A quiet revolution is brewing in veterinary oncology, and it starts with three blood draws and seven dogs. Researchers at the University of California, Davis have delivered the first evidence in dogs that the variability in doxorubicin exposure between different patients is dramatically larger than the variability within the same patient across repeated treatment cycles. The finding, published in Veterinary Oncology, strikes at the heart of a dosing convention that has governed cancer chemotherapy for more than six decades: calculating drug doses from body surface area. If the results hold up in larger cohorts, they could pave the way toward personalized doxorubicin dosing in dogs, mirroring a shift already underway in human oncology.
Doxorubicin is an anthracycline antitumor antibiotic and one of the most widely used injectable chemotherapeutics in veterinary medicine. It acts on cancer cells through a battery of mechanisms, including DNA intercalation, inhibition of DNA and RNA polymerases, inhibition of topoisomerase II, DNA alkylation, and the generation of reactive oxygen species. That versatility makes it a staple against many canine malignancies, both as a single agent and within multi-drug protocols. But its toxicity profile is equally well known. Gastrointestinal adverse events and myelosuppression, the suppression of bone marrow activity, are the dose-limiting toxicities, and dogs receiving doxorubicin as a single agent experience gastrointestinal side effects more frequently than with most other intravenous chemotherapeutics.
The problem with the current standard lies in its assumptions. Doses of doxorubicin, like many chemotherapies, are calculated using body surface area, a practice originally adopted to normalize doses across species on the grounds that surface area better reflects basal metabolic rate than body weight alone. Yet body surface area cannot account for the factors that actually determine how much drug circulates in a patient: distribution, metabolism, and excretion. The evidence that something is amiss has been accumulating for years. Smaller dogs suffer more severe adverse events when dosed by body surface area, which is why dogs under 15 kilograms are typically treated with a weight-based dose of 1 milligram per kilogram instead. The formula, in short, is imperfect.
Pharmacokinetics offers a way out. Rather than asking how much drug was administered, pharmacokinetic-guided dosing asks how much drug the patient actually absorbed and retained over time, quantified as the area under the drug concentration-time curve, or AUC. Previous work in canine patients demonstrated that predicted doxorubicin AUC correlates with neutropenia, the dangerous drop in neutrophils that marks bone marrow suppression, as well as with total white blood cell counts and the surviving fraction of neutrophils at the nadir, the lowest point of the count. Human medicine has already capitalized on this logic. Pharmacokinetic-guided dosing of docetaxel reduced variability in bone marrow suppression by up to 50 percent, and individualized fluorouracil dosing in metastatic colorectal cancer significantly improved objective response rates compared with conventional dosing.
The Davis team, led by Sridhar Madan Veluvolu together with Robert B. Rebhun, Jaeyoung Kim, and Luke Anthony Wittenburg, built on a tool developed earlier in their laboratory: a limited sampling model that predicts total doxorubicin exposure from just three blood samples. Under the protocol, dogs received a standard 30 milligrams per square meter dose by 20-minute intravenous infusion, and blood was drawn at five, 45, and 60 minutes after the infusion ended. Serum doxorubicin concentrations were measured with a validated liquid chromatography tandem-mass spectrometry assay, linear from 10 to 750 nanograms per milliliter, and plugged into a weighted equation that estimates the AUC over the first six hours. The elegance of the approach is its practicality: three time points instead of an exhaustive sampling series makes prospective clinical use feasible.
The prospective study ran from June 2021 to August 2022 at the UC Davis Veterinary Medical Teaching Hospital, enrolling tumor-bearing dogs at least 15 kilograms in weight with favorable performance scores and no cardiac dysfunction or multidrug-resistance gene mutations. Ten dogs met the criteria, but three were withdrawn after a single dose because of owner-perceived adverse events, leaving seven patients who completed three full cycles of doxorubicin with complete blood counts at baseline and at the seven-day nadir after each dose. Owners filled out standardized questionnaires about gastrointestinal signs after every treatment and were asked to keep their pets on a consistent diet to make those observations as comparable as possible.
