Mechanisms of mammalian hydroxylation: some novel metabolites of chlorobenzene

Regioselectivity of cytochrome P-450 catalyzed hydroxylation of fluorobenzenes predicted by calculated frontier orbital substrate characteristics

In the present study, a hypothesis is presented for the prediction of the regioselectivity of cytochrome P-450 catalyzed hydroxylation of fluorobenzenes. The regioselectivity of the in vivo hydroxylation of fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,2,3-triluorobenzene, and 1,2,4-triflurobenzene could be predicted within 6% accuracy on the basis of the substrate's frontier orbital characteristics for electrophilic attack. The in vivo regioselectivity of the hydroxylation of fluorobenzene was not significantly influenced by changes in the cytochrome P-450 enzyme pattern. This implies that the regioselectivity is not predominantly determined by the juxtaposition of the relatively small substrates in the active sites of the cytochrome P-450s catalyzing the reaction. Additional in vitro experiments using 1,2-difluorobenzene as the model substrate demonstrated that minor factors influencing the regioselectivity and possibly responsible for the 6% deviation from the calculated values in in vivo experiments might be (i) the influence of biotransformation routes occurring in vivo but not of importance in in vitro microsomal incubations and (ii) a small variation due to influences of the contribution of various cytochrome P-450 enzymes. On the basis of the results obtained, it is concluded that the aromatic hydroxylation of fluorobenzenes proceeds through an initial electrophilic attack of (FeO)3+ on the aromatic substrate, and not through initial electron abstraction followed by attack of the (FeO)2+ species on the substrate radical cation. The fact that the regioselectivity observed could be predicted and/or explained by the site of initial (FeO)3+ attack also argues against epoxides as important intermediates in the formation of phenol metabolites from fluorobenzenes.

Quantitation of urinary chlorobenzene metabolites by HPLC: concentrations of 4-chlorocatechol and chlorophenols in urine and of chlorobenzene in biological specimens of subjects exposed to chlorobenzene

A simple method for the determination of 4-chlorocatechol (ClCh, 4-chloro-1,2-benzenediol) and chlorophenols (ClPh), metabolites of monochlorobenzene (ClBz), in urine by high performance liquid chromatography (HPLC) is described. Enzymatic hydrolysates of urine were applied to a stainless-steel column packed with octadecyl-silanized silica gel, and a mixed solution of 20 mM potassium phosphate monobasic: acetonitryl (75:25, v/v) was used as a mobile phase. The procedures for ether extraction and evaporation of extract could be omitted. The accuracy and precision of the present HPLC method were satisfactory. The excretion kinetics of ClCh and p-ClPh were investigated over 35 h after cessation of ClBz inhalation. Proportional relationships between concentrations of ClBz in air and of its metabolites in urine were observed. The slopes of regression lines predicting the levels of ClCh, p-ClPh and total ClPh in urine taken during the last 2 h of exposure to ClBz in air were 6.56, 1.13 and 2.83 mg/g creatinine for 1 ppm ClBz, respectively. ClBz in the blood and the end exhaled air of subjects at the end of exposure were identified by gas chromatography (GC) and mass spectrometry. A proportional relationship was observed between the concentration of ClBz in air and that in blood. The validity of the threshold limit value (TLV) for ClBz as evaluated from the subjective and objective symptoms is discussed.

Identification of urinary metabolites in rats treated with p-chloronitrobenzene

Urinary metabolites in rats treated with p-chloronitrobenzene were identified by gas chromatography-mass spectrometry. A single dose of 100 mg/kg body wt p-chloronitrobenzene was administered intraperitoneally to male Sprague-Dawley rats and urine samples were collected from the 8th to 24th hour after the administration. Urinary metabolites were extracted with diethylether at pH 1.0 and pH 10.0 from urine samples hydrolyzed with acid and base and from intact urine samples. Aliquots of the ethereal extracts were injected into a gas chromatograph-mass spectrometer. Nine substances were identified: p-chloroaniline, 2,4-dichloroaniline, p-nitrothiophenol, 2-chloro-5-nitrophenol, 2-amino-5-chlorophenol, p-chloroformanilide, 4-chloro-2-hydroxyacetanilide, a small amount of p-chloroacetanilide and traces of unchanged p-chloronitrobenzene.

