In January, the FDA released a ‘Dear veterinarian’ letter about a new canine allergy drug called Zenrelia. The letter updated safety information, citing a response study that determined it was not safe to administer vaccinations to dogs taking the drug.
Elanco Animal Health, the company that produces Zenrelia, conducted the study using 10-month-old laboratory beagles. Each received primary vaccinations while being treated with the drug at three times the recommended dose, and five dogs were severely immunosuppressed while two were euthanized due to complications. Out of eight, only two beagles were able to respond to the vaccines successfully — a number too low to ignore for researchers.
Even with the recommended dosage, Elanco is advising that it is not safe to administer vaccines while taking Zenrelia due to risk of an inadequate immunity to severe infections including pneumonia, canine distemper and rabies. A resurgence of a zoonotic disease like rabies could also increase human health risks.
“These animal and public health concerns can be mitigated by withholding Zenrelia for a certain amount of time before and after vaccination,” the agency said in a statement to the American Veterinary Medical Association.
Christina Gentry, DVM, is an assistant clinical professor of dermatology at Texas A&M’s Small Animal Teaching Hospital.
“In most medication studies, it is required that researchers use three to five times the dosing for safety protocol,” Gentry said. “I can imagine that having three times of the appropriate dose, for a long, persistent period of time, would touch more of the immune system and could lead to an adverse response.”
According to a 2018 report from Banner Pet Hospital, there has been a 30% increase in canine dermatitis over the past 10 years. Certain breeds are more prone to allergies and scientists speculate climate change and rising temperatures may have also contributed to the increase.
Zenrelia is a drug that must be prescribed by a licensed veterinarian. It uses ilunocitinib tablets, which are specific enzyme inhibitors, to treat dogs with allergic dermatitis which presents as excessive itching, leading to open wounds.
The drug is classified as a nonselective temporary Janus kinase, or JAK, inhibitor that blocks inflammatory signaling in the body, breaking the itch-scratch cycle. Gentry compares the process to sending emails.
“When environmental allergens like pollen penetrate the skin, your body’s receptors are going to send out an email that it’s time to freak out and start itching,” Gentry said. “Since we can’t avoid dogs going outside, that’s where JAK inhibitor drugs come in. The inhibitors block emails from cells that say, ‘start itching.’”
Like Zenrelia, the drug Apoquel, made by the company Zoetis, is commonly prescribed by veterinarians to treat a broader range of cells. When Apoquel has too narrow of a range, Zenrelia is used to ensure blockage of enough signals to effectively stop itching.
“Certain drugs, for both humans and dogs, prefer specific pathways to block messages from,” Gentry said. “Zenrelia targets JAK 1 and 2 receptors to stop inflammation. Problems only arise when the drug blocks too many receptors and affects the immune system, in extreme cases inhibiting things like bone marrow production and white blood cell count.”
Gentry also noted that dogs who are prescribed Zenrelia must be one year or older and have received their initial rounds of vaccinations as a puppy. As for booster shots later in life, a recent study found that dogs treated with ilunocitinib tablets were still able to be protected against diseases after receiving their booster vaccinations.
Melissa Eisenschenk, DVM, is a board-certified veterinary dermatologist with Pet Dermatology Clinic. She said that if adverse effects were to occur with drugs like Zenrelia and Apoquel, there is a relatively low risk.
“Interestingly, I have seen Apoquel improve mobility in dogs with arthritis,” Eisenschenk wrote in a blog post on Pet Dermatology Clinic’s website. “I am not sure if this is just because they overall feel better when their skin is not inflamed or if some dogs with allergic inflammation of the skin also have inflammation of the joints as well. It is common for me to hear a dog ‘acts like a puppy again.’”
The FDA’s letter was issued to prevent the spread of misinformation and to update pet owners and veterinarians on the new study. Gentry urged pet owners to always follow their veterinarian’s guidance.
“We have strategies to mitigate risk with this medication,” Gentry said. “We talk about these potential risks and benefits to the dog with their owners, and then we put a monitoring strategy in place that matches the use of the drug. We’re not just prescribing this medication that may cause side effects and waving people on their merry way. That’s not what we do.”
Gentry said allergies affect 10%, if not more, of the canine population. Since Zenrelia is a newer drug, researchers are still testing its long-term effects to find adverse responses or potential problems. For now, Zenrelia is approved by the FDA and, when used with the proper, updated guidelines, is safe for dogs.
“I wouldn’t hesitate to prescribe this drug to the right patient,” Gentry said.
Editor’s Note: A previous version of this article incorrectly described ilunocitinib tablets as immunosuppressants. The description has been corrected.
