Molecular imaging is an emerging technology that allows for the visualization of the interactions between molecular probes and biological targets. Positron emission tomography (PET), in particular, is a useful modality that enables in vivo biological information to be obtained in a noninvasive manner using a variety of PET radiopharmaceuticals.1 Radiolabeled peptides are becoming increasingly important in nuclear oncology, where they are used in the diagnosis and treatment of several different cancers. Cyclic RGD peptides, such as cyclo[Arg-Gly-Asp-D-Tyr-Lys], are potent antagonists for the αvβ3 integrin receptor.2 A variety of different cyclic RGD peptides conjugated to a radioactive tracer have been reported for the PET imaging of tumors that over-express the αvβ3 integrin receptor.3
We reported here radiosynthesis and preliminary PET study of a 11C-labelled peptide containing [1-11C]1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid ([1-11C]Tpi), from corresponding cyclo[Arg-Gly-Asp-D-Tyr-Lys(Trp)] hydrochloride via a Pictet-Spengler reaction by using [11C]formaldehyde. We proceeded to investigate the remote-controlled radiosynthesis of cyclo[Arg-Gly-Asp-D-Tyr-Lys([1-11C]Tpi)] ([11C]1) using an automatic production system to generate the [11C]CH3I. From a starting point in the range of 21.0-22.2 GBq for the [11C]CO2, [11C]1 was obtained at the end of synthesis in the range of 0.8-1.4 GBq. The average time required for the synthesis was found to be 35 min from the end of the bombardment. The radiochemical purity of [11C]1 was found to be greater than 98% and its specific activity was 85.7±9.4 GBq/μmol.
The usefulness of our synthesis was demonstrated PET studies using [11C]1 for the mouse bearing MIAPaca-2 and Bx-PC3, a pancreatic cancer cell line. The Bx-PC3 cell-to-muscle and MIAPaca-2 cell-to-muscle of [11C]1 at 22 min were retained at 2.8 and 1.3, respectively. Further details about the tumor imaging studies by PET will be reported.