Automated One-pot Radiosynthesis of [11C]S-adenosyl Methionine

Abstract

Background: Glycine N-methyltransferase is an enzyme overexpressed in some neo-plastic tissues. It catalyses the methylation of glycine using S-adenosyl methionine (SAM or AdoMet) as substrate. SAM is involved in a great variety of biochemical processes, including transmethylation reactions. Thus, [ 11 C]SAM could be used to evaluate transmethylation activity in tumours. The only method reported for [ 11 C]SAM synthesis is an enzymatic process with several limitations. We propose a new chemical method to obtain [ 11 C]SAM, through a one-pot synthesis. Method: The optimization of [ 11 C]SAM synthesis was carried out in the automated TRACERlab ® FX C Pro module. Different labelling conditions were performed varying methylating agent, precursor amount, temperature and reaction time. The compound was purified using a semi-preparative HPLC. Radiochemical stability, lipophilicity and plasma protein binding were evaluated. Results: The optimum labelling conditions were [ 11 C]CH 3 OTf as the methylating agent, 5 mg of precursor dissolved in formic acid at 60 ºC for 1 minute. [ 11 C]SAM was obtained as a diastereo-meric mixture. Three batches were produced and quality control was performed according to specifications. [ 11 C]SAM was stable in final formulation and in plasma. Log P OCT obtained for [ 11 C]SAM was (-2,01 ± 0,07) (n=4), and its value for plasma protein binding was low. Conclusion: A new chemical method to produce [ 11 C]SAM was optimized. The radiotracer was obtained as a diastereomeric mixture with a 53:47 [(R,S)-isomer: (S,S)-isomer] ratio. The compound was within the quality control specifications. In vitro stability was verified. This compound is suitable to perform preclinical and clinical evaluations.