Redox‐sensitive contrast agents for MRI based on reversible binding of thiols to serum albumin

Abstract

DOTA-based complexes of gadolinium (Gd) bearing a thiol moiety on a propyl or hexyl arm were synthesized. It was hypothesized that these complexes would form reversible covalent linkages with human serum albumin (HSA), which contains a reactive thiol at cysteine-34. The binding constant of the hexyl complex to HSA was measured to be 64 mM ؊1 and decreased to 17, 6.1, and 3.6 mM ؊1 in the presence of 0.5, 1, and 2 mM homocysteine, respectively. The binding constant of the propyl complex to HSA was significantly lower (5.0 mM ؊1 ) and decreased to 2.0, 1.5, and 0.87 mM ؊1 in the presence of 0.5, 1, and 2 mM homocysteine, respectively. The longitudinal water-proton relaxivities of the hexyl and propyl complexes at 37°C and 4.7 T were 2.3 and 2.9 mM ؊1 s ؊1 , respectively, in saline. The relaxivities of the HSA-bound forms of the hexyl and propyl complexes were calculated to be 5.3 and 4.5 mM

A gadolinium triacetic monoamide DOTA derivative with a methanethiosulfonate anchor group. Relaxivity properties and conjugation with albumin and thiolated particles

“…This tripeptide is the most abundant reducing molecule in the circulation (44) and is also the most abundant non-proteinaceous thiol in mammalian cells (45). The reaction between disulfide links and GSH leads to the release of the gadolinium chelate through a thiol-disulfide exchange as also observed for a gadolinium complex featuring a free thiol function in the presence of homocysteine (8). The relaxation time changes of a solution of the reduced BSA conjugate with GdMTS-ADO3A upon the addition of 100 equivalents of GSH follow the pseudo-first-order reaction kinetics…”

“…On the contrary, covalent bonding proved to have a detrimental effect on the clearance of the metal complexes and it was thus suggested to create reversible covalent bonds with macromolecules that are cleaved by endogenous biomolecules or after administration of exogenous substances following the imaging examination. The disulfide bond appears particularly interesting in this respect as it is easily cleaved by thiols present in the body (3,7,8). The latter are found in relatively low concentrations in the circulation [about 15 mM (3)] and the residence times in the body of thiolated contrast agents should thus remain sufficiently long to allow imaging.…”

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.…”

Metal Complexes as MRI Contrast Enhancement Agents

“…This relaxivity change can report on the different redox status of the blood pool or of tumor tissue. 275 In another report, a GdDO3A was conjugated to a 2-pyridine disulfide group via a flexible linker (Scheme 19). The reaction of the disulfide with glutathione (GSH), and the subsequent reduction coordination of one GSH carboxylate group to the paramagnetic center, results in a q ¼ 1 hydration state.…”