Spiropyran‐based Nanocarrier: A New Zn2+‐Responsive Delivery System with Real Time Intracellular Sensing Capabilities

A new spiropyran-based stimuli-responsive delivery system is presented that encapsulates and then releases an extraneous compound in response to elevated levels of Zn2+, a critical factor in cell apoptosis. A C12-alkyl substituent on the spiropyran promotes self- assembly into a micelle-like nanocarrier in aqueous media, with nanoprecipitation and encapsulation of added payload. Zn2+ binding occurs to an appended bis(2-pyridylmethyl)amine group at biologically relevant micromolar concentration. This leads to switching of the spiropyran (SP) isomer to the strongly fluorescent ring opened merocyanine-Zn2+ (MC-Zn2+) complex, with associated expansion of the nanocarriers to release the encapsulated payload. Payload release is demonstrated in solution and in HEK293 cells by encapsulation of a blue fluorophore, 7-hydroxycoumarin, and monitoring its release using fluorescence spectroscopy and microscopy. Furthermore, we demonstrate the use of the nanocarriers to deliver a caspase inhibitor, Azure B, into apoptotic cells in response to an elevated Zn2+ concentration. This then inhibits intracellular caspase activity, as evidenced by confocal microscopy and in real-time by time-lapsed microscopy. Finally, the nanocarriers are shown to release an encapsulated proteasome inhibitor (5) in Zn2+-treated breast carcinoma cell line models. This then inhibits intracellular proteasome and induces cytotoxicity to the carcinoma cells.