The headline result is stark. Measured and dose-normalized doxorubicin concentrations varied nearly tenfold between patients at the sampled time points, and there was no significant correlation between the total milligram dose a dog received and its predicted AUC. When the coefficient of variation was calculated after normalizing for dose, the average within-patient variability across three cycles was just 4.7 percent, while the average between-patient variability was 25.4 percent, a difference that was highly statistically significant. Before dose normalization the within-patient figure was 10.7 percent against 22.5 percent between patients. Notably, the within-patient variability observed in these dogs, ranging from 2.8 to 9 percent, is lower than the 6 to 59 percent reported in humans, while the between-patient variability in dogs was tighter than the 37 to 93 percent seen in human studies.
The model also proved its worth as a predictive instrument. Although no significant relationship emerged between predicted AUC and absolute neutrophil counts across all data points, the picture sharpened when the analysis focused on dogs whose seven-day neutrophil counts were genuinely lower than baseline, suggesting the true nadir had been captured. For those patients, higher exposure correlated significantly with a lower surviving fraction of neutrophils. The limited sampling model, using baseline neutrophil count and predicted AUC together, successfully predicted the absolute nadir neutrophil count with a mean prediction error of just 3.3 percent. Three neutropenic events were captured during the study, two of which occurred at the highest predicted exposures, and one patient who received a 25 percent dose reduction after severe neutropenia subsequently showed a 25 to 32 percent drop in exposure, an encouraging hint that the model tracks dose changes faithfully.
The gastrointestinal findings add a patient-centered dimension. Using owner-reported scores, the researchers found that higher predicted AUC correlated significantly with decreased appetite and increased nausea, though not with vomiting or diarrhea, and no significant relationship emerged for composite gastrointestinal scores. The authors caution that owner scoring of subjective signs carries inherent bias, but the correlations suggest that the patients with the highest drug exposure are also the ones feeling the worst, a pattern that personalized dosing could potentially flatten. The study had clear limitations, including its small size of seven completers, strict eligibility criteria that may not represent the broader population of treated dogs, and unexplored genetic factors, since doxorubicin disposition involves influx and efflux transporters and metabolizing enzymes whose variants in dogs remain poorly characterized.
What comes next is the crucial question of targets. The researchers emphasize that no one yet knows what AUC a dog should hit to maximize tumor response while minimizing toxicity, and that target may differ from patient to patient. In human docetaxel therapy, AUC-guided dose adjustment dramatically increased the proportion of patients within the therapeutic window and improved disease control rates. Neutropenia itself may serve as a double-edged biomarker, since in canine lymphoma it has been associated with longer remission and survival times, hinting that the right amount of bone marrow suppression may signal effective dosing. For now, the Davis team’s message is simple and provocative: the same dose produces wildly different exposures in different dogs, but each dog is remarkably consistent with itself. That consistency is precisely what makes individualized dosing achievable, and the first treatment cycle may be all the information a clinician needs to get there.
Subject of Research: Pharmacokinetic variability of doxorubicin exposure in dogs with cancer using a limited sampling model
Article Title: Comparison of interpatient and intrapatient variability in doxorubicin exposure using a validated limited sampling model in dogs with cancer
Article References: Veluvolu, S. M., Rebhun, R. B., Kim, J., & Wittenburg, L. A. (2025). Comparison of interpatient and intrapatient variability in doxorubicin exposure using a validated limited sampling model in dogs with cancer. Veterinary Oncology, 2(1), Article 24. https://doi.org/10.1186/s44356-025-00038-z
Image Credits: AI Generated
DOI: 10.1186/s44356-025-00038-z
Keywords: doxorubicin, veterinary oncology, pharmacokinetics, limited sampling model, chemotherapy dosing, body surface area, neutropenia, AUC, dogs, adverse events, personalized medicine, canine cancer
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Nathaniel Bowman. (October 1, 2026). Doxorubicin Doses Hit the Same Dogs Differently, Landmark Canine Study Reveals. Scienmag. https://scienmag.com/doxorubicin-doses-hit-the-same-dogs-differently-landmark-canine-study-reveals/
Nathaniel Bowman. “Doxorubicin Doses Hit the Same Dogs Differently, Landmark Canine Study Reveals.” Scienmag, 1 October 2026, https://scienmag.com/doxorubicin-doses-hit-the-same-dogs-differently-landmark-canine-study-reveals/. Accessed 1 October 2026.
Nathaniel Bowman. “Doxorubicin Doses Hit the Same Dogs Differently, Landmark Canine Study Reveals.” Scienmag. October 1, 2026. https://scienmag.com/doxorubicin-doses-hit-the-same-dogs-differently-landmark-canine-study-reveals/
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