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.

Studies on the reactivity of acyl glucuronides--II. Interaction of diflunisal acyl glucuronide and its isomers with human serum albumin in vitro

A major metabolite of diflunisal (DF) is its reactive acyl glucuronide conjugate (DAG) which can undergo hydrolysis (regeneration of DF), intramolecular rearrangement (isomerization via acyl migration) and intermolecular reactions with nucleophiles. We have compared the fate of DAG and its individual 2-, 3- and 4-O-acyl positional isomers (at ca. 55 micrograms DF equivalents/mL) after incubation with human serum albumin (HSA, 40 mg/mL) at pH 7.4 and 37 degrees. Initial half-lives (T1/2) for DAG and its 2-, 3- and 4-isomers were 53, 75, 61 and 26 min, respectively. DAG was more labile to hydrolysis than any of its isomers but the latter, in particular the 4-isomer, were much better substrates for formation of covalent DF-HSA adducts. After a 2-hr incubation, 2.4, 8.2, 13.7 and 36.6% of substrate DAG and its 2-, 3- and 4-isomers (respectively) were present as DF-HSA adducts. With long term incubation, the concentrations of adducts so generated in situ declined in a biphasic manner, with apparent terminal T1/2 values of ca. 28 days. DAG was much more labile to transacylation with methanol (i.e. formation of DF methyl ester) than an equimolar mixture of its isomers after incubation in a 1:1 methanol:pH 7.4 buffer solution at 37 degrees (T1/2 values of 5 and 70 min, respectively). The data do not support direct transacylation with nucleophilic groups on protein as the predominant mechanism of formation of covalent DF-HSA adducts in vitro.

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

1. The disposition of diflunisal (DF) was investigated in bile-exteriorized and intact homozygous Gunn rats given 10 and 50 mg/kg doses i.v. and in Wistar rats given 10 mg/kg doses i.v. 2. In Gunn rats, DF sulphate, DF acyl glucuronide, and a hitherto unidentified metabolite of DF, a conjugate of 3-hydroxy-DF, were identified as the major metabolites, accounting for approximately 37%, 16% and 11% respectively of 10 mg/kg doses and 35%, 24% and 15% respectively of 50 mg/kg doses in bile-exteriorized animals. There was no evidence for formation of DF phenolic glucuronide. 3. Total plasma clearance of DF and formation clearances of DF to DF sulphate and 3-hydroxy-DF were little affected by increase of dose from 10 to 50 mg DF/kg, whereas formation clearance of DF to DF acyl glucuronide was increased, but not significantly. 4. In Gunn rats with undisturbed bile flow into the gut, recoveries of DF sulphate and total 3-hydroxy-DF in urine increased to approximately 48% and 25% dose respectively at the expense of DF acyl glucuronide through enterohepatic recirculation. 5. In bile-exteriorized Wistar rats, DF phenolic glucuronide, DF acyl glucuronide, DF sulphate and 3-hydroxy-DF accounted for 16%, 27%, 14% and 2%, respectively, of 10 mg/kg doses. In intact Wistar rats, urinary recoveries of the metabolites were 15%, 13%, 23% and 5%, respectively. 6. Thus in comparison to Wistar rats, phenolic glucuronidation of DF was absent or negligible in homozygous Gunn rats, acyl glucuronidation was significantly decreased, sulphation was unchanged, and the 3-hydroxylation of DF was significantly enhanced.

Noninvasive approaches to tracing pathways in carbohydrate metabolism

Compounds that can be given safely in large quantity, conjugate with intermediates of carbohydrate metabolism in liver, and are excreted, allow large amounts of those intermediates to be isolated noninvasively. By administering labeled compounds that form those intermediates and determining the amount and/or distribution of label in those intermediates, the metabolism of those compounds can be traced. Thus, glucuronide formation has been used to sample hepatic uridine diphosphate glucose (UDP-glucose) and study glycogen metabolism and the pentose pathway, phenylacetate to sample hepatic alpha-ketoglutarate and estimate relative flux through the Krebs cycle, and acetylation to sample hepatic acetyl CoA. Interpretations require knowledge of the anatomical sites of formation of the intermediates, since more than one pool of an intermediate can exist in liver. The extent the labeled compound is metabolized in extrahepatic tissues also must be taken into account.

