Enhancing PET Signal at Target Tissue in Vivo: Dendritic and Multimeric Tris(hydroxypyridinone) Conjugates for Molecular Imaging of α_vβ₃ Integrin Expression with Gallium-68

Tris­(hydroxypyridinone) chelators conjugated to peptides can rapidly complex the positron-emitting isotope gallium-68 (⁶⁸Ga) under mild conditions, and the resulting radiotracers can delineate peptide receptor expression at sites of diseased tissue in vivo. We have synthesized a dendritic bifunctional chelator containing nine 1,6-dimethyl-3-hydroxypyridin-4-one groups (SCN-HP₉) that can coordinate up to three Ga³⁺ ions. This derivative has been conjugated to a trimeric peptide (RGD₃) containing three peptide groups that target the α_vβ₃ integrin receptor. The resulting dendritic compound, HP₉-RGD₃, can be radiolabeled in 97% radiochemical yield at a 3-fold higher specific activity than its homologues HP₃-RGD and HP₃-RGD₃ that contain only a single metal binding site. PET scanning and biodistribution studies show that [⁶⁸Ga­(HP₉-RGD₃)] demonstrates higher receptor-mediated tumor uptake in animals bearing U87MG tumors that overexpress α_vβ₃ integrin than [⁶⁸Ga­(HP₃-RGD)] and [⁶⁸Ga­(HP₃-RGD₃)]. However, concomitant nontarget organ retention of [⁶⁸Ga­(HP₉-RGD₃)] results in low tumor to nontarget organ contrast in PET images. On the other hand, the trimeric peptide homologue containing a single tris­(hydroxypyridinone) chelator, [⁶⁸Ga­(HP₃-RGD₃)], clears nontarget organs and exhibits receptor-mediated uptake in mice bearing tumors and in mice with induced rheumatoid arthritis. PET imaging with [⁶⁸Ga­(HP₃-RGD₃)] enables clear delineation of α_vβ₃ integrin receptor expression in vivo.