmunoassay method was employed to determine fentanyl in plasma from six volunteers after an intravenous bolus of 0.2 mg, and in plasma from dogs treated both intravenously and subcutaneously with 0.02 mg/kg. The plasma level of fentanyl could be followed for up to 6 h after a therapeutic dose in dogs and man.

Radioimmunoassay of the new opiate analgesics alfentanil and sufentanil. Preliminary pharmacokinetic profile in man

The development of two analogous radioimmunoassay (RIA) procedures based on dextran‐charcoal separation is described for the quantification of two fentanyl‐like analgesics, alfentanil and sufentanil. Immunization of rabbits with conjugates of bovine serum albumin and carboxy‐derivatives of the respective drugs resulted in the production of antisera capable of detecting less than 0.05 ng ml−1 of the parent analgesics with high specificity and almost no cross‐reactivity with major metabolites. Excellent agreement was obtained between RIA—without prior extraction—and gas chromatography for alfentanil concentrations in human plasma. Because of sufentanils low therapeutic plasma levels, no comparison could be made between its RIA and an alternative assay, however, there was strong evidence for the specificity of the assay when applied directly to plasma. With these RIA methods preliminary information was obtained on plasma concentrations and elimination of alfentanil or sufentanil in patients given an intravenous bolus injection of 50 μg kg−1 of alfentanil, or 5 μg kg−1 of sufentanil. For both analgesics, the pharmacokinetic profile in man could be described by a three‐compartment model. The terminal elimination half‐life was 88 min for alfentanil and 140 min for sufentanil. Six hours after a therapeutic dose, plasma levels were in the order of 3 and 0.3 ng ml−1 for alfentanil and sufentanil respectively.

On the pharmacokinetics of domperidone in animals and man. IV. The pharmacokinetics of intravenous domperidone and its bioavailability in man following intramuscular, oral and rectal administration.

SummaryThe pharmacokinetics and bioavailability of domperidone, a novel gastrokinetic, were studied in healthy male subjects by comparing plasma concentrations and urinary excretion following intravenous, intramuscular, oral and rectal administration. Two oral dosage forms were studied: 10-mg tablets and a 10-mg/ml oral solution. The influence of a meal on the oral bioavailability and the dose-proportionality were also investigated.Plasma levels of intravenous domperidone could be described by a three-compartment model with a rapid distribution of 40% of the dose to as «shallow» peripheral compartment. The final elimination half-life was 7.5 hours. Peak plasma levels were reached within 30 minutes following intramuscular and oral administration and at 1–4 hours following rectal administration. Since domperidone showed an extensive first-pass elimination, AUC-values -a measure for the bioavailability- were consider-ably lower after oral than after parenteral administration. Equal oral and rectal doses gave a similar bioavailability. AUC-values increased proportionally with the dose over a 10–60 mg range. Cumulative urinary excretion of unchanged domperidone was proportional to corresponding AUC-values.The bioavailability was discussed in the light of the therapeutic results.

The Metabolism and Fate of Closantel (Flukiver) in Sheep and Cattle

Closantel was reasonably well absorbed in sheep and cattle. After oral (10 mg/kg) or parenteral (5 mg/kg) administration, similar peak times (8-48 h) and peak plasma levels (45-55 micrograms/mL) are observed. Plasma level-time curves are superimposable for either route and increase linearly with the dose. The elimination half-life of closantel is 2 to 3 weeks. The relative bioavailability of 50% of oral closantel can partly be explained by incomplete absorption. Experiments in sheep with 14C-closantel revealed that the plasma radioactivity is almost exclusively due to the unmetabolized drug, metabolites accounting for less than 2%. At least 80% of the dose was excreted with the feces over the investigational period of 8 weeks, and less than 0.5% with the urine. Closantel was only poorly metabolized. Over 90% of the fecal radioactivity was due to the parent compound. Two monoiodoclosantel isomers were the only fecal metabolites detected with radio-HPLC. The distribution of closantel to tissues was limited by its high protein binding. Closantel bound strongly (greater than 99.9%) and almost exclusively to plasma albumin. Accordingly, tissue concentrations were many times lower than the corresponding plasma levels. Residual radioactivity in sheep in all tissues but liver was entirely due to closantel. About 30% to 40% of the liver radioactivity could be attributed to monoiodoclosantel. In both sheep and cattle, residual tissue concentrations decline parallel to the plasma concentrations. Consequently, the plasma kinetics of closantel reliably reflect its depletion from tissues. Independently of the dosing scheme and route of administration, the maximum daily intake by the consumer was always below the acceptable daily intake within 4 weeks after the last dose.

On the pharmacokinetics of domperidone in animals and man III. Comparative study on the excretion and metabolism of domperidone in rats, dogs and man

SummaryThe excretion and metabolism of the novel gastrokinetic and antinauseant drug domperidone were studied after oral administration of the14C-labelled compound to rats, dogs and man, and after intravenous administration to rats and dogs.Excretion of the radioactivity was almost complete within four days. In the three species, the radioactivity was excreted for the greater part with the faeces. Biliary excretion of the radioactivity amounted to 65% of the dose 24 hours after intravenous administration in rats.Unchanged domperidone as determined by radioimmunoassay, accounted in urine for 0.3% in dogs, 0.4% in man, and in faeces for 9% in dogs and 7% in man. The main metabolic pathways of domperidone in the three species were the aromatic hydroxylation at the benzimidazolone moiety, resulting in hydroxy-domperidone -the main faecal metabolite-, and the oxidativeN-dealkylation at the piperidine nitrogen, resulting in 2,3-dihydro-2-oxo-1H-benzamidazole-1-propanoic acid the major radioactive urinary metabolite- and 5-chloro-4-piperidinyl-1,3-dihydro-benzimidazol-2-one. In urine the two first metabolites were present partly as conjugates.A mass balance for the major metabolites in urine, faeces, bile and plasma samples was made up after radio-HPLC (reversephase HPLC with on-line radioactivity detection) of various extracts. Only minor species differences were detected.

