DRUG DELIVERY: FUNDAMENTALS AND APPLICATIONS

Vision problems constitute a major burden to society, although some sight-threatening diseases (such as cataracts) are now largely successfully treated. The leading causes of blindness and low vision in the Western world are primarily diseases of the elderly population, including age-related macular degeneration (AMD), diabetic retinopathy, and glaucoma. As the population demographic changes, treating these diseases becomes increasingly important as a health-care priority.

Effective drug delivery to the eye is extremely challenging, and unlike the other epithelial routes described in this book, ophthalmic drug delivery is used only for the treatment of local conditions of the eye and is not considered as a portal of drug entry to the systemic circulation. Significant advances have been made to optimize the localized delivery of medication to the eye, so that the route is now associated with highly sophisticated drug delivery technologies, some of which are unique to the eye. We begin this chapter with a consideration of the anatomical and physiologic barriers of the eye and the concomitant limitations they impose for successful ophthalmic delivery. The pathology of the most important ophthalmic diseases is described, as well as their location in the eye, as this has important implications for the delivery route chosen. This is followed by a discussion of the various routes of ophthalmic drug delivery and the associated drug delivery and targeting technologies for each route.

13.2 ANATOMY AND PHYSIOLOGY OF THE EYE: IMPLICATIONS FOR OPHTHALMIC DRUG DELIVERY

The function of the eye is to produce a clear image of the external world and to transmit this to the visual cortex of the brain. In order to do this, the eye must have constant dimensions, an unclouded optical pathway, and the ability to focus light on the retina. These requirements, and the need for protection of the globe, determine the special structure of the eye and its associated physiological responses.

13.2.1 ANATOMY OF THE EYE

The eye is a spherical fluid-filled structure composed of three layers: (1) the sclera/cornea, (2) the uvea, and (3) the retina (Figure 13.1). The sclera, the tough outer “white” of the eye, is a layer of dense connective tissue that covers the entire eyeball, except for the cornea. Its stiff nature helps maintain the globe’s shape, making it more rigid, and protecting its inner parts. Extraocular muscles attach to the sclera through tendons, allowing the eye to move. The cornea is a transparent avascular layer at the front of the eye (FOTE), which covers the colored iris. Its curved structure helps to focus light onto the retina (Elkington and Frank 1999).

The anterior cavity (anterior to the lens; FOTE) is subdivided into two chambers: (1) the anterior chamber, between the cornea and the iris, and (2) the posterior chamber, which lies behind the iris, but in front of the lens. The lens is composed of proteins called crystallins, arranged like the layers of an onion; it lacks blood vessels and is completely transparent. Both chambers of the anterior cavity are filled with circulating aqueous humor, described further in the following texts.

The vitreous chamber (posterior to the lens; back of the eye [BOTE]) is the larger, posterior cavity of the eye. It contains a clear, jellylike, viscoelastic gel, the vitreous humor, which is formed during embryonic development and is not replaced thereafter. The character of this gel changes through life, becoming less structured, and therefore more liquid, in the elderly.

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FIGURE 13.1 Sagittal section of the eye.

The second layer is the uvea or uveal tract, which is composed of the choroid, iris, and ciliary body. The choroid is highly pigmented (the melanin limits reflections within the eye) and contains many blood vessels that supply the retina. Anteriorly, the choroid meets the iris and the ciliary body. The iris is the thin, pigmented structure in the eye, the shape of a flattened doughnut, responsible for controlling the diameter of the pupil and thus the amount of light reaching the retina. The ciliary body comprises both the ciliary muscle and the ciliary processes. Contraction of the ciliary muscle relaxes the zonular fibers, resulting in a change in the curvature of the lens and facilitating fine focus (accommodation).

The ciliary processes are capillary-rich protrusions on the internal surface, which produce and secrete the aqueous humor. This transparent, slightly alkaline fluid contains less protein and glucose than plasma, but more ascorbic acid, an important antioxidant (Civan and Macknight 2004). Aqueous humor nourishes the lens and cornea, carrying oxygen and nutrients and removing waste, as it circulates through the anterior cavity. It subsequently drains at sclerocorneal trabecula into Schlemm’s canal, to be resorbed into the blood. Normally, aqueous humor is completely replaced every 90 minutes.

The third layer of the eye is the retina, which contains a pigmented layer and a neural layer. The neuroretina comprises three distinct layers of neurons, interconnected by synapses: the photoreceptor layer, the bipolar cell layer, and the ganglion cell layer (Figure 13.2). Visual data are transduced by the photoreceptors and processed through the other neural layers before being transmitted as nerve impulses along the axons that form the optic nerve. There are two types of photoreceptor: rods (especially important for “gray” night vision) and cones (daytime vision). The larger cones only make up 5% of the total population of visual receptors and thus the retina is dominated by rods (about 100 million in man). The 5–6 million cones comprise S-type cones detecting blue light, M-type cones most sensitive to green light, and L-type cones detecting red light. The central retina, known as the macula, has a high density of cones, thus allowing for highresolution central vision.

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FIGURE 13.2 Microscopic structure of the retina.

13.2.2 LIMITING FACTORS FOR OPHTHALMIC DELIVERY

A considerable number of anatomical and physiological barriers exist to protect the eye from the entry of foreign substances. These barriers act as major limiting factors for ophthalmic drug delivery, which will be discussed in the following texts. Further reviews of the barrier properties of the eye are discussed in Wilson and Tan (2012) and Mains and Wilson (2013).

13.2.2.1 Visibility Requirements

More than half the sensory receptors in the human body are located in the eyes and a large part of the cerebral cortex is devoted to processing visual information. Transduction of the light energy into a receptor potential occurs in the outer segment of rods and cones. Visual signals are subsequently transmitted through the neuronal cell layers to the retinal ganglion cells, which provide retinal output to the brain. Light must have an unobstructed path to reach the photoreceptors; subsequent neuronal processing and nerve transmission should also be unimpeded.

The eye then affords a particularly challenging route for drug delivery, as a drug delivery system (DDS) cannot interfere with this vital sensory role. For example, other epithelial routes such as the skin, or oral cavity, can use sustained-release technology such as an adhesive patch to provide drug release for a prolonged period of time. The situation is difficult for the eye as even if the device is transparent and well tolerated, any resulting refractive change would cause blinking, possible lacrimation, and attempted removal. This places design limitations for a DDS in the central axis of vision. Similar limitations are encountered within the eye. Delivery systems formulated as colloids and particulates such as nanoparticles and liposomes can be used for controlled release and drug targeting via many epithelial routes. However, within the eye, there is a risk that movement of fluid would transport particulate systems posteriorly, where they could adhere to the lens, causing opacification, or appear as free-floating objects in the visual path, scattering light and decreasing visual acuity.

The extreme sensitivity and delicacy of the eye is a further difficulty and contrasts with more robust drug delivery sites such as those of the skin or the oral cavity. For example, the cornea is one of the most sensitive surfaces in the body, containing 300–600 more pain receptors than the skin (Belmonte and Cervero 1996). Thus, some chelating agents and surfactants, widely explored at high concentrations as a means of enhancing epithelial permeability via the oral, nasal, pulmonary, and buccal routes cannot be used in ocular topical preparations.

13.2.2.2 Tear Film, Nasolacrimal Drainage, and Reflex Larimation

The tear film comprises a three-layer structure: an inner mucin layer produced by conjunctival goblet cells that adheres to the cornea, a middle aqueous layer secreted by the lacrimal gland, and an external lipid layer produced by meibomian glands of the eyelid (Figure 13.3).

In the physiological process of nasolacrimal drainage, the tear film spreads over the surface of the eyeball due to the blinking of the eyelids. The tears are then cleared away as rapidly as they are produced, draining first into the lacrimal canals, then the lacrimal sac, and finally into the nasal cavity (Figure 13.4). The tear film and the process of nasolacrimal drainage combine to protect the eye by removing foreign material and form the first optical surface. Tears also lubricate and moisten the eyeball and carry nutrients to the cornea and conjunctiva. Each blink partially redistributes the tear film over the surface of the cornea and conjunctiva, and nutrients are replenished.

This process has important implications for drug delivery to the eye surface. Any topical medication must be retained on the surface of the eye despite a constant flow of tears and continuous eye blinking. In practice, nonviscous topical medications are rapidly cleared by this mechanism: up to 90% of the dose administered from conventional eyedrops is lost in the first 15 seconds after administration. Clearance by nasolacrimal drainage results in drug loss and also raises the possibility of systemic side effects, due to drug absorption through the mucous membrane of the nasolacrimal duct. This represents a particular concern for drugs such as the beta-blockers used in the management of glaucoma: unwanted systemic absorption can result in adverse cardiovascular and respiratory effects.

