Spectrofluorimetric determination of nomifensine in human plasma and urine by derivatization with fluorescamine.

A sensitive, simple and rapid spectrofluorimetric method was developed for the determination of nomifensine in human plasma and urine. The present method was based on the derivatization by fluorescamine in phosphate buffer at pH 4.0 to produce a highly fluorescent product which was measured at 488 nm (excitation at 339 nm). The method was validated according to the ICH guidelines with respect to linearity, limit of detection, limit of quantification, accuracy, precision, recovery and robustness. The assay was linear over the concentration ranges 100-2,000 and 50-2,000 ng/mL for plasma and urine, respectively. The limits of detection were calculated to be 13.9 and 7.5 ng/mL for plasma and urine, respectively. The method was successfully applied to the analysis of the drug in human plasma and urine.

A two-compartment description and kinetic procedure for measuring regional cerebral [11C]nomifensine uptake using positron emission tomography.

S-[11C]Nomifensine (S-[11C]NMF) is a positron-emitting tracer suitable for positron emission tomography, which binds to both dopaminergic and noradrenergic reuptake sites in the striatum and the thalamus. Modelling of the cerebral distribution of this drug has been hampered by the rapid appearance of glucuronide metabolites in the plasma, which do not cross the blood--brain barrier. To date, [11C]NMF uptake has simply been expressed as regional versus nonspecific cerebellar activity ratios. We have calculated a "free" NMF input curve from red cell activity curves, using the fact that the free drug rapidly equilibrates between red cells and plasma, while glucuronides do not enter red cells. With this free [11C]NMF input function, all regional cerebral uptake curves could be fitted to a conventional two-compartment model, defining tracer distribution in terms of [11C]NMF regional volume of distribution. Assuming that the cerebellar volume of distribution of [11C]NMF represents the nonspecific volume of distribution of the tracer in striatum and thalamus, we have calculated an equilibrium partition coefficient for [11C]NMF between freely exchanging specific and nonspecific compartments in these regions, representing its "binding potential" to dopaminergic or noradrenergic uptake sites (or complexes). This partition coefficient was lower in the striatum when the racemate rather than the active S-enantiomer of [11C]NMF was administered. In the striatum of patients suffering from Parkinson's disease and multiple-system atrophy, the specific compartmentation of S-[11C]NMF was significantly decreased compared with that of age-matched volunteers.

Metabolism of nomifensine after oral and intravenous administration.

The metabolism of nomifensine was studied after single oral and intravenous administration and after 2 weeks of oral dosing. The three principal metabolites reached maximum plasma concentrations rapidly (in 1 to 1.5 hours) after nomifensine administration. Less than 10% was detected as a free, unconjugated form. All three metabolites were eliminated rapidly (elimination t1/2 values between 6.8 and 9.0 hours). Only very low concentrations of free metabolites were found in plasma after 24 hours of nomifensine administration. AUC values for free metabolites were between 0.27 to 0.46 hr X mumol/L after all nomifensine schedules. Two weeks of dosing had no significant influence on the elimination t1/2 or AUC values of the metabolites, indicating no change in the hydroxylation and methylation reactions. In addition, there were no changes in the conjugation reactions during prolonged nomifensine dosing. Nomifensine has a very short t1/2 and no tendency for accumulation after repeated doses. We conclude that nomifensine's clinical pharmacokinetic profile is not significantly changed by the kinetic behavior of its three main metabolites after the usual maintenance doses.

Disposition of nomifensine after acute and prolonged dosing.

