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- Section XV. Ophthalmology
Chapter 66. Ocular Pharmacology
Overview
This chapter focuses on specific pharmacodynamic, pharmacokinetic, and drug delivery issues
relevant to ocular therapy and imparted by the unique anatomy and function of this sensory organ,
introduced at the outset of this chapter. Many of the pharmacological agents discussed here have
been discussed in earlier chapters. Autonomic agents have several uses in ophthalmology, including
diagnostic evaluation of anisocoria and myasthenia gravis, as adjunctive therapy in laser and
incisional surgeries, and in the treatment of glaucoma. These agents are discussed in detail in
Chapters 6: Neurotransmission: The Autonomic and Somatic Motor Nervous Systems, 7:
Muscarinic Receptor Agonists and Antagonists, 8: Anticholinesterase Agents, 9: Agents Acting at
the Neuromuscular Junction and Autonomic Ganglia, and 10: Catecholamines, Sympathomimetic
Drugs, and Adrenergic Receptor Antagonists. The antimicrobial agents employed for chemotherapy
of orbital cellulitis, conjunctivitis, keratitis, endophthalmitis, retinitis, and uveitis also are discussed
in Chapters 43: Antimicrobial Agents: General Considerations, 44: Antimicrobial Agents:
Sulfonamides, Trimethoprim-Sulfamethoxazole, Quinolones, and Agents for Urinary Tract
Infections, 45: Antimicrobial Agents: Penicillins, Cephalosporins, and Other -Lactam Antibiotics,
46: Antimicrobial Agents: The Aminoglycosides, 47: Antimicrobial Agents: Protein Synthesis
Inhibitors and Miscellaneous Antibacterial Agents, 48: Antimicrobial Agents: Drugs Used in the
Chemotherapy of Tuberculosis, Mycobacterium avium Complex Disease, and Leprosy, 49:
Antimicrobial Agents: Antifungal Agents, and 50: Antimicrobial Agents: Antiviral Agents
(Nonretroviral). The vitamins and trace elements used in adjunctive eye therapy are discussed in
Chapters 63: Water-Soluble Vitamins: The Vitamin B Complex and Ascorbic Acid and 64: Fat-
Soluble Vitamins: Vitamins A, K, and E, and immunomodulatory agents important in treating
vitreoretinopathy, retinitis, and uveitis are discussed in Chapter 53: Immunomodulators:
Immunosuppressive Agents, Tolerogens, and Immunostimulants. Also included in this chapter are
the wetting agents and tear substitutes used to treat dry eye syndrome, as well as drugs and osmotic
agents affecting ocular electrolyte metabolism (see also Chapter 29: Diuretics). The chapter
concludes with a prospectus on the future of ocular therapeutics, including gene transfer,
immunomodulation, molecular- and cellular-based therapies including inhibitors of protein kinase C
for diabetic retinopathy, and neuroprotection.
Ocular Pharmacology: Introduction
History
Records from Mesopotamia (ca. 3000–4000 B.C.) reveal that mysticism—combined with vegetable,
animal, and mineral matter—was used to treat spirits and devils causing eye disease. During the
classical Greek era (ca. 460–375 B.C.) when Hippocrates revolutionized the therapeutics of disease,
several hundred remedies were described for afflictions of the eye. Galen and Susruta categorized
eye diseases on an anatomical basis and applied medicinal as well as surgical remedies advocated
by Hippocrates (see Duke-Elder, 1962; Albert and Edwards, 1996).
With this empirical approach to treat disease, ophthalmic therapeutics took root from remedies
- discovered for systemic diseases. For instance, silver nitrate was used medicinally in the early
seventeenth century. Credé later instituted the use of silver nitrate in newborns as prophylaxis
against neonatal conjunctivitis, a potentially blinding condition, which during his time was
primarily caused by Neisseria gonorrhoeae. In the nineteenth century, numerous organic substances
were isolated from plants and introduced to treat eye diseases. The belladonna alkaloids were used
as poisons, for asthmatic therapy, and for cosmetic effect; hyoscyamus and belladonna were used to
treat iritis in the early 1800s. Atropine was isolated and used therapeutically in the eye in 1832. In
1875, pilocarpine was isolated; the therapeutic effect of lowering intraocular pressure was
recognized in 1877, providing the basis for a safe and effective treatment of glaucoma that is
stillbreak efficacious.
Overview of Ocular Anatomy, Physiology, and Biochemistry
The eye is a specialized sensory organ that is relatively secluded from systemic access by the blood-
retinal, blood-aqueous, and blood-vitreous barriers. Because of this anatomical isolation, the eye
offers a unique, organ-specific pharmacological laboratory to study, for example, the autonomic
nervous system and effects of inflammation and infectious diseases. No other organ in the body is
so readily accessible or as visible for observation; however, the eye also presents some unique
opportunities as well as challenges for drug delivery (see Robinson, 1993).
Extraocular Structures
The eye is protected by the eyelids and by the orbit, a bony cavity of the skull that has multiple
fissures and foramina that conduct nerves, muscles, and vessels (Figure 66–1). In the orbit,
connective (i.e., Tenon's capsule) and adipose tissues and six extraocular muscles support and align
the eyes for vision. The area behind the eye (or globe) is called the retrobulbar region.
Understanding ocular and orbital anatomy is important for safe periocular drug delivery, including
subconjunctival, sub-Tenon's, and retrobulbar injections. The eyelids serve several functions.
Foremost, their dense sensory innervation and eyelashes protect the eye from mechanical and
chemical injuries. Blinking, a coordinated movement of the orbicularis oculi, levator palpebrae, and
Müller's muscles, serves to distribute tears over the cornea and conjunctiva. In human beings, the
average blink rate is 15 to 20 times per minute. The external surface of the eyelids is covered by a
thin layer of skin; the internal surface is lined with the palpebral portion of the conjunctiva, which is
a vascularized mucous membrane continuous with the bulbar conjunctiva. At the reflection of the
palpebral and bulbar conjunctiva is a space called the fornix, located superiorly and inferiorly
behind the upper and lower lids, respectively. Topical medications usually are placed in the inferior
fornix, also known as the inferior cul-de-sac.
Figure 66–1. Anatomy of the Globe in Relationship to the Orbit and
Eyelids. Various routes of administration of anesthesia are demonstrated by the
blue needle pathways. (Adapted from Riordan-Eva and Tabbara, 1992, with
permission.)
- The lacrimal system consists of secretory glandular and excretory ductal elements (Figure 66–2).