Under normal conditions, the human tear volume is about 7–9 μL and is relatively constant. The maximum amount of fluid that can be held in the lower eyelid sack is 15–20 μL, but only 3 μL of a solution can be incorporated in the precorneal film without causing it to destabilize. Eyedrop administration results in a sudden increase in tear volume, causing rapid reflex blinking. The volume of a drop is typically 33–38 μL and consequently some of the eyedrop can be spilled on the cheeks and eyelashes, resulting in drug wastage. Further, a large proportion of the retained instilled volume is pumped into the nasolacrimal duct, resulting in drug loss and the risk of systemic side effects.

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FIGURE 13.3 The structure of the tear film.

FIGURE 13.4 Lacrimal flow and drainage. The lacrimal gland produces tears, which are distributed by blinking. At normal rates of flow, tears are cleared by nasolacrimal drainage. Sudden increases produced by instillation, or reflex tearing, overspill the lower fornix and roll over the cheek. (Modified from Blamb/Shutterstock.com.)

Any irritation of the eye surface will induce a reflex lacrimation and blinking, as a protective mechanism. Again, this has implications for topical drug delivery, in particular for the formulation of topical products. The exposure of the eye surface to an acidic fluid may cause protein denaturation, forming insoluble complexes. Alkalization of the tear film tends to cause a saponification of lipids due to an interaction of the hydroxyl ions with cell membranes. At a high pH, alkalis behave as hydrophobic species and are more damaging to the cornea than acids (since precipitation of protein limits further ingress). Mildly acidic solutions are therefore better tolerated than slightly alkaline solutions, and reflex tears neutralize the irritant solution by dilution but will also wash away the instilled drug.

13.2.2.3 Epithelial Permeability Barriers

The eye presents sequential epithelial barriers, which reduce flux of nonnutrients into the eye, as a further natural defense mechanism against the entry of hazardous agents. As described in detail in Chapter 4 (Section 4.3), drug permeation across epithelial barriers is generally restricted to two main routes: the paracellular route (between adjacent epithelial cells) and the transcellular route (across the epithelial cells). The presence of tight junctions with resistances in excess of 120 Ωcm² between the epithelial cells of the eye severely restricts paracellular transport, confining the route to very small, hydrophilic molecules. The alternative, transcellular transport, is a viable transport pathway for drugs that possess the requisite physicochemical properties to allow for passive diffusion across many layers of epithelial cells. The physicochemical properties that favor transcellular diffusion are broadly the same as those affecting transepithelial absorption at any site and have been discussed in Chapter 4. In summary, favorable transport properties for transcellular passive diffusion include high lipid solubility and partition coefficient, low molecular weight and compact molecular volume, absence of charge, aqueous solubility, and chemical stability.

In medicines development through the turn of the century, massive jumps were made in the production of materials from cell sources rather than by the chemical synthetic route. As described in other chapters, these new “biologics” include peptide and protein drugs, as well as DNA-based therapeutics. For the eye, an early interest was in agents that restricted the uncontrolled growth of immature vessels following retinal ischemia—notably compounds that regulated expression of vascular endothelial growth factor (VEGF) and gene vectors. Such compounds had already appeared in cancer compound selection, since anti-VEGF molecules restrict the growth of solid cancers. All the drugs in this group are large and polar and therefore tend to have very low flux across membranes. Usually, they must be introduced by intravitreal injection, as the epithelial barriers restrict their penetration to the BOTE (see Section 13.7.1.1).

The principal epithelial barrier to drug transport in the eye was thought to be the cornea, but since it makes up only 20% of the outer eye area in contact with an instilled drop, the conjunctiva must also play a significant role in the absorption process. The conjunctiva is a thin, transparent mucous membrane composed of highly vascularized, nonkeratinized, stratified, columnar epithelium, which is approximately 5–15 cell layers thick. It also contains goblet cells that help lubricate the eye by producing the mucous component of the tear film. The conjunctiva lines the inside of the eyelids (tarsal conjunctiva) and the anterior one-third of the eyeball, apart from the cornea (bulbar conjunctiva). The tight junctions and multilayered nature of the conjunctiva make it an important barrier against infection but also limit the permeation of topically applied drugs. Since the conjunctiva is highly vascularized, significant drug loss to the systemic circulation can occur, resulting in drug wastage and the risk of systemic adverse effects.

The adult cornea measures 11–12 mm horizontally and 9–11 mm vertically. Corneal thickness varies, being thinnest centrally (550 μm) and gradually increases to the periphery (700 μm). It is an avascular five-layered structure comprising (anteriorly to posteriorly) the epithelium, Bowman’s membrane, stroma, Descemet’s membrane, and endothelium (Figure 13.5).

The outer corneal epithelium of the cornea constitutes the most important barrier for corneal drug flux. It comprises five to six layers of nonkeratinized, stratified, squamous epithelium, with an abundance of tight junctions. As introduced earlier, the tight junctions form a highly resistant barrier to paracellular transport, so that transport across the cornea is usually limited to the transcellular route. The inner stroma layer of the cornea (about 90% of the thickness of the cornea in most mammals) is composed of a modified connective tissue, rich in collagen, other proteins, and mucopolysaccharides. About 70%–80% of the wet weight is water, which presents a barrier to highly lipophilic drugs. The innermost layer of the cornea is a monolayer of endothelium, but this has leaky endothelial junctions that do not impede drug transit.

13.2.2.4 Other Physical Barriers of the Eye

Deep to the outer corneal and conjunctival epithelial barriers, the eye presents further obstacles to drug transport.

The sclera: The sclera is a layer of dense connective tissue, consisting primarily of collagen and elastin fibers, as well as proteoglycans, embedded in an extracellular matrix. It is thinnest at the site of extraocular muscle attachment (0.3 mm) and thickest at the posterior pole (1 mm). The essentially hydrophilic proteoglycan extracellular matrix can be expected to impede the diffusion of hydrophobic drugs through its tissue. Positively charged drugs may also interact with negatively charged proteoglycans and thus show reduced flux.

The retina: The retinal pigment epithelium (RPE) comprises a monolayer of cuboidal epithelial cells, lining the back of the retina (Figure 13.2). Tight junctions between the RPE cells form a blood/retina barrier and limit the access of drug molecules from the blood circulation Descemet’s membrane endothelium to the retina. The RPE also contains several efflux pumps including P-glycoprotein and multidrug resistance–associated protein, which further limit the passage of compounds into the vitreous. The inner retina receives a blood supply from the central retinal artery, which passes with the optic nerve into the eye. These blood vessels are lined with tightly packed endothelial cells thus forming the inner limiting membrane (ILM). The barrier in the inner globe is the meshwork of ILM and neuronal cells.

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FIGURE 13.5 The corneal epithelial barrier.

Vitreous humor: The vitreous is a viscoelastic gel composed of Type II collagen fibers, which trap coiled hyaluronic acid and water molecules. As the volume of adult human vitreous is around 4 mL, the vitreous humor presents a large diluting sink. Additionally, the network of collagen and hyaluronic acid presents a diffusional barrier, limiting the passage of cells and macromolecules, particularly cationic molecules and cationic carriers, due to charge interactions.

13.3 DISEASES OF THE EYE

The eye is prone to a wide variety of diseases, which may be of a systemic origin, such as diabetes or hypertension, or peculiar to the eye, such as glaucoma, cataract, and macular degeneration. Many of these conditions, such as macular degeneration, glaucoma, and diabetic retinopathy are chronic conditions that require continuous therapy for treatment. Furthermore, since the eye is located on the surface of the body, it is also easily injured and infected. According to the location of the disease, ocular disorders are grouped as disease of the anterior cavity or vitreous chamber.

13.3.1 DISEASES OF THE ANTERIOR CAVITY

Dry eye is a common problem, estimated to affect over 4 million Americans (Schein et al. 1997) and its incidence increases with age. Dry eye will result if the composition of tears is changed, or an inadequate volume of tears is produced. Dry eye conditions are not just a cause for ocular discomfort, but prolonged dry eye can lead to corneal damage, keratitis, and compromised vision.

A variety of different inflammatory eye conditions are described, according to the region of the eye affected. For example, blepharitis is an inflammation of the eyelid, most commonly caused by a bacterial infection, usually Staphylococcus aureus. Conjunctivitis is an inflammation of the conjunctiva, resulting in redness to the visible white of the eye. Allergic conjunctivitis (AC) is defined as inflammation of the conjunctiva in response to an allergen. AC may be seasonal or perennial. Primary symptoms are itching, burning, and then a watery discharge. Bacterial conjunctivitis can result in pain and a purulent discharge, while viral conjunctivitis results in a clear discharge. Trachoma is an infectious cause of conjunctivitis caused by the organism Chlamydia trachomatis. The resulting chronic granular inflammation causes a fibrosis of the conjunctival lid surface, resulting in inversion of the lid margin. Subsequent abrasion from eye lashes results in keratitis and can lead to blindness unless treated; it is the most common cause of blindness in North Africa and the Middle East (Burton and Mabey 2009). Keratitis is an inflammation or infection (bacterial, viral, or fungal) of the cornea. Patients with keratitis present with decreased vision, ocular pain, foreign body sensation, red eye, and often a cloudy/opaque cornea.

