Structure-activity relationship for deacetylation of a homologous series of phenacetin analogs and their N-hydroxy derivatives

Deacetylation of a homologous series of alkoxy acetanilides (p-methoxy-, p-ethoxy- (phenacetin), p-(n)-propoxy- and p-(n)-butoxyacetanilide) and three of the corresponding N-hydroxy derivatives was examined in microsomal fractions from the livers and kidneys of C57BL/6J mice. The rates of deacetylation of the phenacetin analogs to the corresponding amines were found to increase with increasing alkyl chain length. With the N-hydroxy derivatives, the apparent KM was found to decrease with increasing chain length, while the Vmax was relatively unaffected. Treatment of the microsomes with the esterase inhibitor bis-p-nitrophenylphosphate resulted in quite similar extents of inhibition of the deacetylation of the phenacetin analogs and their N-hydroxy derivatives.

Inhibition of glutathione excretion, bile flow, and alterations of the glutathione status by 4-nitrosophenetol during perfusion of rat livers

(1) Hemoglobin-free single-pass perfusion of isolated rat livers was carried out with various concentrations of 4-nitrosophenetol (NOPt). (2) NOPt, up to 2 mumol/min/g liver wet wt., was reduced by the liver with formation of N-hydroxy-4-phenetidine (NHOHPt), 4-phenetidine (NH2Pt), phenacetin and polar metabolites. (3) Three per cent of NOPt was irreversibly bound to liver proteins after a load of 20 mumol/g liver wet wt. After 30 min perfusion, 0.2 mumol of liver glutathione was lost by 1 mumol NOPt infused. (4) Bile flow and glutathione release by the bile decreased rapidly during NOPt perfusion. (5) Glutathione release of the livers into venous effluent was diminished by NOPt and was stimulated only slightly by t-butylhydroperoxide (BOOH). Because BOOH reduction and glutathione peroxidase were not altered and intracellular glutathione disulfide (GSSG) levels were elevated, inhibition of the GSSG excretory mechanism is assumed.

Species-specific activation of phenacetin into bacterial mutagens by hamster liver enzymes and identification of N-hydroxyphenacetin O-glucuronide as a promutagen in the urine

Phenacetin was mutagenic in Salmonella typhimurium TA100 in plate assays when liver fractions from Aroclor-treated hamsters, but not rats, were used. Its known or putative metabolites were synthesized; of these, N-hydroxyphenacetin and N-acetoxyphenacetin were found to be mutagenic in liquid and plate assays, both requiring activation by liver fractions from Aroclor-treated hamsters. 2-Hydroxyphenacetin and 2-acetoxyphenacetin were nonmutagenic. N-Hydroxyphenetidine (the deacetylated metabolite of phenacetin) and p-nitrosophenetole were the only products that were found to be mutagenic per se when assayed under N2 in either Salmonella TA100 and TA100 NR (nitroreductase-deficient) strains. Phenacetin was administered to male BDVI rats and Syrian golden hamsters, and its urinary metabolites were deconjugated with beta-glucuronidase:arylsulfatase. After reactivation by hamsters liver fractions, mutagenicity was demonstrated in S. typhimurium TA100 with urine from phenacetin-treated hamsters, but not with that from rats. After treatment with deconjugating enzymes, N-hydroxyphenacetin was isolated from hamster urine by high-performance liquid chromatography and identified by mass spectral analysis. The data support the conclusions that (a) N-hydroxyphenacetin is a proximate mutagenic metabolite of phenacetin which, after N-deacylation, is responsible for the mutagenicity observed in vitro and in the urine of hamsters and (b) the higher yield of N-hydroxyphenacetin that is formed in the liver of hamsters as compared to rats explains the pronounced species-specific activation of phenacetin into bacterial mutagens.

