Ocular pharmacokinetic, drug bioavailability, and intraocular drug delivery systems

Rocío Herrero-Vanrell, PhD and Jose A. Cardillo, MD

CHAPTER

9

INTRODUCTION

Successful ophthalmic therapy requires therapeutic concentrations of the active substance on the target site. Depending on the ocular disease the drug must reach a targeted zone where the therapeutic action is completed. For the treatment of ocular surface diseases, such as infection or inflammation, and for anterior-segment diseases, effective concentrations of the drug must be maintained in the cornea and conjunctiva, or in the anterior chamber, respectively. In contrast, to treat chronic ocular diseases as uveitis, macular edema, age-related macular degeneration, retinitis and intraocular malignancy, the active substance must reach the posterior segment of the eye.

Selection of the administration route depends on the location of the target tissue. Topical administration of medication is accepted best by patients. However, these agents present unique and challenging hurdles for drug delivery and only 5% of the administered dose penetrates the eye.1 For this reason, when higher concentrations of drugs, such as antibiotics, corticoids, antiviral agents, and biopharmaceuticals, are required inside the eye, it is necessary to employ intracameral or intravitreous injections.

A potential barrier to adequate drug delivery after eye drop instillation is diffusion through the cornea.2,3 Exploiting the permeability of the sclera, periocular routes may also offer a promising alternative to boost drug penetration and tissue targeting.4–9 Periocular administration includes subconjunctival, sub-Tenon’s capsule, peribulbar, posterior juxtascleral, and retrobulbar injections. In these cases, the drug must cross the sclera to reach the intraocular tissues. However, access of drugs to the tissues in the posterior segment of the eye from periocular injections, as well as from doses administered systemically, is repressed by the blood–retina barrier (BRB). Because any disease affecting the posterior segment of the eye may represent visual impairment and blindness, it is necessary to achieve, as fast as possible, effective drug concentrations that are only possible by intravitreous injections.

This chapter comprises an ocular pharmacokinetic overview and describes new topical and intraocular drug delivery systems aimed to obtain successful ocular therapy.

TOPICAL AND PERIOCULAR

ADMINISTRATION

Corneal, conjunctival, and scleral routes are used for ocular penetration of active substances administered topically. The topical ophthalmic route is the first therapeutic choice because it is noninvasive and tolerance is generally good. However, drug bioavailability to intraocular tissues is low and only about 5% of the dose is able to reach the anterior segment due to the elimination process of the ocular surface.1 Furthermore, the conjunctival and scleral routes avoid the counterflow of the aqueous humor and the lens barrier. The sclera is highly permeable to active substances, even several macromolecules. However, transscleral drug penetration to the interior of the eye is limited by choroidal blood flow and the retinal pigment epithelium (RPE) barrier.2

The cornea comprises three zones: epithelium, stroma, and endothelium. The thickness in humans is 0.5–0.6 mm in its central portion.3 A great number of low-molecular-weight substances are able to reach the aqueous humor through the transcorneal route by passive diffusion. The diffusion rate follows Fick’s law and is conducted by a concentration gradient, conditioned by the hydrophilic or lipidic nature of the different layers of the cornea and by the nature of the drugs administered (hydrophilic/lipophilic balance).4 While epithelium and endothelium possess a lipidic nature, the stroma has hydrophilic properties, being highly permeable for hydrophobic or hydrophilic substances respectively. Some substances can pass through the cornea, by the paracellular route. Nevertheless, the tight junctions of corneal epithelial cells limit passage through the intercellular spaces. Absorption promoters can be used to modify the integrity of the tight junctions, improving drug absorption. However, these agents can produce epithelial damage.

Currently the conjunctiva is considered an attractive route for ocular drug absorption.5 It is a tissue with an endogenous transport machinery to allow penetration of active substances. Furthermore, the conjunctiva surface and pore radius in epithelium are larger than those found in the cornea.

The transscleral route consists of the injection of the drug into a periocular space (subconjunctival, sub-Tenon, peribulbar, posterior juxtascleral, and retrobulbar spaces).6 Subconjunctival injection has demonstrated poorly sustained vitreous and retinal drug levels in animal studies. Transport barriers in the transscleral route have been categorized as static and dynamic barriers.7 Static barriers comprise tissues in which the active substance diffuses to reach the retina. In order to get the retina the drug must diffuse through sclera, Bruch’s membrane, and RPE. The primary dynamic barriers to transscleral drug delivery are the conjunctival lymphatic/blood vessels and the choroids.

