A Novel Viscosity-Responsive Fluorescent Photosensitizer with Imidazole−Coumarin Conjugation Induces Lysosome-Mediated Ferroptosis in Cancer Cells

Abstract Lysosomes play a crucial role in ferroptosis, presenting a dual opportunity for disease diagnosis and targeted cancer therapy. However, existing lysosomal probes generally lack the photosensitizing capabilities necessary to induce ferroptosis. Herein, we report the development of a novel series of benzimidazole−coumarin conjugates acting as dual-mode fluorescent photosensitizers. Structure−activity relationship (SAR) studies identified NIC-6, featuring an extended naphthimidazole core, as the optimal candidate, exhibiting exceptional photo-induced cytotoxicity with a phototoxicity index (PI) exceeding 1000 across multiple cancer cell lines. NIC-6 demonstrated highly specific lysosomal localization and enabled monitoring of microenvironmental viscosity changes. Upon targeted light irradiation, NIC-6 generated reactive oxygen species (ROS) that triggered lysosomal membrane permeabilization (LMP) and subsequent intracellular lipid peroxidation. Biochemical evaluations in 4T1 cells confirmed that NIC-6 profoundly depleted GSH and downregulated GPX4, culminating in severe ferroptotic cell death. Furthermore, NIC-6 effectively penetrated 3D tumor spheroids and significantly arrested tumor growth in a 4T1 murine xenograft model while maintaining an excellent biosafety profile. These findings highlight NIC-6 as a promising theranostic agent for precision oncology.