Investigation of the complexation of natZr(IV) and 89Zr(IV) by hydroxypyridinones for the development of chelators for PET imaging applications
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 pKa of L1H or L1′H (pKa = 5.01) in comparison with L2H and L3H (pKa = 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.