Glaucoma refers to a group of different eye conditions, in which damage to the optic nerve leads to progressive, irreversible, vision loss. It is sometimes categorized as a stand-alone disease, separate from the posterior segment diseases, since it is treated by local topical therapy to the anterior eye. It is considered one of the major global ophthalmic clinical problems; it is the second leading cause of blindness, after cataracts, worldwide. In the United States, primary open-angle glaucoma (POAG) is the most common form of the disease, accounting for 90% of all cases. It occurs mainly in the over 50-year-olds and is characterized by an increase in the intraocular pressure (IOP) caused by a restriction of outflow and decreasing oxygenation. The associated nerve damage involves loss of retinal ganglion cells. Visual loss develops first in the peripheral visual field and then progresses to the central visual field. POAG is a painless, insidious disease in which injury develops over years. Symptoms are absent until extensive optic nerve damage has been produced. If the condition is left untreated, blindness will result.

13.3.2 DISEASES OF THE VITREOUS CHAMBER AND RETINA

Intraocular infections are those infections of the inner eye, including the aqueous humor, iris, vitreous humor, and retina. They may occur after ocular surgery, trauma, or may arise from systemic infections. Intraocular infections carry a high risk for loss of vision and may result in the spread of infection from the eye into the brain.

Uveitis refers to an inflammation of the anterior uvea (that portion of the eye including the anterior chamber, iris, and ciliary body) and surrounding tissues. In addition, a posterior segment component (retinochoroidal) may be involved. Uveitis may arise due to an autoimmune disorder, following an infection, or other causes. If untreated, inflammation in the anterior segment may lead to glaucoma and cataracts, while inflammation in the posterior segment may lead to macular edema.

Cytomegalovirus retinitis (CMVR) is an inflammation of the retina of the eye, which may lead to blindness. It is caused by an infection of human CMV (HCMV), a herpes-type virus. HCMV opportunistically infects patients with a weakened immune system; it came to the attention of the public with the appearance of HIV in the 1990s and may be masked by the concomitant use of corticosteroids. Much of the early research on ocular implants was stimulated by the urgent need to treat this disease.

AMD* is a degenerative disorder of the retina, typically in persons aged 50 years and older. Central vision becomes increasingly blurred, thereby limiting perception of fine detail, which is required for activities like reading, driving, recognizing faces, and seeing the world in color. The disease begins as the more common dry AMD (85% of patients) but can later progress to the second, much more severe form, known as wet AMD (neovascular AMD; 15% of patients), which is associated with acutely progressive vision loss.

In dry AMD, debris associated with the turnover of photoreceptors surrounds the most lightsensitive region of the retina, the macula lutea. The pigmented layer atrophies and degenerates, resulting in a gradual deterioration of central vision. In wet AMD, degeneration of the macula is due to the growth of new subretinal blood vessels, which are fragile and leaky; fluid leakage causes permanent macula injury.

The emergence of CMVR (as a consequence of HIV infection) in the late 1970s was a significant driver at the time for the development of new ocular delivery technologies such as ocular inserts. In a similar manner, a large section of current ophthalmic drug delivery research is driven by the need to achieve successful drug delivery to the BOTE in order to treat AMD. AMD remains a leading cause of irreversible blindness and visual impairment in the world (Agarwal et al. 2015). As many as 11 million people in the United States have some form of macular degeneration. Although previous estimates suggested that age is a prominent risk factor, with 2% risk of contracting the disease for those ages 50–59, to nearly 30% for those over the age of 75, the incidence of AMD in the United States is declining in each successive generation and adults born after World War II have a significantly lower risk of early disease progression (Cruickshanks et al. 2014).

Diabetic retinopathy is one of the major complications of diabetes and results from hyperglycemia-induced changes in the retinal microvasculature. Initially, the damage is limited to microaneurysms in the retinal capillary walls, which can leak fluid. It then progresses to scarring, followed by the proliferation of a new leaky vasculature, stimulated by VEGF. The overgrowth of new, unstable, leaky retinal capillaries reduces visual acuity. Diabetic macular edema (DME) is caused by leaking macular capillaries, which cause fluid and protein deposits to collect on, or under, the macula of the eye, causing it to thicken and swell, and distorting central vision. Retinopathy is accelerated by hyperglycemia, hypertension, and smoking.

13.4 OVERVIEW OF DRUG DELIVERY TO THE EYE

In the search for an efficient route for ocular drug delivery to the eye, various complex interrelated factors must be considered. These considerations include the limitations generated by the anatomy and physiology of the eye, the pathology and location of the ocular disease, and the accessibility of a site for drug delivery. Thus, a number of different approaches are possible, which are summarized in Figure 13.6 and described in detail in the following texts.

13.4.1 TOPICAL DELIVERY

Topical delivery is useful in the treatment of local conditions of the surface of the eye, for example, conjunctivitis, but it is limited by the short residence time of conventional eyedrops. Mechanisms to enhance residence time include the incorporation of mucoadhesive polymers into eyedrop formulations, as well as the use of micro- and nano-DDS, contact lenses, and intraocular inserts.

Applying a drug topically to the eye can also, in principle at least, be used to treat BOTE targets such as the retina and macula; in animals such approaches seem to be successful due to the size of the globe. In humans, topically applied drugs may fail to achieve sufficient con-junctival and corneal penetration to access the required BOTE targets. A successful drug would have to be extremely potent and have an exemplary safety profile. Strategies to increase corneal penetration after topical administration include the use of micro- and nanoparticulate DDS and prodrugs.

13.4.2 INTRAVITREAL DELIVERY

For the treatment of BOTE conditions such as AMD and diabetic retinopathy, an intravitreal injection provides therapeutic concentrations of the drug adjacent to the intended site of activity; a much smaller dose is required than via the topical route. Intravitreal injections are invasive, and repeated treatment carries the small risk of retinal damage and infection. Consequently, current research is focused on the use of long-acting depots and implants, in order to minimize injection frequency.

FIGURE 13.6 Different routes of drug delivery to the eye.

13.4.3 TRANSSCLERAL ROUTE

In recent years, attention has turned to forming a scleral depot for achieving sustained drug delivery to the BOTE, as an alternative to topical and intravitreal delivery. Mechanisms to achieve delivery to the retina include transscleral and suprachoroidal injections. Newly emerging technologies to achieve transscleral delivery include iontophoresis and implantable diffusion pumps (Figure 13.6). Other approaches investigate the placement of a device within the sclera, for electroporation and iontophoresis. The suprachoroidal space (SCS) can also be targeted using short needles and microneedle arrays. Some of the methods require highly advanced drug delivery technologies and are still at an early stage of exploration.

13.4.4 SYSTEMIC ROUTE

The systemic route is another potential means by which medications can gain access to the BOTE. A drug administered orally or intravenously (i.v.) that is circulating in the blood can distribute into the eye, although distribution is poor as the eye only receives a relatively small proportion of the total blood flow. Drug entry into the posterior segment is also often limited by two significant barriers (Figure 13.2): (1) the outer barrier of the retinal pigment epithelium (RPE) and (2) the inner blood–retinal barrier, comprising the endothelial cells of the retinal blood vessels.

Assuming that the drug can achieve access to the eye via the bloodstream, the route still suffers from the disadvantage that only a very small volume of tissue in the eye requires treatment, yet all the organs of the body are subjected to the action of the drug. Interactions of the drug with other body systems may result in adverse, or even toxic, effects, particularly if the drug has a narrow therapeutic index. High doses of a drug are also required (resulting in increased drug costs and the risk of adverse effects), due to drug dilution in the bloodstream and possible premetabolism of the drug, prior to reaching the eye. For these reasons, there was a move away from oral carbonic anhydrase inhibitors such as Diamox^® (acetazolamide) at the end of the 1990s, and currently the systemic route is not generally used for the delivery of pharmacologically active materials to the eye.

There is, however, an important exception: ocular photodynamic therapy (PDT). In this treatment, a nontoxic light-sensitive compound is administered i.v. and selectively accumulates in the retina. It is then exposed to laser light, whereupon it becomes toxic to targeted malignant (or other disease) cells (Kim and Morley 2006). On shining the light, free radicals and super oxide ions generate singlet oxygen, a primary mediator of tissue damage. Singlet oxygen is highly reactive and therefore can only exert effects within 10–20 nm of its generation, which confines the cytotoxic effects to a very close locus near to the target.

