The sulphate conjugate of diflunisal: its synthesis and systemic stability in the rat

Hepatobiliary transport of diflunisal conjugates and taurocholate by the perfused rat liver: the effect of chronic exposure of rats to diflunisal

Acyl glucuronides are reactive electrophilic metabolites of carboxylate drugs which can form covalent adducts with endogenous macromolecules such as serum albumin and hepatic proteins. Such adducts have been suggested as initiating factors in certain immune and toxic responses to acidic drugs. In the present study, pretreatment of rats with high daily doses (50 mg/kg orally) of the non-steroidal anti-inflammatory drug (NSAID) diflunisal (DF) for 35 days, followed by perfusion of the isolated liver with 3 mg DF for 3 hr, resulted in appreciable concentrations of covalent adducts of DF with hepatic tissue (3.68 microg DF/g liver). Immunoblotting using a rabbit polyclonal DF antiserum showed the major DF-modified bands at about 110, 140 and 200 kDa. A vehicle-pretreated control group achieved adduct concentrations of only 0.37 microg DF/g liver, with the 200 kDa band not detectable in immunoblots. Elimination of DF from perfusate of the isolated perfused rat liver (IPRL) preparation was the same (t1/2 about 3.4 hr) in both DF- and vehicle-pretreated groups. Appearance of the sulfate (DS) conjugate, the major metabolite in perfusate, was also similar. However, higher concentrations of the acyl glucuronide (DAG) and phenolic glucuronide (DPG) conjugates were found in perfusate at later times, though a statistically significant difference in area under the concentration-time curve was found only in the case of DAG. At 3 hr, recoveries of dose as DAG and DPG were significantly higher in perfusate, but not in bile. No significant differences in uptake and biliary excretion of taurocholate were found between the two groups. The finding of higher perfusate concentrations of DAG and DPG could signal a minor compromise to biliary excretion processes for the glucuronides, though whether such a result is simply coincident with or attributable to DAG-derived covalent DF-protein adducts in liver remains indeterminate.

Vesico-hepato-renal cycling of acidic drugs via their reactive acyl glucuronide metabolites? Studies with diflunisal in rats

1. Deconjugation-reconjugation cycling of acidic drugs is known to occur in vivo via the hydrolysis of their reactive acyl glucuronide metabolites during their circulation in the blood (systemic cycling) or during their passage through the gut after biliary excretion (enterohepatic cycling). Whether such cycling occurs after renal excretion via hydrolysis in the urinary bladder followed by absorption of liberated drug (vesico-hepato-renal cycling) was investigated in rats using diflunisal (DF) and its acyl glucuronide (DFAG) as model compounds. 2. After administration of DF (1 mg/0.5 mL buffer, pH 7) into the bladder of anaesthetized bile-exteriorized rats, DF appeared rapidly in plasma, achieving peak concentrations of 7 micrograms/mL at 1 h. At 4 h, 30% of the dose was recovered as metabolites, mainly DFAG and DF phenolic glucuronide (DFPG) in bile, while 30% was recovered as unchanged DF from the bladder. 3. By contrast, after intravesical administration of an equimolar amount of DFAG at pH 7 or 5, DFAG itself was not detectable in plasma. Plasma concentrations of DF were barely detectable, with only approximately 1% of the administered dose recovered as metabolites in bile. 4. The data thus show that, although DF itself undergoes facile absorption from the urinary bladder of healthy rats, vesico-hepato-renal cycling of DF via DFAG appears to be of only minor quantitative importance.

