Naloxone

Mechanism

Pharmacological Group and Molecular Target

Pure competitive antagonist of the high-affinity opioid receptor (mainly on Mu [µ] receptors, and to a lesser extent Kappa [κ] and Delta [δ]).

Detailed Mechanism of Action

Exogenous opioids bind to Gi/o protein-coupled metabotropic presynaptic and postsynaptic receptors, reducing intracellular cAMP concentration, decreasing calcium entry, and increasing potassium efflux. This reduces synaptic excitability in the ventilatory nuclei of the brainstem. Naloxone acts as a competitive antagonist with high affinity for Mu receptors, immediately displacing exogenous agonists (such as heroin, fentanyl, morphine or methadone). It blocks receptor signal transduction, restoring basal respiratory tone, the response to carbon dioxide in the respiratory centers of the medulla, and raising the patient's alertness.

Pharmacokinetics

Pharmacokinetic and Toxicokinetic Profile

• Absorption: Extremely low oral bioavailability (<2%) due to an almost absolute hepatic first-pass metabolism, which makes it ideal for combined oral formulations (to prevent abuse of the drug by injection). Excellent absorption via intranasal (IN), subcutaneous (SC), intramuscular (IM) or intravenous (IV).
• Distribution: It is rapidly distributed in all body tissues, reaching high brain concentrations immediately. Crosses the placental barrier.
• Metabolism: Mainly hepatic by direct glucuronidation mediated by the enzyme UGT2B7 to form naloxone-3-glucuronide, which lacks biological activity.
• Excretion: Renal, 70% complete within 24-48 hours.
• Elimination half-life: Between 30 and 90 minutes in adults (very short compared to methadone or fentanyl).

Gentle Titration Strategy to Avoid Acute Withdrawal Syndrome

In patients with established physical dependence on opioids, the administration of a high dose of naloxone can suddenly precipitate a severe hyperadrenergic withdrawal syndrome (characterized by massive vomiting with risk of aspiration, severe hypertension, sinus tachycardia, extreme agitation, and acute non-cardiogenic lung edema).

Titration recommendation: If the patient breathes spontaneously at rates greater than 8 breaths per minute and does not present severe cyanosis, the use of high boluses should be avoided. It is preferred to dilute 0.4 mg of naloxone in 10 mL of saline solution and administer doses of 0.04 mg to 0.08 mg IV every 2-3 minutes, stopping the administration at the exact moment in which the patient's basal respiratory pattern normalizes, maintaining a state of protective conscious sedation.

Indicators and dose

Specific Clinical Indications

• Complete or partial reversal of central nervous system depression and, as a priority, severe respiratory depression induced by natural or synthetic opioids.
• Presumptive diagnosis of opioid intoxication in patients with coma of unknown origin.

Dosage and Routes of Administration

• Direct Intravenous Route (Bolus in Respiratory Arrest): 0.4 mg to 2 mg IV; If there is no response after 2-3 minutes, the dose can be repeated. Cumulative doses greater than 10 mg without clinical response require reconsideration of the diagnosis of opioid overdose.
• Intranasal Route: 2 mg to 4 mg in a single spray into a nostril, preferred for pre-hospital first response settings.
• Continuous Infusion: Given the risk of resedation when the effect of the antidote against opioids with a long half-life (such as Methadone, prolonged-release Oxycodone) wears off, it is recommended to calculate 2/3 of the dose of naloxone that successfully awakened the patient and infuse it per hour intravenously diluted in physiological saline.