The pharmacokinetics of nomifensine were studied after single oral and intravenous doses. The effect of prolonged oral dosing on the pharmacokinetics of nomifensine was also evaluated. Nomifensine was rapidly absorbed from the gastrointestinal tract. The peak concentration of free nomifensine (0.18 mumol/L) was reached at 1.13 hours after dosing. The highest concentration after the intravenous dose was 1.21 mumol/L. The elimination t1/2 after a single dose was about 4 hours regardless of the route of administration. Nomifensine was extensively distributed in body fluids and tissues, with an apparent volume of distribution of 8.69 L/kg. The AUC of free nomifensine after oral dosing was only 26.5% of that after intravenous infusion. Absorption from the gastrointestinal tract was complete, and the AUCs of total nomifensine were equal after all treatments. The main reason for limited bioavailability seems to be extensive first-pass metabolism during the absorption process. The AUC of free nomifensine decreased substantially (from 0.78 to 0.32 hr X mumol/L) and the elimination t1/2 was shortened (from 4.39 to 2.11 hours) after a 2-week dosing period. These effects suggest marked induction of the metabolizing enzymes. An increase in nomifensine dosage may be needed in some patients to maintain a full therapeutic effect.

Selective and sensitive high-performance liquid chromatographic assay for the metabolites of nomifensine in human plasma.

A selective high-performance liquid chromatographic method for the determination of the three metabolites of nomifensine in human plasma is described. All metabolites and the internal standard, mexiletine, are extracted with diethyl ether and then back-extracted into an acidic aqueous phase. After subsequent extraction into diethyl ether the metabolites are analysed by high-performance liquid chromatography. A reversed-phase C18 column is used with a mobile phase of dioxane-methanol-potassium phosphate buffer (pH 2.25). The sensitivity of the method is 0.007 micromol/l for all metabolites. Extraction efficiencies are 84.6%, 75.8%, and 78.2% for 4'-hydroxynomifensine, 4'-hydroxy-3'-methoxynomifensine and 3'-hydroxy-4'-methoxynomifensine, respectively. The reproducibility of the method is good, the coefficients of variation (%) varying between 2.1% and 9.9% in the concentration range 0.05-1.00 micromol/l. The procedure was applied to human plasma samples from a volunteer who had received a single oral dose of nomifensine. The method is accurate and sensitive for pharmacokinetic studies on the metabolites of nomifensine.

First-pass metabolism of nomifensine in dogs.

Nomifensine (1 and 5 mg/kg) was administered to dogs orally and intravenously. The pharmacokinetics of the drug was evaluated. Nomifensine was rapidly absorbed from the gastro-intestinal tract reaching maximum concentration at 0.5-1 h. The peak levels were directly proportional to the doses administered. The elimination half-life was 6 h and only very small amounts were found in blood at 24 h after administration. The apparent volume of distribution (Vd) was 120-149 1, suggesting an extensive distribution of the drug throughout body fluids and tissues. The area under the serum concentration-time curve (AUC) obtained after oral administration was significantly smaller than that after intravenous administration indicating incomplete bioavailability of the drug in oral form. The conjugation of nomifensine after the two different administration routes was also studied: the conjugation reaction was in equilibrium at 15 min after oral administration, while after intravenous administration, equilibrium was not reached until 1-1.5 h. The metabolism of nomifensine occurred in the gastrointestinal membranes and or in the liver during the absorption process; the first-pass effect was marked.

Kinetics and metabolism of nomifensine.

Metabolic and pharmacokinetic studies of nomifensine maleate, a tetrahydroisoquinoline derivative with antidepressant properties, are reviewed. Results of pharmacokinetic studies indicate that nomifensine has a short distribution phase and a large volume of distribution. It is rapidly metabolized to its N-glucuronide. Plasma levels of nomifensine-N-glucuronide are up to 100-fold higher than those of nomifensine, obviously because of a smaller volume of distribution. As nomifensine-N-glucuronide is extremely unstable and cleaved to nomifensine, determinations of nomifensine are easily falsified. It is therefore recommended only to determine the sum of nomifensine and its N-glucuronide (total nomifensine) in clinical trials. Kinetics of total nomifensine can best be described by the open two-compartment model: Maximum plasma levels are obtained 1-2 hours postadministration; mean elimination half-life is 2 hours. Excretion is almost entirely by the kidneys, with approximately 88% of an oral dose excreted within 24 hours.

Determination of nomifensine in human serum. A comparison of high-performance liquid and gas--liquid chromatography.

