Dual PI3K/mTOR Inhibitor VDC597 Shows Promise Against Canine Lymphoma and Leukemia in Lab Studies

Lymphoma is one of the most common diagnoses in veterinary oncology, accounting for roughly 7 to 24 percent of all canine cancers and fully 80 percent of hematopoietic tumors in dogs. Although the standard CHOP chemotherapy protocol, built around cyclophosphamide, doxorubicin, vincristine, and prednisone, reliably produces short-term remissions, the field has seen little genuine progress in years: median survival times hover near twelve months, and only about 20 to 25 percent of patients are alive two years after diagnosis. Now, a team at Colorado State University’s Flint Animal Cancer Center reports that a dual inhibitor of PI3K and mTOR, an experimental drug called VDC597, meaningfully suppresses the growth of canine B-cell lymphoma and chronic lymphocytic leukemia cells in laboratory experiments, offering a potential new line of attack against these stubborn diseases.

The drug targets the phosphatidylinositol-4,5-bisphosphate 3-kinase, AKT, mechanistic target of rapamycin signaling cascade, a central growth and survival pathway that is frequently dysregulated in both canine and human cancers. When this pathway runs amok, it drives neoplastic cells to proliferate relentlessly, evade apoptosis, crank up metabolism, and, in the case of lymphoid malignancies, stimulate the production of vascular endothelial growth factor, a molecule that supports tumor neovascularization. Because single-node inhibitors of the pathway often deliver only brief or incomplete responses, likely due to well-known feedback loops within the signaling network, the researchers turned to VDC597, a compound that blocks PI3K along with both mTOR complexes, mTORC1 and mTORC2, thereby hitting the pathway at multiple points simultaneously.

In their experiments, the team tested VDC597 against two canine cell lines: 1771, derived from a spontaneously occurring B-cell lymphoma, and CLL1390, derived from chronic lymphocytic leukemia. Both lines had previously been documented to carry mutations in or constitutively express activated proteins of the PI3K-AKT-mTOR pathway, making them appropriate models for pathway-targeted therapy. Western blot analysis showed that a 24-hour treatment with the drug produced a clear, dose-dependent reduction in phosphorylated AKT at the serine 473 residue, the molecular signature of pathway activation. The leukemia line proved more sensitive, showing significant suppression at concentrations as low as 0.2 micromolar, while the lymphoma line required a full micromolar dose to achieve statistically significant inhibition.

Timing experiments revealed that the drug’s effects on signal transduction are both rapid and durable. Following a single 1 micromolar dose, phosphorylation of AKT dropped quickly and remained at roughly half of control levels for a full 24 hours. Downstream phosphorylation of 4EBP1, a translation-regulating target of mTORC1, fell even more sharply, with statistically significant suppression measurable from half an hour after treatment through the two-hour mark. The researchers verified these biochemical findings by immunohistochemistry on fixed cell pellets, observing reduced perinuclear and cytoplasmic staining for both phosphoproteins after drug exposure, an encouraging confirmation that the compound shuts down signaling across the entire pathway rather than just a single branch.

The functional consequences were equally striking. After 72 hours of exposure, VDC597 halved the viable cell population at concentrations within or near what is pharmacologically achievable, with mean half-maximal inhibitory concentrations of 1.41 micromolar for CLL1390 and 2.56 micromolar for 1771, values informed by an earlier phase I trial of the drug in tumor-bearing dogs. Live-cell imaging, tracking fluorescently labeled nuclei and dead-cell stains over 48 hours, showed that the drug did not merely slow proliferation: it actively killed the lymphoma cells, with cell death increasing in a dose-dependent manner and becoming significantly elevated at concentrations of 0.25 micromolar and above. Enzyme-linked immunosorbent assays further demonstrated that VDC597 suppressed vascular endothelial growth factor production in both cell lines, again in a dose-dependent fashion, suggesting the drug may undermine the angiogenic support system that lymphoid tumors rely upon.

Because combination therapy, not single agents, ultimately determines how well targeted drugs translate into clinical benefit, the team paired VDC597 with the three chemotherapy drugs most commonly used in canine lymphoma protocols: dexamethasone, doxorubicin, and vincristine. The results were revealing. Combining VDC597 with dexamethasone produced genuinely synergistic growth inhibition, quantified by combination indices below 1 using the Chou-Talalay method, with the effect strongest at lower drug concentrations. Pairing the drug with doxorubicin yielded additive to mildly synergistic effects at lower doses, though the highest doxorubicin concentration produced antagonism, possibly because PI3K inhibition dampens the reactive oxygen species generation through which doxorubicin promotes apoptosis. Vincristine, whose mechanism overlaps with the pathway’s role in microtubule function during mitosis, showed only additive to mildly antagonistic interactions.

