The scale of the problem is considerable. Canine oral tumors account for up to 12 percent of all cancers seen in dogs, with melanoma, squamous cell carcinoma, acanthomatous ameloblastoma, and fibrosarcoma representing the most common diagnoses of the oral cavity and pharynx. When these tumors are caught early, wide surgical excision remains the most effective local treatment. But many oral cancers are diagnosed at a locally advanced stage, when complete surgical resection is no longer possible. Radiation and chemotherapy become the fallback options, and while radiation can produce high response rates in tumors such as oral melanoma and squamous cell carcinoma, one-year survival rates for some of the most common oral cancers fall below 50 percent. Novel adjuvant therapies are urgently needed to make those responses last.

The drug at the heart of the study, CB101, is a proprietary hydrogel-based injectable formulation of resiquimod, a molecule known as a toll-like receptor 7/8 agonist. Toll-like receptors sit on the surface or within certain immune cells and act as sentinels for danger signals. In humans, TLR8 is uniquely expressed on myeloid dendritic cells, monocytes, and natural killer cells, making it an attractive lever for pulling the immune system into a fight against cancer. Activating these receptors may disrupt the immunosuppressive microenvironment that tumors build around themselves. Resiquimod itself has never been approved by the FDA, but topical formulations have been tested in human clinical trials for skin diseases and as a vaccine adjuvant, generating an extensive body of toxicology and pharmacology data supporting its safety profile.

The rationale for pairing this immune agonist with radiation rests on a phenomenon called immunogenic cell death. High-dose radiation, such as the 8 Gy per fraction used in some hypofractionated protocols, can kill tumor cells in a way that makes them more visible to the immune system, releasing tumor antigens that dendritic cells can engulf and cross-present to CD8-positive cytotoxic T lymphocytes. Research suggests that doses around 8 Gy per fraction are particularly well suited to synergizing with immunotherapeutics. In principle, this cascade can generate an adaptive, tumor-specific immune response capable of affecting not just the irradiated tumor but also distant, non-irradiated lesions, the so-called abscopal effect. Preliminary mouse data showed that the combination of radiation and CB101 improved local tumor control compared with either treatment alone and elicited such abscopal responses. Injecting the drug directly into the tumor is designed to limit systemic exposure and minimize off-target immune side effects.

Three dogs with histologically confirmed head and neck cancers were prospectively enrolled in the pilot study. Each animal received a baseline CT scan for radiation planning, followed by palliative radiation therapy delivered as four weekly 8 Gy fractions over weeks one through four. CB101 was administered intratumorally at a fixed dose of 10 micrograms in 500 to 1,000 microliters of volume at one-week intervals during weeks two through five, with the first dose given immediately before the second radiation fraction. Treatment planning relied on CT-based software, and dogs were anesthetized and immobilized in vacuum cushions with bite blocks to ensure reproducible setup. A follow-up CT scan at week 12 assessed tumor response, and serial thoracic radiographs at week 24 monitored for distant metastasis. All owners provided informed consent under institutional animal care protocols.

Feasibility proved to be the study’s clearest success. Little to no difficulty occurred during the intratumoral injections, and leakage of the formulation was negligible. Tumors arising from or containing bone were more challenging to infuse; one dog with mandibular fibrosarcoma required both intratumoral and peritumoral administration because of the density of the tumor and its proximity to the normal mandible. Injections were generally performed under general anesthesia for patient comfort, and no pain was noted after the procedure. The volume of each injection was chosen based on the expected ability of the hydrogel to diffuse into the tumor and surrounding tissue, guided by physical examination, CT evidence of bone infiltration, and three-dimensional tumor measurements.

Safety outcomes were equally encouraging. Toxicity was graded using the Veterinary Cooperative Oncology Group criteria and a veterinary radiation morbidity scoring scheme. Only one adverse event was attributable to CB101 itself: a case of grade 1, mild pain during a fourth injection performed under light sedation rather than general anesthesia. Every other adverse event documented was an expected side effect of palliative radiation, and no dog experienced toxicity higher than grade II. One dog developed grade I skin and mucosal toxicity along with grade II ocular toxicity requiring eye drops and antibiotics, an outcome anticipated from the radiation dose distribution, which included a large portion of the left eye. Another dog experienced no toxicity at all.

