Absence of phenolic glucuronidation and enhanced hydroxylation of diflunisal in the homozygous Gunn rat

The role of glucuronidation in N-(3,5-dichlorophenyl)succinimide (NDPS) nephrotoxicity: nephrotoxic potential of NDPS and NDPS metabolites in Gunn, Wistar, and Fischer 344 rats

The agricultural fungicide N-(3,5-dichlorophenyl)succinimide (NDPS) is an acute nephrotoxicant in rats. Although the mechanism of NDPS nephrotoxicity is not clear, our previous studies have strongly suggested that glucuronide conjugation of NDPS metabolite(s) is an important biotransformation reaction leading to the ultimate nephrotoxicant metabolite(s) mediating NDPS nephrotoxicity. In this study, the nephrotoxic potential of NDPS and its nephrotoxicant metabolites, N-(3,5-dichlorophenyl)-2-hydroxysuccinimide (NDHS) and N-(3,5-dichlorophenyl)-2-hydroxysuccinamic acid (NDHSA), was examined in Gunn rats, which contain a genetic deficiency in bilirubin uridine diphosphate-glucuronosyltransferase (UDPGT), to explore further the role of glucuronidation in NDPS nephrotoxicity. The nephrotoxic potential of NDPS, NDHS, and NDHSA was also examined in Wistar rats, the parent strain for Gunn rats and which generally have normal UDPGT activity. Comparisons were then made with the nephrotoxicity induced by these compounds in Fischer 344 (F344) rats. Age-matched male F344, homozygous (j/j) Gunn, and Wistar rats were used. Rats (four to eight rats/group) of each strain were administered NDPS (0.4 mmol/kg ip), NDHS (0.1 or 0.2 mmol/kg ip), NDHSA (0.1 mmol/kg ip), or vehicle, and renal effects were monitored functionally and morphologically for 48 h. NDPS and its nephrotoxicant metabolites, NDHS and NDHSA, were much weaker nephrotoxicants in Gunn rats than in F344 rats, while Wistar rats were susceptible to the nephrotoxicity induced by NDPS, NDHS, or NDHSA. These results suggest that the lack of NDPS nephrotoxicity observed in Gunn rats is due to the deficiency in UDPGT in this strain rather than the parent Wistar strain being inherently nonresponsive to NDPS nephrotoxicity. Therefore, it appears that glucuronide metabolite(s) of NDHS and/or NDHSA contribute(s) to NDPS nephrotoxicity, although the exact nature of the nephrotoxicant glucuronide metabolite(s) of NDPS remains to be determined.

Effect of glucuronidation substrates/inhibitors on N-(3,5-dichlorophenyl)succinimide nephrotoxicity in Fischer 344 rats

The agricultural fungicide N-(3,5-dichlorophenyl)succinimide (NDPS) is an acute nephrotoxicant in rats. Our previous studies have strongly suggested that glucuronide conjugation of NDPS metabolites might be a bioactivation step mediating NDPS nephrotoxicity. In this study, effects of substrates and/or inhibitors of primarily glucuronidation on NDPS nephrotoxicity were examined to explore further the role of glucuronidation in NDPS nephrotoxicity. Male Fischer 344 rats (4-6/group) were administered one of the following intraperitoneal (i.p.) pretreatments (dose, pretreatment time) prior to NDPS (0.4 mmol/kg) or NDPS vehicle (sesame oil, 2.5 ml/kg): (1) no pretreatment; (2) borneol (900 mg/kg, 30 min); (3) eugenol (500 mg/kg per day, 3 days); (4) clofibric acid (400 mg/kg, 15 min before (1/2 dose) and 3 h after (1/2 dose)), or (5) valproic acid, sodium salt (1.0 mmol/kg, 15 min). Following NDPS or NDPS vehicle administration, renal function was monitored at 24 and 48 h. Pretreatment with borneol or eugenol, substrates for ether glucuronidation and sulfation (mainly glucuronidation), afforded complete protection against NDPS nephrotoxicity. Substrates for acyl glucuronidation, clofibric acid or valproic acid, mildly reduced or had little effect on NDPS nephrotoxicity, respectively. These results suggest that ether glucuronide conjugates of NDPS metabolites, rather than acyl glucuronide conjugates, may be the primary ultimate nephrotoxicant species mediating NDPS nephrotoxicity.

