Carbonyl reduction of naltrexone and dolasetron by oxidoreductases isolated from human liver cytosol.

The opioid receptor antagonist naltrexone and the antiemetic 5-HT(3) receptor antagonist dolasetron are ketonic drugs that are efficiently reduced to their corresponding alcohols in-vivo. These experiments aimed at characterizing the role in these reactions of individual oxidoreductases present in human liver cytosol. Aldo-keto reductases (AKRs) and carbonyl reductase (CR, EC 1.1.1.184) purified from human liver cytosol were incubated with varying substrate concentrations and 6beta-naltrexol or reduced dolasetron were analysed by HPLC. AKR1C1, AKR1C2, and AKR1C4 were able to reduce both substrates. On the basis of k(cat)/K(m) values, AKR1C4 was nearly 1000-fold more efficient in reducing naltrexone than was AKR1C1, while AKR1C2 was of intermediate efficiency. Substrate inhibition was observed on incubating AKR1C2 or AKR1C4 with naltrexone. In contrast, dolasetron was also a substrate of CR. AKR1C1 and AKR1C4 were the most efficient enzymes in producing reduced dolasetron. We concluded that the efficient reduction of naltrexone by AKR1C4 probably causes the high 6beta-naltrexol/naltrexone ratio in man. The rapid disappearance from human plasma of dolasetron given intravenously and its virtual absence after oral dosage are explained by its liability to reduction by several enzymes, including CR which shows widespread expression in human tissues.

Tertiary N-glucuronides of clozapine and its metabolite desmethylclozapine in patient urine.

In experiments with expressed human UDP-glucuronosyltransferase 1A4 (UGT1A4), the antipsychotic clozapine proved to be conjugated to two different glucuronides, one of which was identified as the quaternary ammonium glucuronide derivatized at the N-methylpiperazine group; this compound had previously been isolated from patient urine. An additional glucuronide produced in larger quantity was assumed to be conjugated at the secondary nitrogen of the central ring to form 5-N-glucuronide, but this was not proven. The analogous olanzapine 10-N-glucuronide was found to make a major contribution to urinary metabolites in human volunteers. In the present investigation, tertiary 5-N-glucuronides were isolated from incubations of clozapine and desmethylclozapine with human liver microsomes fortified with UDP-glucuronic acid, and their structures were confirmed by mass and (1)H NMR spectrometry. The same conjugates could also be purified from patient urine. Their approximate quantities in urine from four patients ranged between 0.1 and 0.5% of the dose, as did those of the quaternary ammonium glucuronide of clozapine. Analogous to olanzapine 10-N-glucuronide, the tertiary clozapine 5-N-glucuronide was resistant toward enzymatic hydrolysis but was labile under acidic conditions.

Isolation and identification of clozapine metabolites in patient urine.

Biotransformation products of the atypical neuroleptic clozapine were isolated from urine samples of three schizophrenic patients by solid-phase extraction, liquid-liquid extraction for the separation of unpolar and polar metabolites, and thin-layer chromatography followed by final purification by high-performance liquid chromatography. Their structures were elucidated by mass spectrometry and (1)H NMR spectroscopy and in some cases by enzymatic deconjugation. Besides the known metabolites desmethylclozapine, clozapine N-oxide, 8-deschloro-8-hydroxyclozapine, and 8-deschloro-8-hydroxydesmethylclozapine, the unpolar fraction contained 7-hydroxyclozapine and a compound in which the piperazine ring of clozapine was partially degraded to an ethylenediamine derivative. Novel metabolites identified in the polar fraction were the sulfate and glucuronide conjugates of 7-hydroxyclozapine N-oxide, 8-deschloro-8-hydroxyclozapine-O-glucuronide, and the O-glucuronide of N-hydroxydesmethylclozapine; further conjugates were tentatively identified as 9-hydroxydesmethylclozapine-O-sulfate and 6-hydroxyclozapine-O-sulfate. In addition, the previously described conjugates 7-hydroxydesmethylclozapine-O-sulfate, 7-hydroxyclozapine-O-glucuronide and -O-sulfate, 8-deschloro-8-hydroxydesmethylclozapine-O-glucuronide, and the quaternary ammonium glucuronide of clozapine were detected.

Purification and characterization of oxidoreductases-catalyzing carbonyl reduction of the tobacco-specific nitrosamine 4-methylnitrosamino-1-(3-pyridyl)-1-butanone (NNK) in human liver cytosol.

