Ian C. Calder

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

Summary 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.

The nephrotoxicity of p-aminophenol. II. The effect of metabolic inhibitors and inducers.

Inducers and inhibitors of the microsomal mixed function oxidase system have no consistent effect upon the nephrotoxicity of p-aminophenol, or on binding of the compound in vivo to cell protein. p-[ring-3H]Aminophenol was bound in vitro to kidney microsomal protein and to a lesser extent to liver. The binding was enhanced by preincubation of the p-aminophenol in air and inhibited by ascorbate, GSH, N2 and NADPH. These findings indicate that in contrast to paracetamol hepatoxicity which is dependent upon the mixed function oxidase system, that nephrotoxicity of p-aminophenol is dependent upon oxidation to a toxic metabolite by some other pathway. A similar metabolite may be responsible for the nephrotoxic action of phenacetin.

The nephrotoxicity of p-aminophenol. I. The effect on microsomal cytochromes, glutathione and covalent binding in kidney and liver.

p-Aminophenol administration lowered the microsomal cytochrome P-450 and b5 content and decreased the activity of NADPH cytochrome c reductase in kidney, but not in liver. Kidney GSH was depleted to 29% of the control value at 2 h, and only partly restored (50% of control) at 24 h. Liver GSH was transiently decreased, the lowest levels (77% of control) occurring at 30 min. The maximum level of covalently bound radioactivity was at two hours when 16.8% of the total radioactivity in kidney, 1.5% in liver and 3.6% in plasma was protein bound. At this time 81% of the total radioactivity in kidney and 95% of that in the liver was present in the soluble fraction.

Liver and kidney damage induced by N-hydroxyparacetamol.

1. Liver and kidney glutathione are depleted in rats and mice following administration of N-hydroxyparacetamol. 2. Centrilobular hepatic necrosis and necrosis of renal proximal convoluted tubules were also found, the liver lesion predominantly in mice and the renal lesion predominantly in rats. Glutathione depletion was not responsible for this species difference. 3. These results indicate that N-hydroxyparacetamol is the metabolic precursor of the reactive toxic intermediate of paracetamol. They are also relevant to the pathogenesis of the renal damage associated with long term abuse of phenacetin containing compound analgesics.

Nephrotoxicity and Molecular Structure

The nephrotoxicity of a number of aminophenols, quinols and catechols has been assessed from the extent of necrosis of proximal convoluted tubules produced by intravenous injection in rats, and the toxicity correlated with the oxidation-reduction potentials of the compounds.

Peroxisome proliferators protect against paracetamol hepatotoxicity in mice.

Abstract The hepatotoxicity of paracetamol is thought to be mediated by an electrophilic intermediate N-acetyl-p-benzoquinone imine (NAPQI). Conjugation of NAPQI with reduced glutathione (GSH) is a primary detoxicarion mechanism, but it is possible that other cellular antioxidants may play a role, by scavenging oxyradicals produced by a NAPQI-related redox cycling reaction. Peroxisome proliferation is a condition in which excess oxyradical formation occurs when the H 2 O 2 -producing enzymes involved in fatty acid β-oxidation are markedly induced while the H 2 0 2 -degrading enzymes are either increased marginally (catalase) or reduced in activity (GSH peroxidase) [1]. The purpose of this study was to determine whether a change in oxidant status associated with hepatic peroxisome proliferation in mice could alter susceptibility to paracetamol-induced hepatotoxicity.

An Experimental Model of Analgesic-Induced Renal Damage—Some Effects of ρ-Aminophenol on Rat Kidney Mitochondria

1. p-Aminophenol, a known nephrotoxin, has been studied as a model for phenacetin-induced renal damage. 2. Respiration, oxidative phosphorylation and ATPase activity were inhibited in mitochondria isolated from the kidneys of treated rats; this could not be reversed by the addition of exogenous loosely bound cofactors and bovine serum albumin to the assay medium. 3. After treatment the mitochondrial levels of sodium and calcium were increased, potassium decreased and magnesium unaltered. 4. Mitochondria isolated from treated rats showed ultrastructural damage. 5. The results are interpreted to indicate that renal tubular cell mitochondrial injury is important in triggering cortical analgesic renal damage.

N-Hydroxyphenacetin as a precursor of 3-subst1tuted 4-hydroxyacetanilide metabolites of phenacetin

Abstract N -Hydroxyphenacetin on acid treatment or after N -ester formation will react with methionine via N -acetyl- p -benzoquinoneimine (VI) to give 4-hydroxy-3-methyl-thioacetanilide, a new urinary metabolite of phenacetin.

Paracetamol and the isolated perfused kidney: Metabolism and functional effects

1. Renal metabolism of paracetamol has been studied in the isolated perfused rat kidney. 2. The major metabolites of paracetamol normally observed in vivo were present in low concentrations in the urine of the isolated perfused rat kidney. 3. Paracetamol was bound covalently to kidney protein in a linear relationship to dose up to 31.9 mM paracetamol in the perfusate. 4. Using Michaelis-Menten kinetics, the apparent Km of 5.5 mM and Vmax of 139 nmol/h/g wet wt. for renal paracetamol oxidation were observed. 5. Concn. of paracetamol greater than 14.2 mM induced immediate diuresis and diminished sodium reabsorption. Lower concn. were without effect on function.

The metabolism and toxicity of paracetamol in Sprague-Dawley and wistar rats

SummaryUrinary paracetamol metabolites from Sprague-Dawley and Wistar rats were analysed by reversed-phase HPLC. Variations in the metabolic profile were observed as a function of dose, age, sex, species and route of administration. In addition the effect of 3-methylcholanthrene as an inducer of cytochrome P450 mixed function oxidase on paracetamol metabolism was also studied. Increased oxidative metabolism which lead to the formation of 3-thiomethylparacetamol conjugates along with paracetamol mercapturic acid could be correlated with increased susceptibility to hepatic demage. Furthermore it appears that the length of time taken for excretion and the level of free drug excreted may be involved in the aetiology of chronic renal damage.