Hepatotoxicity of 6-mercaptopurine (6-MP) and azathioprine (AZA) in adult IBD patients

OBJECTIVE: 6-Mercaptopurine (6-MP) and azathioprine (AZA) are effective in the treatment of IBD; however, drug-induced hepatotoxicity has been reported in 10-15% of pediatric patients and has been associated with the 6-MP metabolite 6-methylmercaptopurine ribonucleotide (6-MMPR) at levels >5,700 pmol/8 × 10⁸ RBC. The aim of this study was to assess the prevalence of 6-MP/AZA hepatotoxicity and its correlation with serum 6-MMPR levels in adult IBD patients. METHODS: Aminotransferases, bilirubin, and 6-MP metabolite levels were measured in 173 adult IBD patients treated with 6-MP or AZA from November 2002 to December 2003. Hepatotoxicity was defined as AST and/or ALT >2x upper limit of normal or cholestasis. RESULTS: Eight patients (4.6%) met criteria for a diagnosis of 6-MP/AZA-induced hepatotoxicity. The mean 6-MMPR level in these 8 patients was 10,537 pmol/8 × 10⁸ RBC versus 3,452 pmol/8 × 10⁸ RBC in the nonhepatotoxic group (P < 0.001). Risk of hepatotoxicity above the third quartile (6-MMPR > 5,300) was 5 times that below the third quartile (11.4% vs 2.3%, P < 0.05); however, nearly 90% of all patients with 6-MMPR > 5,300 pmol/8 × 10⁸ RBC had no hepatotoxicity, while almost 40% of subjects with hepatotoxicity had 6-MMPR levels below this cutoff. CONCLUSIONS: 6-MP/AZA-induced hepatotoxicity is uncommon in the adult population. Although hepatotoxicity is associated with higher mean 6-MMPR levels, the sensitivity and specificity of 6-MMPR for drug-induced hepatotoxicity was poor. Monitoring liver tests in patients on 6-MP/AZA is suggested, and dose reduction or cessation of 6-MP/AZA, even with high 6-MMPR levels, should be reserved for patients with elevated aminotransferases.