The problem of racemization in the stereospecific assay and pharmacokinetic evaluation of ketorolac in human and rats.
PURPOSE: A comparison of a previously reported indirect (precolumn derivatization) assay for ketorolac (KT) and a new direct method described here was made to establish the conditions under which KT may undergo racemization and to explain the observed discrepancies in the pharmacokinetics of KT reported in the literature. METHODS: A previously reported pre-column derivatization method and a new direct method were employed to determine the effect of pH and ionic strength on racemization. Using the conditions where no racemization occurred, the pharmacokinetics in humans and rats, and protein binding of KT enantiomers were determined. RESULTS: Under the chromatographic conditions employed for the direct assay, no racemization was observed. Under high pH and ionic strength, however, both methods resulted in KT racemization. The indirect method resulted in rapid and complete racemization due to the strong basic conditions required for derivatization. In both humans and rats, the pharmacokinetics of racemic KT were stereoselective with the R enantiomer being predominant (AUC S/R: humans, 0.26; Rats: 0.45). This is likely due to more extensive plasma protein binding of S than its antipode (unbound S/R: 1.35). CONCLUSIONS: The discrepancies in the literature may be explained by rapid racemization of KT that occurs during sample preparation for the pre-column derivatization method. Considerations should be given to the possibility of racemization during the assay development and validation.