MARKED ENANTIOSELECTIVE PROTEIN-BINDING IN HUMANS OF KETOROLAC IN-VITRO - ELUCIDATION OF ENANTIOMER UNBOUND FRACTIONS FOLLOWING FACILE SYNTHESIS AND DIRECT CHIRAL HPLC RESOLUTION OF TRITIUM-LABELED KETOROLAC

The protein binding of the enantiomers of the nonopiate analgesic, ket orolac, was investigated in vitro using human plasma and solutions of human serum albumin (HSA) at physiological pH and temperature. In orde r to detect the very low levels of unbound enantiomers in protein solu tions, tritium-labelled rac-ketorolac was synthesised by regiospecific isotopic exchange of the parent drug with tritiated water as the isot ope donor. Radiochemical purification of this compound by reversed-pha se HPLC followed by direct resolution using a chiral alpha(1)-acid gly coprotein (Chiral-AGP) HPLC column afforded labelled enantiomers of hi gh specific activity. The in vitro use of (R)- and (S)-[H-3(4)]ketorol ac enabled reproducible radiometric detection of enantiomers in protei n solution ultrafiltrate. The unbound fractions of (R)- and (S)-ketoro lac [fu(R) and fu(S), respectively] were determined when drug was adde d to various plasma or albumin solutions as either the separate enanti omers or as the racemate. Over an enantiomeric plasma concentration ra nge of 2.0-15.0 mu g/ml, fu(S) (mean range: 1.572-1.795%) was more tha n 2-fold greater (P < 0.001) than fu(R) (mean range: 0.565-0.674%). Bo th fu(R) and fu(S) were constant over this concentration range, and ea ch was unaffected by the presence of the corresponding antipode (P > 0 .05). At a concentration of 2.0 mu g/ml in 40.0 g/liter fatty acid-fre e HSA, fu(R) and fu(S) were approximately 0.5 and 1.1%, respectively, and both values declined with increasing concentrations of the long ch ain fatty acid, oleic acid. We have previously shown that the pharmaco kinetics of ketorolac in humans are markedly enantioselective and sugg est in this report that these differences are largely the result of su bstantial differences in the protein binding of ketorolac enantiomers. These findings stress the importance of monitoring the unbound concen trations of the enantiomers of chiral drugs if correct interpretations are to be made of enantioselective pharmacokinetic data. (C) 1994 Wil ey-Liss, Inc.