Microsomal Dealkylation of Drugs

Synthesis, characterization, computational calculation and biological studies of some 2,6-diaryl-1-(prop-2-yn-1-yl)piperidin-4-one oxime derivatives

A new series of 2,6-diaryl-1-(prop-2-yn-1-yl)piperidin-4-one oximes (17-24) were designed and synthesized from 2,6-diarylpiperidin-4-one oximes (9-16) with propargyl bromide. Unambiguous structural elucidation has been carried out by investigating IR, NMR ((1)H, (13)C, (1)H-(1)H COSY and HSQC), mass spectral techniques and theoretical (DFT) calculations. Further, crystal structure of compound 17 was evaluated by single crystal X-ray diffraction analysis. Single crystal X-ray structural analysis of compound 17 evidenced that the configuration about CN double bond is syn to C-5 carbon (E-form). The existence of chair conformation was further confirmed by theoretical DFT calculation. All the synthesized compounds were screened for in vitro antimicrobial activity against a panel of selected bacterial and fungal strains using Ciprofloxacin and Ketoconazole as standards. The minimum inhibition concentration (MIC) results revealed that most of the 2,6-diaryl-1-(prop-2-yn-1-yl)piperidin-4-one oximes (17, 19, 20 and 23) exhibited better activity against the selected bacterial and fungal strains.

Biotransformation in review: applications in ocular disease and drug design

The purpose of this paper is to review the biochemical processes of systemic biotransformation and describe their relevance to ocular disease and drug metabolism. The diverse nature of the biochemical pathways, commonly found in enzyme metabolism, is discussed. The occurrence of these processes in the eye has significance in that the products of metabolism may accumulate locally and exert deterimental effects, presumably by altering the cellular structure and/or function of crucial visual elements. The manipulation of these metabolic pathways within the eye has ramifications in the development of novel drug design for both ocular disease treatment and, perhaps more importantly, disease prevention.

On the mechanism of amine oxidations by P450

1. Experimental data previously used to support an electron/proton transfer mechanism for oxidative dealkylation of amines by P450 are critically analysed with the conclusion that the mechanistic evidence is indecisive. 2. A new mechanistic criterion recently proposed to distinguish between electron/proton transfer and hydrogen atom transfer mechanisms is discussed. It is based on isotope effect profiles determined for the deprotonation of a series of para-substituted N-methyl-N-trideuteriomethyl)aniline cation radicals by pyridine and for hydrogen atom abstraction from the corresponding neutral amines by the tert-butoxyl radical. These reactions model the steps proposed in the two P450 mechanisms. 3. Isotope effect profiles measured for the demethylation of substituted NN-bis(dideuteriomethyl)anilines by four different forms of P450 were found to be experimentally indistinguishable from the hydrogen atom transfer profile, and distinctly different from the cation radical deprotonation profile. This provides strong evidence that P450 oxidatively dealkylates the amines by a hydrogen atom transfer mechanism and not by an electron/proton transfer mechanism.

Identification of urinary metabolites of ecabapide in rat

1. 14C-Ecabapide, 3-[[[2-(3,4-dimethoxyphenyl)ethyl]carbamoyl]methyl]amino-N- methyl[14C]benzamide, was dosed orally to rat (100 mg/kg). Within 48h after dosing, 36.7 +/- 5.4 and 55.7 +/- 11.8% of the administered radioactivity was recovered from urine and faeces respectively. 2. The unchanged drug was the major compound excreted in the urine and accounted for 37% of the urinary radioactivity. Seven urinary metabolites were purified by preparative hplc and their structures were elucidated by mass and 1H-nmr spectrometry. 3. The major metabolic pathway of ecabapide was found to be the formation of 3-amino-N-methylbenzamide produced by N-dealkylation of the secondary amine at the 3-position of the benzamide moiety followed by acetylation. 4. Further metabolic pathways of the N-methylbenzamide moiety were N-demethylation via the carbinolamine derivatives, and/or aromatic hydroxylation followed by glucuronidation.