1. Four enzymes were purified to homogeneity from human liver cytosol and were demonstrated to be responsible for carbonyl reduction of the tobacco-specific nitrosamine 4-methylnitrosamino-1-(3-pyridyl)-1-butanone (NNK). 2. Carbonyl reductase (EC 1.1.1.184), a member of the short-chain dehydrogenase/reductase (SDR) superfamily, was compared with three isoenzymes of the aldo-keto reductase (AKR) superfamily in terms of enzyme kinetics, co-substrate dependence and inhibition pattern. 3. AKR1C1, 1C2 and 1C4, previously designated as dihydrodiol dehydrogenases (DD1, DD2 and DD4), showed lower K(m) (0.2, 0.3 and 0.8 mM respectively) than did carbonyl reductase (7 mM), whereas carbonyl reductase exhibited the highest enzyme efficiency (Vmax/K(m)) for NNK. Multiplication of enzyme efficiencies with the relative quantities of individual enzymes in cytosol resulted in a rough estimate of their contributions to total alcohol metabolite formation. These were approximately 60% for carbonyl reductase, 20% each for AKR1C1 and 1C2, and 1% for AKR1C4. 4. Except for AKR1C4, the enzymes had a strong preference for NADPH over NADH, and the highest activities were measured with an NADPH-regenerating system. Carbonyl reductase activity was extensively inhibited by menadione, rutin and quercitrin, whereas medroxyprogesterone acetate, phenolphthalein and flufenamic acid were potent inhibitors of AKR1C1, 1C2 and 1C4. 5. In conclusion, cytosolic members of the SDR and AKR superfamilies contribute to reductive NNK detoxification in human liver, the enzymes responsible being carbonyl reductase and aldoketo reductases of the AKRIC subfamily.

Comparative N-glucuronidation kinetics of ketotifen and amitriptyline by expressed human UDP-glucuronosyltransferases and liver microsomes.

Like other basic amphiphilic drugs, the (S)-enantiomer of the antiallergic drug ketotifen exhibited biphasic kinetics when it was converted to two isomeric quaternary ammonium-linked glucuronides in human liver microsomes. For (R)-ketotifen this applied when incubations were carried out in the absence of a detergent. Two UDP-glucuronosyltransferases (UGTs) present in human liver, UGT1A4 and UGT1A3, were previously shown to catalyze tertiary amine N-glucuronidation when expressed in HK293 cells. Therefore, the conjugation kinetics of (R)- and (S)-ketotifen were investigated with the two expressed proteins. When homogenates of HK293 cells expressing UGT1A4 were incubated without detergent, N-glucuronidation kinetics were monophasic with K(M) values of 59 +/- 5 microM for (R)- and 86 +/- 26 microM for (S)-ketotifen. In experiments with membranes containing expressed UGT1A3, somewhat higher K(M) values were obtained. These values correspond to the high rather than to the low K(M) components of ketotifen glucuronidation in liver microsomes, the latter exhibiting K(M) values around 2 and 1 microM, respectively, with (R)- and (S)-ketotifen. With amitriptyline as the substrate, N-glucuronidation kinetics in the absence of detergent were biphasic in human liver microsomes and monophasic with a high K(M) value in cell homogenates containing UGT1A4. The results suggest that UGT1A4 and UGT1A3 catalyze high-K(M) N-glucuronidation of tertiary amine drugs, whereas the low-K(M) reaction requires either an alternative enzyme or a special conformation of UGT1A4 or UGT1A3 that can be attained in liver microsomes, but not in HK293 cell membranes.

High-affinity stereoselective reduction of the enantiomers of ketotifen and of ketonic nortriptyline metabolites by aldo-keto reductases from human liver.

Aldo-keto reductases (AKR) form an enzyme superfamily catalyzing the reduction of carbonyl compounds and in some cases the reverse oxidation of alcohols as well. In particular, a role in drug metabolism has been considered for the AKR1C family, but published data failed to reveal low Km drug substrates. Moreover, structure activity relationships using chemically related substrates have not been established. In the present investigation, a modified procedure was developed for the isolation of AKR1C1, 1C2, and 1C4 (dihydrodiol dehydrogenases 1, 2, and 4) from human liver cytosol along with carbonyl reductase (EC 1.1.1.184), a member of the short-chain alcohol dehydrogenase superfamily. The kinetics of NADPH-dependent reduction by the closely related enzymes AKR1C1 and 1C2 were studied with the structurally similar substrates (R)- and (S)-ketotifen and E- and Z-10-oxonortriptyline by HPLC measurement of the products. Km values varied between 2.6 and 53 microM and Vmax values between 5 and 313 mU/mg protein; substrate inhibition with Ki around 30 microM occurred in the reduction of E- and Z-10-oxonortriptyline by AKR1C1. The reactions were strictly stereospecific with production of one enantiomeric alcohol from each ketotifen enantiomer and of the (+)-enantiomers of E- and Z-10-hydroxynortriptyline. Enzymatic NADP+ -dependent oxidation of the alcohols mirrored the reduction with regard to stereochemical specificity. All four ketones were no or poor substrates of carbonyl reductase, whereas haloperidol was reduced by this enzyme with low affinity, but high efficiency.

