Oral administration of pimobendan in healthy horses results in low plasma concentrations

Objective To describe the pharmacokinetics of a single dose of pimobendan (Vetmedin) in healthy adult horses and provide preliminary evidence of the absorption of 2 compounded pimobendan formulations. Methods From August 2022 through November 2022, 6 healthy adult horses were administered a single dose of 0.5 mg/kg pimobendan orally. A subset of 2 horses was also administered pimobendan, compounded as an oil-based suspension and a capsule formulation. Plasma was collected at times 0, 15, 30, and 45 minutes and 1, 2, 4, 8, 12, and 24 hours after administration. The plasma concentrations of pimobendan and its active metabolite, O-desmethyl-pimobendan, were determined by UPLC-MS-MS. Data were analyzed using noncompartmental pharmacokinetics. Results Pimobendan reached a mean maximum plasma concentration (C max ) of 4.96 ± 2.13 ng/mL at time 2.17 ± 0.98 hours, with an area under the concentration-versus-time curve from time 0 to infinity (AUC 0–∞ ) of 22.1 ± 8.8 h·ng/mL. The compounded formulations in the 2 horses receiving them resulted in a C max ranging from 2.66 to 4.58 ng/mL and an AUC 0–∞ ranging from 11.78 to 16.82 h·ng/mL. O-desmethyl-pimobendan was not detected in any sample. Conclusions The mean C max and AUC 0–∞ of pimobendan in the plasma of these horses after administration of Vetmedin and compounded formulations was considerably lower than the C max reported in dogs administered the same dose. Clinical Relevance Low plasma concentrations of pimobendan in horses were measured regardless of formulation. Further investigations of other doses and routes are recommended prior to clinical use.