Brimonidine, Apraclonidine

Mechanism

💊 Commercial Names

Alphagan, Komboglyze Ophthalmic, Simbrinza (brimonidine + brinzolamide).

🔬 Pharmacological Group

Selective α-adrenergic receptor agonists₂. Indirect sympathomimetics.

🧪 Standard Formulations

Brimonidine tartrate 0.15% and 0.20%, apraclonidine hydrochloride 0.5% and 1.0% (single-dose).

Mechanism of Action

Brimonidine is selective for α receptors₂ approximately 1000 times higher than that of α receptors₁:

• Activation of α_2A,B,C Presynaptic and Postsynaptic:
  1. Postsynaptic Effect on the Ciliary Processes: Binding to α receptors₂ of the ciliary epithelium, linked to G proteins of the inhibitory type (G_i), blocks adenylate cyclase. This reduces the synthesis of intracellular cAMP and decreases the production of aqueous humor immediately.
  2. Presynaptic Effect: It inhibits the release of norepinephrine from sympathetic nerve terminals in the eye, reducing ocular constitutive sympathetic tone.
• Increased Uveoscleral Outflow: After repeated doses, it stimulates the synthesis of endogenous prostaglandins (PGE₂) in the iris and ciliary body, promoting local remodeling of the collagen matrix and facilitating secondary uveoscleral drainage.
• Retinal Neuroprotection (Theory): Brimonidine diffuses into the retina in animal models, where activation of α receptors₂ positively regulates cell survival factors (bcl-2) and suppresses the accumulation of excitotoxic glutamate, preventing cell apoptosis. retinal ganglion cells (RGCs).

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.