Contrasting systemic stabilities of the acyl and phenolic glucuronides of diflunisal in the rat

1. Diflunisal (DF) is metabolized in humans and rats primarily to its acyl glucuronide, phenolic glucuronide and sulphate conjugates. 2. After i.v. administration of DF acyl glucuronide to pentobarbitone-anaesthetized rats, DF and its phenolic glucuronide and sulphate conjugates appeared rapidly in plasma, indicating ready systemic hydrolysis of the acyl glucuronide and subsequent biotransformation of liberated DF. 3. Approximately 72% of the acyl glucuronide dose was recovered in bile and urine over 6 h: 52% as acyl glucuronide, 6% as phenolic glucuronide, 5% as sulphate, and 8% as isomers of the acyl glucuronide arising from intramolecular acyl migration. 4. Blockage of excretion routes by ligation of the ureters, bile duct, and both ureters and bile duct, decreased plasma clearance of the acyl glucuronide from 7.8 ml/min per kg to 6.0, 3.2 and 2.2 ml/min per kg respectively, and increased the apparent terminal plasma half-life of DF from 2.1 h to 2.6, 3.4 and 6.3 h, respectively. 5. By contrast, DF phenolic glucuronide was quite stable after i.v. administration at the same dose. 6. This study shows that systemic cycling between DF and its acyl glucuronide exists in the rat in vivo, with portions of each cycle of unstable acyl glucuronide through DF yielding stable phenolic glucuronide and (presumptively stable) sulphate conjugate.

The effect of multiple dosage on the kinetics of glucuronidation and sulphation of diflunisal in man

1. The single (250 and 500 mg) and multiple dose (250 and 500 mg twice daily for 15 days) pharmacokinetics of diflunisal were compared in young volunteers. 2. The plasma clearance of diflunisal was lowered significantly after multiple dose administration (5.2 +/- 1.2 and 4.2 +/- 0.7 ml min-1 for the 250 and 500 mg twice daily regimens, respectively) as compared with single dose administration 11.4 +/- 3.1 and 9.9 +/- 2.0 ml min-1 for the 250 and 500 mg single doses, respectively). 3. The partial metabolic clearances of diflunisal by acyl and phenolic glucuronide formation were lowered significantly (greater than 50%) after multiple dose administration. 4. The urinary recovery of diflunisal sulphate increased as a function of dose: 6.1 +/- 2.8 and 9.1 +/- 3.5% following the 250 and 500 mg single dose, respectively, and 10.9 +/- 3.1 and 15.9 +/- 3.6% following the 250 and 500 mg twice daily regimens. The partial metabolic clearance of diflunisal by sulphate conjugation was unchanged following multiple dose administration. 5. The plasma protein binding of diflunisal was concentration-dependent. Analysis of unbound plasma clearances of diflunisal showed that its total plasma clearance following 500 mg twice daily was affected by both saturable glucuronidation and concentration-dependent plasma binding.

The effect of diethyl ether, pentobarbitone and urethane anaesthesia on diflunisal conjugation and disposition in rats

1. The disposition of diflunisal (DF) at 10 mg/kg i.v. was investigated over 4 h in bile-exteriorized male rats continuously anaesthetized with (a) diethyl ether inhalation (as required), (b) pentobarbitone sodium i.p. (55 mg/kg initially), (c) urethane i.p. (1500 mg/kg initially) or (d) urethane i.v. (750 mg/kg initially), and compared to that obtained in conscious rats. 2. Diethyl ether decreased the plasma clearance of DF to about 30% of control values, by inhibition of both glucuronidation and sulphation of DF. 3. Pentobarbitone anaesthesia caused only modest inhibition of DF elimination, with plasma clearance decreased to about 80% of control values. 4. Plasma profiles and biliary recovery of DF and its conjugates were little altered by urethane i.p. anaesthesia, but urinary recovery was low and variable because of the nearanuria produced by urethane via this administration route. 5. Urinary recovery of DF and its conjugates was satisfactory in rats given urethane i.v., but tissue distribution of DF was substantially decreased. 6. Pentobarbitone was considered to interfere least with DF disposition at the 10 mg/kg dose, and was selected as the most suitable anaesthetic agent for ongoing studies of disposition of DF and its conjugates in anaesthetized rats.

