Chirality and nonsteroidal anti-inflammatory drugs

Chromatographic resolution, chiroptical characterization and preliminary pharmacological evaluation of the enantiomers of butibufen: a comparison with ibuprofen

Enantiomeric resolution of butibufen has been achieved on a cellulose tris(3,5-dimethylphenylcarbamate) chiral stationary phase with hexane-isopropanol-trifluoroacetic acid, 100:1.2:0.02 (v/v/v) as mobile phase at a flow rate of 1.0 mL min(-1). Semi-preparative isolation of the enantiomers then chiroptical characterization indicated that the order of elution was (-)-R- before (+)-S-butibufen. When tested for their effects on the cyclooxygenase and 5-lipoxygenase pathways of eicosanoid metabolism in calcium ionophore-activated rat peritoneal leukocytes it was found that (+)-S-butibufen inhibited generation of thromboxane B2 (TXB2) and prostaglandin E2 (PGE2) (cyclooxygenase pathway), with an IC50 of 1.5 microM (approx.), whereas the (-)-R enantiomer was essentially inactive. Neither enantiomer inhibited the 5-lipoxygenase pathway. In this regard, (+)-S-butibufen was approximately five times less potent as a cyclooxygenase inhibitor than (+)-S-ibuprofen. These results show the enantiomeric specificity and pathway selectivity of this novel non-steroidal anti-inflammatory drug.

Effects of R- and S-enantiomers of chiral non-steroidal anti-inflammatory drugs in experimental colitis

Prostaglandins appear to play an important role in down-regulating intestinal inflammation and promoting repair of injury. In experimental colitis, inhibition of prostaglandin synthesis with nonsteroidal anti-inflammatory drugs (NSAID) leads to marked exacerbation of tissue injury. It has been suggested that the ability of chiral NSAID to inhibit prostaglandin synthesis is completely attributable to the s-enantiomer, while the r-enantiomer is a much weaker inhibitor. Thus, it is possible that r-enantiomers of chiral NSAID will have reduced intestinal toxicity and reduced ability to exacerbate colitis. In the present study, we compared r- and s-enantiomers of two chiral NSAID (flurbiprofen and etodolac) in terms of their ability to exacerbate colitis in the rat. We found that r-flurbiprofen and r-etodolac did not exacerbate colitis, in contrast to the s-enantiomers or racemates. The r-enantiomers also had significantly less inhibitory activity on prostaglandin synthase. Reduced biliary excretion of r-etodolac may have also contributed to the lack of detrimental effects in this model. The results support the hypothesis that prostaglandins play an essential role in down-regulating colonic inflammation and promoting repair.

Perioperative analgesia with non-steroidal analgesics

Non-steroidal anti-inflammatory drugs are widely used for perioperative analgesia as a sole analgesic after minor surgical procedures and in combination with other types of analgesics after major surgery. The development and use of new selective cyclo-oxygenase-2 inhibitors hopefully reduce the likelihood of perioperative adverse effects that may prevent the use of non-steroidal anti-inflammatory analgesics in many patients. It remains to be determined, however, whether they can offer effective analgesia with minimal risks of side-effects in everyday clinical use.

Pharmacological importance of stereochemical resolution of enantiomeric drugs

Drug enantiomers have identical properties in an achiral environment, but should be considered as different chemical compounds. This is because they often differ considerably in potency, pharmacological activity and pharmacokinetic profile, since the modules with which they interact in biological systems are also optically active. Within biological systems, the metabolism of one isomer may be via a different pathway or occur at a different rate from that of the other isomer. Preferential binding of one isomer to plasma proteins may cause differences in circulating free drug and hence alter concentrations at active sites. Interactions of both isomers may differ at the active sites through which pharmacological action is mediated. Actions and levels of activity of the stereoisomers in vivo may also differ. All the pharmacological activity may reside in a single enantiomer, whereas several possibilities exist for the other enantiomer-- it may be inactive, have a qualitatively different effect, an antagonistic effect or produce greater toxicity. Two isomers may have nearly identical qualitative pharmacological activity, qualitatively similar pharmacological activity but quantitatively different potency, or qualitatively different pharmacological activity. To avoid adverse effects and optimise the therapeutic value of enantiomeric drugs, it is necessary that methods for the resolution of racemates be evolved and devolved to determine isomeric purity, establish the effectiveness of isomers of the drug, and detect the presence of an enantiomer with lower therapeutic activity and undesirable adverse effects. Even if a drug is given as a pure enantiomer, methods to discriminate between enantiomers are required because racemisation can occur both in vitro and in vivo. Methods developed for resolution of drug enantiomers should facilitate routine testing of single isomers and their metabolites, studies of pharmacological, toxicological and clinical effectiveness, routine analysis of racemates, pure enantiomers or intermediates in manufacturing processes, and investigation of the potential for inversion of an enantiopure drug substance during the early stages of drug development and therapeutic drug monitoring.