Tetrazine-based bioorthogonal reactions to image hypoxia in cancer cells

Suboptimal oxygen supply in solid tumors controls a variety of cancer hallmarks and impairs responsiveness to anticancer treatments. Yet, tools to visualize hypoxia in cancer are still limited. Here we present an application of the tetrazine-dienophile inverse-electron demand Diels-Alder (IEDDA) reaction for hypoxia visualization. In this approach, trans-cyclooctene-clickable 2-nitroimidazoles work in combination with tetrazine-functionalized probes for live imaging of differential levels of hypoxia in two-dimensional (2D) and three-dimensional (3D) models. We used tetrazine-based fluorescent probes to determine oxygen levels in cell monolayers, spheroids, and patient-derived organoids, and applied a "click-to-release" approach to evaluate the diffusion of the released probe in the surrounding peripheral regions, allowing to quantify the bystander effect. Given the compatibility of the tetrazine-dienophile click reaction with living organisms, the combination of clickable 2-nitroimidazoles with tetrazine-based reporters opens a new avenue for in vivo hypoxia imaging.