Recent Advances in Photodynamic Therapy: Metal-Based Nanoparticles as Tools to Improve Cancer Therapy

Cancer remains a significant global health challenge, with traditional therapies like surgery, chemotherapy, and radiation often accompanied by systemic toxicity and damage to healthy tissues. Despite progress in treatment, these approaches have limitations such as non-specific targeting, systemic toxicity, and resistance development in cancer cells. In recent years, nanotechnology has emerged as a revolutionary frontier in cancer therapy, offering potential solutions to these challenges. Nanoparticles, due to their unique physical and chemical properties, can carry therapeutic payloads, navigate biological barriers, and selectively target cancer cells. Metal-based nanoparticles, in particular, offer unique properties suitable for various therapeutic applications. Recent advancements have focused on the integration of metal-based nanoparticles to enhance the efficacy and precision of photodynamic therapy. Integrating nanotechnology into cancer therapy represents a paradigm shift, enabling the development of strategies with enhanced specificity and reduced off-target effects. This review aims to provide a comprehensive understanding of the pivotal role of metal-based nanoparticles in photodynamic therapy. We explore the mechanisms, biocompatibility, and applications of metal-based nanoparticles in photodynamic therapy, highlighting the challenges and the limitations in their use, as well as the combining of metal-based nanoparticles/photodynamic therapy with other strategies as a synergistic therapeutic approach for cancer treatment.

Reactive Oxygen Species and Folate Receptor-Targeted Nanophotosensitizers Composed of Folic Acid-Conjugated and Poly(ethylene glycol)-Chlorin e6 Tetramer Having Diselenide Linkages for Targeted Photodynamic Treatment of Cancer Cells

Folic acid-conjugated nanophotosensitizers composed of folic acid (FA), poly(ethylene glycol) (PEG) and chlorin e6 (Ce6) tetramer were synthesized using diselenide linkages for reactive oxygen species (ROS)- and folate receptor-specific delivery of photosensitizers. Ce6 was conjugated with 3-[3-(2-carboxyethoxy)-2,2-bis(2-carboxyethoxymethyl)propoxy]propanoic acid (tetra acid, or TA) to make Ce6 tetramer via selenocystamine linkages (TA-sese-Ce6 conjugates). In the carboxylic acid end group of the TA-sese-Ce6 conjugates, FA-PEG was attached again using selenocystamine linkages to make FA-PEG/TA-sese-Ce6 conjugates (abbreviated as FAPEGtaCe6 conjugates). Nanophotosensitizers were fabricated by a dialysis procedure. In the morphological observations, they showed spherical shapes with small diameters of less than 200 nm. Stability of the aqueous FAPEGtaCe6 nanophotosensitizer solution was maintained (i.e., their particle sizes were not significantly changed until 7 days later). When H₂O₂ was added to the nanophotosensitizer solution, the particle size distribution was changed from a monomodal pattern to a multimodal pattern. In addition, the fluorescence intensity and Ce6 release rate from the nanophotosensitizers were also increased by the addition of H₂O₂. These results indicated that the nanophotosensitizers had ROS-sensitive properties. In an in vitro cell culture study, an FAPEGtaCe6 nanophotosensitizer treatment against cancer cells increased the Ce6 uptake ratio, ROS generation and light-irradiated cytotoxicity (phototoxicity) compared with Ce6 alone against various cancer cells. When the folic acid was pretreated to block the folate receptors of the Y79 cells and KB cells (folate receptor-overexpressing cells), the intracellular Ce6 uptake, ROS generation and thereby phototoxicity were decreased, while the MCF-7 cells did not significantly respond to blocking of the folate receptors. These results indicated that they could be delivered by a folate receptor-mediated pathway. Furthermore, an in vivo pulmonary metastasis model using Y79 cells showed folate receptor-specific delivery of FAPEGtaCe6 nanophotosensitizers. When folic acid was pre-administered, the fluorescence intensity of the lungs was significantly decreased, indicating that the FAPEGtaCe6 nanophotosensitizers had folate receptor specificity in vitro and in vivo. We suggest that FAPEGtaCe6 nanophotosensitizers are promising candidates for a targeted photodynamic therapy (PDT) approach against cancer cells.