The secretory system is composed of the main lacrimal gland, which is located in the temporal outer
portion of the orbit, and accessory glands, also known as the glands of Krause and Wolfring (see
Figure 66–1), located in the conjunctiva. The lacrimal gland is innervated by the autonomic nervous
system (see Table 66–1 and Chapter 6: Neurotransmission: The Autonomic and Somatic Motor
Nervous Systems). The parasympathetic innervation is clinically relevant since a patient may
complain of dry eye symptoms while taking medications with anticholinergic side effects, such as
antidepressants (see Chapter 19: Drugs and the Treatment of Psychiatric Disorders: Depression and
Anxiety Disorders), antihistamines (see Chapter 25: Histamine, Bradykinin, and Their Antagonists),
and drugs used in the management of Parkinson's disease (see Chapter 22: Treatment of Central
Nervous System Degenerative Disorders). Located just posterior to the eyelashes are meibomian
glands (see Figure 66–1), which secrete oils that retard evaporation of the tear film. Abnormalities
in gland function, as in acne rosacea and meibomitis, can greatly affect tear film stability.
Figure 66–2. Anatomy of the Lacrimal System. (Adapted from Riordan-Eva and
Tabbara, 1992, with permission.)
Conceptually, tears constitute a trilaminar lubrication barrier covering the conjunctiva and cornea.
- The anterior layer is composed primarily of lipids secreted by the meibomian glands. The middle
aqueous layer, produced by the main lacrimal gland and accessory lacrimal glands (i.e., Krause and
Wolfring glands), constitutes about 98% of the tear film. Adherent to the corneal epithelium, the
posterior layer is a mixture of mucins produced by goblet cells in the conjunctiva. Tears also
contain nutrients, enzymes, and immunoglobulins to support and protect the cornea.
The tear drainage system starts through small puncta located on the medial aspects of both the upper
and lower eyelids (Figure 66–2). With blinking, tears enter the puncta and continue to drain through
the canaliculi, lacrimal sac, nasolacrimal duct, and then into the nose. The nose is lined by a highly
vascular mucosal epithelium; consequently, topically applied medications that pass through this
nasolacrimal system have direct access to the systemic circulation.
Ocular Structures
The eye is divided into anterior and posterior segments (see Figure 66–3A). Anterior segment
structures include the cornea, limbus, anterior and posterior chambers, trabecular meshwork,
Schlemm's canal, iris, lens, zonules, and ciliary body. The posterior segment comprises the vitreous,
retina, choroid, sclera, and optic nerve.
Figure 66–3. A. Anatomy of the Eye. B. Enlargement of the Anterior Segment
Revealing the Cornea, Angle Structures, Lens, and Ciliary Body. (Adapted from
Riordan-Eva and Tabbara, 1992, with permission.)
- Anterior Segment
The cornea is a transparent and avascular tissue organized into five layers: epithelium, Bowman's
membrane, stroma, Descemet's membrane, and endothelium (see Figure 66–3B).
Representing an important barrier to foreign matter, including drugs, the epithelial layer is
composed of five to six layers of epithelial cells. The basal epithelial cells lie on a basement
membrane that is adjacent to Bowman's membrane, a layer of collagen fibers. Constituting
approximately 90% of the corneal thickness, the stroma, a hydrophilic layer, is uniquely organized
with collagen lamellae synthesized by keratocytes. Beneath the stroma lies Descemet's membrane,
- the basement membrane of the corneal endothelium. Lying most posteriorly, the endothelium is a
monolayer of cells adhering to each other by tight junctions. These cells maintain corneal integrity
by active transport processes and serve as a hydrophobic barrier. Hence, drug absorption across the
cornea necessitates penetrating the trilaminar hydrophobic-hydrophilic-hydrophobic domains of the
various anatomical layers.
At the periphery of the cornea and adjacent to the sclera lies a transitional zone (1 to 2 mm wide)
called the limbus. Limbal structures include the conjunctival epithelium, which contain the stem
cells, Tenon's capsule, episclera, corneoscleral stroma, Schlemm's canal, and trabecular meshwork
(Figure 66–3B). Limbal blood vessels, as well as the tears, provide important nutrients and
immunological defense mechanisms for the cornea. The anterior chamber holds approximately 250
l of aqueous humor. The peripheral anterior chamber angle is formed by the cornea and the iris
root. The trabecular meshwork and canal of Schlemm are located just above the apex of this angle.
The posterior chamber, which holds approximately 50 l of aqueous humor, is defined by the
boundaries of the ciliary body processes, posterior surface of the iris, and lens surface.
Aqueous Humor Dynamics and Regulation of Intraocular Pressure
Aqueous humor is secreted by the ciliary processes and flows from the posterior chamber, through
the pupil, into the anterior chamber, and leaves the eye primarily by the trabecular meshwork and
canal of Schlemm. From the canal of Schlemm, aqueous humor drains into an episcleral venous
plexus and into the systemic circulation. This conventional pathway accounts for 80% to 95% of
aqueous humor outflow and is the main target for cholinergic drugs used in glaucoma therapy.
Another outflow pathway is the uveoscleral route (i.e., fluid flows through the ciliary muscles and
into the suprachoroidal space), which is the target of selective prostanoids (see Chapter 26: Lipid-
Derived Autacoids: Eicosanoids and Platelet-Activating Factor and later in this chapter).
The peripheral anterior chamber is an important anatomical structure for differentiating two forms
of glaucoma: open-angle glaucoma, which is by far the most common form of glaucoma, and angle-
closure glaucoma. Current medical therapy of open-angle glaucoma is aimed at decreasing aqueous
humor production and/or increasing aqueous outflow. The preferred management for angle-closure
glaucoma is surgical iridectomy, either by laser or by incision, but short-term medical management
may be necessary to reduce the acute intraocular pressure elevation and to clear the cornea prior to
laser surgery. As mentioned in other chapters, acute angle-closure glaucoma may be induced rarely
in anatomically predisposed eyes by anticholinergic, sympathomimetic, and antihistaminic agents.
Interestingly, however, individuals with those susceptible angles do not know they have them. As
far as they know, they do not have glaucoma and are not aware of a risk of angle-closure glaucoma.
Yet, drug warning labels do not always specify the type of glaucoma for which this rare risk exists.
