Investigation of the complexation of ^natZr(<scp>iv</scp>) and ⁸⁹Zr(<scp>iv</scp>) by hydroxypyridinones for the development of chelators for PET imaging applications

Abstract

Three hydroxypyridinone (HOPO) positional isomers - 1,2-HOPO (L1H) and its water soluble analogue (L1'H), 3,2-HOPO (L2H) and 3,4-HOPO (L3H) have been investigated for the complexation of Zr(iv). Potentiometric and UV-Vis spectrometric studies show a higher thermodynamic stability for the formation of Zr(L1')₄ in comparison with Zr(L2)₄ and Zr(L3)₄ as well as a higher kinetic inertness in competition studies with EDTA or Fe³⁺ at a radiotracer concentration with ⁸⁹Zr. Besides the low pK_a of L1H or L1'H (pK_a = 5.01) in comparison with L2H and L3H (pK_a = 8.83 and 9.55, respectively), the higher stability of Zr(L1')₄ can be attributed in part to the presence of the amide group next to the chelating oxygen that induces intramolecular H-bond and amide/π interactions that were observed by X-ray crystallography and confirmed by quantum chemical calculations. The data presented here indicate that the 1,2-HOPO L1' exhibits the best characteristics for Zr(iv) complexation. However, 3,2-HOPO and 3,4-HOPO patterns, if appropriately tuned, for instance with the addition of an amide group as in the 1,2-HOPO ligand, may also become interesting alternatives for the design of Zr(iv) chelators with improved characteristics for applications in nuclear imaging with ⁸⁹Zr.