Stereoselective systemic disposition of ibuprofen enantiomers in the dog

Treatment of ibuprofen intoxication with charcoal haemoperfusion in two dogs

Case history: Two dogs presented separately to the Small Animal Hospital, University of Florida (Gainsville, FL, USA) for ingestion of ibuprofen. The first dog ingested 561.8 mg/kg ibuprofen in addition to paracetamol and caffeine and vomited prior to admission. This patient also received fluid therapy for 8 hours prior to charcoal haemoperfusion. The second dog ingested 500 mg/kg of ibuprofen and the owners induced vomiting with hydrogen peroxide prior to presentation. Due to the severity of clinical signs, both patients were treated with charcoal haemoperfusion.Clinical findings: The concentrations of ibuprofen in the blood of the dogs prior to treatment were 478 and 301 mg/L. During the treatment ibuprofen concentrations were reduced by 95.8% and 45.5%, respectively, with no treatment side effects and minimal clinical signs after treatment.Diagnosis: Toxicity due to ingestion of ibuprofen toxicity that was successfully treated with charcoal haemoperfusion.Clinical relevance: In the cases described here minimal benefit was seen after 3 hours of treatment using one haemoperfusion cartridge. This is in contrast to a previously published report in which dogs were treated for 6 hours with two charcoal haemoperfusion cartridges. This suggests that one cartridge may be sufficient. The amount of ibuprofen ingested was not a reliable predictor of the concentration in blood at the initiation of treatment. Charcoal haemoperfusion is an effective means of reducing plasma concentrations of ibuprofen, however, its use may be limited by its cost and availability.

Stereoselective Pharmacokinetics and Chiral Inversion of Ibuprofen in Adjuvant-induced Arthritic Rats

2-Arylpropionic acid (2-APA) nonsteroidal anti-inflammatory drugs are commonly used in racemic mixtures (rac) for clinical use. 2-APA undergoes unidirectional chiral inversion of the in vivo inactive R-enantiomer to the active S-enantiomer. Inflammation causes the reduction of metabolic activities of drug-metabolizing enzymes such as cytochrome P450 (P450) and UDP-glucuronosyltransferase. However, it is unclear whether inflammation affects the stereoselective pharmacokinetics and chiral inversion of 2-APA such as ibuprofen (IB). We examined the effects of inflammation on the pharmacokinetics of R-IB and S-IB after intravenous administration of rac-IB, R-IB, and S-IB to adjuvant-induced arthritic (AA) rats, an animal model of inflammation. The plasma protein binding of rac-IB, glucuronidation activities for R-IB and S-IB, and P450 contents of liver microsomes in AA rats were determined. Total clearance (CL_tot) of IB significantly increased in AA rats, although the glucuronidation activities for IB, and P450 contents of liver microsomes decreased in AA rats. We presumed that the increased CL_tot of IB in AA rats was caused by the elevated plasma unbound fraction of IB due to decreased plasma albumin levels in AA rats. Notably, CL_tot of R-IB but not S-IB significantly increased in AA rats after intravenous administration of rac-IB. These results suggested that AA could affect drug efficacies after stereoselective changes in the pharmacokinetics of R-IB and S-IB.

Pharmacokinetic interaction of ibuprofen enantiomers in rabbits

The potential interaction between two ibuprofen enantiomers was studied after intravenous administration of R-(–)-, S-(+)- and racemic ibuprofen to rabbits. The total body clearance values calculated by compartmental model analysis (0.65+0.21 for R-(–)-ibuprofen and 0.63+0.34 for S-(+)-ibuprofen) after intravenous administration of the racemate of ibuprofen were significantly smaller than those of individual enantiomers (0.95+0.23 for R-(–)-ibuprofen and 1.03+0.23 for S-(+)-ibuprofen), indicating that the enantiomer–enantiomer interaction results in a mutual inhibition. The enantiomeric interaction in the pharmacokinetic behaviour of ibuprofen after racemic administration is considered to be a result of an alteration in the metabolic or excretion phase (or both) rather than stereoselective protein binding in the systemic distribution.

