Novel CDTA-based, Bifunctional Chelators for Stable and Inert Mn^II Complexation: Synthesis and Physicochemical...

In the search for Mn^II MR and PET/MR imaging agents with optimal balance between thermodynamic stability, kinetic inertness, and relaxivity, two novel bifunctional Mn^II chelators (BFMnCs) based on CDTA (trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid) were synthesized. A six-step synthesis, involving the buildup of a functionalized trans-1,2-diaminocyclohexane core, provided CuAAC-reactive 6a and 6b bearing an alkyne or azide substituent on the cyclohexane ring, respectively (CuAAC = Cu^I-catalyzed azide-alkyne 1,3-dipolar cycloaddition). Thermodynamic, kinetic, and relaxometric studies were performed with 4-HET-CDTA (8a) as a "model chelator," synthesized in two steps from 6a. The protonation constants revealed that 8a is slightly less basic than CDTA and forms a Mn^II complex of marginally lower thermodynamic stability (log K_MnL = 13.80 vs 14.32, respectively), while the conditional stability constant is almost identical for both chelates (pMn = 8.62 vs 8.68, respectively). Kinetic assessment of the Cu^II-mediated transmetalation of [Mn(4-HET-CDTA)]^2- showed that proton-assisted complex dissociation is slightly slower than for [Mn(CDTA)]^2- (k₁ = 297 vs 400 M^-1 s^-1, respectively). Importantly, the dissociation half-life near physiological conditions (pH 7.4, 25 °C) underlined that [Mn(4-HET-CDTA)]^2- is ∼35% more inert (t_1/2 = 16.2 vs 12.1 h, respectively). Those findings may be accounted for by a combination of reduced basicity and increased rigidity of the ligand. Analysis of the ¹⁷O NMR and ¹H NMRD data attributed the high relaxivity of [Mn(4-HET-CDTA)]^2- (r₁ = 4.56 mM^-1 s^-1 vs 3.65 mM^-1 s^-1 for [Mn(CDTA)]^2-; 20 MHz, 25 °C) to slower rotational dynamics (τ_R²⁹⁸ = 105 ps). Additionally, the fast water exchange of the complex correlates well with the value reported for [Mn(CDTA)]^2- (k_ex²⁹⁸ = 17.6 × 10⁷ vs 14.0 × 10⁷ s^-1, respectively). Given the exquisite compromise between thermodynamic stability, kinetic inertness, and relaxivity achieved by [Mn(4-HET-CDTA)]^2-, appropriately designed CuAAC-conjugates of 6a/6b are promising precursors for the preparation of targeted, bioresponsive, or high relaxivity manganese-based PET/MR tracers (^52g/55 Mn^II) and MR contrast agents (Mn^II).