Pharmacodynamics and enantioselective pharmacokinetics of racemic carprofen in the horse

Carprofen is a nonsteroidal anti‐inflammatory drug of the 2‐arylpropionate subclass. It contains a single chiral centre and exists in two enantiomeric forms. In this study rac‐carprofen, at two dosages, 0.7 and 4.0 mg/kg, and placebo were administered i.v. to six New Forest horses in a three period cross‐over study. The concentration–time profiles were established for R(–) and S(+)‐carprofen for plasma and both inflamed (exudate) and noninflamed (transudate) tissue cage fluids. R(–)‐carprofen was the predominant enantiomer in all three fluids, as indicated by plasma area under the curve ( AUC ) values for R(–) and S(+)‐carprofen of 117.4 and 22.6 μg h/mL (low dose carprofen) and 557.5 and 138.1 μg h/mL (high dose carprofen) respectively. Penetration of both enantiomers into exudate was slow and limited and passage into transudate was even lower. The pharmacodynamics of rac‐carprofen was investigated at both the molecular level and in terms of the ability to suppress components of the tissue cage inflammatory response. Low dose carprofen produced only moderate and transient inhibition of serum thromboxane (Tx)B 2 but failed to affect exudate prostaglandin (PG)E 2 concentrations, whilst suppression of exudate leukotriene (LT)B 4 and β‐glucuronidase was not significant. High dose carprofen produced greater and more persistent inhibition of serum TxB 2 and virtually abolished exudate PGE 2 synthesis. Some inhibition of LTB 4 and β‐glucuronidase in exudate was also obtained. At both dosages rac‐carprofen reduced the swelling produced by intradermal bradykinin injection but only high dose carprofen was anti‐inflammatory as indicated by suppression of temperature rise over exudate tissue cages and neither dose affected leucocyte numbers in exudate. When considered in conjunction with previous data on carprofen, the present findings indicate that carprofen is not a selective inhibitor of cyclooxygenase (COX) isoenzymes, COX‐1 and COX‐2 in the horse, although it may show some preference for COX‐2 inhibition. Because low dose carprofen, which is the clinically recommended dosage, produces minimal inhibition of COX, it is likely to achieve its therapeutic effects at least partially through other pathways, possibly including weak to moderate inhibition of 5‐lipoxygenase and of enzyme release. The good safety margin of carprofen in clinical use might also be explained by weak COX inhibition and by other actions at the molecular level.