Structure-activity relationships in the formation of amides from substituted N-benzylanilines

1. The in vitro hepatic microsomal metabolism of certain substituted N-benzylanilines was studied in the male hamster to establish the mechanism(s) and process(es) involved in the formation of the corresponding amides. 2. N-Benzyl-2,4,6-trihalogeno, N-benzyl-4-cyano- and N-benzyl-4-nitroanilines were only metabolized by N-debenzylation. However, N-benzyl-4-methyl- and N-benzyl-2,4,6-trimethylanilines gave rise to both the corresponding amide and nitrone metabolites together with dealkylation products. These latter two substrates also produced hydroxymethyl metabolites as major products. Metabolism of N-(2,4,6,-trimethylbenzyl)aniline also led to the formation of an amide metabolite. The dealkylation products, the corresponding imine and an unknown metabolite, probably an hydroxylated product were also detected with this substrate. 3. N-(2,4-Dichlorobenzyl) and N-(2,6-dichlorobenzyl) anilines yielded the corresponding nitrone metabolites; but no amide metabolite was detected. Oxidative dealkylation leading to the formation of the corresponding primary anilines and aldehydes, together with para hydroxylation of aniline rings, were established as major routes of metabolism for both compounds. Similarly, neither N-(2,4,6-trifluorobenzyl) nor N-(4-nitrobenzyl) anilines produced any amide metabolite although dealkylation products were detected. 4. The pattern of amide formation observed for these N-benzylsubstituted anilines is discussed in terms of the steric and electronic effects of their aromatic substituents.

The recognition of a diarylimine as a metabonate produced during incubation of N-benzyl-4-chloroaniline with hepatic microsomal preparations

Evidence is presented for the formation of N-benzylidene-4-chloroaniline as a metabonate during the metabolism of N-benzyl-4-chloroaniline. Control studies suggest that the diarylimine is formed as a chemical artifact from the debenzylation products (benzaldehyde and 4-chloroaniline). This novel observation indicates a possible pathway to amide formation from N-benzylanilines via diarylimines as intermediates.

N-hydroxymethylnorcotinine, a new primary in vitro metabolite of cotinine

1. N-Hydroxymethylnorcotinine; 5-(3'-pyridyl)-1-hydroxymethyl-pyrrolidone-2, was found as a new primary metabolite of cotinine in vitro. 2. N-Hydroxymethylnorcotinine was synthesized and characterized by gas chromatography-mass spectrometry, mass spectra, nuclear magnetic resonance and ultraviolet spectroscopy. 3. This new metabolite is formed by incubation of cotinine with hamster hepatic microsomes in the presence of NADPH and oxygen.

N-oxidation and N-demethylation of methylephedrine by rat-liver microsomes

1. The metabolism of methylephedrine was studied using rat-liver microsomes; methylephedrine N-oxide and ephedrine were isolated as metabolites. 2. The formation of methylephedrine N-oxide was inhibited by methimazole or by preincubation of microsomes, but that of ephedrine was not; whereas the formation of ephedrine was inhibited by either SKF 525-A or CO, but the formation of the N-oxide was not. 3. Pretreatment of rats with phenobarbital increased N-demethylation, but decreased N-oxidation. Pretreatment with 3-methylcholanthrene did not influence either reaction. 4. The results indicate that the formation of the N-oxide was mediated by the flavin-containing monooygenase system and that of ephedrine was mediated by the cytochrome P-450 system.

Conformational analysis of 9-substituted adenines in relation to their microsomal N1-oxidation

Metabolic N-oxidation of adenine, 9-methyladenine, 9-benzyladenine, 9-benzhydryladenine and 9-trityladenine has been investigated using hepatic microsomes from hamster, guinea-pig, rabbit, mouse, rat, and dog. N1-Oxide formation occurs with 9-benzyladenine and 9-benzhydryladenine using liver preparations of all species examined, although to different extents. The N-oxidase activity was found, amongst rodents, in the order hamster greater than mouse greater than rabbit greater than rat greater than guinea-pig. Microsomal preparations from dog liver contained a small quantity of P-450 and yet produced a relatively large amount of the N-oxides, possibly indicating that other cytochromes in addition to P-450 may be involved in the N-oxidation of these compounds. The most favourable conformations of these 9-substituted analogues have been established using computer graphics modelling and 1H NMR techniques. Results obtained confirmed the importance of the stereochemical properties of these compounds in relation to N1-oxidation. These observations substantiate and extend our previous findings on the electronic, lipophilic, and stereochemical factors affecting the N-oxidation of adenine derivatives.

[The pharmacokinetics of antilipemic agents. 5. Is 2-(4-hydroxyphenoxy)-2-methylpropionic acid a metabolite of the antilipemic ciprofibrate?]

