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

Abstract

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.