INTRAVITREAL ADMINISTRATION

Intravitreal administration is a direct deposit of the drug in the vitreous by injection. Drugs are mainly eliminated from the vitreous by the anterior route. If the active substance has an adequate permeability in the retinal capillaries and in RPE it can be eliminated from the retina through the BRB to the systemic circulation. Drugs can reach the retina from the choroid, although high systemic doses are needed.

OCULAR PHARMACOKINETICS

Most of the ocular pharmacokinetic studies corresponds to topical administration of active substances and they are based on the determination of the drug concentration in the ocular tissues. The number of compartments depends on the physicochemical properties of the active principle and the tissues. From a clinical point of view, the development of pharmacokinetic models with a small number of parameters is of great interest. If the ocular pharmacokinetic study following a topical administration includes noncompartmental treatments, the most difficult parameter to evaluate is distribution volume because it is nec-

essary to know the whole amount of drug able to reach the aqueous humor. In order to determine the distribution volume, the active substance can be administered in the anterior chamber or by topical infusion.

Incorporation of the drug to systemic circulation after topical administration represents a loss in the biophase. Elimination includes any process that implies a loss of drug from the target tissue, such as distribution to adjacent tissues, drainage of aqueous humor, incorporation in the systemic circulation, and biotransformation. Ocular bioavailability is related to drug access to the target side or biophase, and systemic exposure refers to incorporation of the active substance in the general circulation.

If the drug is in the blood stream through the conjunctival blood capillaries after systemic, topical, or periocular administration, the blood–aqueous barrier limits its access to the anterior chamber. The blood–aqueous barrier is composed of the uveal capillary endothelia and ciliary epithelia. The BRB limits the diffusion of the active substance to the retina. BRB is composed of an outer barrier and an inner barrier. The outer barrier is formed by the RPE the inner barrier by the endothelial cells of retinal vessels. Cells in RPE are connected by tight junctions. Overall flux of active substances across the BRB depends on its concentration gradient, its permeability in the BRB, and the BRB surface area. RPE is a tighter barrier than the sclera for hydrophilic and larger molecules. However, for small lipophilic molecules the RPE and sclera behave similarly.

The most important limitations of ocular pharmacokinetics studies are the sampling and the number of animals. For evaluation of intraocular concentrations each sample comes from one different animal.8 This means that a large number of animals are necessary to carry out a complete pharmacokinetic study.

TOPICAL FORMULATIONS

In contrast to intestinal mucosa, the ocular surface is not a favorable place for drug penetration. Just after administration of the ophthalmic formulation several processes are triggered.All phenomena are directed to the elimination of the ophthalmic formulation from the ocular surface. Conventional ophthalmic pharmaceutical forms for topical administration include solutions, suspensions, emulsions, aqueous gels, and ointments. Solutions are employed most frequently because they are easily prepared, filtered, and sterilized. Furthermore, the cost of the elaboration process is not as expensive. Slightly viscous liquid formulations are widely employed and solutions with viscosities around 15–18 mPa/s are well accepted by patients. Suspensions are employed for poorly soluble drugs such as corticosteroids. The main disadvantage of ointments are that they produce blurred vision. For this reason they are almost exclusively used at night.

For the past two decades, one of the main objectives in the ocular field has been the optimization of ophthalmic formulations. Efforts have been directed towards the development of systems that are able to improve the bioavailability of the active substance by increasing the contact time of the formulation with the ocular surface or facilitating the transcorneal penetration of the drug. Among the strategies employed to improve ocular bioavailability are formulations of prodrugs, soft contact lenses, inserts and patches, mucoadhesive polymers, in situ gelification systems, absorption promoters, vesicular and colloidal systems. Prodrugs are usually obtained by ester linkage of the active substance. In this way the ester penetrates the cornea and hydrolysis occurs in the anterior chamber.

Soft contact lenses are prepared from polymers that act as a reservoir for the drug. Once the device is in contact with the cornea, the active substance is released from the system. Soft lenses are deposited in the conjunctival sac, releasing the drug gradually.

Ophthalmic inserts can be elaborated with biodegradable or nonbiodegradable polymers. Collagen, polyacrylamide, and cellulose derivatives such as hydroxypropylmethylcellulose are the polymers most used for biodegradable inserts. Previously, a nonbiodegradable device (Ocusert) has been developed to release pilocarpine.

