Early detection is key to eradicating most diseases, especially cancers. Magnetic Resonance Imaging (MRI) is a powerful, noninvasive method to detect aberrant tissues within the body. Current clinically approved contrast agents, however, lack specificity for cancer cells, undergo inefficient relaxivity, and can be cytotoxic. We propose a new class of smart MRI agents that become activated selectively at cancer sites. For this proposal, the MRI agents will be turned on through selective enzymatic hydrolysis via cathepsin B, an enzyme that is overexpressed by many cancer cells. Not only will this method greatly enhance the intensity of the signal at cancerous over healthy cells and tissues, but the proposed agents should be less toxic and demonstrate greater relaxivity as compared to traditional Gd3+ based MRI agents. Furthermore, the proposed agents could enter cells in an energy independent process to give smart MRI agents that operate within cells. Future applications include equipping the MRI agents with targeting agents for greater cell/tissue selectivity. To detect cancers that do not overexpress suitable enzymes, different types of switches will be attached that will respond to redox chemistry or changes in pH. The specific aims of this proposal are to create MRI agents, which can be optimized in the off and on modes, test their response to cathepsin B, measure their relaxivity values, and to determine their toxicities in cells and mice. The long-term goal is to provide researchers and the medical community tailor made smart MRI agents to advance the study of cellular functions and provide early detection of diseases, such as cancer. PUBLIC HEALTH RELEVANCE: MRI is a widely used imaging tool to detect cancers at early stages within the body. Combining smart MRI agents that light up selectively at diseased tissues would greatly advance MRI, saving lives, reducing costs, and reducing debilitation. Presented in this proposal is a new class of smart MRI agents that turn on at sites that over express cathepsin B, a biomarker of several cancers. The long-term goal is to construct smart MRI agents that detect the unique features of cancers, such as enzyme expression, hypoxia, and acidic pH.