N, N-dimethyltryptamine forms oxygenated metabolites via CYP2D6 - an in vitro investigation

N, N-dimethyltryptamine (DMT) is a psychedelic compound that has shown potential in the treatment of depression. Aside from the primary role of monoamine oxidase A (MAO-A) in DMT metabolism, the metabolic pathways are poorly understood. Increasing this understanding is an essential aspect of ensuring safe and efficacious use of DMT.This work aimed to investigate the cytochrome 450 (CYP) mediated metabolism of DMT by incubating DMT with recombinant human CYP enzymes and human liver microsomes (HLM) followed by analysis using high-resolution mass spectrometry for metabolite identification.DMT was rapidly metabolised by CYP2D6, while stable with all other investigated CYP enzymes. The metabolism of DMT in HLM was reduced after inclusion of harmine and SKF-525A whereas quinidine did not affect the metabolic rate, likely due to MAO-A residues present in HLM. Analysis of the CYP2D6 incubates showed formation of mono-, di- and tri-oxygenated metabolites, likely as a result of hydroxylation on the indole core.More research is needed to investigate the role of this metabolic pathway in vivo and any pharmacological activity of the proposed metabolites. Our findings may impact on safety issues following intake of ayahuasca in slow CYP2D6 metabolizers or with concomitant use of CYP2D6 inhibitors.

INFLUENCE OF ERYTHROMYCIN ON THE PHARMACOKINETICS OF XIMELAGATRAN MAY INVOLVE INHIBITION OF P-GLYCOPROTEIN-MEDIATED EXCRETION

A pharmacokinetic interaction between erythromycin and ximelagatran, an oral direct thrombin inhibitor, was demonstrated in this study in healthy volunteers. To investigate possible interaction mechanisms, the effects of erythromycin on active transport mediated by P-glycoprotein (P-gp) in vitro in Caco-2 and P-gp-over-expressing Madin-Darby canine kidney-human multidrug resistance-1 cell preparations and on biliary excretion of melagatran in rats were studied. In healthy volunteers (seven males and nine females; mean age 24 years) receiving a single dose of ximelagatran 36 mg on day 1, erythromycin 500 mg t.i.d. on days 2 to 5, and a single dose of ximelagatran 36 mg plus erythromycin 500 mg on day 6, the least-squares mean estimates (90% confidence intervals) for the ratio of ximelagatran with erythromycin to ximelagatran given alone were 1.82 (1.64-2.01) for the area under the concentration-time curve and 1.74 (1.52-2.00) for the maximum plasma concentration of melagatran, the active form of ximelagatran. Neither the slope nor the intercept of the melagatran plasma concentration-effect relationship for activated partial thromboplastin time statistically significantly differed as a function of whether or not erythromycin was administered with ximelagatran. Ximelagatran was well tolerated regardless of whether it was administered with erythromycin. Erythromycin inhibited P-gp-mediated transport of both ximelagatran and melagatran in vitro and decreased the biliary excretion of melagatran in the rat. These results indicate that the mechanism of the pharmacokinetic interaction between oral ximelagatran and erythromycin may involve inhibition of transport proteins, possibly P-gp, resulting in decreased melagatran biliary excretion and increased bioavailability of melagatran.

Absorption, distribution, metabolism, and excretion of ximelagatran, an oral direct thrombin inhibitor, in rats, dogs, and humans

The absorption, metabolism, and excretion of the oral direct thrombin inhibitor, ximelagatran, and its active form, melagatran, were separately investigated in rats, dogs, and healthy male human subjects after administration of oral and intravenous (i.v.) single doses. Ximelagatran was rapidly absorbed and metabolized following oral administration, with melagatran as the predominant compound in plasma. Two intermediates (ethyl-melagatran and OH-melagatran) that were subsequently metabolized to melagatran were also identified in plasma and were rapidly eliminated. Melagatran given i.v. had relatively low plasma clearance, small volume of distribution, and short elimination half-life. The oral absorption of melagatran was low and highly variable. It was primarily renally cleared, and the renal clearance agreed well with the glomerular filtration rate. Ximelagatran was extensively metabolized, and only trace amounts were renally excreted. Melagatran was the major compound in urine and feces after administration of ximelagatran. Appreciable quantities of ethyl-melagatran were also recovered in rat, dog, and human feces after oral administration, suggesting reduction of the hydroxyamidine group of ximelagatran in the gastrointestinal tract, as demonstrated when ximelagatran was incubated with feces homogenate. Polar metabolites in urine and feces (all species) accounted for a relatively small fraction of the dose. The bioavailability of melagatran following oral administration of ximelagatran was 5 to 10% in rats, 10 to 50% in dogs, and about 20% in humans, with low between-subject variation. The fraction of ximelagatran absorbed was at least 40 to 70% in all species. First-pass metabolism of ximelagatran with subsequent biliary excretion of the formed metabolites account for the lower bioavailability of melagatran.

Characterisation of the human liver in vitro metabolic pattern of artemisinin and auto-induction in the rat by use of nonlinear mixed effects modelling

AIMS

The aims of the study were to characterise the metabolic pattern of artemisinin in human and rat liver microsomes and to assess the magnitude of auto-induction in the rat.

METHODS

(14)C-artemisinin was incubated with human liver microsomes and with liver microsomes from rats pretreated with oral artemisinin or placebo. The metabolic fate of (14)C-artemisinin in microsomes from human B-lymphoblastoid cell lines transformed with CYP2A6, CYP2B6 and CYP3A4 was also investigated. The human liver microsome data and the rat liver microsomes data were analysed by nonlinear mixed effects modelling and naïve pooling using NONMEM, respectively.

RESULTS

Four metabolites were radiometrically detected in experiments with rat liver microsomes. The model that best described the data involved three primary metabolites of which one metabolite was further metabolised to a secondary metabolite. The formation of the four metabolites was induced 2.8, 7.2, 4.8 and 2.5-fold, respectively, in liver microsomes from rats pre-treated with artemisinin. Three metabolites were formed in human liver microsomes; having the same retention times as three of the metabolites formed in the rat. The final model consisted of two primary metabolites and a secondary metabolite with CYP2B6 and CYP2A6 influencing the formation rates of the major and minor primary metabolites, respectively.