PDT is currently used for the treatment of wet AMD, whereby a light-sensitive agent, verteporfin (Visudyne^®) is administered i.v. and preferentially accumulates in the abnormal submacular blood vessels of patients with AMD. Laser treatment then selectively generates free oxygen radicals that cause cytotoxic damage and the occlusion of new vessels. A recent update on PDT and the applications of the range of therapeutic approaches in managing AMD is published in an excellent review by Agarwal et al. (2015).

13.5 TRADITIONAL TOPICAL DRUG DELIVERY

Traditional FOTE drug delivery involves instilling drops of a solution (or suspension) of the drug from an eyedrop bottle onto the surface of the eye. 95% of all ophthalmic drugs are delivered using a traditional eyedrop bottle. Drugs for topical delivery include beta-blockers and anticholinergics for the management of glaucoma, NSAIDs and corticosteroids for pain and inflammation, antibacterials and antivirals for eye infections, and antihistamines for allergies. Topical drug delivery localizes the drug effects, facilitates drug entry that is otherwise hard to achieve with systemic delivery, and avoids first-pass metabolism. Furthermore, ophthalmic solutions are easy to prepare, filter, and sterilize.

Pragmatically, the delivery method is imprecise, inaccurate, and inefficient. In practice, topical application frequently fails to establish a therapeutic drug level for a desired length of time within the target ocular tissues and fluids. This is because the eye is extremely efficient at eliminating topically instilled medications. As described in Section 13.2.2.2, the eye has a built-in drainage system designed to protect it from damage and irritation. Eyedrops are subjected to the physiological processes of reflex lacrimation, blinking, and nasolacrimal drainage. Due to the nasolacrimal clearance system, most of an administered dose is lost within seconds of instillation. This means that to produce a therapeutic effect, conventional eyedrops must be administered several times a day, which is cumbersome for the patient and decreases patient compliance. It also leads to issues of poor efficacy and safety problems, with the danger of systemic adverse effects if the drug is absorbed.

In order to optimize drug delivery via conventional eyedrops, a number of key parameters must be controlled. Proper instillation is essential to ensure efficacious treatment. A recent study showed that 90% of glaucoma patients were not administering their drops correctly (Gupta et al. 2012). An eyedrop should be placed in the inferior fornix by tilting the head back and gently pulling the lower lid away from the globe and creating a pouch to receive the drop. The lid is then gently returned to the globe, entrapping a small amount of liquid in the inferior conjunctival sac. Gentle pressure should be applied at the inner corner of the eye to prevent tears from diluting the eyedrops and also to reduce unwanted systemic absorption of the drug. Administration in this way means the drop is retained in the eye up to twice as long as if it is simply dropped over the superior sclera.

A reduced instilled volume can also contribute to improved efficacy in ocular delivery. Volumes of 30–60 μL are typically dispensed from eyedrop bottles, yet an introduced volume of more than 10 μL will result in nasolacrimal drainage. A larger volume will induce reflex blinking and increased lacrimation, which dilute the drug and may cause it to be washed out of the eye, resulting in drug wastage and possibly systemic side effects. The size of the drop is affected by multiple parameters, including the diameter of the opening of the bottle, the handling angle of the bottle, the concentration of polymer in the formulation (higher concentrations giving rise to larger drops), and the viscosity and surface tension of the formulation. Tips capable of delivering a drop of 8–10 μL have been designed by varying the relationship between the inner and outer diameters of the end of the tip. The use of smaller eyedroppers in commercial containers has not been popular and they may pose a safety risk due to the acute angle of the bottle tip. Although a smaller drop may be retained longer in the conjunctival sac, an instilled volume less than 8 μL is not recommended due to the difficulty in making up a suitable drug concentration in such a small volume.

The normal osmolality of tears is almost equivalent to that of normal saline solution. Variations in osmotic pressure between 100–340 mOsm/kg appear to be well tolerated by the eye. Beyond these values irritation takes place, again eliciting reflex tears and reflex blinking. Accordingly, the ophthalmic preparation should be formulated with optimal pH, surface tension, and osmolality values (see USP guidance in Aldridge et al. 2013). The pH should be between 7.0 and 7.7. Some drugs are unstable in this pH range, and therefore need to be formulated at other pH values, but it is preferred that little or no buffering is employed. The surface tension of tear fluid at the eye temperature has been measured as 43.6–46.6 mN/m for normal eyes and 49.6 mN/m for patients with dry eye. The instillation of a solution that lowers the surface tension may disrupt the outermost lipid of the tear film into numerous oily droplets, which become solubilized. The protective effect of the oily film against evaporation of the tear film aqueous layer disappears and dry spots will be formed. The dry spots are painful, irritant, and elicit reflex blinking. The symptoms in sufferers are worsened by activities in which the rate of blinking is reduced, including watching television. In severe dry eye, punctal plugs may be employed to increase the resident tear volume.

Self-administration of eyedrops is often problematic, with one study reporting at least 50% of patients admitting to difficulty in instilling their own eyedrops (Connor and Severn 2011). The most common reported difficulties include problems with aiming the bottle accurately (with patients missing the eye entirely, or dropping the dose onto the eyelids, eyelashes, or cheeks) and being unable to squeeze it sufficiently to expel the dose. Elderly patients and those suffering from rheumatoid arthritis, osteoarthritis, carpal tunnel disease, and stroke have lower grip strengths, which make squeezing the bottle particularly difficult. A patient’s hand may wobble because they have to squeeze the bottle at the limit of their capability, and this may compromise their ability to aim accurately.

Eyedrop containers have to be flexible to allow squeezing, yet rigid enough to prevent flooding of the ocular surface. Factors that influence the force requirements include the rigidity of the bottle, the ratio of bottle height to width, the viscosity of the medication, and the length of the dispensing tip. Small, single-dose units tend to have higher force requirements. The force exerted on the bottle determines, in part, the drop size and number of drops actually expelled: wide variability in both these parameters has been described, leading to imprecise drug dosing. There are also psychological factors at play: the eye is such a delicate sensory organ that many patients have a natural aversion to using eyedroppers, being instinctively nervous that they will damage their eyes. Again, this is especially true of the elderly, with their weak eyesight, limited strength, and poor balance.

In order to improve FOTE delivery and reproducibility, some bottles have flexible areas, or a pump action, to facilitate the action of dispensing a drop accurately. A number of eyedrop delivery aids have been developed, including, for example, the Xal-Ease^® plastic eyedrop dispenser. This fits over the existing eye bottle and helps with the removal of the bottle cap and the positioning of the drug over the eye; it then dispenses only a single drop of the medication into the eye. The VersiDoser^™ is capable of delivering a precise dose of drug into the eye (ranging from 10 to 50 μL) in virtually any hand/head orientation. Other eyedrop delivery aids include a system of mirrors to help the patient see the dropper tip and devices such as the Autosqueeze^™ Eye Drop Dispenser, a tongs-like device to assist with bottle squeezing.

Suspensions are also used in ophthalmic formulations, particularly for anti-inflammatory steroids, which typically exhibit poor aqueous solubility. They are commonly formulated by dispersing micronized drug powder (<10 μm in diameter) in a suitable aqueous vehicle. Suspensions provide a longer duration of action than solutions, as the particles persist in the conjunctival sac, giving rise to a sustained-release effect; however, the presence of particles may result in a foreign body sensation in the eye causing reflex lacrimation and blinking.

13.6 ADVANCED TOPICAL DELIVERY

A wide variety of technologies have been developed in recent years to improve the efficacy of topical delivery to the eye. These have been reviewed extensively, although it must be noted that many of the texts cover the same material from similar viewpoints. A good review of the field is found in Nakhlband and Barar (2011), and other sources are cited later in this chapter. It is established that simple aqueous formulations have relatively low efficacy with regard to drug delivery, which gives scope for innovation. The principal technologies focus on mechanisms to (1) prolong residence time at the ocular surface, to prevent premature clearance, and (2) enhance transcorneal penetration, to deliver topically applied drugs to the interior ocular tissues.

13.6.1 POLYMERIC EXCIPIENTS

Viscosity enhancing polymers provide a thickened solution that reduces lacrimal drainage and increases residence time at the ocular surface. This results in improved efficacy, less frequent dosing, and improved patient compliance. Hydrophilic polymers hold water by weak hydrogen bonding and if they wet surfaces, resist dehydration. A further application of polymeric solutions as functional ingredients is therefore in the supplementation of mucin-deficient tears in postmenopausal dry eye.

The function of a polymer is to interpenetrate tear and surface mucins: chain length, polymer flexibility, and chain segment mobility are key properties of nonionic polymers. The most well-known classes of polymers in this group include cellulosic polymers, such as methylcellulose (MC), hydroxyethyl cellulose, hydroxypropyl MC, and hydroxypropyl cellulose (HPC). They provide a wide range of viscosities (400–15,000 cps) and are compatible with many topically applied drugs. Polyvinyl alcohol (PVA) is also widely used as a drug delivery vehicle and a component of artificial tear preparations. This polymer can reduce interfacial tension at the oil/water interface, enhance tear film stability, be easily sterilized, is compatible with a range of ophthalmic drugs, and is nontoxic.

