Mechanism of Action
Chemically, naloxone is a phenanthrene derivative structurally similar to opioid molecules. During metabolism, it undergoes reduction reactions (specifically, reduction of the carbonyl group).
The drug acts as a full competitive antagonist at all types of opioid receptors ($\mu$, $\delta$, $\kappa$). The direct competitive antagonism mechanism involves binding to receptors and physically displacing full agonists (e.g., morphine, fentanyl, methadone, codeine, promedol) and partial agonists. The primary target for saving a patient's life is the neuronal $\mu$-opioid receptors of the respiratory center.
Pharmacological Effects
The drug does not produce intrinsic opioid-like effects; rather, it reverses the actions of narcotic analgesics.
- Respiratory Restoration: Reverses drug-induced respiratory depression, restoring the sensitivity of the respiratory center to carbon dioxide ($CO_2$).
- Reversal of Central Effects: Eliminates analgesia, euphoria, and sedation caused by narcotic drugs.
- Effects on Brainstem Reflex Centers: Reverses miosis (pupillary constriction). Miosis results from oculomotor nerve center excitation during opiate use and serves as a characteristic diagnostic sign of intoxication.
Indications for Use
The primary indication is acute intoxication (overdose) with narcotic analgesics. In opioid poisoning, respiration initially becomes slow and deep (therapeutic doses), then very slow and superficial (toxic doses), progressing to respiratory center paralysis and death.
Naloxone is the drug of choice and is administered intravenously or intramuscularly for the emergency restoration of vital functions.
Comprehensive Detoxification: In addition to administering the antidote, management logic includes eliminating the toxin from the body:
- Gastric Lavage: A 0.05% potassium permanganate solution (which chemically oxidizes morphine) or an activated charcoal suspension in warm water (toxin adsorption) is used.
- Bowel Cleansing: Administration of a saline laxative following lavage.
- Resuscitative Measures: Mechanical ventilation is applied in cases of profound respiratory depression.
Characteristics and Limitations
- Short Duration of Action: The effect following intravenous administration lasts from 1 to 4 hours. Because many opioids have a longer duration of action, close patient monitoring and potential repeat administration of the drug are required.
- Challenges with Buprenorphine: Naloxone is less effective in buprenorphine overdose (an agonist-antagonist) because the latter forms a very tight bond with the receptors.
- Differentiation of Agents: Pure antagonists (naloxone, naltrexone) must be clearly distinguished from agonist-antagonists (nalbuphine, pentazocine, butorphanol) and full agonists (omnopon). Unlike naltrexone, which acts for up to 36 hours and is administered orally for long-term dependency therapy, naloxone is strictly an injectable emergency medication.
Prescription Guidelines
The drug is available as an injection solution administered intravenously (IV) or intramuscularly (IM).
Example of administration for opiate poisoning: IV 400 mcg over 2–3 minutes. The total dose should not exceed 10 mg.
Rp.: Sol. Naloxoni hydrochloridi 0.04% - 1 ml D.t.d. N. 10 in amp. S. Administer intravenously 1 mL (400 mcg) over 2–3 minutes in case of opiate poisoning.