Abstract To produce effective therapeutic treatment, drugs need to reach and engage the cognate target in the cells of interest. Incomplete target occupancy may lead to cell survival and disease progression. Therefore, for maximum efficacy, it is critical to achieve high target occupancy within each cell of a tumor. However, the tumor microenvironment likely creates a dynamic, heterogenous distribution of drug that limits target occupancy in some cells. Unfortunately, we lack the tools to determine target engagement at the cellular level, which prevents studies on heterogeneous target occupancy and contributes to the poor understanding of drug failure mechanisms. Here, we developed an approach to measure target engagement of small molecule inhibitors within single cells in mouse xenograft in vivo tumor models. Our method uses fluorescently labeled, target specific companion imaging drugs, fluorescence polarization imaging, and intravital microscopy, to measure target occupancy of unlabeled, clinical drugs in single cells. We calculated the equivalent in vitro exposure of ibrutinib, a covalent BTK inhibitor, in vivo, but found increased cellular heterogeneity, highlighting differences between in vivo activity and in vitro measurements. Additionally, following systemic delivery of the reversible PARP inhibitor olaparib, we found cells with substantial uninhibited target when average measurements indicated all target was occupied, illustrating drug activity heterogeneity that may lead to tumor survival. These results demonstrate that our technique, which is the first to measure unlabeled drug engagement with target in single cells, can quantitate cellular fractional occupancy heterogeneity during drug treatment. We expect this approach to serve as a valuable tool when measuring cellular pharmacology in vivo, providing insight on dosing, drug activity in the tumor and sources of heterogeneous drug distribution. Citation Format: Matt Dubach, Ralph Weissleder. Imaging single cell drug target engagement in vivo reveals heterogeneous fractional occupancy. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2093.