FDA approves first and only second-generation Janus kinase inhibitor

Merck Animal Health announced today that the FDA has approved atinvicitinib tablets (Numelvi), making it the first and only second-generation Janus kinase (JAK) inhibitor indicated for the control of pruritus associated with allergic dermatitis in canine patients older than 6 months.

“With more than 70 years of innovation and commitment to animal health, we are confident that Numelvi will offer veterinary professionals a groundbreaking, first-of-its-kind solution aimed at improving the health and well-being of dogs—from as young as 6 months to mature dogs—and ultimately providing peace of mind for the people who care for them,” Christine McKinney, DVM, DACVD, senior professional services veterinarian at Merck Animal Health, said in an organizational release.¹

“Numelvi also offers added convenience for veterinary clinics; most dogs, starting at weights as low as 4.4 lb, require only 1 or half of a tablet, and there is no need to adjust vaccination schedules. Plus, Numelvi comes in easy-to-dispense bottles and can be stored under standard conditions. Its active ingredient is intrinsically stable, eliminating the need for a protective film coat or specialized packaging and providing the longest shelf life among JAK inhibitor tablets in veterinary medicine.”

Numelvi is a once-daily, first-line treatment that reduces itch starting with the first dose. According to data from Merck Animal Health, the product is at least 10 times more selective for JAK1 than it is for JAK2, JAK3, and tyrosine kinase 2,² and obstructs the effects of JAK1-dependent cytokines involved in inflammation and itch in allergic dermatitis. High selectivity for JAK1 can shrink interference with the beneficial immune functions of other JAKs, which can create a safety profile with proven efficiency.

Numelvi is currently the only JAK inhibitor approved for use in canine patients as young as 6 months and weighing up to 4.4 lb (2 kg). This product will be available nationwide for veterinary clinics and hospitals in spring 2026.

In a veterinary general practice, skin conditions are extremely common in dogs and can make up to 20% of the clinic’s daily cases, causing animals discomfort, emotional distress, and interruptions to sleep and other regular daily routines, and can have a detrimental effect on the human-animal bond.

RELATED: The itchy dog and the caregiver's burden³

“Merck Animal Health is continuously working to expand its portfolio to include innovative veterinary medicinal products that meet the needs of dogs and their owners,” said Tim Kowalski, distinguished scientist, discovery research, Merck Animal Health.¹ “Numelvi, a second-generation JAK inhibitor, is the first of its kind to enter the market in veterinary medicine, giving it a unique profile compared with other JAK inhibitors. Second-generation JAK inhibitors are already considered the standard of care in human health, and Numelvi brings that same advanced approach to veterinary medicine.”

References

1. FDA approves Numelvi (atinvicitinib tablets) for dogs from Merck Animal Health – the first and only second-generation Janus kinase (JAK) inhibitor for the control of pruritus associated with allergic dermatitis. News release. Merck Animal Health. February 25, 2026. Accessed February 26, 2026. https://www.businesswire.com/news/home/20260211583513/en/FDA-Approves-NUMELVI-atinvicitinib-tablets-for-Dogs-from-Merck-Animal-Health-The-First-and-Only-Second-Generation-Janus-Kinase-JAK-Inhibitor-for-the-Control-of-Pruritus-Associated-with-Allergic-Dermatitis
2. Merck Animal Health. Data on file. 2026
3. McCafferty C. The itchy dog and the caregiver’s burden. dvm360. March 21, 2025. Accessed February 26, 2026. https://www.dvm360.com/view/the-itchy-dog-and-the-caregiver-s-burden