Chapter 13  ·  Module 4

Mixed Agonist-Antagonists, Partial Agonists, and Opioid Antagonists

Buprenorphine, mixed agonist-antagonists, naloxone, and naltrexone compared

Four Drug Classes Compared

Class Examples Mu Activity Ceiling Effect Abuse Potential Precipitates Withdrawal
Full mu agonists Morphine, oxycodone, fentanyl, methadone Full agonist None High No
Partial mu agonist Buprenorphine Partial agonist Yes (respiratory depression) Lower Yes — high affinity displaces full agonists
Mixed agonist-antagonists Nalbuphine, butorphanol, pentazocine Antagonist or weak partial agonist Yes Lower Yes — displaces mu agonists
Pure antagonists Naloxone, naltrexone Antagonist (no activation) N/A (no agonist effect) None Yes — abrupt displacement of agonists

Buprenorphine: Key Properties

Mechanism

Receptor Profile

  • Partial mu receptor agonist
  • Kappa receptor antagonist
  • Very high mu receptor binding affinity
  • Displaces full agonists from mu receptors

Advantage

Ceiling Effect

  • Ceiling effect on respiratory depression
  • Safer than full agonists in overdose
  • Kappa antagonism reduces dysphoria
  • Sublingual and injectable formulations

Critical Risk

Precipitated Withdrawal

  • Displaces full agonists from mu receptors
  • Causes immediate severe withdrawal in dependent patients
  • Patient must be in spontaneous withdrawal before induction
  • More intense than natural withdrawal

Pure Antagonists: Naloxone vs Naltrexone

Acute Use

Naloxone

  • Acute overdose reversal
  • Onset 1–5 min intravenous; duration 30–90 min
  • Poor sublingual bioavailability → basis of Suboxone combination
  • Intranasal spray for community use
  • Repeat dosing needed for long-acting opioids

Long-Term Use

Naltrexone

  • Opioid use disorder and alcohol use disorder maintenance
  • Oral daily or extended-release monthly injection
  • Requires full detoxification first (7–10 days minimum)
  • Precipitates severe withdrawal if given to dependent patient
  • Blocks reward from any opioid taken during treatment

Suboxone Rationale: Why Naloxone Is Added to Buprenorphine

When buprenorphine/naloxone is taken sublingually as prescribed, naloxone is poorly absorbed and does not interfere with buprenorphine's therapeutic effect. If the tablet is crushed and injected, naloxone becomes fully bioavailable and precipitates immediate withdrawal in any opioid-dependent individual. This formulation strategy substantially reduces diversion and injection misuse.