Leflunomide with prednisone or nonsteroidal anti-inflammatory drug therapy is safe and tolerated for long-term treatment of immune-mediated polyarthritis in 27 dogs

OBJECTIVE To retrospectively evaluate safety and tolerance of leflunomide for long-term treatment of canine idiopathic immune-mediated polyarthritis (IMPA). ANIMALS 27 dogs with clinical signs and synovial fluid cytology supportive of IMPA with >= 6 months' follow-up after starting leflunomide. METHODS Medical records were reviewed to identify dogs prescribed leflunomide for treatment of IMPA from February 2012 to May 2022. Initial leflunomide doses of 2 to 4 mg/kg once daily were prescribed and were titrated to the lowest effective dose with concurrent anti-inflammatory therapy. Complete blood count, serum chemistry, and clinical signs were monitored throughout the course of treatment. RESULTS Adverse effects potentially attributable to leflunomide noted in 9 of 27 dogs (33%) included vomiting, diarrhea, lethargy, decreased or absent appetite, polyuria and polydipsia, and secondary antibiotic responsive infection and were self-limiting or resolved with outpatient therapy. Alkaline phosphatase (ALP) and alanine aminotransferase (ALT) elevation were documented in all dogs prescribed leflunomide plus prednisone, with persistent liver enzyme elevation in 6 of 9 dogs (67%) and normalization after antibiotic therapy in 3 of 9 dogs (33%). The majority of dogs prescribed leflunomide plus NSAID (11/17 [65%] dogs) did not experience liver enzyme elevation; 2 of 17 (12%) dogs developed transient antibiotic-responsive liver enzyme elevations, and 4 of 17 (23%) dogs had persistent liver enzyme elevation. CLINICAL RELEVANCE Leflunomide was well tolerated for long-term management of IMPA. A significant difference in liver enzyme elevation was identified between dogs prescribed prednisone versus NSAID in combination with leflunomide. Leflunomide with NSAID therapy resulted in less hepatotoxicity compared with leflunomide with prednisone.