Synthesis of three 11C-labelled methionine-containing enkephalin analogues

The synthesis of three S-[methyl- 11C]-labelled enkephalin analogues, Tyr-D-Ala-Gly-Phe-Met-NH2, Tyr-D-Ala-D-Ala-Phe-Met-NH2 and Tyr-D-Met-Gly-Phe-Pro-NH2, from the corresponding S-benzyl-homocysteine-containing peptides is reported. The protected pentapeptide amides were prepared by (3+2) fragment condensations in solution. These peptides were subsequently deprotected with sodium in liquid ammonia and the sulphide anions formed alkylated with [11C]-methyl iodide to give the S-[methyl-11C]-labelled enkephalins. After purification by preparative LC, these labelled peptides were obtained in 55 to 75% radiochemical yield. The radiochemical purities of the products were higher than 98%, and the specific activity was in the order of 20-200 mCi/μmol. (author)