Acetophenetidin-induced methemoglobinemia

Methaemoglobinaemia associated with the use of cocaine and volatile nitrites as recreational drugs: a review

Methaemoglobinaemia can cause significant tissue hypoxia, leading to severe, potentially life-threatening clinical features and/or death. Over recent years there have been increasing reports of methaemoglobinaemia related to recreational drug use. There have been 25 articles describing methaemoglobinaemia related to recreational use of volatile nitrites (poppers) and more recently, four reports of methaemoglobinaemia in association with recreational cocaine use. In this article we discuss the mechanisms by which methaemoglobinaemia occurs in relation to the use of both volatile nitrites and cocaine, and summarize the published cases of recreational drug-related methaemoglobinaemia. The volatile nitrites can cause methaemoglobinaemia directly through their activity as oxidizing agents. However, with cocaine, methaemoglobinaemia is related to adulterants such as local anaesthetics or phenacetin, rather than to the cocaine itself. Clinicians managing patients with acute recreational drug toxicity should be aware of the potential for methaemoglobinaemia in these patients, particularly in patients with cyanosis or unexplained low oxygen saturations on pulse oximetry, and ensure that appropriate and timely management is provided, including, where appropriate, the use of methylthioninium chloride (methylene blue).

Methemoglobinemia secondary to topical silver nitrate therapy--a case report

Methemoglobinemia is a rare complication in individuals exposed to nitrates or nitrites. Whereas methemoglobinemia is a recognized potential complication in burn patients treated with topical 0.5% silver nitrate solution, no report of methemoglobinemia in burn patients has been present in the literature for more than 15 years. We raise consciousness about this complication with a case report of a 12-month-old child with necrotizing fasciitis resulting from a cutaneous flank infection. The patient developed cyanosis 20 days after initiation of topical treatment with 0.5% silver nitrate solution. Intravenous injection of methylene blue can restore normal blood oxygenation.

Metabolic polymorphisms

Polymorphisms have been detected in a variety of xenobiotic-metabolizing enzymes at both the phenotypic and genotypic level. In the case of four enzymes, the cytochrome P450 CYP2D6, glutathione S-transferase mu, N-acetyltransferase 2 and serum cholinesterase, the majority of mutations which give rise to a defective phenotype have now been identified. Another group of enzymes show definite polymorphism at the phenotypic level but the exact genetic mechanisms responsible are not yet clear. These enzymes include the cytochromes P450 CYP1A1, CYP1A2 and a CYP2C form which metabolizes mephenytoin, a flavin-linked monooxygenase (fish-odour syndrome), paraoxonase, UDP-glucuronosyltransferase (Gilbert's syndrome) and thiopurine S-methyltransferase. In the case of a further group of enzymes, there is some evidence for polymorphism at either the phenotypic or genotypic level but this has not been unambiguously demonstrated. Examples of this class include the cytochrome P450 enzymes CYP2A6, CYP2E1, CYP2C9 and CYP3A4, xanthine oxidase, an S-oxidase which metabolizes carbocysteine, epoxide hydrolase, two forms of sulphotransferase and several methyltransferases. The nature of all these polymorphisms and possible polymorphisms is discussed in detail, with particular reference to the effects of this variation on drug metabolism and susceptibility to chemically-induced diseases.

A form of cytochrome P450 in man, orthologous to form d in the rat, catalyses the O-deethylation of phenacetin and is inducible by cigarette smoking

