(489b) Imaging Drug Target Engagement in Vivo

Dubach, J. M., Massachusetts General Hospital/Harvard Medical School
Weissleder, R., Massachusetts General Hospital/Harvard Medical School

The pathway of successfully treating cancer with small molecule drugs involves: drugs reaching the intended target (pharmacokinetics); drugs binding to the target (engagement); and inhibition or activation of the target effecting cellular function (pharmacodynamics).  In vivo, it is difficult to parse out these mechanisms when studying fate and activity of drug treatment.  Therefore, we developed a novel imaging approach to quantitatively determine target engagement of small molecule drugs at the subcellular level in vivo.  Through the creation of a two photon fluorescence polarized microscope we can measure the anisotropy of fluorescent compounds, which reflects the brownian rotation of the molecule.  With this technique we measured target engagement of fluorescently labeled Olaparib, a PARP inhibitor currently in clinical trials for various cancers, at the subcellular level in tumor models in vivo.  This approach, which is the first to enable imaging of target engagement, provides a method to analyze potential mechanisms of cancer drug success or failure.  Through determination of target engagement, the mechanism of drug failure, whether it stems from improper pharmacokinetics or ineffective pharmacodynamics, can be more easily resolved, shedding light on how to more successfully attack the disease.  Further, this technique is generally applicable to nearly any fluorescently labeled drug.