In an effort to improve on those dismal numbers, veterinary oncologists have tested a series of combination chemotherapy protocols incorporating agents such as vincristine, cyclophosphamide, and dacarbazine alongside doxorubicin. None has proven clearly superior to single-agent doxorubicin, and most carry greater toxicity. One combination, however, generated genuine excitement: doxorubicin paired with dacarbazine, administered as a true concurrent combination for dogs with advanced-stage hemangiosarcoma, produced an objective response rate of 47.4 percent in a prospective study, although its impact on overall survival remains undefined. The catch is logistical and toxic. Dacarbazine must be given as an eight-hour intravenous infusion every three weeks, and beyond the usual myelosuppressive risks of neutropenia and thrombocytopenia, it is notorious for causing acute gastrointestinal upset that requires prophylactic antiemetic therapy.

That is where temozolomide enters the story. Temozolomide is an oral chemotherapeutic closely related to dacarbazine, but far easier to administer and generally well tolerated in dogs, with favorable safety data already available when it is combined with doxorubicin in canine lymphoma patients. In human medicine, temozolomide is the first-line chemotherapy for glioblastoma multiforme, a deadly brain cancer, and its mechanism of action is well characterized. Once in the bloodstream, the drug spontaneously hydrolyzes into its active metabolite, 5-(3-methyl-1-triazeno) imidazole-4-carboxamide, or MTIC. MTIC methylates nucleotide bases in DNA, with guanine as the most important target. The cell’s mismatch repair pathway then detects these methylated bases and attempts to repair them in a futile cycle that ultimately triggers apoptosis, the programmed death of the cancer cell.

That mechanism also explains why some tumors resist the drug. In cells with an intact mismatch repair system, sensitivity to temozolomide is governed largely by a DNA repair protein called O-6-methylguanine-DNA methyltransferase, or MGMT. MGMT works by directly removing the methyl groups that temozolomide installs, effectively erasing the damage before it can become lethal. When MGMT is overexpressed, the drug’s fingerprints are wiped away almost as fast as they are written, and the tumor survives. In human glioblastoma, this relationship is so reliable that testing MGMT status, particularly epigenetic silencing of the MGMT promoter, is routinely used to decide whether temozolomide is a viable option for a patient. The question for veterinary researchers was whether the same logic could be imported into canine hemangiosarcoma: if MGMT expression varied among tumors, it might serve as a biomarker to identify which dogs would benefit from adding temozolomide to doxorubicin.

A new study led by Brianna Moore, Julie Nguyen-Edquilang, and Matthew R. Berry at the University of Illinois set out to test exactly that hypothesis in the laboratory. The team worked with five canine hemangiosarcoma cell lines, named DHSA-1426, Emma Brain, Emma Spleen, FITZ, and SBHSA, along with a canine aortic endothelial cell line as a comparison. The cell lines had been shared by collaborators at the University of Minnesota, Colorado State University, and the University of Wisconsin-Madison. To anchor their measurements, the researchers used a human cervical cancer cell line, HeLa, and a pair of mouse glioma cell lines, one wild type and one engineered to carry a human MGMT knock-in, as controls. All cells were cultured under standard conditions at 37 degrees Celsius in a humidified incubator with five percent carbon dioxide.

The researchers measured MGMT at two levels. Quantitative PCR, using canine-specific TaqMan primers with GAPDH as the reference gene, quantified MGMT transcript abundance across three biological replicates with five technical replicates each. Western blotting, performed with a rabbit monoclonal antibody and normalized to the loading control beta-actin, assessed protein expression. The two approaches told a generally concordant story. Four of the five hemangiosarcoma cell lines, DHSA-1426, Emma Brain, Emma Spleen, and SBHSA, showed low or absent MGMT transcript and protein. FITZ was the striking exception, displaying the highest MGMT expression of the panel, roughly twenty-fold greater protein expression than the canine endothelial cell line based on normalized densitometry. The protein bands appeared at the expected sizes, approximately 37 kilodaltons in human controls and approximately 40 kilodaltons in canine cells, a slight species difference confirmed against UniProt database entries.