The excretion of chlorophenylmercapturic acid, chlorophenols and a guanine adduct in the urine of chlorobenzene-treated rats after phenobarbital pretreatment

Chlorobenzene (CB) was administered to male Wistar rats. Twenty-four-hour urine samples were collected over a period of 7 days. p-Chlorophenylmercapturic acid, chlorophenols and a guanine adduct were determined by chromatographic methods. The excretion pattern of p-chlorophenyl-mercapturic acid did not seem to be significantly affected by phenobarbital. To analyze for the chlorophenols and guanine adducts, diluted urine was subjected to cation exchange chromatography using UV-detection. Fractions were found containing chlorophenols and a compound showing chromatographic properties similar to those of the synthetic N7-phenylguanine. The results of the HPLC analyses suggested the presence of a guanine adduct excreted on days 1 and 2 and between days 4 and 6 post-administration. There were no compounds detected which were identical with p-, m- or o-isomers of N7-chlorophenylguanine. The excretion of p- and m-chlorophenols by phenobarbital-pretreated animals was twice as high as that in untreated rats. A 4-fold increase was detected for o-chlorophenol. It is assumed that it is mainly direct hydroxylation that can be induced by phenobarbital. Dehalogenated phenolic metabolites may be capable of covalent binding to DNA.

Specific determination of bromobenzene metabolites by high-performance liquid chromatography

A chromatographic method for the quantitative and simultaneous determination of phenolic and mercapturic acid type metabolites of bromobenzene is presented. Acid hydrolysis (1.5 N HCl, 100 degrees C, 10 min) yielded unconjugated bromophenols. After extraction (ethyl acetate, pH 2), evaporation to dryness and dissolution in methanol, samples were analysed by high-performance liquid chromatography (HPLC) with a c18 reverse-phase column and a phosphate buffer (0.1 M,pH 9 + 5 mM tetrabutylammonium dihydrogen phosphate) - acetonitrile gradient with ultraviolet detection (225 nm). Benzylmercapturic acid and o-chlorophenol were used as internal standards. Using this method, it was possible to determine simultaneously o, m- and p-bromophenols together with o-, m- and p-bromophenylmercapturic acids. Additional validation data were obtained by analysis of urine samples from rats treated with bromobenzene. This new method was compared to existing procedures.

Toxic responses to acute, subchronic, and chronic oral administrations of monochlorobenzene to rodents

Acute (single exposure), 14-d repeated exposure, 91-d subchronic, and 103-wk chronic toxicity studies of orally administered (gavage, in corn oil) monochlorobenzene were conducted in male and female Fischer-344 rats and B6C3F1 hybrid mice. A single exposure to 4000 mg/kg was lethal to male and female rats, while a single exposure to a dose as low as 1000 mg/kg was lethal to mice. Fourteen daily exposures to 1000 mg/kg caused death in rats of both sexes, but neither survival nor clinical health were compromised at 500 mg/kg in rats or mice. In the 91-d studies, wherein monochlorobenzene was administered once daily, 5 d/wk, survival was reduced by doses of 500 mg/kg and higher in rats, and by doses of 250 mg/kg and higher in mice. Dose-dependent necrosis of the liver (hepatocytes), degeneration or focal necrosis of the renal proximal tubules, and lymphoid or myeloid depletion of the spleen, bone marrow, and thymus (mild to severe) were produced by doses of 250 mg/kg or greater of monochlorobenzene in both sexes of rats and mice, although the incidences of these lesions varied considerably by sex and species. Consistent changes in the circulating blood components were not observed, but a mild porphyrinuria was detected at the higher doses. No toxic effects were observed at doses of 125 mg/kg or less. In the 2-yr studies, wherein monochlorobenzene was administered once daily, 5 d/wk, doses of 30 or 60 mg/kg in male mice and 60 or 120 mg/kg in female mice and male and female rats did not produce any evidence of toxicity. Doses of 60 or 120 mg/kg caused slight (statistically significant at 120 mg/kg; p less than 0.05) increases in the frequencies of male rats with neoplastic nodules of the liver. Increased tumor frequencies were not observed in female rats or in male or female mice receiving monochlorobenzene.

