Prevalence of hepatotoxicity and myelosuppression with alternate day use of azathioprine and glucocorticoids for treatment of dermatological conditions in dogs

Background The use of azathioprine (AZA) in dogs is limited by the potential for hepatotoxicity and myelosuppression. Hypothesis/Objectives To determine the prevalence of AZA-associated hepatotoxicity in dogs with dermatological conditions receiving alternate-day AZA. The hypothesis was that dogs receiving AZA every other day (EOD) would have a lower prevalence of hepatotoxicity compared to published data for dogs receiving daily AZA. A secondary aim was to determine the prevalence of AZA-associated myelosuppression over the same time period and population. Animals Forty-one client-owned dogs with dermatological conditions treated with AZA EOD and glucocorticoids with clinical and haematological follow-up available for a minimum of two months of AZA therapy. Methods Retrospective analysis of data from April 1994 to July 2020. Hepatotoxicity was defined as elevation of alanine aminotransferase (ALT) at least twofold above the reference range. Results Azathioprine-associated hepatotoxicity was observed in two of 41 dogs (4.9%), with onset at 18 and 40 days, respectively. One dog receiving AZA at 1.9 mg/kg EOD had a fourfold increase in ALT. The other dog (AZA dose 2.3 mg/kg EOD) had a 30-fold increase in ALT. Azathioprine was not associated with thrombocytopenia, anaemia or neutropenia in any dogs. Lymphopenia developed in one dog (2.4%) with onset at 105 days. Conclusions and clinical relevance Alternate-day AZA administration with tapering glucocorticoids was well-tolerated in dogs with dermatological conditions.