Insights into host-guest interactions and enhanced MRI contrast applications of water-soluble tetrahedral metal-organic cages

Metal-organic cages (MOCs) are emerging as pivotal components in the design of innovative magnetic resonance imaging (MRI) contrast agents. This study introduces a novel, water-soluble metal-organic cage, K₁₂[Fe₄L₄], synthesized using a co-planar triangular ligand, N-{4-[4,6-bis(4-{[(2,3-dihydroxyphenyl)carbonyl]amino}phenyl)-1,3,5-triazin-2-yl]phenyl}-2,3-dihydroxybenzamide (H₆L). This cage represents the first instance of a gadolinium(III)-free M₄L₄ structure optimized for T₁-weighted imaging. We provide a detailed analysis of its structural attributes, aqueous stability, magnetic properties, and host-guest interactions. Relaxometric evaluations in vitro reveal that K₁₂[Fe₄L₄] significantly elevates the longitudinal relaxivity of water protons, achieving a relaxivity of 5.41 mM⁻¹ s⁻¹ at 33 °C and 0.5 T. This enhancement increases to 9.24 mM⁻¹ s⁻¹ in the presence of human serum albumin (HSA), indicating potent interaction dynamics with binding constant (K_a) of (6.1 ± 1.4) × 10⁴ M⁻¹. Comparative studies highlight that even at low doses of 0.010 mmol kg⁻¹, K₁₂[Fe₄L₄] surpasses the imaging efficacy of the commercial agent ProHance, while also demonstrating prolonged in vivo retention suitable for extended diagnostic procedures. These findings not only underscore the utility of M₄L₄ cages in T₁-weighted MRI but also lay foundational insights for the development of multinuclear, gadolinium(III)-free contrast agents, paving the way for safer and more effective diagnostic imaging solutions.