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⁻¹ s⁻¹, respectively. The in vivo pharmacokinetics of both thiol complexes were altered by a chase of homocysteine but not saline, while the washout of GdDTPA was unaffected by either chase. Such redox‐sensitive reversible binding of Gd complexes to plasma albumin can be exploited for imaging tissue redox and the blood‐pool by MRI. Magn Reson Med, 2006.