J.F. Newton

Chemically Induced Nephrotoxicity: Role of Metabolic Activation

Renal xenobiotic metabolism can result in production of electrophiles or free radicals that may covalently bind macromolecules or initiate lipid peroxidation. The mechanisms of renal xenobiotic metabolism may vary in different anatomical regions. Kidney cortex contains a cytochrome P-450 system while medulla contains a prostaglandin endoperoxidase. Recently cysteine conjugated-lyase has been implicated in production of reactive intermediates. Metabolic activation may be amplified by accumulation of xenobiotics within renal cells due to tubular concentrating and/or secretory mechanisms. Additionally, renal xenobiotic detoxicification can occur by conjugation with glucuronide, sulfate or glutathione.

Acetaminophen nephrotoxicity in the rat: I. Strain differences in nephrotoxicity and metabolism☆

Acetaminophen (APAP) produced renal necrosis restricted to the straight segment of the proximal tubule in Fischer 344 (F344) rats but not in Sprague-Dawley (SD) rats. APAP-induced renal functional changes (elevation in blood urea nitrogen and reduction in the accumulation of p-aminohippurate by renal cortical slices) also correlated with strain-dependent histopathological changes. Such strain differences have been attributed to differences in renal P-450 activation of APAP or the deacetylation of APAP to the nephrotoxic metabolite, p-aminophenol (PAP). Kidneys from F344 rats displayed greater concentrations of P-450 and greater ethoxycoumarin-o-deethylase activity than kidneys from SD rats. However, covalent binding of [ring-14C]APAP to renal and hepatic microsomal protein in vitro was similar for both SD and F344 rats. Deacetylation of APAP to PAP was similar in renal and hepatic homogenates from SD and F344 rats. Furthermore, isolated kidneys from SD and F344 rats perfused with APAP excreted PAP at similar rates. PAP excretion, over a 24-hr period following APAP administration, was greater in F344 rats than in SD rats only at the highest dose (900 mg/kg) of APAP. Thus, strain differences in APAP-induced nephrotoxicity apparently cannot be attributed to differences in P-450 activation of APAP or in deacetylation to the nephrotoxic metabolite, PAP.

Acetaminophen nephrotoxicity in the rat. II. Strain differences in nephrotoxicity and metabolism of p-aminophenol, a metabolite of acetaminophen.

Acetaminophen (APAP) produces renal necrosis restricted to the straight segment of the proximal tubule in Fischer 344 (F344) rats. On the other hand, Sprague-Dawley (SD) rats are extremely resistant to the nephrotoxic effects of APAP. Such strain differences may be due to different susceptibilities to the nephrotoxic metabolite, p-aminophenol (PAP). PAP administration in both strains of rats resulted in a renal lesion indistinguishable from the APAP-induced renal lesion in F344 rats. The PAP-induced renal lesions in F344 rats, however, were generally more severe than those in SD rats. PAP-induced renal functional changes (elevation in blood urea nitrogen and reduction in the accumulation of p-aminohippurate by renal cortical slices) correlated with strain-dependent histopathological changes. Analysis of urinary metabolites over a 24-hr period following PAP administration (200 and 400 mg/kg) indicated that more PAP was excreted as APAP in SD than in F344 rats. Covalent binding of PAP to renal microsomes in vitro was much greater in F344 rats than in SD rats at substrate concentrations less than 5 mM. These results suggest that strain differences in PAP-induced nephrotoxicity may be related to differences in the intrarenal activation of PAP. Furthermore, strain differences in APAP-induced nephrotoxicity may be related to strain differences in the activation of the nephrotoxic metabolite, PAP.

Metabolism and pharmacokinetics of acetaminophen in a severely poisoned young child

A 1-year-old child with severe acetaminophen (APAP) poisoning after ingestion of 10 gm APAP demonstrated central nervous system depression, shock, hypothermia, and metabolic acidosis. There was dramatic improvement during treatment with intravenously administered N-acetylcysteine (NAC) and hemodialysis, and the patient recovered without sequelae. A detailed study of APAP metabolism was carried out during the initial 72 hours after ingestion. APAP-sulfate and APAP-glucuronide accounted for 29% and 33%, respectively, of total drug in urine, whereas cysteine and NAC conjugates accounted for only 12%. The low incidence of severe toxicity in children after overdoses of APAP may be related to greater capacity to metabolize APAP via a nontoxic pathway.

