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-labelled ketorolac.

The protein binding of the enantiomers of the nonopiate analgesic, ketorolac, was investigated in vitro using human plasma and solutions of human serum albumin (HSA) at physiological pH and temperature. In order to detect the very low levels of unbound enantiomers in protein solutions, tritium-labelled rac-ketorolac was synthesised by regiospecific isotopic exchange of the parent drug with tritiated water as the isotope donor. Radiochemical purification of this compound by reversed-phase HPLC followed by direct resolution using a chiral alpha 1-acid glycoprotein (Chiral-AGP) HPLC column afforded labelled enantiomers of high specific activity. The in vitro use of (R)- and (S)-[3H4]ketorolac enabled reproducible radiometric detection of enantiomers in protein solution ultrafiltrate. The unbound fractions of (R)- and (S)-ketorolac [fu(R) and fu(S), respectively] were determined when drug was added to various plasma or albumin solutions as either the separate enantiomers or as the racemate. Over an enantiomeric plasma concentration range of 2.0-15.0 micrograms/ml, fu(S) (mean range: 1.572-1.795%) was more than 2-fold greater (P < 0.001) than fu(R) (mean range: 0.565-0.674%). Both fu(R) and fu(S) were constant over this concentration range, and each was unaffected by the presence of the corresponding antipode (P > 0.05). At a concentration of 2.0 micrograms/ml in 40.0 g/liter fatty acid-free HSA, fu(R) and fu(S) were approximately 0.5 and 1.1%, respectively, and both values declined with increasing concentrations of the long chain fatty acid, oleic acid.(ABSTRACT TRUNCATED AT 250 WORDS)