Blue Fluorescent Siloxytecans Exhibit Potent Anticancer Activity and Enable Direct Real-Time Quantification of Intracellular Uptake

Notwithstanding the natural abundance of silicon on Earth, silicon-containing compounds comprise relatively few pharmaceutical drugs, though several silylated natural products have demonstrated greater bioavailability and lipophilicity. Here, we apply this strategy in the preparation and biological evaluation of synthetic camptothecin analogs involving a C-10 silyl ether on SN-38, which not only blocks a site of metabolism but also enhances bright-blue fluorescence properties in such compounds. These siloxytecans exhibit comparable dose- and time-dependent antiproliferative activity in a broad panel of cancer cells. Uniquely, we demonstrate that the enhanced fluorescence of these compounds enables real-time, quantitative visualization of the dynamics and selectivity of intracellular uptake through fluorescence microscopy without the need for extensive sample preparation or installation of auxiliary fluorophores. We further demonstrate that the kinetics of cellular uptake observed by fluorescence microscopy are consistent with time-course washout experiments with subtle differences in anticancer potency in several cell lines. Further cell cycle analysis by flow cytometry and cell-free topoisomerase inhibition studies suggests that these siloxytecans retain the topoisomerase-inhibiting properties of camptothecin and other related topoisomerase I inhibitors. Collectively, these studies highlight the utility of quantitative fluorescence microscopy in investigating mechanisms of biological transport and the anticancer activity of such siloxytecans.