Tumor responses varied, as expected in such a small and histologically diverse cohort. The dog with oronasal squamous cell carcinoma showed a partial response at week 12 but developed distant progressive disease, first in lymph nodes and then in the lungs, between weeks 12 and 24, and was euthanized 198 days after enrollment. The dog with oral melanoma experienced local progression at week 12 and pulmonary metastasis by week 24, surviving 254 days. The dog with oral fibrosarcoma had stable disease at the week 12 scan, with eventual progression outside the radiation field at week 26 and a survival time of 371 days. All three dogs were free of distant metastases when treatment began.

Intriguingly, serum cytokine analysis revealed negligible changes in the immune signaling molecules the team measured, including TNF-alpha, IL-6, IL-15, interferon-alpha, IL-1 beta, and IL-12p40. The researchers suspect the sampling schedule missed any transient cytokine peaks, which could be expected within hours to three days after dosing rather than a full week later. The absence of clinical signs of cytokine release suggests the weekly 10 microgram dose was well tolerated but may have fallen below a therapeutic threshold. The starting dose was chosen conservatively because a human trial using 12.5 micrograms intralesionally had reportedly triggered cytokine storm and hypotension in some patients. Other veterinary researchers have used far higher resiquimod doses, with a University of Kansas group treating dogs with cutaneous mast cell tumors using doses ranging from 0.07 to 1.36 milligrams and observing one complete response and three partial responses among six dogs.

The Penn team acknowledges several confounders, including the use of NSAIDs in two of the three dogs, which could have either blunted the innate immune response or improved outcomes through COX-2 inhibition, and antimicrobial treatment in one dog, which in human melanoma patients has been associated with worse survival on immunotherapy. Future studies, the authors say, should standardize these medications, sample blood at multiple early time points after injection, and consider analyzing tumor tissue to capture local immune effects within the microenvironment. A dose escalation study appears warranted, and the researchers envision eventually combining TLR agonists with checkpoint inhibitors, an approach that could shift immunologically cold tumors toward hot ones. As academic and industry groups work toward commercializing canine checkpoint inhibitors, this modest pilot study in three dogs may mark the first step of a translational path that runs from the veterinary clinic to human oncology and back again.

Subject of Research: Combining intratumoral resiquimod immunotherapy with hypofractionated radiation therapy in canine head and neck cancer

Article Title: Preliminary evaluation of the safety and feasibility of toll-like receptor ligand CB101 combined with hypofractionated radiation therapy in canine head and neck cancer: a pilot study

Article References: Ghanian, A., DiBona, J., Duda, L., Xu, X., Lukens, J. N., Pearce, T., Ehrhardt, M., Rook, A., Durham, A., Maity, A., & Flesner, B. (2025). Preliminary evaluation of the safety and feasibility of toll-like receptor ligand CB101 combined with hypofractionated radiation therapy in canine head and neck cancer: a pilot study. Veterinary Oncology, 2(1), Article 18. https://doi.org/10.1186/s44356-025-00031-6

Image Credits: AI Generated

DOI: 10.1186/s44356-025-00031-6

Keywords: canine cancer, head and neck cancer, radiation therapy, resiquimod, CB101, toll-like receptors, immunotherapy, intratumoral injection, veterinary oncology, dendritic cells, abscopal effect, pilot study

The challenge the researchers set out to address is well known to immunotherapy researchers. Checkpoint-blocking drugs have transformed outcomes for some patients with melanoma, lung cancer and other malignancies, but many solid tumours remain resistant because their surroundings actively suppress immune activity. Tumour-associated macrophages, or TAMs, are abundant white blood cells found inside tumours that, in their tumour-associated state, help the cancer evade destruction. Professor Zhao explained the central problem succinctly: the immune system may be fully capable of attacking a tumour, but the tumour environment can stop those immune cells from doing their job. Her team’s answer was to change that environment from within, delivering treatment precisely to the cells that maintain the immunosuppressive order and persuading them to switch sides.

