Dual-Photosensitizer Platform Significantly Boosts Immunogenic Cell Death and Photodynamic Therapy Efficacy by Targeting ER and Other Organelles Concurrently

Photoimmunotherapy has recently emerged from photodynamic therapy (PDT) by leveraging the intrinsic immunogenic potential of PDT to stimulate the host immune system against cancer. An ideal photosensitizer (PS) should not only trigger efficient tumor regression but also elicit robust antitumor immunity, thereby preventing metastasis and tumor recurrence. Here, we report a rationally designed dual-photosensitizer system comprising (i) P-Nap-COOH, a diffusely localizing PS-targeting major subcellular organelles such as lysosomes and mitochondria, and (ii) P-Nap-ER, an endoplasmic reticulum (ER)-targeted PS with strong immunogenic cell death (ICD)-inducing capacity. PDT using this dual-PS strategy markedly reduced the IC50 and IC70 values of P-Nap-ER in breast cancer cells and enhanced ROS-mediated multiorganelle stress, thus demonstrating clear synergism. In addition, the dual system induced robust ICD, characterized by elevated ecto-calreticulin exposure, release of High Mobility Group Box 1 (HMGB1) from the nucleus, extracellular release of ATP, upregulation of C/EBP homologous protein (CHOP), and significantly increased phagocytosis of treated cancer cells by antigen-presenting cells. By concurrently activating distinct apoptotic and ER-stress pathways, this approach reduces drug dosage requirements and enhances antitumor immune response. Moreover, there was notable reduction in cell mobility indicating epithelial mesenchymal transition (EMT) reversal potential. Furthermore, there was marked reduction in PD-L1, a key modulator of immune escape mechanisms. These findings position the dual-PS platform as a promising strategy for in situ photoimmunotherapy as well as for the development of PDT-based therapeutic cancer vaccines or dendritic cell vaccines.