Metabolism of imipramine in vitro: synthesis and characterization of N-hydroxydesmethylimipramine

Sequential metabolism of secondary alkyl amines to metabolic-intermediate complexes: opposing roles for the secondary hydroxylamine and primary amine metabolites of desipramine, (s)-fluoxetine, and N-desmethyldiltiazem

Three secondary amines desipramine (DES), (S)-fluoxetine [(S)-FLX], and N-desmethyldiltiazem (MA) undergo N-hydroxylation to the corresponding secondary hydroxylamines [N-hydroxydesipramine, (S)-N-hydroxyfluoxetine, and N-hydroxy-N-desmethyldiltiazem] by cytochromes P450 2C11, 2C19, and 3A4, respectively. The expected primary amine products, N-desmethyldesipramine, (S)-norfluoxetine, and N,N-didesmethyldiltiazem, are also observed. The formation of metabolic-intermediate (MI) complexes from these substrates and metabolites was examined. In each example, the initial rates of MI complex accumulation followed the order secondary hydroxylamine > secondary amine >> primary amine, suggesting that the primary amine metabolites do not contribute to formation of MI complexes from these secondary amines. Furthermore, the primary amine metabolites, which accumulate in incubations of the secondary amines, inhibit MI complex formation. Mass balance studies provided estimates of the product ratios of N-dealkylation to N-hydroxylation. The ratios were 2.9 (DES-CYP2C11), 3.6 [(S)-FLX-CYP2C19], and 0.8 (MA-CYP3A4), indicating that secondary hydroxylamines are significant metabolites of the P450-mediated metabolism of secondary alkyl amines. Parallel studies with N-methyl-d(3)-desipramine and CYP2C11 demonstrated significant isotopically sensitive switching from N-demethylation to N-hydroxylation. These findings demonstrate that the major pathway to MI complex formation from these secondary amines arises from N-hydroxylation rather than N-dealkylation and that the primary amines are significant competitive inhibitors of MI complex formation.

Particular ability of cytochromes P450 3A to form inhibitory P450-iron-metabolite complexes upon metabolic oxidation of aminodrugs

The ability of 21 drugs containing an amine function to form inhibitory P450-iron-metabolite complexes absorbing around 455 nm was studied on liver microsomes from rats treated with various P450 inducers. These drugs belong to different chemical and therapeutic series and exhibit very different structures. In the case of eight compounds (diltiazem, lidocaine, imipramine, SKF 525A, fluoxetine, L-alpha-acetylmethadol, methadol and desmethyltamoxifen) whose oxidation by microsomes from rats treated with several inducers was studied, only dexamethasone (DEX)-treated rat microsomes and, to a lesser extent, phenobarbital (PB)-treated rat microsomes, were able to give significant amounts of 455 nm absorbing complexes. Ten of the 21 compounds studied gave such complexes with DEX-treated rat microsomes, while only three compounds gave complexes (in low amounts) with PB-treated rat microsomes only. For all compounds leading to complexes both with DEX- and PB-treated rat microsomes, much higher amounts of complexes were obtained with DEX-treated rat microsomes. DEX-treated rat microsomes also led to the most intense type I spectral interactions with most of the compounds studied, and very often exhibited the highest N-dealkylation activities towards the tertiary or secondary amine function of the drugs used. A few exceptions aside, there generally exists a qualitative relationship between the ability of P450 3As, induced by DEX, to bind and N-dealkylate amino compounds and their propensity to lead to 455 nm absorbing complexes. This was confirmed by in vivo experiments showing that rats treated with diltiazem, tamoxifen or imipramine accumulated large amounts of 455 nm absorbing complexes in their liver only after pretreatment with DEX and, to a lesser extent, with PB. This particular ability of P450 3As to oxidize amino drugs with formation of inhibitory P450-metabolite complexes could be of great importance for the appearance of drug interactions in man.

Metabolism of lofepramine and imipramine in liver microsomes from rat and man

1. The in vitro metabolism of lofepramine was studied in comparison with imipramine. Both compounds were hydroxylated and demethylated by a NADPH-generating system in rat and human liver microsomes. 2. Three metabolites were in common for the two drugs, namely desipramine (DMI), 2-hydroxydesipramine (2-OH-DMI) and didesmethylimipramine (DDMI). 3. Lofepramine was also metabolized to three unique tricyclic metabolites. Comparisons with authentic reference compounds suggested that two of these metabolites were 2-hydroxylofepramine and desmethyllofepramine. 4. The ratio between the concentrations of DDMI and DMI was higher for lofepramine than imipramine. This is probably due to DDMI formation via two parallel metabolic pathways of lofepramine, i.e. DMI and desmethyllofepramine, respectively. 5. It is speculated that the different metabolic pattern of lofepramine as compared with desipramine and imipramine is of importance for the therapeutic profile of the drug.

Metabolism of chlorpromazine and promazine in vitro: isolation and characterization of N-oxidation products

1. The syntheses of the secondary hydroxylamines of nor1chlorpromazine and nor1promazine via their corresponding primary hydroxylamines and oximes are described. 2. The N-oxidation products are unstable to analysis by g.l.c. without prior derivatization; the decomposition products and the structures of the trimethylsilyl (TMS) and trifluoroacetyl (TFA) derivatives were characterized by g.l.c.-mass spectrometry. 3. Chlorpromazine, promazine and their demethylated products were shown to undergo metabolic N- and alpha-C-oxidation, to yield hydroxylamines and carboxylic acids, on incubation with fortified 9000 g liver homogenates of male New Zealand white rabbits. 4. A condensation product, an artifact formed by reaction of the metabolically derived primary hydroxylamines with acetaldehyde, an impurity in the extraction solvent, diethyl ether, was identified. 5. N-hydroxynor1- and N-hydroxynor2chlorpromazine undergo metabolic reduction to the parent amines, and the secondary hydroxylamine undergoes N-demethylation to yield the corresponding primary hydroxylamine.

Methadone metabolism in the rat in vivo: identification of a novel formamide metabolite

Deuterium-labelled methadone and metabolites were used for the g.l.c.-mass spectrometry detection and identification of biliary conjugated methadone metabolites in rats. After beta-glucuronidase hydrolysis the bile extract contained an unknown metabolite that was not ring hydroxylated and retained an intact keto group. Chemical oxidation of the methadone metabolite 2-ethylidene-N,5-dimethyl-3,3-diphenylpyrrolidine, perchlorate salt (EDDP) with m-chloroperbenzoic acid in chloroform, gave a compound identical by g.l.c.-mass spectrometry to the new metabolite. The chemical oxidation product was identified as 2-(4',4'-diphenylheptan-5'-one-2'-yl)oxaziridine by spectroscopic methods. The oxaziridine was shown to quantitatively isomerize to a secondary formamide (2-formamido-4,4-diphenyl-5-heptanone) during g.l.c.-mass spectrometry analysis. The formamide was also isolated by flash column chromatography after reflux of the oxaziridine in m-xylene, and then characterized by spectroscopy. The formamide and oxaziridine g.l.c.-mass spectrometry characteristics were identical. It was concluded on the basis of g.l.c.-mass spectrometry that the metabolite is the secondary formamide.