Pharmacokinetics and pharmacodynamics of intravenous and oral apixaban in horses

Large vessel and microvascular thrombi are common complications in systemically ill horses contributing to patient morbidity and mortality. Apixaban, an oral factor Xa inhibitor, shows excellent efficacy against stroke and deep vein thrombosis in humans. The purpose of this study was to determine serum apixaban concentrations and anti-factor Xa activity in horses after orally administered apixaban. Five horses received a single dose of intravenous (0.09 mg/kg) and oral (1 mg/kg) apixaban in a cross-over design. Serum apixaban concentrations and anti-Xa activity were measured serially via liquid chromatography-tandem mass spectrometry and a commercial assay, respectively, for 12 hr following oral administration. Apixaban was detected in all horses after both oral and intravenous administration. Oral administration yielded a mean maximum concentration of 60.3 ng/ml (59.4–111 ng/ml), mean time to maximum concentration of 0.5 hr (0.5–2), mean half-life of 6.2 hr (4.6–8.3), and mean oral bioavailability of 10% (3.8–17.4). After oral administration, anti-Xa activity had a strong positive relationship with serum apixaban and was best represented by a dose-response model with the following parameters: E₀ = 5.00 ng/ml, E_MAX = 311 ng/mL, EC₅₀ = 267 ng/ml, and n = 1.58. Anti-Xa activity was significantly higher 2 hr post-administration compared with baseline (p =.032). Despite low oral bioavailability, administration of 1 mg/kg oral apixaban, in healthy horses, achieves serum concentrations similar to those reported in humans. Apixaban has potential clinical utility in horses and warrants further investigation.