The potential influence of 5-aminosalicylic acid on the induction of myelotoxicity during thiopurine therapy in inflammatory bowel disease patients.

ABSTRACT

OBJECTIVE: To investigate the potential influence of 5-aminosalicylic acid (5-ASA) on the induction of myelotoxicity during thiopurine therapy in inflammatory bowel disease patients. METHODS: (a) The retrospective study included inflammatory bowel disease patients treated with azathioprine (AZA)/6-mercaptopurine (6-MP). Thiopurine methyltransferase (TPMT) activity and 6-thioguanine nucleotide (6-TGN) levels were detected at stable medication points. (b) The prospective study was performed in active disease patients 4 weeks of AZA 50 mg/day followed by concomitant 5-ASA 3 g/day for another 4 weeks. 6-TGN was analyzed at weeks 4 and 8. RESULTS: (a) Of the 139 retrospective study patients, 45 were on AZA/6-MP+5-ASA and 94 on AZA/6-MP alone. The myelotoxicity rates were 47 and 16%, respectively. Multivariates regression analysis indicated that the administration of concomitant 5-ASA was the only risk factor associated with myelotoxicity (odds ratio=3.45, 95% confidence interval 1.31-9.04, P=0.01). (b) Thiopurine methyltransferase activity was not significantly different between patients on AZA/6-MP+5-ASA and patients on AZA/6-MP alone (P=0.78). (c) 6-TGN levels were significantly higher in samples on AZA/6-MP+5-ASA than those on AZA/6-MP (P=0.003) alone. (d) Sixteen patients completed the prospective study. After 4 weeks on AZA 50 mg/day, 6-TGN levels of 13 patients were less than 230 pmol/8×10 RBC. After another 4 weeks' cotreatment with mesalazine 3 g/day, 12 patients had 6-TGN levels at least 230 pmol/8×10 RBC, five patients had 6-TGN levels at least 420 pmol/8×10 RBC, and two of these five patients developed myelotoxicity. CONCLUSION: The risk of thiopurine-induced myelotoxicity markedly increases in patients treated with combined 5-ASA and 2 mg/kg/day AZA therapy, which may be correlated to the increase in 6-TGN. 50 mg daily AZA when concomitant 5-ASA might help maintain an effective 6-TGN level without increasing the risk of myelotoxicity.