Opioid Pharmacology Summary

Opioids are a class of drugs that bind to opioid receptors in the central and peripheral nervous system to produce analgesia, sedation, and other physiological effects. Their pharmacology includes mechanisms of action, receptor interactions, pharmacokinetics, and clinical effects.

Mechanism of Action

• Receptor Interaction: Opioids primarily act on three types of G-protein coupled receptors:
  • Mu (μ): Primary receptor for analgesia, euphoria, respiratory depression, and physical dependence.
  • Kappa (κ): Associated with spinal analgesia, dysphoria, and sedation.
  • Delta (δ): Modulates analgesia and has a role in mood regulation.
• Opioids inhibit adenylyl cyclase, reduce cyclic AMP levels, and lead to decreased neurotransmitter release (e.g., substance P, glutamate).
• They hyperpolarize neurons by increasing potassium efflux and reducing calcium influx, decreasing neuronal excitability.

Pharmacokinetics

1. Absorption:
   • Routes: Oral, intravenous (IV), intramuscular (IM), transdermal, subcutaneous, and epidural.
   • Oral opioids often have first-pass metabolism, reducing bioavailability.
2. Distribution:
   • Lipid solubility determines CNS penetration. Highly lipophilic drugs (e.g., fentanyl) cross the blood-brain barrier more rapidly.
   • Protein binding affects distribution (e.g., morphine is less protein-bound than fentanyl).
3. Metabolism:
   • Most opioids are metabolized in the liver by cytochrome P450 enzymes (e.g., CYP3A4 for fentanyl and CYP2D6 for codeine).
   • Morphine undergoes glucuronidation to active metabolites (e.g., morphine-6-glucuronide).
   • Some opioids, like remifentanil, are metabolized by plasma esterases.
4. Elimination:
   • Renal excretion is the primary route for most opioids and their metabolites.
   • Impaired renal or hepatic function can prolong opioid activity.

Clinical Effects

1. Desired Effects:
   • Analgesia: Effective for nociceptive and some neuropathic pain.
   • Sedation and Anxiolysis: Beneficial in perioperative and intensive care settings.
   • Cough Suppression: E.g., codeine and dextromethorphan.
2. Adverse Effects:
   • Respiratory Depression: Dose-dependent; mediated by μ-receptors.
   • Nausea and Vomiting: Activation of the chemoreceptor trigger zone.
   • Constipation: Reduced gastrointestinal motility.
   • Pruritus: Histamine release (common with morphine).
   • Physical Dependence and Tolerance: Develop with prolonged use.
3. Special Considerations:
   • Partial Agonists and Antagonists:
     • Buprenorphine: Partial agonist at μ-receptors, antagonist at κ-receptors.
     • Naloxone: Pure antagonist used to reverse opioid effects.
   • Synthetic vs. Natural Opioids:
     • Natural: Morphine, codeine.
     • Semi-synthetic: Oxycodone, hydromorphone.
     • Synthetic: Fentanyl, remifentanil.

https://www.researchgate.net/figure/The-pharmacology-of-opioid-agonists-and-gut-selective-MOR-antagonists-in-relation-to_fig2_375903868

Opioids (eg, morphine) can generate a pseudoallergic response by directly stimulating mast cell degranulation, releasing histamine and other vasoactive mediators.  This nonimmunologic reaction can cause itching, urticarial rash, wheezing, hypotension, and tachycardia that closely mimic true IgE-mediated type 1 hypersensitivity (eg, anaphylaxis).  However, true IgE-mediated allergic reaction is rare with opioids, and suggestive symptoms rarely indicate the need to avoid opioids altogether.

• Because direct mast cell degranulation tends to be most prominent with relatively low-potency opioids (eg, morphine, meperidine), a relatively high-potency opioid (eg, fentanyl) is usually better tolerated.

Clinical Application

• Choice of Opioid: Determined by pain severity, patient factors (e.g., age, organ function), and route availability.
• Special Populations: Renal or hepatic impairment, pregnancy, and opioid-naïve patients require tailored dosing.