Aromatic oxidation of some phenothiazines

Curiosities in drug metabolism

1. It is inevitable that during some xenobiotic biotransformation studies, a certain metabolite or degradation product arises of which the identity is uncertain, the route of formation is ambiguous, or it is just a plain mystery. 2. The following communication draws attention to three drugs reported in the literature, chlorphentermine, phenothiazine and aminopyrine, where after many years of investigation there still exists uncertainty over some of their metabolites. Noticeably, these three examples probably involve (potential) interaction of a nitrogen centre within the drug molecule. 3. It is hoped that the resurrection and assemblage of these data will offer interesting reading and that these examples may prove sufficiently intriguing to motivate further exploration and some resolution of these lingering concerns.

Glycerolysis of acyl glucuronides as an artifact of in vitro drug metabolism incubations

During an investigation of the in vitro glucuronidation of benoxaprofen by human liver S-9 fraction, an unusual drug-related entity possessing a protonated molecular ion that was 74 mass units greater than the parent drug was observed. It was identified as the glycerol ester of benoxaprofen. Formation of this entity required inclusion of uridine diphosphoglucuronic acid (UDPGA) in the incubation, suggesting the formation of benoxaprofen acyl glucuronide followed by transesterification with the glycerol present in the incubation due to its presence as a stabilizer for liver subcellular fractions. Formation occurred during the sample work-up procedure while the samples were subjected to evaporation in vacuo, which does not remove glycerol. Conversion of purified benoxaprofen acyl glucuronide to the glycerol ester was demonstrated in glycerol at 37 degrees C. Other drugs that are converted to acyl glucuronides in vitro (diclofenac, mefenamic acid, tolmetin, and naproxen) were also shown to form corresponding glycerol esters when incubated with human liver S-9 fraction and UDPGA. The potential formation of glycerol esters of carboxylic acid drugs undergoing acyl glucuronidation in vitro represents an experimental artifact to which drug metabolism scientists should be aware.

The toxicity of phenothiazine

Phenothiazine, the parent compound of a multitude of present-day drugs, has been employed on an extensive scale for its insecticidal, fungicidal, antibacterial and anthelmintic properties. Almost a catholicon, its widespread use in animals and man has led to the uncovering of many adverse reactions encompassing effects on blood elements, neuromuscular problems and photosensitization. The high lipophilicity of phenothiazine and the formation of two redox systems amongst its many metabolites can facilitate the occurrence of generalised macromolecular disruption. Information from the literature has been garnered and appraised in this review to enable an insight into the possible mode(s) of interaction of phenothiazine with living systems.

Oxidation of phenothiazine and phenoxazine by Cunninghamella elegans

1. To determine the ability of fungi to metabolize sulphur- and oxygen-containing azaarenes, Cunninghamella elegans ATCC 9245 was grown in 125-ml flasks containing fluid Sabouraud medium. The cultures and controls were incubated at 28 degrees C with shaking and dosed with 16.7 mM phenothiazine or phenoxazine. After incubation for 72h, the mycelia and filtrates were extracted with ethyl acetate and the combined residues analysed by high-performance liquid chromatography. Residual phenothiazine and phenoxazine were 21 and 22%, respectively, of the total UV absorbance at 254 nm. 2. The metabolites were identified by mass spectrometry and proton nuclear magnetic resonance spectroscopy. The fungus oxidized phenothiazine to phenothiazine sulphoxide, 3-hydroxyphenothiazine sulphoxide, phenothiazin-3-one, and 3-hydroxyphenothiazine and oxidized phenoxazine to phenoxazin-3-one. 3. Three of the four compounds produced by C. elegans from phenothiazine were identical to those produced by mammals, supporting the use of the fungus as a microbial model for drug metabolism.

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.

Inhibition of the mutagenicity and metabolism of 6-methyl-benzo[a]pyrene and 6-hydroxymethyl-benzo[a]pyrene

