Simultaneous Monitoring of Azathioprine Metabolites in Erythrocytes of Crohn's Disease Patients by HPLC-UV.

The conversion of azathioprine (AZA) to active 6-thioguanine nucleotides (6-TGN) is essential for its clinical efficacy; however, another metabolite formed, 6-methylmercaptopurine (6-MMP), is related to hepatotoxicity. Blood samples were collected from 37 patients under AZA's treatment, and a new HPLC-UV method was validated and applied for simultaneous quantification of 6-TGN and 6-MMP in erythrocytes of Crohn's disease (CD) patients. The concentration of 6-TGN and 6-MMP found ranged from 4.5 to 2,456 ρmol/8 × 108 red blood cells (RBCs) for 6-TGN and from 170 to 53,951 ρmol/8 × 108 RBCs for 6-MMP. Reduced levels of 6-MMP in patients into combo therapy with AZA and allopurinol (2,031 ρmol/8 × 108 RBCs) have been observed when compared to patients undergoing monotherapy with AZA (9,098 ρmol/8 × 108 RBCs). Additionally, there was a negative correlation (r = -83.7%, P < 0.05) between lymphocyte count and 6-TGN levels. The method developed is reliable, accurate and reproducible and can be used as an important tool in the monitoring routine of patients with CD under AZA treatment, allowing the individualization of the dose, monitoring adherence to the treatment and the evaluation of the clinical outcome of these patients.