High-performance liquid (HPLC) and gas--liquid chromatographic (GLC) methods for the measurement of the antidepressant nomifensine in human serum were developed and compared for precision, accuracy, sensitivity and convenience. No significant difference was found between these two techniques with regard to sensitivity and precision. Both methods can accurately measure serum nomifensine concentrations down to 8 nmol/1. The coefficient of variation (C.V.) for intra-assay variability of nomifensine was 4.8% (HPLC) and 5.5% (GLC) at 150 nmol/1. The HPLC method proved to be both simpler and more selective than the GLC method. The calibration graph was linear over the range 8-1000 nmol/1 in the HPLC method, but only up to 150 nmol/1 in the GLC method. The selectivity and simplicity of the HPLC method make it useful for both pharmacokinetic studies and therapeutic serum level monitoring of nomifensine. The HPLC method was applied to the analysis of serum samples obtained from four healthy individuals receiving therapeutic dosages of nomifensine.

Clinical effects and serum levels of nomifensine in depressive patients.

Twenty depressed out-patients were treated with nomifensine in an open clinical trial of 6 weeks duration. After an initial placebo week the daily doses of nomifensine varied from 50 to 300 mg divided into 2-3 doses. Mean score on the Hamilton Rating Scale for Depression (HRS) declined from 19.7 to 10.3 at Week 4. At the same time-point morning serum levels of nomifensine varied from less than 8.0 to 196.0 nmol/l. No correlation was found between drug concentrations and clinical outcome or side-effects. There was also a pronounced intra-individual variation in nomifensine serum levels. No tendency for accumulation of the drug was observed. Thus, the measuring of nomifensine trough levels in the morning does not give useful clinical information.

Determination of nomifensine plasma concentrations: a comparison of radioimmunoassay and gas chromatography.

1 A gas-liquid chromatography (g.l.c.) method for measurement of the antidepressant nomifensine was developed and compared for precision, accuracy, sensitivity and convenience with radioimmunoassay (RIA). 2 No significant difference was found between the two techniques for unconjugated nomifensine (y = 1.07x = 2.7; r = 0.996) or total nomifensine (y = 1.;02x + 0.02; r = 0.999). 3 RIA proved more sensitive than g.l.c. (detection limit of RIA being 1 microgram/l and g.l.c. 5 micrograms/l). 4 RIA was found to be a more convenient technique (up to 50 samples per day with RIA, and 16 samples with g.l.c.).

A comparison of the pharmacodynamic profiles of nomifensine and amitriptyline in normal subjects.

1 Six healthy male volunteers participated in a double-blind placebo crossover comparison of the pharmacodynamic profiles of single oral doses of 75 mg nomifensine and 50 mg amitriptyline. 2 Nomifensine treatment did not influence salivary flow and did not significantly affect psychomotor performance (critical flicker fusion, pursuit rotor and reaction time): in addition nomifensine had no significant effect on subjective measurements of sedation and concentration. 3 By contrast, amitriptyline treatment significantly reduced salivary flow and was associated with significant sedation and reduced concentration: significant changes in psychomotor performance were also noted. 4 Plasma concentrations of amitriptyline and nomifensine were measured at 2 h. The respective median concentration values were 55.0 ng/ml and 52.0 ng/ml. 5 Ex vivo platelet amine uptake of dopamine (DA) and 5-hydroxytryptamine (5HT) was measured 2 h after each treatment. Both nomifensine and amitriptyline treatment significantly inhibited DA uptake to a similar extent. Amitriptyline treatment additionally inhibited 5-HT uptake.

Plasma nomifensine concentration. Cardiological effects and clinical response.

Cardiological effects of nomifensine were studied in 10 endogenously depressed in-patients receiving up to 200 mg/day for 3 weeks. No significant cardiac effects were observed using His' bundle electrography and electrocardiography. 9 of the 10 patients showed a significant recovery from their depressive illness. Plasma nomifensine concentrations at day 22 were not correlated with clinical response, blood pressure, or electrocardiographic parameters. A negative correlation between A-H interval, in the His' bundle electrogram, and day 22 nomifensine concentration was observed.