An alternative methodology for the determination of the radiochemical purity of (11)C-methionine

BACKGROUND: [(11)C]Methionine ([(11)C]MET) acts as an oncological radiopharmaceutical for measuring increased protein synthesis in brain tumours while having low uptake in normal brain an. In several Latin American countries, the official pharmacopoeias are the United States of America Pharmacopoeia (USP) and the European Pharmacopoeia (EP). For determination of the radiochemical purity, we used the monograph of L-methionine injection ([(11)C] methyl-methionine) of the EP (01/2002: 1617) because it is not available in the USP. We present herein an alternative methodology for the determination of the radiochemical purity, which allows the assessment of the [(11)C]CH(3)I and of the degradation radioactive products. RESULTS: The proposed method enabled a greater number of impurities to be determined than could be identified using the EP method. Under the conditions proposed by EP, it is not possible to determine the presence of [(11)C]CH(3)I or other lipophilic radiochemical impurities in the final product. For the determination of enantiomeric purity according to EP a chiral thin-layer chromatography (TLC) allows the determination of D and L methionine between the established limits. This is a slow method and presents an inadequate detection due to the decay of the sample. An alternative HPLC method was optimized. CONCLUSION: The proposed radiochemical purity method allowed the identification of a greater number of radiochemical impurities compared to the method proposed by the EP monograph in 16 min. The proposed enantiomeric purity method enables to perform the determination in a reasonable amount of time. A discussion concerning the limit of 95% radiochemical purity proposed by the EP is necessary. If more radiochemical impurities can be detected, it is more reasonable to propose a limit of 90% for this parameter to perform clinical examinations.