Design of bifunctional pyridinophane ligands as Mn²⁺ MRI agents for diagnostic imaging of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder whose prevalence is rising with the aging of the global population. Among the proposed pathological hallmarks, the beta-amyloid (Aβ) peptide aggregates and soluble Aβ oligomers are established biomarkers and remain valuable diagnostic targets. While positron emission tomography (PET) imaging agents dominate AD diagnostic imaging, there are no FDA-approved MRI agents for AD. Herein, we report five bifunctional chelators built on the 2,11-diaza[3.3](2,6)pyridinophane framework, and which were evaluated as chelators for Mn²⁺-based MRI contrast agents. Based on in vitro studies, including thermodynamic stability and kinetic inertness measurements, T ₁ relaxivity and ¹⁷O transverse relaxivity measurements to extract hydration numbers and water-exchange parameters, we obtained a clear structure-activity correlation for the corresponding bifunctional chelators: anionic picolinate and acetate arms increase thermodynamic stability and kinetic inertness, while the benzothiazolyl-phenol arm accelerates water exchange. Importantly, a high hydration number alone is insufficient, as a rapid water exchange is also needed for an appreciable contrast. Moreover, we show both the bifunctional chelators and their Mn²⁺ complexes exhibit appreciable affinity for Aβ aggregates, both in vitro and in 5xFAD mouse brain sections. [Mn(TE-8)], the most kinetically inert complex with favorable relaxivity, log D, and Aβ affinity, was advanced to in vivo MRI studies. Unlike MnCl₂, which accumulates non-specifically, [Mn(TE-8)] cleared through renal and hepatobiliary routes and produced measurable brain contrast enhancement. Together, these results establish the diazapyridinophane scaffold as a viable first-generation platform for blood-brain barrier (BBB)-permeable Mn²⁺ MRI contrast agents.