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.

PROJECT NARRATIVE Magnetic resonance imaging (MRI) is one of the most powerful diagnostic tools available in clinics. However, the recent rising concern about the toxicity of gadolinium-based contrast agents has lead to renewed interest in finding alternatives. This project proposes to develop a new class of metal-free MRI contrast agents based on biocompatible and biostable organic triarylmethyl radicals.