Effects of sodium sulfate on acute N-(3,5-dichlorophenyl)succinimide (NDPS) nephrotoxicity in the Fischer 344 rat

The agricultural fungicide N-(3,5-dichlorophenyl)succinimide (NDPS) induces acute polyuric renal failure in rats. Results of previous studies have suggested that NDPS may induce nephrotoxicity via conjugates of NDPS metabolites. Thus, the purpose of this study was to examine if administered sodium sulfate could alter NDPS nephrotoxicity. Male Fischer 344 rats (four rats per group) were administered a single intraperitoneal (i.p.) injection of sodium sulfate (0.035, 0.07, 0.35 or 3.5 mmol/kg) or sodium chloride (7.0 mmol/kg) 20 min before NDPS (0.2, 0.4 or 0.8 mmol/kg) or NDPS vehicle (sesame oil, 2.5 ml/kg) and renal function monitored at 24 and 48 h. High dose sodium sulfate (3.5 mmol/kg) markedly attenuated NDPS nephrotoxicity, while sodium chloride had no effect on NDPS-induced renal effects. NDPS nephrotoxicity was also attenuated by a pretreatment dose of 0.35 mmol/kg sodium sulfate, while 0.07 mmol/kg sodium sulfate pretreatment potentiated NDPS 0.2 mmol/kg to produce nephrotoxicity without markedly attenuating NDPS 0.4 mmol/kg to induce renal effects. A dose of 0.035 mmol/kg sodium sulfate did not potentiate NDPS 0.2 mmol/kg to induce nephrotoxicity. These results suggest that sulfate conjugates of NDPS metabolites might contribute to NDPS nephrotoxicity.

Conjugation-deconjugation cycling of diflunisal via beta-glucuronidase catalyzed hydrolysis of its acyl glucuronide in the rat

The role of beta-glucuronidase catalyzed hydrolysis of glucuronides on the in vivo disposition kinetics of xenobiotics was studied in the rat. The metabolic disposition kinetics of diflunisal, a compound undergoing transformation to an acyl and phenyl glucuronide, were studied in rats under control conditions and following administration of D-glucaro-1,4-lactone, a potent and specific beta-glucuronidase inhibitor. D-glucaro-1,4-lactone treatment resulted in a significant decrease in beta-glucuronidase activity in plasma, urine, and hepatic microsomes. Total (i.e. urinary and biliary) recovery of the acyl glucuronide following i.v. injection of diflunisal (10 mg/kg) was significantly higher in D-glucaro-1,4-lactone treated rats (41 +/- 3%, n=6) compared to control rats (29 +/- 2%, n=6) whereas for diflunisal phenyl glucuronide this total recovery was very similar in both groups of rats (16.0 +/- 1.0% vs. 18.0 +/- 0.2%, n=6, respectively). The partial clearance of diflunisal associated with the formation of the acyl glucuronide was significantly higher in D-glucaro-1,4-lactone treated rats (0.413 +/- 0.024 ml/min/kg) compared to control animals (0.269 +/- 0.042 ml/min/kg). The partial clearance related to the formation of the phenyl glucuronide, on the contrary, was not significantly affected by D-glucaro-1,4-lactone treatment. These results shows that the in vivo glucuronidation of diflunisal to the acyl glucuronide, unlike diflunisal glucuronidation to the phenyl glucuronide, is subject to a futile conjugation-deconjugation cycle. Such futile cycling may have significant therapeutic and toxic implications.

Glucuronidation of diflunisal in liver and kidney microsomes of rat and man

1. The glucuronidation of diflunisal to its phenolic (DPG) and acyl glucuronide (DAG) was measured in vitro using microsomes prepared from rat (n = 4) and human (n = 6) liver and kidney tissue. UGT activities towards bilirubin, 4-nitrophenol and (-)-morphine were also determined. 2. beta-Glucuronidase activity towards phenolphthalein glucuronide was much lower in microsomes prepared from human liver (45.2 +/- 3.1 Fishman Units/mg protein), human kidney (22.0 +/- 3.3 FU/mg), and rat kidney (25.1 +/- 2.5 FU/mg) as compared with rat liver (118.7 +/- 8.8 FU/mg). 3. The formation rate of DAG significantly increased when saccharo-1,4-lactone, a beta-glucuronidase inhibitor, was added to the rat liver microsomal incubation medium. beta-Glucuronidase inhibition, however, had little effect on the formation rate of DAG in human liver microsomes, and no effect in rat and human kidney microsomes. The formation of DPG was not affected by the microsomal beta-glucuronidase activity. 4. Unlike rat kidney microsomes, which only formed DAG, human kidney microsomes formed both diflunisal glucuronides. Formation of both diflunisal glucuronides in human kidney microsomes (Vmax = 0.97 +/- 0.21 and 0.27 +/- 0.07 nmol/min/mg for formation of DAG and DPG respectively) represented 60-70% of the activity found in liver microsomes (Vmax = 1.58 +/- 0.32 and 0.40 +/- 0.08 nmol/min/mg for formation of DAG and DPG respectively). 5. These results demonstrate that the in vitro glucuronidation rate of diflunisal may be affected by the microsomal beta-glucuronidase activity particularly when using rat liver microsomes. Our results also demonstrate that the human kidney has an important UGT-activity towards diflunisal.

