Dynamic alterations in cell surface glycosylation occur in numerous biological processes that involve cell–cell communication and cell migration. We report here imaging of cell surface glycosylation in live mice using double click chemistry. Cell surface glycans were metabolically labeled using peracetylated azido-labeled N-acetylgalactosamine and then reacted, in the first click reaction, with either a cyclooctyne, in a Huisgen [3 + 2] cycloaddition, or with a Staudinger phosphine, via Staudinger ligation. The second click reaction was a [4 + 2] inverse electron demand Diels–Alder reaction between a trans-cyclooctene and a tetrazine, where the latter reagent had been fluorescently labeled with a far-red fluorophore. After administration of the fluorescent tetrazine, the bifunctional cyclooctyne-cyclooctene produced significant azido sugar-dependent fluorescence labeling of tumor, kidney, liver, spleen, and small intestine in vivo, where the kidney and tumor could be imaged noninvasively in the live mouse.

Cell death is an important target for imaging the early response of tumors to treatment. We describe here the validation of a phosphatidylserine-binding agent for detecting tumor cell death in vivo based on the C2A domain of synaptotagmin-I. Methods: The capability of nearinfrared fluorophore-labeled and 99m In-labeled derivatives of C2Am for imaging tumor cell death, using planar near-infrared fluorescence imaging and SPECT, respectively, was evaluated in implanted and genetically engineered mouse models of lymphoma and in a human colorectal xenograft. Results: The fluorophorelabeled C2Am derivative showed predominantly renal clearance and high specificity and sensitivity for detecting low levels of tumor cell death (2%-5%). There was a significant correlation (R . 0.9, P , 0.05) between fluorescently labeled C2Am binding and histologic markers of cell death, including cleaved caspase-3, whereas there was no such correlation with a site-directed mutant of C2Am (iC2Am) that does not bind phosphatidylserine. 99m In-C2Am also showed favorable biodistribution profiles, with predominantly renal clearance and low nonspecific retention in the liver and spleen at 24 h after probe administration. 99m In-C2Am generated tumor-to-muscle ratios in drug-treated tumors of 4.3· and 2.2·, respectively, at 2 h and 7.3· and 4.1·, respectively, at 24 h after administration. Conclusion: Given the favorable biodistribution profile of 99m In-labeled C2Am, and their ability to produce rapid and cell death-specific image contrast, these agents have potential for clinical translation.