Stability of thiopurine metabolites: a potential analytical bias

In a report recently published in this journal (1), Reinshagen et al., who investigated 6-thioguanine nucleotide (6-TGN) concentrations in patients with inflammatory bowel disease (IBD), concluded that standard and adapted dosing of azathioprine led to identical 6-TGN concentrations and remission rates and that therapeutic drug monitoring of thiopurine therapy was of no clinical benefit in patients with a wild-type thiopurine S-methyltransferase (TPMT) genotype. Apart from these analytical issues, we have again demonstrated an essential and clinically relevant decrease in both 6-TGN and 6-MMP concentrations attributable to sample storage/ shipping conditions, findings that are of pivotal importance for the use of therapeutic monitoring in (future) multicenter studies and for interpretation of pharmacological data in clinical practice in patients on thiopurine therapy.