Incorporation of Carboxylate Pendant Arms into 18‐Membered Macrocycles: Effects on [nat/203Pb]Pb(II) Complexation

We present a detailed investigation on the coordination chemistry of [nat/203Pb]Pb(II) with chelators H4PYTA and H4CHX‐PYTA. These chelators belong to the family of ligands derived from the 18‐membered macrocyclic backbone PYAN and present varying degrees of rigidity due to the presence of either ethyl or cyclohexyl spacers. A complete study of the stable Pb(II) complexes is carried out via NMR, X‐Ray crystallography, stability constant determination and computational studies. While these studies indicated that Pb(II) complexation is achieved, and the thermodynamic stability of the resulting complexes is very high, a certain degree of fluxionality does exist in both cases. Nevertheless, radiolabeling studies were carried out using SPECT (single photon emission computed tomography) compatible isotope lead‐203 (203Pb, t1/2=51.9 h), and while both chelators complex the radioisotope, the incorporation of carboxylate pendant arms appears to be detrimental towards the stability of the complexes when compared to the previously described amide analogues. Additionally, incorporation of a cyclohexyl spacer does not improve the kinetic inertness of the system. Chelators based on an 18‐membered macrocyclic unit form very stable Pb(II) complexes from the thermodynamic point of view. The studies performed with [203Pb]Pb(II) show that the fluxionality of the complexes is a key factor that needs to be considered for the development of Pb(II)‐based radiopharmaceuticals.