When extensive hydrogen bonding occurs between a polymer and surface or a solute macromolecule, the polymer may be classified as a bioadhesive. Bioadhesion is an interfacial phenomenon in which a synthetic or natural polymer becomes attached to a biological substrate by means of interfacial forces. If it involves mucin or mucous-covered membrane, the narrower term “mucoadhesion” is employed. Mucoadhesion has been used to enhance bioavailability of drugs via various other routes, including oral/gastrointestinal, oral cavity, nasal, and vaginal routes; further information can be found in the relevant chapters describing these routes. The presence of mucin in the tear film means that this phenomenon can also be exploited for ophthalmic drug delivery and charged anionic polymers can be utilized. As with the nonionic polymers, it seems that the length of the polymer tails must be long enough and mobile to facilitate molecular entanglement. The threshold has been defined as around 100,000 Da in flexible chain motif polymers. When anionic polymers are used, the maximum interaction occurs at an acid pH, suggesting that the polymer must be in its protonated form for viscoelastic synergy with surface mucins.

Sodium hyaluronate is a high-molecular-weight polymer extracted by a patented process from sources including animal sources, bacterial fermentation, and most recently directly using hyaluronate synthetase acting on UDP-sugar monomers. It consists of a linear, unbranched, nonsulfated, polyanionic glycosaminoglycan, composed of one repeating disaccharide unit of D-sodium glucuronate and N-acetyl-D-glucosamine. The polymer is mucoadhesive: the carboxyl groups of hyaluronate form hydrogen bonds with sugar hydroxyl groups of mucin, producing an intimate contact with the cornea. Furthermore, hyaluronate solutions exhibit pseudoplastic behavior (where viscosity is higher at the resting phase), which provides a thickened tear film, slows drainage, and ensures an improved distribution on the cornea during blinking. Sodium hyaluronate is also mixed with xanthan gum, the objective being to produce a tear mucin mimetic with similar viscoelastic and rewetting properties. Addition of an antibiotic, such as netilmicin, is used in the treatment of corneal abrasions (Faraldi et al. 2012).

In situ gelling systems are also used for ophthalmic delivery. A sol–gel transition can be triggered by a change in pH, temperature, or ionic strength of the formulation, upon instillation in the eye. One of the early materials investigated was cellulose acetate phthalate, which provided a pH-triggered system. The polymer has a very low viscosity up to pH 5 but coacervates in contact with the tear fluid at pH 7.4, forming a gel in few seconds and releasing the active ingredient in a sustained manner. Unfortunately, high polymer concentrations (25%) are required, and the instilled solution has a low pH, both of which cause discomfort to the patient.

Poloxamer F127 is a solution at room temperature, but when it is instilled onto the eye surface, the elevated temperature (34°C) causes the solution to become a gel, thereby prolonging its contact with the ocular surface. Again, the system suffers from the disadvantage that it requires a high polymer concentration (25% poloxamer); also, the surfactant properties of poloxamer may be detrimental to ocular tolerability.

Timoptic XE^® is an in situ gel-forming solution of the beta-blocker timolol maleate and Gelrite^® gellan gum. Gellan gum is an anionic polysaccharide formulated in aqueous solution, which forms clear gels under the influence of an increase in ionic strength. The gelation increases proportionally to the amount of either monovalent or divalent cations. The concentration of sodium in human tears (≈2.6 μg/μL) is particularly suitable to induce gelation of gellan gum following topical instillation into the conjunctival sac. The reflex tearing further enhances the viscosity of the gellan gum by increasing the tear volume and thus increasing the cation concentration. Scintigraphic studies showed that Gelrite (0.6% w/v) significantly prolongs ocular retention in man by forming a gelled depot on the scleral margin (Greaves et al. 1990).

Carbomers comprise poly(acrylic acid) polymers that undergo both temperature- and pH-dependent changes in structure. They are acidic, low viscosity, aqueous dispersions that transform into stiff gels when instilled into the conjunctival sac upon instillation. Carbomers offer several advantages for ophthalmic delivery, including high viscosities at low concentrations, strong adhesion to mucosa, thickening properties, compatibility with many active ingredients, and low-toxicity profiles. DuraSite^® is a synthetic polymer of cross-linked poly(acrylic acid) that stabilizes drug molecules in an aqueous matrix, maintaining therapeutic doses of a drug on the eye surface for up to 6 hours. The technology is used in Azasite^® (azithromycin ophthalmic solution) for the ocular delivery of the antibiotic azithromycin in the treatment of bacterial conjunctivitis: only once a day dosing is required (Friedlaender and Protzko 2007).

Combinations of different phase-transition polymers are also being investigated, in order to improve the gelling properties while also reducing the total polymer payload in the system, thereby improving tolerability and reducing discomfort. Simple mixing sometimes produces synergistic effects on thickening but may result in reduced mucoadhesion.

13.6.2 CONTACT LENSES AND CUL-DE-SAC INSERTS

Drug-soaked contact lenses can be placed on the corneal surface, where they remain for up to 12 hours, thereby providing a reservoir of drug that desorbs following the Arrhenius equation and sustains drug release over a short period of time (Bengani and Chauhan 2013). Many methods of drug loading have been explored; the most common is presoaking the lens in a solution of the drug. Drug loading by soaking of lenses in ophthalmic formulations not designed for the task may cause toxicity to the corneal epithelium because preservatives such as benzalkonium chloride have a great affinity for the hydrophilic contact lens material and can become concentrated in the lens. Alternative drug-loading approaches include (1) using molecularly imprinted polymeric hydrogels; (2) chemical conjugation of the drug, or drug-loaded microparticles, to the lens surface; and (3) using liposome-loaded contact lenses.

There can be difficulty in adequately controlling drug release from the lens over the desired time frame. The drug on the inner surface has a different microenvironment to the outer surface, much of which will be lost to the external ocular tissue. There are also problems associated with patient comfort, acceptability, and compliance. The lenses must not interfere with optical clarity. Many people, particularly the elderly, find wearing lenses uncomfortable and inconvenient. They also cause foreign-body sensations and blurring and may decrease oxygen tension on the corneal surface. Dry eye syndrome and infections due to poor hygiene have been associated with prolonged lens use, which is a further concern. Furthermore, the wearing of contact lenses is contraindicated in many inflammatory conditions, which thus limits their applicability in a wide variety of ocular disorders.

Cul-de-sac inserts are designed to be left in the conjunctival sac and release their drug load over time. One of the first systems utilizing this principle was the wafer-like insoluble implant, Ocusert^® from Alza, commercialized back in 1974 (see also Chapter 1). The system was designed to release pilocarpine at a constant rate for a week, to treat chronic glaucoma. It consisted of an inner layer containing pilocarpine in an alginate gel with di-(ethylhexyl) phthalate as a release enhancer, which was sandwiched between two outer, rate-controlling layers of poy(ethylene-co-vinylacetate) (EVA). Although the system represented very sophisticated drug delivery technology for the time, it nevertheless proved unpopular with patients, who complained of foreign body sensation, as well as difficulties in handling and insertion of the inserts. Approximately, 20% of all patients accidentally removed the device without being aware of the loss.

In spite of the poor patient acceptance of Ocusert, a number of other ocular inserts are almost always being studied. One of the much-copied originators, soluble ocular drug insert (SODI), consists of a small oval wafer of polyacrylamide used to deliver a variety of drugs, including ciprofloxacin and acyclovir. Porcine collagen shields have been designed to promote corneal healing but the use has not been accepted. Hybrid systems have also been developed; for example, liposomes containing cyclosporin A have been incorporated into collagen shields and shown enhanced drug delivery in comparison to both the free drug and liposomally associated drug.

Although not used as a drug delivery carrier, Lacrisert^® is a rod-shaped ocular insert, which is used for treatment of dry eye syndromes. Made of HPC, the insert begins to dissolve within minutes of being inserted in the conjunctival sac. As it slowly dissolves, the inserts soften, stabilizing the tear film.

13.6.3 USE OF MICRO- AND NANOPARTICULATE DRUG DELIVERY SYSTEMS

Micro- and nanoparticulate DDS have been described in detail in Chapter 5 for parenteral delivery. They comprise an extensive array of drug carriers in the micro- and nanometer size range, including microparticles, nanoparticles, liposomes, niosomes, and dendrimers. Many of these systems have also been investigated for ophthalmic delivery, both for topical delivery as described here, as well as for intravitreal injection to the BOTE (described in Section 13.7).

