Synthesis and evaluation of novel bifunctional chelating agents are primarily designed to sequester Ga(III) isotopes, but these ligands are also evaluated to address the chelation of transition metals for both radiopharmaceutical and chemotherapeutic applications. For example, the bifunctional 1,4,7-triazacyclononane-N,N',N''- triacetic acid (NOTA) has been shown to be an exceptionally stable sequestering agent for Ga-66 in vivo. Evaluation of C-functionalized NOTA for Ga(III) isotopes continues with use of Ga-68 and Ga-66 for use in PET imaging. Novel chelating agents, based on cis,cis-1,3,5-triaminocyclohexane (tach) being used as a platform for introducing a wide variety of metal binding functional groups, continue to be exploited. Numerous novel chelating agents based upon tach have been synthesized, characterized, and evaluated for forming metal complexes with a variety of transition metal ions. Specifically, tris(pyridyl)triamine derivatives of tach (tachpyr) continue to be investigated for chemotherapeutic applications. These ligands disrupt of cellular iron transport and storage mechanisms as a pathway for apoptotic cytotoxicity. Studies with Fe(II)[tachpyr] have also demonstrated the reactive oxidative nature of the ligand with Fe(III) forming Fe(II) and then cycling through redox cycles and Fenton chemistry. Preliminary structure activity relationship (SAR) studies into tuning lipophilicy and electronic nature of the pyridine donors of tachpyr have been initiated through the introduction of methyl substituents onto the aromatic rings of TACHpyr. These studies on the fundamental structure and stability of the metal complexes formed with added methyl groups produced preliminary SAR information indicating severe limitations of the 6-position of the pyridyl ring, but also enhancement of activity with substitution at the 3-position. Further studies to introduce electron-withdrawing groups to perturb the electronic nature of the environment of the chelated Fe metal ion as well as to alter the overall charge of the complex are ongoing. In parallel, modifications are also being planned to increase the biological half-life of these agents. Copper complexes of several TACH ligands that demonstrated the ability to hydrolytically cleave DNA phosphate ester bonds in model compounds, to cleave plasmid DNA, and to exert significant cytotoxicity in vitro continue to be investigated. The TACHpyr complex has also been evaluated as a potential radiopharmaceutical along with all of the other heterocyclic hexacoordinating tach based ligands with Cu-64 and Cu-67. Significant in vitro stability has been noted for the tachpyr analogs not substituted with methyl groups at the 6-position. Also evaluated has been a tachpyr analog wherein the pyridyl moieties have been replaced with imidazole groups. These ligands were found to efficiently transchelate Cu(II) from the complex formed with TETA, a macrocyclic chelating agent employed in clinical trials that has been reported compromised due to in vivo transchelation. Further in vivo evaluation of the Cu(II) radio-metal complexes of these ligands are planned to fully evaluate their potential prior to embarking of syntheses of bifunctional analogs for protein modification.
