We propose to use liposomes and paramagnetic contrast agents to enhance the diagnostic information obtainable from in vivo NMR images. Liposomes have properties that provide potential advantages as carriers of NMR contrast agents. These include the capability of shielding the contrast agents from interactions with plasma and cells, modulating access of water, and targeted delivery to specific regions of the body or organs. Paramagnetic metal cations, nitroxide free radicals, and oxygen will be examined to determine the feasibility of their use in liposomes for enhancing NMR imaging. The physical and chemical characteristics of liposomes will be analyzed with respect to their ability to capture and target contrast agents. Preliminary studies have demonstrated the feasibility of our approach. Stable liposomes were formed containing paramagnetic metal cations (Mn2+) or nitroxide free radicals. Encapsulation of these contrast agents was found to change significantly the interaction between the contrast agent and the water protons in a manner dependent on the properties of the liposome. These studies were performed using ESR and NMR techniques. Liposomes containing nitroxides or MnC12 were taken-up by macrophages in a time-dependent manner, and the relaxation rate of the intracellular water was increased. It was also demonstrated that liposomes containing MnC12 can be used to modify the relaxation rate of mouse liver to a degree sufficient to enhance the NMR image. In the proposed studies we plan to 1) establish methods for the efficient encapsulation of paramagnetic contrast agents, 2) systematically determine the effect of liposome encapsulated paramagnetic contrast agents on proton relaxation in vitro and in vivo, 3) evaluate the ability of liposomes to deliver paramagnetic contrast agents to specific organs (principally liver and spleen), and 4) develop new methods for assessing organ function using liposomes, especially hepatobiliary clearance, phagocytosis, and redox metabolism via reduction of paramagnetic contrast agents. The proposed studies will be performed by a team of investigators with extensive experience in the areas of liposome technology, ESR and NMR spectroscopy, Nuclear Medicine, and Magnetic Resonance Imaging. This group conducted the preliminary studies for this proposal. Our goal is to enhance the capabilities of NMR imaging to detect disease for both clinical applications and basic science studies.
