Joseph J. P. Heykants

Effect of Food on the Pharmacokinetics of a New Hydroxypropyl‐β‐Cyclodextrin Formulation of Itraconazole

Study Objective. To compare the pharmacokinetics of a single 100‐mg oral dose of itraconazole administered as 10 ml of a 10‐mg/ml itraconazole solution in hydroxypropyl‐β‐cyclodextrin under fasting versus postprandial conditions.

Plasma protein binding of risperidone and its distribution in blood

The plasma protein binding of the new antipsychotic risperidone and of its active metabolite 9-hydroxy-risperidone was studied in vitro by equilibrium dialysis. Risperidone was 90.0% bound in human plasma, 88.2% in rat plasma and 91.7% in dog plasma. The protein binding of 9-hydroxy-risperidone was lower and averaged 77.4% in human plasma, 74.7% in rat plasma and 79.7% in dog plasma. In human plasma, the protein binding of risperidone was independent of the drug concentration up to 200 ng/ml. The binding of risperidone increased at higher pH values. Risperidone was bound to both albumin andα1-acid glycoprotein. The plasma protein binding of risperidone and 9-hydroxy-risperidone in the elderly was not significantly different from that in young subjects. Plasma protein binding differences between patients with hepatic or renal impairment and healthy subjects were either not significant or rather small. The blood to plasma concentration ratio of risperidone averaged 0.67 in man, 0.51 in dogs and 0.78 in rats. Displacement interactions of risperidone and 9-hydroxy-risperidone with other drugs were minimal.

Alfentanil pharmacokinetics and metabolism in humans

The metabolism of alfentanil was studied in three healthy subjects after a 1-h infusion of 2.5 mg alfentanil-3H. One of the subjects was a poor hydroxylator of debrisoquinc. Pharmacokinetic parameters were similar in the three subjects and were in the same range as those reported for volunteers. The majority of the administered radioactivity was excreted in the urine (90% of the dose), but unchanged alfentanil represented only 0.16–0.47% of the dose. Alfentanil and metabolites were characterized by HPLC co-chromatography with reference compounds and/or by mass spectrometry and quantified by GLC and radio-HPLC. The main metabolic pathway was N-dealkylation at the piperidine nitrogen, with formation of noralfentanil (30% of the dose). Other Phase I pathways were aromatic hydroxylation, N-dealkylation of the piperidinc ring from the phenylpropanamide nitrogen, O-demelhylation, and amide hydrolysis followed by N-acetylatton. Glucuronic acid conjugation of aromatic or aliphatic hydroxyl functions was the main Phase II pathway. The second major metabolite was the glucuronide of N-(4-hydroxyphenyl) propanamide (14% of the dose). The metabolite pattern in these subjects was qualitatively very similar to that described previously in rats and dogs. Differences in the mass balance of urinary metabolites between the three subjects were very small, and there was no qualitative or quantitative evidence for a deficiency in the metabolism of alfentanil in the subject who was a poor metabolizer of debrisoquine.

Identification of the cytochrome P450 enzymes involved in the metabolism of cisapride: in vitro studies of potential co‐medication interactions

Cisapride is a prokinetic drug that is widely used to facilitate gastrointestinal tract motility. Structurally, cisapride is a substituted piperidinyl benzamide that interacts with 5‐hydroxytryptamine‐4 receptors and which is largely without central depressant or antidopaminergic side‐effects. The aims of this study were to investigate the metabolism of cisapride in human liver microsomes and to determine which cytochrome P‐450 (CYP) isoenzyme(s) are involved in cisapride biotransformation. Additionally, the effects of various drugs on the metabolism of cisapride were investigated. The major in vitro metabolite of cisapride was formed by oxidative N‐dealkylation at the piperidine nitrogen, leading to the production of norcisapride. By using competitive inhibition data, correlation studies and heterologous expression systems, it was demonstrated that CYP3A4 was the major CYP involved. CYP2A6 also contributed to the metabolism of cisapride, albeit to a much lesser extent. The mean apparent Km against cisapride was 8.6±3.5 μM (n=3). The peak plasma levels of cisapride under normal clinical practice are approximately 0.17 μM; therefore it is unlikely that cisapride would inhibit the metabolism of co‐administered drugs. In this in vitro study the inhibitory effects of 44 drugs were tested for any effect on cisapride biotransformation. In conclusion, 34 of the drugs are unlikely to have a clinically relevant interaction; however, the antidepressant nefazodone, the macrolide antibiotic troleandomycin, the HIV‐1 protease inhibitors ritonavir and indinavir and the calcium channel blocker mibefradil inhibited the metabolism of cisapride and these interactions are likely to be of clinical relevance. Furthermore, the antimycotics ketoconazole, miconazole, hydroxy‐itraconazole, itraconazole and fluconazole, when administered orally or intravenously, would inhibit cisapride metabolism.

