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

Drug structure-transport relationships

Malcolm Rowland has greatly facilitated an understanding of drug structure–pharmacokinetic relationships using a physiological perspective. His view points, covering a wide range of activities, have impacted on my own work and on my appreciation and understanding of our science. This overview summarises some of our parallel activities, beginning with Malcolm’s work on the pH control of amphetamine excretion, his work on the disposition of aspirin and on the application of clearance concepts in describing the disposition of lidocaine. Malcolm also spent a considerable amount of time developing principles that define solute structure and transport/pharmacokinetic relationships using in situ organ studies, which he then extended to involve the whole body. Together, we developed a physiological approach to studying hepatic clearance, introducing the convection–dispersion model in which there was a spread in blood transit times through the liver accompanied by permeation into hepatocytes and removal by metabolism or excretion into the bile. With a range of colleagues, we then further developed the model and applied it to various organs in the body. One of Malcolm’s special interests was in being able to apply this knowledge, together with an understanding of physiological differences in scaling up pharmacokinetics from animals to man. The description of his many other activities, such as the development of clearance concepts, application of pharmacokinetics to the clinical situation and using pharmacokinetics to develop new compounds and delivery systems, has been left to others.

Hepatic disposition and metabolite kinetics of a homologous series of diflunisal esters

The hepatic disposition and metabolite kinetics of a homologous series of diflunisal O-acyl esters (acetyl, butanoyl, pentanoyl, and hexanoyl) were determined using a single-pass perfused in situ rat liver preparation. The experiments were conducted using 2% BSA Krebs-Henseleit buffer (pH 7.4), and perfusions were performed at 30 mL/min in each liver. O-Acyl esters of diflunisal and pregenerated diflunisal were injected separately into the portal vein. The venous outflow samples containing the esters and metabolite diflunisal were analyzed by high performance liquid chromatography (HPLC). The normalized outflow concentration-time profiles for each parent ester and the formed metabolite, diflunisal, were analyzed using statistical moments analysis and the two-compartment dispersion model. Data (presented as mean +/- standard error for triplicate experiments) was compared using ANOVA repeated measures, significance level P < 0.05. The hepatic availability (AUC'), the fraction of the injected dose recovered in the outflowing perfusate, for O-acetyldiflunisal (C2D = 0.21 +/- 0.03) was significantly lower than the other esters (0.34-0.38). However, RN/fu, the removal efficiency number RN divided by the unbound fraction in perfusate fu, which represents the removal efficiency of unbound ester by the liver, was significantly higher for the most lipophilic ester (O-hexanoyldiflunisal, C6D = 16.50 +/- 0.22) compared to the other members of the series (9.57 to 11.17). The most lipophilic ester, C6D, had the largest permeability surface area (PS) product (94.52 +/- 38.20 mL min-1 g-1 liver) and tissue distribution value VT (35. 62 +/- 11.33 mL g-1 liver) in this series. The MTT of these O-acyl esters of diflunisal were not significantly different from one another. However, the metabolite diflunisal MTTs tended to increase with the increase in the parent ester lipophilicity (11.41 +/- 2.19 s for C2D to 38.63 +/- 9.81 s for C6D). The two-compartment dispersion model equations adequately described the outflow profiles for the parent esters and the metabolite diflunisal formed from the O-acyl esters of diflunisal in the liver.

Do the pharmacodynamics of the nonsteroidal anti-inflammatory drugs suggest a role in the management of postoperative pain?