Micro- and nanoparticulate DDS can offer a number of advantages for ophthalmic drug delivery, including the potential to provide (1) enhanced retention time at the corneal surface, which can facilitate drug diffusion across the corneal barrier and access to the posterior compartment; (2) high drug loading, thereby ensuring a high concentration gradient that will drive transcorneal drug permeation via passive diffusion; (3) the incorporation of mucoadhesive polymers, which also enhance corneal retention; (4) the incorporation of targeting moieties to target specific areas of the eye; (5) the protection of labile drug molecules within the carrier construct; and (6) sustained release of the drug from the carrier, thereby providing a prolonged release profile and minimizing the frequency of dosing.

Against these advantages, there are limitations associated with their use. For example, they can cause clouding of the corneal surface and interfere with the visual field. Further problems include the limited drug-loading capacity that is possible in many cases, as well as difficulties in sterilizing the formulations and the limited shelf life of the products.

Piloplex is a nanoparticle formulation comprising poly(methyl methacrylate) (PMMA)—acrylic acid copolymers, loaded with the antiglaucoma drug pilocarpine, and was one of the first DDS formulations to be studied for ophthalmic drug delivery (Mazor et al. 1979). Although the formulation necessitated fewer applications than conventional pilocarpine eyedrops, it was discontinued due to various formulation-related problems, including its nonbiodegradability, local toxicity, and the difficulty of preparing a sterile formulation.

Since then, many other nanoparticle systems have been investigated, with particular emphasis on biodegradable systems. For example, poly-e-caprolactone nanoparticles were shown to improve the efficacy of betaxolol in the treatment of glaucoma, compared to betaxolol delivered in commercial eyedrops. The enhancement was ascribed to two factors: (1) the nanoparticles increased the precorneal retention of the drug by agglomeration and (2) the entrapped drug was in the nonionized form in the oily core of the carrier and thus could diffuse at a faster rate into the cornea. Pilocarpine-loaded poly(DL-lactic-co-glycolic acid) nanoparticles have also shown potential for controlled drug delivery, with an enhanced ocular pharmacological response than that of the free drug (Bourges et al. 2003).

The first application of liposomes in ocular drug delivery involved the use of a liposomal suspension of idoxuridine for the treatment of herpes simplex keratitis in rabbits: the liposomal formulation was found to give more efficient results compared to the aqueous solution (Smolin et al. 1981). Chitosan-coated liposomes (chitosomes) improved precorneal retention due to mucoadhesion and reduced drug metabolism at the precorneal epithelial surface. Liposomal formulations have shown improved efficacy over the free drug in the ophthalmic delivery of ganciclovir and fluconazole solution, in the candidiasis-associated keratitis model in rabbits (Habib et al. 2010). In the rabbits treated with fluconazole solution, 50% healing was observed in 3 weeks, whereas 86.4% healing was observed in rabbits treated with fluconazole-encapsulated liposomes.

13.6.4 PRODRUGS

Prodrugs are pharmacologically inactive derivatives of drug molecules that require a chemical or enzymatic transformation, in order to release the active drug within the body. In most cases, prodrugs are simply chemical derivatives that are one or two steps away from the parent drugs. In ophthalmic delivery, research has focused on producing more lipophilic derivatives of the parent drug, which display enhanced transcorneal permeability characteristics. Many ocular drugs contain hydroxyl or carboxyl groups that can be esterified to lipophilic ester prodrugs, which are subsequently acted on by esterases in the eye, to produce the parent drug.

An example is dipivefrine, a dipivalyl ester prodrug of epinephrine, which has now taken the place of epinephrine in the treatment of glaucoma. The prodrug is 600 times more lipophilic at pH 7.2 than epinephrine, and the penetration rate across the cornea is about 20 times higher. As such, a much smaller dose, with far less side effects, can be administered. Other prodrugs conferring increased corneal permeability have been developed for a wide variety of drugs including (1) the acetyl, propionyl, butyryl, and pivalyl ester prodrugs of timolol, (2) aliphatic acyl ester prodrugs of acyclovir, and (3) a range of dexamethasone esters (Ye et al. 2013).

Ocular prodrugs are also associated with a number of disadvantages, including poor aqueous stability and solubility, and an increased incidence of eye irritation. Also, from the standpoint of regulatory agencies, chemical derivatization of the drug results in the formation of a new chemical entity, which puts a further regulatory burden on development.

13.7 INTRAVITREAL DRUG DELIVERY

Intravitreal drug delivery allows a direct application of the drug near to the retina, thus eliminating the access barriers encountered when using topical administration. As a result, a much higher dose of administered drug can actually reach the target site and yield a more efficacious treatment of posterior eye diseases. Adverse drug effects are also substantially reduced. This route is currently the most acceptable and effective method to treat vitreoretinal disease. Intravitreal drug delivery can be achieved by (1) direct injection and (2) using implantable systems (Wilson et al. 2011). A review of approaches from a joint working party of Association for Research in Vision and Ophthalmology illustrates many of these approaches (Edelhauser et al. 2010).

13.7.1 INTRAVITREAL INJECTIONS

Intravitreal injection uses a small hypodermic needle to penetrate across the sclera, choroid, and retina into the vitreous body (Figure 13.6). Following administration of a small molecule, the drug diffuses from the center of vitreous to the edge of the retina in around 1.5 hours. For small molecules, the diffusion coefficient through the vitreous humor approaches that for water, whereas for larger antibodies and particulates, evidence of steric restriction is noted, particularly if the molecules or carriers are cationic. This reveals the structural characteristics of the vitreous, which consists of a network of hyaluronate holding collagen nanofibrils apart. Drug loss from the center of the vitreous takes place via two routes: (1) an anterior route involving diffusion forward into the anterior chamber, followed by drainage with the aqueous humor and removal to the systemic circulation and (2) a posterior route involving passive permeability and/or active secretion across the retina. All molecules can access the anterior route, whereas only some materials engage with the transport processes. Small drugs, such as the floxacillins, are lost primarily by anterior chamber diffusion and have a short half-life in the vitreous, which can be as little as 2–3 hours. In contrast, we have measured the half-life of 150 kDa fluorescein dextran as 30 days in the rabbit vitreous (Tan et al. 2011). With large molecules and particulate carriers, the influence of synchisis becomes important. In the ageing eye, the vitreous liquefies as the hyaluronans separate from the collagen fibers that collapse toward the back surface of the lens, allowing greater convective clearance forward. In this population, sustained delivery devices may fail to be effective.

Repeated intravitreal injection carries small but significant risks, such as infection, clouding of the vitreous humor, nonclearing vitreous hemorrhage, injury to delicate ocular structures (such as retinal detachment), and endophthalmitis. The procedure also requires specialist administration and may be painful, and the necessity of repeated injections can be highly unpleasant for patients. Drugs given by intravitreal injection include many anti-inflammatory corticosteroids, as well as antibiotics and antivirals. New advances involve the delivery of anti-VGEF drugs and gene therapies; these specific applications are described next.

13.7.1.1 Intravitreal Injections for Anti-VEGF Drugs

VGEF is an endogenous compound that induces angiogenesis, increases vascular permeability, and promotes inflammation. Recently, four new anti-VEGF drugs have been introduced for the management of wet AMD and DME (Table 13.1). The first three drugs listed in Table 13.1 are either monoclonal antibodies (mAb) or antibody fragments (Fab). On injection, these drugs penetrate the RPE, then bind and neutralize VGEF. The fourth drug (pegaptanib) represents a gene therapy approach, described in the next section. Anti-VGEF injections are needed every month/6 weeks, which is a significant health-care burden for this chronic condition.

TABLE 13.1
Antivascular Endothelial Growth Factor Drugs for Wet Age-Related Macular Degeneration

Angiogenesis Inhibitor	Type of Molecule	Dosage
Bevacizumab (Avastin^®)	mAb against VEGF	Once a month
Aflibercept (Eylea^®)	Fab/VEGF receptor hybrid	Once a month
Ranibizumab (Lucentis^®)	Fab	Once a month
Pegaptanib (Macugen^®)	Oligonucleotide aptamer	Every 6 weeks
Abbreviation: VEGF, vascular endothelial growth factor.

13.7.1.2 Intravitreal Injections for Gene Therapy

The use of gene therapy in ophthalmic drug delivery is based on the retinal delivery of specific nucleotide sequences, which include sequences of DNA, RNA, and their modifications. The nucleotide sequences may work by a number of molecular transcription mechanisms, such as (1) induction of gene expression (gene therapy), (2) suppression of translation of target mRNA (antisense oligonu-cleotides), and (3) binding to specific protein targets (aptamers). In this way, the therapeutic problem is corrected at the level of molecular expression, in contrast to alleviation of symptoms as in conventional therapy. A significant problem concerns the difficulty in the administration of gene therapies, which are generally hydrophilic, large molecules, which exhibit suboptimal cellular loading when administered as simple solutions in saline. Progress is relatively slow, and although antisense oligonucleotides in particular have moved as far as proof of concept in small rodent models, access to the retina is easier in rodents than in man, which tends to overstate expectations (Short 2008).

