Shunting (hypermethylating) thiopurine metabolism, characterized by excessive 6-MMPR production and (sub)therapeutic 6-TGN levels, poses a significant challenge in the treatment of inflammatory bowel disease (IBD). This study evaluates the metabolic outcomes of switching to low-dose thiopurine with allopurinol (LDTA) or thioguanine (TG) in IBD patients with shunting metabolism.

This retrospective study analysed demographic data, thiopurine metabolite profiles, and adverse event rates in shunting IBD patients before and after switching to LDTA or TG therapy. Metabolite variability was assessed by examining alterations in 6-MMPR and 6-TGN levels and their correlation before and after therapy switch.

Both therapies significantly reduced toxic 6-MMPR levels, with 100% of patients achieving non-toxic levels (6-MMPR-level below 7000 pmol/8 × 10E8 RBC) post-therapy. For TG, 93% of patients attained non-toxic 6-TGN levels (6-TGN < 1000 pmol/8 × 10E8 RBC). In contrast, LDTA showed greater variability in 6-TGN outcomes, with 50% of patients reaching therapeutic levels, 31% remaining subtherapeutic, and 19% within toxic range. The slope for LDTA was significantly positive (1.044, R2 = 0.4515, P = 0.004), reflecting proportional increases in 6-TGN levels.

Switching to LDTA or TG therapy in shunting IBD patients resulted in favourable alterations in thiopurine metabolism. However, LDTA appears to have a slightly less favourable metabolite profile, with some residual 6-MMPR formation and fewer patients achieving normal 6-TGN levels compared to TG. Given the challenges associated with achieving optimal thiopurine metabolite levels with LDTA and the need for closer monitoring of 6-TGN levels, TG appears a more suitable option for thiopurine shunting IBD patients.

In the treatment of diseases such as acute childhood leukaemia (ALL) and inflammatory bowel disease (IBD), the thiopurines azathioprine, 6-mercaptopurine, and 6-thioguanine are used. Thiopurines are antimetabolites and immunomodulators used to maintain remission in patients. They are all prodrugs and must be converted into the competing antimetabolites thioguanosine triphosphate and deoxythioguanosine triphosphate for final incorporation into RNA or DNA. The current therapeutic drug monitoring (TDM) method measures the sum of the formed metabolites in the sample, after acidic hydrolysis at high temperature. In this work, the goal is to measure these drugs closer to their pharmacological endpoints, once incorporated into DNA. After extracting DNA from whole blood, followed by DNA hydrolysis, 2'-deoxythioguanosine (dTG) and the complementary natural nucleobase 2'-deoxycytidine (dC) were measured. Chromatographic separation on a HSS T3 column followed by mass spectrometric detection was performed in multi-reaction monitoring (MRM) mode on a Xevo TQ-XS with ESI in positive mode, within 5 min. The concentration range for dTG was 0.04-5 nmol/L, and for dC, 0.1-12.5 µmol/L. The lower limit of detection was determined to a concentration of 0.003 nmol/L for dTG and 0.019 µmol/L for dC. The intra- and inter-assay imprecision for the quality controls ranged between 3.0 and 5.1% and between 8.4 and 10.9%, respectively. Sample stability for up to 4 years is shown. In summary, a sensitive method to quantify the thiopurines incorporated into DNA as dTG has been developed and will be used in further clinical studies for a better understanding of the mode of action of the thiopurines and the use of this method in TDM.

Thiopurines have been a cornerstone in the treatment of inflammatory bowel disease (IBD). Although they have been used for more than 50 years, there are still some unsolved issues about their efficacy and, also, some safety concerns, mainly the risk of myelosuppression and life-threatening lymphoproliferative disorders. Furthermore, the development of biological therapy raises the question whether there is still a role for thiopurines in the IBD treatment algorithm. On the other hand, limited cost and wide availability make thiopurines a reasonable option in settings of limited resources and increasing prevalence of IBD. In fact, there is a growing interest in optimizing thiopurine therapy, since pharmacogenomic findings suggest that a personalized approach based on the genotyping of some molecules involved in its metabolism could be useful to prevent side effects. Polymorphisms of thiopurine methyltransferase enzyme (TPMT) that result in low enzymatic activity have been associated with an increased risk of myelotoxicity, especially in Caucasians; however, in Asians it is assumed that the variants of nudix hydrolase 15 (NUDT15) are more relevant in the development of toxicity. Age is also important, since in elderly patients the risk of complications seems to be increased. Moreover, the primo-infection of Epstein Barr virus and cytomegalovirus under thiopurine treatment has been associated with severe lymphoproliferative disorders. In addition to assessing individual characteristics that may influence thiopurines treatment outcomes, this review also discusses other strategies to optimize the therapy. Low-dose thiopurines combined with allopurinol can be used in hypermethylators and in thiopurine-related hepatotoxicity. The measurement of metabolites could be useful to assess compliance, identify patients at risk of adverse events and also facilitating the management of refractory patients. Thioguanine is also a rescue therapy in patients with toxicity related to conventional thiopurine therapy. Finally, the current indications for thiopurines in monotherapy or in combination with biologics, as well as the optimal duration of treatment, are also reviewed.

Thiopurine therapy can be optimised by determining the concentration of the drug's metabolites.

Retrospective analysis on a prospective database of 31 patients with inflammatory bowel disease who failed therapy with thiopurines. Thiopurine metabolites (6-thioguanine, 6-TGN and 6-methylmercaptopurine, 6-MMP) were measured by high-performance liquid chromatography (Laboratorios Cerba, Barcelona) and treatment was duly adjusted in accordance with the results. Clinical response was reassessed after six months.

Despite the appropriate theoretical dose of thiopurines being administered, the dose was insufficient in 45.6% of patients (nonadherence to treatment suspected in 6.45%) and 16.2% received an excessive dose or the drug was metabolised by other metabolic pathways. After treatment was optimised based on metabolite levels, only 25.8% (8/31) were prescribed a biological agent, while 74.2% of cases (23/31) were managed through dose optimisation alone.

Monitoring thiopurine metabolite levels may help clinicians to assess non-responsive patients before adding or switching to another drug (generally a biological agent), thereby avoiding any additional costs or potential toxicity. This strategy may also help to identify patients receiving an insufficient dose and those with an alternative metabolic pathway, who could be candidates for low-dose AZA with allopurinol, as well as patients who are suspected of being non-adherent. In three out of four patients, switching to a biological agent can be avoided.