Sex-difference and the effects of smoking and oral contraceptive steroids on the kinetics of diflunisal

The single dose pharmacokinetics of diflunisal were studied in 4 groups of 6 young volunteers: control men, control women, women taking low estrogen oral contraceptive steroids (OCS), and women smokers (10-20 cigarettes day). The plasma clearance of diflunisal was significantly higher in men (0.169 ml.min-1.kg-1) and in women on OCS (0.165 ml.min-1.kg-1) as compared to control women (0.108 ml.min-1.kg-1). Partial metabolic clearances of diflunisal by the three conjugative pathways (phenolic and acyl glucuronide formation, sulphate conjugation) were all increased in men and women OCS users as compared to control women. Statistically significant increases, however, were only observed for the partial metabolic clearance of diflunisal by phenolic glucuronidation between men and women (2.91 vs. 1.85 ml.min-1 respectively), and for the partial clearance by acyl glucuronidation between OCS users and control women (4.81 vs. 3.01 ml.min-1 respectively). Smoking resulted in a moderate increase (35%) in plasma diflunisal clearance. However, a significant reduction in total urinary recovery of diflunisal and its glucuronide and sulphate conjugates was found in smokers (70.5% in smokers as compared to 84.2-87.2% in the 3 other study groups). Consequently, smoking may have induced hydroxylation, a minor oxidative metabolic pathway of diflunisal recently discovered in man.

Elimination of diflunisal as its acyl glucuronide, phenolic glucuronide and sulfate conjugates in bile-exteriorized and intact rats

1. The disposition of diflunisal (DF) was investigated in both bile-exteriorized and intact rats given 10 and 100 mg/kg doses intravenously (i.v.). 2. In addition to the phenolic glucuronide (DPG) and acyl glucuronide (DAG) conjugates, the sulfate conjugate (DS) was found to be a major metabolite. The glucuronides were excreted preferentially in bile, whereas DS was excreted almost exclusively in urine. 3. In bile-exteriorized animals, recoveries of DPG, DAG and DS in bile were 12.2%, 23.8%, 0.4%, respectively, and in urine, 10.3%, 5.6% and 15.2%, respectively, at the 10 mg/kg dose; and in bile, 11.3%, 41.6% and 1.0% respectively, and urine 2.9%, 1.1% and 17.0%, respectively, at the 100 mg/kg dose. 4. Total plasma clearance of DF and formation clearance of DF to DPG were reduced at the higher dose, suggesting saturation of this glucuronidation pathway. Formation clearances of DF to DAG and DS were little affected by the dose change. 5. Considerable enterohepatic recirculation of DF was apparent from the prolongation of DF and its conjugates in plasma of rats with an intact bile flow into the gut. The net metabolic effect of such cycling was enhancement of overall DS formation, from 15.6% and 18.0% of the 10 and 100 mg/kg doses, respectively, in bile-exteriorized rats to 28.5% and 42.1% of the doses respectively, in the intact animals.

Reactivity considerations in the analysis of glucuronide and sulfate conjugates of diflunisal

Reactivity of glucuronide and sulfate conjugates was taken into account in development of a simple isocratic HPLC method for direct assay of diflunisal (DF) and its acyl glucuronide (DAG), phenolic glucuronide (DPG), and sulfate (DS) conjugates. Whereas DPG was stable over the pH range 0-9, DAG was highly labile at neutral to slightly alkaline pH, undergoing rearrangement (isomerisation via acyl migration), hydrolysis, and in the presence of methanol, transesterification to DF methyl ester. The 2-, 3-, and 4-O-acyl positional isomers of DAG appeared as three pairs of peaks. Interconversion between partners of each pair occurred even under acidic conditions inhibitory to acyl migration, implicating mutarotation. DS was stable at neural to slightly alkaline pH, but underwent hydrolysis under relatively strongly acidic conditions. However, this hydrolysis was remarkably catalyzed (e.g., by 1,000-fold) in the presence of solvents (i.e., solvolysis) such as diethyl ether and ethyl acetate. DS (an acid) could not be extracted from aqueous solution because of this acidic solvolysis. Suitable conditions for simultaneous direct analysis (nonextractive, nonconcentrative) of DF and its reactive (DAG and DS) and unreactive (DPG) conjugates were achieved by working at pH of approximately 4.5. The procedure thus developed is suitable for plasma, urine, and bile samples, and has revealed the presence of new, as yet unidentified, metabolites of DF.

