Oxidation of polycyclic N-arylacetamides to glycolamides and hydroxamic acids in rabbits

Metabolism of 2-acetylaminofluorene. I. Metabolism in vitro of 2-acetylaminofluorene and 2-acetylaminofluoren-9-one by hepatic enzymes

1. 2-Acetylaminofluorene (AAF) was converted by rat liver microsomal and cytosolic enzymes to 2-aminofluorene (AF), 2-glycoloylaminofluorene (GAF), 2-acetylaminofluoren-3-, -7-, and -9-ol (3-, 7-, 9-hydroxy-AAF), and 2-acetylaminofluoren-9-one (AAF-9-one). In addition, a new metabolite MX1 was detected. 2. AAF was converted by rabbit liver microsomal and cytosolic enzymes to N-hydroxy-AAF, GAF, 5-, 7-, and 9-hydroxy-AAF, AAF-9-one, 5- and 7-hydroxy-AAF-9-one (new compounds), and AF, indicating species differences in the N- and ring-hydroxylation of AAF and secondary oxygenation of AAF. In addition, an unknown metabolite MX2 was detected. 3. AAF-9-one was converted by rat liver microsomal and cytosolic enzymes to optically active 9-hydroxy-AAF and 7-hydroxy-AAF-9-one; in addition MX1 was found. 4. Rabbit liver microsomal and cytosolic enzymes converted AAF-9-one to 2-aminofluoren-9-one (AF-9-one), 9-hydroxy-AAF, N-hydroxy-AAF-9-one, GAF-9-one, 7-hydroxy-AAF-9-one, and 7,9-dihydroxy-AAF. In addition, metabolite MX1 and its dihydro-dihydroxy derivative were found. 5. These results indicate that AAF and AAF-9-one have common metabolic pathways, as AAF after primary oxygenation to 9-hydroxy-AAF and partial dehydrogenation to AAF-9-one, undergoes secondary oxygenation to 7-hydroxy-AAF-one and MX1 as well as the corresponding dihydro-dihydroxy derivatives.

The in vitro/in vivo comparative metabolism of 4-aminobiphenyl using isolated hepatocytes

The metabolism of 4-aminobiphenyl by isolated hepatocytes from various species was compared with urinary metabolite profiles in the same species. Radioactive compounds in concentrates of ether extracts from hepatocytes or urine following hydrolysis were analysed by TLC and reversed phase HPLC in conjunction with radioactivity monitoring and synthetic standards. The major metabolites from hepatocytes and in urine were 4-acetamidobiphenyl, 3-hydroxy-4-aminobiphenyl 4'-hydroxy-4-aminobiphenyl and 4'-hydroxy-4-acetamidobiphenyl. Oxidation of the amine nitrogen gave hydroxylamino, nitroso and nitro compounds. Minor metabolites were 2'-hydroxy amine and amide, the hydroxamic acid and the oxamic acid. The urinary metabolite profiles correlated well with those from hepatocytes for each species.

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.

N-hydroxy-N-arylacetamides. II: Molecular aspects of ferrihaemoglobin formation by N-hydroxy-N-arylacetamides and arylhydroxylamines in the rat

Ferrihaemoglobin (HbFe3+) formation in rats after i.p. injection of 6 N-hydroxy-N-arylacetamides has shown that N-hydroxy-4-chloroacetanilide(N-hydroxy-4ClAA) was the most active and N-hydroxy-2-acetylaminofluorene(N-hydroxy-2AAF) the least active compound tested. As N-hydroxy-N-arylacetamides were thought to produce HbFe3+ only after enzymic N-deacetylation, the corresponding arylhydroxylamines were also tested for HbFe3+-forming activity and were found to be more active, N-hydroxy-4-chloroaniline(N-hydroxy-4ClA) being one of the most active and N-hydroxy-2-aminofluorene(N-hydroxy-2AF) the least active compound tested. N-Hydroxy-4-chloroacetanilide given i.p. to rats more rapidly invaded the blood and produced larger amounts of ferrihaemoglobin than did N-hydroxy-2-acetylaminofluorene, due to differences in their availability in plasma. Injection of 50 mg/kg of N-hydroxy-4-chloroacetanilide gave similar concn of HbFe3+ and 4-chloronitrosobenzene(4-CINOB) as injections of 8 mg/kg of N-hydroxy-4-chloroaniline, indicating that the arylhydroxylamine, after N-deacetylation, was the active molecule in vivo. The concn of 4-chloronitrosobenzene declined faster than HbFe3+ concn. 4-Chloronitrosobenzene therefore is a further example of a 'hit-and-run' chemical. Inhibition by the microsomal carboxylesterase inhibitor, bis(4-nitrophenyl)phosphate(BNPP), indicated that ferrihaemoglobin formation by 4-chloroacetanilide, but not by N-hydroxy-4-chloroacetanilide, depends on the enzymic activity of hepatic microsomal carboxylesterases.

