There is a great need for positive contrast agents for magnetic resonance imaging (MRI) that respond to levels of hypoxia in heterogeneous environments like those found in many tumors for the purpose of predicting therapeutic outcomes, developing new therapies, and enabling the testing of hypotheses relevant to our collective fundamental understanding between hypoxia and human health. Our long-term goal is to develop positive contrast agents for MRI to fill this void in diagnostic medicine by focusing on EuII-containing complexes that are among the most promising areas of study. The overall objective of this application is to establish the groundwork necessary for translation of our new 19F-EuII-based complexes into useful hypoxia-responsive contrast agents by studying the influence of the position of fluorine on ligands for europium and characterizing in vitro and in vivo indices of hypoxia. The rationale that underpins the proposed research is that EuII- containing complexes labeled with fluorine differentially (T1 or 19F) influence MRI as a function of the oxidation state of europium as we have recently demonstrated both in vitro and in vivo. The expected outcome of this proposal is the establishment of 19F-EuII-based contrast agents with defined hypoxia indices. This outcome is expected to have a positive impact by contributing to the NIH's mission in the understanding of human diseases. We plan to achieve the objective of the proposal by pursuing three specific aims: (1) to synthesize a series of redox-active Eu-based multimodal contrast agents designed rationally from our initial successful agent; (2) to characterize temperature-dependence of relaxivity and in vitro hypoxia indices for multimodal redox-active 19F-Eu-based probes for MRI; and (3) To define in vivo hypoxia indices in healthy mice, hypoxic mice, and mouse models of osteosarcoma. The new 19F-EuII-based probes will be significant because they are expected to enable changes in hypoxia resulting from therapies to be imaged, consequently aiding in assessing therapeutic efficacy and influencing the care and management of cancer patients. Furthermore, because hypoxia is relevant to a wide-range of diseases, this proposal is expected to maximize returns in many other investments of the NIH.