Opioid Analgesics and Antagonists

Opioid Receptors & Classification - Pain's Off-Switches

• Receptors: Mu (μ), Kappa (κ), Delta (δ) - G-protein coupled receptors (GPCRs) in CNS & periphery.
  • Mechanism: ↓cAMP, ↑K+ efflux, ↓Ca2+ influx → neuronal inhibition.
• Endogenous Opioids: Endorphins (μ-affinity), Enkephalins (δ-affinity), Dynorphins (κ-affinity).
• Drug Classes (Examples):
  • Agonists (e.g., Morphine, Fentanyl)
  • Partial Agonists (e.g., Buprenorphine)
  • Mixed Agonist-Antagonists (e.g., Pentazocine)
  • Antagonists (e.g., Naloxone, Naltrexone)

⭐ Endogenous opioids like endorphins, enkephalins, and dynorphins are naturally occurring peptides that modulate pain and are targeted by opioid analgesics.

Opioid MOA & Pharmacokinetics - Body's Opioid Journey

• Mechanism of Action (MOA):
  • Receptor binding: μ, κ, δ GPCRs.
  • Presynaptic effects: ↓ Ca²⁺ influx → ↓ neurotransmitter release.
  • Postsynaptic effects: ↑ K⁺ efflux → hyperpolarization → ↓ excitability.

⭐ Opioids exert their analgesic effect by binding to G-protein coupled receptors (GPCRs), primarily μ (mu), κ (kappa), and δ (delta), leading to decreased presynaptic Ca²⁺ influx and increased postsynaptic K⁺ efflux.

• Pharmacokinetics (ADME):
  • Absorption: Variable; significant first-pass metabolism (e.g., Morphine).
  • Distribution: Lipid solubility dictates CNS entry (Fentanyl > Morphine).
  • Metabolism: Hepatic (CYP450, glucuronidation). Codeine → Morphine (CYP2D6).
  • Excretion: Renal.

Key Opioid Agonists - The Relief Squad

• Morphine (Prototype µ-agonist):
  • Strong analgesia (severe pain e.g. MI, cancer).
  • AEs: Resp. depression, miosis, constipation, histamine.
  • ⚠️ Avoid: Head injury, asthma.
• Pethidine (Meperidine):
  • Synthetic µ-agonist; shorter action. Atropine-like (mydriasis).
  • Norpethidine metabolite → seizures in renal failure.

  ⭐ Pethidine (Meperidine) is an opioid agonist that, unlike morphine, has atropine-like effects, may cause less biliary spasm, and its metabolite norpethidine can accumulate in renal failure leading to seizures.

• Fentanyl & congeners:
  • Potent (~100x M); rapid onset, short duration.
  • Uses: Anesthesia, pain patches. Risk: chest rigidity.
• Methadone:
  • Long-acting µ-agonist; opioid dependence, chronic pain.
  • Risk: QT prolongation.
• Codeine:
  • Weak µ-agonist (→morphine by CYP2D6); antitussive.
• Tramadol:
  • Weak µ-agonist + SNRI; moderate pain.
  • Risk: Seizures, serotonin syndrome.
• Tapentadol:
  • µ-agonist + NRI; less GI AEs.
• Buprenorphine:
  • Partial µ-agonist, κ-antagonist; resp. depression ceiling.
  • Uses: Dependence, pain. Can precipitate withdrawal.

Opioid ADRs & Toxicity - The Dark Side

• Common ADRs: Constipation (most persistent), N/V, sedation, dizziness, pruritus, urinary retention.
• Respiratory depression: Dose-dependent; most serious acute ADR.
• Tolerance: ↓ effect with repeated use.
• Dependence: Withdrawal syndrome on cessation.
• 📌 Mnemonic for opioid toxicity triad: CPR - Coma, Pinpoint pupils, Respiratory depression.
• Acute Toxicity:
  • Triad: Coma, pinpoint pupils (miosis), respiratory depression.
  • Management: Airway support, Naloxone (0.4-2 mg IV/IM/SC).

⭐ The classic triad of acute opioid poisoning is coma, pinpoint pupils (miosis), and respiratory depression; naloxone is the specific antidote.

Opioid Antagonists & Clinical Use - Counter & Cautions

• Naloxone: Reverses OD (respiratory depression). Short t½ (1-2h).
• Naltrexone: Oral, long-acting. For opioid/alcohol dependence.
• PAMORAs (Methylnaltrexone): For OIC, peripheral.
• OD Management:

• ⚠️ Cautions: Acute withdrawal; Naloxone: renarcotization; Naltrexone: hepatotoxic.

⭐ Naloxone is a pure opioid antagonist with a rapid onset and short duration of action (1-2 hours), primarily used to reverse opioid-induced respiratory depression in overdose situations.

High‑Yield Points - ⚡ Biggest Takeaways

• Morphine: Prototype μ-agonist; causes analgesia, euphoria, respiratory depression, miosis, constipation.
• Codeine: Weak opioid, metabolized to morphine by CYP2D6; effective antitussive.
• Fentanyl: Highly potent, rapid-acting μ-agonist; used in anesthesia, breakthrough pain.
• Tramadol: Dual action (weak μ-agonist, SNRI); risk of seizures, serotonin syndrome.
• Naloxone: Pure antagonist for acute opioid overdose reversal; short duration of action.
• Naltrexone: Orally active, long-acting antagonist for opioid and alcohol dependence treatment.