An image contrast agent selectively activated by prostate specific antigen.

A family of image contrast agent conjugates designed to undergo enzymatic activation has been synthesized. The agents underwent activation both with enzymatically active prostate specific antigen (PSA) and alpha-chymotrypsin, releasing free fluorophore via cleavage of a three-component system. A hexapeptide derivative showed exclusive activation by PSA and constitutes a method for tracking PSA activity in vitro.