Some new aspects of the metabolism of phenacetin in the rat

A critical review of the acetaminophen preclinical carcinogenicity and tumor promotion data and their implications for its carcinogenic hazard potential

In 2019 the California Office of Environmental Health Hazard Assessment (OEHHA) initiated a review of the carcinogenic hazard potential of acetaminophen, including an assessment of the long-term rodent carcinogenicity and tumor initiation/promotion studies. The objective of the analysis herein was to inform this review process with a weight-of-evidence assessment of these studies and an assessment of the relevance of these models to humans. In most of the 14 studies, there were no increases in the incidences of tumors in any organ system. In the few studies in which an increase in tumor incidence was observed, there were factors such as absence of a dose response and a rodent-specific tumor supporting that these findings are not relevant to human hazard identification. In addition, we performed qualitative analysis and quantitative simulations of the exposures to acetaminophen and its metabolites and its toxicity profile; the data support that the rodent models are toxicologically relevant to humans. The preclinical carcinogenicity results are consistent with the broader weight of evidence assessment and evaluations of multiple international health authorities supporting that acetaminophen is not a carcinogenic hazard.

Bio-isosteric replacement of amide group with 1,2,3-triazole in phenacetin improves the toxicology and efficacy of phenacetin-triazole conjugates (PhTCs)

AIM

Inflammatory algesia and pyresia are common pathological consequences of physiological defense. Phenacetin introduced as effective analgesic anti-pyretic agent, was proscribed from therapeutic use because of associated systemic toxicity. The aim of the study was to evaluate the potency of 1,2,3-triazole conjugation in reducing toxicity and increasing efficacy of the phenacetin nucleus.

MAIN METHODS

The amide bond implicated as the cause of phenacetin toxicity was bioisosterically replaced with 1,2,3-triazoles to yield a series of PhTCs(PhTC1, PhTC2 and PhTC3). The toxicology of the synthesized conjugates in reference to phenacetin was evaluated in accordance with OECD test guidelines 420, 425 and 407. For the purpose of evaluating anti-inflammatory potency carrageenan induced paw edema and croton oil induced ear edema models were evaluated. Anti-nociceptive efficacy was assessed using Eddy's hot plate and acetic acid induced writhing experimental models. For anti-pyretic efficacy, the conjugates were submitted to Brewer's yeast antipyretic assay.

KEY FINDINGS

Toxicological examination of PhTCs in comparison to phenacetin revealed that, phenacetin treatment caused considerable nephrotoxicity and hepatotoxicity in experimental models PhTCs were devoid of such toxic manifestations. Results of pharmacological assays showed that the entire series of PhTCs possessed better anti-inflammatory, anti-nociceptive and anti-pyretic potential than phenacetin. Furthermore it was revealed that the pharmacological profile of PhTC1 with triazole substitution at para position of the phenol ring exhibited potency even better than that exhibited by the reference standards.

CONCLUSION

Bioisosteric replacement of amide bond by 1,2,3-triazole in the phenacetin moiety yields conjugates with superior efficacy and diminished toxicity, thus opening neo avenues in treatment of inflammatory syndromes.

Toxicological implications of polymorphic drug metabolism

The occurrence of genetic polymorphisms of drug metabolism means that populations contain subgroups (phenotypes) that differ sharply in their abilities to effect a number of metabolic reactions. Because of this, major interphenotype differences occur in responsiveness to drugs and toxic substances. The well established genetic polymorphisms of acetylation and hydrolysis illustrate the important association that exists between phenotype and propensity to develop toxic and exaggerated responses to some substances. Recently, for metabolic oxidation, a new genetic polymorphism of drug metabolism has been described and it promises to provide a better understanding of inter-individual variability in the metabolic handling of, and responsiveness to, drugs and toxic substances. The following effects of the polymorphism are described here: (a) its influence in determining variable presystemic metabolism and hence systemic drug availability; (b) its role in determining alternative toxic pathways of metabolism in individuals who have a genetically determined impairment of oxidative capacity and (c) its influence on the development of agranulocytosis associated with metiamide administration.