Pentose pathway in human liver

[1-14C]Ribose and [2-14C]glucose were given to normal subjects along with glucose loads (1 g per kg of body weight) after administration of diflunisal and acetaminophen, drugs that are excreted in urine as glucuronides. Distributions of 14C were determined in the carbons of the excreted glucuronides and in the glucose from blood samples drawn from hepatic veins before and after glucagon administration. Eighty percent or more of the 14C from [1-14C]ribose incorporated into the glucuronic acid moiety of the glucuronides was in carbons 1 and 3, with less than 8% in carbon 2. In glucuronic acid from glucuronide excreted when [2-14C]glucose was given, 3.5-8.1% of the 14C was in carbon 1, 2.5-4.3% in carbon 3, and more than 70% in carbon 2. These distributions are in accord with the glucuronides sampling the glucose unit of the glucose 6-phosphate pool that is a component of the pentose pathway and is intermediate in glycogen formation. It is concluded that the glucuronic acid conjugates of the drugs can serve as a noninvasive means of sampling hepatic glucose 6-phosphate. In human liver, as in animal liver, the classical pentose pathway functions, not the L-type pathway, and only a small percentage of the glucose is metabolized via the pathway.

Biliary excretion of diflunisal conjugates in patients with T-tube drainage

The urinary and biliary excretion of diflunisal and its glucuronide and sulphate conjugates were studied in 10 patients following cholecystectomy. Total urinary excretion (0-24 h) was 36.6 +/- 16.4% of the 250 mg dose. Biliary excretion (0-24 h) was restricted to the phenolic and acyl glucuronides and accounted for 3.7 +/- 2.3% of the dose. An inverse relationship existed between urinary and biliary excretion of diflunisal and its conjugates. The data indicate that the reduced plasma clearance of diflunisal in patients with renal failure may, at least in part, be due to increased biliary excretion of diflunisal glucuronides followed by hydrolysis in the gut and reabsorption of diflunisal i.e. enterohepatic cycling.

Differential effects of phenobarbital on ester and ether glucuronidation of diflunisal in rats

The relative contribution of ether and ester glucuronidation to diflunisal metabolism was assessed by studying the effects of enzyme inducers, phenobarbital (PB), 3-methylcholanthrene (3-MC) and beta-naphthoflavone (BNF). Treatment with either PB, 3-MC or BNF increased markedly the unbound intrinsic clearance of diflunisal. Saline-treated control rats showed a greater unbound intrinsic clearance of diflunisal than oil-treated controls indicating that repetitive treatment with oil had an effect on enzyme activity. Treatment with 3-MC and BNF appeared to cause a decrease in the biliary clearance of ether and ester glucuronide, but PB had little effect on the biliary clearance of glucuronides. Rats pretreated with PB showed a 3-fold increase in the fractional metabolite formation clearance of ether glucuronide and a 2-fold increase in the fractional metabolite formation clearance of ester glucuronide, suggesting differential effects of PB on ester and ether glucuronidation. A similar trend, but to a smaller extent, was also observed for 3-MC- and BNF-treated rats. These results suggest the possibility of selective induction of multiple forms of UDP-glucuronyltransferase involved in metabolism of diflunisal.

A diflunisal related fatality: a case report

A case is reported where the death of an individual resulted from the ingestion of diflunisal. Diflunisal was identified by a combination of liquid chromatography, UV spectrophotometry and colorimetry. Diflunisal was quantified in blood (260 mg/l), bile (71 mg/l), kidney (350 mg/kg), liver (400 mg/kg), stomach contents (34 mg) and urine (78 mg/l). No previous literature references discussing diflunisal related fatalities were available.

Evaluation of the teratogenicity and pharmacokinetics of diflunisal in cynomolgus monkeys

This study examined the pharmacokinetics and potential teratogenicity of the nonsteroidal antiinflammatory drug, diflunisal, in cynomolgus monkeys. Pregnant cynomolgus monkeys were administered 0.5% methyl cellulose, 20 mg/kg/day diflunisal, or 80 mg/kg/day diflunisal on Days 25 to 48 of gestation. There was no evidence of maternal toxicity, increased abortion rate, fetal growth retardation, or malformation. These data demonstrate that diflunisal is not teratogenic in cynomolgus monkeys over a dosage range of 20 to 80 mg/kg/day. Peak plasma levels of diflunisal were found 1 hr after oral administration of [14C]diflunisal at a dosage of 60 mg/kg and declined to low levels by 24 hr. The plasma elimination half-life was calculated to be 10.2 hr over the period of 1 to 8 hr postadministration. Intact diflunisal accounted for 96.4% of total plasma radioactivity at 0.5 hr and declined to a value of 74% at 8 hr. Plasma protein binding averaged greater than 99% over a concentration range of 62.5 to 250 micrograms/ml. Urinary excretion of diflunisal and metabolites averaged 66.5% of the dosage over the first 4 days postadministration, compared with 0.8% in the feces. The majority of activity represented conjugates of diflunisal. Embryo concentrations of diflunisal on Days 35 to 37 of gestation were 0.7 and 1.1% of maternal plasma level at 4 hr postadministration of 20 or 60 mg/kg, respectively.