Conjugation of the enantiomers of ketotifen to four isomeric quaternary ammonium glucuronides in humans in vivo and in liver microsomes.

The antiallergic drug ketotifen is chiral due to a nonplanar seven-membered ring containing a keto group. Earlier studies have revealed glucuronidation at the tertiary amino group as a major metabolic pathway in humans. Chemical synthesis of glucuronides from racemic ketotifen now led to four isomers separable by HPLC of which two each could be ascribed to (R)-(+)- and (S)-(-)-ketotifen by synthesis from the enantiomers. According to (1)H NMR analysis of the (S)-ketotifen N-glucuronides, the conformation of the piperidylidene ring differs between the two isomers. Enzymatic hydrolysis with Escherichia coli beta-glucuronidase proceeded at a lower rate with the slower eluting (S)-ketotifen glucuronide than with the three other isomers. On incubation of the ketotifen enantiomers (0.5-200 microM) with human liver microsomes in the presence of UDP-glucuronic acid and Triton X-100, the N-glucuronides of (R)-ketotifen were produced with an apparent K(M) 15 microM and V(max) 470 pmol/min/mg protein. The two (S)-ketotifen glucuronides were formed by two-enzyme kinetics with K(M1) 1.3 microM and K(M2) 92 microM and V(max) values of 60 and 440 pmol/min/mg protein. After ingestion of 1 mg of racemic ketotifen, 10 healthy subjects excreted in urine 17 +/- 5% of the dose in the form of N-glucuronides. The (R)-ketotifen glucuronide isomers contributed one-sixth only, whereas the remainder consisted primarily of the (S)-ketotifen glucuronide isomer, which eluted last. Differential hydrolysis or membrane transport may be responsible for the discrepancy between N-glucuronide isomer ratios in vitro and in vivo.

Pharmacokinetics of clozapine and its metabolites in psychiatric patients: plasma protein binding and renal clearance.

AIMS: N-Desmethylclozapine and clozapine N-oxide are major metabolites of the atypical neuroleptic clozapine in humans and undergo renal excretion. The aim of this study was to investigate to what extent the elimination of these metabolites in urine contributes to the total fate of clozapine in patients and how they are handled by the kidney. METHODS: From 15 psychiatric patients on continuous clozapine monotherapy, blood and urine samples were obtained during four 2 h intervals, and clozapine and its metabolites were assayed in serum and urine by solid-phase extraction and h.p.l.c. Unbound fractions of the compounds were measured by equilibrium dialysis. RESULTS: The following unbound fractions in serum were found (geometric means): clozapine 5.5%, N-desmethylclozapine 9.7%, and clozapine N-oxide 24.6%. Renal clearance values calculated from unbound concentrations in serum and quantities excreted in urine were for clozapine on average 11% of the creatinine clearance, whereas those of N-desmethylclozapine and clozapine N-oxide amounted to 300 and 640%, respectively. The clearances of unbound clozapine and N-desmethylclozapine increased with increasing urine volume and decreasing pH. All renal clearance values exhibited large interindividual variations. The sum of clozapine and its metabolites in urine represented on average 14% of the dose. CONCLUSIONS: Clozapine, N-desmethylclozapine and clozapine N-oxide are highly protein-bound in serum. Clozapine is, after glomerular filtration, largely reabsorbed in the tubule, whereas the metabolites undergo net tubular secretion. Metabolic pathways alternative or subsequent to N-demethylation and N-oxidation must make major contributions to the total fate of clozapine in patients.

Variability of diphenhydramine N-glucuronidation in healthy subjects.