Interaction of miconazole ketoconazole and itraconazole with rat-liver microsomes

The interaction of the antimycotics miconazole, ketoconazole and itraconazole with liver microsomes from untreated rats or from rats pretreated with phenobarbital or 3-methylcholanthrene, gave rise to type II difference spectra. The interactions of the antimycotics with control, phenobarbital-induced or 3-methylcholanthrene-induced microsomes were biphasic, except for the monophasic binding of ketoconazole to phenobarbital-induced microsomes. The N-demethylation of N,N-dimethylaniline, the O-demethylation of p-nitroanisole and the hydroxylation of aniline in microsomes from untreated and inducer-treated rats were lowered by miconazole and ketoconazole, the former being the more potent inhibitor. Control microsomes were less sensitive than induced microsomes. Itraconazole was almost devoid of inhibitory properties. The three antimycotics were non-competitive (mixed) inhibitors of enzyme activities in phenobarbital-induced microsomes. The Ki values were of the same order of magnitude as the Ks values, except for itraconazole. For the latter drug, Ki values were much greater than could be expected from the spectral studies. It is concluded that the antimycotics affect microsomal enzyme activities via a direct interaction of an azole-nitrogen with the haem group of cytochrome P-450. The interaction with mammalian cytochrome P-450 decreases from miconazole greater than ketoconazole much greater than itraconazole and is much weaker than the interaction of the antimycotics with yeast cytochrome P-450.

Pharmacokinetics and Dose Proportionality of Domperidone in Healthy Volunteers

Domperidone is a potent gastrokinetic agent and antinauseant currently undergoing clinical trials in the United States. The bioequivalence of 20 mg of domperidone given as free‐base tablets and maleate salt tablets, and the bioavailability of base and maleate tablets relative to a solution, were studied in 21 fasting men using a crossover design. Plasma samples collected for up to 48 hours were analyzed for domperidone levels, using a sensitive and specific radioimmunoassay (RIA). The absorption of domperidone was very rapid, with mean peak plasma concentration (Cmax) values of 18.8, 15.0, and 20.7 ng/mL attained at 0.9, 1.2, and 0.6 hours after the administration of base tablet, maleate tablet, and solution, respectively. The mean elimination half‐life (t1/2) ranged from 12.6 to 16.0 hours. The mean oral clearance (CL/F) after the solution dose was 4,735 ± 2,017 mL/min and the mean apparent volume of distribution (Vd/F) was 6,272 ± 5,100 L, indicating an extensive distribution of domperidone in the body. The area under the plasma concentration‐time curve (AUC) data demonstrated bioequivalence of base and maleate tablets. The relative bioavailability for base tablet and maleate tablet was 107 ± 50% and 116 ± 47%, respectively, of that of the solution. Dose proportionality of domperidone was also studied in 12 subjects at solution doses of 10, 20, and 40 mg. Linear correlations between the dose and Cmax and AUC values were observed. Mean CL/F remained relatively constant after doses of 10, 20, and 40 mg (5,255 ± 3,159, 4,842 ± 1,774, and 4,380 ± 1,289 mL/min, respectively), indicating linear pharmacokinetics of domperidone over the dose range studied.

Assay methods for sufentanil in plasma : radioimmunoassay versus gas chromatography-mass spectrometry

BackgrondThe terminal pharmacokinetic parameters of sufentanil have, until now, been poorly characterized. This is probably because of the poor sensitivity or unreliability of the assay methods used. Radioimmunoassay (RIA) can be a very helpful assay method for sufentanil. However, before application to key pharmacokinetic studies, it requires adequate validation, e.g., by comparison with a method of proven sensitivity and specificity, such as gas chromatography-mass spectrometry (GC-MS). MethodsSpiked control plasma samples and 135 plasma samples obtained from five patients receiving intravenous doses of 500 or 750 μg sufentanil, as a 10–20-min infusion, were analyzed by an improved, sensitive RIA and capillary GC-MS. ResultsBoth techniques had comparable limits of quantitation (0.02 ng/ml). Between-day coefficients of variation in the 0.05–10-ng/ml concentration range were 8.5–10.5% for the RIA and less than 10% for the GC-MS method. The patient plasma concentrations determined by RIA (y) and GC-MS (x) showed a good agreement (y = 1.01x + 0.002) and a correlation coefficient of 0.97. ConclusionsThe results demonstrate the validity of the improved RIA method for the determination of sufentanil plasma concentrations.