Studies on the reactivity of acyl glucuronides--VIII. Generation of an antiserum for the detection of diflunisal-modified proteins in diflunisal-dosed rats

Acyl glucuronide metabolites of carboxylic drugs such as the salicylate derivative diflunisal (DF) have been shown to react with proteins to produce covalent adducts. To aid in the study of the formation and distribution of these adducts in both humans and rats, we raised an antiserum against human serum albumin modified by covalent attachment of DF via an amide bond, using a carbodiimide reagent. This antiserum had wide reactivity, reacting with all types of DF-modified proteins tested and with free DF (albeit at a lower affinity). It did not cross-react with other salicylates or other non-steroidal anti-inflammatory drugs. The antiserum has been used in immunoblotting to detect proteins covalently modified by DF in the plasma and livers of rats treated with the drug for 7 days. Although some cross-reactivity was apparent on the blots, a series of DF-modified proteins was found in cytosolic, mitochondrial and mixed membrane fractions of hepatocytes, with molecular weights ranging from 28 to 130 kDa.

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.

Studies on the reactivity of acyl glucuronides--VI. Modulation of reversible and covalent interaction of diflunisal acyl glucuronide and its isomers with human plasma protein in vitro

Acyl glucuronide conjugates are chemically reactive metabolites which can undergo hydrolysis, rearrangement (isomerization via acyl migration) and covalent binding reactions with protein. The present study was undertaken to identify factors modulating the reactivity of diflunisal acyl glucuronide (DAG) with human serum albumin (HSA) in vitro, by comprehensively evaluating the interplay of the three pathways above when DAG and a mixture of its 2-, 3- and 4-isomers (iso-DAG) were incubated with protein. Buffer, plasma, fraction V HSA, fatty acid-free HSA, globulin-free HSA and fatty acid- and globulin-free HSA were investigated at pH 7.4 and 37 degrees, each in the absence and presence of warfarin, diazepam and diflunisal (DF) as reversible binding competitors. DAG and iso-DAG were highly reversibly bound (ca. 98-99.5%) in plasma and HSA solutions. The binding was primarily at the benzodiazepine site, since displacement occurred in the presence of diazepam and fatty acids but not warfarin. DAG degradation, via rearrangement, hydrolysis and covalent adduct formation (in that order of quantitative importance), was retarded in plasma and HSA solutions compared to buffer. The protective effect of protein was afforded by the high reversible binding to the (non-catalytic) benzodiazepine site. The warfarin site appeared to be catalytic for DAG hydrolysis, whereas rearrangement appeared to be hydroxide ion-catalysed only. In contrast to DAG, iso-DAG degradation was greatly accelerated in the presence of protein, through both covalent binding and catalysis of hydrolysis. Covalent binding via DAG was increased in the presence of warfarin but decreased in the presence of diazepam, DF and fatty acids. The opposite effects were found for covalent binding via iso-DAG. The data suggest that covalent binding of DF to HSA via DAG and iso-DAG occurs by different mechanisms (presumably transacylation and glycation, respectively) at different sites (benzodiazepine and warfarin, respectively) whereas reversible binding occurs primarily at the same site (benzodiazepine).

Studies on the reactivity of acyl glucuronides--V. Glucuronide-derived covalent binding of diflunisal to bladder tissue of rats and its modulation by urinary pH and beta-glucuronidase

Acyl glucuronide conjugates of acidic drugs have been shown to be reactive metabolites capable of undergoing non-enzymic hydrolysis, rearrangement (isomerization via acyl migration) and covalent binding reactions with plasma protein. In an earlier study (King and Dickinson, Biochem Pharmacol 45: 1043-1047, 1993), we documented formation of covalent adducts of diflunisal (DF), a salicylate derivative which is metabolized in part to a reactive acyl glucuronide (DAG), with liver, kidney, skeletal muscle and small and large intestine (in addition to plasma protein) of rats given the drug i.v. twice daily at 50 mg DF/kg for 7 days. The present study shows that covalent adducts of DF were also formed with urinary bladder tissue of these rats, achieving concentrations (ca. 5 micrograms DF equivalents/g tissue) higher than those found in the other tissues noted above. After cessation of dosing, the adduct concentrations declined with an apparent T 1/2 value of ca. 20 hr. Adducts were also formed ex vivo in excised rat bladders in which DAG or a prepared mixture of its acyl migration isomers (iso-DAG) were incubated at pH 5.0, 6.5 and 8.0. After 8 hr incubation, the highest concentrations (ca. 11 micrograms DF equivalents/g) were produced with iso-DAG at pH 5.0, and the lowest (ca. 2.3 micrograms DF equivalents/g) with DAG at pH 5.0. However, a major competing reaction for DAG (at least at pH 5.0) was hydrolysis by beta-glucuronidases originating from bladder tissue. By contrast, iso-DAG was quite resistant to such hydrolysis. The phenolic glucuronide conjugate, another important metabolite of DF, was hydrolysed only slowly. Similar results were obtained in fresh rat urine adjusted to pH 5.0. The results support covalent DF adduct formation in rat bladder originating from both DAG and iso-DAG as ultimate reactants, though the extent of binding is modulated by both urinary pH and beta-glucuronidases.