1. In previous studies (Boobis et al., 1985b) it was shown that a monoclonal antibody (MAb 3/4/2), raised against rat cytochrome P450 form c, reacts with an isoenzyme(s) of cytochrome P450 in human liver. It was predicted that the epitope with which this antibody reacts should be present on both isoenzymes of the P450IA gene sub-family (the orthologues of forms c and d) in man (Edwards et al., 1987). 2. This antibody was used to probe 45 different samples of human liver, by the technique of Western blotting. With one exception, all of the samples contained immunoreactive protein, a single band at Mr 54,000 (orthologous to rat form d), which ranged in content from less than 0.5 to 33.5 pmol mg-1 microsomal protein. The content of the human orthologue of form c was below 0.5 pmol mg-1, the limit of detection of the assay. 3. Thirteen of the samples were from patients of known smoking status. Immunoreactive P450 content was 3.5-fold higher, and phenacetin O-deethylase activity was four-fold higher, in the smokers than in the non-smokers. 4. There was a highly significant correlation between the amount of immunoreactive cytochrome P450 and the high affinity component of phenacetin O-deethylase activity in both smokers and non-smokers. 5. It is concluded that the high affinity component of phenacetin O-deethylase activity in man is catalysed by the orthologue of rat cytochrome P450d, and that this isoenzyme is inducible by cigarette smoking. 6. In a number of previous publications it has been suggested that there is an association between the poor metaboliser (PM) phenotype for debrisoquine and impaired phenacetin O-deethylation. In the present study it was shown that not all subjects PM for debrisoquine are poor metabolisers of phenacetin.

Phenacetin O-deethylase activity of the rat: strain differences and the effects of enzyme-inducing compounds

Phenacetin O-deethylase activity in microsomal fractions from liver of DA and Fischer rats has been determined. No major sex or strain differences were found. Kinetic analysis revealed two major components of O-deethylase activity in the liver of both strains of rats. Michaelis-Menten analysis revealed no major difference between the strains. Phenacetin O-deethylase activity is inducible by both 3-methylcholanthrene and phenobarbitone in DA and Fischer rats. 3-Methylcholanthrene selectively increases the high-affinity component of activity, by 20- to 25-fold, whereas phenobarbitone selectively increases the low-affinity component, by two- to three-fold. It is concluded that there is no major difference between the DA and Fischer strains in their ability to O-deethylate phenacetin. Thus, unlike poor metabolizers of debrisoquine in the human population, who appear also to have impaired phenacetin O-deethylase activity, the DA rat is deficient in only the former activity.

Phenacetin O-deethylase: an activity of a cytochrome P-450 showing genetic linkage with that catalysing the 4-hydroxylation of debrisoquine?

Phenacetin O-deethylase activity was impaired, both in vivo and in vitro, in poor metabolisers of debrisoquine, consistent with the work of others. No impairment was observed in the oxidation of acetanilide, amylobarbitone or antipyrine in the PM phenotype. There was a good correlation (r = 0.804) between the high affinity component of phenacetin O-deethylase and debrisoquine 4-hydroxylase activities. No such correlation was observed with the low affinity component of phenacetin O-deethylase activity. Although debrisoquine was a competitive inhibitor of phenacetin O-deethylase activity, phenacetin was without effect on debrisoquine 4-hydroxylation. There was also marked differences in the effects of sparteine, guanoxan and alpha-naphthoflavone on the two activities. Cigarette smoking was associated with a significant, two-fold, increase in phenacetin O-deethylase activity whilst debrisoquine 4-hydroxylase activity was not affected. It is concluded that the high affinity component of phenacetin O-deethylase and debrisoquine 4-hydroxylase activities are catalysed by different isozymes of cytochrome P-450 but that these are most probably regulated by closely linked genes.

The toxicity and biotransformation of single doses of acetaminophen in dogs and cats

The biotransformation of single oral doses of acetaminophen (APAP) was studied in dogs an cats. Each animal received APAP at a no-effect (low), mildly toxic (medium), and severely toxic (high) dosage; dosages for each species were selected to produce similar clinical effects at each respective dosage. For dogs, these dosages were 100, 200, and 500 mg APAP/kg, while for cats, the similar effective dosages were 20, 60, and 120 mg APAP/kg. Plasma half-lives in dogs remained constant at the lower two dosages, but nearly tripled at the high dosage. The plasma half-lives in cats rose with increased dosage. Although the cats were given lower APAP dosages than the dogs, the plasma half-lives of cats were greater than those of the dogs at the medium and high dosages. Both species excreted about 85% of the administered single dose within the first 24 hr. APAP-glucuronide was the principal metabolite excreted in the urine of dogs; its fraction of the total metabolites excreted in urine remained constant at the three dose levels. In cats, APAP-sulfate was the major metabolite in urine at all three dosage levels, but the fraction of the total urinary metabolites represented by APAP-sulfate decreased as the dosage increased. Hepatic centrilobular pathology was seen in dogs, while cats had more diffuse liver pathologic changes. The results indicate that the cat is at increased risk from APAP exposure because of impaired glucuronidation and saturation of its sulfate conjugation pathway.

