Study finds emodepside formulations aren’t interchangeable in dogs

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A newly published AJVR study is putting harder data behind a question many parasitologists and small animal clinicians have been wrestling with: whether different emodepside formulations can be used interchangeably when treating dogs with multi-anthelmintic drug-resistant hookworms. The answer, based on this pharmacokinetic work, is no. Investigators reported that the FDA-approved feline topical solution and the EMA-approved canine modified-release tablet were not bioequivalent when administered orally to dogs, and that the feline topical product produced substantially higher systemic exposure when given by mouth. (pubmed.ncbi.nlm.nih.gov)

That matters because U.S. veterinarians have had few reliable options for dogs with persistent or confirmed resistant Ancylostoma caninum infections. Emodepside has emerged as a last-line consideration in some of these cases, even though in the U.S. it is approved only as a topical combination product for cats. It has attracted interest in part because it belongs to the octadepsipeptide class, with a mechanism of action distinct from other currently marketed anthelmintics. Prior clinical guidance and industry commentary have already stressed that oral extra-label use in dogs should be approached cautiously, especially after failure of combination protocols and with confirmation that resistance, rather than reinfection or compliance issues, is driving persistent egg shedding. (yourpetandyou.elanco.com)

In the new crossover study, seven healthy client-owned dogs were enrolled from May through August 2023. Dogs received single doses in three 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, with plasma pharmacokinetics tracked for 21 days. Oral dosing with the feline topical formulation resulted in markedly greater absorption than the canine tablet, failing bioequivalence testing as defined by FDA standards for equal rate and extent of absorption. More specifically, the oral feline product produced an approximately 3-fold higher Cmax and a 2.4- to 2.8-fold greater AUC than the canine tablet at the same 1 mg/kg dose. By contrast, topical administration of the feline product yielded plasma concentrations 36- to 122-fold lower than oral dosing, suggesting that simply applying the U.S. cat product topically is unlikely to produce the exposures associated with oral treatment approaches for resistant hookworms. (pubmed.ncbi.nlm.nih.gov)

The broader backdrop is a resistant hookworm problem that has been building for years, particularly in some greyhound and foxhound populations, and that now has wider relevance in companion animal practice. Published work has documented multiple-anthelmintic drug-resistant A. caninum in U.S. dogs, while the Companion Animal Parasite Council notes suspected drug-resistant hookworms as an active clinical issue. More recent commentary in the parasitology literature has also pointed to the safety implications of using the feline topical product orally in dogs, including the potential for substantially higher peak concentrations than with the canine oral product marketed in other countries. (pubmed.ncbi.nlm.nih.gov)

Expert commentary before this paper had already flagged the same practical concerns. AAHA coverage, citing parasitology experts including Cassan Pulaski as well as Pablo David Jimenez Castro and Ray Kaplan, noted that emodepside may be effective in some refractory cases but should not be treated as a routine solution because of limited safety data. Those experts highlighted the need to screen for heartworm infection, consider MDR1 status in at-risk dogs, document failure of more conventional combination therapy, and counsel pet parents carefully about the extra-label nature and risks of treatment. The new AJVR data now give those cautions a clearer pharmacokinetic basis by showing that same-dose oral use of the feline topical formulation does not mirror the canine oral tablet. (aaha.org)

Why it matters: For veterinary professionals, this study is a reminder that formulation is not a technical footnote, it’s the clinical story. A topical cat product, even when it contains the same active ingredient, may not mimic the exposure profile of a canine oral tablet, and that difference can affect both efficacy assumptions and toxicity risk. In practical terms, the data support a more conservative, evidence-based approach to emodepside use in dogs with resistant hookworms: confirm the diagnosis, rule out reinfection and pseudo-resistance, exhaust recommended combination strategies first, and if emodepside is considered, treat it as a high-caution extra-label intervention rather than a straightforward substitution. (pubmed.ncbi.nlm.nih.gov)

What to watch: The next step is likely more work on dose optimization, safety, and real-world outcomes in dogs with confirmed resistant hookworm infections, along with continued discussion about whether U.S. clinicians need a more appropriate approved canine formulation or clearer consensus protocols for last-line use. The study also reinforces a broader point likely to shape future guidance: route of administration and product design can materially alter emodepside exposure, even when the nominal dose is the same. (pubmed.ncbi.nlm.nih.gov)