Synthesis and characterization of the tetraazamacrocycle 4,10-dimethyl-1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (H2Me2DO2A) and of its neutral copper(II) complex [Cu(Me2DO2A)]. A new 64Cu-labeled macrocyclic complex for positron emission tomography imaging

Abstract

The new macrocycle 4,10-dimethyl-1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (H₂Me₂DO2A, H₂L) has been synthesized and characterized. The basicity constants of the macrocycle and the stability constants of its complex formation with copper(II) ions have been determined in aqueous solution by potentiometry. The protonation sequence of the macrocycle has been studied by ¹H and ¹³C-{¹H} NMR spectroscopy. The data obtained are consistent with the first and second protonations taking place at the methyl-substituted nitrogen atoms, while the third and fourth protonations involve the two carboxylate arms. High-resolution NMR studies have revealed that the ethylenic protons in the tetraprotonated H₄L²⁺species are inequivalent due to multiple hydrogen bonds within the macrocycle ring, indicating that the system is locked in a rigid conformation on the NMR timescale. The molecular structure of the copper(II) complex [Cu(Me₂DO2A)]·2H₂O has been determined by a single crystal X-ray analysis. The macrocycle wraps the metal in a distorted octahedral geometry using the four nitrogen atoms and two oxygen atoms from the two carboxylate groups. These latter are cis to each other. The excellent stability of [Cu(Me₂DO2A)] at physiological pH is confirmed by a cyclic voltammetry study that shows the complex to be irreversibly reduced at E_p = −0.81 V. The radioactive ⁶⁴Cu-labeled complex [⁶⁴Cu(Me₂DO2A)] has been synthesized using ⁶⁴CuCl₂. Animal biodistribution studies have been performed by bolus injection in rabbit and followed by Positron Emission Tomography (PET), an in vivo imaging technique used in nuclear medicine to detect the regional distribution and concentration of positron emitting radionuclides. PET images showed that the tested compound has high uptake in liver, kidneys and bladder. A significant uptake of radioactivity is observed in brain.