The role of p-aminophenol in acetaminophen-induced nephrotoxicity: Effect of bis(p-nitrophenyl) phosphate on acetaminophen and p-aminophenol nephrotoxicity and metabolism in Fischer 344 rats☆

Acetaminophen (APAP) produces proximal tubular necrosis in Fischer 344 (F344) rats. Recently, p-aminophenol (PAP), a known potent nephrotoxicant, was identified as a metabolite of APAP in F344 rats. The purpose of this study was to determine if PAP formation is a requisite step in APAP-induced nephrotoxicity. Therefore, the effect of bis(p-nitrophenyl) phosphate (BNPP), an acylamidase inhibitor, on APAP and PAP nephrotoxicity and metabolism was determined. BNPP (1 to 8 mM) reduced APAP deacetylation and covalent binding in F344 renal cortical homogenates in a concentration-dependent manner. Pretreatment of animals with BNPP prior to APAP or PAP administration resulted in marked reduction of APAP (900 mg/kg) nephrotoxicity but not PAP nephrotoxicity. This result was not due to altered disposition of either APAP or acetylated metabolites in plasma or renal cortical and hepatic tissue. Rather, BNPP pretreatment reduced the fraction of APAP excreted as PAP by 64 and 75% after APAP doses of 750 and 900 mg/kg. BNPP did not alter the excretion of APAP or any of its non-deacetylated metabolites nor did BNPP alter excretion of PAP or its metabolites after PAP doses of 150 and 300 mg/kg. Therefore, the BNPP-induced reduction in APAP-induced nephrotoxicity appears to be due to inhibition of APAP deacetylation. It is concluded that PAP formation, in vivo, accounts, at least in part, for APAP-induced renal tubular necrosis.

Acetaminophen nephrotoxicity in the rat. Renal metabolic activation in vitro

High doses of acetaminophen (APAP) result in hepatic centrilobular and renal cortical necrosis in man and the F344 rat. Hepatic necrosis is considered to be due to the generation of an arylating intermediate via a microsomal cytochrome P-450 dependent system. Renal microsomes also metabolize APAP to an arylating intermediate via a P-450 dependent mechanism. Thus, at least part of the renal damage from APAP may be due to a biochemical mechanism similar to that in liver. Additionally, APAP is deacetylated to p-aminophenol (PAP) in renal and hepatic cytosol and microsomes. Previous results demonstrated that PAP may be activated in renal microsomes via an NADPH-independent mechanism. Therefore, significant metabolic activation of APAP in the kidney may occur subsequent to deacetylation. Covalent binding of [ring-14C]APAP to renal subcellular fractions was used to substantiate this hypothesis. Under appropriate incubation conditions, enzymatic NADPH-independent covalent binding of [ring-14C]APAP could be demonstrated in renal microsomes but not in 100,000g supernatant fractions. Combination of these subcellular fractions resulted in greater covalent binding of [ring-14C]APAP than in the individual subcellular fractions alone. Addition of glutathione, bis(p-nitrophenyl)phosphate (a deacetylase inhibitor), or PAP inhibited this covalent binding. In contrast, NADPH-independent covalent binding of [ring-14C]APAP could not be demonstrated in any combination of hepatic subcellular fractions. Experiments comparing [ring-14C]APAP and [acetyl-14C]APAP covalent binding to renal 10,000g supernatant fractions indicate that the compound which binds to renal macromolecules is derived from PAP. Thus, these results are consistent with the hypothesis that APAP can be metabolically activated in the kidney after deacetylation to PAP.

Acetaminophen nephrotoxicity in the rat: Quantitation of renal metabolic activation in vivo☆

Renal cortical necrosis induced by acetaminophen (APAP) may be related to generation of reactive intermediates by two mechanisms of metabolic activation, direct P-450 dependent metabolic activation (P-450) or metabolic activation subsequent to deacetylation of APAP to p-aminophenol (PAP). Generation of arylating intermediates by both pathways of metabolic activation was quantified in cyclohexamide (HEX)-pretreated or naive rats in vivo with specifically labeled [14C]APAP. The association of each type of metabolic activation with APAP-induced nephrotoxicity was determined in Fischer 344 (F344) and Sprague-Dawley (SD) rats, strains that are susceptible and resistant to APAP-induced nephrotoxicity, respectively. Covalent binding of [ring-14C]APAP to renal cortex was approximately four times greater than [acetyl-14C]APAP in HEX-pretreated F344 rats. In contrast, in SD rats pretreated with HEX covalent binding of [ring-14C]APAP and [acetyl-14C]APAP in the renal cortex was not different. Furthermore, covalent binding of [ring-14C]APAP to renal cortical protein was approximately four times greater in F344 rats than in SD rats. Arylation of hepatic protein by either [ring-14C]APAP or [acetyl-14C]APAP was similar regardless of strain or pretreatment regimen. These studies demonstrated arylation of renal macromolecules in vivo by reactive intermediates resulting from PAP in F344 but not SD rats. Since F344, but not SD, rats are susceptible to APAP-induced nephrotoxicity, it appears the formation of arylating intermediates by PAP is a requisite step in APAP-induced nephrotoxicity.

