Rac1 as a Potential Pharmacodynamic Biomarker for Thiopurine Therapy in Inflammatory Bowel Disease

Background: Azathioprine and mercaptopurine (MP) are effective in treating patients with inflammatory bowel disease (IBD). Immunosuppressive effects of thiopurines involve T-cell apoptosis after inhibition of GTPase Ras-related C3 botulinum toxin substrate 1 (Rac1). This study aimed to assess whether expression and activity of Rac1 or phosphorylated ezrin-radixin-moesin (pERM) in patients with IBD could provide a useful biomarker for the pharmacodynamic thiopurine effect and might be related to clinical effectiveness. Methods: This was a 2-stage study: stage 1 concerned a cross-sectional cohort of patients with IBD clinically in remission and treated with (n = 10) or without stable weight-based thiopurine therapy (n = 11) and healthy controls (n = 6); stage 2 concerned a prospective study regarding IBD patients with clinically active disease who initiated MP therapy (n = 11) compared with healthy controls (n = 11). Expression and activity of Rac1 and ERM and pERM were determined. Results: The median Rac1 expression was statistically significantly reduced by thiopurine maintenance therapy {0.54 [interquartile range (IQR) 0.47–0.88] versus 0.80 arbitrary units [IQR 0.64–1.46]} compared with patients without immunosuppressive therapy (P = 0.042), but not Rac1 activity and pERM. In responders to MP therapy (n = 6), both median active Rac1 [93 (IQR 81–151) to 76 ng Rac1/mg protein (IQR 62–98)] and Rac1 expression [16.2 (8.8–29.4) to 1.5 arbitrary units (0.9–5.3)] decreased (P = 0.028). In nonresponders (n = 3), Rac1 expression and activity increased. Conclusions: IBD patients treated with thiopurines had a lower expression of Rac1 compared with those not treated with thiopurine. Effective MP therapy led to decreasing concentrations of Rac1-GTP and Rac1 expression. Therefore, Rac1-GTP and expression of Rac1, but not phosphorylation of ERM, form potentially pharmacodynamic markers of therapeutic thiopurine effectiveness in patients with IBD.