As described in Chapter 16, gene delivery systems can be classified into viral and nonviral vectors. To augment the delivery of both vector types, techniques are used that span the full range of ocular administration technologies and include topical administration, gene gun, electroporation, sonoporation, intrastromal injection, and iontophoresis. Potential viral vectors for ocular delivery include adenovirus, adeno-associated virus (AAV), retrovirus, and lentivirus vectors. The AAV vector has been used to deliver the gene RPE65 to the retina and has restored vision in human patients suffering from Leber’s congenital amaurosis, an autosomal recessive blinding disease (Simonelli et al. 2010). A recent review of the output from current clinical trials has concluded that although AAV approaches are useful, there remains a need for combinatorial approaches, including cotreatment with neuroprotective factors, antiapoptotic agents, and antioxidants (Dalkara and Sahel 2014).

Although viral vectors are more efficient in the delivery of genes, nonviral systems provide a complementary approach, offering easier production, unlimited gene size, and minimized immune reactions. The principal negative aspect of nonviral vectors is their poor transduction efficiency compared to viral vectors. Nonviral vectors include plasmid DNA, dendrimers, lipids, polymers, and nanoparticles (see also Chapter 16).

At the time of writing, there are two ocular gene-based drugs in clinical use, both delivered via intravitreal injection. The first one is Vitravene^® (fomivirsen sodium), a phosphorothioate oligonucleotide for the treatment of CMV infection in acquired immunodeficiency syndrome (AIDS) patients. The second is Macugen^® (pegaptanib sodium), an anti-VEGF aptamer for the treatment of wet AMD (Table 13.1). In addition, two siRNA molecules (bevasiranib and Sirna-027), which modify the activity of VEGF and its receptor (VEGFR-1), are also given by intravitreal injection and have recently entered clinical trials. For a current review on gene therapy in ocular applications, see the article by Solinís and colleagues (2014).

13.7.1.3 Long-Acting Intravitreal Injections

Risks, inconvenience, and other problems associated with intravitreal injections have been outlined earlier. To improve therapy, long-acting intravitreal injections have been developed. These are usually formulated by restricting the solubility of the active material. More advanced systems are described here.

The Verisome^™ delivery technology is dependent on a phase change in the formulation to effect sustained release. Verisome^™-based products can be injected into the vitreous as a liquid, using a standard 30-gauge needle. On injection, the liquid coalesces into a sphere that settles to the bottom of the globe, where it can be directly observed by the physician. The depot is biodegradable and studies have demonstrated, for example, the sustained release of triamcinolone acetate over a 1-year period.

Cortiject is a preservative-free emulsion formulation, composed of an oily carrier with phospholipid as a surfactant, which encapsulates a dexamethasone prodrug. The prodrug is de-esterified at the target site by a retina-specific esterase and activated to dexamethasone. A single intravitreal injection has been shown in clinical studies to provide sustained release over 6–9 months (see the excellent review by Haghjou et al. 2011).

Dispersed systems including liposomes and microparticulates can release their drug payloads gradually and over an extended period of time, with improved safety profiles. For example, liposome-encapsulated amphotericin B produced less toxicity than the commercial amphotericin B solution injected intravitreally. Microspheres formulated from poly(lactide-co-glycolide) (PLGA) loaded with 1 mg triamcinolone acetonide (TA) were injected via the intravitreous route into patients with diffuse DME. The efficacy measured by resolution of the macular edema was superior to conventional TA 4 mg injections. Intravitreal injection of liposomal tacrolimus was highly effective in suppressing the process of experimental autoimmune uveoretinitis in a rat model, without any side effects on retinal function or systemic cellular immunity (Zhang et al. 2010). Genentech and SurModics have collaborated on an investigation of biodegradable microspheres loaded with the anti-VGEF agent, ranibizumab. Preliminary data show that ranibizumab-loaded microparticles can deliver the agent over a period of approximately 4–6 months.

Larger particles can provide a longer period of treatment, but progress in this area will carry a risk of poor patient acceptance if the DDS interferes with vision, for example, appearing as “floaters” in the visual field. Using smaller particles may still cause vision hazing, as well as lessening the period of treatment.

13.7.2 INTRAVITREAL IMPLANTS

Another approach to providing a prolonged intravitreal action is to use an intravitreal implant (Kuno and Fujii 2011). As described in Chapter 6, implants usually have a matrix or reservoir architecture and are largely diffusion-driven devices, although erosion is a further important release mechanism for biodegradable systems. Using implants, predictable and reproducible delivery rates are possible for prolonged periods of time.

One of the earliest systems, the Vitrasert^® implant, is a sustained-release intravitreal implant of the antiviral ganciclovir, which is indicated for the treatment of CMVR, in individuals with AIDS. The implant comprises ganciclovir pellets dispersed in a PVA matrix, coated with a discontinuous film of the hydrophobic polymer poly(ethylene-co-vinylacetate) (EVA). The entire assembly is further coated with PVA, to which a suture tab made of PVA is attached, so that the implant can be sutured into place after insertion into the vitreous cavity. The drug is released gradually over a 5- to 8-month period (Dhillon et al. 1998).

Many intravitreal implants have been developed for the sustained release of anti-inflammatory corticosteroids. For example, Retisert^® delivers fluocinolone acetonide (FA) over a 30-month period for the treatment uveitis and other retinal diseases. The implant, about the size of a grain of rice, comprises an FA core, encased in a silicone elastomer cup. A single- or double-release orifice is covered by a PVA membrane to control drug release. The entire device is attached to a PVA suture tab that again anchors the implant inside the eye.

FA is also delivered via intravitreal implant using the Iluvien^® implant, which has recently been granted FDA approval for the treatment of DME. The implant, a tiny cylindrical polyimide tube, is so small that it can be injected directly into the vitreous cavity with a proprietary 25-gauge needle, in an office-based procedure. This is in contrast to the larger-sized Retisert^®, which requires surgical implantation in the operating room. Controlled release of FA from Iluvien^® is possible for up to 36 months. As the device is nonbiodegradable, it remains in the vitreous cavity even after the drug payload has expended. With chronic dosing, it is thus possible that many empty devices may accumulate in the vitreous of a patient over time.

The I-vation intravitreal implant was designed to release TA for up to 36 months. Made of titanium, the device has a corkscrew design and is coated with TA and the rate-limiting, nonbiodegradable polymers, PMMA, and EVA copolymer. The helical coil shape is designed to both maximize the surface area for drug release and also facilitate tissue attachment: once screwed into the eye, the implant is anchored to the sclera, which keeps it out of the visual field. Due to major complications, including raised IOP and cataract development, the Phase 2b trials were terminated.

Nonbiodegradable implants require either surgical removal once the drug is depleted, or in some cases the empty implant remains in the eye. As an alternative, biodegradable implants can be used. A recent FDA-approved example is the Ozurdex^® intravitreal implant, which delivers the corticosteroid dexamethasone using the Novadur^™ solid polymer delivery technology, based on biodegradable PLGA. Ozurdex^® is preloaded into a single-use, specially designed applicator to facilitate injection of the rod-shaped implant directly into the vitreous cavity. The PLGA matrix slowly degrades in vivo to lactic acid and glycolic acid, releasing dexamethasone over a 6-month period. PLGA is also used for the manufacture of a rod-shaped biodegradable implant for the delivery of brimonidine, an alpha agonist. Brimonidine causes the release of various neurotrophins in vivo, which have the potential to prevent apoptosis of photoreceptors and/or RPE. A brimonidine-loaded PLGA intravitreal implant, dose 400 μg, is now in clinical trials.

The PRINT^™ (Particle Replication in Non-Wetting Templates) technology under development by Envisia is an interesting idea using microtemplates that are filled with polymer–drug mixture to produce a range of shapes, including rods, disks, and doughnut shaped particles. The technology appears to be suitable for encapsulating large biologics, and the company has reported preliminary data for the ophthalmic delivery of prostaglandin analogs in animals.

13.7.2.1 Implanted Cells

Encapsulated cells, or stem cells, implanted subretinally, are under investigation at several centers (Tao 2006). Neurotech Pharmaceuticals has described the delivery of proteins by genetically engineered human retinal pigment epithelial cells entrapped in polymeric microcapsules, or hollow fibers, and implanted directly to the back of the eye. The implanted cells subsequently produce therapeutic proteins for the posterior segment. Renexus^® (NT-501) consists of a semipermeable hollow fiber membrane capsule, surrounding a scaffold of strands of poly(ethylene terephthalate) yarn, which can be loaded with cells. The cells (NTC-200) are a human cell line of RPE, genetically modified to secrete recombinant human ciliary neurotrophic factor (CNTF). The device is surgically implanted in the vitreous through a scleral incision and is anchored by a suture at one end of the device. Once in place, the NTC-200 RPE cells continuously produces CNTF at the site of implantation, allowing for its controlled, continuous delivery. The concept exploits the function of the semipermeable membrane, which facilitates oxygen and nutrient influx into the cells and the efflux of CNTF, while simultaneously protecting the implanted cells from host cellular immunologic attack.

