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Ocular Pharmacology

Ocular Pharmacology

Ocular Pharmacology: Introduction of drugs used in Eyes 

Author: Dr. Gyanendra Lamichhane, MD, Lumbini Eye Institute

Pharmacodynamics 

It is the biological and therapeutic effect of the drug (mechanism of action)
Most drugs act by binding to regulatory macromolecules, usually neurotransmitters or hormone receptors or enzymes.
If the drug is working at the receptor level, it can be agonist or antagonist.
If the drug is working at the enzyme level, it can be activator or inhibitor.

Pharmacokinetics 

It is the absorption, distribution, metabolism, and excretion of the drug
A drug can be delivered to ocular tissue as:
1. Locally: 

  • Eye drop 
  • Ointment
  • Periocular injection
  • Intraocular injection

2. Systemically: 

  • Orally 
  • IV (Intravenous) 

Drug Delivery in Eyes

1. Topical (Drop, Ointment, Gel)
2. Periocular (Subconj., Subtenon, Peribulbar)
3. Intraocular (Intracameral, Intravitreal)
4. Systemic (oral, intravenous, Intramuscular)

Factors influencing local drug penetration into ocular tissue

  • Drug concentration and solubility: the higher the concentration the better the penetration e.g pilocarpine 1-4% but limited by reflex tearing
  • Viscosity: addition of methylcellulose and polyvinyl alcohol increases drug penetration by increasing the contact time with the cornea and altering corneal epithelium
  • Lipid solubility: because of the lipid-rich environment of the epithelial cell membranes, the higher lipid solubility the more the penetration  (Amphipathic- epithelium/endothelium----lipophilic, stroma---hydrophilic)
  • Surfactants: the preservatives used in ocular preparations alter cell membrane in the cornea and increase drug permeability e.g. benzylkonium and thiomersal
  • pH: the normal tear pH is 7.4 and if the drug pH is much different, this will cause reflex tearing
  • Drug tonicity: when an alkaloid drug is put in the relatively alkaloid medium, the proportion of the uncharged form will increase, thus more penetration
  • Molecular weight and size:

TOPICAL

Drop (Gutta)
  • simplest and more convenient
  • mainly for day time use
  • 1 drop=50 microlitre
  • Conjunctival sac capacity=7-13 microliter (so, even 1 drop is more than enough
Method
  1.  Hold the skin below the lower eyelid
  2.  Pull it forward slightly
  3.  Install One drop
Measures to increase drop absorption:
-Wait 5-10 minutes between drops
-Compress lacrimal sac
-Keep lids closed for 5 minutes after installation

Ointments:
  • Increase the contact time of ocular medication to ocular surface thus better effect
  • It has the disadvantage of vision blurring
  • The drug has to be high lipid soluble with some water solubility to have the maximum effect as the ointment.
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