Automated solid phase radio-labeling of peptides with 4-[18F]fluorobenzoic and 2-[18F]fluoropropionic acid

Abstract

1887

Objectives: Incorporation of positron emitting radionuclide 18F (t1/2 110min) into peptides for target specific PET imaging is exclusively carried out by employing a variety of prosthetic groups. We developed earlier a solid phase approach for conjugation of 4-[18F]fluorobenzoic (FBA) and 2-[18F]fluoropropionic (FPA) acids to the protected peptides attached to a solid phase synthesis support. Our method provides regio-selectively radio-labeled peptides in high purity as opposed to the labeling of the non-protected peptides in solution using N-hydroxy succinimidyl 4-[18F]fluorobenzoate. Solid phase synthesis allows easy automation of many synthetic procedures, hence robotic synthesizers are widely used in the synthetic peptide chemistry. We developed a simple solid phase synthesizer for the conjugation of FPA or FBA to the peptide followed by the cleavage of the product from the solid support.

Methods: The 4-[18F]fluorobenzoic acid was synthesized using the CTI/CPCU using modified parameter settings and purified on C-18 Sep-Pak cartridge using an automatic syringe pump. The 2-[18F]fluoropropionic acid was synthesized manually. We used Kloehn programmable automatic syringe pump equipped with an 8-port head to assemble the continuous flow synthesizer for radio-labeling of the peptide on solid support.

Results: The resin is enclosed in the Teflon syringe filter and connected to port 6 of the automatic syringe pump. The reagents and washing solvents are added to the resin through ports 1-5. During the coupling and cleavage steps, the mixture is pumped continuously through the resin using ports 6 and 7. Port 8 is used to discard the reagents to the waste container. Linux was chosen as a reliable platform for developing the control software for the syringe pump and the graphical user interface was developed using GTK+ libraries. The program allows the user to set specific synthetic parameters and run the steps one by one or to perform the whole synthesis.

Conclusions: The continuous flow synthesizer was used to conjugate peptides with FPA and FBA. The crude products were obtained in purity greater than 90% and yields comparable with manually performed synthesis.