Human cytochromes P-450

Studies in vivo have provided evidence for a multiplicity of cytochromes P-450 in man, some of which are under independent monogenic control. Although the activity of cytochromes P-450 in man are generally lower than those of rat, this is by no means always the case. There are several important exceptions including the N-hydroxylation of 2-acetamidofluorene. Studies in vitro by a number of different techniques have confirmed the evidence from studies in vivo that there are multiple forms of human cytochrome P-450. In addition to differences in Vmax, the different forms of cytochrome P-450 may also exhibit marked differences in their apparent Km values. The implications that this may have for pharmacokinetics and toxicology are discussed. The polymorphism in the 4-hydroxylation of debrisoquine observed in vivo has been shown to be due to a defect in a specific form of cytochrome P-450 which appears to be under monogenic regulation. Cross-inhibition studies have enabled the specificity of this isozyme to be characterized. Such studies have also enabled the contribution of this isozyme of cytochrome P-450 to the oxidation of other substrates to be determined. Compounds investigated include bufuralol and phenytoin. Evidence from studies both in vivo and in vitro suggest that selective induction of different forms of cytochrome P-450 can occur in man. However, the number of different classes of inducer in man is not yet known. Human cytochromes P-450 have been purified to near homogeneity in several laboratories. Different forms of cytochrome P-450 purified from the same liver sample vary in molecular weight, chromatographic characteristics and substrate specificities.

Differences in the reactions of isomeric ortho- and para-aminophenols with hemoglobin

The metabolites of phenacetin, 2-hydroxyphenetidine and 4-nitrosophenetol, rapidly produced ferrihemoglobin both in vivo (dogs) and in vitro. At low concns, 2-hydroxyphenetidine was superior to 4-nitrosophenetol in ferrihemoglobin formation. The kinetics of ferrihemoglobin formation by 2-hydroxyphenetidine in solutions of purified human hemoglobin was biphasic and exhibited an unusual dose response. Similar to p-aminophenols, 2-hydroxyphenetidine was oxidized by oxyhemoglobin, and the oxidation product(s) were reduced by ferrohemoglobin with the formation of ferrihemoglobin. In addition, these oxidation products condensed to 2-amino-7-ethoxy-3H-phenoxazine-3-one (u.v., i.r., 1H-NMR and mass spectroscopy). This metabolite produced ferrihemoglobin by itself and was responsible for the slow phase of ferrihemoglobin formation observed with 2-hydroxyphenetidine. This condensation reaction, which was also observed with 2-aminophenol, prevented thioether formation of the transient o-quinonimines with the cysteine residues of hemoglobin and reduced glutathione as observed with 4-aminophenol and 4-dimethylaminophenol. Phenoxazone formation, which depends on the square of the o-quinonimine concn, was negligible at micromolar concns. At similar concns addition reactions to thiols prevailed also with 2-hydroxyphenetidine and 2-aminophenol. Other electrophilic reactions, e.g. with primary amino groups of amino acids, were insignificant. These dose-dependent differences in the reactions of isomeric aminophenols may explain the low nephrotoxicity of those o-aminophenols capable of forming phenoxazones when given in a single dose. This self-detoxication of some o-quinonimines, however, should not function during long-term exposure to repetitive low doses of such o-aminophenols.

