Validation of an LC-MS assay for the quantification of the enantiomers of Org 4428 in human plasma.

A sensitive and selective liquid chromatographic mass spectrometric assay has been validated for the quantification of Org 4428 enantiomers in human plasma. The assay employs n-hexane extraction from alkalinized plasma, separation on a narrow-bore enantioselective normal phase Chiralpak AD column and APCI MS-MS detection. The lower limit of quantification is 0.5 ng ml-1 plasma for the individual enantiomers.

Pharmacokinetics and biotransformation of mirtazapine in human volunteers.

This paper investigated the pharmacokinetics and biotransformation of mirtazapine in healthy human volunteers. The results showed that the area under the plasma drug concentration-time curve (AUC) of mirtazapine in human plasma appeared to be three times higher than the AUC of demethylmirtazapine. As mirtazapine is marketed as a racemic mixture and both enantiomers possess pharmacological properties essential for the overall activity of the racemate, the pharmacokinetics of mirtazapine were examined and appeared to be enantioselective. The R(-)-enantiomer showed the longest elimination half-life from plasma. This was ascribed to the preferred formation of a quaternary ammonium glucuronide of the R(-)-enantiomer. This glucuronide may be deconjugated, leading to a further circulation of the parent compound, thus causing a prolongation in the elimination half-life. The S(+)-enantiomer was preferentially metabolised into an 8-hydroxy glucuronide. Other metabolic transformation pathways found for mirtazapine were demethylation and N-oxidation. Mirtazapine was extensively metabolised and almost completely excreted in the urine (over 80%) and faeces within a few days after oral administration.

Mirtazapine pharmacokinetics with two dosage regimens and two pharmaceutical formulations.

PURPOSE: To compare, in a clinical study of a special design, the pharmacokinetic profile of mirtazapine in 20 young healthy male volunteers on two treatment regimens with homothetic oral tablets at steady state: NOCTE (1 x 30 mg at 21.00 h) and BID (15 mg at 21.00 h and 15 mg at 09.00 h). METHODS: Pharmacokinetic parameters were calculated from mirtazapine plasma levels assayed by gas chromatography with nitrogen-sensitive detection. A special analysis of variance allowed interesting interactions to be distinguished. RESULTS: The steady state was reached after 4 and 6 days for NOCTE and BID respectively; the difference was presumably due to intersubject variability. In accordance with pharmacokinetic theory, the peak-to-trough ratio at steady state was significantly lower (twofold) for BID than for NOCTE. Within BID, a small difference (approx. 10%) was found in the extent of absorption between evening and morning administration. Although statistically significant, this difference meets strict bioequivalence requirements. The regimens NOCTE and BID were found to be bioequivalent for the steady-state area-under-the-curve-curve and the peak time. Bioequivalence testing for the peak level was not meaningful due to the difference in dosing regimens. The observed elimination half-lives of 19.7 +/- 3.0 h and 20.8 +/- 2.7 h (n = 20) for NOCTE and BID, respectively are in agreement with previous results. CONCLUSIONS: Differences (if any) were found to meet strict bioequivalence requirements and were so small that they are of no clinical consequences.

Pharmacokinetics of steroidal muscle relaxants in isolated perfused rat liver.

Both in humans and animals hepatic elimination is an important factor determining the duration of action of non-depolarizing neuromuscular blocking drugs. To elucidate the hepato-biliary disposition of muscle relaxants the pharmacokinetics of several structurally related but physicochemically distinct steroidal neuromuscular blocking drugs were studied in isolated perfused rat livers. Pharmacokinetics analysis with the DIFFIT computer program enabled the simultaneous fitting of independently measured perfusate disappearance and biliary excretion rate curves using a numerical approach. The hepatic disposition of the steroidal muscle relaxants could be adequately described by a three compartment model with elimination from the peripheral compartment V2 (biliary excretion) and storage in a deep compartment (V3) connected to V2. In addition, for vecuronium only slow ester hydrolysis occurring in V2 and V3 was included in the model. The lipophilicity rather than the relative mobility of the muscle relaxants showed a positive relationship with biliary clearance (Cl20) and the initial hepatic uptake (Cl12), indicating that hepato-biliary transport of these organic cations is highly dependent on the hydrophobic character of the compounds. In addition, net hepatic uptake of the steroidal cations was influenced markedly by transport from the liver to perfusate (hepatic efflux). This hepatic efflux (k21) decreased with increasing lipophilicity. In contrast, the extent of intracellular sequestration into deep compartments, indicated by high k23/k32 ratios, seemed to be inversely related to the lipophilicity of the muscle relaxants and might explain the observed prolonged hepatic storage of some of these compounds. In combination with data from subfractionation studies the results indicate that the pharmacokinetic analysis of the hepatic disposition of steroidal muscle relaxants may be used to evaluate actual transport phenomena participating in the hepatic disposition of these drugs.