Itraconazole pharmacokinetics in the female genital tract: plasma and tissue levels in patients undergoing hysterectomy after a single dose of 200 mg itraconazole

Twenty patients who underwent hysterectomy received a single dose of 200 mg itraconazole at different moments before surgery. At the moment of surgery, a urine sample, blood sample and tissue samples of different organs of the female genital tract were collected. Blood levels and tissue levels of itraconazole were measured by means of an HPLC method. Itraconazole blood levels were lower than the corresponding tissue levels in the various organs of the female genital tract. This finding for itraconazole is different from findings with ketoconazole, indicating that itraconazole has a higher affinity for tissue than ketoconazole. Urine levels of itraconazole were virtually undetectable in all samples. None of the patients complained of side-effects, and blood biochemical parameters all remained within the normal limits. The premedication with itraconazole had no effect whatsoever on the induction and maintenance of and recovery from the anaesthesia. No abnormal effects on ECG were observed.

Comparative metabolism of flunarizine in rats, dogs and man: an in vitro study with subcellular liver fractions and isolated hepatocytes

1. The biotransformation of 3H-flunarizine ((E)-1-[bis(4-fluorophenyl)methyl]-4-(3-phenyl-2-propenyl)piperazine dihydrochloride, FLUN) was studied in subcellular liver fractions (microsomes and 12,000 g fraction) and in suspensions or primary cell cultures of isolated hepatocytes of rats, dogs and man. The major in vitro metabolites were characterized by h.p.l.c. co-chromatography and/or by mass spectrometric analysis. 2. The kinetics of FLUN metabolism was studied in microsomes of dog and man. The metabolism followed linear Michaelis-Menten kinetics over the concentration range 0.1-20 microM FLUN. 3. A striking sex difference was observed for the in vitro metabolism of FLUN in rat. In male rats, oxidative N-dealkylation at one of the piperazine nitrogens, resulting in bis(4-fluorophenyl) methanol, was a major metabolic pathway, whereas aromatic hydroxylation at the phenyl of the cinnamyl moiety, resulting in hydroxy-FLUN, was a major metabolic pathway in female rats. In incubates with hepatocytes, these two metabolites were converted to the corresponding glucuronides. 4. In human subcellular fractions, aromatic hydroxylation to hydroxy-FLUN was the major metabolic pathway. In primary cell cultures of human hepatocytes, oxidative N-dealkylation at the 1- and 4-piperazine nitrogen and glucuronidation of bis(4-fluorophenyl)methanol were observed. The in vitro metabolism of FLUN in humans, resembled more than in female rats and in dogs than that in male rats. 5. The present in vitro results are compared with data of previous in vivo studies in rats and dogs. The use of subcellular fractions and/or isolated hepatocytes for the study of species differences in the biotransformation of xenobiotics is discussed.

Induction potential of fluconazole toward drug-metabolizing enzymes in rats.

The induction of drug-metabolizing enzymes in rat liver was studied after subchronic administration of the new triazole antifungal agent fluconazole. The administered doses were 10, 40, and 160 mg/kg per day for 7 days. Fluconazole behaved as a high-magnitude inducer and significantly increased cytochrome P-450 concentrations already at 10 mg/kg (+42%). Cytochrome P-450 induction by fluconazole was dose dependent and reached a value of 302% of the control value at the dose of 160 mg/kg. The induction effects on cytochrome P-450 were also reflected in the drug-metabolizing enzyme activities in hepatic microsomes of pretreated rats. Fluconazole (160 mg/kg per day) preferentially induced the demethylase activities of N,N-dimethylaniline and p-nitroanisole to 258 and 281% of the control values, respectively. The detoxification enzyme UDP-glucuronosyltransferase was significantly lowered by fluconazole at the highest dose. A possible link between the induction potential and the pharmacokinetic properties of triazole antifungal agents is discussed.