ATC Vet Code: QN02AF01
Pharmacotherapeutic group: Morphinan derivatives, butorphanol.
Butorphanol tartrate (R(-) enantiomer) is a centrally acting analgesic. Its action is agonist-antagonist at the opiate receptors in the central nervous system, agonist at the kappa (κ) opioid receptor subtype and antagonist at the mu (µ) receptor subtype.
The kappa (κ) receptors control analgesia, sedation without depression of cardiopulmonary system and body temperature, whereas the mu (µ) receptors control supraspinal analgesia, sedation and depression of cardiopulmonary system and body temperature.
The agonist component of butorphanol activity is ten times more potent than the antagonist component.
Onset and duration of analgesia:
Analgesia generally occurs within 15 minutes following administration in horse, dog and cat. After a single intravenous dose in the horse, analgesia usually lasts for 15 – 60 minutes. In the dog, it lasts for 15-30 minutes after a single intravenous administration. In cats with visceral pain, analgesic effect for 15 minutes up to 6 hours after butorphanol administration has been demonstrated. In cats with somatic pain, the duration of analgesia has been considerably shorter.
In the horse, butorphanol has a high clearance (on average 1.3 L/h/kg) after intravenous administration. It has a short terminal half-life (mean <1 hour), indicating that 97% of a dose will be eliminated after intravenous administration in, on average, less than 5 hours.
In the dog, butorphanol administered by the intramuscular route has a high clearance (around 3.5 L/h/kg). It has a short terminal half-life (mean <2 hours), indicating that 97% of a dose will be eliminated after intramuscular administration in, on average, less than 10 hours. Repeated dose pharmacokinetics and the pharmacokinetics following intravenous administration have not been studied.
In the cat, butorphanol administered by the subcutaneous route has a low clearance (<1320 mL/kg/h). It has a relative long terminal half life (around 6 hours) indicating that 97% of the dose will be eliminated in approximately 30 hours.
Repeated dose pharmacokinetics have not been studied.
Butorphanol is metabolised extensively in the liver and excreted in the urine. The volume of distribution is large, suggesting wide distribution into tissue.