Ophthalmic Use
The primary clinical application of timolol is the management of glaucoma. As a non-selective $\beta$-blocker, it effectively lowers intraocular pressure.
The mechanism of action is exclusively related to the reduction of aqueous humor production (secretion). An important clinical feature of the drug is that, unlike cholinergic agents, timolol has no effect on accommodation or pupil size. This makes its use more comfortable for patients, as visual acuity is not impaired by blurred vision from cycloplegia or miosis.
The pharmacokinetics of the eye drop formulation are fairly predictable:
- Onset of action: Rapid, occurring 10–30 minutes after instillation into the conjunctival sac.
- Peak effect: Maximum pressure reduction is observed within 1–2 hours.
- Duration of action: The effect persists for up to 24 hours, allowing for once- or twice-daily administration.
Adverse Effects and Systemic Absorption
Despite local administration, timolol can cause both local and systemic adverse reactions.
Local side effects include:
- Reduced tear production (dry eye syndrome);
- Conjunctivitis;
- Local allergic reactions.
Systemic effects (resorptive action): Although the class of hydrophilic $\beta$-blockers generally penetrates barriers poorly, timolol is specifically a lipophilic substance. Due to its high lipophilicity, it is easily and rapidly absorbed through the vascular-rich mucosa of the nasolacrimal duct into the systemic circulation.
This can lead to systemic reactions typical of $\beta$-blockers: bradycardia, hypotension, and generalized fatigue. The risk is especially high for patients with bronchial asthma, as systemic absorption of a non-selective $\beta$-blocker can trigger severe bronchospasm.
Systemic Administration
In addition to ophthalmology, timolol is also prescribed systemically (per os). In cardiology, the drug is used to treat:
- Hypertension;
- Angina pectoris.
Properties of Hydrophilic $\beta$-Blockers (Reference)
In contrast to lipophilic timolol, it is important to understand the properties of truly hydrophilic $\beta$-blockers. They possess low lipophilicity, which determines their pharmacokinetics:
- Absorption: Gastrointestinal absorption is incomplete, with bioavailability limited to 30–35%.
- Distribution: Drugs in this group poorly cross the blood-brain barrier (BBB). The clinical consequence is an absence of central nervous system (CNS) depression, whereas lipophilic analogues may cause central side effects.
- Excretion: Weakly metabolized, eliminated primarily unchanged by the kidneys.
- Safety profile: General contraindications and side effects are similar to propranolol, but central nervous system side effects are significantly milder with hydrophilic agents.