Isolation and identification of a new major metabolite of diflunisal in man. The sulfate conjugate

A new metabolite of diflunisal has been identified in volunteers and patients after multiple dose administration. The metabolite was isolated from human urine by silica gel chromatography and was further purified by reversed phase HPLC. Arylsulfatase from Helix pomatia and from Aerobacter aerogenes completely hydrolyzed the isolated metabolite to diflunisal, although hydrolysis by bacterial arylsulfatase was extremely slow. Electron impact mass spectra for diflunisal and its sulfate conjugate were virtually identical. Negative ion fast atom bombardment mass spectra clearly showed the quasimolecular ion [M-H]- at m/z 329 (base peak) as well as a large fragment ion (90% relative intensity) at m/z 249 corresponding to the loss of the sulfate moiety. Urinary excretion patterns in volunteers and rheumatoid arthritis patients revealed that sulfate conjugation of diflunisal is a minor metabolic pathway after single 500-mg dose administration (less than 10% of the dose), whereas it becomes a major pathway (21.3-44.3% of the dose) following multiple doses (500 mg b.i.d.). In one volunteer, who ingested 500 mg diflunisal b.i.d. for 5 weeks, it was shown that the percentage of the dose excreted as diflunisal sulfate gradually increased during the first week to approximately 30% and stayed virtually unchanged for the remaining 4 weeks of diflunisal intake. These preliminary observations are not compatible with the idea that sulfate conjugation is capacity-limited at lower substrate concentrations than glucuronide conjugation, nor do they suggest that sulfation of diflunisal is rate-limited by depletion of inorganic sulfate body stores.

Dose-dependent pharmacokinetics of diflunisal in rats: dual effects of protein binding and metabolism

The purpose of this study was to define the dual effects of saturable metabolism and saturable protein binding on the pharmacokinetics of diflunisal. Steady-state diflunisal concentration and its unbound fraction were examined in seven groups of rats to determine the relationships of infusion rate, concentration and total and unbound clearances. The total body plasma clearance decreased initially and then went up as the concentration of diflunisal increased, whereas the intrinsic clearance of unbound drug decreased with increasing concentration. The former is a consequence of saturable metabolism as well as saturable protein binding; the latter is a consequence of saturable metabolism. The fraction of unbound diflunisal increased with concentration. The biliary excretion data of ester and ether glucuronide suggested that both the ester and ether glucuronidation processes are capacity-limited, although the enzyme system for ether glucuronide has a lower Km and capacity than the system responsible for the ester glucuronidation.

Multiple cobinding of two ligands to serum albumin: a stoichiometric description of binding equilibria

Binding equilibria for simultaneous binding of several molecules of two anionic ligands, sulfamethizole/warfarin in one series and sulfamethizole/diflunisal in another, to human serum albumin were studied by equilibrium dialysis. It was found that Klotz's stepwise binding equilibrium concept, extended to cover interaction of two ligands with one carrier, could be used for a quantitative description of binding equilibria. Reciprocity of ligand effects was established at all levels. Heterotropic anticooperativity was present among these pairs of ligands. The experiments were supplemented with observations of albumin binding equilibria for traces of warfarin in the presence of varying amount of oleate, up to 6 mol/mol albumin, by measuring dialysis rates for unbound warfarin. Binding of warfarin to albumin is enhanced upon binding of oleate up to 4 mol/mol albumin, and decreases at higher oleate concentrations. Using stoichiometric (stepwise) binding constants for oleate previously published by Ashbrook et al. [(1975) J. Biol. Chem. 250, 2333-2338], the reverse effect, of warfarin on binding of oleate, was calculated. Simultaneous binding of these ligands to albumin could be described according to the stoichiometric principles as used above for sulfamethizole/warfarin and sulfamethizole/diflunisal.

