Synthesis of a new bifunctional NODA for bioconjugation with PSMA ligand and one-step Al18F labeling

Abstract

The Al¹⁸F labeling method is a relatively new approach that allows radiofluorination of biomolecules such as peptides and proteins in a one-step procedure and in an aqueous solution. However, instability of the complex of [AlF]²⁺ with hexadentate chelator NOTA may attribute to the disassociation of free ¹⁸F^- and [Al¹⁸F]²⁺ and accumulation in bone. In this study, we designed and synthesized a new bifunctional pentadentate AlF-chelator p-SCN-PhPr-NODA as well as its nitro form p-NO₂-PhPr-NODA. Chelator p-NO₂-PhPr-NODA exhibited increased Al (III) complexation kinetics determined by AA III complexation kinetic studies and stronger coordination ability towards [AlF]²⁺ according to DFT calculation studies in comparison with hexadentate chelator NOTA. As a proof of concept, bifunctional chelator p-SCN-PhPr-NODA was furthermore conjugated to a PSMA targeting moiety Glu-urea-Lys to form NODA-PrPh-GuL. The conjugated peptide showed acceptable radiochemical yield (12.5-16.4%) and efficiency with an excellent radiochemical purity (∼100% after SPE purification) in Al¹⁸F labeling. The labeled peptide exhibited good in vitro stability and significant specificity for PSMA. Biodistribution study and MicroPET scan in healthy Kun Ming mice with the labeled peptide were performed and demonstrated excellent in vivo stability of Al¹⁸F-labeled construct. In general, the successful application of the new bifunctional chelator in labeling dipeptide Glu-urea-Lys with Al¹⁸F could facilitate its possibility in conjugating with other peptides for PET imaging with enhanced in vivo stability, thus providing better in vivo performances.

Keywords: Aluminum fluoride; Chelator; Fluorine-18; PET; PSMA.