Evaluation of Stability of Thiopurine Metabolites Using a Validated LC-MS/MS Method.

Measurement of thiopurine metabolites is helpful to monitor adverse effects and assess compliance in patients on thiopurine treatment. The purpose of this study was to develop and validate an analytical method for measurement of thiopurine metabolites, thioguanine nucleotides (6-TGN) and 6-methylmercaptopurine nucleotide (6-MMPN), in RBCs. We developed and validated a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the quantification of 6-TGN and 6-MMPN and evaluated the stability of the thiopurine metabolites in RBC and whole blood states without any preprocessing at various storage conditions. The linear range was 0.1-10 µmol/L and 0.5-100 µmol/L for 6-TGN and 6-MMPN, respectively. The mean extraction recovery at the two concentrations was 71.0% and 75.0% for 6-TGN, and 102.2% and 96.4% for 6-MMPN. Thiopurine metabolites in preprocessed RBC samples were stable at 25°C and 4°C after storage for 4 hours and at -70°C for up to 6 months. However, 6-TGN decreased by 30% compared with the initial concentration when stored at -20°C for 180 days. In whole blood states, 6-TGN decreased by about 20% at four days after storage at 4°C. We validated a reliable LC-MS/MS method and recommend that the patient's whole blood sample be preprocessed as soon as possible.

[Determination of urinary iodine concentration by inductively coupled plasma-mass spectrometry in thyroid cancer patients on low-iodine diet].

BACKGROUND: Thyroid cancer patients should be on low-iodine diet (LID) before radioactive iodine therapy (RAIT) to maximize the effect of RAIT. Urinary iodine excretion is the most accurate marker of very recent dietary iodine intake. We developed and evaluated the analytical performance of inductively coupled plasma-mass spectrometry (ICP-MS) to determine urinary iodine concentration. METHODS: We evaluated the linearity, precision, accuracy, and lower limit of quantification (LLOQ) of an ICP-MS method (Agilent 7500ce) to determine urinary iodine concentration in accordance with the Food and Drug Administration (FDA) guidelines for bioanalytical method validation. This method was used to determine and compare the iodine concentration in random urine samples of 120 thyroid cancer patients on LID for 1 week and 80 healthy adults on normal diet. RESULTS: Our ICP-MS method showed good linearity (1.0-1,913 microg/L; R(2)>0.999). Both intra-day and inter-day precision CV were within 20% for the LLOQ (1 microg/L) and within 15% for the other concentrations. Accuracy was 110-120% for the LLOQ and 95-115% for the other concentrations. The median concentration of iodine in random urine samples from thyroid cancer patients on LID (38.7 microg/L) was significantly lower than that of healthy subjects (238.8 microg/L) (P<0.0001). CONCLUSIONS: Urinary iodine analysis by ICP-MS showed good linearity, precision, accuracy, wide measuring range of detection, and lower LLOQ. This method will be very useful to evaluate the status of dietary iodine intake and the appropriateness of LID in thyroid cancer patients, thereby maximizing the effect of RAIT.