Coumarin and Curcumin-Metal Complexes as Next-Generation Photosensitizers in Cancer Photodynamic Therapy

Abstract

To explore the emerging role of natural ligands, specifically coumarin and curcumin, and their coordination with the transition metals ruthenium (Ru) and iridium (Ir) as photosensitizers (PSs) in photodynamic therapy (PDT) for cancer. This highlights the integration of natural compounds and transition metals to overcome limitations in photophysical properties, hypoxia tolerance, and clinical translation. Regarding PDT oncology, this examines an immunological effect on Ru/Ir complexes and natural ligand-metal hybrids. They induce immunogenic cell death (ICD) through reactive oxygen species (ROS) generation, calreticulin exposure, extracellular ATP release, and HMGB1 secretion. These damage-associated molecular patterns act as “danger signals” to recruit dendritic cells, prime CD8+ cytotoxic T-cells, and establish systemic antitumor immunity. This study compares natural ligand-metal complexes with conventional Ru(II)/Ir(III) complexes and clinical PSs to assess their translational potential as immune-activating agents in PDT oncology, as well as focusing on the integration of nanotechnology with natural ligand-metal complexes to enhance delivery, biocompatibility, and clinical translation. A narrative review was conducted of the literature published between 2010 and 2025 across multiple electronic databases, including WanFang Data, PubMed, ScienceDirect, Scopus, Web of Science, Springer Link, SciFinder, and CNKI, without language restrictions. Studies focusing on coumarin, curcumin, Ru(II), Ir(III), and PDT were analyzed. Extracted data included chemical structures, absorption and emission spectra, singlet oxygen yields, biological activities, and therapeutic outcomes. Comparative evaluation was performed between free natural ligands, their Ru(II)/Ir(III) complexes, and nanodelivery systems to assess efficacy, biocompatibility, and translational potential. Coumarin and curcumin exhibited intrinsic antioxidant, anti-inflammatory, and anticancer properties but were limited by short absorption/emission ranges, poor photostability, and low singlet oxygen yields, restricting preclinical application. Coordination with Ru(II) and Ir(III) significantly enhanced intersystem crossing, extended absorption into the near-infrared region, and improved singlet oxygen quantum yields (ΦΔ up to ~0.78). These complexes demonstrated potent photocytotoxicity under normoxia and hypoxia, achieving IC50 values in the nanomolar range, which indicated organelle-specific targeting (mitochondria, lysosomes, ER), induced ICD, and synergized with checkpoint blockade. Nanocarrier encapsulation further improved solubility and tumor selectivity, and reduced systemic toxicity. Coumarin- and curcumin-based Ru/Ir complexes represent promising next-generation or immune activating PDT agents by combining natural pharmacological activity with superior photophysical performance. The ability to generate reactive oxygen species under hypoxia and achieve multimodal therapeutic effects positions them as strong candidates for clinical translation. Clinical approval of natural ligand-Ru/Ir complexes depends on rigorous safety, pharmacokinetic, and nanodelivery validation, but these complexes clearly extend PDT beyond local cytotoxicity toward durable immune protection. Future research should prioritize ligand engineering, nanotechnology integration, and translational models to bridge preclinical promise with safe and effective clinical applications.