Assay of 6-thioguanine in human plasma.

1. A fluorimetric method has been developed for measuring therapeutic concentrations of 6-thioguanine in human plasma. The drug was measured as an oxidized derivative, guanine 6-sulphonate. 2. 6-thioguanine was extracted from plasma by a novel procedure which isolated the drug from an interfering plasma components. This method involved the formation of the thioguanine phenyl mercury derivative in alkaline plasma and its extraction into toluene. The free drug was released by back-extraction into hydrochloric acid. 3. The assay is specific and shows a limit of sensitivity of 5 ng/ml. This was shown to be adequate for measuring plasma concentrations of 6-thioguanine after the administration of a 160 mg dose in a patient with leukaemia.