Opioid Antagonist

OPIOID ANTAGONIST


OPIOID ANTAGONISTS

Drugs included:

  • Naloxone, Naltrexone & Nalmefene:
    • Potent µ-receptor antagonist
    • Significant blocking action at κ and δ-receptors.
  • Methylnaltrexone, Alvimopan & Naloxegol:
    • Peripheral opioid antagonists

Individual drugs:

  • Naloxone:
    • Ineffective orally & given parenterally.
    • Very short acting drug.
  • Nalmefene:
    • Given parenterally.
    • Longer half-life.
  • Naltrexone:
    • Orally-effective.
    • Long-acting.

Actions of opioid antagonist:

  • Mainly reverses opioid effects on i.v. administration.
  • Precipitates withdrawal symptoms in opioid-dependent subjects.

Uses:

1. Naloxone:

  • Drug of choice for acute opioid poisoning. Yet, dosage needs repeated administration.
  • Neonatal resuscitation for reversing opioid effects (opioids used during labor).
  • Should not be used, when mother (baby in utero) is dependent on opioid.
  • Disadvantages: Precipitates withdrawal.

2. Naltrexone:

  • As a maintenance drug for opioid poisoning.
  • Used for preventing relapse, after opioid de-addition.
  • Used to decrease craving in chronic alcoholics.

3. Combination drugs:

  • Naltrexone + bupropion – Obesity treatment.

Effects of combination:

  • On oral combination of naloxone + opioid:
    • Only opioid is absorbed, not naloxone. 
    • Hence, opioid produces its desired action.
  • On i.v. route of addiction:
    • Both opioid & naloxone are absorbed. 
    • Naloxone stops euphoria.

4. Uses of peripheral opioid antagonist:

  • Methylnaltrexone, Naloxegol & Alvimopan – Indicated for opioid-induced constipation.

Contraindications:

C/I peripheral opioid antagonist:

  • Biliary colic – 
    • Due to increased intrabiliary pressure.
  • Asthmatics – 
    • Due to bronchoconstriction aggravate in by releasing histamine (C/I in asthmatics).
  • Spinal or epidural opioid administration – 
    • Results in intense pruritus over lips & torso.
    • Due to histamine release.

NOTE ON OPIOID DE-ADDICTION:

  • Chronic intake of opioids → Causes physical & psychological dependence.
  • On sudden stoppage → Severe withdrawal symptoms.
  • Aim: 
    • 1st aim – Stop further drug usage → Followed by maintenance of de-addiction & prevent relapse.

1. For short duration/small doses of addiction:

  • Sudden stoppage of drug therapy can be attempted.
  • Mild withdrawal symptoms.
  • Treatment:
    • β-blockers/clonidine/lofexidine.
2. For long duration/large dosage addiction:
  • Sudden withdrawal of offending drug is dangerous.
  • Cause severe withdrawal symptoms.
  • Treatment:
    • Methadone maintenance.
    • Addictive drug replaced by equivalent dose of methadone.
    • Prevents withdrawal symptoms by opioid receptors stimulation, yet with much less addiction. 
    • Methadone dose gradually decreased & finally stopped.

3. For relapse prevention after de-addiction:

  • Naltrexone – Mainly prevents euphoric action by blocking µ receptors.

4. On retaking opioid (after de-addiction):

  • No euphoria.
  • Person’s resolution to quit addiction strengthens.

Exam Important

OPIOID ANTAGONISTS

  • Naloxone, Naltrexone & Nalmefene are potent µ-receptor antagonist
  • Methylnaltrexone, Alvimopan & Naloxegol are peripheral opioid antagonists
  • Naloxone is ineffective orally & given parenterally.
  • Nalmefene is given parenterally.
  • Naltrexone is orally-effective.
  • Opioid antagonist mainly reverses opioid effects on i.v. administration.
  • Main uses of naloxone are drug of choice for acute opioid poisoning & neonatal resuscitation for reversing opioid effects.
  • Naloxone precipitates withdrawal symptoms.
  • Naltrexone used as for maintenance drug for opioid poisoning, for preventing relapse, after opioid de-addition & decreases craving in chronic alcoholics.
  • Naltrexone along with bupropion for obesity treatment.
  • Peripheral opioid antagonist indicated for opioid-induced constipation.
  • For short duration/small doses of opioid addiction – Stoppage of drug attempted & treated with β-blockers/clonidine/lofexidine.
  • For long duration/large dosage addiction – Sudden withdrawal is dangerous & treated with Methadone maintenance.
  • For relapse prevention after de-addiction, Naltrexone is used.
  • Naltrexone mainly prevents euphoric action by blocking µ receptors.

 

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