The H1-antagonist diphenhydramine can undergo direct glucuronidation at its tertiary amino group with formation of a quaternary ammonium glucuronide. The intraindividual variability in the amount of N-glucuronide excretion in urine was investigated in two female volunteers who repeatedly took single doses of 25 mg diphenhydramine hydrochloride without and with concomitant administration of ascorbic acid or ammonium chloride for urine acidification. Another two female and four male subjects underwent single tests without and with additional ascorbic acid. Diphenhydramine N-glucuronide quantities in urine differed significantly among subjects and ranged between 2.7% and 14.8% of the dose within 8 h. Neither ascorbic acid nor ammonium chloride significantly influenced the quantity of N-glucuronide in urine, but ammonium chloride, that in contrast to ascorbic acid proved effective in lowering urinary pH, increased the excretion of the parent drug.

Biphasic kinetics of quaternary ammonium glucuronide formation from amitriptyline and diphenhydramine in human liver microsomes.

The tricyclic antidepressant amitriptyline and the H1-receptor antagonist diphenhydramine are conjugated in human liver microsomes fortified with UDP-glucuronic acid at their tertiary amino groups with the formation of quaternary ammonium glucuronides. The kinetics of the reactions were found to be biphasic with apparent KM1 and KM2 values of 1.4 microM and 311 microM for amitriptyline and 2.6 microM and 1180 microM for diphenhydramine in four liver samples. Vmax1 values varied between 2 and 17 pmol-mg protein-1.min-1 for the two substrates and Vmax2 values between 80 and 740 pmol-mg protein-1.min-1. A close correlation existed between amitriptyline and diphenhydramine glucuronidation rates in microsomes from seven livers at concentrations corresponding to 10-40% of KM2. At low concentrations, diphenhydramine competitively inhibited the glucuronidation of amitriptyline. Vmax/K(M) values of the high-affinity UDP-glucuronosyltransferase(s) (UGTs) exceed those of the low-affinity enzyme(s) severalfold, such that the former should make the major contribution to N-glucuronidation of the drugs at therapeutic concentrations in vivo.

[Amalgam--"etiology" of psychiatric disorders?]. / Amalgam - "Ursache" psychischer Störungen?

In a liability lawsuit an expertise had to answer the question whether a mania in the course of an affective psychosis could have been caused by chronic mercury intoxication resulting from dental amalgam fillings. On the basis of current psychiatric and toxicological knowledge, such an association can be disproved. Mercury intake from amalgam fillings does not lead to toxic concentrations in organs or body fluids. Therefore physicians and dentists should avoid alarming patients and thus causing iatrogenic harm.

Elevated norharman plasma levels in alcoholic patients and controls resulting from tobacco smoking.

Plasma norharman and harman levels were measured by solvent extraction and HPLC with fluorescence detection in alcohol-dependent patients undergoing in-patient abstinence treatment and in control subjects. In both groups, randomly collected samples from smokers contained higher mean norharman levels than those from non-smokers. In three volunteers norharman concentrations rose sharply after smoking of one or two cigarettes and declined to near-basal levels within one hour after one cigarette. When 12 patients kept a smoking-free interval of at least 6 h, they had similarly low plasma norharman concentrations (20 +/- 8 pg/ml) as 18 non-smoking control subjects (17 +/- 8 pg/ml) or as 13 smoking controls who had abstained from smoking (20 +/- 6 pg/ml). Ten of the patients smoked one cigarette and within 5-10 min attained norharman levels of 177 +/- 147 pg/ml plasma. The high prevalence of smokers among chronic alcoholics probably explains the previous finding of elevated norharman plasma levels in these patients.

Stereoselective reversible ketone formation from 10-hydroxylated nortriptyline metabolites in human liver.

1. E- and Z-10-hydroxynortriptyline are major metabolites of amitriptyline and nortriptyline in man. Upon incubation with human liver microsomes or cytosol, these metabolites were oxidized to the corresponding ketones, E- and Z-10-oxonortriptyline. (+)-E- and (+)-Z-10-hydroxynortriptyline were distinctly preferred over the (-)-isomers as substrates. NADP+ supported the oxidation in cytosol, whereas in microsomes NAD+ was the best cofactor. 2. Incubation of E- and Z-10-oxonortriptyline with NADPH and cytosol resulted in the nearly exclusive formation of (+)-E- and (+)-Z-10-hydroxynortriptyline. Kinetic analysis revealed high-affinity reduction (K(m) 1-2 microM) of the two ketones and an additional low-affinity component with the E-isomer. 10-Oxonortriptyline reduction was also catalysed by rabbit, but not by rat or guinea pig liver cytosol. 3. With [4-3H]NADPH as cosubstrate, tritium was incorporated into E- and Z-10-hydroxynortriptyline preferentially from the pro-4R position. Redox cycling of (+)-E- and (+)-Z-10-hydroxynortriptyline in cytosol in the presence of NAD- and NADPH was indicated by 3H incorporation from [pro-4R-3H]NADPH. 4. Recombinant human carbonyl reductase catalysed low-affinity reduction of E-10-oxonortriptyline with preferential transfer of the pro-4S-3H of labelled NADPH. 5. Ketone reduction in cytosol was strongly inhibited by 9,10-phenanthrenequinone and dehydrolithocholic acid and moderately by other 3-oxo steroids and some anti-inflammatory drugs. 6. The high-affinity reduction of E- and Z-10-oxonortriptyline and the oxidation of the alcohols in cytosol are probably mediated by a member of the aldo-keto reductase family of enzymes.

