Zydol Mechanism of Action

Opioid Analgesic.
Pharmacology: Pharmacodynamics: The analgesic effect of butorphanol varies with the route of administration. Onset occurs within minutes when given intravenously and within 15 minutes when given by intramuscular injection.
Peak analgesic effect is reached within 30 to 60 minutes for both intravenous and intramuscular routes.
The duration of analgesia, influenced by both the pain model and administration route, generally lasts 3 to 4 hours, based on the time by which 50% of patients required re-medication. In postoperative studies, the duration of analgesia with IV or IM butorphanol was comparable to that of morphine, meperidine, and pentazocine when administered at equipotent doses.
Mechanism of Action: Butorphanol acts as a partial agonist at the mu-opioid receptor and a full agonist at the kappa-opioid receptor. Its primary therapeutic effect is analgesia. Clinically, doses are adjusted to achieve effective pain relief, though use may be limited by side effects such as respiratory and central nervous system (CNS) depression.
While the exact mechanism of its analgesic effect is unknown, specific CNS opioid receptors for endogenous compounds with opioid-like activity have been identified throughout the brain and spinal cord are thought to mediate the analgesic effects of this drug.
Pharmacokinetics: Butorphanol Tartrate Injection is rapidly absorbed following intramuscular (IM) administration, with peak plasma concentrations reached within 20 to 40 minutes. After the initial absorption and distribution phase, the pharmacokinetics of butorphanol are similar for both intravenous and intramuscular administration.
Serum protein binding is independent of concentration over the range achieved in clinical practice (up to 7 ng/mL) with a bound fraction of approximately 80%.
The volume of distribution of butorphanol ranges from 305 to 901 liters and total body clearance from 52 to 154 liters/hr.
Metabolism and Elimination: Butorphanol is extensively metabolized in the liver, with similar metabolic profiles observed after both intravenous and intramuscular administration. Due to significant first-pass metabolism, oral bioavailability is low-ranging from only 5% to 17%.
The primary metabolite is hydroxybutorphanol, while norbutorphanol is formed in smaller amounts. Both metabolites are detectable in plasma, although norbutorphanol typically appears at only trace levels. Hydroxybutorphanol has an elimination half-life of approximately 18 hours, which can lead to moderate accumulation (less than 5-fold) with repeated dosing to steady state.
Elimination of butorphanol occurs through both urine and feces. Following administration of radiolabeled butorphanol, 70-80% of the dose is recovered in urine, and about 15% in feces. Only about 5% of the dose is excreted unchanged in urine, while 49% is excreted as hydroxybutorphanol and less than 5% as norbutorphanol.