Young Dogs With Splenic Cancer Live Far Longer on Chemotherapy, Landmark Trial Reveals

Canine hemangiosarcoma is one of the most feared diagnoses in veterinary medicine. This aggressive cancer of blood vessel cells strikes an estimated 50,000 dogs in the United States each year, most often in the spleen, the right auricle of the heart, the liver, or the skin. The tumors form irregular, leaky vessels that impede normal blood flow, causing necrosis and catastrophic internal hemorrhage. Even with surgical removal followed by doxorubicin chemotherapy, the current standard of care, median survival remains around four to six months, and only about ten percent of dogs live beyond one year. Against this grim backdrop, a new clinical trial has produced an unexpected finding that could reshape how veterinarians think about this disease: age at diagnosis appears to matter profoundly, with young adult dogs experiencing durable, long-term remissions that have never before been clearly documented.

The findings come from the PRO-DOX study, a phase I clinical trial led by researchers at the University of Minnesota, with participation from the University of Pennsylvania and Purdue University, and published in the journal Veterinary Oncology. The trial was designed to test a different question altogether: whether propranolol, a cheap and widely used beta-blocker, could be repurposed as an adjunct to doxorubicin chemotherapy. The rationale was compelling. Propranolol blocks beta-adrenergic receptors, which are highly expressed in angiosarcoma, a rare human cancer that closely mirrors canine hemangiosarcoma in its clinical presentation and morphology. In human patients with angiosarcoma, propranolol combined with conventional chemotherapy has produced remarkable responses, including in cases with metastatic disease. Preclinical work by the same team had shown that propranolol inhibits proliferation and induces apoptosis in both angiosarcoma and canine hemangiosarcoma cell lines, and that the two drugs act synergistically in laboratory models.

The clinical trial enrolled twenty client-owned dogs with stage 1 or stage 2 splenic hemangiosarcoma, confirmed by histopathology after splenectomy, with no evidence of gross metastatic disease. Dogs began oral propranolol three times daily at escalating doses of 0.8, 1.0, or 1.3 milligrams per kilogram, using an adaptive Bayesian continual reassessment design that allowed dose assignments to be adjusted as safety data accumulated. Once the target dose was reached, each dog received doxorubicin at 30 milligrams per square meter of body surface area intravenously every three weeks for five cycles, while propranolol was continued for up to one year. The treatment was well tolerated: propranolol monotherapy produced only two mild adverse events, a single case of diarrhea and one persistent cough, and none of the dogs experienced the hypotension that investigators had flagged as the primary anticipated risk. Doxorubicin-related neutropenia occurred in just two dogs.

When the researchers compared survival between the treated dogs and a combined historical control group of forty dogs given standard surgery and doxorubicin-based care at the two universities, the results were deflating for the propranolol hypothesis. Median survival was 134 days in the PRO-DOX group versus 152 days in the comparison group, a difference that was not statistically significant. Even preclinical work seemed to echo the clinical result: in immunodeficient mice bearing canine hemangiosarcoma xenografts, propranolol and doxorubicin each slowed tumor growth independently, but the combination offered no advantage over either drug alone. The researchers note that their mouse model, lacking functional T cells, could not capture propranolol’s immune-mediated anti-tumor effects, which prior studies suggest are central to its activity, a limitation that may be compounded in older dogs whose immune systems are already in decline.

But buried within the negative result was a striking signal. All three dogs in the trial that had been diagnosed at age five survived more than two years, each enrolled in a different dose cohort, ruling out any relationship to propranolol dosing. When the team stratified the dogs by life stage, using veterinary guidelines and canine epigenetic aging studies to define young adult dogs as younger than seven years, older adult dogs as seven to under eleven years, and senior dogs as eleven or older, the pattern became statistically robust. Young adult dogs survived significantly longer than the older groups, a finding that held in the historical control group as well, where two young dogs survived 576 and 1,704 days. Focusing on exceptional survival, defined as living beyond one year, five of five young adult dogs achieved it, compared with only five of fifty-two older dogs, an association that reached high statistical significance.