After oral administration of Ciprofibrate (1) to volunteers (n = 5) only its glucuronide, but no 1 and no 2-(4-Hydroxyphenoxy)-2-methyl-propionic acid (3) could be detected in the fresh urine. Its formation from 1 seems to be impossible on the basis of established biochemical reactions. Therefore, its presence in the urine of volunteers reported by other authors is refuted.

Modelling of chemical reactions catalysed by membrane-bound enzymes. Determination and significance of the kinetic constants

A model of multiphasic systems, based on the assumption of zero-order partition of substrates and products into the membranes, is applied to reversible mono-substrate and bi-substrate reactions catalysed by membrane-bound enzymes. Apart from replacement of single-phase kinetic constants by apparent kinetic constants, the derived kinetic expressions are formally identical with those for corresponding single-phase systems. The model confers to the apparent kinetic constants an experimentally verifiable meaning. For full characterization of membrane-kinetic systems, experiments at various concentrations of enzyme-embedding phospholipid are required. Extrapolation to zero phospholipid concentration of each Km app then yields the corresponding true kinetic constant characteristic of the membrane-bound enzyme and also provides a technique for determination of the membrane-partition constants. The procedure implies that the phospholipid content should be assayed for full characterization of membrane-bound enzymes. If, for practical reasons, the assays have to be limited to a single enzyme concentration, correction of the apparent kinetic constants is still possible provided the phospholipid concentration and the partition constants of the reactants are known. The model has permitted prediction of a number of previous observations reflecting the multiphasic nature of the systems. The assumptions, underlying the model, and their implications are examined as well as some commonly used experimental designs for determination of the type of enzymic site.

Kinetic deuterium isotope effects on deamination and N-hydroxylation of cyclohexylamine by rabbit liver microsomes

Deuterium isotope effects on the kinetic parameters for deamination and N-hydroxylation of cyclohexylamine (CHA) catalyzed by rabbit liver microsomes with NADPH are investigated. Both reactions are inhibited by carbon monoxide and have the characteristics of typical cytochrome P450-dependent monooxygenase reactions. A small and significant deuterium isotope effect operates in the oxidative deamination of CHA. The apparent isotope effects, i.e., VH/VD and (V/K)H/(V/K)D ratios for deamination, are 1.75 and 1.8-2.3, respectively. On the basis of N-hydroxylation, the VH/VD and (V/K)H/(V/K)D ratios are 0.8-0.9. The N-hydroxylation rate of alpha-deuterated CHA (D-CHA) is somewhat higher than that of CHA. The increased increment of hydroxylamine formation seems to coincide with the decreased amount of deamination. Substitution of deuterium in the alpha-position of CHA results in metabolic switching of cytochrome P450 from deamination to N-hydroxylation with low deuterium isotope effects. The data are interpreted in terms of an initial one-electron abstraction from the nitrogen to form an aminium cation radical followed by recombination with iron-bound hydroxyl radical leading to N-hydroxylamine, or followed by alpha-carbon deprotonation to form a neutral carbon radical. The latter can lead to a carbinolamine intermediate for deamination by way of imine or recombination with nascent iron-bound hydroxyl radical. The relative rates of the reactions depend on the alpha-carbon deprotonation rates of amines.

Participation of cytochrome P-450 isozymes in N-demethylation, N-hydroxylation and aromatic hydroxylation of methamphetamine

1. Five isozymes of cytochrome P-450 were purified from liver microsomes of phenobarbital-pretreated (P-450-SD-I and -II), 3-methylcholanthrene-pretreated (P-450-SD-III) and untreated rats (P-450-SD-IV and -V) to determine their catalytic activities in metabolic reactions of methamphetamine. 2. All the isozymes except P-450-SD-III showed considerably high N-hydroxylating activity of methamphetamine. The cytochromes P-450 initiate N-demethylation of this drug by two metabolic pathways, C-hydroxylation and N-hydroxylation. 3. Both N-demethylation and N-hydroxylation of methamphetamine were efficiently catalysed by the phenobarbital-inducible forms P-450-SD-I and -II and constitutive forms P-450-SD-IV and -V. 4. The constitutive forms P-450-SD-IV and -V revealed high catalytic activities of p-hydroxylation of methamphetamine, but phenobarbital- and 3-methylcholanthrene-inducible isozymes showed only low activities. 5. The present results indicate that the different extents of the metabolic intermediate complex formation with cytochrome P-450 (455 nm complex) in the microsomes from phenobarbital-, 3-methylcholanthrene-pretreated, and untreated rats is not attributable to the activities of the respective isozymes of cytochrome P-450 to form the precursor of the complex, N-hydroxymethamphetamine.