CONCLUSIONS

CYP2B6 and CYP2A6 activities described variability in the formation of the major and minor primary metabolites, respectively, in human liver microsomes. All artemisinin metabolic pathways in rat liver microsomes were induced in artemisinin pretreated animals. We suggest modelling as a method for the discrimination and detection of more complex metabolic patterns from in vitro metabolism rate data.

An assessment of human liver-derived in vitro systems to predict the in vivo metabolism and clearance of almokalant

The ability of various human derived in vitro systems to predict various aspects of the in vivo metabolism and kinetics of almokalant have been investigated in a multicenter collaborative study. Although almokalant has been withdrawn from further clinical development, its metabolic and pharmacokinetic properties have been well characterized. Studies with precision-cut liver slices, primary hepatocyte cultures, and hepatic microsomal fractions fortified with UDP-glucuronic acid all suggested that almokalant is mainly glucuronidated to the stereoisomers M18a and M18b, which is in good agreement with the results in vivo. Both in vivo and in vitro studies indicate that the formation of M18b dominates over that of M18a, although the difference is more pronounced with the in vitro systems. Molecular modeling, cDNA-expressed enzyme analysis, correlation analysis, and inhibition studies did not clearly indicate which P450 enzymes catalyze the oxidative pathways, which may indicate a problem in identifying responsible enzymes for minor metabolic routes by in vitro methods. All of the in vitro systems underpredicted the metabolic clearance of almokalant, which has previously been reported to be a general problem for drugs that are cleared by P450-dependent metabolism. Although few studies on in vivo prediction of primarily glucuronidated drugs have appeared, in vitro models may consistently underpredict in vivo metabolic clearance. We conclude that in vitro systems, which monitor phase II metabolism, would be beneficial for prediction of the in vivo metabolism, although all of the candidate liver-derived systems studied here, within their intrinsic limitations, provided useful information for predicting metabolic routes and rates.

Compound mixtures in Caco-2 cell permeability screens as a means to increase screening capacity

The purpose of this investigation was a study of simultaneous permeability measurement using compound mixtures (cassette dosing) as an alternative to single compound evaluation in order to increase the capacity of screens for intestinal drug permeability. Drug transport across Caco-2 monolayers was studied, both in the apical to basolateral and the basolateral to apical direction. The apparent permeability coefficients for ten compounds displaying different intestinal transport mechanisms were determined, first as single compounds and then as components of a mixture. Seven beta-adrenoceptor antagonists and baclofen were analysed simultaneously using reversed phase HPLC with UV detection, D-glucose and mannitol were measured by scintillation counting. The results indicated that the Papp from the mixture as donor phase correlated well with that of the single compounds and merely small changes in the Papp of each compound were observed between the single compound and mixture experiments. This minor variation resulted in a change in rank-order of the poorly permeable compounds in the mixture, however, without affecting their association with the permeability class according to the biopharmaceutics classification system (BCS). It can be concluded that the use of compound mixtures is a suitable method for improving the capacity in permeability screens. Further improvement of the throughput may be expected upon automatisation of permeability measurements using robotics combined with increased selectivity using LC-MS analysis.

Presystemic elimination of the beta-blocker pafenolol in the rat after oral and intraperitoneal administration and identification of a main metabolite in both rats and humans

Pafenolol is a beta 1-adrenoreceptor antagonist exhibiting some interesting oral absorption properties in both rat and humans. The blood concentration-time profile exhibits two peaks, and the bioavailability is low and dose-dependent due to an incomplete and nonlinear intestinal uptake. The origin of the presystemic metabolism was studied in rats after oral and intraperitoneal administration of tritium-labeled pafenolol with reference to the intravenous route by means of urinary excretion data of pafenolol and metabolites specifically assayed by HPLC and radioisotope detection. The oral-bioavailability increased from 15.8 +/- 4.1% (1.0 mumol/kg) to 33.3 +/- 5.8% (25 mumol/kg, p < 0.001). This was primarily due to a change in the fraction of the absorbed dose (fa) from 21.9 +/- 4.6 to 39.5 +/- 7.9% (p < 0.01). The bioavailability following an intraperitoneal dose was almost complete indicating that the presystemic metabolism was due to gut wall metabolism. Saturation of the presystemic metabolism contributed only by approximately 15-20% to the 2-fold increase of bioavailability. This clearly indicates that the underlying mechanism for the low and dose-dependent bioavailability was an incomplete and nonlinear intestinal uptake. The metabolic pattern showed that at least eight metabolites are formed in the rat. One of these is an alpha-OH pafenolol, identified as the main metabolite in human urine by mass spectrometry.

Determination of a renin inhibitor in plasma by solid-phase extraction using acetone as protein binding displacer followed by on-line high-performance liquid chromatography

H 218/54 is a potent inhibitor of human renin activity (pIC50 = 8.3 at pH 6) and is therefore a potential agent for blood pressure reduction. This lipophilic compound is highly bound to plasma proteins, e.g. 99.7% in rats and 99.6% in humans. For pharmacokinetic studies, a quantitative assay for 3H-H 218/54 in plasma has been developed. On top of an AASP phenyl solid-phase cartridge 70 microliters of rat plasma or 1 ml of cynomolgus plasma was mixed with 200 microliters of water containing 20% acetone. The acetone displaced the substance from plasma proteins without precipitation of the sample and clogging of the extraction column. The mixture was passed through the cartridge, which adsorbed 3H-H 218/54. The cartridge was placed in an AASP autosampler connected to a reversed-phase LC system, with a Vydac C-18 column and CH3CN-H2O-TFA (60:40:0.1, v/v/v) as mobile phase. The effluent from the separation column was collected in fractions for radioactivity counting. Recovery, as measured after adding various amounts of tritium-labelled H 218/54 to blank plasma followed by repeated analysis of the samples, was close to 100% with relative standard deviations between 1.4 and 3.0%. At the lowest level tested, 200 dpm per sample, the recovery was 120% with a relative standard deviation of only 10%. The sensitivity of the method will depend on the specific radioactivity of the dose given.

