Tumor necrosis factor-α (TNFα) antagonists have advanced the management of inflammatory bowel diseases patients leading to an improvement of patient's quality of life with the reduction of number of surgeries and hospitalizations. Despite these advances, many patients do not respond to the induction therapy (primary non-response—PNR) or lose response during the treatment (secondary loss of response—LOR). In this paper we will provide an overview of the definition, epidemiology and risk factors for PNR and LOR, as well as discuss the therapeutic options for managing LOR.

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.

Pregnancy has a major effect on maternal thiopurine metabolism. In utero the unborn child is exposed to 6-TGN, but not to 6-MMP. Sixty per cent of the infants were born with anaemia, which raises the question whether infants should be tested for possible anaemia immediately after birth.

Thiopurines are frequently used for the treatment of IBD. The complex pharmacology, metabolism, mechanism of action and toxicity profile of these immunosuppressive drugs have now been partly elucidated. The activity of thiopurines is partly mediated by the metabolite 6-thioguanosine 5'-triphosphate, which inhibits the function of the small GTPase Rac1, leading to apoptosis of activated T cells, and influences the conjugation of T cells with antigen-presenting cells. The activity of the enzyme thiopurine S-methyltransferase has a major influence on the bioavailability and toxicity of thiopurines, and several thiopurine metabolites might have adverse effects in patients. Myelotoxicity can be caused by grossly elevated levels of 6-thioguanine nucleotides, and elevated levels of 6-methylmercaptopurine ribonucleotides have been associated with hepatotoxicity. The sensitivity and specificity of these methylated metabolites for predicting thiopurine-induced liver enzyme abnormalities are, however, poor. 6-Thioguanine has been suggested as an alternative to the classical thiopurines azathioprine and 6-mercaptopurine for the treatment of IBD, but there are concerns about its toxicity profile, especially with regard to the induction of nodular regenerative hyperplasia of the liver. Data now suggest that the induction of nodular regenerative hyperplasia of the liver during 6-thioguanine therapy might be dose-dependent or dependent on the level of 6-thioguanine nucleotides.

Background and aims The COVID-19 risk and disease course in inflammatory bowel disease (IBD) patients remains uncertain. Therefore, we aimed to assess the clinical presentation, disease course and outcomes of COVID-19 in IBD patients. Second, we determined COVID-19 incidences in IBD patients and compared this with the general population. Methods We conducted a multicenter, nationwide IBD cohort study in the Netherlands and identified patients with COVID-19. First, we assessed the COVID-19 disease course and outcomes. Second, we compared COVID-19 incidences between our IBD study cohort and the general Dutch population. Results We established an IBD cohort of 34,763 patients. COVID-19 was diagnosed in 100/34,763 patients (0.29%). 20/100 patients (20%) had severe COVID-19 defined as admission to the intensive care unit, mechanical ventilation, and/or death. Hospitalization occurred in 59/100 (59.0%) patients and 13/100 (13.0%) died. All patients who deceased had comorbidities and all but one were > 65 years. In line, we identified > 1 comorbidity as an independent risk factor for hospitalization (OR 4.20, 95% CI 1.58-11.17, p = 0.004). Incidences of COVID-19 between the IBD study cohort and the general population were comparable (287.6 (95% CI 236.6-349.7) versus 333.0 (95% CI 329.3-336.7) per 100,000 patients, respectively; p = 0.15). Conclusions Of 100 cases with IBD and COVID-19, 20% developed severe COVID-19, 59% was hospitalized and 13% died. A comparable COVID-19 risk was found between the IBD cohort (100/34,763 = 0.29%) and the general Dutch population. The presence of > 1 comorbidities was an independent risk factor for hospitalization due to COVID-19.