Thus, unwarranted concern is raised among patients who have open-angle glaucoma, by far the
most common form of glaucoma in the United States, and who need not be concerned about taking
these drugs. In any event, in anatomically susceptible eyes, anticholinergic, sympathomimetic, and
antihistaminic drugs can lead to partial dilation of the pupil and a change in the vectors of force
between the iris and the lens. The aqueous humor then is prevented from passing through the pupil
from the posterior chamber to the anterior chamber. The result can be an increase in pressure in the
posterior chamber, causing the iris base to be pushed against the angle wall, thereby closing the
filtration angle and markedly elevating the intraocular pressure.
Iris and Pupil
looseness1The iris is the most anterior portion of the uveal tract, which also includes the ciliary
- body and choroid. The anterior surface of the iris is the stroma, a loosely organized structure
containing melanocytes, blood vessels, smooth muscle, and parasympathetic and sympathetic
nerves. Differences in iris color reflect individual variation in the number of melanocytes located in
the stroma. Individual variation may be an important consideration for ocular drug distribution due
to drug-melanin binding (see"Distribution," below). The posterior surface of the iris is a densely
pigmented bilayer of epithelial cells. Anterior to the pigmented epithelium, the dilator smooth
muscle is oriented radially and is innervated by the sympathetic nervous system (see Figure 66–4)
which causes mydriasis (dilation). At the pupillary margin, the sphincter smooth muscle is
organized in a circular band with parasympathetic innervation which, when stimulated, causes
miosis (constriction). The use of pharmacological agents to dilate normal pupils (i.e., for clinical
purposes such as examining the ocular fundus) and to evaluate the pharmacological response of the
pupil (e.g., unequal pupils, or anisocoria, seen in Horner's syndrome or Adie's pupil) is summarized
in Table 66–2. Figure 66–5 provides a flowchart for the diagnostic evaluation of anisocoria.
Figure 66–4. Autonomic Innervation of the Eye by the Sympathetic (a) and
Parasympathetic (b) Systems. (Adapted from Wybar and Kerr Muir, 1984, with
permission.)
Figure 66–5. Anisocoria Evaluation Flowsheet. (Adapted with permission from
Thompson and Pilley, 1976.)
Ciliary Body
- The ciliary body serves two very specialized roles in the eye: secretion of aqueous humor by the
epithelial bilayer and accommodation by the ciliary muscle. The anterior portion of the ciliary body,
called the pars plicata, is composed of 70 to 80 ciliary processes with intricate folds. The posterior
portion is the pars plana. The ciliary muscle is organized into outer longitudinal, middle radial, and
inner circular layers. Coordinated contraction of this smooth muscle apparatus by the
parasympathetic nervous system causes the zonules suspending the lens to relax, allowing the lens
to become more convex and to shift slightly forward. This process, known as accommodation,
permits focusing on near objects and may be pharmacologically blocked by muscarinic cholinergic
antagonists, through the process called cycloplegia. Contraction of the ciliary muscle also puts
traction on the scleral spur and, hence, widens the spaces within the trabecular meshwork. This
latter effect accounts for at least some of the intraocular pressure-lowering effect of both directly
acting and indirectly acting parasympathomimetic drugs.
Lens
The lens, a transparent biconvex structure, is suspended by zonules, specialized fibers emanating
from the ciliary body. The lens is approximately 10 mm in diameter and is enclosed in a capsule.
The bulk of the lens is composed of fibers derived from proliferating lens epithelial cells located
under the anterior portion of the lens capsule. These lens fibers are continuously produced
throughout life.
Posterior Segment
Because of the anatomical and vascular barriers to both local and systemic access, drug delivery to
the eye's posterior pole is particularly challenging.
Sclera
The outermost coat of the eye, the sclera, covers the posterior portion of the globe. The external
surface of the scleral shell is covered by an episcleral vascular coat, by Tenon's capsule, and by the
conjunctiva. The tendons of the six extraocular muscles insert into the superficial scleral collagen
fibers. Numerous blood vessels pierce the sclera through emissaria to supply as well as drain the
choroid, ciliary body, optic nerve, and iris.
Inside the scleral shell, the vascular choroid nourishes the outer retina by a capillary system in the
choriocapillaris. Between the outer retina and the choriocapillaris lies Bruch's membrane and the
retinal pigment epithelium, whose tight junctions provide an outer barrier between the retina and the
choroid. The retinal pigment epithelium serves many functions, including vitamin A metabolism
(see Chapter 64: Fat-Soluble Vitamins: Vitamins A, K, and E), phagocytosis of the rod outer
segments, and multiple transport processes.
Retina
The retina is a thin, transparent, highly organized structure of neurons, glial cells, and blood vessels.
Of all structures within the eye, the neurosensory retina has been the most widely studied (see
Dowling, 1987). The unique organization and biochemistry of the photoreceptors have provided a
superb model for investigating signal transduction mechanisms (see Stryer, 1987). Rhodopsin has
been intensely analyzed at the level of its protein and gene structures (see Khorana, 1992). The
wealth of information about rhodopsin has made it an excellent model for the G protein–coupled
receptors (see Chapter 2: Pharmacodynamics: Mechanisms of Drug Action and the Relationship
- Between Drug Concentration and Effect). Such detailed understanding holds promise for targeted
therapy for some of the hereditary retinal diseases.
Vitreous
The vitreous is a clear medium that makes up about 80% of the eye's volume. It is composed of
99% water bound with collagen type II, hyaluronic acid, and proteoglycans. The vitreous also
contains glucose, ascorbic acid, amino acids, and a number of inorganic salts (see Sebag, 1989).
Optic Nerve
The optic nerve is a myelinated nerve conducting the retinal output to the central nervous system. It
is composed of (1) an intraocular portion, which is visible as the 1.5-mm optic disk in the retina; (2)
an intraorbital portion; (3) an intracanalicular portion; and (4) an intracranial portion. The nerve is
ensheathed in meninges continuous with the brain. At present, pharmacological treatment of some
optic neuropathies is based on management of the underlying disease. For example, optic neuritis
may be treated best with intravenous methylprednisilone (Beck et al., 1992, 1993); glaucomatous
optic neuropathy is medically managed by decreasing intraocular pressure.
Pharmacokinetics and Toxicology of Ocular Therapeutic Agents
Drug Delivery Strategies
Factors that affect the bioavailability of ocular drugs include pH, salt form of the drug, various
structural forms of a given drug, vehicle composition, osmolality, tonicity, and viscosity. Properties
of varying ocular routes of administration are outlined in Table 66–3. A number of delivery systems
have been developed for treating ocular diseases. Most ophthalmic drugs are delivered in aqueous
solutions. For compounds with limited solubility, a suspension form facilitates delivery.