Determination of four carboxylic acid metabolites of felodipine in plasma by high-performance liquid chromatography

A reversed-phase high-performance liquid chromatography method with ultraviolet detection at 220 nm was developed to determine four carboxylic acid metabolites in plasma following therapeutic doses of the calcium antagonist felodipine. After the addition of an internal standard the analytes were isolated by liquid-liquid and solid-phase extraction. The metabolites were applied to a C2 cartridge in their free acid form, but they were transformed and retained as ion pairs with tetrabutylammonium during a wash with phosphate buffer (pH 7), prior to automated elution and injection by the Varian AASP system onto the analytical C18 column. Using a sample volume of 1 ml of plasma, the lower limit of determination for the metabolites was about 20 nmol/l. The influence of the pH of the mobile phase on the retention time of the metabolites and the structural requirements for the internal standard were studied. The method was applied to plasma samples from four dogs collected after an oral dose of felodipine. The plasma concentration-time profiles of the metabolites gave useful information about the mechanisms by which they were formed and eliminated.

Direct coupled column separation and determination of the diastereomeric glucuronides of almokalant, a new class III antiarrhythmic drug, in human urine

A reversed-phase coupled column separation (CCS) system for the analysis of two diastereomeric glucuronides of almokalant, a new class III antiarrhythmic drug, in human urine is described. After direct injection of urine samples (50 microliters) the glucuronides were isolated by complex formation on a terbium(III) loaded strong cation exchanger at alkaline pH. The solutes were eluted from the precolumn by an acidic mobile phase, enriched and separated on Hypercarb (porous graphitic carbon) as analytical column with 0.1 M acetic acid pH 2.8 and 30% acetonitrile as mobile phase. The calibration graph was linear (r2 = 0.9999) and the detection limits were in the low picomole (UV) or femtomole (fluorescence) range. Optimization of the analytical column revealed that elution order and selectivity for the glucuronides were dependent on the buffer agent and temperature used. By appropriate choice of mobile phase conditions all four diastereomers could be separated.

Pharmacokinetics of the enantiomers of felodipine in the dog after oral and intravenous administration of a pseudoracemic mixture

1. A pseudoracemic mixture of deuterated (S)-felodipine and unlabelled (R)-felodipine was administered as single i.v. or oral doses to four dogs. Plasma concentrations of the enantiomers and their corresponding pyridine metabolites were determined by g.l.c.-mass spectrometry. 2. No isotope effects were observed after oral administration of equimolar amounts of deuterated and unlabelled (S)-felodipine. 3. The pharmacokinetic parameters of the enantiomers were similar after i.v. administration, indicating that the disposition of felodipine was not stereoselective. 4. After oral administration the bioavailability of (R)-felodipine was slightly higher than that of (S)-felodipine in two of the dogs, presumably due to a lower first-pass extraction of the (R)-enantiomer, while no difference was observed in the other two dogs. 5. No substantial differences in Cmax or AUC were observed between the deuterated and unlabelled pyridine metabolites, indicating that the oxidative clearances of the felodipine enantiomers were similar.

Mechanistic studies on the metabolic activation of acetaminophen in vivo

Three analogues of acetaminophen (APAP), labeled at specific positions with either oxygen-18 or deuterium, were administered by ip injection to male BALB/c mice at the moderately hepatotoxic dose of 200 mg kg-1 in order to probe the mechanism by which APAP undergoes metabolic activation in vivo. The thioether conjugates of APAP present in bile, urine, and feces, which are believed to derive from the electrophilic intermediate N-acetyl-p-benzoquinone imine (NAPQI), were isolated following aqueous-phase derivatization, separated by HPLC, and converted to a common volatile derivative for analysis by GC-MS. The observed labeling patterns of these conjugates indicated that APAP undergoes metabolism to NAPQI by a process that does not involve the generation of a free oxygenated intermediate, but which more likely entails the sequential removal of two electrons from the substrate. On the basis of these findings, an integrated metabolic scheme is proposed which invokes initial cytochrome P-450 mediated generation of a caged oxygen-centered APAP radical species. Subsequent reactions of this intermediate may account for the formation of all known oxidative metabolites of APAP.

Analysis and stereoselective metabolism after separate oral doses of tocainide enantiomers to healthy volunteers

The potential of stereoselective metabolism of tocainide was studied in six healthy volunteers after separate oral administration of the pure enantiomers in solution. A method was developed to convert the N-carbamoylglucuronide of tocainide in plasma and urine by base treatment to a hydantoin derivative which after extraction and silation was analysed by selected ion monitoring using a deuterated internal standard. Analytical problems concerning side-reactions during derivatization of the conjugate are discussed. The peak plasma levels of the enantiomers, observed at less than or equal to 2 h after dosing, were similar but plasma clearances and terminal half-lives were different after oral administration of (R)-tocainide (195.5 +/- 20.1 ml min-1 and 9.7 +/- 0.8 h) and (S)-tocainide (110.2 +/- 10.5 ml min-1 and 14.5 +/- 1.7 h). Over 0-96 h the averaged urinary recovery of (R)-tocainide was 36 per cent and of (S)-tocainide 50 per cent. Stereoselective metabolism was a likely mechanism for the observed differences as the urinary recovery of the conjugate formed from (R)-tocainide differed substantially from that of (S)-tocainide (45 vs 1.2 per cent of given dose). Plasma t1/2 of the (R)- and (S)-conjugate were 9.9 and 18.7 h, respectively, indicating formation rate limited kinetics of the metabolite. The renal clearances of the conjugates were not significantly different (131 vs 97 ml min-1).

