OCULAR ANATOMY AND PHYSIOLOGY RELEVANT TO THE NONCORNEAL ROUTE

II. OCULAR ANATOMY AND PHYSIOLOGY RELEVANT TO THE NONCORNEAL ROUTE

The ocular anatomy and physiology relevant to ocular drug delivery was reviewed by Robinson (95). Some of the main points relevant to noncorneal drug delivery are summarized below.

A. Noncorneal Drug Penetration and Tissue Distribution The shape of the eye in humans and rabbits, the primary animal model used

in ocular research, approximates that of a globe, as shown in Figure 1. From the perspective of ocular drug delivery, the eye can be regarded as consisting of two parts: an anterior and a posterior segment. The tear fluid, the cornea, anterior chamber filled with aqueous humor, iris-ciliary body, and the lens comprise the anterior segment. The posterior segment of the eye consists of

The Noncorneal Route in Ocular Drug Delivery 337

Figure 1 Anatomy of the eye. (Adapted from Ref. 154.)

four structures—the conjunctiva, sclera, choroid, and retina—surrounding the vitreous cavity that contains the vitreous humor. Taking into considera- tion the geometry of the eyeball and the diffusional pathways, drug pene- trating via the corneal route has direct access to the anterior segment tissues, providing high levels in the cornea, aqueous humor, and the iris-ciliary body. In contrast, drug entering via the noncorneal route traverses the conjunctiva and sclera entering the choroid, retina, and eventually the vit- reous humor. This selective tissue distribution of drugs entering the eye via the noncorneal route may be promising for drug delivery targeting the posterior eye (23,96).

338 Ahmed

B. Barriers to Noncorneal Drug Penetration Intraocular entry via the noncorneal route requires drug penetration across

the conjunctiva and the sclera. There are are three factors that can be barriers to the noncorneal penetration of ocularly applied drugs: (a) drug removal from the precorneal areas due to lacrimation and tear drainage, (b) the barriers to drug diffusion offered by the structures making up the outer coat of the eye, and (c) drug loss to the systemic circulation via the ocular vasculature.

1. Precorneal Fluid Dynamics Topically applied drugs are commonly administered as an eye drop formu-

lated as a solution, suspension, gel, ointment, and occasionally as a solid insert (86–94). The first barrier to intraocular penetration of topically applied drugs is tear turnover in the precorneal area. Under normal, una- nesthetized conditions, the human tear volume averages 7 mL, with the estimated maximum volume that the cul-de-sac can momentarily contain with eye drop administration at about 30 mL (97). The human tear film is a lightly buffered aqueous fluid with a pH of approximately 7.2–7.5 and with an estimated thickness of 4–9 mm (98,99). The average tear turnover rate in humans is about 16% per minute under basal conditions but may be increased to 30% per minute due to stimulation resulting from drop instilla- tion. The restoration of normal tear volume in the human requires an esti- mated 2–3 minutes, with 80% or more of the administered eye drops lost to drainage in the first 15–30 seconds after instillation (98). The resulting short contact times of drugs with absorbing membranes of the eye is the primary reason that typically less than 5% of a topically applied drug reaches the intraocular tissues (87–89).

2. Diffusion Across Ocular Membranes

a. Conjunctiva The conjunctiva is a thin, transparent mucous mem- brane that starts at the corneoscleral junction (limbus) and extends to the eyelid margin. The portion of the conjunctiva loosely attached to the ante- rior surface of the globe is referred to as the bulbar conjunctiva. The more firmly adhering segment lining the inside of the eyelids is called the tarsal or palpebral conjunctiva. The conjunctiva can be divided into three layers: (a) an outer epithelium, forming a permeability barrier, (b) the substantia propria, containing structural and cellular elements, nerves, lymphatics and blood vessels, and (c) the submucosa, providing a loose attachment to the underlying sclera. The conjunctival epithelium is a stra- tified epithelium, squamous at the lids and columnar towards the cornea.