Bioadhesive polymers are formulations including anionic or cationic polymers whose end groups are able to interact with the sialic acids of mucins. Then, the ocular contact time of formulation is increased. Among the bioadhesive polymers cellulose derivatives (carboxymethylcellulose), polyacrylic acids (carbopol), and hyaluronic acid are used the most frequently.9 Due to their good tolerance related to their ocular surface, these polymers are used as components of artificial tears. The in situ gel systems are liquid formulations that contain polymers in solution. Once the formulation is instilled the solution changes to gel because of the pH, tears, electrolytes, or increase in temperature. An example is Gelrite, a polysaccharide that is able to form gel systems at low concentration in the presence of cationic molecules of tears.10

Absorption promoters are substances that improve the permeability of the epithelium by modifying the cellular membrane or opening the tight junctions of the corneal epithelial cells. Some ionic preservatives, such as benzalconium chloride (BAC) or the sodium salt ethylenediamine tetraacetic­ acid (EDTA), have been demonstrated to promote the penetration of several substances.11 However, these substances present adverse effects and can cause damage to ocular surface tissues.

If the active substance is soluble in water it can be included in complex systems such as cyclodextrins (cyclic oligosaccharides). Nanocarriers, such as liposomes, nanoparticles, and dendrimers, are used to enhance ocular drug delivery.12 They can be administered as eye drops, providing an extended residence time on the ocular surface after their instillation. Liposomes are vesicular systems composed of bilayers of phospholipids organized as cellular membranes. Several substances can be included in the vesicles because they have aqueous and lipid zones. Nanoparticles are polymeric systems of 1–1000 nm size in which the active substance is encapsulated or adsorbed.

The polymers usually employed to prepare nanoparticles for the topical ophthalmic route are poly(acrylic acid) derivatives (polyalquilcyanocrylates), albumin, poly-ε-caprolactone, and chitosan. Dendrimers are macromolecules made up of a series of branches around a small molecule, ending in a large number of cationic or anionic groups. Microemulsions are water and oil disperse systems in which a surfactant and a cosurfactant are employed. The size of the inner phase is less than 1 µm. They have a clear dispersion and are thermodynamically stable.

INTRAOCULAR DRUG

DELIVERY SYSTEMS

Pathologies affecting the posterior segment are devastating and the topical treatment is not successful. Therapeutically, success is conditioned for the difficult access to target tissues. When the topical route is employed the cornea, lens, aqueous and BRB yield noneffective drug levels in vitreous, retina, and choroids. Administration of the drug in the vitreous or adjacent structures (transscleral routes) is necessary for prompt effects. While steady-state concentrations are easy to achieve by the oral route, successive intravitreal injections are poorly tolerated, being associated with important side-effects such as vitreous hemorrhage, intraocular pressure increase, optic atrophy, retinal detachment, cataracts, and endophthalmitis. Therefore, the treatment has to be administered promptly and effective concentrations of the active substance maintained as long as possible.

CONVENTIONAL FORMULATIONs

Conventional intravitreal formulations include solutions, vitreous substitutes, and suspensions. However, the main disadvantage of solutions is the impossibility to establish long treatments to maintain drug concentrations in the therapeutic window with a single injection. Moreover intravitreal administration does not eliminate the risk of extraocular infections and infection in the other eye. In the past 20 years substantial effort has been put towards developing new drug delivery systems able to release the drug over a long time, avoiding successive intravitreal injections.

Delivery Drug Retinal for Routes and Models Animal • 2 section

Systems Delivery Drug Intraocular and Bioavailability, Drug Pharmacokinetic,•chapteOcular9

Triamcinolone acetonide

Cs

Figure 9.1  Example of a depot system. The poorly soluble drug remains insoluble after intravitreal injection and is slowly solubilized in the aqueous phase (vitreous). The effect is maintained for several months.

Figure 9.2  Ocular fundus after intravitreal injection of triamcinolone acetonide. (Courtesy of Dr. Jose Manuel Benitez del Castillo.)

DEPOT FORMULATIONs AND

VITREOUS SUBSTITUTES

Currently, depot formulations are employed to treat macular edema and intraocular inflammation. These formulations include poor aqueous solubility drugs such as fluocinolone acetate and triamcinolone acetate that are administered as suspensions. The drug precipitates after injection in the vitreous body (Figure 9.1). Then, the active substance drug is slowly solubilized and the effect lasts for months (Figure 9.2). The main problem in relation to these formulations is the formation of crystalline structures in the proximal areas to the retina. Vitreous substitutes allow the incorporation of lipophilic drugs in vehicles such as silicone oil and aqueous soluble drugs in solutions of hyaluronic acid and cellulosic derivatives.

