ABC of Ocular Pharmacology for Optometrists in Nigeria.

Drugs in pharmacology are chemical substances used in the treatment, cure, prevention and/or diagnosis of disease or used to otherwise enhance physical or mental well-being.
["Drugs." Dictionary.com Unabridged (v1.1)].
Stedman's Electronic Medical Dictionary denotes drug as a"Therapeutic agent; any substance, other than food, used in the prevention, diagnosis, alleviation, treatment, or cure of disease."
Ocular drugs are by implication therapeutic/chemical agents meant to prevent, treat, cure or diagnose eye diseases.
Clinical pharmacology studies the pharmacology of therapeutic agents in the treatment, cure, prevention, management or diagnosis of a disease.
Historically, clinical ocular pharmacology was developed in the early nineteenth century but advances in this field has since became the norm and drug therapy based mainly on empiric, palliative and often toxic outcomes are today being advanced to be curative, less toxic and better enhanced to actualize set objectives of the drugs. {Williams BP, et al. (abstract) . The birth of ocular Pharmacology in the 20th Century.}
The term ocular pharmacology or ocular clinical pharmacology on this blog discusses the pharmacokinetics and pharmacodynamics of drugs to manage, treat, cure, prevent or used for diagnosis purposes in the eye and its adnexa viz the lacrimal apparatus, the upper and the lower lid, the orbital fats and the orbital cavity and ocular vascular supplies.
The process of absorption, distribution, metabolism (biotransformation), and elimination of drugs by the body is known as pharmacokinetics. Pharmacodynamics of drug is the quantitative relationship between drug and effect (mechanism of drug action).
[Iain L.O. Buxton, (2006). The pharmacological basis of therapeutics.]
The cornea, the conjunctival epithelium, the sclera, the ocular blood barrier and the ocular brain barrier remains the most important barrier to drug interaction (topical or systemic) with the host site of action in the process of drug pharmacodynamics of ocular intended medications.
The ocular blood barrier is formed by the endothelium of the retinal vessels, the cilliary epithelium and the retinal pigment epithelium.
(Peiffer, et al., (2001). Small animal ophthalmology: problem oriented-approach. Elsevier Hlth Sci. p. 46).
Ocular drug therapy is administered via
a) Eye drops
b) Eye ointments
c) Ocular injections.

a) & b) above are known as topical route of drug administration in the sense that they come in contact with the cornea, the conjunctival epithelium and the sclera and most often effective on the anterior 1/6th of the ocular globe viz the the iris, the cilliary process & cilliary body, the aqueous humour, the cornea, the conjunctival body and other superfacial ocular vascular supplies.
To enhance their pharmakokinetics of eye drops & eye ointments, the following factors are considered:
1) Concentration of the drops
2) Solubility
3) Viscosity
4) Lipid/Aqueous affinity
5) Surfactants
6) Reflex tearing factor
7) Tissue Binding factor etc.

On the other hand, Ocular injections bypass the corneal barrier & the conjunctival epithelium to reach the posterior 5/6th of the globe. This route of drug administration  usually results to more toxic than the eyedrops or the eye ointments but peak serum concentration is reached on time and efficacy of drugs is enhanced and prognosis of disease is often more clearer as therapeutic level of drug administered is usually high.
Ocular injections could be:
i) Peri-ocular {sub-conjunctival, sub-Tenon's & Retrobulbar}
ii) Intraocular {intercameral & intravitreal}
iii) Systemic.
Note: Usually refer to an experienced Ophthalmologist if you encounter conditions that require ocular injections.

Finally it should be noted that the corneal epithelium & corneal endothelium are hydrophobic while the corneal stroma is hydrophilic.
Thanks for your time... See you next time.

Dr Ezebuiroh Okwudiri Victor.

N/B: This write-up is a not-for-profit article.