Efficacy and Field Safety of the Second‐Generation, Highly Selective JAK1 Inhibitor Atinvicitinib for the Treatment of Canine Atopic Dermatitis: A Multicentre, Blinded, Randomised, Placebo‐Controlled Clinical Trial in Client‐Owned Dogs

ABSTRACT Background Janus kinase (JAK) inhibitors alleviate pruritus and clinical signs of canine atopic dermatitis (cAD). Objective To confirm the optimal dosing regimen of atinvicitinib, a second‐generation highly selective inhibitor of JAK1 in dogs. Animals Sixty‐one client‐owned dogs, ≥ 12 months and ≥ 2 kg, with cAD. Materials and Methods In a randomised, masked, placebo‐controlled clinical trial, dogs received atinvicitinib, with food, at 0.8–1.2 mg/kg, once daily for 28 days (q.d. group, n = 21) or 0.4–0.6 mg/kg twice daily for 14 days followed by once daily dosing for 14 days (b.i.d.–q.d. group, n = 21); or placebo b.i.d.–q.d. (control group, n = 19). Owners assessed pruritus (Day [D]0–7, D14, D28) using a Visual Analog Scale (PVAS); veterinary surgeons assessed skin lesions (D0, D28) using the cAD Extent and Severity Index, 4th iteration (CADESI‐04). On D0, D14 and D28, blood and urine samples were collected. Treatment success was defined as a ≥ 50% reduction from baseline PVAS or CADESI‐04 on D28. Proportions of dogs with ≥ 2 PVAS reduction, and reduction in PVAS and/or CADESI‐04 from baseline, also were assessed. Results Treatment success was 87.5% for the q.d. group ( p = 0.011) and 73.3% for the b.i.d.–q.d. group ( p = 0.033) (23.1% control group). There was ≥ 2 decrease in PVAS in 80.0% of q.d. dogs. (16.7% controls, p = 0.042). Compared to controls, atinvicitinib‐treated dogs had a significantly greater reduction in PVAS (q.d., p = 0.003, b.i.d.–q.d., p = 0.001) and CADESI‐04 (q.d. group, p = 0.046). Mean results of clinical pathological values remained within reference ranges. Treatment was well‐tolerated. Conclusions and Clinical Relevance Once‐daily atinvicitinib was safe, well‐tolerated and effective at rapidly reducing pruritus and resolving cAD‐associated skin lesions.