Ronald T. Coutts

Identification of kaempferol as a monoamine oxidase inhibitor and potential neuroprotectant in extracts of Ginkgo biloba leaves.

The effects of Ginkgo biloba leaf extract on rat brain or livermonoamine oxidase (MAO)‐A and ‐B activity, biogenic amine concentration in nervous tissue, N‐methyl‐d‐aspartate (NMDA)‐ and N‐(2‐chloroethyl)‐N‐ethyl‐2‐bromobenzylamine (DSP‐4)‐induced neurotoxicity and antioxidant activity was investigated to determine the effects of the extract on monoamine catabolism and neuroprotection.

Gas chromatographic analysis of trace phenols by direct acetylation in aqueous solution

Abstract Acetate esters of six phenolics were formed by the direct addition of 500 μl acetic anydride to 250 ml of a dilute aqueous phenolic solution containing 10 g sodium bicarbonate. In the concentration range of 0.08–0.24 μmoles/1, phenol, o -cresol, m -cresol, p -cresol, 2,4-dichlorophenol and 1-naphthol easily formed acetate esters which provided for improved gas chromatographic characteristics and virtually quantitative recovery from aqueous solution. On extraction with small volumes of methylene chloride, phenol could only be recovered to the extent of 28–41% in the 0.2–2.0 mg/1 range. On the other hand, phenyl acetate, formed in water prior to extraction, was 100% recovered. The stable acetate esters can be analyzed using standard gas chromatographic solumns such as OV-17 or OV-101 while phenolics generally require specially deactivated packings. On 1% SP-1240 DA all of the derivatives, with the exception of m - and p -cresyl acetate, could be separated.

Involvement of CYP3A4 and CYP2D6 in the metabolism of haloperidol.

Abstract1. Human cytochrome P450 (CYP) isoenzymes expressed in a human cell line were used to elucidate their involvement in the metabolism of haloperidol (HAL).2. It was found that CYP3A4 catalyzes the metabolism of HAL to HAL 1,2,3,6-tetrahydropyridine (HTP). HTP is further metabolized to HAL pyridinium (HP+) by both CYP3A4 and CYP2D6.3. CYP3A4 and CYP2D6 are also responsible for the N-dealkylation of HAL. The N-dealkylation of reduced HAL (RH) was observed, which is catalyzed by CYP3A4. In addition, CYP3A4 also catalyzes the oxidation of RH back to HAL.4. These results are discussed in terms of the metabolic interactions of HAL with other drugs and how this knowledge may be used to reduce the movement disorders induced by HAL.

Pharmacokinetics of Ibuprofen Enantiomers in Humans Following Oral Administration of Tablets with Different Absorption Rates

Ibuprofen (IB) is a racemic drug and is administered as such. While activity is due mainly to the S enantiomer, pharmacokinetic interpretations, as well as criteria to assess the bioequivalence of IB formulations, are based on measurements of the total (S + R) drug concentrations. IB enantiomers possess different disposition properties mainly as a result of R-to-S isomeric bioinversion. Inversion is maximal during the absorption phase, suggesting, perhaps, involvement of a presystemic process. This concept was evaluated in healthy subjects by crossover administration of four IB tablets having different absorption rates. The plasma concentrations of the individual isomers were measured using a stereospecific gas chromatographic assay. Differences among the products were insignificant with respect to the extent to the absorption. The S:R concentration ratios rose for 4 to 6 hr and then remained relatively unchanged. This observation was consistent with equal terminal t1/2 values for the enantiomers. There were significant differences between the peak times (Tmax) of the products. The S:R ratios of the concentrations at Tmax of S and AUC also differed; significant positive correlations were found between Tmax and the S:R ratios of Cmax. Thus the extent of R-to-S inversion, and hence the potency of a racemic dose of IB, may be absorption rate dependent.

Comparison of chemical components and antioxidant capacity of different Echinacea species

Alcoholic extracts of the roots and leaves of three Echinacea species (E. purpurea, E. angustifolia and E. pallida) were analysed for the presence of characteristic chemicals by HPLC directly coupled to ultraviolet absorbance and electrospray mass spectrometric detectors. The method permitted rapid characterization and tentative identification of a large number of caffeoyl conjugates and alkamides in all the samples investigated. The roots of the three species differed markedly in their contents of characteristic compounds. Cichoric acid and verbascoside predominated in extracts of E. purpurea root whereas cynarine and dodeca‐2E,4E,8Z,10Z/E‐tetraenoic acid isobutylamide were the major chemicals characteristic of E. angustifolia root extracts. Echinacoside and 6‐O‐caffeoylechinacoside predominated in extracts of E. pallida roots. Characteristic alkamides were also examined by electrospray tandem mass spectrometry (MS/MS) and these compounds provided characteristic fragmentation patterns. Extracts of the roots and leaves of all three species were found to have antioxidant properties in a free radical scavenging assay and in a lipid peroxidation assay.