Mutagenicity of N-hydroxy-2-acetylaminofluorene and N-hydroxy-phenacetin and their respective deacetylated metabolites in nitroreductase deficient Salmonella TA98FR and TA100FR

The mutagenicity of N-hydroxy-2-acetylaminofluorene and N-hydroxyphenacetin and their respective deacetylated metabolites, N-hydroxy-2-aminofluorene and 2-nitrosofluorene, and N-hydroxyphenetidine and rho-nitrosophenetole was determined in nitroreductase deficient Salmonella tester strains TA 98FR and TA100FR. The mutagenicity of N-hydroxy-2-acetylaminofluorene medicated by either rat liver microsomes or rat liver 105 000 g supernatant fractions was no different in either TA98 (nitroreductase proficient) or TA98FR (nitroreductase deficient). Similarly the mutagenicity of N-hydroxyphenacetin mediated by hamster liver microsomes was not affected by either the presence or absence of nitroreductase activity in TA100. N-Hydroxy-2-aminofluorene and 2-nitrosofluorene were equipotent direct acting mutagens in both TA98 and TA98FR, as were both N-hydroxyphenetidine and rho-nitrosophenetole in TA100 and TA 100FR. Ascorbate (5 mM) and NADPH (1 mM) had no significant effect on the mutagenicity of either N-hydroxy-2-acetylaminofluorene, N-hydroxy-2-aminofluorene, or 2-nitrosofluorene in TA98 or TA98FR whereas ascorbate and NADPH markedly inhibited the mutagenicity of both N-hydroxyphenetidine and rho-nitrosophenetole in both TA100 and TA100FR. Ascorbate appears to be inhibiting the mutagenicity of N-hydroxyphenetidine and rho-nitrosophenetole as a result of the nonenzymatic chemical reduction of these compounds to non-mutagenic derivatives.

Activation of N-hydroxyphenacetin to mutagenic and nucleic acid-binding metabolites by acyltransfer, deacylation, and sulfate conjugation

N-Hydroxyphenacetin was activated to a mutagen in the Salmonella-Ames test by rabbit liver acyltransferase, rat liver cytosol, and rat liver microsomes. N-[ring]3H]-Hydroxyphenacetin was bound to transfer RNA when activated by acyltransferase from rabbit or rat liver or rat liver microsomes. The acyltransferase-catalyzed binding was not inhibited by paraoxon, a deacetylase inhibitor. The use of N-hydroxyphenacetin radioactively labeled in the acetyl group, as well as the ring, indicated that deacetylation was involved in the microsome-catalyzed binding reaction. In addition, the microsome-catalyzed binding was inhibited 90% by paraoxon. p-Nitrosophenetole, a deacetylated derivative of N-hydroxyphenacetin, was synthesized and bound to transfer RNA without enzymatic activation. Activation of N-hydroxyphenacetin by sulfate conjugation was also found to lead to binding to transfer RNA. The data implicated acyl transfer, deacetylation, and sulfate conjugation as possible routes for the activation of N-hydroxyphenacetin.

N-hydroxyphenacetin, a new urinary metabolite of phenacetin in the rat

N-Hydroxyphenacetin has been found in the urine of rats dosed with phenacetin, extending previous reports that phenacetin is N-hydroxylated by liver microsomes in vitro. After an oral dose of phenacetin (500 mg/kg) urine was collected for 24 hr, conjugates hydrolyzed with extract of Helix pomatia, and the metabolites extracted with dichloromethane and treated with diazomethane. Methylation of N-hydroxyphenacetin produced a stable derivative, N-methoxyphenacetin, which was separated from most other metabolites by thin layer chromatography. Identification of N-methoxyphenacetin was by combined gas chromatography-mass spectrometry and comparison with the synthetic reference compound. Quantification by gas chromatography with flame-ionization detection showed that 0.023% of the dose phenacetin was recovered from urine as N-hydroxyphenacetin. It is probable that this value considerably underestimates the extent of phenacetin N-hydroxylation in vivo, inasmuch as N-hydroxyphenacetin is known to be rapidly degraded in biological systems.

