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Opioid Pharmacology - Detailed Review

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1. Terminology

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2. Opioid Receptors

Mu-receptor subtypes:

• μ1 (supraspinal) - analgesia, euphoria
• μ2 (spinal) - respiratory depression, constipation

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3. Receptor Distribution

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4. Signal Transduction Mechanisms

Acute effects of opioid receptor activation:

1. Inhibition of adenylyl cyclase → ↓ cAMP → ↓ protein kinase A activity
2. Activation of inward-rectifying K+ channels (GIRK) → membrane hyperpolarization → neuronal inhibition
3. Inhibition of voltage-gated Ca2+ channels (N-type, P/Q-type) → ↓ presynaptic neurotransmitter release
4. Inhibition of substance P and glutamate release presynaptically

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5. Endogenous Opioid System

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6. Drug Classification by Receptor Activity

Full Agonists (μ)

Partial Agonist

Mixed Agonist-Antagonists

CRITICAL: Never combine with full agonists - can precipitate acute withdrawal and worsen pain.

Antagonists

Atypical Dual-Mechanism Agents

• Tramadol: Weak μ agonist + inhibits norepinephrine and serotonin reuptake. Risk of seizures. Has a ceiling effect.
• Tapentadol: ~2x stronger than tramadol; μ agonist (with ceiling effect) + norepinephrine reuptake inhibitor.

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7. Pharmacological Effects of Opioids

CNS Effects

Respiratory System

Cardiovascular

GI Effects

• Constipation (μ2 receptors in enteric NS) - does NOT develop tolerance
• ↓ GI motility and secretion
• Spasm of sphincter of Oddi (biliary system)
• Nausea/vomiting early in treatment

Endocrine

• ↓ GnRH, LH, FSH → hypogonadism, sexual dysfunction
• ↑ prolactin, ADH
• Chronic use → accelerated osteoporosis

Genitourinary

• Urinary retention (↓ detrusor tone, ↑ sphincter tone)

Pruritus

• Histamine release (morphine > others); also central μ receptor mechanism
• Not prevented by antihistamines when central

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8. Individual Drugs - Key Pharmacology

Morphine (Prototype)

• Onset: Rapid (especially IV); Duration: 2-4 hours
• Oral bioavailability: ~25% (high first-pass metabolism)
• Metabolism: Liver glucuronidation → morphine-6-glucuronide (M6G) (active, accumulates in renal failure) + morphine-3-glucuronide (M3G)
• Caution in renal failure - M6G accumulates → prolonged respiratory depression
• Sustained release: MS Contin (12h), Kadian (24h)

Fentanyl

• 100x more potent than morphine
• Highly lipophilic → rapid CNS penetration; short duration with IV bolus
• Transdermal patch: 12, 25, 50, 75, 100 μg/h; plasma levels rise over 12-18 hours; 72-hour patch; elimination half-life ~21 hours
• Safe in renal failure (hepatic metabolism to inactive metabolites)
• Illicit fentanyl analogues (carfentanil) driving overdose epidemic

Codeine

• Prodrug - converted to morphine by CYP2D6
• Ultrarapid metabolizers (CYP2D6 duplications) can develop acute morphine toxicity
• Poor metabolizers get no analgesia
• Antitussive action partially independent of conversion

Methadone

• Long-acting μ agonist + NMDA receptor antagonist
• Highly variable half-life: 4-130 hours
• Slow onset of peak effect (~2-6 hours)
• Oral bioavailability ~80%
• QTc prolongation risk (blocks hERG K+ channels) → arrhythmia
• Used for maintenance treatment of OUD; doses ≥60 mg/day more effective
• Reduces craving and induces cross-tolerance to other opioids
• Safe in renal failure

Meperidine (Pethidine)

• Active metabolite normeperidine accumulates → myoclonus, seizures, dysphoria
• Avoid for chronic use; serotonin syndrome risk with MAOIs
• Avoid in renal failure

Buprenorphine

• Partial μ agonist - ceiling effect on respiratory depression (major safety advantage)
• High receptor affinity → can displace full agonists; precipitates withdrawal if given when dependent patient still has opioids aboard
• Transdermal (chronic pain) and sublingual/buccal (OUD - Suboxone with naloxone)

Hydromorphone

• 5x more potent than morphine orally; 1.5 mg IV ≈ 30 mg oral morphine ≈ 7.5 mg oral hydromorphone

Naloxone

• Competitive μ (and κ, δ) antagonist
• Rapid reversal of opioid overdose (IV, IM, intranasal)
• Short duration: 30-90 minutes - may require repeat dosing or infusion for long-acting opioids

