Assay of an active metabolite of 6-thioguanine, 6-thioguanosine 5'-monophosphate, in human red blood cells.

A flow-fluorimetric, liquid chromatographic assay for 6-thioguanosine 5'-monophosphate (TGMP) in human red blood cells (RBCs) has been developed. This compound is an active metabolite of 6-thioguanine, 6-mercaptopurine and the immunosuppressant, azathioprine. The samples were prepared for chromatography by a novel isolation method which extracts the RBC constituents into an organic solvent and leaves the TGMP in the aqueous layer where it is oxidised to give a highly fluorescent species which is then separated by liquid chromatography. The method is sensitive to below 200 ng ml-1 in RBCs which is below the levels encountered following a therapeutic dose of 6-thioguanine or azathioprine. The assay is simple and rapid enough for routine use.