Versatile Diphosphine Chelators for Radiolabeling Peptides with Tc and Cu.

We have developed a diphosphine (DP) platform for radiolabeling peptides with Tc and Cu for molecular SPECT and PET imaging, respectively. Two diphosphines, 2,3-bis(diphenylphosphino)maleic anhydride (DP) and 2,3-bis(di--tolylphosphino)maleic anhydride (DP), were each reacted with a Prostate Specific Membrane Antigen-targeted dipeptide (PSMAt) to yield the bioconjugates DP-PSMAt and DP-PSMAt, as well as an integrin-targeted cyclic peptide, RGD, to yield the bioconjugates DP-RGD and DP-RGD. Each of these DP-PSMAt conjugates formed geometric /-[MO(DP-PSMAt)] (M = Tc, Tc, Re; X = Ph, Tol) complexes when reacted with [MO] motifs. Furthermore, both DP-PSMAt and DP-PSMAt could be formulated into kits containing reducing agent and buffer components, enabling preparation of the new radiotracers /-[TcO(DP-PSMAt)] and /-[TcO(DP-PSMAt)] from aqueous TcO in 81% and 88% radiochemical yield (RCY), respectively, in 5 min at 100 °C. The consistently higher RCYs observed for /-[TcO(DP-PSMAt)] are attributed to the increased reactivity of DP-PSMAt over DP-PSMAt. Both /-[TcO(DP-PSMAt)] and /-[TcO(DP-PSMAt)] exhibited high metabolic stability, and SPECT imaging in healthy mice revealed that both new radiotracers cleared rapidly from circulation, via a renal pathway. These new diphosphine bioconjugates also furnished [Cu(DP-PSMAt)] (X = Ph, Tol) complexes rapidly, in a high RCY (>95%), under mild conditions. In summary, the new DP platform is versatile: it enables straightforward functionalization of targeting peptides with a diphosphine chelator, and the resulting bioconjugates can be simply radiolabeled with both the SPECT and PET radionuclides, Tc and Cu, in high RCYs. Furthermore, the DP platform is amenable to derivatization to either increase the chelator reactivity with metallic radioisotopes or, alternatively, modify the radiotracer hydrophilicity. Functionalized diphosphine chelators thus have the potential to provide access to new molecular radiotracers for receptor-targeted imaging.