The pharmacokinetics, pharmacological responses and behavioral effects of acepromazine in the horse.

This research delves into the effects and characteristics of acepromazine—a tranquilizing drug—on horses, revealing that it has a wide dispersion rate in horses, binds to over 99% of plasma proteins after injection, and impacts physiological responses such as penile protrusion and hematocrit levels. Further work is needed to understand its elimination in equine urine due to potentially prolonged urinary disposal.

Study of Acepromazine Pharmacokinetics

• The researchers examined how acepromazine, a tranquilizing drug, behaves in the bodies of horses. After an intravenous injection, they found the drug was widely distributed in the horse’s body. Acepromazine returned over 99% binding with plasma proteins, indicating strong interaction with these molecules.
• The decline in plasma drug levels was divided into two phases: the alpha half-life, which lasted 4.2 minutes, and the beta or elimination half-life, which spanned 184.8 minutes. The drug was detectable in plasma for up to 8 hours post administration at a dosage of 0.3 mg/kg.
• Acepromazine distribution within the blood constituted 46% in the plasma phase and 54% within the erythrocyte or red blood cell phase, demonstrating a near-even distribution between these two components of blood.

Pharmacological Responses and Behavioral Effects

• The research further identifies significant physiological responses to acepromazine in horses. Its administration led to evident penile protrusion at doses ranging from 0.01 mg/kg to 0.4 mg/kg. The duration and extent of the protrusion were linked to the dosage.
• The administration of the drug also significantly reduced hematocrit levels, representing the proportion of red blood cells in blood. These levels were lowered by over 20% from control levels with increasing dosages.
• Horses’ behavioral responses also changed after the administration of acepromazine, indicated by a clear reduction in the variable interval (VI 60) responding rate in all horses tested.

Impact and Further Research

• The results of the study suggest that hematocrit alterations are the most sensitive pharmacological responses to acepromazine administration in horses, followed by changes in penile extension, respiratory rate, VI responding, and locomotor responses.
• Given its wide distribution within the body and strong binding with plasma proteins, acepromazine is difficult to detect in plasma at regular clinical doses. Because of its large volume of distribution, it is likely that this drug’s elimination through urine may be prolonged.
• Researchers conclude that further work is needed to fully understand how acepromazine is eliminated in equine urine. A greater understanding of this process would contribute to improved recommendations on dosing and potential side effects when using this drug for tranquilizing horses.