AZATHIOPRINE-RELATED BONE-MARROW TOXICITY AND LOW ACTIVITIES OF PURINE ENZYMES IN PATIENTS WITH RHEUMATOID-ARTHRITIS

Objective. Azathioprine (AZA) metabolism largely parallels the endogen ous purine pathways. To date, thiopurine methyltransferase (TPMT) defi ciency has been reported as a cause of AZA-related bone marrow toxicit y in 1 patient with rheumatoid arthritis (RA). We therefore studied pu rine enzyme activities in 3 patients with RA who experienced AZA-relat ed bone marrow toxicity. Methods. Lymphocyte activity of purine nucleo side phosphorylase and 5'-nucleotidase (5NT) and erythrocyte activity of TPMT, key enzymes in thiopurine catabolism, were measured in 3 RA p atients who had experienced AZA-related bone marrow toxicity and in 16 RA patients without signs of toxicity despite at least 6 months of tr eatment with AZA. Results. Two patients with AZA-related home marrow t oxicity were found to have a TPMT deficiency, 1 partial and 1 total. I n the third patient, 5NT activity was found to be well below the lowes t level observed in the control subjects. Conclusion. All 3 patients w ith 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.