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Objective: Azathioprine (AZA) metabolism largely parallels the endogenous purine pathways. To date, thiopurine methyltransferase (TPMT) deficiency has been reported as a cause of AZA-related bone marrow toxicity in 1 patient with rheumatoid arthritis (RA). We therefore studied purine enzyme activities in 3 patients with RA who experienced AZA-related bone marrow toxicity., Methods: Lymphocyte activity of purine nucleoside phosphorylase and 5'-nucleotidase (5NT) and erythrocyte activity of TPMT, key enzymes in thiopurine catabolism, were measured in 3 RA patients who had experienced AZA-related bone marrow toxicity and in 16 RA patients without signs of toxicity despite at least 6 months of treatment with AZA., Results: Two patients with AZA-related bone marrow toxicity were found to have a TPMT deficiency, 1 partial and 1 total. In the third patient, 5NT activity was found to be well below the lowest level observed in the control subjects., Conclusion: All 3 patients with severe AZA-related bone marrow toxicity had abnormal purine enzyme activities. Deficiency of purine enzymes, including TPMT and 5NT, may be a cause of AZA-related bone marrow toxicity in patients with RA.

We conducted a double-blind, randomized trial comparing azathioprine (AZA) and methotrexate (MTX) in the treatment of patients with rheumatoid arthritis in whom parenteral gold and/or D-penicillamine treatment had been unsuccessful. Patients were randomly assigned to receive either AZA (100 mg daily) or oral MTX (7.5 mg weekly). After 8 weeks, the dosage was increased depending on the clinical improvement. Sixty-four patients were followed up for 48 weeks (33 AZA, 31 MTX). Comparison of values at week 24 with baseline values revealed significant improvement in 12 of 13 disease variables in the MTX group and in 6 of 13 in the AZA group. Comparison between the 2 treatment groups at 24 weeks, by area-under-the-curve analysis, showed significantly more improvement in the MTX group in terms of the swollen joint count, pain score, erythrocyte sedimentation rate, C-reactive protein level, hemoglobin level, thrombocyte level, and disease activity score. A significant overall clinical improvement (disease activity score) was found in 7 of 20 patients treated with AZA and 18 of 30 patients treated with MTX after 24 weeks of therapy, and in 6 of 12 AZA-treated patients and 19 of 25 MTX-treated patients after 48 weeks. The number of withdrawals due to side effects was significantly higher in the AZA group. After 48 weeks, only 12 patients from the AZA group (36%), but 25 from the MTX group (81%), were still using the initial drug. These results demonstrate MTX to be superior to AZA in the treatment of rheumatoid arthritis, with a more rapid clinical improvement which is sustained after 1 year, accompanied by a lower rate of serious adverse reactions.

NZB/NZW mice were treated with various immunosuppressive drugs used in human SLE. Cyclophosphamide (5 mg/kg 6 days out of 7), alone or with prednisolone, was better than azathioprine, prednisolone, or azathioprine-plus-prednisolone, in prolonging survival and/or reducing proteinuria, Coomb's antibodies, antinuclear antibodies, and glomerular deposits of gamma-globulin. Intermittent bolus therapy with cyclophosphamide (59 mg/kg/10 days) was as effective as daily therapy. However, 61% of the mice receiving any cyclophosphamide regimen developed malignant tumors compared to none in the other groups.