ASSAY OF AZATHIOPRINE, 6-MERCAPTOPURINE AND A NOVEL THIOPURINE METABOLITE IN HUMAN-PLASMA

1 A simple, specific assay for 6‐mercaptopurine (6‐MP) in human plasma with a sensitivity of 10 ng/ml (66 nmol/1) has been developed. 2 6‐MP was extracted directly from plasma into toluene using a novel extraction procedure. This involves conversion of 6‐MP into a phenyl mercury derivative by its reaction with phenyl mercuric acetate in alkaline plasma and extracting into toluene. Back‐extraction of the toluene layer with 0.1N HCl regenerates 6‐MP, which is then oxidised to purine‐6‐sulphonate and assayed fluorimetrically. 3 This assay has been modified to measure azathioprine and a new thiopurine metabolite in plasma. 4 In a kidney transplant patient given azathioprine, 50 mg i.v., conversion to 6‐MP was rapid and the plasma half‐life of 6‐MP was 36 min. 5 These assays are suitable for studying the pharmacokinetics of azathioprine in patients with kidney transplants. The 6‐MP assay should also prove useful for studying the pharmacokinetics of the drug in patients with leukaemia. 1979 The British Pharmacological Society