Feasibility of a stereoselective synthesis of [11C](S, S)-S-adenosylmethionine ([11C](S,S)-SAM) catalyzed by an immobilized enzyme

Abstract

This work aims to develop a stereoselective enzymatic alternative for the radiosynthesis of [11C](S,S)-S-ade-nosylmethionine ([11C](S,S)-SAM), a potential PET-CT radiotracer for monitoring particularly aggressive prostate tumors. Conventional synthesis of this compound has been carried out at Uruguayan Center of Molecular Imaging, resulting in an almost racemic mixture 53:47 ratio of (R,S) to (S,S) isomer. Producing the radiotracer in an optically pure form is a requirement for administration to humans and additionally it would enhance diagnostic sensitivity when administered to the patient. The main challenges were designing a biocatalyst capable of withstanding the harsh conditions of the radiotracer synthesis module and achieving the reaction in a very short time due to the rapid decay of 11C. A mutant of E. coli methionine adenosyltransferase (I303V MAT) with enhanced SAM synthesis was cloned, expressed, and immobilized on agarose using an irreversible covalent isourea bond. This immobilized enzyme synthesized [11C](S,S)-SAM from [11C]L-methionine in an automated module, with the labeled methionine produced in situ from [11C]CH3I and L-homocysteine thiolactone. The product was obtained with an enantio and diasteromeric excess greater than 99 % and average conversion of 80 %. The reuse of the immobilized enzyme was studied, showing that after three cycles of reuse the radiosynthesis performance remained unchanged.