Effect of enterohepatic circulation on the pharmacokinetics of diflunisal in rats

The purpose of this study was to examine the effect of enterohepatic circulation on the pharmacokinetics of diflunisal in rats. The "linked animals" experiments provided evidence that diflunisal exhibits an enterohepatic circulation. Within 26 hr after iv administration of diflunisal (10 mg/kg) to rats, excretion was as follows: 42.2% of the dose, bile; 2.3%, unchanged drug; 27.8%, ester glucuronide; and 12.1%, ether glucuronide. On the average, approximately 65% of the amount of the drug and its glucuronides excreted in bile was reabsorbed from the gut. Biliary excretion and plasma data showed that biotransformation of diflunisal to its glucuronides is the rate-limiting step in their elimination. A concentration-dependent decrease in the partial formation clearance to ester glucuronide was observed with decreased concentration of diflunisal. These concentration-dependent kinetics can be at least partly explained by the nonlinear protein binding of diflunisal.

Diflunisal-induced maternal anemia as a cause of teratogenicity in rabbits

Diflunisal [5-(2,4-difluorophenyl)-salicylic acid] is a new analgesic antiinflammatory drug that, when administered orally to rabbits at 40 and 60 mg/kg/day, caused terata, most commonly axial skeletal defects. These same dosage levels also caused a severe maternal hemolytic anemia following a dramatic decrease in erythrocyte ATP levels. The teratogenicity, anemia, and depletion of ATP were unique to the rabbit among species examined. To test the possible causality between the teratogenic effects and anemia induced by diflunisal, a single dose of 180 mg/kg diflunisal was administered to rabbits on gestation day 5. This treatment produced an anemia that persisted through gestation day 15 in addition to causing the characteristic axial skeletal defects. Since diflunisal was cleared from maternal blood before gestation day 9, the critical day for induction of similar axial skeletal defects by hypoxia, the skeletal malformations probably resulted from maternal hypoxia secondary to anemia and not from a direct and specific effect of the drug on the embryo. In addition, we observed that the diflunisal level in the embryo was less than 5% of the peak maternal blood level probably as a result of high plasma protein binding of diflunisal in the maternal blood (greater than 98%). This relatively low placental transfer may explain the lack of diflunisal teratogenicity in rats and mice compared to aspirin which crosses the placenta more readily. These studies demonstrate that a species that exhibits unusually severe drug-specific maternotoxicity is probably an unsuitable model for the prediction of the teratogenic potential of that drug in humans.

Glucuronidation and elimination of diflunisal in the isolated perfused rat liver: effect of pretreatment with phenobarbitone, clofibric acid and spironolactone

The effects of pretreatment of rats with phenobarbitone, clofibric acid and spironolactone on the metabolism and biliary excretion of the salicylate derivative diflunisal have been studied using the isolated perfused liver. The clearance of diflunisal was increased significantly by pretreatment with each of the drugs. Biliary excretion of diflunisal acyl and phenolic glucuronides followed apparent first-order and Michaelis-Menten kinetics, respectively. Pretreatment with clofibric acid or spironolactone resulted in a significant increase in the biliary excretion of the phenolic glucuronide. Pretreatment with phenobarbitone or spironolactone enhanced biliary excretion of the acyl glucuronide, particularly during the first 2 hr. The concentration of the phenolic glucuronide in the perfusate increased steadily during the 6 hr perfusions, whereas the acyl glucuronide concentration was relatively stable, reflecting the ease of hydrolysis of acyl glucuronides under physiological conditions.

Diflunisal disposition and hypouricemic response in osteoarthritis

The disposition of diflunisal was studied at daily doses of 250, 500, 750, 1000 mg/day in 24 male patients (mean age 65 yr and mean creatinine clearance 72 ml/min). Each dose was given for 14 days and diflunisal apparent oral clearance and serum urate was measured on the last day of each dosing regimen. There was a dose-dependent decrease in mean diflunisal apparent oral clearance with dose from 628 ml/hr at 250 mg/day to 426 ml/hr at 1000 mg/day, with most of the decrease occurring at the lower doses and becoming less pronounced at doses of 750 and 1000 mg/day. There was a strong positive correlation between diflunisal apparent oral clearance and creatinine clearance. Diflunisal induced a hypouricemic effect at all doses, but the responses at doses of 750 and 1000 mg/day did not differ.