Simultaneous measurement of dolasetron and its major metabolite, MDL 74,156, in human plasma and urine

A selective and sensitive analytical method for the simultaneous measurement of dolasetron (I) and its major metabolite, MDL 74,156 (II), in human plasma and urine samples has been developed using a structural analogue. MDL 101,858, as internal standard (I.S.). The compounds were extracted from plasma and urine using solvent extraction after the addition of the I.S. Chromatographic separation was carried out on a reversed-phase HPLC column and detection and quantification was by fluorescence with excitation and emission wavelengths of 285 and 345 nm, respectively. Linear responses were obtained over concentration ranges of 5 to 1000 pmol/ml for plasma samples and 20 to 1000 pmol/ml for urine samples with correlation coefficients for the calibration curves exceeding 0.999 in all cases. Intra-day and inter-day reproducibility yielded limits of quantification of 10 pmol/ml for I and 5 pmol/ml for II plasma and 50 pmol/ml for I and II in urine. The method has been applied to the simultaneous analysis of both compounds in plasma and urine samples coming from clinical pharmacokinetic studies.

Selectivity of MDL 72,974A for MAO-B inhibition based on substrate and metabolite concentrations in plasma

Plasma concentrations of 3,4-dihydroxyphenylethylglycol (DOPEG), noradrenaline (NA), adrenaline (A), 3,4-dihydroxyphenylalanine (DOPA), 3,4-dihydroxyphenylacetic acid (DOPAC), dopamine (DA) and phenylethylamine (PEA) were analyzed in samples taken prior to, during and following the administration of single, daily doses of 12 or 24 mg MDL 72,974A to healthy male volunteers. No effects on the concentrations of DOPA, A, DA or DOPAC were seen during the administration of either dose over 10 days. No treatment-related changes in the concentration of NA were evident at either dose. No changes in DOPEG or PEA concentrations were seen with the 12 mg dose; however, small but significant decreases in plasma DOPEG concentrations and a significant increase in PEA were seen during the administration of the 24 mg dose. This would suggest that at the 24 mg dose some intraneuronal inhibition of MAO-A may be occurring although the lack of increases in NA and A concentrations indicates no accompanying change in sympatho-adrenal activity. Plasma PEA concentrations do not provide a more sensitive or functional indication of MAO-B inhibition. The increase in PEA concentrations at the higher dose may suggest that the inhibition of both forms of the enzyme is necessary to increase its plasma concentration.

Kinetics and metabolism of p-tyramine during monoamine oxidase inhibition by mofegiline

The effects of monoamine oxidase B (MAO-B) inhibition by mofegiline on the pharmacokinetics of p-tyramine and its major metabolite, p-hydroxyphenylacetic acid, were investigated in 24 healthy male volunteers. p-Tyramine doses were administered before and after a 14-day treatment period of 1, 12, or 24 mg mofegiline or placebo. Normalized p-tyramine for area under the plasma concentration-time curve after treatment were not significantly different from their respective before-treatment values for any of the dose groups. The relative bioavailability of p-tyramine after treatment was not significantly different from before treatment, although a tendency to a greater bioavailability was seen with the 12 and 24 mg doses. There were no significant differences between pharmacokinetic parameters for p-hydroxyphenylacetic acid. The data suggest that mofegiline maintains its selectivity for MAO-B in the intestine and liver at doses up to and including 24 mg. Therefore these doses would not be expected to be associated with the hypertensive crises normally associated with the "cheese effect."