An updated review of folate-functionalized nanocarriers: A promising ligand in cancer

The uncontrolled release of drugs in conventional drug delivery systems has led to the introduction of new nanotechnology-based drug delivery systems and the use of targeted nanocarriers for cancer treatment. These targeted nanocarriers, which consist of intelligent nanoparticles modified with targeting ligands, can deliver drugs to specified locations at the right time and reduce drug doses to prevent side effects. Folate is a suitable targeting ligand for folate receptors overexpressed on cancer cells and has shown promising results in the diagnosis and treatment of cancer. In this review, we highlight the latest developments on the use of folate-conjugated nanoparticles in cancer diagnosis and treatment. Moreover, the toxicity, biocompatibility and efficacy of these nanocarriers are discussed.

Comparative Toxicity, Biodistribution and Excretion of Ultra-Small Gold Nanoclusters with Different Emission Wavelengths

The exponentially increased use of gold nanoclusters in diagnosis and treatment has raised serious concern about their potential threat to living organisms. However, the mechanisms of toxicity of gold nanoclusters in vitro and in vivo remain poorly understood. In this work, comparative toxicity studies, including biodistribution and excretion, were carried out with mildly and chemically synthesized ultra-small L-histidine-protected and bovine serum albumin (BSA)-protected gold nanoclusters in an all-aqueous process. These nanoclusters did not induce a remarkable impact on cell viability, even at relatively high concentrations (100 μg/mL). The haemolytic assay demonstrated that the gold nanoclusters could not destroy blood cell at 600 μg/mL. After intravenous injection with mice, the biocompatibility, biodistribution, and excretion were determined. Quantitative analysis results showed that accumulation varied in the liver, spleen, kidney, and lung, though primarily in the liver and spleen. They were excreted in urine and faeces, but mainly excreted through urine. In our study, no obvious abnormalities were found in body weight, behavioral changes, blood and serum biochemical indicators, and histopathology. These findings suggested that both gold nanoclusters showed similar effects in vivo and were safe and biocompatible, laying the foundation for safe biomedical application in the future.

Efficient Delivery of Chlorin e6 by Polyglycerol-Coated Iron Oxide Nanoparticles with Conjugated Doxorubicin for Enhanced Photodynamic Therapy of Melanoma

Chlorin e6 (Ce6) is a promising photosensitizer for tumor photodynamic therapy (PDT). However, the efficacy of Ce6 PDT is limited by Ce6's poor water solubility, rapid blood clearance, and inadequate accumulation in the tumor tissue. This problem is tackled in this work, wherein functionalized superparamagnetic iron oxide nanoparticles (IO-NPs) were used as carriers to deliver Ce6 to melanoma. The IO-NPs were coated with polyglycerol (PG) to afford good aqueous solubility. The chemotherapeutic agent doxorubicin (DOX) was attached to the PG coating via the hydrazone bond to afford affinity to the cell membrane and thereby promote the cell uptake. The hydrophobic nature of DOX also induced the aggregation of IO-NPs to form nanoclusters. Ce6 was then loaded onto the IO nanoclusters through physical adsorption and coordination with surface iron atoms, yielding the final composites IO-PG-DOX-Ce6. In vitro experiments showed that IO-PG-DOX-Ce6 markedly increased Ce6 uptake in mouse melanoma cells, leading to much-enhanced photocytotoxicity characterized by intensified reactive oxygen species production, loss of viability, DNA damage, and stimulation of tumor cell immunogenicity. In vivo experiments corroborated the in vitro findings and demonstrated prolonged blood clearance of IO-PG-DOX-Ce6. Importantly, IO-PG-DOX-Ce6 markedly increased the Ce6 distribution and retention in mouse subcutaneous melanoma grafts and significantly improved the efficacy of Ce6-mediated PDT. No apparent vital organ damage was observed at the same time. In conclusion, the IO-PG-DOX NPs provide a simple and safe delivery platform for efficient tumor enrichment of Ce6, thereby enhancing antimelanoma PDT.