Microsomal N-oxidation of long chain N,N-dimethylalkylamines: quantitative structure-activity relationships

N,N-Dimethylalkylamines CmH2m+1N(CH3)2 (m = 4 to 18) undergo in vitro N-oxidation to the corresponding amine oxides by mouse liver microsomes. The relative oxidisability of these compounds is parabolically dependent on structural and physico-chemical characteristics (with a maximum at m 12) and is controlled by at least three factors: lipophilicity, stereochemistry and the nucleophilicity of the compounds under evaluation. The results from the QSAR study support this multifactorial view.

The metabolism of N-benzyl-4-substituted anilines: factors influencing in vitro C- and N-oxidation

N-Benzyl-4-substituted anilines are metabolized by N-debenzylation, aniline-ring hydroxylation and N-oxidation. Factors affecting metabolism of these secondary anilines have been studied in vitro. Enzymes responsible for C- and N-oxidation are present mainly in the liver, but also occur in lung and kidney and are localized in the endoplasmic reticulum. Quantitative and qualitative species differences were observed in microsomal N-benzylaniline metabolism. In hamsters and mice N-debenzylation is the major route of metabolism. In guinea pig and rabbit, ring hydroxylation is the major pathway whereas the rat uses both pathways to equal extents. The hamster has particularly high N-oxidase activity. A sex difference in microsomal N-benzylaniline metabolism is evident in rats but not mice. Kinetic constants for C- and N-oxidation of N-benzylanilines are reported.

The metabolism of aromatic amines

Aromatic amines are of general interest in drug metabolism and some are a health hazard, particularly as bladder carcinogens. Conditions for the biological ring- and N-oxidation of aniline and its derivatives are reviewed. The metabolism of 2-naphthylamine and aminobiphenyls and the involvement of metabolites of aromatic amines in bladder cancer is discussed.

Bioerodible polyanhydrides as drug-carrier matrices. II. Biocompatibility and chemical reactivity

The biocompatibility of bioerodible polyanhydrides and toxicology of the polymer breakdown products were assessed. Poly-[bis (p-carboxy-phenoxy) propane anhydride] (PCPP), Poly(terephthalic acid anhydride) (PTA), and their copolymers with sebacic acid were tested. The polymers did not provoke inflammatory responses in the corneas of rabbits over a six week implantation period. Subcutaneous implantation studies of PCPP in rats over a six month period showed no evidence of inflammatory cells and only slight tissue encapsulation by layers of fibroblastic cells. The degradation products of the polymers were nonmutagenic, noncytotoxic, and had a low teratogenic potential. The in vitro growth of mammalian cells on the polymers was unaffected as measured by cell morphology and cell growth rate. The chemical reactivity of the polyanhydrides with reactive model drugs, para substituted anilines, was also examined. Amides were formed when the drugs were injection molded with the polyanhydrides at 120 degrees C. However, no reaction was observed using compression molding at room temperature. No reaction occurred between the polymer and the drug during the hydrolytic degradation of the matrix at 37 degrees C.

Microsomal N- and C-oxidation of 4-substituted N-benzylanilines

Using direct specific analytical techniques microsomal metabolism of N-benzyl-4-substituted anilines has been investigated and found to include both N- and C-oxidation. N-Debenzylation was observed with all substrates and species examined. N-Oxidation usually yielded aryl nitrones, although the N-hydroxy derivative of the 4-chloro-substituted substrate was identified in some species. This is the first direct evidence of microsomal N-hydroxylation of a secondary aniline. The metabolic formation of amides from these secondary amines was observed and is believed to be a novel class of metabolite for these substrates.