Identification of two main urinary metabolites of [14C]omeprazole in humans

The excretion and metabolism of [14C]omeprazole given orally as a suspension was studied in 10 healthy male subjects. An average of 79% of the dose was recovered in the urine in 96 hr, with most of the radioactivity (76% of dose) being eliminated in the first 24 hr. Pooled urine (0-2 hr) from five subjects, containing about 47% of the dose, was analyzed by reverse phase gradient elution LC with radioisotope detection. Omeprazole was completely metabolized to at least six metabolites. The two major metabolites were extensively purified by LC and their structures were determined by MS with derivatization and use of stable isotopes, 1H NMR, and comparison with synthetic references. They were formed by hydroxylation of a methyl group in the pyridine ring, followed by further oxidation of the alcohol to the corresponding carboxylic acid. Both metabolites retained the sulfoxide group of omeprazole, rendering them as unstable as the parent compound at pH less than 7. They accounted for approximately 28% (hydroxyomeprazole) and 23% (omeprazole acid) of the amount excreted in the 0-2-hr collection interval. Based on in vitro studies with the synthetic metabolites in isolated gastric glands, it is unlikely that M1 and M2 will contribute to the pharmacological effect of omeprazole in humans.

The use of alkoxycarbonyl derivatives for the mass spectral analysis of drug-thioether metabolites. Studies with the cysteine, mercapturic acid and glutathione conjugates of acetaminophen

Alkoxycarbonyl derivatives of the cysteine-, N-acetylcysteine- and glutathione conjugates of acetaminophen have been prepared in aqueous buffer solutions and their chromatographic and mass spectrometric properties examined. Structurally informative fragmentation patterns of the cysteine- and N-acetylcysteine derivatives were obtained when their methyl esters were subjected to analysis by direct insertion chemical ionization (CH4) mass spectrometry, although field desorption and liquid secondary ion mass spectrometric techniques were required in order to obtain satisfactory spectral data for derivatives of the glutathione adduct. Treatment of ethoxycarbonyl derivatives of the three acetaminophen metabolites with N-methyltrifluoroacetamide-based silylating reagents led to the formation of a common volatile product which was ideally suited to analysis by gas chromatography/electron impact mass spectrometry. A mechanism is proposed for the formation of this novel derivative, which appears to possess a benzo-1,3-thioxalane structure, and its mass spectral characteristics are reported. Finally, the utility of alkoxycarbonyl derivatives for the analysis of drug-thioether conjugates in biological fluids is discussed in terms of their advantages for aqueous phase derivatization, purification by high-performance liquid chromatography and characterization by mass spectrometry.

Covalent binding of acetaminophen to mouse hemoglobin. Identification of major and minor adducts formed in vivo and implications for the nature of the arylating metabolites

When hepatotoxic doses of [ring-U-14C]acetaminophen ([ring-U-14C]APAP) were administered to mice, radioactivity became bound irreversibly to hemoglobin as well as to proteins in the liver and kidney. The covalent binding to hemoglobin was dose-dependent, and in phenobarbital-pretreated mice occurred to the extent of approximately 8% of the corresponding binding to liver proteins. Degradation of the modified globin by acid hydrolysis yielded 3-cystein-S-yl-4-hydroxyacetanilide as the major radioactive product, accounting for approximately 70% of protein-bound drug residues. This finding is consistent with the view that the majority of covalent binding of APAP to proteins is mediated by N-acetyl-p-benzoquinone imine (NAPQI), a reactive metabolite which preferentially arylates cysteinyl thiol residues. However, after administration of [acetyl-3H]APAP to mice, it was found that approximately 20% of the drug bound to hemoglobin had lost the N-acetyl side-chain, indicating the existence of a second type of APAP-protein adduct. One minor component of the globin hydrolysate was identified as S-(2,5-dihydroxyphenyl)-cysteine, which most likely arises from binding to hemoglobin of p-benzoquinone, a hydrolysis product of NAPQI. The two adducts reported represent the first identified examples of arylating drugs binding to hemoglobin. Experiments on the influence of different cytochrome P-450 inducing agents on the ratio of drug bound to hemoglobin versus hepatic proteins suggested that the reactive metabolites of APAP are formed in the liver and migrate to the erythrocyte, rather than being produced by hemoglobin-catalyzed oxidation of APAP. These findings imply that the reactive metabolites of APAP escape from hepatocytes in some latent forms, which then participate in the arylation of protein thiols in red blood cells and, possibly, at other remote sites.

Analysis of alpha-hydroxy metabolites of metoprolol in human urine after phosgene/trimethylsilyl derivatization

Three metoprolol metabolites containing an alpha-hydroxy group were identified in human urine by capillary column gas chromatography/mass spectrometry. After aqueous phase cyclization with phosgene the neutral or acidic derivatives formed were isolated by solvent extraction at pH 10 or 3, respectively. Following silylation the electron impact mass spectra of the metabolites exhibited a characteristic ion at m/z 336 of high abundance which originated from cleavage of the bond adjacent to the alpha-OTMS group. Most probably the identified compounds were formed by further biotransformations of alpha-hydroxy metoprolol, which is a primary metabolite. The analytical method is applicable to detect the metoprolol metabolites reported so far. A quantitative assay for one of the metabolites (H 119/72) with nitrogen selective detection is described. The total amount of this metabolite excreted by one subject within 24 h after dosing was about 0.25% of the given dose.

Metabolism of [14C] felodipine, a new vasodilating drug, in healthy volunteers

After oral administration of [14C] felodipine (27.5mg) to 4 healthy volunteers, 6 main urinary metabolites were identified by gas chromatography-mass spectrometry. The compounds were isolated by solvent extraction at pH 2.2 and silylated prior to analysis. They were formed by dehydrogenation of felodipine followed by ester hydrolysis, hydroxylation of the alkyl groups and conjugation. These metabolites were excreted both as free acids and as conjugates accounting on average for 37% of the excreted amount (23% of the dose). A specific liquid chromatographic assay with radioactive detection was developed to determine the acidic metabolites in all collected samples. The urinary excretion rate declined biphasically for the mono-acids III and IV, whereas the excretion rates of metabolites VI, VII and VIII, formed via aliphatic hydroxylation, were better fitted to equations of first-order processes.

