Conditional Relay Activation of Theranostic Prodrug by Pretargeting Bioorthogonal Trigger and Fluorescence‐guided Visible Light Irradiation

Bioorthogonalized light-responsive click-and-uncage platform has enabled precise cell surface engineering and timed payload release, but most of such photoactivatable prodrugs have "always-on" photoactivity leading to the dark toxicity. On the other hand, the conditionally activatable photocage is limited to the application of fluorogenic probe/photosensitizer liberation. Herein, we devise a conditionally activatable theranostic platform based on the tetrazine (Tz)-boron-dipyrromethene (BODIPY) construct, in which tetrazine serves as a quencher motif to disable both the fluorescence and photoresponsivity of BODIPY. The uncaged BODIPY upon the bioorthogonal transformation of the tetrazine reinstates fluorescence on-target to guide the subsequent visible light irradiation thereby releasing the therapeutic payload in situ. The proof of concept for such "light where is lit up"-activated theranostics was demonstrated by the tetrazine-BODIPY conjugated SN-38 (1) with masked fluorescence, up to 70 times-reduced cytotoxicity, and good photostability. Upon an inverse electron-demand Diels-Alder (IEDDA) reaction between tetrazine unit on 1 and a biotinylated trans-cyclooctene (TCO) as a pretargeted trigger on tumor cell surface, the green fluorescence from BODIPY was fully restored for cell-selective imaging, which guided the green light irradiation (500~520 nm) to activate the caged drug SN-38 exerting potent cytotoxicity against tumor cells, offering high spatiotemporal precision of disease diagnosis and therapy.