Sulfonamide-functionalized gadolinium DTPA complexes as possible contrast agents for MRI: A relaxometric investigation

A novel Gd-DTPA derivative with a built-in sulfonamide (SA) was synthesized as a contrast agent for MRI. The complex was designed to selectively target the enzyme carbonic anhydrase. It is shown that the longitudinal relaxation rates of aqueous solutions of Gd-DTPA-SA in the presence of carbonic anhydrase increase significantly. The binding constant is determined to be 15,000 ± 5,000 M–1. This value ensures substantial formation of the carbonic anhydrase adduct at imaging concentrations of Gd-DTPA-SA. The complex interacts with erythrocytes, presumably due to a high affinity for the carbonic anhydrase present on the outer surface of the latter. This takes place even though the enzyme has a low abundance and is easily saturated by small amounts of Gd-DTPASA. The interaction of Gd-DTPA-SA with serum proteins is negligibly small. Therefore, the complex could potentially be tested as a selective contrast agent for compartments outside the blood pool.