Neutralizing copper(II) complex charge through sulfonation: no1py2pyS as a new chelator for 64Cu radiopharmaceuticals

We report the synthesis and comprehensive characterization of a novel sulfonated TACN-based ligand, no1py2pyS (L2S), designed to combine high aqueous solubility, neutral complex charge, and strong Cu(II) affinity. L2S was synthesized via an orthoamide pathway coupled with regioselective sulfonation of picolyl pendants. Protonation equilibria were investigated using 1H NMR and UV–Vis spectroscopies. The Cu(II) complex ([CuL2S]) formed rapidly under acidic (1 M HCl) and physiological (PBS, pH 7) conditions and remained stable across a wide pH range (0−12), even under highly acidic environment (5 M HCl). DFT calculations revealed two diastereoisomeric forms, Δ(λλλ) and Λ(λλλ), with a negligible free energy difference (0.04 kcal mol−1), suggesting significant population of both forms at room temperature. However, EPR spectroscopy indicated the predominance of the Δ(λλλ)/Λ(δδδ) enantiomeric pair (87%), which displays a Jahn-Teller distorted octahedral geometry. X-ray crystallography validated the structure, revealing macrocyclic and pendant arm coordination (N6 donor set) and sulfonate-mediated copper bridging. Cyclic voltammetry demonstrated quasi-reversible redox behavior, a reduction potential (E1/2 = −614 mV vs. Ag/AgCl/ 3 M KCl) below the physiological threshold and a reduced risk of demetallation, as no complex dissociation was observed. Finally, radiolabeling studies with [64Cu]Cu2+ showed quantitative incorporation (>99%) under mild conditions (pH 4.5, RT, 10 min) at low concentration (10−6 M), and high stability in human serum for up to 2 h. Taken together, these findings identify L2S as a promising platform for the development of copper-based radiopharmaceuticals.