Development of a Simple,Rapid, and Robust Isocratic Liquid Chromatographic Method for the Determination of Pyrimethamine and its Synthetic Impurities in Bulk Drugs and Pharmaceutical Formulations

Pyrimethamine is an important antiparasitic drug in the treatment of malaria and toxoplasmosis and is often used in combination with either sulfadoxine, sulfalene, or sulfadiazine. Determining the content of pyrimethamine and investigating the related substances is currently possible applying either a compendial monograph utilizing thin layer chromatography as well as liquid chromatographic methods used by the respective manufacturers. To provide a simple method which is capable of determining the content of pyrimethamine and of resolving four of its potential synthetic impurities a very simple, cheap, precise, and accurate isocratic RP-HPLC method was developed. All analytes can be separated within a total runtime of 30 min and the method was linear within the concentration ranges of 0.12–0.740, 0.104–0.621, 0.120–0.710, 2.0–11.8, and 1.01–5.80 µg mL^?1 for pyrimethamine, impurity A, impurity B, impurity C, and impurity D, respectively. These substances were separated by employing a Eurospher-II C₁₈H column (250 × 4.6 mm, 5 µm particle size), a mobile phase being a mixture of a 0.05 M KH₂PO₄ buffer solution (pH 2.6) and methanol in the ratio 40:60 (v/v). The analysis was carried out at 30 °C, applying a flow rate of 1.2 mL min^?1, and a detection wavelength of λ = 215 nm. The coefficients of determinations (R ²) for the five analytes were greater than 0.994 for pyrimethamine and all impurities. Results of recovery studies were within the range of 89.1–105.1% for all substances. In all tested genuine batches of pyrimethamine raw material impurities within the specified limits were present which is concurrent with results obtained from using the present manufacturer’s method.