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Barriers to ocular drug delivery
Designing formulations and delivery systems for topically applied ophthalmic drugs is
challenging due to anatomy, physiology and biochemistry of the eye which makes it
almost impervious to foreign substances including drugs.
The bioavailability of the topically applied drugs is very low (1-10%) due to various
protective mechanisms of the eye which include solution drainage, lacrimation, systemic
absorption via conjuctiva and a highly selective corneal barrier.
These barriers can be divided into precorneal, corneal and post-corneal barriers.
(1)Precorneal barriers
(a) Precorneal fluid dynamics
All liquid dosage forms, including aqueous solutions, oily solutions, suspensions and
liposomes, are rapidly drained from the conjunctival sac to the nasolachrimal duct.
The residence time of instilled dose is low. Such rapid drainage results from the
tendency of eye to maintain the residence volume at 7-10µl at all time. Drainage rate
becomes even higher when the formulation is irritating to the eye. (E.g. in case of
positively charged liposomes). To minimize the irritation potential of suspended
particles, the particle size in suspensions should be as small as possible.
Several factors influence the drainage rate:
Instilled volume – the rate of solution drainage is directly proportional to the instilled volume.
Ideally a high concentration of drug in a minimum drop volume would be desirable .however;
there is a practical limit to concept of minimum dosage volume. Droppers delivering small
volume are difficult to design and produce and m.ost patients cannot detect administration of
small volume.
Viscosity- low viscosity solutions are drained more rapidly. Therefore, increasing solution
viscosity is a popular method of prolonging the residence time of an instilled dose in
conjunctival sac. Various polymers can be used such as, poly vinyl alcohol, poly pyrrolidone,
hydroxypropyl cellulose, and other cellulose derivatives. Optimum viscosity is in the range 12-
15 cps, beyond which gain in ocular absorption would be minimal while risk of inaccuracy of
instillation and blurring of vision increases.
pH and tonicity- For stability reasons, most eye drops are formulated at pH other than pH 7.4.
They are therefore potentially irritating to the eye, stimulating tear production. Alkaline pH
induced greater lacrimation than acidic pH. This is consistent with the lower buffer capacity of
tears in the basic than in acidic range. Since tears are poorly buffered, a strategy to minimize
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the impact of induced lacrimation is to use the most dilute buffer possible; i.e. to keep the
tonicity low.
Drugs – drugs that act on the lacrimal gland can affect precorneal fluid dynamics. Examples
pilocarpine, epinephrine, local anesthetics such as tetracaine, certain beta blockers and tear
stimulants.
Induced lacrimation by epinephrine and pilocarpine leads to reduction in ocular absorption of
timolol when used in the same dropper with either of the above two drugs.
Suppression of tear turnover by topical application of 5 drops 0.5% tetracaine increases the
amount of pilocarpine absorbed in aqueous humour of albino rabbit eye.
(b)Drug binding to tear proteins
Out of several proteins in tears, albumin, globulins and lysozymes are involved in binding of
some drugs in tears to a significant extent. This binding leads to reduction in the free drug
concentration available for absorption. This binding becomes more significant when there is an
elevation in tear protein in certain extraocular disease states such as corneal inflammation,
herpes simplex infection and allergic conjunctivitis.
(c) Conjunctival drug absorption
Conjuctiva is vascularized, thin mucous membrane lining the inside of the eyelids and anterior
sclera. It is 17 times larger surface area than cornea and 2 to 30 times greater permeability to
drugs. Therefore conjunctival uptake of drug leads to reduction in amount of drug available for
corneal absorption.
It is possible to reduce conjunctival drug absorption in two ways: varying drug lipophilicity or
changing the drug formulation such as increasing solution ph, lowering solution tonicity, and
lowering the percentage of EDTA and benzalkonium chloride in the formulation.
Conjuctiva plays a minor role in contributing to drug loss due to systemic absorption.
(d) systemic drug absorption
Topically applied drug get into systemic absorption via nasal mucosa (nasolachrimal drainage).
Systemic absorption of drug via nasal mucosa can be minimized by nasolachrimal occlusion for
5 min,with or without eyelid closure or by changing vehicle composition such as incorporating
polymers, changes in vehicle type, alteration in solution ph and tonicity and adjustment of
preservative concentration.
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Co administration of low doses of vasoconstrictors such as phenylephrine and epipherine or by
designing ophthalmic drugs which are poorly absorbed into blood stream or are rapidly
inactivated in the circulation.
(2) Corneal barrier
The majority of topically applied drugs enter the eye through passage across cornea.
The cornea is mainly composed of five sections:
Epithelium, bowman’s membrane, stroma, Descemet’s membrane and endothelium.
Epithelium acts as the principal barrier mainly for hydrophilic drugs.
Stroma, consists of multiple layers of hexagonally arranged collagen fibers containing
aqueous pores or channels. It allow hydrophilic drugs to easily pass through but it acts as
a significant barrier for Lipophilic
Corneal integrity can be compromised by sufficiently high concentration of certain
formulation Excipient such as preservatives (e.g. benzalkonium chloride and other
cationic surfactants) and chelating agents (e.g. edta).
(3) Post corneal barriers
A. drug binding to melanin
Drug binding to melanin pigments in the iris and ciliary body can affect bioavailability of
topically applied drugs both positively and negatively.
The impact is usually negative due to (a) high binding capacity of the pigments relative to the
amount of drug reaching the eye
(b) Slow rate and small fraction of bound drug that is subsequently released.
Drug binding to melanin is saturable.
B. drug metabolism
A number of enzymes participate in ocular drug metabolism such as esterase, monoamine
oxidase, comt, ketone reductase, n aceltyltransferase
Of the anterior segment tissues, the corneal epithelium and iris – ciliary body are metabolically
most active. The enrichment of metabolizing enzymes in these tissues leads to degradation of
drugs during their transport through corneal epithelium and after reaching sites of action in iris
–ciliary body.
