PEGylation of protein-based MRI contrast agents improves relaxivities and biocompatibilities

Magnetic resonance imaging (MRI) has emerged as a leading diagnostic technique in clinical and preclinical settings. However, the application of MRI to assess specific disease markers for diagnosis and monitoring drug effect has been severely hampered by the lack of desired contrast agents with high relaxivities, and optimized in vivo retention time. We have reported the development of protein-based MRI contrast agents (ProCA1) by rational design of Gd3 + binding sites into a stable protein resulting in significantly increased longitudinal (r1) and transverse (r2) relaxivities compared to Gd-DTPA. Here, we report a further improvement of protein contrast agents ProCA1 for in vivo imaging by protein modification with various sizes of polyethylene glycol (PEG) chain. PEGylation results in significant increases of both r1 and r2 relaxivities (up to 200%), and these high relaxivities persist even at field strengths up to 9.4 T. In addition, our experimental results demonstrate that modified contrast agents have significant improvement of in vivo MR imaging and biocompatibilities including dose efficiency, protein solubility, blood retention time and decreased immunogenicity. Such improvement can be important to the animal imaging and pre-clinical research at high or ultra-high field where there is an urgent need for molecular imaging probes and optimized contrast agent.

Modeled structure of designed ProCA1 with Gd3 + (purple) binding site and outer sphere water molecules associated with PEG chain (red).Figure optionsDownload as PowerPoint slideHighlights
► Significant relaxivities increase of protein contrast agents by PEGylation.
► High water number and strong metal binding affinity were obtained.
► The protein contrast agents were able to target cancer cells specifically.
► Enhanced MR imaging with low toxicity were observed in vivo.