INTRAOCULAR DRUG DELIVERY SYSTEMS

Intraocular drug delivery systems allow the release of the drug, bypassing the blood–ocular barrier, and are adequate by the intraocular route (Figure 9.3). The main advantage of these preparations is that they can release the drug over a long time with one single administration, having the same effect as multiple injections. Depending on their size, intraocular drug delivery devices must be implanted through a relatively large surgical incision or through a smaller tissue perforation. These pharmaceutical systems are of great interest in the treatment of posteriorsegment diseases and they can be prepared from biodegradable or nonbiodegradable polymers. Biodegradable polymers have the advan-

Figure 9.3  Intraocular devices. Adapted from Yasukawa T, Ogura Y, Tabata Y, et al. Drug delivery systems for vitreoretinal diseases. Prog Retin Eye Res 2004;23:253–281.

Active substance

Figure 9.4  Example of reservoir and matrix systems. Reservoir structures include a pellet of the active substance surrounded by a polymeric membrane. Matrix structures are formed by a polymeric network in which the drug is dispersed.

tage of disappearing from the site of action after releasing the drug. The majority of intraocular devices are prepared from nonbiodegradable materials and they are able to release controlled amounts of drugs for months. Nonbiodegradable polymers include silicone, polyvinyl alcohol (PVA) and ethylvinylacetate (EVA). Ocular devices can be classified into reservoir and matrix systems (Figure 9.4). Reservoir structures include a pellet of the active substance surrounded by a polymeric membrane. Matrix structures are a polymeric network in which the drug is dispersed. If the size is larger than 1 mm they are called implants.

NONBIODEGRADABLE IMPLANTS

Examples of nonbiodegradable implants are the Vitrasert and Retisert implants (Bausch & Lomb). Both are reservoir systems. Vitrasert, a reservoir implant loaded with ganciclovir, was approved by the Food and Drug Administration in 1996. The release of the drug is controlled by the EVA and PVA polymers. PVA is a permeable polymer and regulates the release rate of the drug. EVA is non permeable. Once in contact with aqueous fluids the active substance is dissolved and a saturated solution is created in the drug pellet. Then, the drug is released out of the device by diffusion.

As long as the solution in the pellet is saturated with the active substance, the release rate is constant. Vitrasert has been effective in controlling the progression of cytomegalovirus retinitis associated with acquired immunodeficiency syndrome (AIDS) for 8 months. However, this implant has been associated with endophthalmitis and an increased rate of retinal detachment.13 Retisert (3 × 2 × 5 mm) is employed for the treatment of chronic noninfectious uveitis of the posterior segment. It is composed of PVA and silicone laminate. The poorly soluble drug (fluocinolone acetonide) is released over a 3-year period.14,15 The

side-effects of this implant are cataract and increased intraocular pressure.

Iluvien (Alimera Sciences and pSivida) is a tube-shaped device (3 mm in length and 0.37 mm in diameter). Its action lasts between 18 and 36 months.16 Recently a nonbiodegradable intrascleral implant prepared with a copolymer of ethylenvinylacetate and betamethasone has been developed.16 The device is deposited in the external zone of the sclera and it is connected with the vitreous through a tube of reduced dimensions. The device released the drug for at least 3 months. The reservoir can hold up to 100 mg of the active substance, and can be recharged. In this case it is recommended to introduce an agent to inhibit cellular growth to avoid cellular blockage of the tube. Implants containing genetically modified human RPE cells have been developed (Neurotech). The device NT 501 (6 mm long) is placed in the pars plana through a small incision. The cells included in the device are able to secrete ciliary neurotrophic factor (CNTF). It can be employed for glaucoma, choroidal neovascularization (CNV) and uveitis.17

The main disadvantage of nonbiodegradable systems is that they have to be implanted and replaced once the drug is released. Implantation requires a surgical procedure.

INTRAOCULAR BIODEGRADABLE DRUG DELIVERY SYSTEMS

Biodegradable polymers are widely employed in the elaboration of controlled drug delivery systems. These devices release the drug while the polymer is being degraded in the target site. Finally, the device disappears avoiding the need of surgery. These materials have been used to prepare implants, liposomes, and injectable particles (nanoparticles and microparticles). Liposomes are biocompatible and biodegradable vesicles. They can be prepared with natural lipids as phospholipids. Hydrophilic and lipophilic active substances can be encapsulated in the aqueous zone or the lipid walls respectively. The liposomes are taken by phagocytic cells, allowing intracellular drug delivery. The surface of the liposomes can be modified to promote preferential binding (i.e., neovascular vessels).

The administration of liposomes by the intraocular route has been the subject of several studies. Liposomes loaded with a prodrug of ganciclovir inhibited cytomegalovirus retinitis in rabbits.18 These vesicles have been loaded with oligonucleotides, protecting them from nucleases.19 Nevertheless, administration of liposomes presents some disadvantages, such as a diminution in visual acuity due to intraocular clouding after intravitreal injection. These side-effects have been described as disappearing after 14–21 days of administration.

