Developing New Multicolor Polypeptide MR Contrast Agents for Stem Cell Tracking

Principal Investigator: Michael McMahon

Development of new contrast materials is important to expand the diagnostic capabilities of MR at the cellular and molecular level. In particular, providing information on cell migration, trafficking and homing would be useful for the development of cell therapies using stem cells and progenitors. Previous cell tracking studies have employed agents containing iron oxide, gadolinium or other metals to generate contrast by influencing the relaxation time of water protons. The drawbacks of using these agents for cell tracking include the feature that cells are detected as positive or negative spots which can be difficult to distinguish from imaging artifacts and also the uncertainty about long-term metal toxicity in-vivo. Chemical Exchange Saturation Transfer (CEST) agents are a new class of MR agents which to date haven't been used for cell tracking. Contrast is generated with these agents by applying a saturation pulse on the agent's protons which rapid chemical exchange with water transferring the signal loss, with the speed of the exchange producing a saturation of multiple water protons from a single exchangeable site on the CEST agent. We are proposing developing a new class of molecular probes for use in cell tracking, inserting these into cells and monitoring their effectiveness in-vivo. Two different types of biopolymer CEST agents will be tested, poly-peptides consisting of D-amino acids (instead of L) and poly-peptides consisting of l-amino acids but with appropriate N-terminal groups resistant to degradation. The advantage of these agents is that their contrast is saturation frequency dependent, allowing multiple "colors" of agent, which would be useful for tracking multiple cell populations independently with MRI (as compared with the simple on/off contrast of metal based agents). We are interested in developing four different "colors" for MR imaging, analogous to the capabilities of optical/fluorescent imaging. We will also optimize the sensitivity and delivery of these agents. We hypothesize that the detection of these agents is possible in-vivo and to achieve this goal we will synthesize a series of these agents, and detect transfected neural stem cells individually in live animals. These contrast agents should prove to be highly sensitive due to the chemical exchange amplification of the contrast. In addition, because these agents are composed exclusively of polypeptides, new delivery/targeting strategies may be employed.