DETERMINATION OF MONTELUKAST (MK-0476) AND ITS S-ENANTIOMER IN HUMAN PLASMA BY STEREOSELECTIVE HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY WITH COLUMN-SWITCHING

A steoreoselective high-performance liquid chromatographic method was developed for the quantification of montelukast (free acid of Singulai r(TM), or MK-0476), a potent and selective leukotriene D-4 (cysLT(1)) receptor antagonist, and it S-enantiomer (L-768,232). The method invol ves protein precipitation and fluorescence detection. Chromatographic separation of the enantiomers from endogenous components in plasma and chiral resolution of the enantiomers are achieved by using column swi tching HPLC and an alpha-acid glycoprotein chiral column. The assay is linear in the range of 28.9-386 ng ml(-1) of free acids of montelukas t and L-768,232. The intraday precision (% relative standard deviation ) values of this method were in the range of 2.5-9.1% for montelukast, and 2.3-6.8% for L-768,232, while the intraday accuracy values were i n the range of 97-103%, for montelukast and 96-104% for L-768,232. The interday precision values of this method at 48.2 and 193 ng ml(-1) we re 5.3 and 3.6%, respectively, for montelukast, and 4.2 and 3.7%, resp ectively, for L-768,232, while the interday accuracy values at these c oncentrations were 97 and 103%, respectively, for montelukast and 99 a nd 102%, respectively, for L-768,232. The utility of the methodology w as demonstrated by analysis of plasma samples from a study in which he althy volunteers received 10 mg per day of montelukast orally for 7 da ys. Results of this study indicate that there is no apparent bioinvers ion of montelukast to its S-enantiomer in humans.