Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

AGONISTS–ANTAGONISTS
First-pass effect destroys some of the drug and the
analgesia is considered to be relatively short-lived (1–
2 h). Because these drugs are κ-agonists and µ-
antagonists, the pain relief is often less than optimal for
chronic discomfort. However, visceral nociception is
considered to be more responsive to κ-agonism, leading
some urologists to advocate butorphanol’s use in chronic
bladder pain.
Butorphanol is a potent antitussive. It may also be
used to antagonize pure µ-agonists with residual analgesia
and reduction in side effects.
Mechanism of action
Butorphanol is a synthetic opioid that is a partial agonist
at µ-receptors as well as being a κ-agonist.
Formulations and dose rates
DOGS AND CATS
• 0.2–0.4 mg/kg IM, SC or IV prn or q.6–12 h
• 0.2–1.0 mg/kg PO q.6–12 h
Pharmacokinetics
Analgesia occurs 20 min after SC administration and
lasts for about 4 h. Duration of analgesia appears to be
longer (2–6 h) in cats than in dogs (1–2 h). Visceral
analgesia is achieved at lower doses (0.1 mg/kg) than
somatic analgesia in cats (up to 0.8 mg/kg).
Adverse effects
● Butorphanol produces less respiratory depression
than morphine because of its ‘ceiling effect’.
● At normal clinical doses, butorphanol produces
mild, transient depression of the cardiovascular
system, which is more profound when combined
with other central depressant drugs.
Known drug interactions
Cardiopulmonary side effects are more pronounced in
the presence of α 2 -agonists and inhalational anesthetic
agents.
Special considerations
In many but not all countries, butorphanol is not a
controlled substance, unlike most of the other opioids,
as there is little potential for human abuse of the
drug.
Pentazocine
Clinical applications
Pentazocine provides moderate pain relief and is most
appropriately used as part of a premedication protocol
for anesthesia.
Mechanism of action
Pentazocine is a synthetic opioid that is a partial agonist
at µ-receptors as well as being a κ-agonist. Its analgesic
efficacy is 25–50% that of morphine.
Formulations and dose rates
DOGS
• 1–3 mg/kg IM, SC or IV
• 2–10 mg/kg PO repeated q.3–4 h prn
CATS
• 0.75–1.5 mg/kg IM, SC or IV
• 2–10 mg/kg PO
Pharmacokinetics
Analgesia occurs 20 min after SC administration and
lasts for 2–4 h. Duration of analgesia is dose related,
lasting 20–120 min in cats after 0.75 and 1.5 mg/kg,
respectively.
Adverse effects
● Pentazocine produces less respiratory depression
than morphine.
● Intravenous administration may cause a transient
decrease in arterial blood pressure.
● Ataxia, inco-ordination and disorientation may
occur in dogs.
● Pentazocine causes dysphoria in cats. Alternative
analgesics are recommended.
Tramadol
Clinical applications
Tramadol provides moderate pain relief. Because of its
dual actions as a µ-agonist and monoamine transport
inhibitor, it produces less respiratory depression for a
given analgesic effect.
Mechanism of action
Tramadol is a synthetic opioid that is a weak agonist at
µ-receptors. Its major metabolites are more potent agonists
at µ-receptors. Tramadol also inhibits monoamine
transporters (principally noradrenaline and serotonin)
which is thought to produce analgesia synergistically
with µ-agonism.
Formulations and dose rates
DOGS
• 1–5 mg/kg PO q.8–12 h
CATS
• 1–2 mg/kg PO q.12 h
Pharmacokinetics
Duration of analgesia is dose related, lasting 4–8 h.
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