Several formulations prolong the time a drug remains on the surface of the eye. These include gels,
ointments, solid inserts, soft contact lenses, and collagen shields. Prolonging the time in the cul-de-
sac facilitates drug absorption. Ophthalmic gels (e.g., pilocarpine 4% gel) release drugs by diffusion
following erosion of soluble polymers. The polymers used include cellulosic ethers, polyvinyl
alcohol, carbopol, polyacrylamide, polymethylvinyl ether-maleic anhydride, poloxamer 407, and
puronic acid. Ointments usually contain mineral oil and a petrolatum base and are helpful in
delivering antibiotics, cycloplegic drugs, or miotic agents. Solid inserts, such as OCUSERT PILO-20
and PILO-40, provide a zero-order rate of delivery by steady-state diffusion, whereby drug is
released at a more constant rate to the precorneal tear film over a finite period of time rather than as
a bolus. Although membrane-controlled drug delivery has advantages and is effective in some
patients, the inserts have not gained widespread use, likely due to their cost and the fact that patients
often have difficulty placing and retaining a solid insert in the cul-de-sac.
Pharmacokinetics
Classical pharmacokinetic theory based on studies of systemically administered drugs (see Chapter
1: Pharmacokinetics: The Dynamics of Drug Absorption, Distribution, and Elimination) does not
fully apply to all ophthalmic drugs (see Schoenwald, 1993; DeSantis and Patil, 1994). Although
similar principles of absorption, distribution, metabolism, and excretion determine the fate of drug
disposition in the eye, alternative routes of drug administration, in addition to oral and intravenous
routes, introduce other variables in compartmental analysis (see Table 66–3 and Figure 66–6).
- Ophthalmic medications are applied topically using a variety of formulations. Drugs also may be
injected by subconjunctival, sub-Tenon's, and retrobulbar routes (see Figure 66–1 and Table 66–3).
For example, anesthetic agents are administered commonly by injection for surgical procedures and
antibiotics and glucocorticoids also may be injected to enhance their delivery to local tissues. 5-
Fluorouracil, an antimetabolite and antiproliferative agent, may be administered subconjunctivally
to retard the fibroblast proliferation related to scarring after glaucoma surgery. Intraocular (i.e.,
intravitreal) injections of antibiotics are considered in instances of endophthalmitis, an intraocular
infection. The sensitivities of the organisms to the antibiotic and the retinal toxicity threshold may
be nearly the same for some antibiotics; hence, the antibiotic dose injected intravitreally must be
carefully titrated.
Figure 66–6. Possible Absorption Pathways of an Ophthalmic Drug Following
Topical Application to the Eye. Solid black arrows represent the corneal route;
dashed blue arrows represent the conjunctival/scleral route; the black dashed line
represents the nasolacrimal absorption pathway. (Adapted from Chien et al.,
1990, with permission.)
Unlike clinical pharmacokinetic studies on systemic drugs, where data are collected relatively easily
from blood samples, there is significant risk in obtaining tissue and fluid samples from the human
eye. Consequently, animal models are studied to provide pharmacokinetic data on ophthalmic
drugs. Commonly, the rabbit is used for such studies (see McDonald and Shadduck, 1977, for
comparison of toxicity, anatomy, and physiology of human and rabbit ocular systems).
Absorption
After topical instillation of a drug, the rate and extent of absorption are determined by the
following: the time the drug remains in the cul-de-sac and precorneal tear film (also known as the
residence time); elimination by nasolacrimal drainage; drug binding to tear proteins; drug
metabolism by tear and tissue proteins; and diffusion across the cornea and conjunctiva (see Lee,
1993). A drug's residence time may be prolonged by changing its formulation. Nasolacrimal
- drainage contributes to systemic absorption of topically administeredbreak ophthalmic medications.
Absorption from the nasal mucosabreak avoids so-called first-pass metabolism by the liver (see
Chapter 1: Pharmacokinetics: The Dynamics of Drug Absorption, Distribution, and Elimination ),
and consequently significant systemic side effects may be caused by topical medications, especially
when used chronically. Possible absorption pathways of an ophthalmic drug following topical
application to the eye are shown schematically in Figure 66–6.
Transcorneal and transconjunctival/scleral absorption are the desired routes for localized ocular
drug effects. The time period between drug instillation and its appearance in the aqueous humor is
defined as the lag time. The drug concentration gradient between the tear film and the cornea and
conjunctival epithelium provides the driving force for passive diffusion across these tissues. Other
factors that affect a drug's diffusion capacity are the size of the molecule, chemical structure, and
steric configuration. Transcorneal drug penetration is conceptualized as a differential solubility
process; the cornea may be thought of as a trilamellar "fat-water-fat" structure corresponding to the
epithelial, stromal, and endothelial layers. The epithelium and endothelium represent barriers for
hydrophilic substances; the stroma is a barrier for hydrophobic compounds. Hence, a drug with both
hydrophilic and lipophilic properties is best suited for transcorneal absorption.
Drug penetration into the eye is approximately linearly related to its concentration in the tear film.
Certain disease states, such as corneal ulcers and other corneal epithelial defects or stromal keratitis,
also may alter drug penetration. Experimentally, drugs may be screened for their potential clinical
utility by assessing their corneal permeability coefficients. These pharmacokinetic data combined
with the drug's octanol/water partition coefficient (for lipophilic drugs) or distribution coefficient
(for ionizable drugs) yield a parabolic relationship that is a useful parameter for predicting ocular
absorption. Of course, such in vitro studies do not account for other factors that affect corneal
absorption, such as blink rate, dilution by tear flow, nasolacrimal drainage, drug binding to proteins
and tissue, and transconjunctival absorption; hence, these studies have limitations in predicting
ocular drug absorption in vivo.
Distribution
Topically administered drugs may undergo systemic distribution primarily by nasal mucosal
absorption and possibly by local ocular distribution by transcorneal/transconjunctival absorption.
Following transcorneal absorption, the aqueous humor accumulates the drug, which is then
distributed to intraocular structures as well as potentially to the systemic circulation via the
trabecular meshwork pathway (see Figure 66–3B). Melanin binding of certain drugs is an important
factor in some ocular compartments. For example, the mydriatic effect of -adrenergic receptor
agonists is slower in onset in human volunteers with darkly pigmented irides compared to those
with lightly pigmented irides (Obianwu and Rand, 1965). In rabbits, radiolabeled atropine binds
significantly to melanin granules in irides of nonalbino animals (Salazar et al., 1976). This finding
correlates with the fact that atropine's mydriatic effect lasts longer in nonalbino rabbits than in
albino rabbits, and suggests that drug–melanin binding is a potential reservoir for sustained drug
release. Another clinically important consideration for drug–melanin binding involves the retinal
pigment epithelium. In the retinal pigment epithelium, accumulation of chloroquine (see Chapter
40: Drugs Used in the Chemotherapy of Protozoal Infections: Malaria) causes a toxic retinal lesion
known as a "bull's-eye" maculopathy, which is associated with a decrease in visual acuity.
