Photodynamic Therapy: Past, Current, Future

Photodynamic therapy (PDT) is a light-activated therapeutic modality that combines photosensitizers, oxygen, and specific light wavelengths to generate reactive oxygen species for selective cellular destruction. Hematoporphyrin derivatives were among the first clinically relevant photosensitizers, and PDT development accelerated after observations that porphyrins accumulated in tumors and induced phototoxicity after light exposure. Current PDT research spans oncology, antimicrobial therapy, nanomedicine, and image-guided therapeutics, reflecting sustained interest across translational life sciences and drug discovery fields. PDT remains significant because it enables spatially controlled treatment with lower systemic toxicity than many conventional therapeutic approaches^[1][4][6][9].

The central PDT mechanism involves excitation of photosensitizers from the singlet ground state to excited singlet and triplet states, followed by energy transfer to molecular oxygen and production of singlet oxygen and additional reactive oxygen species. Singlet oxygen is regarded as a principal cytotoxic mediator responsible for oxidative damage, apoptosis, membrane disruption, and mitochondrial dysfunction during PDT. PDT-induced apoptosis proceeds through extrinsic and intrinsic pathways involving Fas receptor activation, cytochrome c release, APAF-1 signaling, and caspase-3 activation. Reactive oxygen species generated during PDT also modulate immune signaling, vasculature, and inflammatory responses in treated tissues. Recent mechanistic studies further demonstrated that energy transfer from the singlet excited state of porphyrins to oxygen contributes to singlet oxygen production, extending the classical PDT photochemistry model ^{[2][3][5][6][10]}.

Cancer therapy remains the dominant clinical application of PDT, including treatment strategies for nonmelanoma skin cancer, bladder cancer, esophageal cancer, head and neck cancer, and non-small cell lung cancer. Porphyrin, chlorin, and bacteriochlorin photosensitizers have shown strong phototoxicity and low dark toxicity in tumor models, with bacteriochlorins exhibiting longer absorption wavelengths and enhanced antitumor efficacy. PDT is also increasingly investigated for antimicrobial therapy because reactive oxygen species disrupt bacterial membranes, DNA, and biofilms formed by resistant pathogens including Pseudomonas aeruginosa and Staphylococcus aureus. Nanocarrier-based PDT systems further improve tumor targeting, drug delivery, and controlled release while reducing damage to surrounding healthy tissues. PDT additionally induces apoptosis, necroptosis, ferroptosis, pyroptosis, and cuproptosis, expanding its relevance for regulated cell death research and combination therapies ^{[3][4][7][8][9][10][11]}.

Current limitations include incomplete clinical superiority data for several solid tumors, oxygen dependence in hypoxic tissues, and variability in photosensitizer selectivity and tissue penetration. Future research focuses on third-generation photosensitizers, nanomaterial-assisted delivery systems, improved singlet oxygen generation, and multimodal phototherapy platforms with enhanced tumor specificity. Mechanistic integration of immune regulation, regulated cell death pathways, and hypoxia-adapted PDT strategies may improve clinical translation and experimental reproducibility in future therapeutic development ^{[1][3][5][6][9][10]}.