Enhancing PET Signal at Target Tissue in Vivo: Dendritic and Multimeric Tris(hydroxypyridinone) Conjugates for Molecular Imaging of α<sub>v</sub>β<sub>3</sub> Integrin Expression with Gallium-68

Tris­(hydroxypyridinone) chelators conjugated to peptides can rapidly complex the positron-emitting isotope gallium-68 (<sup>68</sup>Ga) 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-HP<sub>9</sub>) that can coordinate up to three Ga<sup>3+</sup> ions. This derivative has been conjugated to a trimeric peptide (RGD<sub>3</sub>) containing three peptide groups that target the α<sub>v</sub>β<sub>3</sub> integrin receptor. The resulting dendritic compound, HP<sub>9</sub>-RGD<sub>3</sub>, can be radiolabeled in 97% radiochemical yield at a 3-fold higher specific activity than its homologues HP<sub>3</sub>-RGD and HP<sub>3</sub>-RGD<sub>3</sub> that contain only a single metal binding site. PET scanning and biodistribution studies show that [<sup>68</sup>Ga­(HP<sub>9</sub>-RGD<sub>3</sub>)] demonstrates higher receptor-mediated tumor uptake in animals bearing U87MG tumors that overexpress α<sub>v</sub>β<sub>3</sub> integrin than [<sup>68</sup>Ga­(HP<sub>3</sub>-RGD)] and [<sup>68</sup>Ga­(HP<sub>3</sub>-RGD<sub>3</sub>)]. However, concomitant nontarget organ retention of [<sup>68</sup>Ga­(HP<sub>9</sub>-RGD<sub>3</sub>)] 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, [<sup>68</sup>Ga­(HP<sub>3</sub>-RGD<sub>3</sub>)], clears nontarget organs and exhibits receptor-mediated uptake in mice bearing tumors and in mice with induced rheumatoid arthritis. PET imaging with [<sup>68</sup>Ga­(HP<sub>3</sub>-RGD<sub>3</sub>)] enables clear delineation of α<sub>v</sub>β<sub>3</sub> integrin receptor expression in vivo.