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PH9.7 | PH9.7 | Ocular Pharmacotherapy and Topical Delivery — SDL Guide — SDL Guide

Learning Objectives

• Describe the mechanism of action, uses, and adverse effects of anti-glaucoma drugs (prostaglandin analogues, beta-blockers, alpha-2 agonists, carbonic anhydrase inhibitors, miotics, hyperosmotic agents)
• Identify contraindications to ocular beta-blockers and brimonidine in specific clinical settings
• Outline the pharmacotherapy for other ocular conditions (bacterial/viral/allergic conjunctivitis, mydriasis, wet AMD)
• Explain the pharmacokinetics of topical ocular drug delivery, including blood-ocular barrier, nasolacrimal drainage, and punctal occlusion

INSTRUCTIONS

Glaucoma is the second leading cause of irreversible blindness worldwide, and in India affects an estimated 12 million people — over half of whom are unaware of their diagnosis. The pharmacotherapy of glaucoma centres on IOP reduction using eye drops, which are among the most widely prescribed medications in ophthalmology. Understanding topical ocular drug delivery — how drugs reach the anterior and posterior segments, how nasolacrimal drainage causes systemic absorption, and when systemic effects from eye drops can be dangerous — is essential for safe prescribing.

References

• Tripathi KD. Essentials of Medical Pharmacology, 9th ed., Ch. 10 (Ocular Pharmacology) (textbook)
• Goodman & Gilman. The Pharmacological Basis of Therapeutics, 13th ed., Ch. 66 (Ocular Pharmacology) (textbook)

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Version 2.0 | NMC CBUC 2024

Aqueous Humour Dynamics and the Pathophysiology of Glaucoma

Understanding aqueous humour (AH) dynamics is the physiological basis for understanding how every anti-glaucoma drug works. Aqueous humour is a clear fluid produced by the ciliary body (via active secretion driven by carbonic anhydrase and Na⁺/K⁺-ATPase) at approximately 2–2.5 mL/minute. It flows from the posterior chamber, through the pupil into the anterior chamber, and drains primarily via two routes: (1) the trabecular meshwork route (conventional drainage, ~80%) — through the trabecular meshwork, Schlemm's canal, and into the episcleral venous system; and (2) the uveoscleral route (unconventional drainage, ~20%) — across the ciliary body face and supraciliary space into the uveal vasculature.

Intraocular pressure (IOP) is determined by the balance between aqueous humour production and drainage. Normal IOP is 10–21 mmHg. Elevated IOP is the primary risk factor for glaucomatous optic neuropathy — progressive death of retinal ganglion cells and their axons (which form the optic nerve), causing characteristic cupped optic disc and visual field loss.

Glaucoma pathophysiology — two main types:
- Open-angle glaucoma (OAG): The anterior chamber angle (where the trabecular meshwork is located) is open and appears structurally normal, but there is increased resistance at the trabecular meshwork, reducing drainage and gradually elevating IOP. The most common form; asymptomatic until late-stage. Most anti-glaucoma drugs target OAG.
- Angle-closure glaucoma (ACG): The iris physically obstructs the trabecular meshwork (the angle 'closes'). Acute ACG presents dramatically: sudden severe eye pain, halos around lights, blurred vision, nausea/vomiting, red eye, a mid-dilated fixed pupil, and corneal oedema — an ophthalmic emergency.

Elevated IOP damages the optic nerve through mechanical compression of axons at the lamina cribrosa and possibly through ischaemia. The damage is cumulative and irreversible — this is why early detection and IOP reduction are life-changing for patients with glaucoma.

Aqueous Humour Circulation and IOP Regulation in the Anterior Eye

Therapeutic Goals in Ocular Pharmacotherapy

The therapeutic goals in ocular pharmacology vary by condition, but for glaucoma they are particularly well defined by both pathophysiology and clinical trial evidence.

