Classification of Blood Pool Agents
A contrast agent is considered a blood pool agent when it is eliminated much more slowly from the circulation than extracellular agents. This results in an increased signal from the blood over an extended period of time. Several possible mechanisms can be responsible for the delay in elimination. The first mechanism is reversible binding to albumin, which prevents immediate leakage into the interstitial space. The second mechanism is based on an increase in the size of the contrast medium molecule which slows down leakage through endothelial pores. A third mechanism involves a change in the route of elimination. An overview of the different classes of blood pool agents is given below. The contrast agents are grouped by their mechanism of action.
Albumin-binding gadolinium complexes
The first agent in this class, gadofosveset trisodium (formerly MS-325, now Vasovist®, Schering AG, Berlin, Germany), is a small-molecular agent that reversibly binds to serum albumin, leading to high relaxivity (19 l/mmol•s on average at clinical field strenghts) and much longer residence times in the blood compared to extracellular agents. (Rohrer et al (2005), Invest Rad 40: 715 - 724)
Because of its high relaxivity, a low dose of just 10–25 μmol/kg produces excellent blood-tissue contrast enhancement. Several clinical studies have demonstrated the agent's efficacy in enhancing the blood vessels, both in first-pass and in delayed steady-state MRA examinations. Gadofosveset was recently approved for use in abdominal and peripheral MRA in Europe and will soon be commercially available.
The second compound in this class is B-22956/1 (Bracco SpA, Milan, Italy). B-22956/1 is a Gd-DTPA derivative with strong, reversible albumin binding. As with gadofosveset, this leads to much longer residence times in the blood, and distinctly higher relaxivity than that of the unbound form of the contrast agent. B-22956/1 has been investigated for MR coronary artery imaging in clinical trials but is not yet commercially available.
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