Can receptors be imaged with MRI agents?

A review of the feasibility of imaging receptors was made using published literature. The results suggest that there is no physical limitation to imaging the classical biochemical receptors using currently available compounds. Limitations occur because of biological constraints. These constraints are those of delivery of contrast material to the site of the receptor in sufficient quantities and the biological implications of saturating receptors. These limitations are reduced by improving the physics through increasing the relaxivity of the contrast agent either by greater intrinsic relaxivity or by attaching many relaxing agents to the ligand. Biochemical constraints are reduced by targeting receptors involved with transport systems such as receptor mediated endocytosis and by targeting those sites that are in or readily accessible to the vascular system. Once targeted MRI agents are developed their clinical use will, most probably, be different than that of the corresponding radiopharmaceuticals.