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

Research Overview

• This study investigated how pimobendan, a heart medication, is absorbed and processed in the blood of healthy adult horses after a single oral dose.
• It also evaluated the absorption of two different compounded formulations of pimobendan to see how their blood concentrations compared to the commercially available form.

Study Objectives

• To describe the pharmacokinetics (how the drug moves through the body) of a single 0.5 mg/kg oral dose of pimobendan (Vetmedin) in healthy adult horses.
• To provide preliminary data on the absorption of two compounded pimobendan formulations: an oil-based suspension and capsule formulation.

Methods

• Subjects: Six healthy adult horses were administered one oral dose of pimobendan at 0.5 mg/kg.
• A subset of two horses received the compounded formulations (oil-based suspension and capsule) in addition to the commercial product.
• Blood samples were collected at multiple time points: baseline (0 minutes), 15, 30, 45 minutes, and 1, 2, 4, 8, 12, and 24 hours post-administration.
• Concentrations of pimobendan and its active metabolite, O-desmethyl-pimobendan, in plasma were measured using ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS-MS).
• Noncompartmental pharmacokinetic analysis was used to interpret the data.

Key Findings

• The commercial pimobendan (Vetmedin) reached a mean maximum plasma concentration (Cmax) of 4.96 ± 2.13 ng/mL, occurring about 2.17 ± 0.98 hours after administration.
• The area under the concentration-time curve (AUC0-∞), which represents overall drug exposure, was 22.1 ± 8.8 hour·ng/mL.
• The two compounded formulations, tested only in the 2 horses receiving them, resulted in slightly lower Cmax values (ranging from 2.66 to 4.58 ng/mL) and AUC0-∞ values (11.78 to 16.82 hour·ng/mL) compared to the commercial prep.
• The active metabolite, O-desmethyl-pimobendan, was not detected in plasma samples from any horse at any time point.
• The measured Cmax and AUC values in horses were considerably lower than those reported for dogs given the same 0.5 mg/kg oral dose, suggesting lower absorption or faster clearance in horses.

Interpretation and Implications

• Low plasma concentrations of pimobendan observed in horses indicate poor oral absorption or possibly rapid metabolism/clearance compared to other species (e.g., dogs).
• The lack of detection of the active metabolite suggests differences in metabolic pathways or a very low conversion rate in horses.
• Compounded formulations did not significantly improve pimobendan plasma levels compared to the commercial product, though sample size for this comparison was very small.
• Given the low plasma drug levels, the clinical effectiveness of oral pimobendan in horses using current dosing and formulations is questionable without further study.
• Further research is required to explore different dosing regimens, alternative delivery routes (e.g., intravenous), or formulations to achieve therapeutic plasma concentrations in horses before pimobendan is used clinically in this species.

Conclusion

• This pharmacokinetic study demonstrated that oral administration of pimobendan in healthy horses results in low plasma concentrations compared to dogs.
• Neither the commercial product nor the compounded formulations achieved high enough blood levels to suggest effective oral absorption at the doses tested.
• Additional investigations with other doses, formulations, or delivery methods are necessary prior to recommending pimobendan for clinical use in horses.