Currently, the treatment of malignant tumors is a significant focus of public concern. In recent years, various tumor treatment methods, including surgical resection, chemotherapy, radiotherapy, and photodynamic therapy (PDT), have gained widespread attention in the biomedical field. Among these, PDT kills tumor cells by generating reactive oxygen species (ROS) through the light activation of photosensitizers (PSs), making it a favored option in clinical tumor treatment due to its minimal damage to patients. Fortunately, some photosensitizers (such as methylene blue and certain porphyrin derivatives) have been approved by the U.S. Food and Drug Administration (FDA) for clinical tumor treatment and have achieved certain results. However, the complex tumor microenvironment (including hypoxia and immune suppression) inevitably limits the efficiency of PDT. Currently, the photosensitizers used clinically are mostly type II photosensitizers, which transfer energy from the T1–S0 state to oxygen to generate singlet oxygen (1O2), thus type II photosensitizers are referred to as energy transfer type photosensitizers. Therefore, type II photosensitizers strictly depend on oxygen, limiting their efficacy in highly hypoxic tumors, and consuming oxygen exacerbates the hypoxic conditions during tumor treatment. In light of this, developing novel photosensitizers that can overcome hypoxic environments and possess clinical translational potential is of great significance.
Type I photosensitizers are a class of compounds that generate various ROS (such as superoxide anion radicals O2•–, hydroxyl radicals ·OH, and hydrogen peroxide H2O2) through electron/hydrogen transfer between the excited state PS and surrounding biological substrates (such as proteins, nucleic acids, lipids, and coenzymes) and oxygen. Specifically, type I photosensitizers can directly oxidize biological substrates and produce ROS through oxygen-independent pathways. Furthermore, the ROS generated in the type I process exhibit cascading reaction characteristics: the initially generated O2•– is converted into H2O2 and O2, and H2O2 is further converted into highly toxic ·OH through Fenton reactions. Clearly, this cascading reaction amplifies oxidative stress while reducing oxygen dependence. Notably, tumors (especially solid tumors) often exist in hypoxic environments, primarily due to tumor cells being distant from blood vessels or adjacent blood vessels malfunctioning. In fact, the oxygen content in the hypoxic core of tumors can be less than 1%. This hypoxic condition severely limits the efficacy of oxygen-dependent type II photosensitizers. In contrast, type I photosensitizers are not limited by molecular oxygen content and are expected to more effectively kill solid tumors, providing an alternative to type II photosensitizers for hypoxic tumor treatment.
In recent years, scientists have actively developed novel type I photosensitizer molecules (including metal complexes and small molecules) and have made certain progress. However, the electron transfer during the molecular excited state transition (T1–S0) process is difficult to regulate, making the systematic construction of type I photosensitizers challenging. Currently, organic small molecule type I photosensitizers are receiving attention due to their excellent biocompatibility, but their pre-design still faces difficulties. Additionally, there are few existing organic small molecule type I photosensitizers, especially lacking guiding molecular construction strategies, which far from meet the biological application needs. Considering that type I photosensitizers are based on electron transfer processes (T1–S0 state), the authors speculate that significantly enhancing spatial electron flow during the molecular excited state transition may promote electron transfer in the T1–S0 process, thus providing possibilities for innovative and systematic construction of type I photosensitizers. Such type I photosensitizers need to possess appropriately prefixed non-conjugated “electron reservoirs” and “electron pumps”. Based on this, the authors propose for the first time an “electron reservoir-pump integrated” molecular design strategy to construct novel organic small molecule type I photosensitizers— integrating the “electron reservoir” with the “electron pump” into a single dye molecule, and directing the design of type I photosensitizers by regulating spatial electron flow (verified through density functional theory [DFT] and spectral experiments).
According to this strategy, the authors designed and synthesized a series of type I photosensitizers, among which Cy5-NF was identified as a representative type I photosensitizer (which can specifically generate a large amount of O2•– under 660 nm laser irradiation). Notably, experimental results indicate that in the absence of sulfonic acid groups (electron reservoir), Cy5-NF0 (one of the derivatives of Cy5-NF) can only generate 1O2; after removing strong electron-withdrawing groups (weakened electron pump), Cy5-NMe dramatically decreased its ability to generate O2•–, fully validating the above strategy.This “electron reservoir-pump integrated” strategy represents a modular approach to constructing organic small molecule type I photosensitizers, providing valuable guidance for future type I photosensitizer construction.
Pyroptosis is a form of programmed cell death distinct from apoptosis, characterized by the upregulation of numerous inflammatory factors. Gasdermin D (GSDMD) and caspase-1 are regarded as key markers of pyroptosis. Notably, pyroptosis is inherently pro-inflammatory and can enhance tumor immunogenicity (given the inherent immune resistance of cancer cells to apoptosis), making it more suitable for tumor treatment. Previous studies have shown that specifically damaging the cell membrane can effectively induce pyroptosis in tumor cells. Experimental results indicate that the highly biocompatible Cy5-NF does not enter tumor cells but rather surrounds the cell membrane. Therefore, Cy5-NF can specifically damage cell membrane integrity under 660 nm laser irradiation (even requiring only 1 minute), achieving excellent anti-tumor effects by activating pyroptosis (i.e., increasing GSDMD/caspase-1 levels) and promoting the infiltration of CD4+ and CD8+ T cells to initiate tumor immunotherapy.
Encouragingly, the results from bilateral model treatments show that Cy5-NF-mediated PDT not only inhibited the growth of primary tumors but also significantly limited the growth of distal tumors and metastasis to different organs through long-term immune memory. More importantly, extending the irradiation time (3 minutes) of Cy5-NF-mediated PDT can almost completely ablate both primary and distal tumors. In summary, this novel type I photosensitizer based on pyroptosis has broad prospects in the field of tumor treatment.
Figure 1: Design Strategy
Figure 2: Schematic Diagram of Cy5-NF Based on the “Electron Reservoir-Pump Integrated” Strategy
Figure 3: Theoretical Calculation Diagrams of Cy5, Cy5-N, Cy5-NMe, and Cy5-NF
Figure 4: Evaluation of ROS Generation Ability and Mechanism of O2− Generation
Figure 5: Induction of Pyroptosis during Cy5-NF PDT Process
Figure 6: In Vivo Anti-Tumor Efficacy, Pyroptosis, and Immune Response
Figure 7: Tumor Pathology and Metastasis of Major Organs after Cy5-NF Mediated PDT
[Literature Details] Li Xu, Haifeng Ge, Fang Zhu, Mingri Zhao, Hongwen Liu, Xiao-Bing Zhang, and Zhe Li. Controlled Electron Transfer: Implementing a Reservoir-Pump Integrated Strategy to Develop a Type I Photosensitizer for Evoking Long-Term Tumor Immunological Memory. J. Am. Chem. Soc., 2025, https://doi.org/10.1021/jacs.5c12631