Extraretinal manifestations of chloroquine toxicity include corneal and crystalline lens opacities and
motility disturbances.
- Metabolism
Enzymatic biotransformation of ocular drugs may be significant since local tissues in the eye
express a variety ofenzymes, including esterases, oxidoreductases, lysosomal enzymes, peptidases,
glucuronide and sulfate transferases, glutathione-conjugating enzymes, catechol-O-methyl-
transferase, monoamine oxidase, and corticosteroid -hydroxylase (see Lee, 1992). The esterases
have been of particular interest because of the development of prodrugs for enhanced corneal
permeability; for example, dipivefrin hydrochloride (Mandell et al., 1978) is a prodrug for
epinephrine, and latanoprost is a prodrug for prostaglandin F2 (Stjernschantz and Resul, 1992);
both drugs are used for glaucoma management. Topically applied ocular drugs are eliminated by the
liver and kidney after systemic absorption.
Toxicology
From the compartmental analysis given in Figure 66–6, it is apparent that all ophthalmic
medications are potentially absorbed into the systemic circulation, so undesirable systemic side
effects may occur. Most ophthalmic drugs are delivered locally to the eye, and the potential local
toxic effects are due to hypersensitivity reactions or to direct toxic effects on the cornea,
conjunctiva, periocular skin, and nasal mucosa. Eyedrops and contact lens solutions commonly
contain preservatives such as benzalkonium chloride, chlorobutanol, chelating agents, and
thimerosal for their antimicrobial effectiveness. In particular, benzalkonium chloride may cause a
punctate keratopathy or toxic ulcerative keratopathy (Grant and Schuman, 1993).
Therapeutic and Diagnostic Applications of Drugs in Ophthalmology
Chemotherapy of Microbial Diseases in the Eye
Antibacterial Agents
General Considerations
A number of antibacterial antibiotics have been formulated for topical ocular use (Table 66–4). The
pharmacology, structures, and kinetics of individual drugs have been presented in detail in
preceding chapters. Appropriate selection of antibiotic and route of administration is dependent on
clinical examination and culture/sensitivity results. Specially formulated antibiotics also may be
available for serious eye infections such as corneal ulcers or keratitis and endophthalmitis.
Preparation of fortified solutions requires a pharmacist familiar with sterile preparation of ocular
drugs.
Therapeutic Uses
Infectious diseases of the skin, eyelids, conjunctiva, and lacrimal excretory system are encountered
regularly in clinical practice. Periocular skin infections are divided into preseptal and postseptal or
orbital cellulitis. Depending on the clinical setting (i.e., preceding trauma, sinusitis, age of patient,
relative immunocompromised state), oral or parenteral antibiotics are administered.
Dacryocystitis is an infection of the lacrimal sac. In infants and children, the disease usually is
unilateral and secondary to an obstruction of the nasolacrimal duct. The physician should be aware
of the changing microbiological spectrum for orbital cellulitis, for example, the sharp decline in the
involvement of Haemophilus influenzae after the introduction in 1985 of the H. influenzae vaccine
- (Ambati et al., 2000). In adults, dacryocystitis and canalicular infections may be caused by
Staphylococcus aureus, Streptococcus species, Candida species, and Actinomyces israelii.
Infectious processes of the lids include hordeolum and blepharitis. A hordeolum, or stye, is an
infection of the meibomian, Zeis, or Moll glands at the lid margins. The typical offending bacterium
is S.aureus, and the usual treatment consists of warm compresses and topical antibiotic ointment.
Blepharitis is a common bilateral inflammatory process of the eyelids characterized by irritation
and burning, and it also is usually caused by a Staphylococcus species. Local hygiene is the
mainstay of therapy; topical antibiotics frequently are used, usually in ointment form, particularly
when the disease is accompanied by conjunctivitis and keratitis.
Conjunctivitis is an inflammatory process of the conjunctiva that varies in severity from mild
hyperemia to severe purulent discharge. The more common causes of conjunctivitis include viruses,
allergies, environmental irritants, contact lenses, and chemicals. The less common causes include
other infectious pathogens, immune-mediated reactions, associated systemic diseases, and tumors of
the conjunctiva or eyelid. The more commonly reported infectious agents are adenovirus and herpes
simplex virus, followed by other viral (e.g., enterovirus, coxsackievirus, measles virus, varicella
zoster virus, vaccinia-variola virus) and bacterial sources (e.g., Neisseria species, Streptococcus
pneumoniae, Haemophilus species, S.aureus, Moraxella lacunata, chlamydial species). Rickettsia,
fungi, and parasites, in both cyst and trophozoite form, are rare causes of conjunctivitis. Effective
management is based on selection of an appropriate antibiotic for suspected bacterial pathogens.
Unless an unusual causative organism is suspected, bacterial conjunctivitis is treated empirically
without obtaining a culture.
Keratitis, or corneal ulcer, may occur at any level of the cornea, e.g., epithelium, subepithelium,
stroma, or endothelium. Numerous microbial agents have been isolated, including bacteria, viruses,
fungi, spirochetes, and cysts and trophozoites. In aggressive forms of bacterial keratitis, immediate
empirical and intensive antibiotic therapy is essential to prevent blindness from corneal perforation
and secondary corneal scarring. Results of culture and sensitivity tests should guide the final drug of
choice.
Endophthalmitis is a potentially severe and devastating inflammatory, and usually infectious,
process of the intraocular tissues. When the inflammatory process encompasses the entire globe, it
is called panophthalmitis. Endophthalmitis usually is caused by bacteria, by fungi, or rarely by
spirochetes. The typical case occurs during the early postoperative course (e.g., after cataract,
glaucoma, cornea, or retinal surgery), following trauma, or by endogenous seeding in the
immunocompromised host or intravenous drug user. Prompt therapy usually includes vitrectomy
(i.e., specialized surgical removal of the vitreous) and empirical intravitreal antibiotics to treat
suspected bacterial or fungal microorganisms (see Peyman and Schulman, 1994; Meredith, 1994).
In cases of endogenous seeding, parenteral antibiotics have a role in eliminating the infectious
source. In trauma or in the postoperative setting, however, the efficacy of systemic antibiotics is not
well established.
