Owing to adverse pharmacokinetics, azathioprine treatment may fail to induce a satisfactory clinical response in systemic connective tissue diseases. The major intracellular cytotoxic metabolites of azathioprine are 6-thioguanine nucleotides (6TGNs).
6-Thioguanine nucleotide accumulation in erythrocytes during azathioprine treatment for systemic connective tissue diseases: a possible index for monitoring treatment.