Mechanism of chloroform nephrotoxicity: IV. Phenobarbital potentiation of in vitro chloroform metabolism and toxicity in rabbit kidneys

Metabolism of chloroform (CHCl3) by a cytochrome P-450-dependent process to a reactive metabolite may be required to elicit hepatic and renal toxicities. Specific inducers or inhibitors of cytochrome P-450 have been employed frequently as tools to demonstrate this relationship between metabolism and toxicity in the liver. The experiments reported herein were designed to identify the relationship between metabolism and toxicity of CHCl3 in the kidney of rabbits, a species in which renal cytochrome P-450 is induced by phenobarbital. Pretreatment with phenobarbital enhanced the toxic response of renal cortical slices to CHCl3 in vitro as indicated by decreased p-aminohippurate and tetraethylammonium accumulation. Phenobarbital pretreatment also potentiated in vitro 14CHCl3 metabolism to 14CO2 and covalently bound radioactivity in rabbit renal cortical slices and microsomes. Addition of L-cysteine significantly reduced covalent binding in renal microsomes from both phenobarbital-treated and control rabbits and was associated with the formation of the radioactive phosgene-cysteine conjugate 2-oxothiazolidine-4-carboxylic acid (OTZ). Formation of OTZ was enhanced in renal microsomes from phenobarbital-pretreated rabbits. Thus, this in vitro model supports the hypothesis that the kidney metabolizes CHCl3 to the nephrotoxic metabolite, phosgene.

Nephrotoxicity of phenolic bromobenzene metabolites in the mouse

Bromobenzene, at doses greater than 5.7 mmol/kg, produced renal proximal tubular necrosis and renal functional changes in mice. p-Bromophenol and o-bromophenol were the major urinary phenolic bromobenzene metabolites although m-bromophenol and 4-bromocatechol were also excreted in detectable quantities. With the exception of o-bromophenol, urinary metabolites were excreted primarily as conjugates. 4-Bromocatechol and the 3 bromophenol isomers were nephrotoxicants (measured as increased blood urea nitrogen and decreased accumulation of organic anions by renal cortical slices) but not hepatotoxicants (measured as serum glutamic pyruvate transaminase) in vivo at 0.56 mmol/kg (i.v.). Preincubation of renal cortical slices with each of these bromobenzene metabolites for 90 min resulted in dose-dependent decreases in the accumulation of p-aminohippurate and tetraethylammonium. At 10 mumol/preincubation (2.4 mM), organic ion accumulation was decreased maximally by all bromobenzene metabolites examined while equimolar amounts of bromobenzene were without effect. 4-Bromocatechol was the most potent nephrotoxicant in vitro. Administration of 0.53-2.12 mmol/kg (i.v.) 4-bromocatechol to mice resulted in a dose-dependent decrease in renal function while hepatic function was altered only slightly at the higher doses. The renal cortical necrosis produced by in vivo administration of 4-bromocatechol could not be distinguished histologically from that induced by bromobenzene. These results demonstrate that 4-bromocatechol and the 3 bromophenol isomers are nephrotoxicants that can be generated from bromobenzene in mice.

Effects of polybrominated biphenyls on metabolism of testosterone by rat hepatic microsomes

Abstract Polybrominated biphenyls (PBBs) have produced alterations in the male reproductive system of several species. These alterations may be a consequence of modified metabolism of androgens. The purpose of this investigation was to determine the effects of PBBs on hepatic microsomal metabolism of testosterone in vitro . Hepatic microsomes were prepared from male and female rats exposed to 0 or 100 ppm PBBs from Day 8 of gestation until they were killed at 1, 2, or 4 months of age. Testosterone and its metabolites were identified and quantified utilizing high-performance liquid chromatography or gas-liquid chromatography. Steroid identification was confirmed with gas chromatography/mass spectrometry. Hydroxylation of testosterone to 7α- and 6β-hydroxytestosterone in microsomes from both sexes at all ages was increased by pretreatment with PBBs. Microsomal conversion of testosterone to 16 α-hydroxytestosterone and androstenedione was enhanced by PBBs in females at all ages and in 1 month-old males but not from males of 2 and 4 months of age. Reduction of testosterone to dihydrotestosterone and dihydroandrosterone was inhibited in microsomes from both sexes at all ages by pretreatment with PBBs. These results support the contention that the effects of PBBs on the male reproductive system may be due, at least in part, to altered metabolism of testosterone.