Triazine-Based Tool Box for Developing Peptidic PET Imaging Probes: Syntheses, Microfluidic Radio labeling, and Structure-Activity Evaluation

This study was aimed at developing a triazine-based modular platform for targeted PET imaging. We synthesized mono- or bis-cyclo(RGDfK) linked triazine-based conjugates specifically targeting integrin alpha(v)beta(3) receptors. The core molecules could be easily linked to targeting peptide and radiolabeled bifunctional chelator. The spacer core molecule was synthesized in 2 or 3 steps in 64-80% yield, and the following conjugation reactions with cyclo(RGDfK) peptide or bifunctional chelator were accomplished using click chemistry or amidation reactions. The DOTA-TZ-Bis-cyclo(RGDfK) 13 conjugate was radiolabeled successfully with Cu-64(OAc)(2) using a microfluidic method, resulting in higher specific activity with above 95% labeling yields compared to conventional radiolabeling (SA ca. 850 vs 600 Ci/mmol). The dimeric cyclo(RGDfK) peptide was found to display significant bivalency effect using I-125-Echistatin binding assay with IC50 value as 178.5 +/- 57.1 nM, which displayed a 3.6-fold enhancement of binding affinity compared to DOTA-TZ-cyclo(RGDfK) 14 conjugate on U87MG human glioblastoma cell. Biodistribution of all four conjugates in female athymic nude mice were evaluated. DOTA-Click-cyclo(RGDfK) 15 had the highest tumor uptake among these four at 4 h p.i. with 1.90 +/- 0.65%ID/g, while there was no clear bivalency effect for DOTA-TZ-BisRGD in vivo, which needs further experiments to address the unexpected questions.