Determination of Association Constants of Macropa-based Chelators for the Stable Complexation of Barium-131 and Lanthanum-133

Objectives
Theranostic concepts and the usage of alpha-particle emitting radionuclides belong to the emerging fields of radiopharmaceutical sciences. Especially and due to its excellent complexation properties, the chelator macropa (mcp)[1,2] was reported to be a suitable complexing agent for 225Ac conjugates[3] and as macrocycle a highly promising starting point for the development of 223/224Ra chelators as well. To follow the theranostic approaches with these alpha emitters, due to their chemical similarities 131Ba[4] for SPECT and 133La[5] for PET are available as diagnostic radionuclide surrogates of radium and actinium, respectively. In our recent study, we aim to establish a new workflow to evaluate and predict the complex stability of new chelating systems by obtaining both protonation constants for respective ligands and association constants for their metal complexes starting with mcp.

Methods
As a prerequisite for the calculation of stability constants (log(K)), the protonation constants (pKa) of mcp were determined by pH‑dependent 1H NMR studies in the first step. Based on these obtained data, the europium-mcp-complex was examined using time-resolved, laser-induced luminescence spectroscopy (TRLFS) and the log(K) was calculated as reference stability constant. Additionally, Eu-TRLFS was used to determine the Eu-mcp species during titration and pH-dependency of the complexation. The method of isothermal titration calorimetry (ITC) was used to measure the complex stability constants of La-, Ba-mcp-complex, as well as Eu-mcp-complex to value the results in comparison with TRLFS data. The evaluation and calculation of the log(K) values were carried out by parallel factor analysis (PARAFAC).

Results
Depending on the protonation ability of the functional groups found in mcp (amines and carboxylates), pKa values were obtained as well as the log(K) values for the respective mcp-complexes with Ba, Eu and La and are shown in Table 1.

Conclusion
By combining 1H-NMR, TRLFS and ITC we demonstrated a new approach to fully characterize the ligand, determine metal complex speciation, pKa values for the mcp chelator and log(K) values of the mcp complexes and agree with previous works by potentiometric titration. Furthermore, this method can be transferred to functionalized and improved chelators and their complexes with a wide variety of metal ions of radionuclides used in nuclear medicine, especially the heavy alkaline earth metal ions Ba2+ and Ra2+. Ultimately, this allows a reliable comparison of the individual affinities of the different chelators to the metal ions and thus a predictability of the complex stabilities for future radiopharmaceuticals.