Gadobenate

Gadobenate (Gd-BOPTA) is a paramagnetic contrast agent for magnetic resonance imaging (MRI) that was approved by the United States Food and Drug Administration (US FDA) in 2004 for imaging the central nervous system to visualize lesions with abnormal blood-brain barrier (BBB) or abnormal vascularity of the brain, spine, and associated tissues (1-3).

Conventional, water-soluble, paramagnetic contrast agents are generally metal chelates with unpaired electrons, and they work by shortening both T₁ and T₂ relaxation times of surrounding water protons to produce the contrast-enhancing effect (1, 2). At normal clinical doses, the T₁ effect tends to dominate. Current agents are water-soluble compounds that distribute in the extracellular fluid and do not cross the intact BBB. They can be used to enhance signals of CNS tissues that lack a BBB (e.g., pituitary gland), extraaxial tumors (e.g., meningiomas), and areas of BBB breakdown (e.g., tumor margins). In these cases, small or multiple CNS lesions are more clearly delineated with contrast enhancement. In addition, contrast enhancement can highlight vasculature, delineate the extent of disease, and confirm the impression of normal or nonmalignant tissues. These contrast agents can also be used in a similar nonspecific manner to enhance contrast between perfused and nonperfused areas in other organs (1, 2, 4).

Gadolinium ion (Gd³⁺), a lanthanide metal ion with seven unpaired electrons, has been shown to be very effective at enhancing proton relaxation because of its high magnetic moment and very labile water coordination (2, 5-7). Gadopentetate dimeglumine (Gd-DTPA) was the first intravenous MRI contrast agent used clinically, and a number of similar gadolinium chelates have been developed in an effort to further improve clinical efficacy, patient safety and patient tolerance. The major chemical differences among these Gd chelates or Gd-based contrast agents (GBCAs) are the presence or absence of overall charge, ionic or nonionic, and their ligand frameworks (linear or macrocyclic). Gd-BOPTA has a linear framework and is ionic. The BOPTA ligand was originally developed as a potential hepatobiliary agent for MRI (8). It is a modification of the DTPA molecule based on the belief that the addition of a benzoylmethyl group to the backbone of DTPA would increase the liver uptake of Gd-BOPTA. The hepatobiliary excretion of Gd-BOPTA appears to be species dependent, and only 2-5% of the dose is excreted via the biliary system in humans. Because of weak/transient interactions between Gd-BOPTA and macromolecules that reduce the tumbling rate of the Gd-BOPTA molecules, Gd-BOPTA demonstrates increased r₁ and r₂ relaxivities (efficiencies in shorteningT₁ and T₂ relaxation times) in solutions containing serum proteins (9-11).

The commercial formulation of Gd-BOPTA is a meglumine salt (Gd-BOPTA/Dimeg) and is available as a 0.5 M injection with a recommended dose of 0.1 mmol/kg (0.2 ml/kg) either as a rapid intravenous infusion or bolus injection for CNS MRI imaging (3, 12). It has an osmolality of 1970 osmol/kg and viscosity of 5.3 mPas at 37^oC.

Both renal and extra-renal toxicities have been reported following the clinical use of gadolinium in patients with underlying kidney disease (13-15). In 2007, the US FDA requested manufacturers of all GBCAs to add new warnings about exposure to GBCAs increases the risk for nephrogenic systemic fibrosis (NSF) in patients with advanced kidney disease.