Contrast agents, methods for preparing contrast agents, and methods of imaging

A contrast agent comprising a modified parvalbumin scaffold polypeptide, wherein one or more amino acids of the scaffold polypeptide form at least one paramagnetic metal ion chelating site, and further comprising a paramagnetic metal ion bound to at least two of the amino acids forming the at least one paramagnetic metal ion chelating site,

• wherein the modified parvalbumin scaffold polypeptide has at least one amino acid substitution that modifies at least one of the metal selectivity of the binding site, increases binding affinity for the metal, or increases serum stability of the contrast agent, and wherein the at least one amino acid substitution is selected from the group consisting of S56D, E60D, G99D, and F103W, wherein the amino acid positions are numbered according to SEQ ID NO:
• 38, and wherein the modified parvalbumin scaffold polypeptide having the paramagnetic metal ion bound thereto is detectable by magnetic resonance imaging (MRI) and
• further comprising a targeting agent attached to the polypeptide, wherein the targeting agent targets a growth factor receptor.