S-ketoprofen was the predominant enantiomer in pig plasma after administration of the 50:50 racemic drug via both routes. The pharmacokinetic parameters of both enantiomers after IM administration were similar to those reported by Fosse  in piglets, despite the pigs in our study being older than those used in their study. Tmax and AUCS/AUCR ratio after oral administration were higher in our study than in the study reported by Neirinckx . The ketoprofen product used in that study was an oral solution, which could have been absorbed faster than the oral powder we used.
The absolute bioavailability of both ketoprofen enantiomers after IM administration is suggested to be almost complete [13, 31]. After PO administration the bioavailability is reported to be approximately 85 % for both enantiomers . The higher relative bioavailability for S than R ketoprofen in our study suggests that some stereoselective absorption and/or first pass metabolism may have occurred. The physiochemical properties of the two enantiomers of ketoprofen are identical and absorption has been regarded to be mainly a passive process. The absorption of ketoprofen is therefore not considered to be stereoselective [18–21, 32]. However, there is also some evidence suggesting that ketoprofen may have an active transport pathway across the intestinal wall . In rats, 84% of the administered dose of R-ketoprofen was inverted to S-ketoprofen in the gastrointestinal tract , while an absence of pre-systemic inversion was reported in pigs . Presystemic inversion in the gastrointestinal tract has also been reported in rats after ibuprofen and fenoprofen administration, and was dependent on the absorption rate [34, 35]. In the present study, the difference in relative bioavailability between enantiomers was smaller, although significant, than that reported in rats after PO and IP administration . The inversion rate of R-ketoprofen to S-ketoprofen is equally high (approximately 70%) in rats and pigs [18, 26]. The possible faster absorption rate of the oral solution used by Neirinckx  may have been partially responsible for the absence of pre-systemic inversion found in their study, whereas in our study the slower absorption rate from the gastro-intestinal tract might have contributed the pre-systemic inversion. The racemic ketoprofen used in our study was an oral powder, which was insoluble in water.
The second peak in S-ketoprofen concentration in plasma after PO and IM administration maybe due enterohepatic recycling. There were individual variation in the sharpness of the second peak and the peak was more evident after PO administration of ketoprofen than after IM administration. In most of the pigs, it was clearly evident within two hours after per oral ketoprofen administration. That confirms our previous findings with total ketoprofen plasma concentrations . The stereoselective enterohepatic circulation of ketoprofen exists at least in rats . Yasui  suggested that glucoronide of S-ketoprofen is hydrolyzed slower than the glucuronide of R-ketoprofen in the intestine. That will lead to longer mean transit time of S-ketoprofen from the bile duct via the intestinal tract and into the systemic circulation and therefore stereoselective enterohepatic recycling may occur. However, the high degree of chiral inversion from S- to R-ketoprofen could also explain some fluctuation in plasma S-ketoprofen concentration.
Since the difference in AUC values between enantiomers was significant for both administration routes, one of the most probable reasons for the lower AUC of R-ketoprofen was higher clearance compared to S-ketoprofen. The enantioselectivity differences in clearance may be attributable to differences in distribution, chiral inversion, hepatic metabolism, renal excretion, or to a combination of these factors. The volume of distribution is low for ketoprofen in pigs, probably due to high protein binding . The degree of stereo selectivity in binding to plasma or tissue proteins, which is species dependent, may result in a significant effect on the amount of drug in the plasma . Enantioselectivity in the binding of ketoprofen to plasma proteins has been reported in humans and camels [37, 38], although contradictory results have also been reported in humans . There have been no reports of possible enantioselectivity protein binding in pigs.
The terminal half-life and MRT of R-ketoprofen were approximately three times shorter and Tmax a half of that for S-ketoprofen after both administration routes. In the present study, pure enantiomers were not administered and chiral inversion and the inversion rate could not be determined. The inversion rate from R-ketoprofen to S-ketoprofen has been previously reported to be 70% in pigs . Ketoprofen is metabolized in the liver and converted into a carbonyl-reduced derivative, 2-(phenyl 3-alphahydroxybenzoyl) propionic acid in swine . In rats, stereo selectivity has been reported for the biliary excretion process . The significance of stereo selectivity in reductive metabolism is difficult to assess, as inversion occurs in most species, and is rapid for ketoprofen . Glucuronidation appears to be an important metabolic pathway for ketoprofen in pigs . Stereo selectivity of glucuronidation has been reported, but it is species and compound dependent . The possible stereo selectivity of ketoprofen glucuronidation in pigs has not been studied, but it could to some extent explain the more rapid elimination of R-ketoprofen than S-ketoprofen.
Fosse  reported an IC50 for S-ketoprofen of 26.7 μg/mL and an IC50 for R-ketoprofen of 1.6 μg/mL from mechanical nociceptive threshold testing in the kaolin-induced inflammation model in neonatal pigs. There have been no other reports on either the total or the enantiospecific therapeutic target concentration of ketoprofen in plasma in pigs. A total ketoprofen plasma concentration of 0.4–6 μg/mL has been recommended as a target therapeutic concentration in humans . A serum concentration of 0.2–0.4 μg/mL of S-ketoprofen is required for the maximum anti-inflammatory effect in adjuvant arthritis in rats , whereas at least 1 μg/mL of total ketoprofen is needed to alleviate pain in orthopedic human patients . In the present study the S-ketoprofen concentrations in plasma in this study were above 0.8 μg/mL for at least 12 hours. The concentration of 1.6 μg/mL of R-ketoprofen was only achieved for two hours. Fosse  reported a biphasic analgesic effect in piglets; an initial comprehensive but short analgesia followed by a moderate but more sustained analgesia. The authors hypothesized that the former was caused by R-ketoprofen and the later by S-ketoprofen. Ketoprofen concentrations persist for longer in inflammatory exudates than in plasma [15, 17, 28, 45, 46], and the clinical effect may therefore last longer than estimated from concentrations in plasma.
In the European Union, the registered ketoprofen dose rate for pigs is 3 mg/kg body weight IM  or 1.5–3 mg/kg body weight PO . In our previous study, increases in AUC and Cmax were proportional in orally administered doses (3 mg/kg and 6 mg/kg) of racemic ketoprofen when the total plasma ketoprofen was measured . However, equivalence was not detected between 3 mg/kg PO and IM. Accordingly, we estimated that the oral dose used in this study, 4 mg/kg, would produce a maximum concentration in plasma similar to the registered dose for intramuscular administration (3 mg/kg).
The relative bioavailability of S-ketoprofen after oral administration was significantly higher than for R-ketoprofen. Since S-ketoprofen is generally regarded as the eutomer regarding cyclooxygenase inhibition, and the terminal half-life of R-ketoprofen is short, in clinical use the administration routes at dose rates used in this study could be considered equally effective.