Metabolites of cardiac antiarrhythmic drugs: their clinical role

Most antiarrhythmic drugs are extensively metabolized, and the accumulation of the metabolites of several of these drugs has been documented. In some cases, the steady-state plasma concentrations of metabolites are considerably greater than is the concentration of the parent drug. Several of these metabolites have been evaluated in animal models for antiarrhythmic activity and their potencies have been defined relative to the activity of their parent compound. Evaluations of activity are generally conducted in animal arrhythmia models, and very few metabolites of antiarrhythmic drugs have been evaluated directly in patients. However, from knowledge of antiarrhythmic activity in animals and the degree to which a metabolite accumulates in the plasma of patients, one can make qualitative judgments about its therapeutic role. Such judgments, however, need to be recognized as tenuous. Quantitative judgments require further information regarding the relationship between the parent drug and metabolite when present simultaneously in the myocardium. One must consider whether the effects of the parent drug and metabolite are additive, synergistic, or even antagonistic. The latter case is most possible with drug-metabolite pairs where the metabolite accumulates substantially, but does not have significant antiarrhythmic potency. Other considerations include noncardiac effects of the metabolites. As in the case of the mono-desethyl metabolite of lidocaine, the significance of its accumulation relates more to central nervous system side effects than to direct cardiac actions. The role of active metabolites also much be considered in regard to differences in the disposition kinetics between the parent drug and metabolite. The most obvious situation where this is important is in designing clinical drug evaluation protocols. As illustrated by the metabolites of encainide and lorcainide, the time course of accumulation and disappearance of the metabolites may be much longer than that of the parent drug. Clinical evaluations at steady state must take into account the time required to achieve steady-state concentrations of the metabolites as well. Similarly, after discontinuation of drug administration, the time required before washout is complete may be totally dependent on the kinetics of the metabolite, and not the parent drug. Variability in metabolic activity also needs to be considered. It has been shown with procainamide and encainide that genetic factors can influence the rate of production of active metabolites and consequently influence the clinical efficacy of these drugs. Another consideration that deserves attention is the question of drug interactions.(ABSTRACT TRUNCATED AT 400 WORDS)

Studies on N-demethylation of methamphetamine by liver microsomes of guinea-pigs and rats: the role of flavin-containing mono-oxygenase and cytochrome P-450 systems

Relative participation of flavin-containing mono-oxygenase and cytochrome P-450 systems in N-hydroxylation of and formaldehyde release from methamphetamine were studied in vitro using liver microsomes of guinea-pigs and rats. In guinea pigs, only methimazole, an inhibitor of flavin-containing mono-oxygenase, significantly suppressed the above reactions. Formaldehyde release from methamphetamine was significantly inhibited not only by methimazole but also by inhibitors of the cytochrome P-450 system in liver microsomes from rats, but not guinea-pigs. Pretreatment of guinea-pigs with phenobarbital and 3-methylcholanthrene did not enhance the metabolism of methamphetamine. Pretreatment of rats with phenobarbital but not 3-methylcholanthrene increased slightly the N-demethylation of methamphetamine by liver microsomes. The results indicate that a marked species difference exists in the enzymes concerned with N-demethylation of methamphetamine. N-Oxidation predominates in guinea-pigs, whereas in rats, N-oxidation and C-oxidation of the methyl group participate equally as the initial reaction of the N-demethylation pathway.

High-performance liquid chromatographic isolation and fast atom bombardment mass spectrometric identification of Di-N-desethylamiodarone, a new metabolite of amiodarone in the dog

Amiodarone is an antiarrhythmic drug which has received considerable attention in recent years. It has been suggested that the unusual pharmacodynamic characteristics of this drug may be due in part to the influence of active metabolites. Using fast atom bombardment (FAB) mass spectrometry we have identified a new metabolite of amiodarone, the di-N-desethyl analog (DDEA). This metabolite was present in the blood of dogs treated with the parent drug, and showed a greater affinity for myocardium than did the parent drug. The unique features of FAB mass spectrometry over electron impact mass spectrometry was an essential element in facilitating the identification of this new metabolite. Whether or not this metabolite has pharmacologic activity or is responsible for some of the side effects occurring during amiodarone administration is not known.

The peroxidatic function of liver microsomal cytochrome P-450: comparison of hydrogen peroxide and NADPH-catalysed N-demethylation reactions

Fifteen different secondary and tertiary methyl amines have been examined as substrates for the cytochromes P-450 of rat-liver microsomes to determine the similarities or differences between the NADPH and oxygen-dependent N-demethylation reaction and the reaction occurring in the presence of hydrogen peroxide. No apparent correlation of the rates of formaldehyde formation using the two different conditions of oxidation was observed. The types of cytochromes P-450 were altered by using rat-liver microsomes from animals treated with various inducing agents. No obvious predictable dependence on the animals treated with various inducing agents. No obvious predictable dependence on the type of cytochrome P-450 present was obtained for the hydrogen peroxide-supported peroxidatic reaction. It is concluded that the hydrogen peroxide-dependent N-demethylation reaction occurs by a reaction mechanism distinct from that occurring during the mixed-function oxidase activity of cytochrome P-450 obtained in the presence of NADPH and oxygen.