Effect of an experimental malaria infection on the metabolism of phenacetin in the rat isolated perfused liver

1. The effect of infection with the rodent malaria parasite Plasmodium berghei on the metabolism of phenacetin has been investigated in a rat isolated perfused liver preparation. 2. A bolus dose of phenacetin (10 mg) was introduced into the perfusate reservoir of both control (n = 4) and malaria-infected (n = 4) liver preparations, and samples of bile and perfusate were collected (0-4 h) for hplc analysis of phenacetin, paracetamol and its phase II metabolites. 3. Whereas malaria had no effect on the hepatic clearance of phenacetin (control: 0.64 +/- 0.15 versus malaria: 0.66 +/- 0.14 ml min-1), there was a significant reduction in the hepatic clearance of generated paracetamol (control: 1.22 +/- 0.15 versus malaria: 0.41 +/- 0.08 ml min-1) and the total recovery in bile and perfusate of paracetamol glucuronide (control: 1.18 +/- 0.44 versus malaria: 0.29 +/- 0.20 mg). There was no significant change during malaria infection in the total recovery of either phenacetin (control: 1.30 +/- 0.73 versus malaria: 0.79 +/- 0.36 mg) or paracetamol sulphate (control: 0.81 +/- 0.25 versus malaria: 0.74 +/- 0.16 mg),

Identification of metabolites of 4,4'-diaminodiphenylmethane (methylene dianiline) using liquid chromatographic and mass spectrometric techniques

The in vitro metabolism of 4,4'-diaminodiphenylmethane (methylene dianiline, MDA) was investigated using rabbit liver microsomes. Minimal clean-up of the microsomal incubations was carried out using zinc sulphate precipitation followed by solid-phase extraction on Sep-Pak C18 cartridges. Three metabolites were detected in hepatic microsomal incubations, namely the azodiphenylmethane (azo) azoxydiphenylmethane (azoxy) and 4-nitroso-4'-aminodiphenylmethane (nitroso) compounds. The azo and azoxy metabolites were produced enzymatically whereas the nitroso compound may have been formed via a non-enzymatic process. Reversed-phase high-performance liquid chromatography-plasma spray mass spectrometry was used to initially detect these metabolites. Fast atom bombardment mass spectrometry and fast atom bombardment tandem mass spectrometry were utilized to further structurally characterise these compounds. Comparison of mass spectral data obtained from synthesised standards with data obtained on the putative metabolites substantiated the characterisation of these compounds.

Genotoxicity of analgesic compounds assessed by an in vitro micronucleus assay

Several analgesic compounds and mixtures of analgesics were examined for both cytotoxicity and ability to induce chromosomal damage in the normal rat-kidney cell line NRK-49F. Chromosomal damage was assessed using an in vitro micronucleus assay. Of all the compounds tested, only N-hydroxyparacetamol caused a high degree of cell death at the concentrations used. 4 analgesic compounds were found to be inducers of micronuclei in NRK cells; in order of decreasing potency these were: N-hydroxyparacetamol, N-hydroxyphenacetin, caffeine and paracetamol. An aspirin, phenacetin, caffeine mixture (APC) failed to induce micronuclei above the background level, and a paracetamol-codeine combination did not increase the level of micronuclei induction above that induced by paracetamol alone. This report suggests paracetamol and some related compounds are capable of inducing chromosomal damage in mammalian cells in vitro, which is consistent with recent reports of a possible paracetamol-DNA interaction.

Renal effects of antipyretic analgesics

Most antipyretic analgesics can cause acute nephrotoxic effects, including acute tubular necrosis, acute interstitial nephritis, glomerular toxicity, and functional changes, such as "salicyl edema," following large doses of sodium salicylate. Most functional changes are related to acute suppression of prostaglandin synthesis, "the acute prostaglandin-effect," and have been primarily noted with the use of indomethacin. The association between prolonged and excessive consumption of compound analgesics and the development of renal disease and renal failure, characterized by renal papillary necrosis, is now well established. Studies in several countries have shown that the incidence of analgesic nephropathy as an indication for dialysis and transplantation corresponds to the per capita consumption of phenacetin in compound analgesics. Analgesic nephropathy, which is part of a wider clinical syndrome, the analgesic syndrome, is uncommon following the use of single analgesics. Analgesic nephropathy and the analgesic syndrome are discussed in detail, including the development of uroepithelial tumors.