Studies on the renal excretion of the acyl glucuronide, phenolic glucuronide and sulphate conjugates of diflunisal

1. In five healthy male volunteers given multiple doses of diflunisal (DF), renal clearances (CLR) of the acyl glucuronide (DAG), phenolic glucuronide (DPG) and sulphate (DS) conjugates were about 42, 25 and 13 ml min-1, respectively. 2. These relatively low CLR values are probably due largely to the very high plasma protein binding of the conjugates, found in vitro to be 99.0%, 97.8% and 99.45%, respectively. 3. Thus glomerular filtration plays the minor and active tubular secretion the major role in renal excretion of the three conjugates. 4. This conclusion was supported by the effect of probenecid co-administration, which decreased CLR of DAG and DPG by about 70%. CLR for DS could not be calculated when probenecid was co-administered (because of interference by probenecid metabolites in the analysis of DS in urine). 5. Water-induced diuresis had no effect on CLR of the DF conjugates, consistent with tubular reabsorption being negligible.

Studies on the reactivity of acyl glucuronides--IV. Covalent binding of diflunisal to tissues of the rat

Acyl glucuronides have been shown to be reactive electrophilic metabolites capable of undergoing hydrolysis, rearrangement (isomerization via acyl migration) and covalent binding reactions to plasma protein. The present study was undertaken to explore the occurrence and extent of in vivo formation of covalent adducts of diflunisal (DF), a salicylate derivative which forms a reactive acyl glucuronide, with tissues and plasma protein of rats. Groups of rats were given 50 mg DF/kg i.v. twice daily for periods of up to 7 days. Steady state plasma concentrations of reversibly bound DF and its conjugates (as measured 6 hr after a dose) were achieved by the third day of dosing. T 1/2 values after cessation of dosing were about 5-10 hr. By contrast, covalent DF-tissue adducts steadily accumulated over the 7-day dosing period. Maximum concentrations, measured 6 hr after the last dose, were 4.8 (liver), 1.0 (kidney), 0.74 (plasma), 0.26 (small intestine minus contents), 0.27 (large intestine minus contents) and 0.20 (skeletal muscle) microgram DF/g tissue or/mL plasma. T 1/2 values of about 50, 67, 18, 38 and 43 hr were obtained for liver, kidney, plasma and small and large intestine (respectively) after cessation of dosing. Thus, the study of acyl glucuronide reactivity and the question of any derived toxicity or immune responses should consider the formation of long-lived adducts in tissues as well as in plasma.

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.

Diflunisal and its conjugates in patients with renal failure

Six patients with renal failure were given a single oral dose (250 mg) of diflunisal. In contrast to the acyl glucuronide, the phenolic glucuronide and sulphate conjugates showed the capacity to accumulate in plasma, suggesting that systemic instability of the acyl glucuronide contributes, via hydrolysis, to plasma concentrations of diflunisal itself. Although earlier studies in renal failure patients have almost certainly underestimated diflunisal clearance (by overestimation of plasma diflunisal concentrations through unrecognized acidic hydrolysis of diflunisal sulphate during analysis), the present results suggest that the reported decrease in clearance was not attributable only to this analytical artifact.