Autoradiographic disposition of [1-methyl-14C]- and [2-14C]caffeine in mice

Male, C57B1/6J mice received either [1-methyl-14C]caffeine or [2-14C]caffeine via the tail vein at a dose of 0.7 or 11 mg/kg, respectively. At 0.1, 0.33, 1, 3, 9, and 24 hr after treatment, the mice were anesthetized with ether and frozen by immersion in dry ice/hexane. The mice were processed for whole-body autoradiography by the Ullberg technique; this procedure does not allow thawing or contact with solvents. All autoradiographs revealed some retention of radioactivity at early time intervals in the lacrimal glands, seminal vesicle fluid, nasal and olfactory epithelium, and retinal melanocytes. The remaining portion of the animal was densitometrically uniform except for the lower levels noted in the CNS and adipose tissues. Excretion of radioactivity by the liver and kidneys seems to be the major routes of elimination. Localization in the liver at late time intervals was confined principally to the centrilobular region. Late sites of retention, observed only after [1-methyl-14C]caffeine administration, included the pancreas, minor and major salivary glands, splenic red pulp, thymal cortex, bone marrow, and gastrointestinal epithelium. Sites of localization present in both studies included the olfactory epithelium, lacrimal glands, hair follicles, and retinal melanocytes. Further studies are needed to determine whether the localization at these various sites is due to metabolic degradation, active transport, or possibly a specific receptor interaction.

The metabolism of 4-substituted-N-ethyl-N-methyl-anilines. II. Some factors influencing alpha-C- and N-oxidation

A g.l.c. method for the quantification of N-demethylated and/or N-de-ethylated (alpha-C-oxidation) and N-oxidation products of several 4-substituted-N-ethyl-N-methylanilines is described. Factors affecting the metabolism of these tertiary anilines in vitro have been studied and conditions which allow maximal metabolism established. The alpha-C- and N-oxidase activity was detected principally in the liver, lung, kidney and bladder microsomal fraction. A species difference in the extent of metabolism was evident, the order of activity not being the same for alpha-C- as that for N-oxidation. The pKa but not log P values determined for the teritary anilines appear to influence the apparent Km for both alpha-C- and N-oxidation.

The formation and metabolism of N-hydroxymethyl compounds--III. The metabolic conversion of N-methyl and N,N,-dimethylbenzamides to N-hydroxymethyl compounds

The stability of metabolically-generated N-(hydroxymethyl) compounds was investigated using a series of N-methylbenzamides as model substrates. N-(Hydroxymethyl)-benzamide was characterized as a major metabolite of N-methylbenzamide in vitro, and was also identified as a urinary metabolite of N-methylbenzamide. N-(Hydroxymethyl) compounds were also found as metabolites of 4-chloro-N-methylbenzamide and 4-t-butyl-N-methylbenzamide in vitro. Thus substitution in the 4-position of the phenyl ring of derivatives of N-(hydroxymethyl)-benzamide did not affect their stability sufficiently to cause degradation to formaldehyde under the conditions used. N-(Hydroxymethyl)-N-methylbenzamide was identified as a metabolite of N,N-dimethylbenzamide in vitro. However, N-(hydroxymethyl)-N-methylbenzamide was less stable than N-(hydroxymethyl)-benzamide under alkaline conditions. Furthermore, N-(hydroxymethyl)-N-methylbenzamide, unlike N-(hydroxymethyl)-benzamide and its 4-substituted derivatives, was positive in the colorimetric assay for formaldehyde, presumably because of its degradation to produce formaldehyde. Thus substitution on the nitrogen atom which bears the methyl group in N-methylbenzamide markedly affected the stability of the N-methylol produced during oxidative metabolism. N-Formylbenzamide was identified as a metabolite of N-methylbenzamide in suspensions of mouse hepatocytes and also in vivo. The mechanism for its production probably involves the generation of N-(hydroxymethyl)-benzamide.

