Pharmacodynamic properties
Butorphanol is a centrally acting analgesic from the group of synthetic opioids with an agonistic-antagonistic effect, agonist at the kappa opioid receptor subtype and antagonist at the mu receptor subtype. The kappa receptors control analgesia, sedation without depression of cardiopulmonary system and body temperature, whereas the mu 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.
Analgesia generally occurs within 15 minutes following administration in horse, dog and cat. After a single intravenous dose in the horse analgesia usually lasts up to 2 hours. In the dog it lasts up to 30 minutes after a single intravenous administration. In cats with visceral pain analgesic effects have been demonstrated for up to 6 hours. In cats with somatic pain duration of analgesia has been considerably shorter.
Increased doses do not correlate with increased analgesia, a dosage of about 0.4 mg/kg leads to a ceiling effect.
Butorphanol has minimal cardiopulmonary depressant activity in the target species. It does not cause histamine release in horses. In combination with α2-agonists it causes additive and synergistic sedation.
Pharmacokinetic particulars
Post parenteral administration absorption of the product is rapid and almost complete with serum peak levels occurring after 0.5 - 1.5 hours. It is highly bound to plasma proteins (up to 80%). Volume of distribution is large after IV administration (2.1 l/kg for horses, 4.4 l/kg for dogs and 7.4 l/kg for cats) suggesting wide distribution into tissues.
Metabolism is rapid and mainly occurs in the liver. Two inactive metabolites are produced. Terminal half life is short: about 44 minutes in the horse, 1.7 hours for dogs and 4.1 hours for cats.
The elimination occurs mainly through urine (to a major extent) and faeces. 97 % of the dose after IV administration in the horse will be eliminated in less than 5 hours.