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9. Pharmacokinetics & Routes of Administration

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10. Tolerance

Mechanisms:

1. Receptor desensitization - GRK (G protein-coupled receptor kinases) phosphorylate the receptor → β-arrestin recruitment → uncoupling from G proteins
2. Receptor internalization/downregulation - reduced surface receptor density
3. cAMP-CREB pathway upregulation - neurons adapt by increasing adenylyl cyclase and protein kinase A expression to counteract the chronic inhibition
4. Central sensitization changes

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11. Physical Dependence and Withdrawal

Mechanism - The cAMP Rebound Model

Opioid Withdrawal Syndrome (Abstinence Syndrome)

Note: Opioid withdrawal is rarely life-threatening in adults (unlike alcohol/benzodiazepine withdrawal) but is intensely dysphoric.

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12. Opioid Tolerance vs. Opioid-Induced Hyperalgesia (OIH)

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13. Opioid Use Disorder (OUD) - Neurobiology

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14. OUD Treatment (FDA-Approved)

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15. Opioid Overdose - The Classic Triad

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16. Special Pharmacology Considerations

Renal Failure

• Avoid: Morphine (M6G accumulates), codeine, meperidine
• Safe to use: Fentanyl, methadone, buprenorphine, hydromorphone (with caution)

Hepatic Failure

• Most opioids metabolized hepatically - reduce doses; monitor closely
• Morphine is relatively preferred

CYP2D6 Interactions

• Codeine, hydrocodone, oxycodone, tramadol are substrates
• CYP2D6 inhibitors (fluoxetine, paroxetine) block conversion → reduced analgesia with codeine
• CYP2D6 ultrarapid metabolizers → toxicity risk

Serotonin Syndrome Risk

• Tramadol + MAOIs, SSRIs, SNRIs → serotonin syndrome
• Fentanyl and meperidine also have some serotonergic activity

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Summary Table - Key Receptor Effects

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• Goodman & Gilman's The Pharmacological Basis of Therapeutics - pp. 462-465 (Opioid Receptors, Signaling, Distribution)
• Bradley and Daroff's Neurology in Clinical Practice - pp. 1084, 1090 (Opioid Receptors, Analgesics)
• Goldman-Cecil Medicine International Edition - pp. 272-273 (Opioid Analgesics, OUD Treatment)
• Adams & Victor's Principles of Neurology, 12th Ed. - p. 1203 (Opioid Addiction)
• Katzung's Basic and Clinical Pharmacology, 16th Ed. - p. 353 (Tolerance, Dependence)
• Kandel's Principles of Neural Science, 6th Ed. - p. 1122 (cAMP-CREB mechanism of tolerance)

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Opioid Side Effects & Contraindications

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SIDE EFFECTS

Overview - What Develops Tolerance and What Doesn't

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1. Respiratory System

• Mechanism: μ2-receptor activation in the pre-Bötzinger complex of the medulla; reduced sensitivity to CO2 (blunted hypercapnic drive) and to hypoxia
• Manifestation: ↓ respiratory rate first (most sensitive indicator), then ↓ tidal volume; apnea at toxic doses
• Falls in O2 saturation represent a critical threshold - requires immediate intervention
• Potentiated by: benzodiazepines (particularly dangerous - avoid co-prescribing), other CNS depressants, alcohol, sleep (loss of cortical drive)
• Tolerance develops with chronic use, but respiratory arrest is still possible even in tolerant patients, especially with co-administered sedatives
• In patients with pain, the pain itself provides a stimulatory counter to sedation - if pain is suddenly relieved (e.g., nerve block), respiratory depression may become unmasked

• Can cause bronchospasm - particularly dangerous in asthmatics
• Fentanyl has minimal histamine release - preferred in patients with asthma/COPD

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2. Gastrointestinal System

Constipation

• Most common and clinically significant chronic side effect
• No tolerance develops - persists indefinitely with continued use
• Mechanism: μ receptors in the enteric nervous system → ↓ propulsive motility, ↑ segmental non-propulsive contractions, ↓ GI secretions, ↑ sphincter tone
• Management: Always prescribe stimulant laxatives prophylactically (bisacodyl, senna, polyethylene glycol); fiber alone is insufficient
• Severe refractory cases: Methylnaltrexone (peripherally acting μ antagonist - does not cross BBB, so doesn't reverse analgesia), naloxegol, lubiprostone