The safety and tolerability of subretinally implanted RPE (derived from human embryonic stem cells), as a cell-replacement therapy for patients with AMD and Stargardt’s disease, was ongoing in early 2015 (Schwartz et al. 2015), but not recruiting patients at the time of writing. In support of this approach, several studies in Yucatan mini-pigs have been reported over the last 2 years, which indicate minimal changes to the retina (e.g., Stefanini et al. 2014).

13.8 TRANSSCLERAL DRUG DELIVERY

Targeting the sclera has received recent attention in the field of ophthalmic drug delivery, particularly in the application of protein and gene delivery. A number of approaches are being investigated.

13.8.1 TRANSSCLERAL INJECTION

Transscleral injections are typically used to deliver a local anesthetic for ophthalmic surgery, or any procedure requiring globe anesthesia and akinesia (Figure 13.6). A retrobulbar injection of anesthetic (retrobulbar block) is an injection of local anesthetic between the inferior and lateral rectus muscles. Subtenons injection (i.e., injection into a fascial sheath of connective tissue between the conjunctiva and episcleral plexus) is increasingly being used as an alternative to retrobulbar in anesthesia, as it is thought to be associated with less risk because a blunt needle is used.

A subconjunctival injection involves an injection of a drug beneath the conjunctiva, bypassing this barrier (Figure 13.6). The drug must then diffuse through the sclera and choroid. This method provides a localized and minimally invasive means of delivery to the posterior eye. The route is being researched for the delivery of bioactive proteins, prostaglandins, and dexamethasone. Recently, a liposomal formulation of the prostaglandin latanoprost (Lipolat) produced a sustained reduction in IOP over 3 months when given by subconjunctival injection in a pilot study in humans (Natarajan et al. 2012).

13.8.2 IONTOPHORESIS AND ELECTROPORATION

Current-assisted drug delivery, which includes iontophoresis and electroporation, has been widely studied in transdermal applications (see Chapter 9, Section 9.3.2) and therefore is of obvious interest in ocular delivery. Iontophoresis is a local, noninvasive drug delivery method that involves the use of low voltages to neutralize the charge on molecules, thereby assisting the movement of weak electrolytes and ions across membranes. By varying the intensity and duration of the electric field, precise amounts of a drug can be delivered, relatively specifically, to local targets. The mechanism of enhancement is thought to proceed through three mechanisms: (1) electric field interaction driving ions through tissue, (2) increase in tissue permeability caused by current flow, and (3) bulk movement of solvent or “electroosmosis” (Behar-Cohen 2012).

Electroporation, in contrast, involves the application of ultrashort high-voltage pulses, which cause a transient opening of the membrane allowing genes and large molecules to travel though the membrane, which then reseals. Plasmid delivery of genes directly to the ciliary muscle of rats has been reported, which then functions as a protein factory for long periods, perhaps several months (Sanharawi et al. 2013).

In ophthalmology, both transscleral and transcorneal iontophoretic drug delivery are possible, but the transscleral route is preferred, as it offers a number of advantages, including (1) a larger surface for transport, (2) enhanced transfer of drugs to the posterior segment, (3) less chance of systemic absorption, and (4) enhanced safety: resistance may cause scarring, so it is more prudent to put the electrode away from the cornea.

The EyeGate^® II Delivery System is an ocular iontophoresis device that uses cathodic iontophoresis with an inert electrode for the ocular delivery of various drugs. The outcomes of clinical trials carried out using the device for the delivery of EGP-437 (a customized dexamethasone phosphate, formulated specifically for iontophoresis), for the treatment of noninfectious uveitis, were reported in 2012 (Behar-Cohen 2012).

13.8.3 MICROELECTROMECHANICAL SYSTEMS

Microelectromechanical systems (MEMs) technology utilizes microfabrication techniques used in the semiconductor industry to produce miniaturized structures, sensors, actuators, and systems. The MEMs can be utilized with miniaturized infusion pumps, to offer very precise control of the volume of drug delivered and the timing of that delivery, even offering the potential for feedback control.

MEMs-enabled implantable drug infusion pumps are refillable, making them amenable to long-term use, in contrast to the sustained-release intravitreal implants described in Section 13.7.2, which suffer from short device lifetime due to the limited drug payloads possible. The pumps being investigated for ophthalmology are driven by electrolysis, a low power-requiring process. The electrochemically induced phase change of water to hydrogen and oxygen gas generates pressure in a drug reservoir, thereby forcing the drug out through a transscleral cannula. The reservoir is implanted in the subconjunctival space and the cannula can be inserted into either the anterior or posterior segment, depending on the delivery requirements.

Such devices are currently at a very early stage of development; a number of prototypes have been described (Saati et al. 2010). For example, Replenish, Inc.™ has developed a small, refillable, implantable ocular drug pump. The device is programmable and can dispense nanoliter-sized doses (a volume sensor gives closed feedback) of drugs every hour, day, or month as needed, before refills. The tiny MicroPump^™ can be “replenished” (hence the name of the technology) using a disposable, proprietary 31-gauge needle tubing kit. Although the device must initially be implanted into the eye, the Replenish device can subsequently last more than 5 years before needing replacement, i.e., much longer than current treatments. A 1-year feasibility trial on the safety profile in beagle dogs was published in 2014 and no inflammatory reactions reported (Gutiérrez-Hernández et al. 2014).

13.8.4 MICRONEEDLE DELIVERY

Microneedles, i.e., individual needles or arrays of needles, in the micrometer size range, have also been widely investigated for transdermal delivery (see also Chapter 9, Section 9.3.3.4), and the technology is now being applied to the ophthalmic route (Patel et al. 2011).

Microneedles can be fabricated out of numerous materials, including metal, polymer, glass, and ceramics, and they can be constructed in a variety of shapes, sizes, and drug-loading modalities. Solid microneedles (or microneedle arrays) can be coated with a drug: once inserted into the eye, the drug coating dissolves from the microneedle support and diffuses into the surrounding tissue, after which the microneedle can be removed. Alternatively, a hollow microneedle can be used to administer liquid formulations, including solutions and suspensions, as well as nanoparticles and microparticles, which also offer the potential of controlled release from the carrier. Biodegradable polymer microneedles can also be used, which do not have to be removed from the eye and can be designed to degrade slowly, thereby providing sustained drug release effects.

The microneedle insertion depth can be adjusted so that they penetrate just a few hundred microns into the sclera, without fully crossing the tissue, making the procedure minimally invasive. Drug released into the sclera then diffuses through the sclera tissue, to access the underlying tissues of the choroid, although retinal exposure is low. Solid microneedles release the drug in the immediate region around the placement site: the sclera then functions as a drug reservoir, for localized drug delivery to the underlying tissues. A hollow microneedle, with a liquid system, means that the drug can potentially flow over a more expanded area.

Increasingly, microneedles are being used to deliver drugs into the suprachoroidal space (SCS), i.e., the space between the sclera and choroid, which goes circumferentially around the eye. Normally, the sclera and the choroid are close together. Pushing fluid into the junction of the tissues creates the SCS, displacing the choroid from its normal position. This route has many attractive features for drug delivery: (1) it can expand and thus accommodate larger volumes of a drug formulation; (2) its proximity to the choroid and retina may provide higher drug concentrations to the chorioretinal tissues, without interfering with the optical pathways; and (3) targeting the SCS decreases exposure of nontarget tissues to the drug. Previous approaches to deliver to the SCS have included using scleral incisions or the use of long cannulas or hypodermic needles. These methods are highly invasive and cannot be performed as a simple office procedure. In contrast, microneedles offer the potential to access the SCS route in a minimally invasive manner. Preliminary results are encouraging. Microneedles, with lengths of 800–1000 μm, have successfully injected micro- and nanoparticles into the SCS, without surgery, in rabbit, pig, and human ex vivo eyes (Chen et al. 2015).

13.9 CONCLUSIONS

In conclusion, the eye continues to present significant challenges to successful drug delivery. Developments are focusing on the need to provide prolonged release of disease modulators, with less risk and easier access than are currently available. In the last few years, the field has been characterized by many exciting new advances, featuring highly sophisticated drug delivery technologies. Such technologies include the use of MEMS, iontophoretic devices, microneedle arrays, nanoparticulate DDS, gene therapies, long-lasting implants, and encapsulated cells. Such innovation and technological expertise offers considerable hope for the future.

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* Age-related macular degeneration is referred to as AMD in the United States and ARMD in much of Europe.