Design, Synthesis, and Evaluation of 1,4,7,10-Tetraazacyclododecane-1,4,7-triacetic Acid Derived, Redox-Sensitive Contrast Agents for Magnetic Resonance Imaging

Abstract

The design and synthesis of three 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A) derivatives bearing linkers with terminal thiol groups and a preliminary evaluation of their potential for use in assembling redox-sensitive Magnetic Resonance Imaging (MRI) contrast agents are reported. The linkers were selected based on computational docking with a crystal structure of human serum albumin (HSA). Gd(III)-DO3A and Eu(III)-DO3A complexes were synthesized, and the structure of one complex was established by X-ray crystallographic analysis. The binding to HSA of a Gd(III)-DO3A complex bearing a thiol-terminated 3,6-dioxanonyl chain was competitively inhibited by homocysteine and by the corresponding Eu chelate. Binding to HSA was abolished when the terminal thiol group of this complex was absent. The longitudinal water-proton relaxivities (r 1 ) of the three Gd(III)-DO3A complexes and of two Gd(III)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) complexes were measured in saline at 7 Tesla. The DO3A complexes exhibited smaller r 1 values, in both bound and free states, than the DOTA complexes.

Redox-active Magnetic Resonance Imaging Contrast Agents: Studies with Thiol-bearing 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetracetic Acid Derivatives

“…The apparent equilibrium binding constant (K A ) for each complex was calculated from the Law of Mass Action as described previously. 12 …”

“…11 We also prepared DO3A gadolinium(III) complexes (not shown) possessing different ring nitrogen-attached sidechains that also terminate with a thiol group. 12 Only one DO3A complex, bearing a 9-mercapto-3,6-dioxanonyl chain, bound in a redox-sensitive fashion. Other DO3A complexes bound additionally, we believe, to surface carboxylates of HSA.…”

Redox‐ and Hypoxia‐Responsive MRI Contrast Agents

“…A series of Gd 3+ -DOTA mono(amide) (DOTA =1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) derivatives with free thiol groups that can form a covalent -S–S- bond with the cysteine-34 (Cys34) residue of HSA have been synthesized (ligand 3 , Figure 2). [30–32] When bound to albumin, the r 1 relaxivity of Gd 3+ complexes is higher than in the unbound form due to the increased rotational correlation time (slower tumbling rate) of the complex (5.3 versus 2.33 mM −1 s −1 , respectively, for complex n =6 at 4.7 T). The thiol/disulfide ratio depends on the extracellular redox state, and in reducing microenvironments, the albumin-bound Gd 3+ complexes are cleaved from HSA by reduction of the disulfide bond.…”

On the synthesis of 1,4,7-tris(tert-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclododecane

“…Most fail to establish the identity and purity of the product. Our interest in the development of redox sensitive MRI CAs25 motivated the development of a simple, concise, scalable synthesis of 3 .…”