Determination of isomeric N-oxide metabolites of some substituted 2,4-diaminopyrimidines by reversed-phase ion-pair high-performance liquid chromatography.

Reversed-phase ion-pair high-performance liquid chromatography has been investigated for the separation, detection, identification and quantitation of the isomeric 1N- and 3N-oxide metabolites of metoprine, pyrimethamine and trimethoprim. A rapid and sensitive analytical method for the simultaneous determination of the isomeric 1N- and 3N-oxides of metoprine and pyrimethamine was devised for in vitro metabolic studies by optimisation of mobile phase pH, pairing-ion concentration, secondary-ion concentration and percentage organic modifier.