Digadoglucitol: A New High Relaxivity MRI Contrast Agent Based on the Dimerization of [Gd(hp-Do3a)] and Activated Intramolecular Catalysis of the Prototropic Exchange

Digadoglucitol is an extracellular macrocyclic dinuclear gadolinium-based contrast agent (GBCA) based on the association of two [Gd(HP-DO3A)] units conjugated through a spacer containing the glucamine moiety. It displays a relaxivity per Gd that is 2 to 3 times higher than the most currently used GBCAs, allowing the use of reduced doses while ensuring a noninferior image contrast. Its high relaxivity is the result of a rational design aimed at exploiting the intramolecular catalysis of the prototropic exchange of the coordinated -OH groups as well as the second sphere contribution brought about by the presence of the hydroxyl functionalities on glucamine. Digadoglucitol maintains the excellent kinetic and thermodynamic properties of the parent [Gd(HP-DO3A)] with an SAP/TSAP ratio of 2/3. A HPLC workup yielded three fractions of diastereoisomers based on the chirality of the 2-hydroxypropyl pendants with similar relaxometric and stability properties. From pH 5 to 9, the deprotonated glucamine nitrogen acts as base to catalyze the prototropic exchange of the coordinating -OH group bringing an enhancement of 1.5-2.0 mM-1 s-1 of the observed relaxivity with respect to the expected value for a q = 1 complex of a similar formula weight. Biodistribution and the Magnetic Resonance Imaging pharmacokinetics of digadoglucitol resulted very similarly to those found for [Gd(BT-DO3A)].