Antiviral Agents
General Considerations
The various antiviral drugs currently used in ophthalmology are summarized in Table 66–5 (see
Chapter 50: Antimicrobial Agents: Antiviral Agents (Nonretroviral) for additional details about
- these agents).
Therapeutic Uses
The primary indications for the use of antiviral drugs in ophthalmology are viral keratitis (Kaufman,
2000), herpes zoster ophthalmicus (Liesegang, 1999; Chern and Margolis, 1998), and retinitis
(Cassoux et al., 1999; Yoser et al., 1993). There are currently no antiviral agents for the treatment
of viral conjunctivitis caused by adenoviruses, which usually has a self-limited course and typically
is treated by symptomatic relief of irritation.
Viral keratitis, an infection of the cornea that may involve either the epithelium or stroma, is most
commonly caused by herpes simplex type I and varicella zoster viruses. Less common viral
etiologies include herpes simplex II, Epstein-Barr virus, and cytomegalovirus. Topical antiviral
agents are indicated for the treatment of epithelial disease due to herpes simplex infection. When
treating viral keratitis topically, there is a very narrow margin between the therapeutic topical
antiviral activity and the toxic effect on the cornea; hence, patients must be followed very closely.
The role of oral acyclovir and glucocorticoids in herpetic corneal and external eye disease has been
examined in the Herpetic Eye Disease Study (Anonymous, 1996, 1997a, 1998). Topical
glucocorticoids are contraindicated in herpetic epithelial keratitis due to active viral replication. In
contrast, for herpetic disciform keratitis, which predominantly is presumed to involve a cell-
mediated immune reaction, topical glucocorticoids accelerate recovery (Wilhelmus et al., 1994).
For recurrent herpetic stromal keratitis, there is clear benefit from treatment with oral acyclovir in
reducing the risk of recurrence (Moyes et al., 1994; Anonymous, 1998).
Herpes zoster ophthalmicus is a latent reactivation of a varicella zoster infection in the first division
of the trigeminal cranial nerve. Systemic acyclovir is effective in reducing the severity and
complications of herpes zoster ophthalmicus (Cobo et al., 1986). Currently, there are no ophthalmic
preparations of acyclovir approved by the United States Food and Drug Administration (FDA),
although an ophthalmic ointment is available for investigational use.
Viral retinitis may be caused by herpes simplex virus, cytomegalovirus (CMV), adenovirus, and
varicella zoster virus. With the highly active antiretroviral therapy (HAART; see Chapter 51:
Antiretroviral Agents: Antiretroviral Agents), CMV retinitis does not appear to progress when
specific anti-CMV therapy is discontinued, but some patients develop an immune recovery uveitis
(Jacobson et al., 2000; Whitcup, 2000). Treatment usually involves long-term parenteral
administration of antiviral drugs. Intravitreal administration of ganciclovir has been found to be an
effective alternative to the systemic route (Sanborn et al., 1992).
Antifungal Agents
General Considerations
The only currently available ophthalmic antifungal preparation is a polyene, natamycin (NATACYN),
which has the following structure:
Other antifungal agents may be specially preparedbreak for topical, subconjunctival, or intravitreal
routes of administration (see Table 66–6). The pharmacology andbreak structures of available
antifungal agents are given inbreak Chapter 49: Antimicrobial Agents: Antifungal Agents.
- Therapeutic Uses
As with systemic fungal infections, the incidence of ophthalmic fungal infections has risen with the
growing number of immunocompromised hosts. Ophthalmic indications for antifungal medications
include fungal keratitis, scleritis, endophthalmitis, mucormycosis, and canaliculitis (see Behlau and
Baker, 1994). Drug selection is based on identifying the pathogenic fungi and, if available,
sensitivity data.
Antiprotozoal Agents
General Considerations
Parasitic infections involving the eye usually manifest themselves as a form of uveitis, an
inflammatory process of either the anterior or posterior segments, and less commonly as
conjunctivitis, keratitis, and retinitis.
Therapeutic Uses
In the United States, the most commonly encountered protozoal infections include Acanthamoeba
and Toxoplasma gondii. In contact-lens wearers who develop keratitis, physicians should be highly
suspicious of the presence of Acanthamoeba (McCulley et al., 2000). Treatment usually consists of
a combination topical antibiotic, such as polymyxin B sulfate, bacitracin zinc, and neomycin sulfate
(e.g., NEOSPORIN), and sometimes an imidazole (e.g., clotrimazole, miconazole, or ketoconazole). In
the United Kingdom, the aromatic diamidines (i.e., propamine isethionate in both topical aqueous
and ointment forms, BROLENE) have been used successfully to treat this relatively resistant
infectious keratitis (Hargrave et al., 1999). Another treatment for Acanthamoeba is the cationic
antiseptic agent polyhexamethylene biguanide, although this is not an FDA-approved antiprotozoal
agent (Lindquist, 1998).
Toxoplasmosis may present as a posterior (e.g., focal retinochoroiditis, papillitis, vitritis, or retinitis)
or occasionally as an anterior uveitis. Treatment is indicated when inflammatory lesions encroach
upon the macula and threaten central visual acuity. Several regimens have been recommended with
concurrent use of systemic steroids: (1) pyrimethamine, sulfadiazine, and folinic acid; (2)
pyrimethamine, sulfadiazine, clindamycin, and folinic acid; (3) sulfadiazine and clindamycin; (4)
clindamycin; and (5) trimethoprim–sulfamethoxazole with or without clindamycin (see Engstrom et
al., 1991; Opremcak et al., 1992).
Other protozoal infections (e.g., giardiasis, leishmaniasis, and malaria) and helminths are less
common eye pathogens in the United States (see DeFreitas and Dunkel, 1994). Systemic
pharmacological management as well as vitrectomy may be indicated for selected parasitic
infections.
Use of Autonomic Agents in the Eye
General Considerations
General autonomic pharmacology has been discussed extensively in Chapters 6: Neurotransmission:
The Autonomic and Somatic Motor Nervous Systems, 7: Muscarinic Receptor Agonists and
Antagonists, 8: Anticholinesterase Agents, 9: Agents Acting at the Neuromuscular Junction and
Autonomic Ganglia, and 10: Catecholamines, Sympathomimetic Drugs, and Adrenergic Receptor
- Antagonists. The autonomic agents used in ophthalmology as well as the responses (i.e., mydriasis
and cycloplegia) to muscarinic cholinergic antagonists are summarized in Table 66–7.