Excretion of 3-hydroxy-diflunisal as a monosulphate conjugate--identification using ESI-MS

Electrospray-ionization mass spectrometry was used to identify a novel, highly polar metabolite of diflunisal isolated from Gunn rat urine. Negative ion spectra were obtained of the sulphate conjugate of diflunisal and the new metabolite, which was identified as a sulphate conjugate of 3-hydroxydiflunisal.

Excretion balance and urinary metabolites of the S-enantiomer of indobufen in rats and mice

The excretion balance and urinary metabolites of the S-enantiomer of indobufen, ((S)2-[p-(1-oxo-2-isoindolinyl)-phenyl]butyric acid), a platelet aggregation inhibitor, were studied in rats and mice after oral administration. The urinary metabolic profile exhibited a marked species difference. The major metabolic pathway in the mouse was acyl glucuronidation followed by renal excretion, whereas in rat urine 5-hydroxylation and subsequent sulphation at the introduced hydroxyl group accounted for almost all recovered radioactivity. Indobufen glucuronide was the major biliary metabolite in the rat, while very little indobufen glucuronide was present in the urine of intact or bile duct-cannulated rats. A marked dose-effect on the elimination and metabolism of S-indobufen was demonstrated in the rat. The recovery (% dose) of 5-hydroxyindobufen and its sulphate after the lower dose of the enantiomer (10 mg/kg) was some 2.8-fold higher compared with the higher dose of 20 mg/kg.

Studies on the reactivity of acyl glucuronides--I. Phenolic glucuronidation of isomers of diflunisal acyl glucuronide in the rat

Diflunisal (DF) is metabolized primarily to its acyl glucuronide (DAG), phenolic glucuronide (DPG) and sulphate (DS) conjugates. Whereas DPG and DS are stable at physiological pH, DAG is unstable, undergoing hydrolysis (regeneration of DF) and rearrangement (intramolecular acyl migration to the 2-, 3- and 4-O-acyl-positional isomers). We have compared the in vivo disposition of DAG with that of an equimolar mixture of its three isomers after i.v. administration at 10 mg DF equivalents/kg to conscious, bile-exteriorized rats. After dosing with DAG, excretion in urine and bile (46% as DAG), hydrolysis (as assessed by recovery of 9% DPG and 8% DS resulting from reconjugation of liberated DF) and rearrangement (17% recovery as isomers of DAG) were important pathways. Highly polar metabolites excreted almost exclusively in bile and accounting for 13% of the dose were identified as an approximate 4:1 mixture of the 2- and 3-O-isomers of DAG which had been glucuronidated at the phenolic function of the salicylate ring i.e. "diglucuronides" of DF. Evidence for trace quantities only of the phenolic glucuronides of the 4-O-isomer of DAG, and of DAG itself, was found. After dosing rats with an equimolar mixture of the isomers, 52% was recovered (as the isomers) in urine and bile in 6 hr. Hydrolysis was less important--less than 3% (total) of the dose was recovered as DPG and DS. The phenolic glucuronides of the 2- and 3-O-isomers (ratio ca. 3:7) accounted for 37%. Evidence for appreciable formation of the phenolic glucuronide of the 4-O-isomer was not found. In one rat dosed with DPG, there was no evidence for further glucuronidation of the salicylate ring at its carboxy function. The data suggest that the 2- and 3-O-isomers of DAG, but not the 4-O-isomer, DAG itself or DPG, are good substrates for further glucuronidation.

Identification of a hydroxy metabolite of diflunisal in rat and human urine

1. A new metabolite of diflunisal, a hydroxy derivative, has been identified in rat and human urine following administration of diflunisal. 2. This hydroxy metabolite of diflunisal is excreted in urine of both species as a polar conjugate, most likely a sulphate. 3. Attempts to isolate the polar conjugate in pure form were unsuccessful due to its rapid hydrolysis in the presence of acid, and organic solvents such as diethyl ether. Its breakdown product, however, was more stable and was isolated and purified by semi-preparative h.p.l.c. Unequivocal identification as 3-hydroxy-diflunisal (i.e. hydroxylation in position 3 of the salicylic acid ring) was accomplished by means of FAB-mass spectrometry and n.m.r. spectroscopy. 4. The contribution of this oxidative metabolic pathway to the overall elimination scheme of diflunisal is more important in rat than in man. Gunn rats excrete more of the hydroxy diflunisal conjugate in urine (20-30% of a 50 mg/kg i.v. dose of diflunisal) than Wistar rats. In healthy humans, hydroxylation of diflunisal contributes only to a small extent to the overall biotransformation of diflunisal.