The contribution of genetically determined oxidation status to inter-individual variation in phenacetin disposition

The oxidative O-de-ethylation and aromatic 2-hydroxylation of phenacetin have been investigated in panels of extensive (EM, n = 13) and poor (PM, n = 10) metabolizers of debrisoquine. The EM group excreted in the urine significantly more paracetamol (EM: 40.8 +/- 14.9% dose/0-8 h; PM: 29.2 +/- 8.7% dose/0-8 h, 2P less than 0.05) and significantly less 2-hydroxylated metabolites (EM: 4.7 +/- 2.3% dose/0-8 h; PM: 9.7 +/- 3.5% dose/0-8 h, 2P less than 0.005) than the PM group. Apparent first-order rate constants, calculated from pooled phenotype data, for overall elimination of phenacetin (k) and formation of paracetamol (kml) were higher in the EM group (EM: k = 0.191 +/- 0.151 h-1; kml = 0.091 +/- 0.025 h-1; PM: k = 0.098 +/- 0.035 h-1, 2P less than 0.05, kml = 0.052 +/- 0.019 h-1, 2P less than 0.05) than the PM group. The apparent first-order rate constant for 2-hydroxylation displayed no significant inter-phenotype differences. Correlation analysis demonstrated that genetically determined oxidation status accounted for approximately 50% of the inter-individual variability in phenacetin disposition encountered in this study.

Eighteen month oral study of aspirin, phenacetin and caffeine, in C57Bl/6 mice

Groups of 40 male and 40 female C57BL/6 mice were maintained for 75-80 weeks on meal form diets containing aspirin, phenacetin and caffeine either singly or in combination. The maximum daily doses of phenacetin alone and the APC combination were approximately one-half of their previously determined respective oral LD50's. Mild, nonprogressive histopathologic changes of the urinary tract were noted with these changes first evident in animals given the highest dose of phenacetin. Sulfhemoglobinemia was also induced in all groups of animals given phenacetin alone or in combination indicating that toxic doses were administered oral LD50's. Mild, nonprogressive histopathologic changes of the urinary tract were noted with these changes first evident in animals given the highest dose of phenacetin. Sulfhemoglobinemia was also induced in all groups of animals given phenacetin alone or in combination indicating that toxic doses were administered oral LD50's. Mild, nonprogressive histopathologic changes of the urinary tract were noted with these changes first evident in animals given the highest dose of phenacetin. Sulfhemoglobinemia was also induced in all groups of animals given phenacetin alone or in combination indicating that toxic doses were administered. Under the conditions of this study, evidence of carcinogens was not demonstrated for any of the drugs given alone or in combination.

Kinetics and metabolism of paracetamol and phenacetin

1 The rate of absorption of oral paracetamol depends on the rate of gastric emptying and is usually rapid and complete. The mean systemic availability is about 75%. 2 Paracetamol is extensively metabolized and the plasma half-life is 1.5-2.5 hours. About 55% and 30% of a therapeutic dose is excreted in the urine as glucuronide and sulphate conjugates, respectively, whereas mercapturic acid and cysteine conjugates (representing conversion to a potentially toxic intermediate metabolite) each account for some 4% of the dose. Paracetamol metabolism is age- and dose-dependent. 3 With hepatotoxic doses, paracetamol metabolism is impaired and the half-life prolonged. Sulphate conjugation is saturated and the proportion excreted as mercapturic acid and cysteine conjugates is increased. 4 The renal clearance of paracetamol depends on urine flow rate by not pH. The renal clearances of the glucuronide and sulphate conjugates often exceed the glomerular filtration rate and are independent of urine flow and pH. 5 Phenacetin absorption depends on formulation. It is extensively metabolized to paracetamol and minor metabolites are probably responsible for toxicity.

Carcinoma of the renal pelvis following the abuse of phenacetin-containing analgesic drugs

Five cases of renal pelvic carcinoma are reported in female patients who abused phenacetin-containing analgesics. The mechanism of carcinogenesis in analgesic abuse is discussed and epidemiological factors are considered. The possibility of development of a renal pelvic tumour in patients with analgesic nephropathy is emphasised.

High-performance liquid chromatographic assay for acetaminophen and phenacetin in the presence of their metabolites in biological fluids

We propose a method in which tracer amounts of a radiolabeled compound are used as the internal standard for the same unlabeled compound in high-performance liquid chromatography. The approach is valuable when a response from the internal standard becomes undesirable due to the presence of interference by the metabolites. We tested our approach with phenacetin and its metabolites, acetaminophen, 2-hydroxyphenacetin, N-hydroxyphenacetin, phenetidine, acetaminophen sulfate conjugate and acetaminophen glucuronide conjugate in biological fluids with the use of [14C]phenacetin and [3H]acetaminophen as the internal standards, and were able to quantitate both phenacetin and acetaminophen simultaneously. We also tested the alternative approach in which the unlabeled drug was used as internal standard for tracer amounts of the same radiolabeled compound, with phenacetin and acetaminophen as the internal standards for tracer amounts of [14C]phenacetin and [3H]acetaminophen. Again, we were able to quantitate the two tracer radiolabeled compounds simultaneously.