The Noncorneal Route in Ocular Drug Delivery 339 It is nonkeratized and has tight junctions that can present a permeability

barrier to the diffusion of drugs. The thickness of the conjunctiva varies from region to region, being 10–15 layers thick towards the cornea and 5–

6 layers thick at the eyelids. The conjunctival epithelium possesses dense microvilli covered with glycoclayx and a mucus layer (100–102). The area of the conjunctival sac in humans has been estimated at 16 cm 2 , of which the cornea constitutes about 10%, whereas that in rabbits is approxi- mately 12–13 cm 2 , of which the cornea accounts for about 20% (103,104). Therefore, the conjunctival surface area exceeds that of the cornea by over fourfold. Unlike the cornea the conjunctiva is highly vascularized. Sys- temic loss can significantly reduce the fraction of drug available for pene- tration via the scleral/conjunctival route.

b. The Sclera The sclera, which is the white, tough outer coat of the eye, is composed largely of connective tissue (105). It has a protective function and maintains the shape of the eyeball by resisting intraocular pressure (106). The sclera is continuous with the cornea and extends pos- teriorly from the limbus, representing nearly 80% of the total surface area of the globe. The relatively softer, outer layer of the sclera is called the episclera. In rabbits, the sclera adjacent to the limbus is about 0.5 mm thick, thinning to as little as 0.2 mm in some areas (107). Structurally, the sclera is very similar to the corneal stroma and is made up of primarily collagen and mucopolysaccharides (108). Numerous channels through which fluid drainage occurs perforate the sclera. Blood vessels enter the uvea and the retina, but the sclera is itself poorly vascularized (109).

3. Systemic Loss via the Ocular Vasculature Although the blood-ocular barrier prevents most systemically administered

drugs from penetrating into the eye (65), ocularly applied drugs can enter the systemic circulation with relative ease. In excess of 70% of the instilled dose of an eye drop can enter the systemic circulation via absorption into the vasculature of the conjunctival and nasal mucosae (62–65). The nasal route is believed to be the primary contributor to the systemic loss of eye drops. For example,the nasal mucosa was 2.5 times more efficient than the conjunctival mucosa in contributing the systemic level of timolol (62). Whereas tight junctions between the endothelial cells of the microvessels result in a very low permeability to most solutes in the retina, the capillaries in the choroid and the ciliary processes there are fenestrated and the perme- ability to low molecular weight compounds is high. By injecting labeled proteins intravenously in rabbits and maintaining a steady-state concentra- tion, Bill et al. (59) showed that the permeability of proteins in the choroid and the ciliary body were high compared to other ocular tissues.

340 Ahmed Table 1 Blood Flow in Ocular Tissues of Primates Blood flow in whole

Tissue

(g/min/tissue) Retina

(mg/min)

a ðN ¼ 15Þ

Iris Ciliary body Ciliary processes Ciliary muscle

N = Number of determinations. a Standard Error of Mean.

Source : Ref. 59.

Radiolabeled microspheres and indicator-dilution techniques have been used to measure ocular hemodynamics (55–57). The blood flow rate through various parts of the eye in primates is reported in Table 1. The kinetic constant of drug transfer from the eye to the circulation usually has a value of 20–50 h

from conventional dosage forms (64). Systemic loss via the ocular vasculature is a major deterrent to noncorneal drug delivery and must be factored into calculating the fraction of drug available for intraocular absorption. Minimizing the systemic loss there can also help to reduce adverse systemic effect following ocular application of potent compounds (66–75).

III. NONCORNEAL ROUTES IN OCULAR DRUG DELIVERY Ahmed and Patton (5) proposed a schematic for the pathways for the

intraocular penetration of topically applied drugs, as shown in Figure 2. Modeling of the intraocular penetration routes and implication on ocular pharmacokinetics and pharmacodynamics was recently reviewed by Worakul and Robinson (14).

Although several investigators had reported a minor route of intra- ocular drug entry via the sclera and the conjunctiva (1–3), Bito and Baroody were the first to present evidence that this noncorneal route may, at least under some circumstances, be more important than the corneal route (4).

They showed that after topical 3 H-PGF 2a application, the choroid, anterior sclera, and the ciliary body contained higher drug concentrations than the aqueous, indicating that the drug was entering the eye by some route other than through the cornea.

The Noncorneal Route in Ocular Drug Delivery 341

Figure 2 Intraocular penetration routes.

Over the past two decades there have been several investigations to further examine the conjunctival/scleral pathway for intraocular entry of drugs. The preferred method has been to mechanically block the cornea from the conjunctiva and sclera in situ with the use of a cylindrical well and introducing a drug solution either inside or outside the well. Hence, the disappearance of drug from the reservoir as well as the appearance of drug in intraocular tissues can be determined to compare the rate and extent of drug penetration via the corneal versus the conjunctival/scleral pathway. This method was employed by Schoenwald et al. (15) to study the ocular penetration pathway for methazolamide analogs, 6-carboxyfluorescein and rhodamine. The conjunctival/scleral route of entry produced higher iris/ ciliary body concentrations for all compounds except for the lipophilic rho- damine. Confocal microscopy results suggested that drug gained entry into the ciliary body through uptake into the blood vessels of the sclera. The clinical implication of the scleral/conjunctival pathway may be important for antiglaucoma drugs where a quicker route to the iris-ciliary body via the blood vessels may result in a faster onset of action. Sasaki et al. (11) used the in situ technique to show that while the b-blocker tilisolol entered the aqueous humor primarily via the corneal route, the access to the vitreous

342 Ahmed body was four times more effective through the sclera than through the

cornea. The application of tilisolol in the conjunctiva or the sclera also showed a high concentration in plasma whereas corneal application pro- duced no systemic levels.

