Simultaneous determination of pyrimethamine and sulphadoxine in human plasma by high-performance liquid chromatography

Pyrimethamine was extracted as a base, and sulphadoxine simultaneous as an ion pair with tetrabutyl-ammonium into methylene chloride. After re-extraction into an acidic aqueous phase separation of the drugs was performed by reversed-phase high performance liquid chromatography using Nucleosil C-18 as the support and acetonitrile-phosphate buffer pH 5.5 (28 + 72) as the mobile phase with tetrabutyl-ammonium as counter ion. Photometric detection at 240 nm was used for both substances. The within-day precision of the method was 5 to 10% for pyrimethamine within 50 to 1000 nmol/l (13 to 250 ng/ml) and for sulphadoxine 3 to 6% within 50 to 1000 mumol/l (16 to 310 micrograms/ml) in plasma. The method is suitable for simultaneous determination of plasma levels of pyrimethamine and sulphadoxine after therapeutic doses of Fansidar.