On the pharmacokinetics of domperidone in animals and man II. Tissue distribution, placental and milk transfer of domperidone in the Wistar rat

SummaryTissue distribution, placental transfer and transition into milk of the gastrokinetic drug domperidone were studied in the Wistar rat after i.v. or p.o. administration of the labelled compound at 2.5 mg/kg. Whole-body autoradiography and liquid scintillation counting were used to investigate the tissue localization of domperidone in pregnant and non-pregnant rats. In lusted rats, tissue levels were maximal within 15 minutes after either route. In non-fasted animals peak time occurred 30 minutes after oral treatment. Large amounts of radioactivity were present in the stomach, and in the bilious contents of the intestine. High activity was further detected in the liver, kidney, lung and some glandular tissues. After a rapid initial decrease radio-activity was eliminated with a half-life of 8–10 hours. Except for brain and testes, plasma levels were markedly lower than corresponding tissue levels.At 1 hour placental levels were 2–2.7 times higher and foetal concentrations 2.1–2.5 times lower than maternal plasma levels. The placentae and foetuses predominantly contained unchanged domperidone. At peak time only 0.2 (i.v.) and 0.08% (p.o.) of the dose crossed the placenta.Blood levels were always lower than milk concentrations. Preferentially metabolites were excreted with the milk. After a 20-hour suckling period of orally dosed dams, 0.2% of the dose was recovered in the combined tissues of 6 suckling pups.

On the pharmacokinetics of domperidone in animals and man. I. Plasma levels of domperidone in rats and dogs. Age related absorption and passage through the blood brain barrier in rats.

SummaryDomperidone, a novel gastrokinetic and antinauseant lacking central side-effects, was administered intravenously to male Wistar rats and orally to fasted rats of either sex and to 1- and 6-day old neonates at doses of 2.5 mg14C-labelled drug/kg. The biphasic absorption of domperidone in fasted rats was extremely rapid suggesting a partial absorption from the stomach. The metabolism of domperidone was sex- and age-related: it was slower in the female rat and in the neonates. The elimination system for the metabolites was still immature in the 1-day old pups. The distribution of domperidone (and related metabolites) to the rat brain was limited, brain concentrations being lower than corresponding plasma levels in all cases. In 1-day old neonates, the blood-brain barrier was less obstructive to the passage of domperidone than in older rats.In Beagle dogs, domperidone pharmacokinetics were described by a two-compartment model with half-lives of distribution and elimination of 6 minutes and 2.45 hours respectively. The time-courses of the drug plasma levels were similar for single and repeated (once daily for 11 months) doses of 2.5, 10 and 40 mg/kg, indicating that chronic administration of domperidone, even at high dose levels, did not alter its pharmacokinetics. AUC-values increased proportionally with the dose pointing to linear pharmacokinetics over a wide dose range.

Radioimmunoassay of the antidiarrhoeal loperamide

Abstract Production of antibodies against loperamide was induced in rabbits that were repeatedly injected with a loperamide-protein conjugate. By using the antiserum, a sensitive and specific radioimmunoassay procedure for loperamide was developed. The drug could be assayed directly in human plasma in amounts as low as 50 picogram. None of the known metabolites of loperamide interferred with the radioimmunologic determination of loperamide in biological fluids. The disposition of loperamide was studied in man. Following a single oral dose of 4 mg in a tablet formulation, peak plasma levels, corresponding to about 0.75 ng/ml, were reached 4 hours after drug intake and drug plasma concentrations could be measured up to 24 hours after administration.

Absorption, excretion and metabolism of oral bromperidol in rats and dogs

SummaryAfter an oral dose of 0.16 mg/kg, the absorption, excretion and metabolism of bromperidol-3H, an analogue of the neuroleptic haloperidol, were studied in dogs. In rats, the tissue distribution, excretion and metabolism were followed.Plasma levels of unchanged bromperidol in the dog were maximal 4–7 hours after dosing and corresponded to 4 ng/ml. Afterwards, elimination of intact drug was biphasic with a terminal half-life of approximately 30 hours. Levels of radioactivity decreased more slowly, probably due to the presence of tritiated water in the plasma.The radioactivity in the urine of both rats (35% of the dose) and dogs (46% of the dose within 4 days), consisted mainly of polar acidic metabolites. Inverse isotope dilution demonstrated that the polar urinary fraction was due to the glycine conjugate ofp-fluorophenylacetic acid almost completely in the rat but only partly in the dog. Unchanged bromperidol was present in trace amounts in the urine, and only 10% of the dose was excreted with the faeces for both rats and dogs. Oxidative JV-dealkylation was the major metabolic pathway in both species and the fate of bromperidol was similar to that of haloperidol.

Simultaneous gas chromatographic determination of lorcainide hydrochloride and three of its principal metabolites in biological samples

A method is described for the determination of the antiarrhythmic drug lorcainide hydrochloride and its three main metabolites in plasma, urine, faeces and tissues from man and animals. The procedure involves the extract of the parent drug, its metabolites and the internal standard from the biological materials at different alkaline pH values, back-extraction into sulphuric acid and re-extraction into the organic phase (heptane--isoamyl alcohol). After silylation of the different phenolic and the N-dealkylated metabolites, analyses were carried out by automated gas--liquid chromatography with electron-capture detection. The method has a sensitivity limit of 5 ng for lorcainide, and 10--20 ng for the various metabolites, per millilitre of plasma. The method was applied to urine, faeces, plasma and tissue samples from man and animals. It was also suitable for automatic sample analysis.