The strong synergy with dexamethasone is mechanistically compelling. Glucocorticoids kill lymphoid cells through receptors whose nuclear translocation and downstream signaling are potentiated by PI3K-AKT-mTOR inhibition, a finding previously documented in human B-cell acute lymphoblastic leukemia. The Colorado State results thus support the hypothesis that adding a dual PI3K/mTOR inhibitor to standard chemotherapy could deepen responses or sensitize resistant tumors, particularly in lymphoma, where the clinical reservoir of effective options is thin.

Importantly, the study went beyond cell culture to examine pathway activation in actual patients. Using tissue microarrays built from 118 lymph node samples collected from 84 dogs with spontaneous B-cell lymphoma, the researchers scored phosphorylated AKT and phosphorylated 4EBP1 by immunohistochemistry, employing both traditional manual Allred scoring and artificial intelligence-assisted whole-slide analysis with Visiopharm software to generate finer-grained H-scores. The two approaches correlated well, validating the use of machine learning tools for quantitative pathology in veterinary specimens. The phosphorylation signals proved highly variable from tumor to tumor, underscoring that not every canine B-cell lymphoma is driven by an equally active PI3K-AKT-mTOR pathway.

Two clinically meaningful correlations emerged. First, tumors with lower expression of major histocompatibility complex class II, a known adverse prognostic marker measured by flow cytometry, showed significantly higher levels of phosphorylated AKT, linking aggressive immune phenotype to stronger pathway activity. Second, samples taken after relapse following CHOP chemotherapy showed higher phosphorylated 4EBP1 than matched pre-treatment samples, a result that retained significance in paired manual scoring. However, no correlation was found between either phosphoprotein and progression-free interval, overall survival, WHO stage or substage, or flow cytometric immunophenotypic markers, and phosphorylated AKT did not rise in parallel with phosphorylated 4EBP1 in the relapse samples. The authors caution against overinterpreting these limited associations and suggest several possible explanations, including AKT phosphorylation at sites not detected by the assay, alternative 4EBP1 kinases such as PIM2 or RAS/MAPK signaling, or mutations downstream of AKT.

The study has clear limitations that its authors acknowledge. Only one cell line each was tested for lymphoma and leukemia, so the full heterogeneity of canine lymphoid malignancies, including T-cell lymphoma and acute lymphoblastic leukemia, remains unexplored. No washout experiments were performed to determine how long signaling stays suppressed after the drug is withdrawn, information that would be essential for designing oral dosing schedules. And the finding of elevated phosphorylated 4EBP1 without corresponding phosphorylated AKT in relapsed tumors awaits mechanistic confirmation. Nevertheless, the core message is clear: VDC597 collapses PI3K-AKT-mTOR signaling, kills lymphoid cancer cells in vitro at achievable concentrations, synergizes with dexamethasone, and may sensitize tumors to standard chemotherapy. As dogs with spontaneous diffuse large B-cell lymphoma are increasingly recognized as valuable models for human non-Hodgkin lymphoma, these results position dual PI3K/mTOR inhibition as a strategy worth advancing toward larger cell line panels, animal studies, and ultimately clinical trials in both species.

Subject of Research: Evaluation of the dual PI3K/mTOR inhibitor VDC597 against canine B-cell lymphoma and chronic lymphocytic leukemia cells in vitro, with immunohistochemical assessment of PI3K-AKT-mTOR pathway activation in spontaneous canine B-cell lymphoma.

Article Title: In vitro effects of the PI3K/mTOR dual-inhibitor, VDC597, in canine lymphoma and leukemia

Article References: Meuten, T., Brill, S. A., Brady, R. V., Farrell, K. B., Rose, B. J., Schlein, L. J., & Thamm, D. H. (2026). In vitro effects of the PI3K/mTOR dual-inhibitor, VDC597, in canine lymphoma and leukemia. Veterinary Oncology, 3(1), Article 13. https://doi.org/10.1186/s44356-026-00065-4

Image Credits: AI Generated

DOI: 10.1186/s44356-026-00065-4

Keywords: canine lymphoma, canine leukemia, VDC597, PI3K, mTOR, AKT, 4EBP1, signal transduction, chemotherapy, immunohistochemistry, veterinary oncology, drug combination