Assay of vecuronium in plasma using solid-phase extraction, high-performance liquid chromatography and post-column ion-pair extraction with fluorimetric detection.

An assay has been developed and validated for the routine monitoring of vecuronium in plasma. It consists of solid-phase extraction using C18 disposables as sample pre-treatment, high-performance liquid chromatography and post-column ion-pair extraction with fluorimetric detection. The fluorescent anion 9,10-dimethoxyanthracene-2-sulphonate (DAS) is used as the counter ion. The detection limit is ca. 5 ng/ml in plasma with a signal-to-noise ratio of 10. The assay is also applicable for monitoring vecuronium and its potential metabolites in other biological media, e.g., urine, bile and tissue (liver, kidney) homogenates.

Determination in plasma of a new antiepileptic drug, dl-(5 alpha,9 alpha,11S*)-5,6,9,10-tetrahydro-N,N-dimethyl-5,9-methanobenzocycloo cten-11-amine hydrochloride, and its N-desmethyl metabolite by liquid-solid extraction and capillary gas chromatography.

We have developed a sensitive and accurate method for the determination in plasma of the antiepileptic drug dl-(5 alpha,9 alpha,11S*)-5,6,9,10- tetrahydro-N,N-dimethyl-5,9-methanobenzocycloocten-11-amine hydrochloride and its N-desmethyl metabolite. The extraction procedure utilizes base-treated disposable C2 solid-phase columns, with the analyte eluted with organic solvent. Nitrogen-selective gas chromatography is used for detection. Linear regression analysis showed that the method is linear between 4 and 1500 ng/ml for the parent drug and between 8 and 3000 ng/ml for the N-desmethyl metabolite. Intra- and inter-day variability, as shown by the coefficient of variation, is less than 8% for both compounds. The method is applicable to routine plasma determination of both these compounds in clinical pharmacokinetic studies.

Determination of Org NC 45 (a myoneural blocking agent) in human plasma using high-performance normal-phase liquid chromatography.

The assay for the quantification of Org NC 45 in human plasma is described in detail. It comprises ion-pair extraction and normal-phase liquid chromatography in conjunction with UV detection. An analogue, 17 beta-deacetyl-Org NC 45, is used as standard in the assay procedure. The accuracy and precision of the assay at 400 ng of Org NC 45 per ml of plasma are 0.7% and 2.8%, respectively. The detection limit is approx. 50 ng Org NC 45 per ml of plasma. The assay can be used for the pharmacokinetic evaluation of Org NC 45 in man.

Separation of geometric isomers of retinyl ester, retinal and retinol, pertaining to the visual cycle, by high-performance liquid chromatography.

A method for the analytical and/or preparative separation of the 13-cis, 11-cis, 9-cis and all all-trans isomers of retinyl palmitate ester, retinal and retinol by high-performance liquid chromatography is described. A straight-phase adsorption system consisting of Si 60 silica gel as the stationary phase and mixtures of n-hexane and dioxane as mobile phases were employed, using photometric detection at either 320 or 360 nm, depending on the nature of the compounds being studied. With simple adaptation of the phase system, all geometric isomers within each group could be separated. The precision of the method was 0.5% at the 10-microgram (about 40-nmole) level and 4% at the 10-ng (about 40-pmole) level (n = 3). The detection limit was about 0.5 ng (about 2 pmole). The suitability of the phase system is demonstrated by chromatograms of test mixtures and of eye extracts. The fractionation of 0.5 mg of an isomeric sample could be achieved on a column of length 250 mm and I.D. 10 mm.