Differences in urinary monochlorobenzene metabolites between rats and humans

The high performance liquid chromatographic method for the determination of p-chlorobenzene mercapturic acid and 4-chlorocatechol conjugates is described. For determination of urinary mercapturic acid, the benzene extract from urine was injected into a liquid chromatograph and for determination of urinary 4-chlorocatechol conjugates, hydrolysate was dissolved in methanol. The methanol solution containing 4-chlorocatechol was injected into a liquid chromatograph. Differences in urinary excretion of monochlorobenzene between rats and humans were studied. Monochlorobenzene was administered to rats intraperitoneally, and to humans orally or by inhalation. Urinary p-chlorophenylmercapturic acid, and 4-chlorocatechol after hydrolysis of its conjugate, were measured. The amount of total metabolites is proportional to the doses administered to rats, rabbits and mice by intraperitoneal injection. The ratio of urinary mercapturic acid to 4-chlorocatechol is in the order of rats, mice and rabbits by intraperitoneal injection, and rats and human beings by oral administration. The excretion of p-chlorophenylmercapturic acid was markedly less than that of 4-chlorocatechol in humans who received monochlorobenzene orally or by inhalation. The results indicate that the 4-chlorocatechol conjugate is a suitable index of metabolites in the urine of workers exposed to monochlorobenzene.

Pharmacokinetics and metabolism of guanfacine in man: a review

1 The fate of guanfacine has been investigated extensively in animals.

2 Pharmacokinetics and metabolism of [¹⁴C]-guanfacine were studied in fourteen subjects given 3 mg orally (seven subjects) and 2.3 mg intravenously. Plasma levels and urinary excretion of radioactivity were measured by liquid scintillation counting. Parent drug was determined by gas chromatography-mass spectrometry. The analytical results were submitted to pharmacokinetic evaluation using the SAAM 26 programme. Metabolites in urine were identified by high pressure liquid chromatography.

3 Guanfacine was rapidly and completely absorbed. Its absolute bioavailability was close to 100%, no evidence of any first-pass effect being found.

4 Its distribution was characterized by low blood levels, low plasma protein binding and a relatively high affinity to the tissues (V_d of 300 l).

5 The elimination half-life of the β-phase was 17 hours. The major route of excretion (80% of the dose) was in the urine. About 1/3 to 1/4 of the total clearance of 11 l/h was renal.

6 The principal metabolite was the 3-hydroxy-derivative of guanfacine conjugated as either O-glucuronide or O-sulphate. The important fraction (30%) of parent drug found in the urine demonstrates a rather moderate biotransformation of guanfacine in man.

7 The results of an additional study after multiple dosing showed that the measured steady-state plasma levels were in agreement with the values predicted from a single dose experiment and proportional to the daily dosage.

Determination of ethinyl estradiol in human urine by radiochemical GLC

A radiochemical GLC analysis was developed for 3H-labeled ethinyl estradiol in human urine. The technique was applied to the unconjugated and aglycone fractions of urine collected from women who were dosed orally with: (a) single capsules containing 2.0 mg of 3H-quinestrol (900 muCi) and 2.5 mg of unlabeled quingestanol acetate dissolved in sesame oil and (b) single tablets containing 100 microgram of 3H-quinestrol (86 muCi). Unconjugated ethinyl estradiol in Day 1 urine collections represented means of 0.02% of the high quinestrol dose and 0.12% of the low dose. Ethinyl estradiol glucuronide in the same collections represented means of 0.55% of the high drug dose and 1.35% of the low dose. The method could detect 1-ng quantities of 3H-ethinyl estradiol and 3H-quinestrol.

Glutathione conjugation of some xenobiotics by Ascaris suum and Moniezia expansa

1. The cestode Moniezia expansa and the nematode Ascaris suum both possess enzymes catalysing the conjugation of glutathione (GSH) with 1-chloro-2,4-dinitrobenzene. 2. The GSH-S-aryl transferase (GSH-S-transferase A) was present in the cytosol of the cestode proglottids and of the nematode intestinal epithelial cells. Other tissues did not contain measurable activity. The enzymes from both species had mol. wt. of about 37 000 and broad pH optima around pH 8.3. Both enzymes were inhibited by Cu2+, Fe3+ and Hg2+ at 1 mM and stimulated by Co2+. 3. Neither M. expansa nor A. suum possessed measurable DDT dehydrochlorinase activity. GSH-S-epoxide transferase (GSH-S-transferase E) activity was indicated in both species; neither species effected the conjugation of bromo- or chlorobenzene. 4. Halogenated anthelmintics were not metabolized to GSH conjugates in the helminths studied and did not inhibit GSH-S-aryltransferase activity towards chlorodinitrobenzene.

Radiochemical GLC assay for nortriptyline in human plasma

A novel method was developed for the assay of nortriptyline in plasma. After nortriptyline was extracted, it was acetylated with 3H-acetic anhydride; the quantity of 3H-acetylnortriptyline in the extract was determined by radiochemical GLC. The method is capable of assaying 5 ng of nortriptyline/ml of plasma. The instrumentation was assembled from commercially available components.