Ocular Pharmacokinetics

Ocular Barriers - Fort Knox Eye

• Tear Film: Initial barrier; drug dilution, reflex tearing, and washout.
• Cornea: Major site for topical drug absorption; a three-layered barrier.
  • Epithelium: Lipophilic; tight junctions resist hydrophilic drugs.
  • Stroma: Hydrophilic; resists lipophilic drugs.
  • Endothelium: Lipophilic; less of a barrier than epithelium.
• Conjunctiva/Sclera: Minor pathway for topical absorption; significant for periocular/subconjunctival injections.
• Blood-Ocular Barriers: Restrict entry from systemic circulation.
  • Blood-Aqueous Barrier: Tight junctions in ciliary body non-pigmented epithelium & iris vessel endothelium.
  • Blood-Retinal Barrier (BRB):
    • Inner BRB: Retinal capillary endothelial cell tight junctions.
    • Outer BRB: Retinal Pigment Epithelium (RPE) tight junctions.

⭐ The corneal epithelium is the major barrier to topical drug absorption for hydrophilic drugs due to its lipophilic nature and tight junctions.

Drug Delivery Routes - Eye Highways

• Topical:
  • Common (drops, ointments) for anterior segment.
  • Limited by corneal barrier, tear washout, nasolacrimal drainage.
  • 📌 Punctal occlusion ↑ absorption, ↓ systemic effects.
• Systemic:
  • Oral/parenteral. Reaches eye via blood.
  • Limited by blood-ocular barriers.
  • For posterior segment, orbital, neuro-ophthalmic conditions.
• Periocular:
  • Injections (Subconjunctival, Sub-Tenon's, Retrobulbar, Peribulbar).
  • Bypass surface barriers; higher local drug levels.
• Intraocular:
  • Direct injection.
  • • Intracameral: Anterior chamber (e.g., post-op antibiotics).
  • • Intravitreal: Vitreous cavity (e.g., anti-VEGF).

  ⭐ Intravitreal injections achieve the highest intraocular drug concentrations in the posterior segment but carry risks like endophthalmitis.

Intraocular Journey - Drug's Eye View

Drug movement governed by Fick's First Law of Diffusion: $J = -D \cdot A \cdot (dC/dx)$.

• Absorption (Cornea - Major Route):

  • 📌 "LIPID drugs love LIPID layers (Epithelium, Endothelium), WATER drugs love WATER layers (Stroma)"
  • Conjunctival/scleral routes: minor for intraocular penetration, major for systemic absorption.

• Distribution:

  • Via aqueous humor circulation.
  • Blood-ocular barriers (blood-aqueous, blood-retinal) limit entry.

  ⭐ Melanin binding of certain drugs (e.g., atropine, timolol) can act as a reservoir, prolonging their effect but also potentially causing toxicity.

• Metabolism:

  • Generally minimal within the eye.
  • Some prodrugs (e.g., latanoprost, dipivefrin) are hydrolyzed to active forms by ocular enzymes.

• Elimination:

  • Primarily via aqueous humor outflow (trabecular meshwork & uveoscleral pathways).
  • Systemic absorption via uveal blood vessels.

PK Modifiers - Eye-Opening Factors

• Corneal Integrity:
  • Epithelial defects (abrasions, ulcers) ↑ drug penetration.
  • Intact epithelium: hydrophilic drug barrier.
• Inflammation:
  • Uveitis, keratitis ↑ vascular permeability & blood flow.
  • Disrupts barriers, ↑ drug entry.

  ⭐ Inflammation significantly increases ocular drug penetration by disrupting barriers and increasing blood flow.

• Iris Pigmentation (Melanin Binding):
  • Binds drugs (e.g., atropine, timolol); acts as reservoir.
  • Dark irides: ↑ binding, prolongs effect; may ↓ initial bioavailability or toxicity.
• Age:
  • Neonates: Immature barriers, ↓ metabolism.
  • Elderly: ↓ tears, ↓ corneal sensitivity, altered systemic clearance.
• Tear Film:
  • Dry eye: ↑ concentration, irritation.
  • Epiphora: ↓ contact time, ↓ absorption.
• Systemic Disease:
  • Renal/hepatic issues alter systemic drug clearance, affecting ocular levels.

High‑Yield Points - ⚡ Biggest Takeaways

• Corneal epithelium is the main barrier to hydrophilic drugs; stroma to lipophilic drugs.
• Topical drugs have low bioavailability (<5%) due to tear washout and nasolacrimal drainage (systemic absorption).
• Prodrugs (e.g., latanoprost, dipivefrin) enhance corneal penetration by altering lipophilicity.
• Intravitreal injections achieve high posterior segment drug levels, bypassing anterior barriers.
• Blood-ocular barriers (blood-aqueous, blood-retinal) restrict entry of systemic drugs.
• Conjunctival and scleral routes offer alternative pathways, especially for larger molecules.