Elucidation of individual cytochrome P450 enzymes involved in the metabolism of clozapine

The atypical antipsychotic clozapine has been reported to be metabolised mainly to its N-oxide and N-demethylated products. In the present study, individual recombinant cytochrome P450 (CYP) and flavin-containing monooxygenase (FMO) enzymes were used to elucidate which enzymes are responsible for these metabolic con-versions. In vitro metabolism of clozapine was investigated using human CYP1A1, CYP1A2, CYP2C8, CYP2EI, CYP2C9-arg144, CYP2C9-cys144, CYP2C19, CYP2D6, CYP3A4, CYP3A5 and FMO3 supplemented with an NADPH generating system. Clozapine and its N-oxidation and N-demethylation metabolites were determined by an HPLC method with a Hypersil CN column and a UV detector. Of the enzymes investigated, CYP1A2, CYP3A4, CYP2D6, CYP2C8, CYP2C19 and, to a lesser extent, CYP2C9-cys, CYP2C9-arg and CYP3A5 were apparently involved in N-demethylation, while CYP1A2, CYP3A4, FMO3 and, to a lesser extent, CYP2C8, CYP2C19 and CYP3A5 were found to catalyse the formation of clozapine N-oxide. A bank of 16 human liver microsome preparations was investigated for ability to catalyze the production of clozapine N-oxide and N-desmethylclozapine. Attempts were made to correlate the rates of formation of these metabolites of clozapine to previously determined catalytic activities of CYP1A2, CYP2C19, CYP2D6, CYP2E1 and CYP3A4. At a clozapine concentration of 20 µM, the rate of formation of clozapine N-oxide showed significant correlations with activities of CYP3A4 (P<0.01) and CYP1A2 (P<0.05). The formation of N-desmethylclozapine exhibited significant correlations with CYP1A2 (P<0.01) and CYP3A4 (P<0.01). Similar correlations were observed when the clozapine concentration was increased to 300 µM except that the formation of clozapine N-oxide no longer correlated with CYP1A2 activity. It can be seen from these results that although some recombinant enzymes individually are capable of metabolising clozapine, the activities of several of these enzymes did not correlate with clozapine metabolism when mixtures of the enzymes are used. By combining the results of the current study and those reported in the literature, it is proposed that CYP3A4 and FMO3 are primarily responsible for the production of clozapine N-oxide, and CYP3A4 and CYP1A2 are primarily responsible for the formation of N-desmethylclozapine. The present study demonstrates the importance of the use of multiple techniques for the elucidation of the enzymes involved in the metabolism of certain drugs.

Effects of the antidepressant phenelzine on brain levels of γ-aminobutyric acid (GABA)

Abstract Time- and dose-response studies were carried out on the effects of the monoamine oxidase-inhibiting antidepressant and antipanic drug phenelzine on GABA levels in rat whole brain. At a dose of 15 mg/kg i.p., phenelzine significantly elevated GABA levels at all time intervals s further investigation indicated that this was a dose-dependent effect. The possible importance of GABA in the etiology and pharmacotherapy of depression and panic disorder is discussed.

Involvement of CYP2D6, CYP3A4, and other cytochrome P-450 isozymes in N-dealkylation reactions

Metabolic N-dealkylation is a commonly observed biotransformation with tertiary and secondary amine drugs and related N-alkylated amides, but surprisingly little is known about the cytochrome P-450 isozymes involved in these dealkylation reactions. In this review, evidence is provided that supports the involvement of various P-450 isozymes, but especially CYP3A4 and other isozymes of the CYP3A subfamily. Although CYP2D6 is generally not considered to be capable of catalyzing the N-dealkylation of basic drugs, some examples of the involvement of this important isozyme in N-dealkylation reactions are identified. Procedures used to identify individual P-450 isozymes involved in N-dealkylation reactions are discussed.

Simultaneous determination of fluoxetine and norfluoxetine enantiomers in biological samples by gas chromatography with electron-capture detection

An electron-capture gas chromatographic procedure was developed for the simultaneous analysis of the enantiomers of fluoxetine and norfluoxetine. The assay involves basic extraction of these enantiomers from the biological samples, followed by their conversion to diastereoisomers using the chiral derivatizing reagent (S)-(-)-N-trifluoroacetylprolyl chloride. The method was utilized to detect and measure the quantity of these enantiomers in plasma and urine of patients and in liver and brain tissue of rats treated with (R,S)-fluoxetine.

Involvement of CYP2D6 in the in vitro metabolism of amphetamine, two N-alkylamphetamines and their 4-methoxylated derivatives

1. Amphetamine (AM) and five amphetamine derivatives, N-ethylamphetamine (NEA), N-butylamphetamine (NBA), 4-methoxyamphetamine (M-AM), 4-methoxy-N-ethylamphetamine (M-NEA) and 4-methoxy-N-butylamphetamine (M-NBA) were incubated with microsomal preparations from cells expressing human CYP2D6 to determine whether the enzyme was capable of catalyzing the direct ring oxidation of all substrates; the N-dealkylation of NEA, NBA, M-NEA and M-NBA; and the O-demethylation of M-AM, M-NEA and M-NBA. 2. None of the six compounds examined was N-dealkylated to any extent. 3. The only metabolites produced from AM, NEA and NBA were the corresponding ring 4-hydroxylated compounds, and the rates of formation were low. 4. All ring 4-methoxylated substrates were efficiently O-demethylated by CYP2D6 to their corresponding phenols. The size of the N-alkyl group influenced the rates of formation of these phenolamines. In contrast to reported findings with 2- and 3-methoxyamphetamines, none of the 4-methoxyamphetamines was ring-oxidized in the CYP2D6 enzyme system to 2- or 3-hydroxy-4-methoxyamphetamines or to dihydroxyamphetamines.