Rational design of ultra-inert Gd-DOTABA probes conjugated with unnatural dipeptides for high-contrast and low-dose tumor and vascular MRI

Molecular Synergy : Two novel GBCAs, Gd-L1 and Gd-L2 , are constructed by integrating a macrocyclic inert Gd-DOTABA core with HSA-targeting biphenylalanine (BiP)-based dipeptides. • High Relaxivity : Both probes exhibit high longitudinal relaxivity ( r 1  = 20.7 and 23.3 mM −1  s −1 in 4.5% HSA at 1.4 T), driven by near-quantitative binding to human serum albumin (>90%). • Superior Kinetic Stability : The complexes demonstrate outstanding inertness, with acid dissociation half-lives 3-5 times longer than the clinical benchmark Gd-DOTA, ensuring a high safety profile. • Potent Low-Dose Efficacy : At merely one-third of the standard clinical dose (0.03 mmol/kg), both agents enable high-contrast tumor delineation and high-resolution angiography in vivo. • Favorable Safety : Comprehensive in vivo studies confirm excellent biocompatibility, efficient dual clearance, and no acute or subacute toxicity, supporting clinical translation potential. The development of gadolinium-based contrast agents (GBCAs) that simultaneously possess high relaxivity and excellent kinetic inertness remains an important challenge for safe and efficient magnetic resonance imaging (MRI). Herein, we report a rational molecular engineering strategy to construct two novel probes, Gd-L1 and Gd-L2 , by conjugating an unnatural biphenylalanine (BiP)-based dipeptide to a macrocyclic Gd-DOTABA scaffold. The introduced BiP-containing dipeptide enhances interaction with human serum albumin (HSA), leading to substantially improved relaxivity. Both complexes exhibit high longitudinal relaxivity ( r 1  = 10.3 and 9.4 mM −1  s −1 in water), which increases to 20.7 and 23.3 mM −1  s −1 in the presence of HSA. In addition, these complexes demonstrate favorable kinetic inertness, with acid dissociation half-lives 3–5 times longer than Gd-DOTA. At a reduced dose of 0.03 mmol/kg, both agents enable high-contrast tumor imaging and magnetic resonance angiography in vivo . Biodistribution, pharmacokinetic, and toxicity studies further support favorable clearance and biocompatibility. These findings demonstrate that unnatural dipeptide-functionalized Gd-DOTABA complexes represent promising macrocyclic albumin-interacting MRI contrast agents with enhanced imaging performance and favorable safety characteristics. Rational Design of Unnatural Dipeptide-Enabled Gd-DOTABA Probes: A Synergistic Approach for Stable, High-Relaxivity Gd Complexes Enabling Low-Dose Tumor and Vascular Imaging. Download: Download high-res image (476KB) Download: Download full-size image