Brimonidine, Apraclonidine

Mechanism

💊 Commercial Names

Alphagan, Komboglyze Ophthalmic, Simbrinza (brimonidina + brinzolamida).

🔬 Pharmacological Group

Agonistas selectivos de receptores adrenérgicos α₂. Simpaticomiméticos indirectos.

🧪 Standard Presentations

Brimonidina tartrato 0.15% y 0.20%, Apraclonidina clorhidrato 0.5% y 1.0% (unidosis).

Mechanism of Action

La brimonidina presenta una selectividad por los receptores α₂ aproximadamente 1000 veces superior a la de los receptores α₁:

• Activation of α_2A,B,C Presinápticos y Postsinápticos:
  1. Efecto Postsináptico en Procesos Ciliares: El acoplamiento a los receptores α₂ del epitelio ciliar, vinculados a proteínas G del tipo inhibidor (G_i), bloquea la adenilato ciclasa. Esto reduce la síntesis de AMPc intracelular y disminuye la producción de humor acuoso de forma inmediata.
  2. Presynaptic Effect: Inhibe la liberación de norepinefrina de los terminales nerviosos simpáticos en el ojo, reduciendo el tono simpático constitutivo ocular.
• Aumento del Flujo de Salida Uveoescleral: Tras dosis repetidas, estimula la síntesis de prostaglandinas endógenas (PGE₂) en el iris y el cuerpo ciliar, promoviendo la remodelación local de la matriz de colágeno y facilitando el drenaje uveoescleral secundario.
• Neuroprotección Retiniana (Teoría): La brimonidina difunde hacia la retina en modelos animales, donde la activación de receptores α₂ regula positivamente factores de supervivencia celular (bcl-2) y suprime la acumulación de glutamato excitotóxico, previniendo la apoptosis de las células ganglionares de la retina (CGR).

Pharmacokinetics

Quantitative Ocular and Systemic Pharmacokinetics

Security

Risk of Central CNS Depression in Pediatrics: Absolute Contraindication

Brimonidine easily crosses the blood-brain barrier of pediatric patients due to the immaturity of their CNS capillary tight junctions. Activation of α₂ receptors in the locus coeruleus and vasomotor center of the brainstem profoundly suppresses central sympathetic activity. This triggers a picture of severe systemic toxicity characterized by extreme somnolence, hyponotic coma, hypothermia, profound bradycardia, orthostatic hypotension and infant apneas that require intensive monitoring and ventilatory support. Brimonidine is absolutely contraindicated in neonates, infants and children under 2 to 6 years of age.

Contraindications

• Absolute: Infants and children under 2 years of age; simultaneous use of drugs Monoamine oxidase inhibitors (MAOIs) such as selegiline or phenelzine (risk of severe hypertensive crisis or cardiovascular collapse due to accumulation of circulating catecholamines).
• Relative: Severe coronary insufficiency or history of myocardial infarction; Raynaud's disease or thromboangiitis obliterans; clinically symptomatic orthostatic hypotension; severe liver or kidney failure.

Adverse Effects (ADR)

• Ocular Sites: Allergic follicular blepharoconjunctivitis (occurs in up to 15-20% of patients chronically treated with brimonidine, characterized by eyelid edema, erythema, giant conjunctival follicles and intense pruritus of immunoallergic origin that requires suspension of treatment); conjunctival dryness; stinging sensation and blurred vision.
• Systemic: Lethargy, intense dry mouth (xerostomia due to sympathetic salivary hyposecretion); chronic fatigue; daytime sleepiness; headache; postural dizziness and mild arterial hypotension.

Drug Interactions

• Tricyclic antidepressants (Amitriptyline, Imipramine): They can inhibit the presynaptic reuptake of norepinephrine, directly interfering with the mechanism of action of brimonidine and substantially reducing its intraocular hypotensive effect.
• CNS depressants (Benzodiazepines, Alcohol): Synergism that enhances the central sedative and drowsy effect.
• Systemic Antihypertensives: Risk of additive hypotension and bradycardia with a silent clinical course.