Pyrrolic and N-oxide metabolites formed from pyrrolizidine alkaloids by hepatic microsomes in vitro: relevance to in vivo hepatotoxicity

An analytical method of improved sensitivity has enabled measurements to be made of N-oxide as well as pyrrolic metabolites formed from a range of unsaturated pyrrolizidine alkaloids in hepatic microsome preparations. Using microsomes from livers of phenobarbitone-pretreated male Fischer rats, all 13 alkaloids tested were metabolised to both N-oxides and pyrroles. The most lipophilic alkaloids gave enhanced rates of metabolism. No consistent relationship existed between rates of N-oxide and of pyrrole formation. The two pathways appeared to be independent. The ratio of N-oxide to pyrrolic metabolites varied, depending on the type of ester: it was highest for 'open' diester alkaloids, lowest for 12 membered macrocyclic diesters and for monoesters. Steric hindrance by the acid moiety could account for these differences, by affecting the balance between microsomal oxidation of the amino alcohol moiety at the nitrogen and C8 positions respectively and could explain the high pyrrole yields given by some macrocyclic diesters. The levels of pyrrolic metabolites bound to liver tissues and responsible for hepatotoxicity in rats given pyrrolizidine alkaloids, did not necessarily reflect the rates of formation of such metabolites measured in vitro. In the animal additional factors could influence the formation and tissue binding of pyrrolic metabolites, including the detoxication of alkaloids by hydrolysis and the chemical reactivity and stability of the toxic metabolites. A comparison of heliotridine esters with retronecine esters showed that the 7-hydroxyl or -ester configuration had a relatively small influence on the balance between formation of pyrrolic metabolites and detoxication by N-oxidation. The results did not support any hypothesis that heliotridine esters should generally be more hepatotoxic than analogous retronecine esters. The structure of the acid moiety was likely to have at least as much influence on toxicity as the base configuration.

The metabolism of tertiary amines

The metabolism of tertiary amines is mediated primarily by cytochrome P-450 and MFAO, leading to alpha-C oxidation and N-oxidation, respectively. We have discussed how lipophilicity, basicity, steric hindrance, and stereochemistry can effect the outcome of metabolism as well as species, sex, and age. The proposed oxidation of tertiary amines to iminium ions by cytochrome P-450 may explain the isolation of various intramolecular and cyanide-trapped metabolites. N-oxides may represent a smaller percentage of the overall in vitro metabolism of tertiary amines due to the postmortem inactivation of MFAO. In addition N-oxide reducing enzymes present in vivo and in vitro may influence the extent of N-oxide formation. In general, definite conclusions about substrate requirements have been difficult to formulate because of the numerous biological and physical parameters affecting the outcome of metabolism. More singularly directed research on a single species of animal and a wide variety of substrates or vice versa would greatly increase our understanding of the potential metabolism of tertiary amine xenobiotics.

Metabolism of 4-substituted-N-ethyl-N-methylanilines: chromatographic and mass spectrometric identification of N-oxidation metabolic products formed in vitro

The electron impact and isobutane and ammonia chemical ionization mass spectral characteristics of the N-oxidation products of a series of 4-substituted-N-ethyl-N-methylanilines are described. The thermolability and the relative non-volatility of N-oxides of this type of tertiary aromatic amine makes their unequivocal identification by gas chromatography and electron impact mass spectrometry difficult. Mass spectral identification is facilitated by employing CI mass spectrometry where, under appropriate conditions, the N-oxides give intense protonated molecular ions. Chromatographic, electron impact and chemical ionization mass spectrometric evidence is presented which shows that these 4-substituted tertiary anilines undergo metabolic N-demethylation, N-de-ethylation and N-oxidation in vitro. The suitability of these tertiary aromatic amines as model substrates for mechanistic studies on C- and N-oxidation is discussed.

Formation of N-(glutathion-S-methylene)-4-aminoazobenzene following metabolic oxidation of the N-methyl group of the carcinogen, N-methyl-4-aminoazobenzene

A major biliary metabolite of the hepatocarcinogen, N,N-dimethyl-4-aminoazobenzene (DAB), in the rat was identified as N-(glutathion-S-methylene)-4-aminoazobenzene (GS-CH2-AB). This conjugate was prepared synthetically by a Mannich condensation of 4-aminoazobenzene (AB), formaldehyde (CH2O) and glutathione (GSH) and has been characterized by chemical analysis and by ultraviolet, visible and 13C-NMR spectroscopy. The same conjugate was also formed in vitro by incubating N-methyl-4-aminoazobenzene (MAB), NADPH, NADH and GSH with rat hepatic microsomes. Evidence is presented that GSH reacted with an intermediate resulting from a cytochrome P-450-dependent oxidation of the N-methyl substituent. This reactive intermediate is presumed to be either an N-methylol or a methimine derivative of AB. The significance of this detoxification mechanism is discussed. The presence of an additional major aminoazo-dye GSH conjugate is also noted.

