Timolol, Betaxolol

Mechanism

💊 Commercial Names

Tusoptin, Cusimolol, Timoptol, Betoptic, Cosopt (timolol + dorzolamide).

🔬 Pharmacological Group

Beta-adrenergic receptor antagonists. Timolol (non-selective β₁/β₂), Betaxolol (selectivo β₁).

🧪 Standard Formulations

Ophthalmic eye drops: timolol 0.25% and 0.50% (solution and extended-release gel), betaxolol 0.25% (suspension) and 0.5%.

Mechanism of Action

Topical beta-blockers inhibit receptors located on the basolateral surface of the nonpigmented epithelium of the ciliary body:

• β Receptor Blockade₂-adrenergic (Timolol): The β receptors₂ in the ciliary body constitutively modulate aqueous humor production. Timolol acts as a pure competitive antagonist. Blocking the receptor inactivates the stimulatory subunit of the G protein (G_s), inhibiting the enzyme adenylate cyclase.
• Decreased Intracellular cAMP: Inhibition of adenylate cyclase reduces intracellular levels of cyclic adenosine monophosphate (cAMP), which inactivates protein kinase A (PKA).
• Reduced Active Secretion: Inactive PKA stops phosphorylating cellular ion channels and membrane transporters (such as the Na exchanger).⁺/H⁺ and the Na cotransporter⁺/K⁺/2Cl^-). This collapses the active osmotic gradient necessary for ultrafiltration and secretion of water from the ciliary stroma into the posterior chamber, reducing the rate of aqueous humor synthesis by 20% to 30%. Betaxolol, by selectively blocking β receptors₁, reduces IOP less potently but minimizes β blockade₂ pulmonar.

Pharmacokinetics

Quantitative Ocular and Systemic Pharmacokinetics

The Punctal Occlusion Maneuver: Prevention of Systemic Absorption

To mitigate systemic absorption and optimize the intraocular bioavailability of timolol and any other topical ophthalmic drug, the patient must be instructed to perform punctal occlusion: immediately after instillation of the drop, press firmly with the index finger on the inner corner of the eye (over the lacrimal sac and lacrimal puncta) for a minimum of 2 to 3 minutes continuous, keeping the eyelids closed without blinking forcefully. This simple physical maneuver mechanically occludes the tear canaliculi, reducing systemic systemic absorption by up to 60-70% and prolonging the contact time of the drug with the cornea.

Security

Critical Systemic Toxicity: The Danger of Timolol in Respiratory and Cardiac Patients

Free ophthalmic timolol in solution is massively absorbed through the nasal mucosa, reaching the heart and lungs without undergoing first-pass metabolism. In patients with bronchial asthma, severe COPD or bronchial hyperreactivity, systemic blockade of bronchial β₂ receptors causes intense bronchoconstriction of vagal origin that can trigger severe bronchospasm with a fatal outcome. At the cardiac level, timolol decreases the automatism and conduction of the sinus and atrioventricular node by blocking β₁ receptors, inducing extreme bradycardia, second or third degree AV block, and acute congestive heart failure. Its use requires auscultation and exhaustive cardiopulmonary history prior to prescription.

Contraindications

• Absolute: Active bronchial asthma or history of severe chronic obstructive pulmonary disease (COPD); clinically significant sinus bradycardia (<50 bpm); second or third degree atrioventricular block not controlled by pacemaker; cardiogenic shock; decompensated heart failure.
• Relative: Labile or poorly controlled diabetes mellitus (systemic timolol masks the autonomic warning symptoms of hypoglycemia, such as tachycardia, tremor, and sweating); myasthenia gravis (may worsen generalized muscle weakness or ocular symptoms such as eyelid ptosis).

Adverse Effects (ADR)

• Eye Locals: Burning, stinging and tearing sensation of immediate onset (associated with the osmolarity of the eye drops and preservatives); punctate keratitis; corneal hypoesthesia (weak local anesthetic block, increases the risk of inadvertent erosions); dry eyes due to alteration of the tear film.
• Systemic: Bradycardia, arterial hypotension, syncope of vasovagal origin, severe bronchospasm, chronic fatigue, CNS depression, erectile dysfunction, insomnia with nightmares and alteration of the serum lipid profile (decrease in HDL).

Drug Interactions

• Systemic Beta Blockers (Metoprolol, Propranolol): Additive pharmacodynamic effect that can induce profound bradycardia or severe hypotension.
• Calcium Antagonists (Verapamil, Diltiazem): Negative synergism that depresses sinus and cardiac conduction function; risk of sinus arrest or hemodynamic collapse.
• CYP2D6 inhibitors (Quinidine, Fluoxetine, Paroxetine): They inhibit the hepatic metabolism of systemically absorbed timolol, drastically increasing its concentration in the blood and multiplying the risk of beta-blocking adverse effects.