Biotransformation of felodipine in liver microsomes from rat, dog, and man

The biotransformation of the calcium antagonist felodipine was investigated in liver microsomes from rat, dog, and man. The metabolites were quantified and identified by gradient elution reverse phase liquid chromatography, liquid scintillation analysis, and GC/MS. Ten metabolites were identified, including the pyridine analogue of felodipine, two carboxylic mono acids, two ester lactones, as well as the corresponding open hydroxy acid forms, and a lactonic compound with a carboxylic acid group. The presence of two decarboxylated products was also verified. Metabolites with an intact dihydropyridine nucleus were not detected. The total pool of metabolites formed in vitro was more lipophilic than that excreted in urine from the same species. The metabolic pathways were similar in the three species studied, although quantitative differences were observed. Comparison between incubations with liver microsomes from male and female rats indicated that the females metabolized felodipine more slowly than the males. From a more detailed quantitative analysis of eight metabolites, in relation to incubation time, it was apparent that the hydroxylation of the 2- and 6-methyl groups occurred at a faster rate (0.027 min-1) than did the ester hydrolysis (0.016 min-1). These hydroxy metabolites rearranged spontaneously to lactones. The results from this study indicate that the open hydroxy acid metabolites were formed enzymatically from the corresponding lactones. A metabolic scheme for the overall metabolism of felodipine is given and discussed with reference to the in vivo situation.

Identification of the main urinary metabolites of omeprazole after an oral dose to rats and dogs

The structures of seven urinary metabolites of omeprazole following high oral doses to rats and dogs were determined unambiguously by combining different analytical and spectroscopic techniques including derivatization and stable isotopes. Omeprazole was metabolized by aromatic hydroxylation at position 6 in the benzimidazole ring followed by glucuronidation. There was also oxidative O-dealkylation of both methoxy groups, and aliphatic hydroxylation of a pyridine methyl group followed by oxidation to the corresponding carboxylic acid. Due to the experimental design, implying no pH control of collected samples, all metabolites were isolated as sulfides. They were formed in both species with quantitative variations in the metabolic pattern. As far as identified metabolites are concerned, aromatic hydroxylation and subsequent glucuronide formation were the major biotransformation routes in the dog. In the rat, aliphatic hydroxylation and the formation of the carboxylic acid represented the major metabolic pathways. The identified metabolites corresponded approximately to 50% (rat) and 70% (dog) of the amount excreted in the 0-24-hr urine (about 12% of the given dose in both species).

Comparative metabolic disposition of oral doses of omeprazole in the dog, rat, and mouse

The metabolic disposition of [14C]omeprazole was studied in dogs, rats, and mice after the administration of pharmacologically active, single oral doses of drug in buffer solutions (pH 9). Averages of 38% (dogs), 43% (rats), and 55% (mice) of the radiolabeled doses were excreted in the urine in 72 hr. Most of the remaining dose was recovered in the feces. Omeprazole was extensively metabolized in all species studied and the metabolites were eliminated rapidly. No unchanged drug could be detected in the urine samples (less than 0.1% of dose). In each species at least 10 metabolites were detected in urine (pH 9) by gradient elution reverse phase HPLC. Based on liquid chromatographic retention data, the metabolic patterns were very complex and exhibited some quantitative differences between species. Bile was collected from rats and from chronic bile-fistulated dogs. Biliary excretion was a major route of elimination of omeprazole metabolites, and four polar metabolites were detected in the rat bile. The stability of omeprazole metabolites at varying pH values is discussed with reference to reductive metabolism of the parent compound.

Identification of felodipine metabolites in rat urine

After intragastric administration of 100 mumol kg-1 [14C]felodipine to rats eight urinary metabolites were isolated. Batch extraction at pH 2.2 and semipreparative reversed-phase liquid chromatography were used for trace enrichment of the metabolites. Trimethylsilylation followed by transesterification with diazomethane blocked the carboxylic acid and alcohol groups selectively before gas chromatography/mass spectrometry (GC/MS) in the electron impact (EI) mode. Deuterated derivatives of the metabolites and chemical ionization measurements added complementary structural information. All metabolites reported in this study were formed from oxidized felodipine by ester hydrolysis. Hydroxylation of the pyridine methyl group represented an important metabolic pathway and metabolites oxidized to the corresponding carboxylic acids were detected as well. Lactone formation from hydroxy acid metabolites in urine as a possible analytical artefact is discussed.

Identification of the major covalent adduct formed in vitro and in vivo between acetaminophen and mouse liver proteins

Improved analytical methodology has been developed for the structural characterization of covalently bound drug-protein adducts and has been applied to an investigation of the conjugates formed in vivo and in vitro between [14C]acetaminophen and mouse liver proteins. The major adduct released by acid hydrolysis of hepatic protein samples, which accounted for approximately 70% of the bound radioactivity in vivo and in vitro, was identified as 3-cystein-S-yl-4-hydroxyaniline, a derivative whose structure reflects the predominance of acetaminophen thioether adducts in drug-modified proteins. It is concluded that the reactive, electrophilic metabolite of acetaminophen, which most likely is N-acetyl-p-benzoquinoneimine, binds with a high degree of selectivity to cysteinyl thiol groups on protein, formally in a Michael-type addition reaction. Cysteine residues thus represent primary target sites for arylation by the reactive metabolite of acetaminophen, and proteins rich in free thiols may be especially vulnerable to damage by this toxic intermediate.

