Improving the stability of 11C–labeled L-methionine with ascorbate

Abstract

Background: Carbon-11 labeled L-methionine (11C–MET) is a popular tracer used in the clinic for imaging brain tumors with positron emission tomography. However, the stability of 11C–MET in its final formulation is not well documented in literature. Recently, we observed fast degradation of HPLC-purified 11C–MET over time, and systematic investigation was conducted to identify the cause. Results: In this study, we verified the degraded product as 11C–labeled methionine sulfoxide (11C–METSO). To minimize oxidation, ascorbate (100 ppm) was added to the HPLC eluant, and the resulting HPLC-purified 11C–MET was stable in the final formulation solution without noticeable degradation for up to 1 h after the end of synthesis. Conclusions: Our data suggest that to minimize degradation, ascorbate can be added to the 11C–MET formulation solution especially if it is not administered into patients soon after the end of synthesis.