Brimonidine

PHARMACOKINETICS

Absorption

Brimonidine is well absorbed through ocular tissues when applied as eye drops, allowing it to act locally in the eye. A small amount may enter systemic circulation, but plasma levels remain low due to limited absorption. When applied topically to the skin, it is minimally absorbed, which helps reduce the risk of systemic side effects.

Distribution

Brimonidine shows moderate distribution after absorption, with about 29% bound to plasma proteins in the bloodstream. It is mainly distributed in ocular tissues when applied as eye drops, with minimal systemic distribution due to low plasma concentrations.

Metabolism

Brimonidine is primarily metabolized in the liver, mainly through aldehyde oxidase and cytochrome P450 enzymes. It undergoes extensive hepatic metabolism to form inactive metabolites, which helps limit its systemic activity after topical administration.

Excretion

Brimonidine is primarily excreted through the kidneys after being metabolized in the liver. Most of the drug is eliminated in the urine in the form of inactive metabolites, with only a small amount excreted unchanged.

PHARMACODYNAMICS

Brimonidine is a selective alpha-2 adrenergic agonist that lowers intraocular pressure by reducing aqueous humor production and increasing its outflow. When applied to the skin, it causes vasoconstriction, which helps reduce facial redness in rosacea. Its effects are mainly local, with minimal systemic impact.

ADMINISTRATION

• Ocular use: Instill eye drops (usually 0.1–0.15%) into the affected eye 1–3 times daily for glaucoma or ocular hypertension.

• Topical skin use: Apply gel (0.33%) thinly to affected facial areas once daily for rosacea.

• Precautions: Avoid contact with eyes (for gel), wash hands after application, and do not apply to broken skin.

DOSAGE AND STRENGTH

• For glaucoma or ocular hypertension (eye drops):

  • 0.1% to 0.15% solution

  • 1 drop in the affected eye 1–3 times daily

• For rosacea (topical gel):

  • 0.33% gel

  • Apply a thin layer once daily to affected areas of the face

FOOD INTERACTIONS

• Oral intake of alcohol may enhance systemic side effects like dizziness, drowsiness, or low blood pressure.

• High-fat meals have no significant effect on its ocular or topical absorption.

DRUG INTERACTIONS

1. CNS depressants (e.g., sedatives, alcohol, opioids) – may increase drowsiness or dizziness.

2. Antihypertensives – combined use can increase risk of low blood pressure or slow heart rate.

3. Monoamine oxidase inhibitors (MAOIs) – may enhance brimonidine’s systemic effects, increasing blood pressure changes or CNS depression.

4. Other ocular drugs – generally safe, but separate eye drops by at least 5 minutes to avoid dilution or interaction.

CONTRAINDICATIONS

1. Hypersensitivity – Allergy to brimonidine or any component of the formulation.

2. Infants and young children (<2 years) – Risk of severe CNS depression.

3. Severe cardiovascular disease – Such as advanced heart block or uncontrolled hypotension.

4. Concurrent use with MAO inhibitors – May increase risk of hypotension or CNS effects.

5. Severe hepatic or renal impairment – Especially for systemic absorption, caution is advised.

SIDE EFFECTS

Common side effects of brimonidine include eye or skin irritation, redness, dry mouth, and mild drowsiness. Less frequently, it may cause headache, low blood pressure, or allergic reactions. Most effects are mild and temporary.

TOXICITY

Brimonidine toxicity is rare in adults but can cause serious effects in children or with overdose, including drowsiness, low blood pressure, slow heart rate, and respiratory depression. Topical or eye use is generally safe due to minimal systemic absorption.