Establishment of specific antibody producing human lines by antigen preselection and Epstein-Barr virus (EBV)-transformation

Specific antibody producing human cell lines were established by preselecting antigen binding B-lymphocytes and subsequently transforming ("immortalizing") them with Epstein-Barr virus (EBV). NNP-binding B-cells were isolated from the blood of three donors with high anti-NNP titers. EBV-transformation led to polyclonal cell lines that had 15-18% NNP receptor positive cells. A similar fraction of the cells produced NNP-specific plaques in the Cunningham-Szenberg assay. Unconcentrated culture media agglutinated NNP-coupled erythrocytes in up to an 1/2,048 dilution and inactivated NNP-coupled T4 bacteriophage in up to an 1/10,000 dilution. EBV-transformed but not similarly NNP-preselected lines of the same donors were completely negative in the rosette, plaque and antibody secretion tests. Supernatants of the antibody producing lines gave no reaction with the non-coupled erythrocytes or with a number of other hapten coupled controls. Anti-NNP antibodies secreted by all three preselected lines were of the IgM kappa type, which was in contrast to the sera of the donors that contained both IgM and IgG antibodies, with both light chain types. Cloning of one NNP-antibody producing line yielded 9 antibody producers out of 30. The positive clones formed NNP rosettes and plaques with 31-86% of the cells and produced anti-NNP of class IgM, Kappa. The cell culture contained 5-16 micrograms IgM/ml cell culture.

Decreased toxicity of the N-methyl analogs of acetaminophen and phenacetin

The N-methyl analogs of p-hydroxyacetanilide (acetaminophen) and p-ethoxyacetanilide (phenacetin) were prepared and tested for toxicity. N-Methylacetaminophen was found to cause no hepatic necrosis in mice, rats, or hamsters in doses that caused massive hepatic necrosis in the same animals when acetaminophen was administered. Neither acetaminophen nor its N-methylated analog caused methemoglobinemia at these dose levels. Fischer rats that were administered large doses of acetaminophen (900 mg/kg s.c.) sustained necrosis in the proximal renal tubules, whereas N-methylacetaminophen caused no renal injury at higher dose levels (1800 mg/kg s.c.). N-Methylphenacetin caused no observable hepatic necrosis in 3-methylcholanthrene (3-MC) pretreated hamsters at dose levels higher than those in which phenacetin caused hepatic necrosis. Also, in contrast to phenacetin, N-methylphenacetin did not cause extensive methemoglobinemia in mice, rats, or hamsters.

Comparative metabolic studies of phenacetin and structurally-related compounds in the rat

1. A comparative study of the metabolism of [acetyl-14C]phenacetin, [acetyl-14C]methacetin, [acetyl-14C]paracetamol and [acetyl=14C]acetanilide in the rat is reported. 2. The extent of N-deacetylation, evidenced by the measurement of respired 14CO2, varied, being greatest with acetanilide (25-31%) and least with paracetamol (6%). 3. The major urinary metabolites in each case were N-acetyl-p-aminophenyl sulphate and N-acetyl-p-aminophenyl glucuronide; the relative proportions varied with the sex of the animals and as a result of extended dosage. 4. The metabolism of [ethyl-14C]phenacetin and [ethyl-14C]phenetidine was investigated and the extent of O-dealkylation determined by measurement of respired 14CO2. 5. The metabolic pathways of some related glycolanilides and oxanilic acids included N-deacylation, and in the glycolanilides, oxidation of the glycollic group.

Neoplasia in the rat induced by N-hydroxyphenacetin, a metabolite of phenacetin

N-hydroxyphenacetin, a phenacetin metabolite, was fed to rats as a 0.05-0.5% dietary supplement. After 9 months, tumours of the liver were found in 36 of 64 animals. One animal also developed a renal tumour. No tumours were found in control animals. The findings implicate phenacetin as a carcinogen and suggest that N-hydroxyphenacetin may be the metabolite responsible.