The delivery vehicle at the heart of the study is the lipid nanoparticle, the same class of fatty droplet that carries mRNA in COVID-19 vaccines. But these particles were given a targeting upgrade: their surface is coated with an antibody that recognises TREM2, a protein expressed at high levels on tumour-associated macrophages. This molecular address label guides the nanoparticles to the very cells responsible for tumour immune suppression. Once inside the macrophages, the particles release two payloads with complementary functions. The first is an mRNA molecule carrying instructions for the macrophage to produce CXCL9, a chemical signalling protein. The second is Resiquimod, a small-molecule compound that pushes macrophages away from their immune-suppressing behaviour and toward a more inflammatory, immune-supportive state.

The two payloads work in concert in a way that illustrates the elegance of the design. CXCL9 acts as a chemical beacon, drawing cytotoxic CD8+ T cells, the immune system’s primary tumour-killing soldiers, into the tumour mass, where they are often excluded or rendered inactive. Meanwhile, Resiquimod shifts the local macrophage population away from suppression, easing the hostile conditions that would normally exhaust or repel those T cells. Rather than depleting the macrophages outright, an approach that can carry inflammatory side effects, the researchers effectively re-educated them, converting a tumour-protective population into an accomplice of the anti-cancer immune response.

In experiments with mouse models, the approach produced measurable shifts in the tumour immune landscape. Treatment reduced the proportion of immune-suppressing macrophages by more than 60 percent, a substantial remodelling of the tumour’s defensive cellular makeup. Levels of CXCL9 within the tumours rose fourfold, confirming that the delivered mRNA was being translated into functional chemical beacon by the targeted cells. The researchers also documented greater numbers and heightened activity of cancer-fighting T cells inside the treated tumours, along with a moderate reduction in tumour growth. While the growth slowdown alone was not dramatic, the immunological changes suggest a tumour environment becoming markedly more permeable and hospitable to immune attack.

The team then tested whether their nanoparticle therapy could amplify the effects of existing immunotherapies. When combined with immune checkpoint-blocking antibodies targeting PD-L1 and CTLA-4, two of the most widely used targets in clinical oncology, the treatment produced further increases in cytotoxic T cells and, notably, the generation of central memory T cells. These long-lived memory cells could give the immune system the ability to recognise and respond to cancer if it returns, a property highly valued in cancer therapy because it hints at durable protection rather than transient tumour shrinkage. Interestingly, however, the combination did not yield additional tumour-growth inhibition in this particular mouse model, a nuance the researchers report candidly and one that will guide future experimental design.

Professor Zhao framed the findings as an important proof of concept that mRNA and nanoparticle technology can reprogramme the immune environment of a tumour. She emphasised that significant work remains before the approach could be considered for patients, but described the results as an encouraging foundation for developing more targeted cancer immunotherapies. That caution reflects the well-known gap between promising mouse studies and clinical reality: nanoparticle manufacturing, dosing, safety profiling and the variability of human tumour environments all present substantial hurdles. Still, the strategy addresses a specific failure mode of current immunotherapy, immune exclusion, and does so with a precision that conventional drugs have struggled to achieve.

The broader significance of the work lies in the expanding repertoire of mRNA medicine. The pandemic demonstrated that lipid nanoparticles can deliver genetic instructions safely and at scale; researchers worldwide are now exploring whether the same platform can carry therapeutic instructions for enzymes, antibodies, cytokines and tumour antigens. This study adds a subtle new use case: not delivering a drug or an antigen, but delivering the blueprint for a signalling molecule that changes the behaviour of the cells receiving it. By targeting TREM2-positive macrophages, the Adelaide team also taps into a growing body of research on macrophage reprogramming, an area increasingly seen as fertile ground for solid tumour therapy where T-cell-focused approaches alone often fall short.