Nausea and Vomiting

• Mechanism: Stimulation of the chemoreceptor trigger zone (CTZ) in the area postrema (μ and δ receptors); also vestibular stimulation (exacerbated by movement - ambulatory patients > recumbent)
• Usually occurs early in treatment; tolerance develops with continued use (though may persist months)
• Management: Metoclopramide, haloperidol, ondansetron, scopolamine, hydroxyzine; switching opioids

Delayed Gastric Emptying

• Opioids slow gastric emptying → contributes to nausea, increases risk of aspiration in perioperative settings

Biliary Spasm

• Opioids increase tone of sphincter of Oddi → can worsen biliary colic or cause elevated LFTs/amylase
• Meperidine historically claimed to cause less biliary spasm (not well supported by evidence)

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3. Central Nervous System

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4. Cardiovascular System

• Peripheral vasodilation - due to histamine release and reduced sympathetic outflow → hypotension
• Bradycardia - at high doses; vagal effects
• Methadone specifically: QTc prolongation via hERG K+ channel blockade → risk of Torsades de Pointes; requires baseline and follow-up ECG monitoring
• Reduced preload may be beneficial in acute pulmonary edema (morphine used adjunctively, though recent evidence challenges this)

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5. Genitourinary System

• Urinary retention - ↑ bladder sphincter tone, ↓ detrusor tone; more common with neuraxial (intrathecal/epidural) administration and in elderly males with BPH
• Antidiuretic effect - via ↑ ADH secretion; tolerance develops
• More common with IV, transmucosal, and epidural routes vs. oral

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6. Integument / Allergic

Pruritus

• Very common with neuraxial (intrathecal/epidural) opioids - incidence 30-100% with intrathecal dosing
• Mechanism: Central μ-receptor activation in the spinal cord/brain - NOT primarily histamine-mediated. This is why antihistamines are often ineffective.
• Treatment:
  • Naloxone or naltrexone (low-dose, antagonist reversal)
  • Nalbuphine (partial agonist - shifts receptor occupancy)
  • Ondansetron (5-HT3 antagonism)
  • Propofol (low dose)
  • Mirtazapine (5-HT3 antagonism + sedation)
  • NSAIDs (diclofenac, tenoxicam) may help
• Morphine, codeine cause more pruritus via histamine release at injection site (urticaria, wheals)

True Allergy

• Rare - urticaria, fixed drug eruptions, anaphylactoid reactions
• Wheals at injection site = local histamine release (not true allergy)
• True anaphylaxis is uncommon but reported with IV codeine/morphine

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7. Endocrine and Metabolic (Chronic Use)

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8. Neonatal/Obstetric Effects

• Opioids cross the placenta → neonatal respiratory depression; have naloxone available at delivery
• Neonatal Opioid Withdrawal Syndrome (NOWS) in infants born to opioid-dependent mothers
• Breastfeeding: No absolute contraindication as a class; however codeine is contraindicated in breastfeeding mothers as CYP2D6 ultrarapid metabolizers can generate toxic morphine levels in breast milk

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9. Drug-Specific Toxicities Summary

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CONTRAINDICATIONS

Absolute Contraindications

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Relative Contraindications / Strong Cautions

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DRUG INTERACTIONS - Key Table

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Management of Side Effects - Quick Reference

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• Goodman & Gilman's Pharmacological Basis of Therapeutics - pp. 475-476 (Contraindications by condition)
• Katzung's Basic & Clinical Pharmacology, 16e - pp. 888-891 (Adverse effects table, drug interactions)
• Harrison's Principles of Internal Medicine, 22e - pp. 988-990 (Side effects overview)
• Bradley & Daroff's Neurology in Clinical Practice - pp. 1090-1091 (CNS side effects, management)
• Miller's Anesthesia, 10e - p. 6125 (Pruritus mechanism and treatment)

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Distribution of Opioids - Detailed Pharmacokinetics

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1. Pharmacokinetic distribution - how the drug distributes from plasma into tissues (governed by physicochemical properties)
2. Receptor distribution - where opioid receptors are located anatomically, which determines the drug's range of effects

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PART 1 - PHARMACOKINETIC DISTRIBUTION

Determinants of Distribution

A small increase in pH (e.g., respiratory alkalosis) increases the non-ionized fraction of morphine, fentanyl, sufentanil, and remifentanil - enhancing CNS entry. Active transport systems in the BBB also efflux opioids from the brain (e.g., cyclosporine blocks morphine's efflux transporter, enhancing its effect).