Comparison of procedures for measuring the quaternary N-glucuronides of amitriptyline and diphenhydramine in human urine with and without hydrolysis.

The activities of beta-glucuronidases from Helix pomatia, Escherichia coli and rat towards the N-glucuronides of amitriptyline and diphenhydramine were considerably lower than those towards standard substrates. The two N-glucuronides were analysed in urine samples by the following procedures: HPLC of the intact conjugate after solid-phase extraction on a cation exchanger cartridge or after direct injection of urine; HPLC of the aglycone after hydrolysis with beta-glucuronidase from H. pomatia or E. coli or after alkaline hydrolysis. Solid-phase extraction led to the highest recovery and precision, and sensitivity can be improved by extracting a larger volume of urine. On application to samples from patients under treatment with amitriptyline, the results of all procedures except alkaline hydrolysis were in good agreement. When diphenhydramine N-glucuronide was analysed in urine samples of volunteers, solid-phase extraction, hydrolysis by E. coli glucuronidase and alkaline hydrolysis resulted in similar values.

Renal clearance of pyridostigmine in myasthenic patients and volunteers under the influence of ranitidine and pirenzepine.

1. To assess the contribution of tubular secretion to the renal excretion of pyridostigmine, and its modification by other cationic drugs, six volunteers were given single oral doses of 60-mg pyridostigmine bromide with and without co-administration of ranitidine or pirenzepine. Renal clearances were determined by h.p.l.c. analysis of pyridostigmine and enzymic measurement of endogenous creatinine in plasma and urine. 2. In patients with myasthenia receiving continuous pyridostigmine therapy, renal clearance values were obtained in the same manner with and without ranitidine (10 patients) or pirenzepine (nine patients) co-medication. 3. Pyridostigmine was not bound to plasma proteins. Its renal clearance averaged 6.65 ml/min per kg (350% of the creatinine clearance) in all subjects, 74% being due to net tubular secretion. 4. Mean values for pyridostigmine renal clearance and for clearance by secretion were decreased in the presence of pirenzepine, but plasma concentrations were not affected significantly. Ranitidine caused a small non-significant decrease of the pyridostigmine clearance in patients. 5. Pyridostigmine had a higher elimination (2 h-1) than the absorption rate constant (0.23 h-1) when administered orally as a non-retarded preparation. 6. The renal clearance of creatinine was slightly increased by pyridostigmine in volunteers and slightly decreased by pirenzepine in the total group of subjects.

Enantioselective amitriptyline metabolism in patients phenotyped for two cytochrome P450 isozymes.

In 26 hospitalized patients with depression, a combined pharmacogenetic test with dextromethorphan, a substrate of cytochrome P450IID6, and mephenytoin, the S-form of which is hydroxylated by a P450IIC isozyme, was carried out before amitriptyline therapy. Metabolites were determined in 24-hour urine samples collected on treatment day 8, and the contributions of individual compounds, including the four isomers of 10-hydroxyamitriptyline and 10-hydroxynortriptyline to total excretion were calculated. Formation of (-)-E-10-hydroxyamitriptyline and (-)-E-10-hydroxynortriptyline apparently depends on the activity of cytochrome P450IID6 because negative correlations existed between the log metabolic ratio of dextromethorphan and the relative quantities of these enantiomers. In contrast, correlations were positive for nortriptyline, (+)-E-10-hydroxynortriptyline, (-)-Z-10-hydroxynortriptyline, and (+)-Z-10-hydroxynortriptyline. The mephenytoin hydroxylase seems to participate in side-chain demethylation to the secondary and primary amines, because the log metabolic ratio of mephenytoin correlated negatively with the relative quantity of E-10-hydroxydidesmethylamitriptyline and positively with that of amitriptyline and its N-glucuronide.