Photodynamic therapy (PDT) is a treatment that uses light-activated molecules called photosensitizers to kill cancer cells. When the photosensitizer is exposed to light at a specific wavelength, it generates toxic molecules called reactive oxygen species (ROS) that destroy cancer cells from the inside. For colorectal cancer, PDT is particularly appealing because it can be delivered endoscopically - through the same tube doctors use for colonoscopies - avoiding the need for surgery.
Colorectal cancer (CRC) is one of the most common and deadly cancers worldwide. While PDT offers advantages over surgery in terms of precision and repeatability, its effectiveness is limited by the immunosuppressive tumor microenvironment (TME) - the cloud of immune-blocking signals that tumors create around themselves to avoid being attacked by the immune system. After PDT kills local tumor cells, immunosuppression often allows the cancer to grow back.
To overcome this problem, researchers have explored combining PDT with immunotherapy - treatments that activate the immune system to fight cancer. The idea is that PDT kills cancer cells and releases tumor antigens (molecular flags that mark cells as cancerous), while immunotherapy trains immune cells to recognize those antigens and mount a lasting anti-tumor response.
Metal-organic frameworks (MOFs) are highly porous crystalline structures composed of metal ion clusters connected by organic molecular linkers. Their sponge-like internal structure makes them exceptional carriers for drug molecules - they can be loaded with therapeutic agents that release slowly over time. In biomedical research, MOFs have attracted enormous interest because their properties can be precisely tuned by changing the metal ions or organic linkers used.
MOF525 is a specific MOF built from zirconium metal nodes and porphyrinic organic linkers (specifically TCPP - tetrakis(4-carboxyphenyl)porphyrin). Porphyrins are naturally light-sensitive molecules found in hemoglobin and chlorophyll. In MOF525, these porphyrin units act as built-in photosensitizers, meaning MOF525 itself generates reactive oxygen species when exposed to 660 nm laser light - without needing a separate photosensitizer drug to be loaded.
Resiquimod (R848) is an immune stimulator drug that activates Toll-like receptors 7 and 8 (TLR7/8) on immune cells. By engaging these receptors, R848 triggers dendritic cells - the immune system's teachers - to mature and present cancer antigens to T cells, effectively training the immune system to recognize and attack the tumor. R848 is commonly used as an adjuvant in cancer immunotherapy.
The research team synthesized MOF525 using a solvothermal method, then loaded R848 into the material's pores by simple immersion - the drug molecules physically diffuse into the porous framework and become trapped. Electron microscopy confirmed that MOF525 formed characteristic cuboctahedral (rounded cube) shapes approximately 320 nm in size, which did not change significantly after R848 loading.
The team confirmed successful drug loading and measured that R848 released sustainably from the framework over the course of a week in laboratory conditions. This slow-release profile is important clinically because it means the drug would remain active at the tumor site for an extended period rather than washing away immediately after injection.
Testing in human colorectal cancer cell lines and bone marrow-derived dendritic cells verified that R848@MOF525 could both generate ROS upon 660 nm laser activation (measuring the PDT component) and trigger dendritic cell maturation (measuring the immunostimulatory component). These in vitro experiments established the two distinct therapeutic functions of the platform before moving to animal testing.
When CT26 colorectal cancer cells (a standard laboratory model of colon cancer) were treated with R848@MOF525 and irradiated with a 660 nm laser, significant PDT-mediated cell death was observed. The cells also released large amounts of ATP (adenosine triphosphate) into their surroundings - a key signal of immunogenic cell death (ICD), a special form of cell death that actively recruits and activates immune cells.
Immunogenic cell death is particularly important because it turns the dying cancer cell into an immune alarm. Released ATP acts as a danger signal that attracts immune cells to the site. This means PDT with R848@MOF525 does not just kill cancer cells directly - it also creates an immune-stimulating environment that can amplify the anti-tumor response.
Both MOF525 and R848@MOF525 strongly promoted dendritic cell maturation in laboratory culture, measured by increased expression of maturation markers. Mature dendritic cells are the key presenters of tumor antigens to T cells, and their activation is critical for generating long-lasting anti-tumor immunity. R848-loaded particles showed enhanced dendritic cell activation compared to MOF525 alone.
In mice bearing subcutaneous CT26 colorectal tumors, PDT with R848@MOF525 produced faster and greater tumor regression compared to PDT with MOF525 alone. The addition of R848's immunostimulatory effect to the light-triggered cell killing created a synergistic therapeutic effect that neither component achieved individually.
After the initial tumors were treated and eliminated, the researchers tested whether the immune response was durable by rechallenging the mice with fresh CT26 cancer cells injected at a different site. Strikingly, mice that had received R848@MOF525 PDT showed no new tumor growth for up to three weeks after rechallenge. This suggests the treatment generated lasting immunological memory against the cancer.
A key practical finding emerged when the researchers extended the time interval between laser irradiations: MOF525 alone failed to control tumor growth under this extended schedule, while R848@MOF525 maintained its anti-tumor efficacy. This suggests that the immunotherapy component of the combination helps sustain anti-tumor activity even when PDT sessions are spread further apart - an important practical advantage for clinical treatment scheduling.
The synergy between PDT and R848 immunotherapy in this platform arises from complementary mechanisms. PDT provides the initial cancer kill and tumor antigen release: when the laser activates MOF525's porphyrin units, the generated ROS destroy cancer cells, releasing cancer-specific proteins that can serve as targets for the immune system.
Simultaneously, R848 released from the MOF framework activates dendritic cells that take up those cancer antigens. These activated dendritic cells then present the antigens to T cells, which learn to recognize cancer cells bearing those same proteins. This trained immune response can then hunt down and kill cancer cells throughout the body, even at sites distant from the original tumor.
This combination elegantly converts a local treatment (PDT at the tumor site) into a potential systemic treatment (immune attack throughout the body). The tumor rechallenge experiment demonstrated this systemic effect, with the immune system blocking new tumor growth at a completely different body site from the original treatment location.
The endoscopic deliverability of PDT is particularly attractive for colorectal cancer, which develops in the colon and rectum - areas accessible through colonoscopy. R848@MOF525 could potentially be injected into colorectal tumors during endoscopy, followed by light activation through the same endoscope, enabling minimally invasive targeted treatment.
The nanoparticle size of R848@MOF525 (approximately 337 nm) is well-suited for tumor accumulation via a phenomenon called the enhanced permeability and retention (EPR) effect, where leaky blood vessels in tumors allow nanoparticles to accumulate preferentially in tumor tissue compared to normal tissue.
Before clinical application, studies examining systemic toxicity, biodistribution, and efficacy in larger animal models and human-relevant colorectal cancer models are needed. The slow, sustained release of R848 from the MOF carrier is a practical advantage because it delivers the immune stimulator directly at the tumor site rather than systemically, potentially reducing side effects associated with systemic immune activation.