Side-by-Side Comparison of Five Chelators for Zr-Labeling of Biomolecules: Investigation of Chemical/Radiochemical Properties and Complex Stability.

In this work, five different chelating agents, namely DFO, CTH-36, DFO*, 3,4,3-(LI-1,2-HOPO) and DOTA-GA, were compared with regard to the relative kinetic inertness of their corresponding Zr complexes to evaluate their potential for in vivo application and stable Zr complexation. The chelators were identically functionalized with tetrazines, enabling a fully comparable, efficient, chemoselective and biorthogonal conjugation chemistry for the modification of any complementarily derivatized biomolecules of interest. A small model peptide of clinical relevance (TCO-c(RGDfK)) was derivatized via iEDDA click reaction with the developed chelating agents (TCO = trans-cyclooctene and iEDDA = inverse electron demand Diels-Alder). The bioconjugates were labeled with Zr, and their radiochemical properties (labeling conditions and efficiency), log, as well as the relative kinetic inertness of the formed complexes, were compared. Furthermore, density functional theory (DFT) calculations were conducted to identify potential influences of chelator modification on complex formation and geometry. The results of the DFT studies showed-apart from the DOTA-GA derivative-no significant influence of chelator backbone functionalization or the conjugation of the chelator tetrazines by iEDDA. All tetrazines could be efficiently introduced into c(RGDfK), demonstrating the high suitability of the agents for efficient and chemoselective bioconjugation. The DFO-, CTH-36- and DFO*-modified c(RGDfK) peptides showed a high radiolabeling efficiency under mild reaction conditions and complete Zr incorporation within 1 h, yielding the Zr-labeled analogs as homogenous products. In contrast, 3,4,3-(LI-1,2-HOPO)-c(RGDfK) required considerably prolonged reaction times of 5 h for complete radiometal incorporation and yielded several different Zr-labeled species. The labeling of the DOTA-GA-modified peptide was not successful at all. Compared to [Zr]Zr-DFO-, [Zr]Zr-CTH-36- and [Zr]Zr-DFO*-c(RGDfK), the corresponding [Zr]Zr-3,4,3-(LI-1,2-HOPO) peptide showed a strongly increased lipophilicity. Finally, the relative stability of the Zr complexes against the EDTA challenge was investigated. The [Zr]Zr-DFO complex showed-as expected-a low kinetic inertness. Unexpectedly, also, the [Zr]Zr-CTH-36 complex demonstrated a high susceptibility against the challenge, limiting the usefulness of CTH-36 for stable Zr complexation. Only the [Zr]Zr-DFO* and the [Zr]Zr-3,4,3-(LI-1,2-HOPO) complexes demonstrated a high inertness, qualifying them for further comparative in vivo investigation to determine the most appropriate alternative to DFO for clinical application.