The oral bioavailability of ibuprofen enantiomers in broiler chickens

Doses of racemic ibuprofen ranging from 5 to 20 mg/kg body weight were administered intravenously (i.v.) and orally to broiler chickens and plasma concentration-time profiles for both ibuprofen enantiomers were determined. The absorption of ibuprofen was evaluated after a bolus administration of a commercially available suspension into the crop and proventriculus, respectively. An enterohepatic circulation as described for other nonsteroidal anti-inflammatory drugs (NSAIDs) in other species could be suggested for both enantiomers after i.v. and oral administration. Significantly higher area under the curve (AUC) values for S(+)-ibuprofen compared with R(-)-ibuprofen were collected after crop and proventriculus administration. Several factors could be responsible for the significant differences in AUC values between both enantiomers.

Validation of a high-performance liquid chromatographic method for the determination of ibuprofen enantiomers in plasma of broiler chickens

To characterise the pharmacokinetic properties of each enantiomer of ibuprofen in broiler chickens, a stereospecific HPLC method based on a alpha1-acid glycoprotein bonded chiral stationary phase has been validated. S-(+)-naproxen was used as internal standard. Enantiomers of ibuprofen and S-(+)-naproxen were baseline separated using a mobile phase consisting of 0.1 M phosphate buffer pH = 7 and 0.4% 2-propanol. The method is precise, specific, accurate and reproducible. Recoveries were higher than 80% and the limits of quantification for R-(-)- and S-(+)-ibuprofen were 1.16 and 1.37 microg ml(-1), respectively. The method seemed suitable for the pharmacokinetic studies of ibuprofen in chickens.

Bioinversion of ibuprofen enantiomers after administration in dogs: estimation of a novel index

This study compares the pharmacokinetics and bioinversion of two chemical forms of ibuprofen administered intravenously or orally. Dogs were given the free acid form of the S(+) isomer p.o. or i.v., or the racemate, as the free acid or sodium salt, p.o., in a cross-over design. The main kinetic parameters were calculated and formation and bioinversion curves plotted. The values of Cmax, Tmax and AUC were higher for the S(+) isomer. The percentage bioinversion averaged between 35-70% according to the form. This study proposes a new index for the calculation of bioinversion, independently of any i.v. administration, and confirms its self-limiting nature.

Stereoselective pharmacokinetics of ibuprofen and its lysinate from suppositories in rabbits

Studies were performed on the effect of ibuprofen racemate ionisation extent on the pharmacokinetics of its enantiomers following administration in suppositories to rabbits. The suppositories, containing 146.3 mg ibuprofen in acidic form (IBP) or 250 mg ibuprofen lysinate (IBPL), equivalent to the above IBP dose, were prepared using lipophilic Witepsol H-15 as a base and administered to rabbits in a crossover design. Compared with IBP, administration of IBPL was followed by faster absorption and elimination of R and S enantiomers. However, significant differences at alpha=0.05 were observed only at the stage of elimination. AUC was markedly higher following administration of suppositories containing IBP than following suppositories with IBPL and this pertained to both R and S enantiomers. Evident inversion of R into S form was noted 30 min following IBPL administration and 1 h after IBP administration. Ionisation extent only insignificantly affected the scope of chiral inversion of ibuprofen R into S form (AUC(S-IBP)/AUC(R-IBP)=1.66, AUC(S-IBPL)/AUC(R-IBPL)=1.57). No presystemic inversion of R into S was observed in rabbits following administration of IBP or IBPL in suppositories. IBP enantiomers were isolated from 0.5 ml serum using solid phase extraction in C(18) columns and were quantified by HPLC using the chiral Whelk O1 column and UV detector (lambda=264 nm).

Stereoselective disposition of suspensions of conventional and wax-matrix sustained release ibuprofen microspheres in rats

PURPOSE

The aim of the study is to evaluate stereoselective in vivo disposition of suspensions of conventional and wax-matrix sustained release ibuprofen microspheres in rats.

METHODS

Male wistar rats were dosed i.v. with 20 mg/kg, or orally with conventional suspension, and three suspension formulations of sustained release microspheres (having three different particle sizes) of racemic ibuprofen. Blood samples were analyzed stereoselectively by reverse phase HPLC.

