Molecular aspects of debrisoquine metabolism studied by gas chromatography mass spectrometry with electron capture negative ion chemical ionisation

Stereoselectivity of the 4-hydroxylation of debrisoquine in man, detected by gas chromatography/mass spectrometry

A stable isotope assay for the quantification of debrisoquine (1) and its major urinary metabolite 4-hydroxydebrisoquine (2) is described. The method consists of extractive derivatization of 1 and 2 by use of 1,3-diketones, chiral derivatization of the 4-hydroxy group of 2, and gas chromatography/negative ion chemical ionization mass spectrometry in the presence of deuterated analogues of 1 and 2. In comparison with synthetic R-(-)-2 and S-(+)-2 it is shown that in vivo benzylic 4-hydroxylation of 1 is highly stereoselective, leading predominantly to S-(+)-4-hydroxydebrisoquine (enantiomeric excess greater than or equal to 90%).

Methoxyphenamine O-demethylase and 5-hydroxylase: a GLC-ECD assay to study their activities and their inhibition by debrisoquine and sparteine

A GLC-ECD method is described for the determination of the O-desmethyl, N-desmethyl and aromatic 5-hydroxy metabolites of methoxyphenamine in liver homogenates. The O-desmethyl and 5-hydroxy metabolites are deficient in poor metabolizers of debrisoquine and sparteine and the Dark Agouti rat model of this human phenotype. The present analytical method can be useful in determining methoxyphenamine O-demethylase and 5-hydroxylase activities as well as identifying those substrates which inhibit these and are worthy of further study.

Attempts to phenotype human liver samples in vitro for debrisoquine 4-hydroxylase activity

Twenty-eight samples of human liver have been characterised for cytochrome P-450 content, aldrin epoxidase, debrisoquine 4-hydroxylase and bufuralol 1'-hydroxylase activities. Evidence is presented here and elsewhere that bufuralol 1'-hydroxylase and debrisoquine 4-hydroxylase are activities catalysed by the same form of cytochrome P-450 in man, and that this form is different from that catalysing the epoxidation of aldrin. Attempts to phenotype liver samples in vitro, in the absence of any metabolic data in vivo for debrisoquine 4-hydroxylation status, met with limited success. A combination of enzyme assays will most probably be required in any such phenotyping of human liver samples.

Drug level monitoring: cardiovascular drugs

Methods for the determination of cardiovascular drugs in blood and plasma are critically reviewed with emphasis on gas and liquid chromatographic techniques. The importance of the various procedures is discussed, in particular sample work-up where the conditions for isolation and derivatization of the compounds are decisive for the accuracy and precision of the methods. Compared with other assay techniques chromatographic methods are generally to be preferred owing to their better selectivity. In the review the following groups are discussed: digitalis glycosides, antiarrhythmic agents, beta-adrenoceptor antagonists, vasodilating agents, antihypertensive compounds, and diuretics.

High affinity of quinidine for a stereoselective microsomal binding site as determined by a radioreceptor assay

The techniques of the radioreceptor binding assay were applied to detect stereoselective binding of quinidine and quinine to a site on human liver microsomes. Binding of 3H-dihydroquinidine was 50% inhibited by 20-100 nM quinidine, while its enantiomer quinine did not displace the 3H-ligand at concentrations up to 500 nM. This stereoselectivity agreed with the affinity values measured by functional enzyme assays of cytochrome P450 activity using sparteine or debrisoquine as substrates.

Bis(trifluoromethyl)aryl derivatives for drug analysis by gas chromatography electron capture negative ion chemical ionization mass spectrometry. Application to the measurement of low levels of clonidine in plasma

A gas chromatographic mass spectrometric assay for clonidine in plasma with a detection limit of a few picograms per ml was required. The p-trifluoromethylbenzyl, pentafluorobenzyl and pentafluorobenzoyl derivatives of clonidine were synthesized and the electron capture negative ion chemical ionization mass spectra of these compounds show extensive fragmentation with prominent ions at m/z 35 and 37 due to the two chlorine atoms in the clonidine molecule. Selected ion monitoring of specific high mass ions in these mass spectra indicated that the required sensitivity could not be obtained with these derivatives. Several bis(trifluoromethyl)pyrimidines were synthesized and these compounds were found to give an intense negative ion current under conditions of resonance electron capture. Consequently, a derivative of clonidine containing a bis(trifluoromethyl)aryl group was synthesized by reacting the drug with 3,5-bis(trifluoromethyl)benzoyl chloride. The negative ion mass spectrum of the reaction product has a base peak at m/z 673 and, when this ion is specifically monitored, an amount of derivative equivalent to 1 picogram of clonidine can be detected. This allowed the development of an assay for clonidine in plasma with a precision of 8% (SD) at 50 pg ml-1, 22% (SD) at 20 pg ml-1 and a lower limit for quantitative determination of 10 pg ml-1. Plasma concentrations of clonidine in 10 subjects given a single 50 micrograms oral dose are reported.