The physicochemical drug properties important to noncorneal pene- tration of topically applied drugs appear to be lipophilicity and molecular size. Using a series of b-blockers Sasaki et al. (16) showed that the perme- ability of penetrants is strongly dependent on lipophilicity for the cornea but

less so for the conjunctiva and the sclera (Fig. 3). Chien et al. (10) studied a 2 - adrenergic agents of varying lipophilicity and observed that the conjuncti- val/scleral pathway was the predominant route for delivery of least lipophi- lic molecule, p-aminoclonidine. The investigators also reported evidence of lateral diffusion of drug from the conjunctiva to the cornea. Pech et al. (18) evaluated a series of amphiphilic timolol prodrugs and observed that the transcleral absorption was the highest with the longest aliphatic chain pro- drugs, which also had the most amphiphilic/lipophilic character. Hence, the in vitro studies suggest that solute lipophilicity is less important for non- corneal drug penetration than it is for transcorneal drug penetration. However, the effect of lipophilicity on the extent of noncorneal penetration

Figure 3 Relationship between logarithmic values of the octanol/water partition coefficient (PC) and permeability coefficient (K p sclera; (*) scraped cornea.

The Noncorneal Route in Ocular Drug Delivery 343 of topically applied drugs in vivo may be difficult to predict from in vitro

studies alone. This requires information on the permeability of the drug across the conjunctiva, sclera, and ocular blood vessels, as well as drug binding to ocular tissues. A suitable predictive model that accounts for all these factors is not yet available.

There is also strong evidence that the noncorneal route may be the preferred pathway for intraocular entry of large, polar molecules that have poor corneal permeability. Ahmed and Patton (5,6) used corneal blocking techniques to demonstrate that the noncorneal pathway was the primary route of intraocular entry for inulin, a molecule that was poorly absorbed across the cornea (Table 2). The permeability of large molecules in conjunc- tiva and sclera is typically higher than in the cornea, suggesting that the noncorneal route may contribute more to the intraocular absorption of large molecules than the corneal route. The permeability of the sclera and conjunctiva will be discussed later in the chapter.

Table 2 Concentration of Inulin in Various Ocular Tissues 20 Minutes Following the Topical Instillation of 25 mL of a 0.65% Inulin Solution, in the Presence and Absence of Corneal Access

Concentration (mg/g) With corneal

Percent access

Without corneal

access

Aqueous humor 2.10 a 0.03 1.4 (0.420,9)

Lens ND

ND

Vitreous humor 0.03 0.02 67 (0.008,9)

Iris-ciliary body 0.79 0.63 80 (0.162,9)

a The mean; standard error of the mean and the number of eyes in parentheses. ND = Not detectable.

Source : Ref. 5.

344 Ahmed Systemic loss via drug absorption into the ocular blood vessels of the

conjunctiva is a nonproductive pathway that diminishes the fraction of drug available for intraocular penetration via the noncorneal route. Accordingly, delivery methods that maximize the drug concentration at the conjunctival surface and minimize nonproductive systemic loss are also expected to improve noncorneal drug penetration. This hypothesis has been supported by some recent studies. Urtti and coworkers (7) pre- sented evidence of application site–dependent noncorneal entry of timolol in rabbit from a topical device that released timolol at 7.2 mg/h. When the device was placed in the inferior conjunctival sac, the resulting timolol concentration in the aqueous humor was nearly 100-fold lower than in the iris-ciliary body. Romanelli et al. (20) showed that bendazac was absorbed into the retina-choroid via the scleral/conjunctival route when delivered topically in polysaccharide vehicles. It was noted that transcor- neal penetration of bendazac was hindered not only by the epithelial bar- rier but also by the strong binding of the drug to the stroma. In another study, Lehr et al. (21) reported formulating gentamicin in the mucoadhe- sive polymer, polycarbophil, facilitated the noncorneal penetration of gen- tamicin probably by intensified contact between the polymer and the underlying bulbar conjunctiva. Dosage design considerations for noncor- neal delivery will be discussed in a subsequent section.