RESULTS

The mean Cmax for (S)- and (R)-ibuprofen decreased with increased particle size of the drug or microspheres in the suspension dosage forms, while the Tmax increased with increased particle size. The mean S/R ratio (AUC0-48) of the suspensions decreased with increase in particle size of the drug or microspheres and these ratios (for both conventional and sustained formulations) were higher than that of the i.v., an indication of presystemic inversion. Decrease in the ratios with increased particle size is suggestive of formulation dependent inversion. The plasma concentration-time data of the sustained release formulations showed bimodal profiles, irrespective of the particle size of the microspheres. The second peak observed after 8 hours is indicative of colonic absorption.

CONCLUSIONS

Stereoselective disposition of ibuprofen microspheres showed higher bioavailability compared to the conventional suspension. Bimodal disposition is influenced by dosage form while presystemic inversion is both site-specific, and dosage form dependent.

Stereoselectivity and enantiomer-enantiomer interactions in the binding of ibuprofen to human serum albumin

Binding of ibuprofen (IB) enantiomers to human serum albumin (HSA) was studied using a chiral fluorescent derivatizing reagent, which enabled the measurement of IB enantiomers at a concentration as low 5 x 10(-8) M. Scatchard analyses revealed that there were two classes of binding sites for both enantiomers. For the high affinity site, the number of the binding sites was one for both enantiomers, and the binding constant of R-IB was 2.3-fold greater than that of S-IB. The difference in the affinity at the high affinity site may result in the stereoselective binding of IB enantiomers at therapeutic concentrations. It was confirmed that the high affinity site of IB enantiomers is Site II (diazepam binding site) by using site marker ligands. Also, significant enantiomer-enantiomer interactions were observed in the binding. The binding data were quantitatively analyzed and a binding model with an assumption of competitive interactions only at the high affinity site simulated the binding characteristics of IB enantiomers fairly well.

Stereoselective pharmacokinetics of ibuprofen in rats: effect of enantiomer-enantiomer interaction in plasma protein binding

Stereoselective pharmacokinetics of ibuprofen (IB) enantiomers were studied in rats. Unidirectional conversion from R-ibuprofen (R-IB) to S-ibuprofen (S-IB) was observed following intravenous administration. S-IB concentrations in plasma following racemate administration were simulated according to a conventional compartmental model using the parameters obtained after the administration of individual enantiomers, and resulted in overestimation of S-IB concentrations. Binding of IB enantiomers measured in rat plasma was stereoselective, the binding of R-IB being more favorable than that of S-IB. Moreover, there are interactions between IB enantiomers in binding, which may cause the increase of distribution volumes of IB enantiomers in the presence of their antipodes. Hence simulated S-IB concentrations according to a conventional compartment model were significantly greater than those observed. Indeed, when the enantiomer-enantiomer interactions were taken into account, simulation of S-IB concentrations in plasma following racemate administration was in good agreement with observed values. Therefore, interactions between stereoisomers as well as dispositional stereoselectivity have to be considered when pharmacokinetics of stereoisomers after administration of the racemate are compared to those after administration of individual isomers.

Stereoselective metabolic pathways of ketoprofen in the rat: incorporation into triacylglycerols and enantiomeric inversion

The enantiomeric bioinversion of ketoprofen (KP) enantiomers and their incorporation into triacylglycerols were investigated in the rat (1) in vitro, using liver homogenates, subcellular fractions, and hepatocytes, and (2) in vivo, in different tissue samples after oral administration of the radiolabelled compounds. In liver homogenates or subcellular fractions, the enantiomer (S)-ketoprofen (S-KP) was recovered unchanged, whereas (R)-ketoprofen (R-KP) was partially converted into its Coenzyme A (CoA) thioester and inverted to S-KP. Both processes occurred mainly in the mitochondrial fraction. This supports the mechanism of inversion via stereoselective formation of CoA thioester of R-KP, already described for other non-steroidal anti-inflammatory drugs. Incorporation into triacylglycerols was detected after incubation with intact hepatocytes in the presence of added glycerol. The process was stereoselective for R-KP vs. S-KP (covalently bound radioactivity 26,742 +/- 4,665 dpm/10(6) cells vs. 6,644 +/- 3,179 dpm/10(6) cells, respectively). However, no incorporation was found in liver samples after oral administration of either R-KP or S-KP. On the contrary, in adipose tissue samples a significant and stereoselective formation of hybrid triacylglycerols was observed: 11,076 +/- 2,790 dpm.g-1 for R-KP vs. 660 +/- 268 dpm.g-1 for S-KP. The incorporated R/S ratio, higher in adipose tissue (R/S = 17) than in hepatocytes (R/S = 4), indicates that fat may be the main tissue store for the xenobiotic R-KP in rats.

