Coumarin–benzothiazole–chlorambucil (Cou–Benz–Cbl) conjugate: an ESIPT based pH sensitive photoresponsive drug delivery system

We have developed an ESIPT based drug delivery system (DDS), Cou-Benz-Cbl conjugate, by incorporating a benzothiazole group at the 8th position of the 7-hydroxy-coumarin moiety for pH sensitive fluorescence properties and photocontrolled release of the anticancer drug chlorambucil. The Cou-Benz-Cbl conjugate exhibited unique photophysical properties like good absorbance at around 350 nm, a large Stokes shift (∼151 nm) and pH sensitive fluorescence properties. The pH sensitive fluorescence properties of the Cou-Benz-Cbl conjugate can be ascribed to an ESIPT turn "on and off" mechanism. At physiological pH, the ESIPT gets turned "off" and a blue fluorescence of the coumarin moiety was observed, but at acidic pH, the ESIPT gets turned "on" and a green fluorescence was noted. Photolysis of the Cou-Benz-Cbl conjugate using UV light of wavelength ≥365 nm resulted in the efficient release of the anticancer drug chlorambucil. Cellular uptake studies revealed that the Cou-Benz-Cbl conjugate was easily internalized inside the cancer cells. Further, an MTT assay showed that the Cou-Benz-Cbl conjugate has a good biocompatibility and low cytotoxicity towards the MDA-MB-231 cell line, whereas upon exposure to UV light, the Cou-Benz-Cbl conjugate exhibited enhanced cytotoxicity compared to the free drug due to the effective release of the anticancer drug chlorambucil inside the cancer cell.