With the expression map in hand, the team turned to cytotoxicity testing using the sulforhodamine B assay. Cells were seeded in 96-well plates, pretreated for two hours with O6-benzylguanine, an inhibitor that depletes the functional pool of MGMT, and then exposed to temozolomide at concentrations ranging from 0.033 micromolar to 1000 micromolar for seven days. Because temozolomide is unstable in solution, it was dissolved fresh immediately before each experiment. Cytotoxicity values were normalized against a DMSO live-cell control and a raptinal control that induces complete cell death, allowing relative responses to be compared across conditions. The control cell lines behaved exactly as biology predicts. GL261 wild-type cells, which lack MGMT, were sensitive to temozolomide even without the inhibitor, while GL261 MGMT knock-in cells were strongly resistant, with IC50 values comparable to the hemangiosarcoma lines, until O6-benzylguanine pretreatment sensitized them, producing the largest drop in IC50 of any line tested.

The hemangiosarcoma results defied expectations. Every canine cell line proved resistant to temozolomide across the entire tested concentration range, regardless of MGMT status. FITZ, the only line expected to be sensitized by MGMT inhibition based on its high expression, remained stubbornly resistant, showing an undulating dose-response curve with no concentration-dependent effect in any of three biological replicates, and no reliable IC50 could be determined. DHSA-1426 likewise showed minimal cytotoxic response across the range, precluding curve fitting. In other words, MGMT expression status could not predict temozolomide sensitivity in canine hemangiosarcoma cells, and blocking MGMT did not rescue drug sensitivity even where the protein was abundant. The data point instead to an MGMT-independent resistance mechanism, and the authors hypothesize that mismatch repair deficiency is the culprit.

That hypothesis has independent support. A comprehensive analysis of microsatellite instability in canine cancers, a hallmark of mismatch repair defects marked by mutations accumulating in repetitive DNA sequences, found a surprisingly high incidence of 63 percent across canine tumors, with hemangiosarcoma among the tumor types showing elevated instability. If canine hemangiosarcoma is indeed frequently mismatch repair deficient, the implications extend well beyond temozolomide. Mismatch repair deficiency also means deficient methylated DNA goes unnoticed by the cell, undermining the drug’s lethal futile-repair cycle. More intriguingly, tumors with high microsatellite instability carry increased mutational burden and therefore heightened immunogenicity, making them particularly susceptible to immune checkpoint blockade. The authors suggest this provides biological rationale for moving beyond chemotherapy combinations entirely and exploring chemo-immunotherapy, pairing doxorubicin with agents such as PD-1 or PD-L1 inhibitors.

The study’s conclusions are a sober but valuable course correction. MGMT remains a highly promising predictive biomarker for temozolomide selection in human glioblastoma, but this in vitro investigation demonstrates that canine hemangiosarcoma cell lines resist the drug through a mechanism that MGMT testing cannot capture. On that basis, the authors conclude the data do not support clinical exploration of temozolomide, alone or combined with doxorubicin, for treating dogs with hemangiosarcoma. Instead, the findings redirect attention toward characterizing mismatch repair deficiency in canine hemangiosarcoma and toward immunotherapy-based strategies that exploit the high mutational burden such deficiency creates. For a disease that claims most of its canine victims within a year of diagnosis, a result that rules out one dead-end combination while pointing toward a biologically grounded alternative is a meaningful step forward for comparative oncology.

Subject of Research: MGMT expression and temozolomide resistance mechanisms in canine hemangiosarcoma cell lines

Article Title: Investigating MGMT expression as a resistance mechanism to temozolomide in canine hemangiosarcoma cell lines

Article References: Moore, B., Nguyen-Edquilang, J., & Berry, M. R. (2026). Investigating MGMT expression as a resistance mechanism to temozolomide in canine hemangiosarcoma cell lines. Veterinary Oncology, 3(1), Article 24. https://doi.org/10.1186/s44356-026-00076-1

Image Credits: AI Generated

DOI: 10.1186/s44356-026-00076-1

Keywords: canine hemangiosarcoma, temozolomide, MGMT, doxorubicin, chemotherapy resistance, mismatch repair deficiency, microsatellite instability, veterinary oncology, DNA repair, immunotherapy, cell lines, dog cancer