Abstract
1059
Objectives Currently applied somatostatin (SST) targeting positron emission tomography (PET) tracers are peptides labeled with radiometals via a coordination ligand.[1] As a consequence the native structure of the peptide is changed which complicates the translation from in vitro to in vivo and ultimately clinical applications. The aim of this project is to establish a direct and general method for native radiolabeling of short peptide sequences with carbon-11 to target SST receptors. A Grignard reaction was used to prepare [11C]benzyl iodide ([11C]BnI) for regioselective and asymmetric alkylation of a prochiral glycine precursor with this carbon-11 electrophile.
Methods The tetrapeptide precursor Ph2N=C-Gly-D-Trp(BOC)-L-Lys(BOC)-L-Thr(tBu)-OtBu was prepared by solution-phase peptide synthesis in 7 steps, resulting in 20% yield and >98% purity. Non-labeled L- and D-Phe-D-Trp-L-Lys-L-Thr-OH were synthesized for reference purposes in 6 steps, with 23-32% yield and >92% purity. [11C]BnI was synthesized in 3 steps starting from PhMgBr and [11C]CO2.[2] The target radiotracer D-[11C]-Phe-D-Trp-L-Lys-L-Thr-OH was prepared from [11C]BnI and the tetrapeptide precursor with the aid of a chiral phase-transfer catalyst. Diastereomeric excess was measured by chiral HPLC by co-elution with single diastereomer standards.
Results [11C]BnI was obtained in high radiochemical yield (52±3%), purity (95±3%) and specific activity (123±17% GBq/µmol).[2] Subsequently, addition of CsOH[asterisk]H2O to the biphenyl-imine Schiff base peptide precursor initiated solid-liquid phase-transfer catalyzed diastereoselective α-alkylation with [11C]BnI (30±8% radiochemical conversion). Acidic deprotection using a cleavage cocktail[4] yielded D-[11C]-Phe-D-Trp-L-Lys-L-Thr-OH at 26 min from the EOB as confirmed by co-injection on the C18 HPLC (>98% radiochemical conversion).
Conclusions Conclusions: We have demonstrated a powerful new radiosynthetic method for radiolabeling native peptide sequences with carbon-11, resulting in the first 11C-labeled native peptide D-[11C]Phe-D-Trp-L-Lys-L-Thr-OH. Current efforts include identification of an optimal chiral phase-transfer catalyst for the preparation of SST peptides. This method obviates the need for time-consuming and low-yielding HPLC purification to obtain single stereoisomers of carbon-11 labeled peptides. Acknowledgements: A.P. is a Fulbright Visiting Student Researcher and a Marie-Curie fellow (project RADIOMI, FP7-PEOPLE-2012-ITN). References: [1] Schottelius M, et al. (2009) Methods, 48, 161-177. [2] Pekošak A, et al. (2015) J. Label. Compd. Radiopharm., 58, 342-348. [3] Ooi T, et al. (2007) Ang. Chem. Int. Ed., 46, 4222-4266. [4] Applied Biosystems: Cleavage, Deprotection, and Isolation of Peptides after Fmoc Synthesis (2007)