The research was led by Adelaide University scientists in collaboration with SA Pathology and the Royal Adelaide Hospital, and is published under the title ‘Targeting tumor-associated macrophages using mRNA lipid nanoparticles for cytotoxic T lymphocyte–mediated cancer immunotherapy’. For patients whose tumours shut out immunotherapy, the study offers a vision of treatment that does not fight the tumour’s fortress head-on but instead quietly converts its guards, arming them with the instructions to raise a signal flare that guides the immune system’s most lethal cells inside. If subsequent studies can translate those beacon-lit results into durable clinical benefit, mRNA nanoparticles may find a second act in oncology as architects of the tumour microenvironment rather than mere couriers of vaccines.

The biology underlying the study helps explain why macrophages have become such a sought-after target. Macrophages are not inherently tumour-friendly; in healthy tissue they patrol, clear debris and coordinate inflammatory responses. Tumours, however, gradually reshape the macrophages they recruit, coaxing them into a state that suppresses cytotoxic T cells, promotes new blood vessel growth and remodels the fibrous matrix surrounding the cancer. High densities of these suppressive macrophages have been associated in many cancer types with poorer responses to checkpoint inhibitors, which is why strategies that convert rather than eliminate them have attracted growing interest.

The choice of TREM2 as a targeting marker reflects recent advances in understanding macrophage identity within tumours. TREM2 is a receptor expressed on a subset of macrophages that accumulates in tumours and is linked to immunosuppressive function, and blocking or depleting TREM2-positive cells has emerged as an active area of preclinical investigation. Using an antibody against TREM2 as a homing device, rather than as a therapeutic agent itself, is a distinctive feature of the Adelaide approach: the antibody serves as an address label that concentrates the therapeutic payload where it is most needed, potentially limiting off-target effects in healthy tissue.

The signalling components of the formulation also draw on established immunology. Resiquimod is a synthetic agonist of toll-like receptors, pathways that act as alarm bells for the innate immune system and have previously been explored as topical treatments and vaccine adjuvants. Encapsulating it alongside mRNA allows the two stimuli to act within the same cell, pairing a behavioural switch with a genetic instruction. CXCL9, meanwhile, belongs to a family of chemokines long known for their role in recruiting CD8+ T cells to sites of inflammation, and low CXCL9 expression has been linked to immune-excluded tumours in human studies, making it a rational choice for restoring T-cell infiltration.

The appearance of central memory T cells in the combination experiments deserves particular attention. Memory T cells persist long after an initial immune response subsides and can mount faster, stronger reactions upon re-encounter with their target. In cancer, inducing such cells is a goal of therapeutic approaches such as cancer vaccines and intratumoural therapies, because tumour recurrence remains a leading cause of treatment failure even when initial therapy is successful. The fact that this memory phenotype emerged despite the absence of added tumour shrinkage suggests that immunological benefit and immediate tumour control may follow different timelines, a distinction that longer animal studies would need to resolve.

The candid reporting of the combination result also illustrates a broader principle in immunotherapy research: immune activation and tumour regression are not always tightly coupled in early experiments. Factors such as the mouse model used, the timing of treatment relative to tumour establishment, and the dose and schedule of each component can all influence whether enhanced T-cell activity translates into measurable growth control. The authors’ decision to publish these nuances alongside the positive findings provides a transparent baseline for other laboratories seeking to refine the formulation, test it across additional tumour types, and determine which patient populations might ultimately benefit most from a macrophage-reprogramming strategy.

Subject of Research: mRNA lipid nanoparticle reprogramming of tumour-associated macrophages for cancer immunotherapy.

Article Title: New ‘smart’ nanoparticles help the immune system better attack tumours

Article References: New ‘smart’ nanoparticles help the immune system better attack tumours. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: lipid nanoparticles, mRNA, tumour-associated macrophages, cancer immunotherapy, CXCL9, TREM2, CD8+ T cells, immune checkpoint inhibitors, tumour microenvironment, Resiquimod, Science Advances, Adelaide University