TY - JOUR T1 - Quantitative Imaging of Receptor-Ligand Engagement in Intact Live Animals JF - bioRxiv DO - 10.1101/228072 SP - 228072 AU - Alena Rudkouskaya AU - Nattawut Sinsuebphon AU - Jamie Ward AU - Kate Tubbesing AU - Xavier Intes AU - Margarida Barroso Y1 - 2017/01/01 UR - http://biorxiv.org/content/early/2017/12/03/228072.abstract N2 - Maintaining an intact tumor environment is critical for quantitation of receptor-ligand engagement in a targeted drug development pipeline. However, measuring receptor-ligand engagement in vivo and non-invasively in preclinical settings is extremely challenging. We found that quantitation of intracellular receptor-ligand binding can be achieved using whole-body macroscopic lifetime-based Förster Resonance Energy Transfer (FRET) imaging in intact, live animals bearing tumor xenografts. We determined that FRET levels report on ligand binding to transferrin receptors conversely to raw fluorescence intensity. We then established that FRET levels in heterogeneous tumors correlate with intracellular ligand binding but strikingly, not with ubiquitously used ex vivo receptor expression assessment. Hence, MFLI-FRET provides a direct measurement of systemic delivery, target availability and intracellular drug delivery in intact animals. Here, we have used MFLI to measure FRET longitudinally in intact animals for the first time. MFLI-FRET is well–suited for guiding the development of targeted drug therapy in heterogeneous intact, live small animals. ER -