ALBUMIN-BINDING SITES FOR ETODOLAC ENANTIOMERS

Non-steroidal anti-inflammatory drugs (NSAIDs) are strongly bound to h uman serum albumin (HSA), mainly to sites I and II. The aim of this st udy was to characterize the binding site(s) of etodolac enantiomers un der physiological conditions (580 mu M HSA) using equilibrium dialysis . The protein binding of etodolac enantiomers, alone or in various rat ios, was studied in order to evaluate the potential competition betwee n them. Our results showed that (S)-etodolac was more strongly bound t o HSA than (R)-etodolac. The displacement of one enantiomer by its ant ipode was observed only at high concentrations of the competitor, and was more pronounced for the (S)-form. Displacement studies of the enan tiomers by specific probes of sites I and II of albumin, dansylamide, and dansylsarcosine, respectively, showed that (R)-etodolac was slight ly displaced by both these probes whereas the free concentration of (S )-etodolac increased markedly in the presence of dansylsarcosine. More over, the binding of ligands to sites I and II is usually affected by alkaline pH, by chloride ions, and by fatty acids. For etodolac, the p resence of 0.1 and 1 M chloride ions and increasing pH (5.5-9) decreas ed the binding of both enantiomers. The same result was obtained with addition of octanoic acid. Conversely, the addition of oleic, palmitic , or stearic acid to the protein solution increased the binding of (R) -etodolac, but decreased that of its antipode. All these findings sugg est that (R)- and (S)-etodolac interact mainly with site II of HSA, an d that the (R)-isomer is also bound to site I under physiological cond itions. (C) 1996 Wiley-Liss, Inc.