Racemization of ketorolac in aqueous solution

The racemization of ketorolac was studied in aqueous buffered solution at 25 and 80 degrees C and analyzed in detail with respect to the catalytic species in solution. The reaction has a U shaped pH rate profile at 80 degrees C with the pH of maximum stability occurring in the region of pH 3.0-7.5. A T90 value of 8 months was observed for a 1.5% (R)-ketorolac tromethamine solution at pH 7.4 and 25 degrees C. Additionally, the data shows that alternative salt forms are necessary in order to prepare a stable single isomer formulation. Alternative buffers, in particular phosphate buffer, provide formulations exhibiting a T90 greater than 2 years.

Nonsteroidal anti-inflammatory drugs in perisurgical pain management. Mechanisms of action and rationale for optimum use

Nonsteroidal anti-inflammatory drugs (NSAIDs) are a group of agents with similar actions but diverse chemical structures. Aspirin (acetylsalicylic acid) and sodium salicylate were the first drugs of this type to be used clinically. However, over the past 3 decades there has been a dramatic increase in the number of NSAIDs available for the treatment of postoperative pain. Tissue injury, such as occurs with surgical intervention, is associated with the release of numerous inflammatory mediators including prostaglandins. Prostaglandins derived from the arachidonic acid cascade are implicated in the production of inflammatory pain, and in sensitising nociceptors to the actions of other mediators. They are synthesised from arachidonic acid via the endoperoxide biosynthesis pathway, the initial step of which is catalysed by the enzyme cyclo-oxygenase. Two forms of the cyclo-oxygenase enzyme (COX-1 and COX-2) have been characterised. COX-1 is important in circumstances where prostaglandins have a protective effect such as gastric mucus production and renal blood flow maintenance. NSAIDs inhibit the synthesis of prostaglandins at 1 or more points in the endoperoxide pathway. Three mechanisms of inhibition of the biosynthetic enzymes have been proposed: (i) rapid, reversible competitive inhibition; (ii) irreversible, time-dependent inhibition; and (iii) rapid, reversible noncompetitive (free radical trapping) inhibition. In addition, there is evidence that NSAIDs have a central antinociceptive mechanism of action that augments the peripheral effect. This may involve inhibition of central nervous system prostaglandins or inhibition of excitatory amino acids or bradykinins. There is considerable variability in the pain relief obtained from NSAIDs. Such variability in drug response may be explained in terms of differences between agents with respect to either pharmacodynamic actions or pharmacokinetic parameters or a combination of both. Stereoisomerism, where preparations exist as racemic mixtures and where only 1 enantiomer is active, may also be important. However, chiral inversion from inactive to active enantiomer may occur and may be rapid or slow. NSAIDs have numerous adverse effects. Gastrointestinal disturbances including ulceration are the commonest adverse responses to NSAIDs and carry the greatest risk of death. Also significant are renal impairment and an increased risk of postoperative haemorrhage. Asthma and allergic reactions are uncommon.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of competitive and non-linear plasma protein binding on the stereoselective disposition and metabolic inversion of ibuprofen in healthy subjects