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Multi-Compartment Distribution Model

Phase 1 - α (rapid distribution):
  Drug leaves plasma → high-flow organs (brain, liver, kidney, heart)
  → rapid initial fall in plasma concentration
  → rapid onset of CNS effect

Phase 2 - β (slower redistribution):
  Drug leaves high-flow organs → lower-flow tissues (muscle)
  → slower fall in plasma concentration
  → termination of CNS effect for most bolus doses

Phase 3 - γ (terminal elimination):
  Drug leaves fat/peripheral tissues back to plasma → liver metabolism
  → slow terminal half-life, clinically relevant with prolonged infusions

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Volume of Distribution (Vd)

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Lipophilicity Comparison - The Clinical Ranking

Morphine is hydrophilic - this means proportionately less morphine crosses the blood-brain barrier under normal conditions. However, when the BBB is compromised (e.g., meningitis, trauma) or in neonates (immature BBB), more enters the CNS. Lipophilic opioids (fentanyl) may give more predictable results in neonates.

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Plasma Protein Binding

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Context-Sensitive Half-Time (CSt½)

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Special Compartments of Distribution

Blood-Brain Barrier (BBB)

• Lipophilic opioids (fentanyl, sufentanil, heroin) cross rapidly → fast onset
• Morphine crosses slowly → delayed onset; peak CNS effect 15-30 min after IV dose
• Compromised BBB → all opioids enter more readily
• Active efflux transporters (P-glycoprotein) pump some opioids OUT of brain → clinically significant with drug interactions

Spinal Cord (Neuraxial Route)

This is why intrathecal morphine is used for postoperative analgesia after major surgery - one dose provides 12-24 hours of analgesia. But it carries a risk of late (6-18h) respiratory depression as drug migrates rostrally to brainstem μ2 receptors.

Fat/Adipose Tissue

• Lipophilic opioids (fentanyl, sufentanil, methadone) accumulate extensively in fat
• In obese patients → larger peripheral reservoir → prolonged duration of action
• In elderly patients → relatively increased fat mass → greater redistribution

Placenta and Breast Milk

• All opioids cross the placenta (lipophilic ones cross faster)
• Appear in breast milk (morphine, codeine at clinically relevant levels)
• Fentanyl in breast milk is generally considered low-risk due to poor oral bioavailability in neonate
• Codeine in breast milk is contraindicated (CYP2D6 ultrarapid metabolizers)

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Interpatient Variability in Distribution

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PART 2 - ANATOMICAL RECEPTOR DISTRIBUTION

Overview

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Mu (μ) Receptor Distribution

• μ1 (supraspinal, neocortex, thalamus) → analgesia, euphoria
• μ2 (spinal cord, brainstem) → respiratory depression, constipation

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Kappa (κ) Receptor Distribution

• Caudate-putamen, nucleus accumbens
• Amygdala - stress responses
• Hypothalamus, pituitary - neuroendocrine effects (diuresis, feeding, stress axis)
• PAG, raphe nuclei, pons, medulla
• Dorsal horn (sparse)
• Minimal in cortex

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Delta (δ) Receptor Distribution

• Olfactory areas - unique distribution among opioid receptors
• Neocortex
• Caudate-putamen, nucleus accumbens, amygdala
• Low levels in dorsal horn

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NOP/ORL1 Receptor Distribution

• Most widely distributed of all four receptor types
• Found in most brain regions and spinal cord
• Areas related to pain, reward, anxiety/stress, memory, and feeding

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Non-Neuronal Distribution

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Summary - Integration of Both Concepts

Physicochemical Property → PK Distribution → Receptor Location → Clinical Effect

Lipophilic (fentanyl):
  Fast BBB crossing → rapid CNS receptor binding → fast onset
  Large Vd (fat depot) → redistribution terminates effect → short bolus duration
  Stays at neuraxial injection site → segmental spinal analgesia

Hydrophilic (morphine):
  Slow BBB crossing → delayed peak CNS effect
  Wide CSF spread → rostral migration → risk of late respiratory depression
  Peripheral conversion → active M6G metabolite (renal accumulation risk)

Both reach μ receptors in PAG, dorsal horn (analgesia),
  medulla (respiratory depression), enteric NS (constipation)

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• Barash, Cullen & Stoelting's Clinical Anesthesia, 9e - pp. 1540-1543 (Opioid Pharmacokinetics, Volume of Distribution, Context-Sensitive Half-Times)
• Goodman & Gilman's Pharmacological Basis of Therapeutics - p. 463 (Opioid Receptor Distribution: μ, κ, δ, NOP)
• Miller's Anesthesia, 10e - p. 6125 (Neuraxial opioid distribution effects)