Versatile Diphosphine Chelators for Radiolabeling Peptides with 99m Tc and 64 Cu.

We have developed a diphosphine (DP) platform for radiolabeling peptides with 99m Tc and 64 Cu for molecular SPECT and PET imaging, respectively. Two diphosphines, 2,3-bis(diphenylphosphino)maleic anhydride (DP Ph ) and 2,3-bis(di- p -tolylphosphino)maleic anhydride (DP Tol ), were each reacted with a Prostate Specific Membrane Antigen-targeted dipeptide (PSMAt) to yield the bioconjugates DP Ph -PSMAt and DP Tol -PSMAt, as well as an integrin-targeted cyclic peptide, RGD, to yield the bioconjugates DP Ph -RGD and DP Tol -RGD. Each of these DP-PSMAt conjugates formed geometric cis / trans -[MO 2 (DP X -PSMAt) 2 ] + (M = 99m Tc, 99g Tc, nat Re; X = Ph, Tol) complexes when reacted with [MO 2 ] + motifs. Furthermore, both DP Ph -PSMAt and DP Tol -PSMAt could be formulated into kits containing reducing agent and buffer components, enabling preparation of the new radiotracers cis / trans -[ 99m TcO 2 (DP Ph -PSMAt) 2 ] + and cis / trans -[ 99m TcO 2 (DP Tol -PSMAt) 2 ] + from aqueous 99m TcO 4 - in 81% and 88% radiochemical yield (RCY), respectively, in 5 min at 100 °C. The consistently higher RCYs observed for cis / trans -[ 99m TcO 2 (DP Tol -PSMAt) 2 ] + are attributed to the increased reactivity of DP Tol -PSMAt over DP Ph -PSMAt. Both cis / trans -[ 99m TcO 2 (DP Ph -PSMAt) 2 ] + and cis / trans -[ 99m TcO 2 (DP Tol -PSMAt) 2 ] + exhibited high metabolic stability, and in vivo SPECT imaging in healthy mice revealed that both new radiotracers cleared rapidly from circulation, via a renal pathway. These new diphosphine bioconjugates also furnished [ 64 Cu(DP X -PSMAt) 2 ] + (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, 99m Tc and 64 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.