Ocular Pharmacology

Ocular Pharmacokinetics & Routes - Eye Drug Journey

• Key Barriers: Cornea (Lipophilic-Hydrophilic-Lipophilic layers), Blood-Aqueous Barrier (BAB), Blood-Retinal Barrier (BRB).
• Topical Route: Commonest. Bioavailability ~5%. Governed by $pH = pKa + log([A-]/[HA])$. Systemic absorption via nasolacrimal duct.
• Periocular Routes: Subconjunctival, Sub-Tenon's. Bypasses cornea, achieving higher local drug levels.
• Intraocular Routes: Intravitreal, Intracameral. Offer 100% bioavailability; direct to target.
• Systemic Route: Limited by BAB/BRB, especially for posterior segment delivery.

⭐ Topical drug absorption: Un-ionized, lipid-soluble forms penetrate the cornea more effectively. oka

Autonomic Agents - Pupil & Pressure Puppeteers

• Cholinergics (Miotics): Miosis, ciliary spasm, ↑outflow.
  • Direct: Pilocarpine (glaucoma), Carbachol.
    • Pilocarpine: 📌 Pilo-pin pupil.
  • Indirect: Physostigmine, Echothiophate.
  • Action: Miosis, accommodation, ↑trabecular outflow.

  ⭐ Pilocarpine 2% helps differentiate 3rd nerve palsy from pharmacological mydriasis.

• Anticholinergics (Mydriatics/Cycloplegics): Mydriasis, cycloplegia (for refraction).
  • Atropine (long), Tropicamide (short).
  • Uses: Refraction (esp. children), uveitis (pain relief, prevent synechiae).
  • ⚠️ Risk: Angle-closure glaucoma.
• Adrenergics:
  • α1 Agonist: Phenylephrine (mydriasis only, no cycloplegia).
  • α2 Agonists: Brimonidine, Apraclonidine (↓aqueous, ↑uveoscleral outflow).
  • β-Blockers: Timolol, Betaxolol (β1 selective) (↓aqueous production).
    • SEs: Bradycardia, bronchospasm.

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Glaucoma Guardians - IOP Busters

Primary goal: Lower Intraocular Pressure (IOP). Medications target aqueous humor (AH) dynamics by decreasing production or increasing outflow.

• Prostaglandin Analogs (PGAs): E.g., Latanoprost. ↑ Uveoscleral outflow. SE: iris pigmentation, lash growth. 📌 "Prosts" for outflow.
• Beta-blockers: E.g., Timolol. ↓ AH production (ciliary body). Caution: systemic effects.
• Alpha-2 Agonists: E.g., Brimonidine. Dual: ↓ AH production & ↑ uveoscleral outflow.
• Carbonic Anhydrase Inhibitors (CAIs): E.g., Dorzolamide (topical), Acetazolamide (systemic). ↓ AH production (inhibits $HCO_3^-$ formation).
• Miotics (Cholinergics): E.g., Pilocarpine. ↑ Trabecular outflow (ciliary muscle contraction).

⭐ Latanoprost is often first-line for open-angle glaucoma due to potent IOP reduction and once-daily dosing.

Anti-Inflammatory & Anti-Infective Arsenal - Soothers & Slayers

• Corticosteroids (Steroids):
  • Mechanism: Inhibit $PLA_2$ → ↓$PGs$, $LTs$.
  • Potent: Dexamethasone, Prednisolone. Soft: Fluorometholone (FML), Loteprednol (less IOP↑).
  • Uses: Uveitis, post-op, severe allergy.
  • SE: ↑IOP, PSC, infection risk.
• NSAIDs (Non-Steroidal Anti-Inflammatory Drugs):
  • Mechanism: Inhibit $COX$ → ↓$PGs$.
  • Examples: Ketorolac, Nepafenac, Diclofenac.
  • Uses: Post-op pain/CME, allergy.
• Anti-Infectives:
  • Antibacterial:
    • Fluoroquinolones (Moxifloxacin): Broad spectrum. 📌 "Floxacins Flock Everywhere" (penetration).
    • Aminoglycosides (Tobramycin): Gram -ve.
    • Macrolides (Azithromycin): Chlamydia.
  • Antifungal: Natamycin (filamentous), Amphotericin B/Voriconazole (Candida/Aspergillus).
  • Antiviral: Acyclovir (HSV), Ganciclovir (CMV).
  • Antiprotozoal: PHMB, Brolene (Acanthamoeba).

⭐ Steroid-induced glaucoma: common with Dexamethasone; less with FML, Loteprednol etabonate (soft steroids).

Miscellaneous Meds - Special Ops Agents

• Anesthetics: Topical (Proparacaine); Injectable (Lignocaine).
• Diagnostic Dyes:
  • Fluorescein: Stains corneal defects; Seidel's. (📌 FADES)

  ⭐ IV Fluorescein for Fundus Angiography (FFA) visualizes retinal circulation.

  • Rose Bengal/Lissamine Green: Stains devitalized cells (dry eye).
• Anti-VEGF: Ranibizumab, Bevacizumab, Aflibercept (neovascularization).
• Lubricants: Artificial tears (CMC, HPMC) for dry eye.

High‑Yield Points - ⚡ Biggest Takeaways

• Autonomic drugs: pilocarpine (miotic), atropine (mydriatic/cycloplegic), timolol (↓IOP).
• Prostaglandin analogs (e.g., latanoprost): first-line for glaucoma, ↑ uveoscleral outflow.
• Topical steroids: potent anti-inflammatory; risk ↑IOP, cataracts.
• Antivirals (e.g., acyclovir) treat herpes simplex keratitis.
• Anti-VEGF agents (e.g., ranibizumab) for wet AMD, diabetic macular edema.
• Fluorescein dye stains corneal abrasions; used in applanation tonometry.
• Benzalkonium chloride (BAK), a common preservative, can cause ocular surface toxicity.