Limited stability of thiopurine metabolites in blood samples: Relevant in research and clinical practise

Monitoring of thiopurine metabolites 6-thioguanine nucleotides (6-TGN) and 6-methylmercaptopurine (6-MMP) is used to assess compliance and explain adverse reactions in IBD-patients. Correlations between dosage, metabolite concentrations and therapeutic efficacy or toxicity are contradictive. Research is complicated by analytical problems as matrices analyzed and analytical procedures vary widely. Moreover, stability of thiopurine metabolites is not well documented, yet pivotal for interpretation of analytical outcomes. Therefore, we prospectively investigated metabolite stability in blood samples under standard storage conditions. Stability at room temperature and refrigeration (22 °C, 4 °C) was investigated during 1 week and frozen samples (−20 °C, −80 °C) were analyzed during 6 months storage. Ten patient samples were analyzed for each study period. Median 6-TGN concentrations on day 7 decreased significantly to 53% and 90% during storage at ambient temperature or refrigeration. Median 6-MMP concentrations on day 7 decreased significantly to 55% and 86%, respectively. Samples stored at −20 °C also showed significant decreases in both 6-TGN and 6-MMP in comparison with baseline values. At −80 °C, only 6-MMP showed a significant decrease in values compared to baseline. The stability of thiopurine metabolites is clearly a limiting factor in studies investigating utilisation of TDM and correlations with therapeutic outcome in IBD-patients. This has to be accounted for in clinical practice and (multi-center) trials investigating thiopurine drugs.