Determination of Plasma Azathioprine and 6-Mercaptopurine in Patients with Rheumatoid Arthritis Treated with Oral Azathioprine

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Abstract
Two specific high-performance liquid chromatography methods for determining plasma concentrations of azathioprine and 6 mercaptopurine after oral administration of azathioprine are presented. It was shown that azathioprine is unstable in the blood samples unless immediately cooled in ice water. The 2-amino analog, guaneran, was used as internal standard for azathioprine, which was extracted from plasma with ethylacetate. A Nucleosil C18 column was used for the separation. The detection limit was 6 nM. For quantification of 6-mercaptopurine, 6-thioguanine was used as internal standard. Plasma was deproteinized with HC104 and the sample was purified on mercurial cellulose. A Beckman ODS column was used and the detection limit was 5 nM. Phar-macokinetic data from two patients are presented. Unchanged azathioprine was seen until 6 h after an oral dose of 32 mg/m2.