Carbon‐11 radiolabeling of an oligopeptide containing tryptophan hydrochloride via a Pictet‐Spengler reaction using carbon‐11 formaldehyde

A procedure for the synthesis of a 11 C‐labeled oligopeptide containing [1‐ 11 C]1,2,3,4‐tetrahydro‐β‐carboline‐3‐carboxylic acid ([1‐ 11 C]Tpi) from the corresponding Trp•HCl‐containing peptides has been developed involving a Pictet‐Spengler reaction with [ 11 C]formaldehyde. The synthesis of [1‐ 11 C]Tpi from Trp and [ 11 C]formaldehyde was examined as a model reaction with the aim of developing a facile and effective method for the labeling of peptides with carbon‐11. The Pictet‐Spengler reaction of Trp and [ 11 C]formaldehyde in acidic media (TsOH or HCl) afforded the desired [1‐ 11 C]Tpi in a moderate radiochemical yield. Herein, the application of a Pictet‐Spengler reaction to an aqueous solution of Trp•HCl gave the desired product with a radiochemical yield of 45.2%. The RGD peptide cyclo[Arg‐Gly‐Asp‐D‐Tyr‐Lys] was then selected as a substrate for the labeling reaction with [ 11 C]formaldehyde. The radiolabeling of a Trp•HCl‐containing RGD peptide using the Pictet‐Spengler reaction was successful. Furthermore, the remote‐controlled synthesis of a [1‐ 11 C]Tpi‐containing RGD peptide was attempted by using an automatic production system to generate [ 11 C]CH 3 I. The radiochemical yield of the [1‐ 11 C]Tpi‐containing RGD at the end of synthesis (EOS) was 5.9 ± 1.9% ( n  = 4), for a total synthesis time of about 35 min. The specific activity was 85.7 ± 9.4 GBq/µmol at the EOS. Copyright © 2013 European Peptide Society and John Wiley & Sons, Ltd.