6 methylmercaptopurine

The CE electrolyte solution was prepared from ammonium acetate adjusted to pH 9.0 by 3% (v/v) ammonium hydroxide and with an addition of 5% (v/v) methanol; all chemicals were obtained from Sigma-Aldrich (Sigma-Aldrich, Steinheim, Germany). The used chemicals were of analytical or LC-MS grade. Demineralized water for the preparation of electrolyte and sample solutions was produced by a Millipore Simplicity 185 (UV) water purification system (Millipore, Molsheim, France). The electrolyte solution was filtered before use through nylon membrane filters of a 0.22 μm pore size (Millipore, Molsheim, France).
The drug standards, namely, prednisone, 6-thioguanine, 6-mercaptopurine monohydrate, allopurinol, azathioprine, mesalazine, and 6-methylmercaptopurine were purchased from Sigma-Aldrich (Sigma-Aldrich, Steinheim, Germany).

6-Mercaptopurine, 6-methylmercaptopurine and 5-fluorouracil were acquired from Sigma-Aldrich (Saint Louis, MO, USA). 6-Thioguanosine-5′-monoposphate (6-TGMP; Figure 1C) was acquired from Jena Bioscience (Löbstedter, Jena, Germany). Acetonitril and methanol high performance liquid chromatography (HPLC)-grade from Merck (Kenilworth, NJ, USA). All water was HPLC-grade and prepared using a Millipore Direct-QTM 5 water system (Millipore, Watford, Hertfordshire, UK). Whole blood was acquired from the Indonesian Red Cross.