Multivalent Bifunctional Chelator Scaffolds for Gallium-68 Based Positron Emission Tomography Imaging Probe Design: Signal Amplification via Multivalency

摘要

The role of the multivalent effect has been well recognized in the design of molecular imaging probes towards the desired imaging signal amplification. Recently we reported a bifunctional chelator (BFC) scaffold design, which provides a simple and versatile approach to impart multivalency to radiometal based nuclear imaging probes. In this work, we report a series of BFC scaffolds (^tBu3->1-COOH, ^tBu3->2-(COOH)2 and ^tBu3->3-(COOH)3) constructed on the framework of 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) for ⁶⁸Ga-based PET probe design and signal amplification via multivalent effect. For proof of principle, a known integrin αvβ3 specific ligand (c(RGDyK)) was used to build the corresponding NOTA conjugates (H3>1, H3>2, and H3>3), which present 1 – 3 copies of c(RGDyK) peptide, respectively, in a systematic manner. Using the integrin αvβ3 binding affinities (IC50 values), the enhanced specific binding was observed for multivalent conjugates (H3>2: 43.9 ± 16.1 nM; H3>3: 14.7 ± 5.0 nM) as compared to their monovalent counterpart (H3>1: 171 ± 60 nM) and the intact c(RGDyK) peptide (204 ± 76 nM). The obtained conjugates were efficiently labeled with ⁶⁸Ga³⁺ within 30 min at room temperature in high radiochemical yields (> 95%). The in vivo evaluation of the labeled conjugates, ⁶⁸Ga->1, ⁶⁸Ga->2 and ⁶⁸Ga->3, was performed using male severe combined immunodeficiency (SCID) mice bearing integrin α_vβ₃ positive PC-3 tumor xenografts (n = 3). All ⁶⁸Ga -labeled conjugates showed high in vivo stability with no detectable metabolites found by radio-HPLC within 2 h post-injection (p.i.). The PET signal amplification in PC-3 tumor by multivalent effect was clearly displayed by the tumor uptake of the ⁶⁸Ga-labeled conjugates (⁶⁸Ga->3: 2.55 ± 0.50%ID/g; ⁶⁸Ga->2: 1.90 ± 0.10 %ID/g; ⁶⁸Ga->1: 1.66 ± 0.15 %ID/g) at 2 h p.i. In summary, we have designed and synthesized a series of NOTA-based BFC scaffolds with signal amplification properties, which may find potential applications in diagnostic gallium radiopharmaceuticals.