Current cancer-targeted imaging agents and therapies require entry inside solid tumors to reach their targets and be most effective. However, endothelial cells (EC) and other biological barriers in vivo restrict passive entry into tumors, leading to very limited intratumoral concentrations and poor efficacy. Nanoparticles (NP) can potentially improve pharmacokinetics and pharmacodynamics of small drugs in part by slowing renal excretion; however, limited penetration across EC barriers and rapid uptake by the reticuloendothelial system (RES) impede NP delivery into tumors. There is an urgent need to devise new probes and systems to study key biological interfaces that restrict delivery in vivo and how they can be overcome and exploited to perform image guided drug delivery (IGDD) that ultimately enhances therapeutic impact for breast cancer. The overall goal of our research program is to explore the use of a newly discovered, active transendothelial transport pathway, the caveolae, to provide an effective solution to target and deliver NP and their therapeutic cargo. The caveolae pumping system enables tissue-specific targeting and penetration to overcome the limitations of current drug delivery paradigms and to improve therapeutic efficacy. Towards this goal, we will study tumor EC barrier function directly in vivo using intravital microscopy (IVM) and determine the degree to which caveolae can be targeted to pump NP into tumors. Our central hypothesis is that linking caveolae-targeting antibodies to NP carriers will increase the capacity of antibodies to deliver small drugs such as radionuclides into tumors and thereby greatly improve therapeutic impact. This hypothesis is formulated on the basis of our prior work and new preliminary data. It will be tested by pursuing two specific aims: 1) To characterize in vivo delivery and immunotargeting properties of dendrimers functionalized with caveolae-specific antibodies; 2) To investigate the utility of targeting dendrimers to tumor endothelial caveolae to enhance radiotherapy and efficacy. We will prepare and characterize immunoconjugated dendrimers of different sizes and use advanced multimodality imaging technologies and novel mammary tumor IVM model systems to evaluate and optimize in vivo targeting, delivery and efficacy. This work will provide the first prototype of caveolae-targeted breast cancer nanotherapies. This contribution is significant because the ability to pump NP across in vivo barriers and escape the RES could create a paradigm shift away from passive transvascular delivery towards using an active pumping pathway to deliver pharmaceutically active agents rapidly and specifically into breast tumors and thereby greatly enhance therapeutic impact. Such findings are expected to have an immediate positive impact through advancing cancer research and oncology with the likelihood that the gained information will provide a novel caveolae-mediated drug delivery system for IGDD to help treat breast cancer and to improve the quality of life of those patients suffering from breast cancer.