Previously reported inhibitors of benzo[a]pyrene (BaP) mutagenicity in Salmonella typhimurium strain TA98 were tested for their effectiveness against the mutagenicity of 6-methyl-benzo[a]pyrene (6-CH3-BaP), 6-hydroxymethyl-benzo[a]pyrene (6-CH2OH-BaP) and 6-acetoxymethyl-benzo[a]pyrene (6-CH3COOCH2-BaP). Dose-response curves obtained for phenothiazine (PTH), 2-chlorophenothiazine (2Cl-PTH), phenylisothiocyanate (PHN), phenethylisothiocyanate (PNE), trans-retinol (TR) and disulfiram (TETD) showed a variety of degrees of inhibition of mutagenicity. Additionally, glutathione (GSH) was found to inhibit the mutagenicity of 6-CH3COOCH2-BaP, and the mutagenicity of 6-CH2OH-BaP was enhanced by the addition of supplemental ATP, Na2SO4 and EDTA. Only 2Cl-PTH was equally as good an inhibitor of 6-CH3-BaP and BaP, reducing revertant colonies to less than 50% of control at 10 X BaP concentration. To probe the mechanism of inhibition, the effect of 2Cl-PTH on the binding of BaP and the 6-substituted benzo[a]pyrenes to cytochrome P-450 was investigated by difference spectroscopy. Also, the effect of 2Cl-PTH on the subsequent metabolism of 6-CH3-BaP and 6-CH2OH-BaP was investigated by rapid scan difference spectroscopy and high-performance liquid chromatographic separation of products. The results are consistent with a major mechanism of inhibition for 2Cl-PTH involving a competition for the cytochrome P-450 binding site.

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

The synthesis of N-hydroxydesmethylimipramine via the corresponding primary hydroxylamine and oxime is described. The N-oxygenated products are unstable to g.l.c. analysis without prior derivatization; the decomposition products are identified by g.l.c.-mass spectrometry. N-Hydroxydesmethylimipramine is shown to be a metabolite of imipramine and desmethylimipramine on incubation of either with fortified 9000 g liver homogenates of male New Zealand white rabbits. The metabolic product is characterized by mass spectrometry and n.m.r. Didesmethylimipramine is shown to undergo metabolic alpha-C-oxidation, to yield the carboxylic acid, 3-(10,11-dihydro-5H-dibenz[b, f]azepin-5-yl)propionic acid, but not N-oxidation. N-Hydroxydesmethylimipramine is metabolically reduced to desmethylimipramine and metabolized further to 10-hydroxydesmethylimipramine, 2-hydroxydesmethylimipramine and the carboxylic acid. The possible role of N-hydroxydesmethylimipramine and 3-(10,11-dihydro-5H-dibenz[b,f]azepin-5-yl)propionic acid in the formation of iminodibenzyl is discussed.

A TLC fluorescence derivatization assay for chlorpromazine and its non-conjugated hepatic microsomal metabolites

Procedures for the assay of chlorpromazine (CPZ) and its metabolites were compared. An improved assay, involving organic extraction, thin-layer chromatography and fluorescence derivatization, was developed. The compounds were extracted from microsomal protein suspensions into 15% n-propanol in dichloromethane by successive extractions at pH 2 and pH 12. CPZ and its mono- and di-desmethyl, 7-hydroxy, N-oxide, sulphoxide and N-oxide-sulphoxide metabolites were separated using TLC on silica gel developed with methanol-acetone-ammonia (50:50:1 v/v/v). The compounds were extracted from the TLC plate with chloroform-methanol (2:1 v/v) and subjected to oxidation with hydrogen peroxide. The highly fluorescent oxidized derivatives were identified as sulphoxides by their fluorescence characteristics. Amounts of derivatized CPZ and metabolites were measured using derivatized promazine as the internal standard.

Metabolic oxidation of desmethylchlorimipramine in vitro by 9000 g rabbit-liver preparations

1. The metabolism of desmethylchlorimipramine (I) has been investigated in vitro using fortified 9000 g liver homogenates of male rabbits. 2. Four metabolic products: N-hydroxydesmethylchlorimipramine (II), 3-(3-chloro-10, 11-dihydro-5H-dibenz[b,f]azepin-5-yl)propanoic acid (III), 10/11-hydroxydesmethylchlorimipramine (IV) and 2/8-hydroxydesmethylchlorimipramine (V) were isolated and identified by electron-impact mass spectrometry and n.m.r. spectroscopy. 3. Desmethylchlorimipramine (I) undergoes both N- and alpha-carbon oxidation in addition to aromatic and alicyclic carbon oxidation.

Metabolism of promethazine in vitro. Identificaton of N-oxidized products

1. Incubation of promethazine (Ia) and desmethylpromethazine (Ib) with 9000g supernatant fractions of rabbit liver homogenate resulted in formation of N-dealkylated, N-oxygenated and ring-hydroxylated products. 2. The N-oxidation products identified by t.l.c. and mass spectra using synthetic reference products are promethazine-N-oxide (IX) and the nitrone (VIII), which is believed to be formed chemically and metabolically from the metabolite N-hydroxydesmethylpromethazine (VII).