Clonidine

Mechanism

Mechanism of Action

Highly selective partial agonist of postsynaptic and presynaptic α_2A, α_2B and α_2C adrenergic receptors in the central nervous system, located mainly in the locus coeruleus of the brain stem and in the nucleus of the solitary tract. Stimulation of presynaptic G protein-coupled receptors inhibits adenylate cyclase and decreases calcium entry, blocking the exocytosis of norepinephrine vesicles. The clinical result is a marked reduction in central sympathetic outflow to the periphery, decreasing systemic vascular resistance, heart rate and blood pressure. In addition, it exhibits affinity for independent imidazoline receptors, which contributes to its antihypertensive actions.

Pharmacokinetics

Key Pharmacokinetics

• Routes: Oral (tablets), transdermal (sustained release patches for 7 days), epidural, intravenous (exceptional).
• Oral absorption: Excellent bioavailability of 75-95% with maximum concentrations in 1 to 3 hours.
• Metabolism: Moderate hepatic (50%) through the cip-450 pathway to inactive metabolites.
• Half-life: 12 to 16 hours, significantly prolonged in patients with advanced renal failure.
• Excretion: Renal, eliminating 65% of the unchanged drug in urine.

Indicators and dose

Approved and Off-label Indications

Approved: Essential arterial hypertension (second or third line); attention deficit hyperactivity disorder (ADHD) in pediatrics; Epidural analgesic adjuvant for intractable cancer pain.

Off-label: Treatment of acute withdrawal syndrome from opioids, alcohol and nicotine; hot flashes associated with menopause; recurrent migraine prophylaxis; Gilles de the Tourette syndrome.

Dosage and Adjustments

Hypertension (Oral): Initial dose of 0.1 mg every 12 hours orally, with weekly increases until reaching a usual maintenance dose of 0.2 to 0.6 mg/day in two divided doses.

Renal adjustment: Reduce the initial dose by half (0.05 mg/dose) if the glomerular filtration rate (eGFR) is < 30 mL/min/1.73m², titrating with caution.

Hepatic adjustment: Systematic adjustment is not required, but close clinical monitoring is advised.

Security

Contraindications

Absolute: Known hypersensitivity to clonidine, severe underlying sinus bradycardia or sinus node disease without a functioning pacemaker.

Relative: Advanced chronic renal failure, Raynaud's disease or other severe peripheral obstructive vasculopathy, active major clinical depression.

Adverse Effects (ADR)

Common: Severe xerostomia (dry mouth due to blockage of sympathetic salivary stimulation), deep sedation, daytime sleepiness, orthostatic dizziness, refractory constipation due to slowing of peristalsis.

Serious: Severe arterial hypotension, symptomatic bradycardia with high-grade atrioventricular block, catastrophic hypertensive rebound phenomenon due to abrupt withdrawal of the drug (hypertensive crisis with extreme tachycardia mediated by excess circulating catecholamines).

Interactions

CNS depressant drugs (alcohol, benzodiazepines, barbiturates): Profound potentiation of drowsiness, sedation and risk of respiratory depression.

Beta blockers: Exacerbated risk of severe bradycardia and hypotension. If both drugs are withdrawn, the beta-blocker must first be discontinued several days before to avoid an unopposed α-adrenergic rebound crisis.

Pregnancy and Breastfeeding

FDA Classification: Category C. Easily crosses the placental barrier; It is associated with the risk of hypotension and bradycardia in the newborn. It is excreted in breast milk; It is recommended to stop breastfeeding or replace the treatment.

Abrupt clonidine withdrawal syndrome

Abrupt interruption of clonidine treatment produces a sudden cessation of tonic central sympathetic inhibition. The result is a hyperadrenergic state characterized by agitation, tremor, severe headache, sweating, and a critical elevation in blood pressure (rebound hypertension) that may culminate in encephalopathy or acute myocardial infarction. The drug should be withdrawn gradually over a minimum period of 2 to 4 days.