Cancer cells damaged by photooxidation activate the unfolded protein response (UPR) endogenously, a critical process for survival that removes cytotoxic misfolded proteins and maintains protein homeostasis. Heat shock protein 90 (HSP90) paralogs—such as GRP94 in the endoplasmic reticulum (ER), TRAP1 in mitochondria, and HSP90A1 in the cytosol—play essential roles in early UPR stages by recognizing irreversibly damaged misfolded proteins and facilitating their refolding or elimination. HSP90 also maintains the conformation, stability, and function of oncogenic client proteins involved in signal transduction pathways, including proliferation, cell cycle progression, apoptosis, invasion, angiogenesis, and metastasis.1 However, conventional HSP90 inhibitors like PU-H71 and 17-AAG non-covalently bind to HSP90, showing limited therapeutic efficacy and off-target toxicity in clinical trials.2
To address this limitation, we synthesized IrPU, a photosensitizer targeting HSP90, and induced targeted elimination through photooxidation to permanently inhibit its function and reduce cellular levels. IrPU exhibited improved photodynamic therapy (PDT) efficacy under hypoxic conditions, as cellular IrPU uptake increased significantly following elevated levels of GRP94 and TRAP1. Upon photoirradiation, IrPU selectively oxidized and eliminated intracellular HSP90s. Consequently, the reduction in oncogenic client proteins and increased misfolded proteins heightened mitochondrial and ER stress. Post-PDT, activation of the mitochondrial UPR (UPRmt) upregulated proteins involved in mitochondrial folding, antioxidant defenses, and protein quality control, potentially inducing resistance to PDT in UPR-activated cancer cells. Similarly, co-treatment with photosensitizers and HSP90 inhibitors showed a synergistic effect in overcoming PDT resistance.3 Dysfunction of crucial UPRmt-associated proteins, which are clients of HSP90, resulted in substantial damage to cancer cells during repeated PDT compared to the non-targeted negative control photosensitizer IrCt. Additionally, reduced GRP94 levels increased ER stress, as evidenced by critical dilation of ER morphology, and promoted overexpression and activated translocation of calreticulin and HMGB1, markers of immunogenic cell death. This study promises enhanced efficacy in PDT by overcoming resistance mechanisms and suggesting a potent strategy for future cancer treatments with reduced off-target effects.