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.

Identifying new indications for existing drugs creates new therapeutic options while bypassing much of the costs and time involved with bringing a new drug to market. The rediscovery of a generic drug, however, is a challenging pursuit because there is no formal regulatory approach and a lack of economic interest by pharmaceutical companies. This played a part in the re-registration of thioguanine as a rescue drug for the treatment of patients with inflammatory bowel disease in The Netherlands. In this article, we aim to underline the importance of drug rediscovery, the difficulties of this procedure in Europe and we attempt to suggest conceivable solutions.

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Thiopurines have spermatotoxic effects in rodents. In humans, overall data are limited and derived from underpowered studies, and therefore not conclusive with regards to the possible effects of thiopurines on spermatogenesis or paternally exposed offspring. Larger, epidemiological trials evaluating the safety of thiopurines to men's fertility and their offspring are mandatory to adequately counsel thiopurine treated men who wish to conceive.