Biotransformation in the monkey of cartazolate (SQ 65,396), a substituted pyrazolopyridine having anxiolytic activity

1. The metabolism of cartazolate (SQ 65,396), an anxiolytic agent, was studied in four male rhesus monkeys after oral administration. 2. Seven metabolites were identified in the pooled urine of the four monkeys. These resulted from a combination of (1) hydrolysis of the 5-carboxylic acid ethyl ester; (2) N-de-ethylation of the pyrazole ring; (3) gamma-hydroxylation of the n-butylamino side-chain; (4) removal of the n-butyl group; and (5) conjugation with beta-glucuronic acid.

Subcellular fractionation of isolated rat hepatocytes. A comparison with liver homogenate

An improved method for the homogenization and the subsequent subcellular fractionation of hepatocytes isolated from adult rat liver is described. The homogenization procedure developed in the present study allows the preservation of the integrity of subcellular structures, as demonstrated by measurement of the activities of representative enzymes as well as by determination of their latency. The activities of representative marker enzymes, as calculated on subcellular fractions obtained by differential centrifugation of the homogenate, are identical whether the homogenate arises from isolated hepatocytes or from the whole liver. Moreover, there is a close similitude between the kinetic parameters (Km and V) of two microsomal cytochrome P450-dependent mixed-function oxidases, namely aniline hydroxylase and aminopyrine demethylase determined on microsomal preparations obtained either from isolated cells or from the whole liver.

Metabolism of norcocaine, N-hydroxy norcocaine and cocaine-N-oxide in the rat

1. The metabolism of [3H]norcocaine, N-hydroxy[3H]norcocaine and cocaine-N-oxide has been investigated in rats after i.v. injection. 2. The biological t 1/2 of norcocaine (dose 2 mg/kg i.v.) in plasma, liver and brain were 0.4, 1.6, 0.5 h, respectively and the compound was not detectable in the central nervous system 6 h after injection. The % dose of norcocaine excreted unchanged in urine and faeces in 96 h were 0.7 and 1.0, respectively. Benzoylnorecgonine, norecgonine, norecgonine methyl ester and an unidentified compound were excreted in urine. 3. The biological t 1/2 of N-hydroxynorcocaine (5 mg/kg i.v.) in brain and plasma were 0.3, 1.6 h respectively and only 1.3 and 1.6% of dose were excreted unchanged in urine and faeces in 96 h. N-Hydroxybenzoylnorecgonine and N-hydroxynorecgonine methyl ester were the major urinary metabolites. N-hydroxynorcocaine was not metabolized to norcocaine in vitro by liver microsomes. Doses of greater than 7.5 mg/kg i.v. resulted in death of rats by cardiorespiratory arrest. 4. Cocaine-N-oxide (50 mg/kg i.v.) yielded ecgonine-N-oxide methyl ester as its major metabolite; other minor metabolites were cocaine (0.5%), norcocaine (1%), benzoylecgonine, ecgonine, ecgonine-N-oxide, along with minor amounts of unmetabolized compound. Lethality of cocaine-N-oxide (100 mg/kg i.v.) was possibly due to metabolism to norcocaine and cocaine.

The N- and alpha- C-oxidation of N,N-dialkylanilines by rabbit liver microsomes in vitro

1. A method for the determination of N,N-dialkylanilines and their metabolites by g.l.c. is described. 2. N-Demethylation and N-oxidation are important routes of metabolism of N-alkyl-N-methylanilines. 3. N-dealkylation, except the removal of a N-t-butyl group, occurs during the metabolism of N,N-dialkylanilines. 4. N-oxidation was found with N,N-dimethyl and N,N-diethylaniline and all N-methyl-N-alkylanilines examined except N-methyl-N-t-butylaniline. 5. A good correlation was found between the hydrophobic bonding constant (pi) and the extent of demethylation of a series of N-alkyl-N-methylanilines.