Structural characterization of the major covalent adduct formed in vitro between acetaminophen and bovine serum albumin

The structure of the covalent adduct formed in vitro between [14C]-acetaminophen ([14C]APAP) and bovine serum albumin (BSA) has been investigated with the aid of new analytical methodology. The APAP-BSA adduct, isolated from mouse liver microsomal incubations to which the radiolabeled drug and BSA had been added, was cleaved using a combination of specific (cyanogen bromide) and non-specific (acid hydrolysis) procedures, following which the mixture of amino acids obtained was derivatized, in aqueous solution, with ethyl chloroformate. The resulting ethoxycarbonyl derivatives were recovered by extraction into ethylacetate, methylated and subjected to profile analysis using both reverse-phase and normal-phase HPLC techniques. In each HPLC step, one major radioactive amino acid adduct was detected and was identified by mass spectrometry as the derivative of 3-cystein-S-yl-4-hydroxyaniline. Based on this finding, and with a knowledge of the behavior under acidic hydrolysis conditions of the 3-cysteinyl conjugate of APAP, it could be concluded that the major APAP-BSA adduct is one in which the drug is bound, via a thioether linkage at the C-3 position, to a sulfhydryl group on the protein. Furthermore, it could be established that this -SH function almost certainly is that associated with the cys-34 residue of BSA.

Absorption, distribution and elimination of felodipine in man

The objectives of these investigations were to study the absorption and disposition characteristics of felodipine in young healthy male volunteers following acute administration of different intravenous and oral doses, and to study urinary metabolites of [14C]felodipine following oral administration. Felodipine is rapidly and extensively absorbed from the gastrointestinal tract but owing to presystemic elimination, probably primarily in the liver, only 15% on average is systemically available. The systemic availability is independent of the oral dose in the 5 to 40 mg dose interval. The major fraction of the felodipine dose is localised extravascularly with a volume of distribution of about 10 L/kg. Less than 1% is confined to the blood. Felodipine is extensively bound to plasma proteins (greater than 99%). The mean elimination half-life of felodipine is greater than 10 hours. The urinary metabolic pattern of felodipine, using high pressure liquid chromatography, reveals 3 major metabolites (carboxylic acids of oxidised felodipine) in human urine.

Simultaneous determination of metoprolol and metabolites in urine by capillary column gas chromatography as oxazolidineone and trimethylsilyl derivatives

A method for the determination of metoprolol and its main metabolites in urine is presented. The method comprises derivatization of the aminopropanol side-chain with phosgene at alkaline pH and isolation in an organic phase at acidic pH. After trimethylsilylation, separation and quantification are performed by capillary column gas chromatography with flame ionization detection. The reaction is performed at pH 12 with 60 microliters of 2 M phosgene in toluene added in three portions. Diethyl ether--dichloromethane is used as extraction medium and bis(trimethylsilyl) acetamide as silylating agent. With spiked samples linear standard curves were obtained for metoprolol and three of its main metabolites with a detection limit varying between 4 and 20 mumol/l of urine. The method was applied to urine samples from a normal individual who had taken 292 mumol of metoprolol as tartrate.

Urinary metabolites of felodipine, a new vasodilator drug, in man, dog, rat and mouse

The urinary excretion of total 14C after oral administration of 25 mg (approximately 1 mumol/kg) 14C-felodipine to man, and intragastric administration (5 mumol/kg) to dog, rat and mouse, was 70, 39, 44 and 53% dose, respectively, in 72 h. Metabolites of felodipine were separated and quantified by h.p.l.c. Unchanged felodipine and its oxidized analogue were not excreted by any of the species studies. Three metabolites, present in all species studied, were isolated from urine and identified as products of the oxidation of felodipine to its pyridine analogue followed by hydrolysis of one or both of the pyridine carboxylic acid esters.

Stereoselective disposition of RS-tocainide in man

The disposition of RS-tocainide in three healthy volunteers has been studied after oral administration of a pseudoracemic mixture containing S(+) [3H] tocainide as a radioactive tracer together with a therapeutic dose of the racemate. Analytical methods based on HPLC have been developed to measure S(+) and R(-) tocainide in urine samples. Selected ion detection has been used for quantification of a tocainide conjugate. The radioactive dose was efficiently absorbed and mainly cleared via the kidneys. The elimination half-life of RS-tocainide was found to be 14.3 hours. The elimination half-lives of the two stereoisomers of tocainide differed significantly, i.e. R(-) tocainide 10 hours, and S(+) tocainide 16.7 hours. The observed t1/2 for the tocainide conjugate of 10.3 hours was close to that of R(-) tocainide, indicating that the metabolite was preferably formed from the R(-) stereoisomer of tocainide. Of the given dose, between 45 and 70% can be accounted for.

Pharmacokinetics of metoprolol and its metabolite alpha-OH-metoprolol in healthy, non-smoking, elderly individuals

The absorption and disposition of metoprolol have been evaluated in 10 healthy, non-smoking, elderly individuals (mean age 73.1 years) by simultaneous determination of [3H]-metoprolol and unlabelled metoprolol. The labelled drug was given as an intravenous tracer dose, immediately followed by oral metoprolol 25 mg. The experiment was preceded by administration of metoprolol 25 mg b.i.d. for 3 days. The volume of distribution, elimination half-life and total body clearance were almost the same as previously observed in healthy, young subjects. The mean systemic availability was about 39% in the elderly, which is lower than the mean of 55% observed in a control group of young volunteers who received 50 mg b.i.d. In the elderly, the mean plasma concentration of alpha-OH-metoprolol was about twice as high as that of the parent drug, whereas the opposite was true of the control group. The results indicate that age-related physiological changes have a negligible effect on the pharmacokinetics of metoprolol.

