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.

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.

Quantitative thin-layer chromatography for routine determination of nomifensine and its metabolites in human urine.

For pharmacokinetic studies with nomifensine, a thin-layer chromatographic (TLC) assay for human urine was introduced. Following acid cleavage of the N-glucuronides, nomifensine and its three main metabolites (M1, M2 and M3) were extracted at pH 10. An aliquot was transferred on to a silica gel plate. After chromatography, irradiation led to intense fluorescent yellow products, which were evaluated using a chromatogram spectrophotometer. Calibration graphs were defined by single parameters of non-linearity. The method is practicable, selective and accurate with detection limits of 0.2 micrograms/ml in urine for the four compounds of interest and can be used for assaying samples up to 24 h following dosage. Total nomifensine urine levels correlated well with those determined by a previous radioimmunoassay method. From cumulative excretions of nomifensine, complete relative bioavailability of a capsule formulation vs. oral solution was shown. Further, sex independence of urine excretion was demonstrated. Pharmacokinetic data were computed using a two-compartment open model for nomifensine and its potent metabolite M1 or a one-compartment open model for M2 and M3.

Clinical trial of a test stick to control patient compliance under nomifensine treatment.

A reliable and rapid way of assessing patient compliance is the use of a semi-quantitative drug determination in the urine. Almost all urines were positive to a microchromatographic test stick 2-6 hours after 25 or 100 mg nomifensine was taken orally by healthy normal volunteers. The test reliability between different observers varied between 76 and 100 per cent. False positive results were regularly observed with carbamazepine and triamterene, and false negatives or weakened reactions with lithium. The test is reliable and highly 'time-specific'. Its applicability is, therefore, restricted to very specific questions.

Metabolism of nomifensine (Alival, Merital): isolation and identification of the conjugates of nomifensine-14C from human urine.

1 It can be stated in regard to the metabolism of nomifensine that the metabolites and also the starting compound are present in alkaline urine almost completely in conjugated form. 2 The metabolites M I-M III represent about 30% of the radioactivity excreted with urine. Two-thirds are N-glucuronides and one-third O-glucuronides. 3 Nearly the entire remainder of the renally eliminated radioactivity in nomifensine which is present in conjugated form as N-glucuronide. 4 Thus over 90% of the metabolites formed in man and excreted via urine have been identified.