Syntheses of β- 11 C-labelled L -tryptophan and 5-hydroxy- L -tryptophan using a multi-enzymatic reaction route

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Multi-enzymatic syntheses of L-[β-11C]tryptophan and 5-hydroxy-L-[β-11C]tryptophan from racemic [3-11C]alanine are reported. 11C-Labelled alanine was prepared by an alkylation of a glycine derivative, N(diphenylmethylene)glycine t-butyl ester, with [11C]methyl iodide obtained from [11C]carbon dioxide, and subsequent hydrolysis. The enzymatic syntheses were carried out in a one-pot reaction using D-amino acid oxidase/catalase, glutamic-pyruvic transaminase, and tryptophanase. The total synthesis time was 50 to 55 min, including h.p.l.c. purification, counted from the start of [11C]methyl iodide synthesis. The yields were ca. 25%, decay corrected, of purified sterilized enantiomerically pure L-[β-11C]tryptophan and 5-hydroxy-L-[β-11C]tryptophan with radiochemical purifies of >98%. The specific activities were in the order of 2.5 GBq/µmol. In a typical run starting with 4.4 GBq [11C]carbon dioxide, 220 MBq of β-11C-labelled L-tryptophan or 5-hydroxy-L-tryptophan was obtained ready for use in human studies.