The stereoselective disposition and metabolic inversion of ibuprofen were studied in 12 healthy subjects under conditions of competitive and non-linear plasma protein binding. Each subject received each of four oral treatments according to a Latin-square design: 300 mg R(-)-ibuprofen, 300 mg S(+)-ibuprofen, 300 mg R(-)(-)+300 mg S(+)-ibuprofen, and 300 mg R(-)(-)+600 mg S(+)-ibuprofen. For a given treatment, the partial clearance of S(+)-ibuprofen was greater than that of R(-)-ibuprofen for all stereoisomeric drug species. Likewise, the unbound partial clearances of S(+)-ibuprofen were greater for most stereoisomeric drug species. There was also less difference among treatment groups when partial clearances were referenced to unbound as opposed to total plasma concentrations of enantiomer. The unbound intrinsic clearance and fractional inversion of R(-)-ibuprofen were unchanged across the four treatments, and chiral inversion was systemic, averaging 69%. In conclusion, stereoselective differences exist for the partial and composite clearances of R(-)- and S(+)-ibuprofen even when corrected for differences in plasma protein binding. However, differences among treatment groups for a particular elimination pathway are largely due to ibuprofen's non-linear binding.

Development and validation of a liquid chromatographic method for the quantitation of ibuprofen enantiomers in human plasma

A method for the quantitation of ibuprofen enantiomers in human plasma has been developed and validated. Separation of R- and S-ibuprofen was achieved on a silica-bonded beta-cyclodextrin column with a mobile phase of acetonitrile-0.02% (v/v) triethylamine in water adjusted to pH 4.0 with glacial acetic acid in water (60:40, v/v). The UV detection was performed at 220 nm. The established linearity range was 1-25 micrograms ml-1 (r > 0.99). The limit of quantitation was designed as 1 microgram ml-1 for each enantiomer. Interday precision and accuracy for the standards were 2.2-5.9% relative standard deviation (RSD) and -2.9(-)+3.5% relative error for R-ibuprofen, and 1.9-6.3% RSD and -7.1(-)+4.4% relative error for S-ibuprofen. Interday precision and accuracy for quality controls at 2.5, 7.5 and 17.5 micrograms ml-1 were 6.1-6.4% RSD and -1.4(-)+0.8% relative error for R-ibuprofen, and 5.7-5.9% RSD and -1.2(-)+2.8% relative error for S-ibuprofen. p-Isopropylbenzoic acid was used as an internal standard. The run time was 26 min. Interference from various lots of human plasma were not observed. Stability results of on-system, re-injection, bench-top, freeze-thaw cycles and sample storage were established.

Evidence of absorption rate dependency of ibuprofen inversion in the rat

Ibuprofen (IB) is a chiral 2-arylpropionic acid derivative used as a nonsteroidal antiinflammatory drug (NSAID). It undergoes substantial R to S chiral inversion in humans and rats. In addition to systemic inversion, presystemic chiral inversion has been suggested for IB in humans but only after administration of formulations with slow absorption rates. In search for a suitable animal model, the absorption rate dependency of the extent of inversion was examined in male Sprague-Dawley rats given 20 mg/kg of racemic IB in aqueous solution (Tmax, 0.6 h), suspension (Tmax, 1 h) or as sustained release granules (Tmax, 2.3 h). In addition, (R)-IB (5 mg/liter) was incubated in the presence of everted rat gut segments in an organ bath at 37 degrees. After sustained release granules, the S:R AUC ratios (7.3 +/- 1.5) were significantly higher than suspension (3.6 +/- 1.1) and solution (3.5 +/- 0.2). Accordingly, AUCS and AUCR, as percent of the total AUC (S+R), significantly increased and decreased, respectively, after administration of the sustained released granules as compared with the solution and suspension. A significant positive linear correlation was found between the S:R AUC ratios and the corresponding Tmax for (R)-IB (r = 0.82). In vitro, (R)-IB was inverted by everted jejunum (12.2 +/- 1.6%), ileum (14.2 +/- 2.0%), and colon (4.4 +/- 0.6%) segments. IB was also glucuronidated in the presence of the intestinal segments. Therefore, similar to earlier observations made in humans, in the rat, the S:R AUC ratio was positively and significantly correlated with the absorption rate from the dosage form.(ABSTRACT TRUNCATED AT 250 WORDS)