Pharmacokinetic interactions with antiepileptic drugs

A large number of pharmacokinetic interactions with antiepileptic drugs have been reported in recent years. Among the interactions affecting the disposition of anticonvulsants, the most important are probably those resulting in inhibition of the metabolism of phenytoin, phenobarbitone and carbamazepine. Drugs which have been shown to inhibit the metabolism of these anticonvulsants and to precipitate clinical signs of intoxication in epileptic patients include sulthiame, valproic acid, chloramphenicol, certain sulphonamides, phenylbutazone, isoniazid and propoxyphene. Interactions affecting the plasma protein binding of antiepileptic drugs are less likely to cause long-lasting alterations in response, but they are important because they change the relationship between serum drug concentrations and clinical effect. Anticonvulsant agents may induce important alterations in the pharmacokinetics of other drugs. Phenytoin and phenobarbitone may decrease the gastrointestinal absorption of frusemide and griseofulvin, respectively. Many of the drugs used in the treatment of the adult epilepsies, including phenytoin, phenobarbitone, primidone and carbamazepine, are potent inducers of the hepatic microsomal enzymes. This results in an increased rate of metabolism and decreased clinical efficacy of a number of drugs, including dicoumarol, steroid oral contraceptives, metyrapone, glucocorticoid agents, doxycycline, quinidine and vitamin D.

Hemolytic anemia and sulfhemoglobinemia due to phenacetin abuse: a case with multivisceral adverse effects

The authors report a case of recurrent sulfhemoglobinemia resulting from a chronic abuse of phenacetin. Cyanosis was accompanied by hemolytic anemia and numerous features of tissue aging. While phenacetin was found to be the oxidizing drug, no source of sulfur was identified. The origin of sulfhemoglobinemia in man is discussed and the possible role of intraerythrocytic glutathione is emphasized.

Effect of a concomitant single dose of ethanol on the hepatotoxicity and metabolism of acetaminophen in mice

A concomitant single dose of ethanol (1 g/kg) protected mice from hepatic injury induced by acetaminophen (250 mg/kg) as evidenced by the lowering of plasma transaminases. Pharmacokinetic studies with [14C]acetaminophen indicated that ethanol enhanced the initial blood concentrations of radiolabel and its rate of elimination. A tissue distribution study suggested that these effects were probably due to an ethanol-induced inhibition of the biliary clearance of acetaminophen from the blood. Examination of the urinary and biliary metabolites indicated that ethanol inhibited the excretion of the degradation products derived from the glutathione-deactivated hepatotoxic acetaminophen intermediate. The decrease in acetaminophen induced hepatotoxicity was therefore attributed to an inhibitory effect of ethanol on the biotransformation of acetaminophen to the toxic intermediate.

A population and familial study of the defective alicyclic hydroxylation of debrisoquine among Egyptians

1. Debrisoquine hydroxylation exhibited profound variation in 72 Egyptian volunteers. 2. The frequency distribution histogram of the metabolic ratio (ratio unchanged drug: 4-hydroxy metabolite in 0-8 h urine) was polymodal. 3. From family data it was possible to define more clearly than before the heterozygous characteristics. 4. Egyptians appear in general to be more extensive oxidizers of debrisoquine than do English subjects. 5. Ramadan fasting was found to lower the absorption of debrisoquine.

Metabolism of [14C]paracetamol and its interactions with aspitin in hamsters

1. The metabolism of [14C]paracetamol (150 mg/kg) and its interactions with aspirin (200 mg/kg) were studied in male hamsters. 2. Aspirin was found to slow the rate of paracetamol absorption from the gastro-intestinal tract, but did not affect the rate of elimination. 3. Metabolism studies showed that greater than 80% of the radioactivity was excreted in the urine in 24 h. Paper chromatography of the urine separated the radioactivity into five peaks, four of which were identified as paracetamol and its glucuronide, sulphate and mercapturate conjugates. 4. The other peak, comprising of less than 10% of the total radioactivity, was a mixture of two or more other metabolites. A major component was isolated and characterized as methyl 2-hydroxy-5-acetamidophenyl sulphone. 5. Aspirin inhibited the metabolism of paracetamol by the sulphate conjugation pathway.