The metabolic disposition of the selective beta 1-adrenoceptor agonist prenalterol in mice, rats, dogs, and humans

The metabolic routes of the selective beta 1-adrenoceptor agonist prenalterol have been studied in mice, rats, dogs, and humans after oral administration. The drug was well absorbed from the gastrointestinal tract and most of the administered radioactivity was excreted in urine from all species within 24 hr. Prenalterol was metabolized to a varying extent in the species studied. About 20% of the 10-mg dose was recovered unchanged in man, the corresponding figures being 1.8% in the mouse, 7% in the rat, and 54% in the dog at 0.263 mg/kg (1 mumol/kg). Three metabolites were characterized and quantified by thin-layer chromatography, high-performance liquid chromatography, nuclear magnetic resonance (1H and 13C), and gas chromatography-mass spectrometry. Pronounced species variations in the metabolic pattern were observed. The phenolic sulfate ester of prenalterol was the main urinary metabolite in man, important in the dog, minor in the rat but not detectable in the mouse. Prenalterol glucuronide was formed in significant amounts in the animals and, in addition, beta-4(hydroxyphenoxy)lactic acid was present in dog urine. In the rat and the mouse the degree of biotransformation of prenalterol was significantly decreased at high oral doses of 2630 mg/kg (10 mmol/kg). The synthesis of prenalterol sulfate ester with use of ion pair extraction techniques is described.

Selected ion monitoring of metoprolol and two metabolites in plasma and urine using deuterated internal standards

A highly sensitive and specific quantitative assay for metoprolol and two of its metabolites, containing an unchanged 2-hydroxy-4-isopropylaminopropoxy sidechain, has been developed. The compounds are isolated from the alkalized sample (plasma or urine) by extraction with dichloromethane, and converted to trifluoroacetyl derivatives by reaction with methyl-bis-(trifluoroacetamide). The reaction mixture is gas chromatographed on an OV-17 column and each substance is assayed by electron impact mass spectrometry using selected ion monitoring, and quantified by comparing the intensity of fragment ion m/z 266 with the intensities of corresponding fragment ions from the deuterated internal standards (m/z 270 and 271). It is possible to determine concentrations as low as 1 nmol l-1 (0.3 ng ml-1) in 1 ml of sample with relative standard deviation of less than 10%.

Pharmacokinetic studies in man of the selective beta 1-adrenoceptor agonist, prenalterol

The pharmacokinetics of prenalterol, a selective beta 1-adrenoceptor agonist, has been studied in healthy subjects, by following the plasma concentration and urinary excretion of the unchanged compound and its total radioactive metabolites after oral and intravenous administration. Each of six healthy subjects received a single i.v. dose (2.5 mg) and three oral doses (2.5, 5.0 and 10 mg) of prenalterol. The oral dose was administered as a solution. Three of the subjects received the intravenous and oral doses of 2.5 mg as tritiated drug. Prenalterol was rapidly and completely absorbed after oral administration. The peak plasma concentration was attained after about 0.5 h. About 25% of prenalterol reached the systemic circulation. Prenalterol was extensively distributed to extravascular tissues with a half-life of the distribution phase close to 7 min. About 90% of the dose was excreted in urine within 24 h irrespective of the route of administration, indicating complete absorption of the drug. On average 60% of the i.v. and 13% of the oral doses were excreted as unchanged drug. The elimination half-life of the compound was 1.8 h, and the decline in the plasma concentration of the metabolites indicated a slower elimination rate than for the unchanged drug. Dose-dependent kinetics were not observed after the oral doses examined.

The effect of impaired renal function on the plasma concentration and urinary excretion of metoprolol metabolites

Plasma concentration and urinary excretion of total and 2 active metabolites of metoprolol have been studied in patients with varying degrees of renal impairment and in healthy subjects after intravenous and oral administration of 20 and 50 mg of 3H-metoprolol tartrate respectively. Renal clearance of total metabolites correlated directly with 51Cr-EDTA clearance (r = 0.95, p less than 0.001). A reduction of glomerular filtration rate (GFR) by 70 to 80% increased the elimination half-life of total metabolites and of the active metabolite alpha-hydroxymetoprolol about 3-fold. Significant accumulation was, however, only observed in the patients with a GFR of about 5 ml/min. Even in these patients, the contribution of alpha-hydroxymetoprolol to the beta-adrenoceptor blocking effect of metoprolol will be negligible. The second active metabolite studied is eliminated via biotransformation, and the urinary excretion as well as the plasma concentration of this metabolite were extremely low in comparison with those of the parent drug.

Plasma levels and beta-blocking effect of alpha-hydroxymetoprolol--metabolite of metoprolol--in the dog

The plasma levels and the beta-blocking effect of metoprolol and its active metabolite alpha-hydroxymetoprolol have been studied after i.v. bolus injections of the substances to dogs. For both substances the beta-blockade increased with the dose, and there was a linear relationship between percent reduction in exercise heart rate and the logarithm of plasma concentration. The dose of the metabolite, however, had to be 5 times higher than that of metoprolol to induce the same degree of beta-blockade. Because of differences in the volume of distribution, 2.0 liters/kg for alpha-OH-metoprolol and 3.5 liters/kg for metoprolol, the 5 times higher dose of alpha-OH-metoprolol resulted in 10 times higher plasma levels of the metabolite than of metoprolol. alpha-OH-Metoprolol was more slowly eliminated (t1/2 approximately 7.0 hr, total body clearance approximately 3.5 ml-kg-1-min-1) than metoprolol (t 1/2 approximately 2.0 hr, total body clearance approximately 20.0 ml-kg-1-min-1). Approximately 5% of an i.v. dose of metoprolol was metabolized to alpha-OH-metoprolol. The half-life of the endogenously formed metabolite was the same as after an i.v. dose of the compound.

Species differences in the metabolism of pamatolol, a cardioselective beta--adrenoceptor antagonist

The metabolism of pamatolol was studied in man, dogs, rats and mice after oral administration of a single dose. The drug was well absorbed in the gastro-intestinal tract and excreted in the urine, mainly in unchanged form, within 24 hrs. Four urinary metabolites were identified by gas chromatographic-mass spectrometric techniques. The metabolic data, in man, dog and mouse was found to be similar, both qualitatively and quantitatively. One metabolism route, involving aliphatic hydroxylation and subsequent oxidation, was found, to a significant extent only in the rat. The species variation between the mouse and the rat with regard to long-term toxicity of pamatolol is discussed. Artefact formation during trace analysis was observed.

