1i). in 43% of Phase II Rabbit Polyclonal to ATRIP failures in a recent study2. Given the complexity ofin vivodrug action3and recent clinical failures of drugs that are not properly characterized4, methods to determine cellular drug binding could, in theory, reduce the considerable clinical failure rates and associated high costs. Direct chemical modification of drugs provides small labels such as biotin or fluorophores enabling Imidafenacin tissue distribution and target engagement measurements by pull down assays or imaging58. However , the addition of a label changes the physiochemical properties of a small molecule, and thus results may not be directly relevant to the parent drug candidate. Imidafenacin Conversely, labeling target proteins with genetic fluorescent labels, such as GFP, may alter protein activity or trafficking9. Among several creative label free approaches to measure target engagement1012PET imaging is currently the most commonly used at multiple stages in drug development13. Radiolabelled drug measures tissue accumulation14while lack of accumulation following drug administration indicates parent drug target occupancy10. However , this approach does not consider non-specific accumulation15, lacks single cell spatial resolution, and some radio-labels, such as carbon-11, have a limiting half-life16. Alternatively, the cellular thermal shift assay (CETSA) measures bound protein thermal stabilization to determine target engagement and can be extended toin vivomeasurements17. Yet, CETSA obtains cell population averages, results are difficult to quantitate andin vivomeasurements have only been demonstrated with covalent drugs. Enzymatic drug inhibition can be measured using activity based probes18or molecules that become fluorescent upon enzyme cleavage19. While these approaches provide valuable insight into target inhibition, they require reactive or cleavable probes, are limited to certain protein classes and lack spatial resolution. Therefore , measuring engagement of clinical drug with target at the cellular level andin vivowith reversible inhibitors has remained elusive. Here we establish a new approach to quantitate target occupancy of unlabeled drugs at cellular resolution using competitive binding with Imidafenacin fluorescently labeled companion imaging probes (CIP) and fluorescence polarization microscopy. Our approach takes advantage of the target specificity of a CIP and the subcellular spatial resolution of microscopy. Importantly, this technique measures unlabeled drug engagement, and, although not a direct measurement of drug concentration in the cell, we determine engagement of drug to the target, which, ultimately, is the therapeutic objective. Here, we quantitate intracellular target engagement of unlabeled covalent and reversible drugs in live cells in culture andin vivo. == Results == == Measuring binding with companion imaging probes == We hypothesized that an unmodified molecularly targeted drug will compete for target binding with a matched fluorescent CIP. Imidafenacin Target engagement of the CIP, detectable by fluorescence polarization microscopy, is thus indicative of unlabeled drug binding. Detection is enabled by the large mass differences between the small fluorescent molecule in free and protein-bound states (Supplementary Results, Supplementary Fig. 1a). With polarized light, only CIP molecules with absorption dipole moments aligned along the plane of excitation become excited through photoselection. Subsequent Brownian rotation of the excited Imidafenacin molecules during the fluorescence lifetime is inversely related to the remaining polarization anisotropy of the emission photons. When bound to the target protein, CIP rotation slows and the excited molecules retain orientation, producing polarization anisotropy. Yet, in the presence of unlabeled drug the CIP cannot bind as completely and the ensemble average polarization, representing the fraction of target bound CIP, becomes more isotropic, indicating unlabeled drug target engagement (Fig. 1a). To obtain spatial information we image cells with polarized two-photon excitation light and collect emission in channels parallel and perpendicular to the excitation polarization (Supplementary Fig. 1b). == Figure 1 . Measuring single cell drug binding through anisotropy competition imaging. == (a)CIP measures unoccupied target through CIP rotational speed slowing when bound to the target, which increases the anisotropy and represents the degree of clinical drug target engagement. Chemical structures of ibrutinib(b)and olaparib(d)and corresponding CIP linked to BODIPY FL(cande). Single cell HT1080 nuclei olBFL intensity(f), anisotropy(g)and rint(h)incubated with different concentrations of olBFL. Data are individual nuclei (green open circle) with mean s. d. (black), n > 68 per olBFL concentration, one technical replicate. (i)Single cell nuclear rint measurements of HT1080 (blue), HCC1937 (red) and MHHES1 (green). Data are individual nuclei with.