Therapeutic Uses
Autonomic drugs are used extensively for diagnostic and surgical purposes and for the treatment of
glaucoma, uveitis, and strabismus.
Glaucoma
In the United States, glaucoma is the leading cause of blindness in African Americans and the third
leading cause in Caucasians. Characterized by progressive optic nerve cupping and visual field loss,
glaucoma is responsible for visual impairment of 80,000 Americans, and at least 2 million to 3
million have the disease (see Tielsch, 1993). Risk factors associated with glaucomatous nerve
damage include increased intraocular pressure, positive family history of glaucoma, African-
American heritage, myopia, and hypertension. The production and regulation of aqueous humor
have been discussed in an earlier section of this chapter. Although particularly elevated intraocular
pressures (e.g., greater than 30 mm Hg) usually will lead to optic nerve damage, certain patients'
optic nerves appear to be able to tolerate intraocular pressures in the mid-to-high twenties. These
patients are referred to as ocular hypertensives; a prospective, multicenter study is being conducted
to determine whether or not early medical treatment to lower intraocular pressure will prevent
glaucomatous optic nerve damage. Other patients have progressive glaucomatous optic nerve
damage despite having intraocular pressures in the normal range, and this form of the disease is
sometimes called normal- or low-tension glaucoma. However, at present, the pathophysiological
processes involved in glaucomatous optic nerve damage and the relationship to aqueous humor
dynamics are not understood.
Current medical therapies are targeted to decrease the production of aqueous humor at the ciliary
body and to increase outflow through the trabecular meshwork and uveoscleral pathways. There is
no consensus on the best therapy for glaucoma. Currently, a National Eye Institute–sponsored
clinical trial, the Collaborative Initial Glaucoma Treatment Study (CIGTS), aims to determine
whether it is best to treat patients newly diagnosed with open-angle glaucoma with filtering surgery
or with medication in terms of preservation of visual function and quality of life (Musch et al.,
1999). This study aside, a stepped medical approach depends on the patient's health, age, and ocular
status. Some general principles prevail in patient management: (1) asthma and chronic obstructive
pulmonary emphysema having a bronchospastic component are relative contraindications to the use
of topical -adrenergic receptor antagonists because of the risk of significant side effects from
systemic absorption via the nasolacrimal system; (2) some cardiac dysrhythmias (i.e., bradycardia
and heart block) also are relative contraindications to -adrenergic antagonists for similar reasons;
(3) history of nephrolithiasis, or kidney stones, is sometimes a contraindication for carbonic
anhydrase inhibitors; (4) young patients usually are intolerant of miotic therapy secondary to visual
blurring from induced myopia; therefore, if a miotic agent is needed in a young patient, the
OCUSERT delivery system usually is preferable; (5) direct miotic agents are preferred over
cholinesterase inhibitors in "phakic" patients (i.e., those patients who have their endogenous lens),
since the latter drugs can promote cataract formation; and (6) in patients who have an increased risk
of retinal detachment, miotics should be used with caution, since they have been implicated in
promoting retinal tears in susceptible individuals; such tears are thought to be due to altered forces
at the vitreous base produced by ciliary body contraction induced by the drug.
With these general principles in mind, a stepped medical approach may begin with a -adrenergic
- receptor antagonist, with the main goal of preventing progressive glaucomatous optic-nerve
damage with minimum risk and side effects from either topical or systemic therapy. When there are
medical contraindications to the use of -receptor antagonists other agents, such as latanoprost
(XALATAN), a prostaglandin F2 prodrug, or an 2-adrenergic receptor agonist may be used as first-
line therapy. The chemical structure of latanoprost is shown below.
Second- and third-line agents include topical carbonic anhydrase inhibitors, epinephrine-related
drugs, and miotic agents. Ironically, epinephrine-related drugs may be used concomitantly with a -
adrenergic receptor antagonist. Epinephrine's main intraocular pressure-lowering effect is to
enhance uveoscleral outflow, but it also may alter trabecular meshwork function and ciliary body
blood flow. If combined topical therapy fails to achieve the target intraocular pressure or fails to
halt glaucomatous optic nerve damage, then systemic therapy with carbonic anhydrase inhibitors
(CAIs) is a final medication option before resorting to laser or incisional surgical treatment. Of the
oral preparations available (see Chapter 29: Diuretics), the best tolerated is acetazolamide in
sustained-release capsules, followed by methazolamide. The least well tolerated are acetazolamide
tablets (Lichter et al., 1978). To reduce side effects, topical CAIs have been developed—
dorzolamide hydrochloride (TRUSOPT), and brinzolamide (AZOPT), whose structures are shown
below. These topical CAIs do not reduce the intraocular pressure as much as do the oral agents.
Toxicity of Agents in Treatment of Glaucoma
Ciliary body spasm is a muscarinic cholinergic effect that can lead to induced myopia and a
changing refraction due to iris and ciliary body contraction as the drug effect waxes and wanes
between doses. Headaches can occur from the iris and ciliary body contraction. Epinephrine-related
compounds, effective in intraocular pressure reduction, can cause a vasoconstriction-vasodilation
rebound phenomenon leading to a red eye. Ocular and skin allergies from topical epinephrine,
- related prodrug formulations, and apraclonidine are common. Systemic absorption of epinephrine-
related drugs can have all the side effects found with direct systemic administration. The -
adrenergic antagonists, while effective in intraocular pressure reduction, can produce systemic side
effects readily through direct absorption in the tissues and via the nasolacrimal system. The use of
CAIs systematically may give some patients significant problems with malaise, fatigue, depression,
paresthesias, and nephrolithiasis; the topical CAIs may minimize these relatively common side
effects. These medical strategies for managing glaucoma do help to slow the progression of this
disease, yet there are potential risks from treatment-related side effects, and treatment effects on
quality of life must be recognized.
Uveitis
Inflammation of the uvea, or uveitis, has both infectious and noninfectious causes, and medical
treatment of the underlying cause (if known) is essential in addition to the use of topical therapy.
Cyclopentolate, or sometimes an even longer-acting antimuscarinic agent such as atropine,
frequently is used to prevent posterior synechia formation between the lens and iris margin and to
relieve ciliary muscle spasm that is responsible for much of the pain associated with anterior uveitis.
If posterior synechiae have already formed, an -adrenergic agonist may be used to break the
synechiae by enhancing pupillary dilation. Topical steroids usually are adequate to decrease
inflammation, but sometimes they must be supplemented by systemic steroids.