To explore whether propranolol’s metabolism might explain the young dogs’ advantage, the team analyzed plasma from nineteen of the twenty dogs, measuring concentrations of propranolol and its primary canine metabolite, 4-hydroxypropranolol, which retains beta-blocking activity. Drug exposure, measured as the area under the concentration-time curve, showed no correlation with survival. However, the maximum concentration of 4-hydroxypropranolol correlated inversely with long-term survival, and older adult and senior dogs had significantly higher peak levels of the metabolite than the young adults. This mirrors human pharmacology, where hepatic clearance of propranolol slows with age. Yet the investigators concluded the effect reflected age-dependent drug metabolism rather than a survival mechanism driven by the drug, because some short-lived older dogs cleared the metabolite just as efficiently. The message was that younger dogs simply process the drug faster, a biomarker of metabolic youth rather than a therapeutic benefit.

The most tantalizing evidence came from the tumors themselves. The researchers extracted RNA from thirteen archived tumor samples and performed unbiased transcriptional profiling, using a method called gene cluster expression summary score analysis to condense thousands of co-regulated genes into interpretable signatures. Tumors from the young, long-lived dogs showed high expression of immune-related gene clusters, including pathways for positive regulation of leukocyte and lymphocyte activation, innate immune responses, and T cell activation. Tumors from older, short-surviving dogs showed the opposite pattern: elevated expression of genes governing the G2/M cell cycle checkpoint, DNA damage response, and DNA repair, with the transcription factor E2F1, a known driver of cell cycle progression and senescence, prominent within that cluster. No individual genes reached significance on their own, likely due to the small sample size, but the pathway-level signatures were clear and consistent.

These molecular findings connect to a growing body of work on why cancer becomes both more common and harder to treat with age. Doxorubicin kills dividing cells by intercalating into DNA and inhibiting topoisomerase II, but it also triggers immunogenic cell death and promotes CD8-positive T cell responses that specifically recognize tumors, a mechanism linked to better outcomes in human breast cancer. Younger dogs, with more robust naive T cell populations and less immunosenescence, may be better positioned to mount that response. Older dogs, by contrast, accumulate senescent cells that secrete inflammatory cytokines, chemokines, and immunosuppressive molecules such as PD-L1, reshaping the tumor microenvironment into one that actively thwarts immunity. The authors suggest that combining DNA repair inhibitors with immune checkpoint blockade, an approach already showing synergy in pancreatic cancer models, could offer older dogs a path that doxorubicin alone cannot.

The study has honest limitations. Only three young adult dogs were enrolled, no six-year-old dogs participated at all, and the timing of chemotherapy, begun an average of 28 days after splenectomy in the trial versus 21 days in the control group, may have fallen just outside a recently proposed optimal window. Immunohistochemical confirmation of the immune signatures was inconclusive, likely because of tumor heterogeneity. Still, the implications are significant for veterinary oncology and for comparative medicine alike, since canine hemangiosarcoma is increasingly valued as a spontaneous model of the human disease. The researchers call for a dedicated prospective trial in dogs under seven, along with studies of age-related differences in doxorubicin pharmacokinetics, and continued exploration of beta-blocker strategies to reinvigorate aged immune systems. For young adult dogs facing this diagnosis, the data offer something hemangiosarcoma rarely provides: reason for hope.

Subject of Research: Age-related survival outcomes and tumor gene expression in dogs with splenic hemangiosarcoma treated with adjuvant doxorubicin chemotherapy

Article Title: Younger age is associated with favorable outcomes in adult dogs with hemangiosarcoma receiving adjuvant doxorubicin chemotherapy: results from the PRO-DOX study

Article References: Borgatti, A., Husbands, B. D., Sarver, A. L., Chacón, J. M., DeFor, T. E., Rendahl, A., Henson, M. S., Modiano, J. F., Stuebner, K. M., Winter, A. L., Scavello, H., Pracht, S., Chehadeh, A., Bergsrud, K., Feiock, C., Anderson, B., Kenney, S., Atherton, M. J., Cheng, S., … Dickerson, E. B. (2025). Younger age is associated with favorable outcomes in adult dogs with hemangiosarcoma receiving adjuvant doxorubicin chemotherapy: results from the PRO-DOX study. Veterinary Oncology, 2(1), Article 35. https://doi.org/10.1186/s44356-025-00049-w

Image Credits: AI Generated

DOI: 10.1186/s44356-025-00049-w

Keywords: canine hemangiosarcoma, doxorubicin, propranolol, veterinary oncology, aging, immunosenescence, DNA damage response, gene expression profiling, tumor microenvironment, splenectomy, dog cancer, clinical trial