Until recently, nonsteroidal anti-inflammatory drugs (NSAIDs) were regarded as weak analgesic agents with a potent antiplatelet effect that severely limited their perioperative usefulness. However, the recent development of injectable NSAIDs has stimulated a re-evaluation of the potential role of this class of drugs in postoperative pain management. In general surgery, NSAIDs have been shown to be effective analgesics when administered after surgery, as judged by either a reduction in pain scores and/or by an opioid sparing effect. Parenteral NSAIDs alone, notably ketorolac and diclofenac, may be adequate or even preferred analgesic agents after minor surgery. In dental surgery, NSAIDs produce greater initial analgesia than steroids, although the latter produce greater suppression of swelling and less functional loss. NSAID pretreatment results in only modest suppression of swelling compared with placebo. These data suggest that the acute analgesic effects of NSAIDs in oral surgery and probably other models result from suppression of a nociceptive process, rather than a generalised anti-inflammatory effect. This view challenges the traditional association between inhibition of prostaglandin synthesis and the therapeutic effects of these drugs. The variety of NSAIDs leads to a range in half-lives from short, e.g. diclofenac (1 h), intermediate, e.g. ketorolac (5h), to long, e.g. tenoxicam (60h), which has implications for both convenience of the dosage regimen and drug accumulation. For some racemic NSAIDs (e.g. ibuprofen), metabolic 'activation' of the inactive R-enantiomer to the active S-enantiomer occurs. Renal dysfunction may increase both the plasma concentration and body residence time of NSAIDs, thereby increasing the risk of adverse effects. The concomitant effects of anaesthesia have not yet been studied. The principal concern regarding the use of perioperative NSAIDs is the risk of decreased haemostasis and wound healing. Although it has been found that NSAIDs prolong bleeding times in patients, values generally remain below the upper limits of those in generally healthy patients. Healing of gastrointestinal anastomoses may be compromised by NSAID administration but corneal healing and bone remodelling are not. There is a need for further research into the potential for renal side effects with NSAIDs in the perioperative setting, where the effects of anaesthesia and surgery may increase the risk of side effects, particularly in elderly patients. The main benefits of NSAIDs derive from opioid sparing (e.g. reduction in perioperative nausea and vomiting and improvement in ventilation), although some studies allude to an enhanced quality of analgesia from the combination compared with either NSAID or opioid alone. The question of pre- vs postinjury treatment with NSAIDs remains unresolved.

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.

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.

Autoxidative injury with loss of cytochrome P-450 following acute exposure of rats to fasting and ether anaesthesia

1. Exposure of fasted rats (20 h) to ether anaesthesia for 4 min resulted in increased exhalation of alkanes, an indication of lipid peroxidation in vivo. 2. Liver and kidney of the fasted rats anaesthetized with ether showed immediate 4-fold increases in luminol-amplified chemiluminescence, reaching maxima 30 min later, indicating the production of reactive oxygen species. 3. Liver and kidney cytosols of the fasted anaesthetized rats similarly showed immediate 4-fold increases of thiobarbituric acid-reactive material (malondialdehyde and other lipid peroxidation breakdown products) which attained maxima 60 min later. 4. Total cytochromes P-450 of liver and kidney of rats were decreased to 25-30% of control values after 20 h fasting and 4 min of ether anaesthesia, but were restored to normal levels 2 h later. Cytochrome P450 I (EROD activity) was decreased to 35-44% of control values by the ether anaesthesia and was restored to 80% of normal levels 2 h later. 5. Diether ether is known to be metabolized by cytochrome P450 IIE1 which is induced by fasting and by diethyl ether, and is possibly involved in the observed radical production, lipid peroxidation, and loss of cytochromes P-450.

Effects of blockage of urine and/or bile flow on diflunisal conjugation and disposition in rats

1. The effects of surgical blockage of either or both of the urinary and biliary excretion routes on the elimination of diflunisal (DF) and its conjugates were investigated in pentobarbitone-anaesthetized rats given DF at 10 mg/kg i.v. 2. In control animals the acyl glucuronide and phenolic glucuronide conjugates were excreted predominantly in bile, whereas the sulphate conjugate was eliminated almost exclusively in urine. 3. Bilateral ureter ligation had little effect on DF elimination, except for accumulation of the sulphate conjugate in plasma. Compensatory biliary excretion did not occur. 4. Total plasma clearance of DF decreased from 1.01 to 0.68 ml/min per kg following bile duct ligation. Plasma concentrations and urinary excretion of the glucuronides were elevated. 5. In rats with blockage of both urinary and biliary excretion routes, total plasma clearance of DF decreased to 0.59 ml/min per kg. Both the sulphate and phenolic glucuronide conjugates accumulated in plasma, whereas the acyl glucuronide peaked at 30 min and then declined in parallel with DF. The latter result indicates systemic instability of DF acyl glucuronide with hydrolytic regeneration of DF as the likely major consequence.