Strabismus
Strabismus, or ocular misalignment, has numerous causes and may occur at any age. In children,
strabismus may lead to amblyopia (reduced vision). Nonsurgical efforts to treat amblyopia include
occlusion therapy, orthoptics, optical devices, and pharmacological agents. The eyes of children
with hyperopia, or farsightedness, must accommodate to focus distant images. In some cases, the
synkinetic accommodative-convergence response leads to excessive convergence and a manifest
esotropia (turned-in eye). This deviated eye does not develop normal visual acuity and is therefore
amblyopic. In this setting, atropine (1%) instilled in the preferred seeing eye every five days
produces cycloplegia and the inability of this eye to accommodate, thus forcing the child to use the
amblyopic eye. Echothiophate iodide also has been used in the setting of accommodative
strabismus. Accommodation drives the near reflex, the triad of miosis, accommodation, and
convergence. A reversible cholinesterase inhibitor such as echothiophate causes miosis and an
accommodative change in the shape of the lens; hence, the accommodative drive to initiate the near
reflex is reduced, and less convergence will occur.
Surgery and Diagnostic Purposes
For certain surgical procedures and for clinical funduscopic examination, it is desirable to maximize
the view of the retina and lens. Muscarinic cholinergic antagonists and 2-adrenergic agonists
frequently are used singly or in combination for this purpose (see Table 66–7).
Intraoperatively, there are circumstances when miosis is preferred, and two cholinergic agonists are
available for intraocular use, acetylcholine and carbachol. Patients with myasthenia gravis may first
present to an ophthalmologist with complaints of double vision (diplopia) or lid droop (ptosis); the
edrophonium test is helpful in diagnosing these patients (see Chapter 8: Anticholinesterase Agents).
Use of Immunomodulatory Drugs for Ophthalmic Therapy
- Glucocorticoids
Glucocorticoids have an important role in managing ocular inflammatory diseases; their chemistry
and pharmacology are described in Chapter 60: Adrenocorticotropic Hormone; Adrenocortical
Steroids and Their Synthetic Analogs; Inhibitors of the Synthesis and Actions of Adrenocortical
Hormones.
Therapeutic Uses
Because of their antiinflammatory effect, topical corticosteroids are used in managing anterior
uveitis, external eye inflammatory diseases associated with some infections and ocular cicatricial
pemphigoid, and postoperative inflammation following intraocular surgery. After glaucoma
filtering surgery, topical steroids are particularly valuable in delaying the wound-healing process by
decreasing fibroblast infiltration, which reduces the potential scarring of the surgical site. Steroids
are commonly given systemically and by sub-Tenon's capsule injection to manage posterior uveitis.
Parenteral steroids followed by tapering oral doses are the preferred treatment for optic neuritis
(Kaufman et al., 2000; Trobe et al., 1999).
Toxicity of Steroids
Extensive discussion has been directed to the toxic effects to the eyes of topical and systemic
corticosteroids. These include the development of posterior subcapsular cataracts and secondary
infections (see Chapter 60: Adrenocorticotropic Hormone; Adrenocortical Steroids and Their
Synthetic Analogs; Inhibitors of the Synthesis and Actions of Adrenocortical Hormones) and
secondary open-angle glaucoma (Becker and Mills, 1963; Armaly, 1963a, 1963b). There is a
significant increase in potential risk for developing secondary glaucoma when there is a positive
family history of glaucoma. If there is no family history of open-angle glaucoma, only about 5% of
normal individuals respond to topical or long-term systemic steroids with a marked increase in
intraocular pressure. With a positive family history, however, moderate to marked steroid-induced
intraocular pressure elevations may be seen in up to 90% of patients. The pathophysiology of
steroid-induced glaucoma is not fully understood, but there is evidence that the GLCIA gene may be
involved (Stone et al., 1997). Typically, steroid-induced elevation of intraocular pressure is
reversible once administration of the steroid ceases.
Nonsteroidal Antiinflammatory Agents
General Considerations
Nonsteroidal drug therapy for inflammation is discussed in Chapter 27: Analgesic-Antipyretic and
Antiinflammatory Agents and Drugs Employed in the Treatment of Gout. The nonsteroidal
antiinflammatory drugs (NSAIDs) are now being applied to the treatment of ocular disease.
Therapeutic Uses
Currently, there are four topical NSAIDs approved for ocular use: diclofenac (VOLTAREN),
flurbiprofen (OCUFEN), ketorolac (ACULAR), and suprofen (PROFENAL). Diclofenac and flurbiprofen
are discussed in Chapter 27: Analgesic-Antipyretic and Antiinflammatory Agents and Drugs
Employed in the Treatment of Gout; the chemical structures of ketorolac, a pyrrolo-pyrolle
derivative, and suprofen, a phenylalkanoic acid, are shown below:
- Flurbiprofen and suprofen are used to counter unwanted intraoperative miosis during cataract
surgery. Ketorolac is given for seasonal allergic conjunctivitis. Diclofenac is used for postoperative
inflammation. Both ketorolac (Weisz et al., 1999a) and diclofenac (Anonymous, 1997b) have been
found to be effective in treating cystoid macular edema occurring after cataract surgery.
Antihistamines and Mast-Cell Stabilizers
Pheniramine (see Chapter 25: Histamine, Bradykinin, and Their Antagonists) and antazoline, both
H1-receptor antagonists, are formulated in combination with naphazoline, a vasoconstrictor, for
relief of allergic conjunctivitis. The chemical structure of antazoline is:
Newer topical antihistamines include emedastine difumarate (EMADINE), olopatadine
hydrochloride (PATANOL), levocabastine hydrochloride (LIVOSTIN), and ketotifen fumarate
(ZADITOR).
Cromolyn sodium (CROLOM), which prevents the release of histamine and other autacoids from
mast cells (see Chapter 28: Drugs Used in the Treatment of Asthma), has found limited use in
treating conjunctivitis that is thought to be allergen-mediated, such as vernal conjunctivitis.
Iodoxamide tromethamine (ALOMIDE), another mast-cell-stabilizing agent, and pemirolast
(ALAMAST), a mast-cell stabilizer that also has other antiinflammatory effects, also are available for
ophthalmic use.
Immunosuppressive and Antimitotic Agents
General Considerations
The principal application of immunosuppressive and antimitotic agents to ophthalmology relates to
the use of 5-fluorouracil and mitomycin C in corneal and glaucoma surgeries. Certain systemic
diseases with serious vision-threatening ocular manifestations—break such as Behçet's disease,
Wegener's granulomatosis, rheumatoid arthritis, and Reiter's syndrome—require systemic
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