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P433. Mercaptopurine therapy in IBD patients modulates GTPase Rac1

M. Seinen1, G. van Nieuw amerongen2, N. de Boer1, J. van Bezu2, C. Mulder1, A. van Bodegraven1, 1VU University medical center, Gastroenterology and Hepatology, Amsterdam, Netherlands, 2VU University medical center, Physiology, Amsterdam, Netherlands


Background and aim: GTPase Rac1 is involved in the pathogenesis of IBD, as Rac1 has been identified as a susceptibility gene for IBD. Loss of Rac1 expression protects mice from the development of experimental IBD. The molecular immunosuppressive mechanism of thiopurine therapy is mainly based on inducing T-cell apoptosis by inhibition of Rac1. The aim of this study is to assess the in vivo effect of thiopurine therapy on Rac1 activity and expression in IBD patients.


Adult IBD patients with active disease, who initiated thiopurine therapy and healthy control subjects were eligible. Activity of disease was assessed by a combination of clinical (including disease activity scores) and laboratory outcomes. Blood was drawn from all patients prior and after 6 weeks of thiopurine therapy. Erythrocytic thiopurine metabolites were measured after 6 weeks of therapy. Blood from healthy controls was drawn with a time-interval of 6 weeks. Cytosol was isolated from leukocytes. Active Rac1 (Rac1-GTP) levels were determined using a G-LISA Rac activation assay. Levels of Rac1 and ERM were measured by Western blot in the IBD patients.


Ten IBD patients (7 Crohn's disease [CD], 3 ulcerative colitis [UC]) and 10 controls were included. All IBD patients were treated with mercaptopurine. No adverse events were observed. The median 6-thio­guanine­nucleotide levels was 570 (190–960) pmol/8×108 RBC. In the majority of patients (6 out of 7) with therapeutic response after initiation of thiopurine therapy, the levels of Rac1-GTP (P = 0.028) and Rac1/ERM expression (P = 0.028) decreased statistically significant. In those without response (N = 3), the levels of Rac1-GTP and Rac1/ERM expression increased. No changes were detected in the healthy controls over time, but levels differed significantly with the IBD patients. No correlations were observed between thiopurine metabolites and Rac1 expression or active Rac1.


Initiation of mercaptopurine therapy in IBD patients modulates the GTPase Rac1 in vivo. In this small scaled study, successful thiopurine therapy led to a decrease in (active) Rac1 expression, while failure led to an increase. These observations implicate that Rac1 may be a (early) pharmacodynamic biomarker of thiopurine efficacy.