Additional routes in the metabolism of phenacetin

omega-Hydroxylation of the ethyl moiety of phenacetin by rabbit-liver microsomal preparations was slow, but was increased 10-fold by pretreatment of the animals with phenobarbitone (PB), and was decreased 2.8-fold by treatment with 3-methylcholanthrene (3-MC) or beta-naphthoflavone (beta-NF). N-[4-(2-hydroxyethoxy)phenyl]acetamide (beta-HAP), the omega-hydroxylation product, which was detected in trace amounts only in the urine of rabbits injected with phenacetin, was converted into [4-(acetylamino)phenoxy]acetic acid (4-APA) by the microsomal and cytosolic fraction of liver homogenate and NADP+ or NAD+. Rabbits excreted 56% of a dose of beta-HAP as 4-APA in the 48 h urine. Phenacetin, injected i.p. into rabbits previously treated with PB, was excreted in the urine as 4-APA (12.2% of dose). beta-HAP formed endogenously or added as substrate in vitro was recovered as the O-acetyl derivative, when ethyl acetate was used for extraction of metabolites from microsomal incubation mixtures. (omega-1)-Hydroxylation of the ethyl moiety of phenacetin, which gave 4-acetamido-phenol, occurred rapidly with rabbit-liver microsomal preparations, and was not increased significantly after pretreatment of animals with either PB or 3-MC. omega-Hydroxylation of the acetic moiety of phenacetin by rabbit-liver preparations to give N-(4-ethoxyphenyl)glycolamide (4-GAP) was slow, but was increased three-fold after pretreatment of animals with 3-MC or beta-NF, whereas PB had no effect. 4-GAP was detected in trace amounts only in the urine of rabbits injected i.p. with phenacetin. N-Hydroxylation of phenacetin by rabbit-liver microsomal preparations was slow, but increased three-fold after treatment of animals with 3-MC, and was unchanged by PB. N-Hydroxylation of phenacetin by hepatic microsomes from 3-MC-treated rabbits was 26 times slower than that of 2-acetylaminofluorene; no N-hydroxy derivatives of N-(4-chlorophenyl)acetamide and propanil were detected in vitro.

N-Hydroxy-N-arylacetamides. I. Toxicity of certain polycyclic and monocyclic N-hydroxy-N-arylacetamides in rats

Of the two carcinogenic N-hydroxy-N-arylacetamides tested, N-hydroxy-4-acetylaminobiphenyl was as active as the monocyclic analogs in the oxidation of hemoglobin, whereas N-hydroxy-2-acetylaminofluorene produced less ferrihemoglobin after IP injection into female and male rats. Monocyclic N-hydroxy-N-arylacetamides, such as N-hydroxy-4-chloroacetanilide or N-hydroxyphenacetin, were more toxic than the parent N-arylacetamides, LD50 in mice being 190 mg/kg for N-hydroxy-4-chloroacetanilide vs 755 mg/kg for 4-chloroacetanilide, and 702 mg/kg for N-hydroxyphenacetin versus 1,220 mg/kg for phenacetin. The higher acute toxicities are probably due, at least in part, to the production of more ferrihemoglobin by the N-hydroxy-N-arylacetamides. Chronic toxicity of N-hydroxy-4-chloroacetanilide was tested on 10 male and 10 female Sprague Dawley rats after IP or SC injection of 20 mg (0.11 mmol)/kg twice weekly for 16 weeks into two groups of 10 animals each (five males, five females, total dose: 3.5 mmol/kg). The experiment, which was terminated after 2 years, did not yield any hint that N-hydroxy-4-chloroacetanilide was carcinogenic in the rat. Subchronic toxicity of N-hydroxyphenacetin was tested in two experiments on male and female Sprague Dawley rats after IP or SC injection of 50 or 100 mg (0.26 or 0.51 mmol)/kg. In the first experiment, two groups of 15 rats each (seven males, eight females) were injected either IP or SC with 50 and 100 mg/kg twice weekly for 29 weeks, and in the second experiment groups of 10 males and 10 females were injected SC with 100 mg/kg twice daily on 5 days a week for 12 weeks. The experiments, which were terminated after 29 weeks and 12 weeks treatment, respectively, did not provide evidence for chronic interstitial nephritis or tumor growth in the kidney. N-Hydroxy-N-arylacetamides were found to be inferior to the corresponding arylhydroxylamines in their ferrihemoglobin-forming capabilities in female rats. Large differences in activity of the arylhydroxylamines and no close relation to the number of rings was observed, N-hydroxy-2-acetylaminofluorene being the least active and N-hydroxy-4-acetylaminobiphenyl being as active as the monocyclic compounds, and exceeding all in the duration of its activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolism of clebopride in vitro. Identification of N-oxidized products

1. N-(1'-Benzyl-4'-piperidyl-N-oxide)-4-amino-5-chloro-2-methoxybenzamide, N-(4'-(N-hydroxylpiperidyl)-4-amino-5-chloro-2-methoxybenzamide and N-(4'-(delta 1'-piperidyl-N-oxide))-4-amino-5-chloro-2-methoxybenzamide were obtained from chloroform extracts of incubation mixtures of clebopride or desbenzyl clebopride with 9000 g supernatant of liver homogenates of male NZW rabbits. 2. These metabolites were identified using electron impact (low and high resolution) and field desorption mass spectrometry, and computer averaged time proton magnetic resonance spectroscopy.