Biotransformation of alprenolol in dog, guinea-pig and rat liver microsomes

1. Metabolites of alprenolol were isolated and identified in dog, guinea-pig and rat liver microsomes by means of g.l.c.-mass spectrometry and comparison with synthetic reference compounds. 2. The compounds were chromatographed as n-butylboronate derivatives, giving a series of diagnostic ions in the mass spectral fragmentation, which was elucidated by using stable isotopes. 3. Alprenolol was metabolized by aromatic ring hydroxylation, oxidation of the allylic function, and degradation of the isopropylaminopropanol side-chain. Alprenolol and four metabolites were quantified by h.p.l.c. and batch extraction techniques based on radioactivity measurements. 4. Five metabolites were detected in rat and guinea-pig liver microsomes and four in the dog. A species variation in the biotransformation of the allyl function in alprenolol was observed. The metabolite formed by oxidation of the allyl double bond was detected in significant amounts in the guinea-pig, and was also formed in the rat but could not be detected in dog liver microsomes.

Identification of urinary and biliary metabolites of alprenolol in the rat

1. After oral administration of alprenolol to rat, 12 metabolites were isolated and characterized as trifluoroacetyl, trimethylsilyl and n-butylboronate derivatives, using a g.l.c.-mass spectrometry-computer system. Fragmentation pathways of derivatives in the mass-spectrometric analysis are discussed. 2. Metabolic reactions involved are oxidative degradation of the propanolisopropylamine side-chain, aromatic hydroxylation, oxidation of the allyl group, and conjugation. A method for direct analysis of epoxide functions in the allyl group is described. 3. In comparison with metabolism of alprenolol in vitro, more polar metabolites are formed in vivo but the same principal metabolic pathways are valid. Structural features for biliary excretion are discussed.

Kinetic studies of dose-dependent metabolism of alprenolol: in vitro and in vivo studies in different species

Kinetic studies of the metabolism of alprenolol were performed with isolated microsomes from the rat, guinea-pig, dog and man at an initial substrate concentration of 0.17--150 micrometer. In all species the rate of aromatic hydroxylation reached a plateu above 50 micrometer of alprenolol in contrast to the rate of desisopropylation, where consistent saturation level was not obtained. The Km-values for the aromatic hydroxylation in the guinea-pig and man, 2,7 micrometer and 1.3 micrometer respectively, showed no concentration dependency in contrast to the rat (Km1 = 0.20 micrometer, Km2 = 26 micrometer) and the dog (Km1 = 0.78 micrometer, Km2 = 66 micrometer). The apparent Km-value of 0.20 micrometer for aromatic hydroxylation in the rat seemed to be of the same order of magnitude as reported spectral dissociation constant (Ks = 0.34 micrometer). In vivo experiments in the rat by oral administration of 7--700 mu mol/kg demonstrated a dose-dependent presystemic elimination of alprenolol. The urinary excretion of hydroxy-alprenolol was significantly lower after the highest dose. It is proposed, that the saturation of the aromatic hydroxylation, catalyzed by a high affinity site or subspecies of cytochrome P-450 with a low capacity, contributes to the dose-dependent kinetics in vivo.

Study of the metabolic pathways of alprenolol in man and the dog using stable isotopes

The metabolic pathways of alprenolol have been investigated in man and the dog, using an ion doublet technique of deuterium labelling combined with gas chromatography mass spectrometry. The drug is eliminated mainly by aromatic hydroxylation and glucuronidation. Specific analytical methods are applied to demonstrate that allylic oxidation and oxidative deamination are quantitatively of minor importance. The mechanism for oxidative deamination via an intermediary aldehyde could be elucidated by using the deuterium labelled compound. A method for characterization of 4-hydroxy-alprenolol glucuronides based on formation of stable derivatives and the following enzymatic hydrolysis is described. This approach has a general applicability to hydroxylated metabolites from compounds with an aminopropanol structure common for beta-adrenoceptor antagonists, for example. The metabolic routes for alprenolol in man and the dog are almost identical and in man more than 95% of a given dose can be accounted for.

Metabolism of metoprolol in the rat in vitro and in vivo

1. Metoprolol was metabolized in rat liver microsomes in vitro by O-demethylation with subsequent oxidation and by aliphatic hydroxylation of the methoxy-ethyl substituent and by oxidative deamination of the propanolisopropylamine side-chain. The same routes of metabolism in the rat in vivo were revealed from urinary metabolites. Eight metabolites were identified by g.l.c.-mass spectrometry by comparison with synthetized reference compounds. 2. Metoprolol binds to cytochrome-P-450 eliciting a type I difference spectrum with KS = 23 +/- 2-0 muM. The apparent Michaelis-Menten constant Km = 39 +/- 4-0 muM and Vmax = 1-28 +/- 0-22 nmol/mg protein X min were not significantly affected by pre-treatment of the rats with metoprolol or phenobarbital. Metoprolol pre-treatment had no effect on the cytochrome-P-450 level in the microsomes nor on the rate of metabolism of four standard substrates. Phenobarbital increased the cytochrome P-450 as expected. 3. Four metabolites representing the three main routes of metabolism were quantitatively determined after metabolism with rat liver microsomes and compared with the urinary levels of the same compounds. The same major metabolites were found in vitro and in vivo. The total amount of metabolites was not influenced by pre-treatment with metoprolol or phenobarbital. The relative amounts of the three main metabolites were slightly affected by pre-treatment.

Fibrin-stabilizing factor inhibitors. 12. 5-Dibenzylaminopentylamine and related compounds, a new type of FSF inhibitors

A series of omegadibenzylaminoalkylamines and related compounds have been prepared and tested as inhibitors of fibrin cross-linking. This structural type was chosen in an attempt to develop noncompetitive inhibitors of fibrinoligase. By the combination of the dibenzylamino moiety at one end and the primary amino group at the other end of a polymethylene chain, the same compound could function both as a pseudo donor substrate and as a noncompetitive alkylating inhibitor. Some of the compounds, notably 74-79, are among the most active fibrinoligase inhibitors described. However, the data indicate that the compounds probably function only as pseudo donor inhibitors.