Metabolism of phenacetin and N-hydroxyphenacetin in isolated rat hepatocytes

The fate of phenacetin and some of tis metabolites have been examined in isolated rat hepatocytes. The overall pattern of metabolism was similar to that found in vivo by others. The major metabolites of phenacetin were paracetamol, free and conjugated, and phenetidine, and about 10% was lost. No N-hydroxyphenacetin was found, but experiments with N-hydroxyphenacetin as substrate showed that at low concentration (as might be formed from phenacetin) it disappeared very rapidly from cell suspensions. N-hydroxyphenacetin was metabolized to its conjugates, and to paracetamol, phenacetin and phenetidine, with a large proportion unaccounted for. With all substrates, increasing concentration resulted in a decreased percentage being metabolized, indicating that the metabolic pathways were saturable. Relatively more phenetidine was found at high phenacetin concentrations, however, apparently because phenetidine is an intermediate metabolite whose own elimination was slowed relatively more than its formation. N-hydroxylated arylamines were toxic to hepatocytes in a dose-dependent manner, suggesting that these cell suspensions could be used to test for hepatotoxicity.

Molecular basis for several drug-induced nephropathies

A recent clinical advance has been the discovery that many drug-induced hepatic diseases result from the metabolic activation of chemically stable drugs to potent alkylating agents by the liver. In addition to the liver, however, the kidney also contains active enzyme systems capable of metabolically activating drugs and other chemicals. For this reason a systematic investigation of the possible role of metabolic activation in the pathogenesis of several drug-induced renal diseases has been undertaken. These laboratory results are reviewed in the light of the clinical spectrum of the renal injuries, and possible therapeutic implications of these new findings are briefly discussed. The potential use of these models of nephrotoxicity to probe a variety of physiologic and pathophysiologic mechanisms of renal function are noted.

4-Acetaminophenoxyacetic acid, a new urinary metabolite of phenacetin

It is shown that 4-acetaminophenoxyacetic acid (APOA) is an urinary metabolite of phenacetin. APOA was isolated by means of silica gel TLC in various solvent systems from the urine of rats, dogs, and humans, collected 24 h after p.o. treatment with phenacetin (rats and dogs: 200 mg/kg; humans: three single doses of 0.5 g). Expressed as a percentage of the dose, APOA was detected at levels of 1% in rats, 0.13% in dogs and 0.04% in humans. 4-Acetaminophenoxyacetic acid was identified as its methylester--synthetized in the reaction of APOA and diazomethene--by thin layer chromatography, UV absorbance, melting point, and mass spectroscopy.

Urothelial changes of the renal papillae in Sprague-Dawley rats induced by long term feeding of phenacetin

Thirty female Sprague-Dawley rats were fed 0.535 per cent phenacetin in the diet for up to 110 weeks. Twenty-six of these rats developed urothelial hyperplasia, partly papillary, of the renal papillae. Twenty-eight rats showed dilatation of the vasa recta frequently associated with thrombus formation and calcification. One phenacetin fed rat had epithelial hyperplasia associated with chronic pyelitis. In 2 of the 30 control rats urothelial hyperplasia was found to be associated with chronic pyelitis. The hyperplastic urothelial changes and vascular changes were often, but not always, present simultaneously. One control rat developed a mammary carcinoma, as compared with 5 rats in the phenacetin group. Four phenacetin fed rats developed carcinoma of the ear duct. The results of the present investigation provide evidence that phenacetin can induce proliferative lesions of the urothelium of the rat renal pelvis with weak carcinogenic activity in the ear duct and mammary glands.

Some effects of phenacetin on the rat

Rats aged from 2 to 4 mth were given phenacetin 100 mg/kg/day and 1-14 g/kg/day, orally, for up to 26 1/2 wk and 3 wk, respectively. Signs of premature ageing such as senile plagues, neurofibrillary tangles and lipofucsin pigment were not demonstrated in the brains of these animals.

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.

Evidence for the nephrotoxicity of analgesics

Analgesic nephropathy in man is a definite but complex and probably multifactorial disorder, which largely follows long-term abuse of mixed analgesics. In North America the disorder has been almost entirely associated with heavy ingestion of the combination acetylsalicylic acid (ASA), phenacetin and caffeine. There is less clinical and experimental evidence for the nephrotoxicity of single analgesics; however, evidence for the nephrotoxicity of ASA alone is greater than that for phenacetin. In view of the evidence for nephrotoxicity of most mild analgesics, removal of phenacetin from the market would be unlikely to eliminate the problem of analgesic nephropathy.