Ocular Pharmacology: Pharmacokinetics, Drug Classes and Safety Monitoring
What the exam tests
This domain covers topical and intraocular pharmacokinetics, the major drug classes used in glaucoma, ocular inflammation, infection, myopia control and retinal neovascular disease, and the safety monitoring each one demands. Questions typically ask you to predict drug behaviour from physicochemical properties, convert a concentration into a delivered dose, or choose the agent that avoids a specific systemic contraindication.
Core rules
- Corneal penetration favours small, moderately lipophilic molecules; the conjunctival and scleral route absorbs larger hydrophilic agents and is the main source of systemic exposure.
- Tear turnover, reflex tearing and nasolacrimal drainage remove most of an instilled drop, and nasolacrimal absorption bypasses hepatic first-pass metabolism.
- Punctal occlusion or gentle eyelid closure for 1-5 minutes markedly reduces systemic absorption.
- Topical beta-blockers (timolol) are contraindicated in asthma and cautioned in bradyarrhythmia and heart block.
- Prostaglandin analogues (latanoprost, travoprost, tafluprost) are dosed once daily in the evening; more frequent dosing reduces efficacy through receptor down-regulation.
- Anti-VEGF agents (ranibizumab, aflibercept, conbercept) are delivered intravitreally; systemic exposure is low but blood pressure and thromboembolic risk are still monitored.
- Low-dose atropine 0.01% is used for paediatric myopia control in Chinese practice, with photophobia and near-vision blur as the main dose-limiting effects.
Worked example
A 0.5% timolol drop of 30 microlitres: 0.5% equals 5 mg/mL, and 30 microlitres equals 0.03 mL, so each drop delivers 5 x 0.03 = 0.15 mg, or 150 micrograms; twice daily gives 300 micrograms per day. Because a large fraction of each drop drains into the nasolacrimal duct and is absorbed systemically, punctal occlusion and eyelid closure can lower plasma timolol several-fold. That is why the technique is taught and checked before abandoning an otherwise effective drug.
Traps that cost marks
- Labelling a corticosteroid intraocular pressure rise as progression of primary open-angle glaucoma rather than a steroid response.
- Prescribing latanoprost twice daily and then interpreting the loss of effect as drug failure.
- Overlooking brimonidine-associated follicular conjunctivitis, and its apnoea risk in infants.
- Assuming a preservative-free formulation is less effective rather than less toxic to the ocular surface.