ABSTRACT
Gadolinium-based contrast agents (GBCA) have revolutionized MRI. They provide contrast and essential
diagnostic information that could not be obtained otherwise. For about 20 years, these compounds were
considered among the safest pharmaceuticals. However, in 2006 the causal relationship between the
development of a devastating and potentially fatal condition named nephrogenic systemic fibrosis (NSF) and
GBCAs in renocompromized patients was established. The FDA has since placed restrictions on the use of
GBCAs for patients with impaired kidney function, virtually eliminating the risk of NSF; however leaving some
patients without having access to potentially lifesaving contrast-enhanced MRI. The more recent reports that
gadolinium deposits cumulatively in the brain, bones, and skin, even in patients with intact blood-brain-barrier
and normal renal and hepatobiliary functions, with unknown long-term hazard significance, have raised many
concerns among the scientific community and patients. The last few years have seen breakthroughs in the
development of metal-free MRI contrast agents based on macromolecular templates loaded with sterically
shielded nitroxide radicals. These metal-free MRI contrast agents have demonstrated high efficacy in vivo with
the ability to provide contrast in disease tissues (e.g., tumors). However, the current designs are limited by the
instability of nitroxides radicals in vivo, leading to reduced diamagnetic hydroxylamines. The PI's lab has
recently reported the synthesis of a highly biocompatible triarylmethyl radical with unmatched in vivo stability,
named OX063. This radical has the capability to solve the stability issues of nitroxide-based organic radical
contrast agents (ORCA). Moreover, Ox063’s trivalent shape makes it a natural dendrimer building block. In
this project, we propose to develop OX063-based dendrimers as the next generation of metal-free MRI
contrast agents. In specific aim 1, we will synthesize and characterize OX063-based dendrimers as dual MRI
and fluorescent agents. A set of ORCAs will be decorated with a tumor-associated macrophage-binding
peptide for active tumor targeting. In specific aim 2, we will perform in vitro and in vivo distribution,
metabolism, excretion, toxicology, and pharmacokinetic studies, and in vivo imaging validation. Finally, in
specific aim 3, we will demonstrate the application of OX063-based dendrimers for in vivo MRI tumor imaging
in a mouse model of breast cancer. Active and passive targeting of the new ORCAs will be compared. The
completion of the project will provide highly biostable metal-free contrast agents which can significantly impact
the field of biomedical imaging.
