Synthesis and Evaluation of New Bifunctional Chelators with Phosphonic Acid Arms for Gallium-68 Based PET Imaging in Melanoma.

Due to the increasing use of generator-produced radiometal Gallium-68 (Ga) in positron-emission tomography/computed tomography (PET/CT), reliable bifunctional chelators that can efficiently incorporate Ga into biomolecules are highly desirable. In this study, we synthesized two new bifunctional chelators bearing one or two phosphonic acid functional groups, named p-SCN-PhPr-NE2A1P and p-SCN-PhPr-NE2P1A, with the aim of enabling facile production of Ga-based radiopharmaceuticals. Both chelators were successfully conjugated to LLP2A-PEG, a very late antigen-4 (VLA-4) targeting peptidomimetic ligand, to evaluate their application in Ga-based PET imaging. NE2P1A-PEG-LLP2A exhibited the highest Ga binding ability with molar activity of 37 MBq/nmol under mild temperature and neutral pH. Excellent serum stability of Ga-NE2P1A-PEG-LLP2A was observed, which was consistent with the result obtained from density functional theory calculation. The in vitro cell study showed that Ga-NE2P1A-PEG-LLP2A had significantly longer retention in B16F10 cells comparing to the reported retention of Cu-NE3TA-PEG-LLP2A, although the uptake was relatively lower. In the biodistribution and micro-PET/CT imaging studies, high tumor uptake and low background were observed after Ga-NE2P1A-PEG-LLP2A was injected into mice bearing B16F10 tumor xenografts, making it a highly promising radiotracer for noninvasive imaging of VLA-4 receptors overexpressed in melanoma.