Glaucoma — IOP reduction: The goal is to reduce IOP to a 'target IOP' that prevents further optic nerve damage. Target IOP is individualised — typically 18–20% reduction from baseline, or an absolute target below 21 mmHg for early disease, but lower (15–16 mmHg) for patients with advanced optic nerve damage or normal tension glaucoma. Lower is generally better. Any anti-glaucoma drug should be evaluated by: (1) magnitude of IOP reduction, (2) systemic contraindications, (3) local tolerability, and (4) dosing convenience (once-daily prostaglandins have better adherence than twice-daily timolol).

Other ocular conditions:
- Bacterial conjunctivitis: goal = kill or inhibit the causative organism (topical antibiotics); most cases are self-limiting, but antibiotics shorten duration.
- Herpes simplex keratitis: goal = suppress viral replication with antiviral (acyclovir ointment) without steroids (which can cause corneal perforation if used alone in herpes keratitis).
- Allergic conjunctivitis: goal = reduce mast cell-mediated histamine release and itch (mast cell stabilisers, antihistamines).
- Fundoscopy (diagnostic): goal = dilate pupil fully (mydriasis) and paralyse accommodation (cycloplegia) using atropine or tropicamide.
- Wet age-related macular degeneration (AMD): goal = stop neovascularisation by blocking VEGF (anti-VEGF injections).

Topical route rationale: Topical ocular administration achieves high drug concentrations at the target tissue (cornea, anterior chamber) while minimising systemic exposure compared to oral dosing. However, it is NOT systemic-exposure-free — nasolacrimal drainage carries unabsorbed drug to the nasolacrimal duct → nasal mucosa → systemic absorption, bypassing first-pass metabolism. For timolol and other drugs with systemic ADR potential, this absorption route is clinically significant.

Classification of Ocular Drugs

Ocular drugs are classified by indication and mechanism. The anti-glaucoma drugs form the most pharmacologically diverse group, as they target different steps in aqueous humour dynamics.

Anti-glaucoma drugs:
- Prostaglandin analogues: latanoprost, bimatoprost, travoprost — most potent single-agent IOP reducers; once-daily evening dosing; increase uveoscleral (unconventional) outflow
- Beta-blockers: timolol (non-selective), betaxolol (β1-selective) — reduce aqueous humour production; first-line in many settings; systemic absorption via nasolacrimal route
- Alpha-2 adrenergic agonists: brimonidine — reduce AH production AND increase uveoscleral outflow; dual mechanism
- Carbonic anhydrase inhibitors (CAIs): dorzolamide, brinzolamide (topical); acetazolamide (oral/IV) — reduce AH production by inhibiting ciliary body CAI
- Miotics (parasympathomimetics): pilocarpine — muscarinic agonist; increases trabecular drainage; less used since prostaglandins became available
- Hyperosmotic agents: mannitol (IV), glycerol (oral) — acute angle-closure emergency; reduces vitreous volume by osmotic dehydration

Drugs for other ocular conditions:
- Antibiotics: chloramphenicol (broad spectrum, topical; aplastic anaemia risk is theoretical with topical use), fluoroquinolones (ciprofloxacin, ofloxacin — first-line for bacterial keratitis and contact-lens-related infections)
- Antivirals: acyclovir ophthalmic ointment — herpes simplex keratitis; ganciclovir gel
- Antiallergic/mast cell stabilisers: olopatadine (dual antihistamine + mast cell stabiliser — first-line for allergic conjunctivitis), ketotifen, cromolyn sodium (pure mast cell stabiliser, prophylactic)
- Mydriatics and cycloplegics: atropine (longest acting — cycloplegia 10–14 days; used for treatment of amblyopia), tropicamide (short-acting 4–6 hours — preferred for routine fundoscopy), cyclopentolate
- Anti-VEGF agents: ranibizumab (Lucentis — Fab fragment), bevacizumab (Avastin — off-label full antibody), aflibercept (VEGF trap) — intravitreal injection for wet AMD and diabetic macular oedema
- Topical NSAIDs: diclofenac, nepafenac — post-operative inflammation, cystoid macular oedema prevention after cataract surgery
- Topical corticosteroids: prednisolone acetate, dexamethasone — for uveitis, post-operative inflammation; risk: steroid-induced glaucoma (IOP elevation) and steroid cataract with prolonged use