Pharmacokinetics of azathioprine following single-dose intravenous and oral administration and effects of azathioprine following chronic oral administration in horses.

Objective—To determine pharmacokinetics of azathioprine (AZA) and clinical, hematologic, and serologic effects of IV and oral administration of AZA in horses. Animals—6 horses. Procedure—In study phase 1, a single dose of AZA was administered IV (1.5 mg/kg) or orally (3.0 mg/kg) to 6 horses, with at least 1 week between treatments. Blood samples were collected for AZA and 6-mercaptopurine (6-MP) analysis 1 hour before and at predetermined time points up to 4 hours after AZA administration. In study phase 2, AZA was administered orally (3 mg/kg) every 24 hours for 30 days and then every 48 hours for 30 days. Throughout study phase 2, blood samples were collected for CBC determination and serum biochemical analysis. Results—Plasma concentrations of AZA and its metabolite, 6-MP, decreased rapidly from plasma following IV administration of AZA, consistent with the short mean elimination half-life of 1.8 minutes. Oral bioavailability of AZA was low, ranging from 1% to 7%. No horses had abnormalities on CBC det...