Study warns emodepside formulation changes exposure in dogs

CURRENT FULL VERSION: A new pharmacokinetic study is adding needed caution to one of the profession’s most closely watched extra-label parasite treatments. In research published online April 4, 2025, in the American Journal of Veterinary Research, investigators reported that the FDA-approved feline topical emodepside formulation is not bioequivalent to the European canine modified-release oral tablet when both are given orally to dogs. The finding matters because emodepside has become a last-line option for some dogs with multi-anthelmintic drug-resistant hookworm infections, even though no canine emodepside product is approved in the U.S. (pubmed.ncbi.nlm.nih.gov)

The backdrop is the growing spread of multidrug-resistant Ancylostoma caninum. An AAVP position paper published in 2025 said resistance has now been documented to all anthelmintic classes registered for dogs in North America, with cases first reported in 2019 and subsequent evidence suggesting wide distribution in U.S. pet dogs, plus reports from Canada. What began as a greyhound-associated problem is no longer confined to that population, which helps explain why veterinarians have looked for alternatives when standard deworming protocols fail. (sciencedirect.com)

That search for alternatives is what pushed emodepside into the spotlight. A 2024 case report described a greyhound with persistent A caninum infection and a benzimidazole resistance marker that improved after emodepside treatment when routine anthelmintics had failed. At the same time, Kansas State investigators had already signaled, in a 2024 research abstract, that they were studying whether the cat topical product given orally in dogs actually matched the canine oral formulation that had shown efficacy elsewhere. The new study answers that question with a clear no. As the authors explained in the AVMA’s Veterinary Vertex podcast, emodepside belongs to the octadepsipeptide class, with a mechanism of action distinct from the other antiparasitic drug classes currently on the market, which helps explain why it has drawn interest as resistance has spread. (pubmed.ncbi.nlm.nih.gov)

In the crossover study, seven healthy client-owned dogs received three regimens across separate phases: the feline topical solution orally at 1 mg/kg, the canine modified-release tablet orally at 1 mg/kg, and the feline topical solution topically at 3 mg/kg. Plasma was sampled for 21 days. Oral administration of the feline topical solution resulted in markedly greater absorption than the canine tablet, meaning the two were not bioequivalent. More specifically, the feline topical product given orally produced a peak concentration about 3 times higher and total systemic exposure (AUC) about 2.4 to 2.8 times greater than the canine tablet at the same dose. Meanwhile, topical administration of the feline product produced emodepside concentrations 36- to 122-fold lower than oral administration. The authors concluded that the higher exposure seen with oral administration of the feline topical product raises potential safety concerns for extra-label use in dogs, while the poor absorption after topical administration suggests that route may be unsuitable for resistant hookworm treatment. (pubmed.ncbi.nlm.nih.gov)

Industry and expert commentary has been moving in the same direction. AAHA’s coverage of extra-label emodepside use for multidrug-resistant hookworms has warned that treatment requires careful patient selection, including screening for heartworm and MDR1 risk. Utah State University, highlighting the new study, similarly noted that emodepside can be dangerous in dogs infected with heartworm microfilariae, depending on the stage of infection. The AVMA podcast discussion of the paper underscored the same practical point in plain terms: formulation and route of administration really matter, and products are not interchangeable just because the labeled mg/kg dose looks similar. Taken together, that commentary underscores that the issue is not whether emodepside can work, but whether clinicians can use the available formulation in a way that is both effective and predictable. (aaha.org)

Why it matters: For veterinary professionals, the study is a reminder that formulation differences can’t be treated as a technical footnote when a drug moves across species, routes, and regulatory markets. A cat spot-on product given orally to a dog may not mimic a canine oral tablet, even at the same nominal mg/kg dose. In practical terms, that affects safety discussions, informed consent, monitoring, and expectations around efficacy. It also complicates decision-making in referral and primary care settings where clinicians may be facing persistent hookworm infections with few remaining options. And because the broader resistant hookworm problem now extends well beyond racing greyhounds, this is becoming a general-practice issue, not just a specialty parasitology concern. It also matters in the U.S. specifically because the only available emodepside product is the feline topical formulation, while the canine oral product used in Europe is not available domestically. (pubmed.ncbi.nlm.nih.gov)

The study also lands at a time when the profession is trying to define more standardized approaches to diagnosing and managing resistant hookworms. The AAVP position paper emphasized that recurrent positive fecal tests do not always equal resistance and called for better diagnostics, more research, and clearer therapeutic pathways. This new pharmacokinetic evidence adds an important piece to that puzzle: if emodepside is used, veterinarians need to think carefully about which formulation, which route, and which patient. That’s especially true in dogs that may have concurrent heartworm risk or genetic susceptibility affecting drug handling. (sciencedirect.com)

What to watch: The next step is likely more work on safe dosing strategies, prospective clinical outcomes, and whether a more standardized U.S. pathway for emodepside use in dogs emerges as resistant hookworm cases continue to spread. (sciencedirect.com)