Determination of thiopurine S-methyltransferase phenotype using thin-layer chromatography and quantitative scanning

Objective: To develop a non-high-performance liquid chromatography method for the determination of thiopurine-S-methyltransferase (TPMT) phenotype using thin-layer chromatography and quantitative scanning. Methods: TPMT reaction was performed using a radiochemical assay. The reaction product [14C]-6-methylmercaptopurine was separated using thin-layer chromatography and quantified by means of radioactive scanning. Day-to-day variance was determined to validate results. Results: Determination of TPMT phenotype using thin-layer chromatography and quantitative scanning is reliable (day-to-day variance 8.5±1.7%, mean±SEM). Mean TPMT activity in 314 randomly selected patient samples was 11.8±3.3 units/ml red blood cells (mean±SD, range 3.5–25 units/ml). Conclusion: We developed a new assay variant for the determination of TPMT phenotype that is easy to perform, reliable and reduces production of radioactive waste. This may lead to more frequent pretreatment determination of TPMT phenotype and increase drug safety and efficacy by individualising thiopurine doses.