Azathioprine and 6-mercaptopurine were considered effective in approximately 40% of IBD patients after 5 years of treatment. A quarter of the patients discontinued thiopurines within 3 months, mostly due to adverse events. A high 6-MMP concentration or 6-MMP/6-TGN ratio was associated with therapeutic failure. If thiopurine use was successfully initiated in the first months, its use was usually extended over many years, as long-term use was associated with continuation of therapy.

Long-term combination therapy with allopurinol and low-dose thiopurines is an effective and well-tolerated treatment in IBD patients with a skewed thiopurine metabolism.

Introduction: Thiopurine therapy is often discontinued in inflammatory bowel disease (IBD) patients. The xanthine oxidase (XO) inhibitor allopurinol has previously shown to enhance thiopurine efficacy and to prevent adverse reactions, the mechanism of this beneficial interaction is not completely clarified. The aim of this study is to observe possible effects of allopurinol and low-dose thiopurine combination therapy on the activity of three pivotal thiopurine metabolizing enzymes. Methods: A prospective study of IBD patients failing thiopurine therapy due to a skewed thiopurine metabolism was performed. Patients were treated with allopurinol and azathioprine or mercaptopurine. Xanthine oxidase, hypoxanthine-guanine phosphoribosyl transferase (HGPRT) and thiopurine S-methyl transferase (TPMT) activities, and thiopurine metabolites concentrations were measured during thiopurine monotherapy, and after 4 and 12 weeks of combination therapy. Conclusion: Allopurinol and thiopurine combination-therapy seems to increase HGPRT and decrease XO activity in IBD patients, which at least in part may explain the observed changes in thiopurine metabolite concentrations.

Summary Background Tioguanine (or thioguanine) is an alternative drug for IBD patients who fail prior conventional immunomodulating therapy. Aim To report effectiveness, safety and therapeutic drug monitoring in a cohort of patients with prolonged tioguanine maintenance therapy. Methods In this nationwide, multicentre study, medical records of tioguanine‐ using IBD patients were retrospectively reviewed. Response to therapy was defined as clinical effectiveness without (re)initiation of corticosteroids, concurrent biological therapy or surgical intervention. All adverse events that occurred during the follow‐up were listed and graded according to the common terminology criteria (CTC). Results Two hundred and seventy‐four patients (female 63%, Crohn's disease in 68%) were included with median treatment duration of 51 months, 1567 patient‐years of follow‐up and median 20 mg/d tioguanine dosage. Tioguanine was tolerated in 79%, clinical effectiveness at 6 months was documented in 66% and sustained clinical effectiveness during 12 months in 51% of patients. Forty‐one per cent of patients developed adverse events: 5% were graded as severe. Adverse events comprised infection requiring hospitalisation in three and skin cancer in eight patients (two melanomas). Asymptomatic nodular regenerative hyperplasia of the liver occurred in two out of 52 patients with liver biopsies (3.8%) and portal hypertension in three whereof one potentially associated with tioguanine (0.4%). Clinical effectiveness was correlated with 6‐thioguanine nucleotide threshold concentrations >682 pmol/8×108 RBC (P < 0.05). Conclusions Long‐term tioguanine therapy for at least 12 months was effective in 51% and well tolerated as a maintenance treatment for IBD in about 70% of patients. Adverse events were common, but mainly mild or moderate. 6‐Thioguanine nucleotide threshold concentration ≥ 700 pmol/8×108 RBC is proposed as target level with higher odds for clinical effectiveness.

Hepatotoxicity does not reoccur during 6-thioguanine treatment in most IBD patients who failed conventional thiopurines due to 6-methyl mercaptopurine associated hepatotoxicity. Hence, at least at short-term, 6-thioguanine appears a justifiable alternative thiopurine for these IBD patients.

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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.

The incidence and prevalence of inflammatory bowel disease (IBD) are increasing. Although the etiology of IBD is unknown, it is thought that genetically susceptible individuals display an inappropriate inflammatory response to commensal microbes, resulting in intestinal tissue damage. Key proteins involved in regulating the immune response, and thus in inflammation, are the small triphosphate-binding protein Rac and its regulatory network. Recent data suggest these proteins to be involved in (dys)regulation of the characteristic inflammatory processes in IBD. Moreover, Rac-gene variants have been identified as susceptibility risk factors for IBD, and Rac1 GTPase signaling has been shown to be strongly suppressed in non-inflamed mucosa compared with inflamed colonic mucosa in IBD. In addition, first-line immunosuppressive treatment for IBD includes thiopurine therapy, and its immunosuppressive effect is primarily ascribed to Rac1 suppression. In this review, we focus on Rac modification and its potential role in the development of IBD, Rac as the molecular therapeutic target in current thiopurine therapy, and the modulation of the Rac signal transduction pathway as a promising novel therapeutic strategy.