Liposomes have been dispersed in thermo-sensible gels of poloxamers to maintain the activity of oligonucleotides and enhance their intracellular penetration.20 Currently, a liposomal formulation of verteporfin is in clinical use. Visudyne (Novartis Pharmaceuticals) is employed for the treatment of age-related macular degeneration and choroidal neovascularization. Liposomes of verteporfin are administered by intra­ venous infusion, causing an occlusion of the targeted vessels after its activation through a nonthermal red laser applied to the retina.21,22 Photrex (Miravant Medical Technologies) contains rostaporfin.23

Among the biodegradable polymers employed to prepare intraocular devices poly(lactic) acid and poly (lactic-co-glycolic) acid (PLGA) are the most popular. They are biodegradable and disappear from the injection site after drug delivery. The polymer chain is broken by hydrolysis in the system. The final products, lactic and glycolic acid, are metabolized to carbon dioxide and water mediated by Krebs cycle. These polymers have been employed to obtain implants, scleral plugs, pellets, discs, films, and rods.13 These implants can be located in different ocular areas such as anterior chamber, peribulbar spaces, scleral surface, or in the vitreous cavity. The size of these devices ranges between 5 mm length and 0.9 mm diameter for scleral plugs; 6 mm length and 0.9 mm diameter for rods; 0.5 mm width and 4 mm diameter for intrascleral discs; and 7 mm length and 0.8 mm diameter for bio­ degradable implants. Scleral plugs require minimal sclerectomy for implantation.

To date, these devices have not been applied clinically due to potential induction of hypotony and wound-healing delay caused during polymer degradation.24 Surodex and Ozurdex (Allergan, USA) are used in clinical practice. They are prepared from PLGA and dexamethasone­ (60 g for Surodex and 700 g for Ozurdex). The effects can last for days (surodex) to weeks (Ozurdex) after their implantation. Surodex is indicated for the treatment of postoperative inflammation filtering surgery on eyes with glaucoma. Ozurdex is currently under inves­ tigation for the treatment of macular edema related to retinal vein occlusion, diabetic macular edema, and uveitis. A dexamethasone posterior-segment drug delivery system (DDS) with the same com­ position of Ozurdex has recently been tested releasing 350 or 700 g of dexamethasone.The implant is inserted into the eye through a small pars plana insertion.25

The toxicity and biocompatibility of the PLGA implants have been evaluated after implantation. In all cases low signs or absence of inflammation have been reported. If inflammation is reported it disappears in several weeks. There is no electoretinograph alteration and no evidence of histopathological change in the retina as a consequence of implantation.26 Injectable particles are classified by size in microparticles (1–1000 m) and nanoparticles (1–1000 nm). By physical structure, the microparticles are classified as microcapsules and microspheres. Microcapsules have a reservoir structure while in microspheres the drug is dispersed through the polymeric matrix. The release kinetic of the active substance from the polymeric matrix depends on the physicochemical properties of the drug, the characteristics of the polymer, percentage of loaded drug, and the surface area.

The great advantage of the particulate systems is that they can be administered as a conventional intraocular injection. Local administration of microand nanoparticles has been carried out by intravitreal27 and periocular injections.28 Intraocular administration of microparticles is carried out as a conventional suspension. The needle is chosen depending on the size of the microparticles. Needles most frequently employed are 25–30 G for sizes 1–106 m and 18 G for particle sizes up to 500 m.27 PLGA microparticles have been prepared with different drugs, such as Adriamycin, 5-fluorouracil (5-FU), and retinoic acid for proliferative retinopathy, dexamethasone for uveitis, aciclovir for herpes infection, ganciclovir for cytomegalovirus retinitis, neurotrophic factors for neuroprotection, and inhibitor of protein kinase C (PKC412) for choroidal neovascularization (Figure 9.5).29,30 PLGA microspheres loaded with triamcinolone have already been injected in humans. Preliminary results have been reported with good tolerance after microparticle injection (Figure 9.6).31 The vehicle employed to suspend particles was isotonic phosphate-buffered saline.

Figure 9.5  Poly(lactic-co-glycolic) acid (PLGA) microspheres loaded with ganciclovir for intravitreal administration. Size 212–300 m

(90 g ganciclovir/mg microspheres). PLGA inherent viscosity

0.39 dl/g.

Delivery Drug Retinal for Routes and Models Animal • 2 section

Systems Delivery Drug Intraocular and Bioavailability, Drug Pharmacokinetic,•chapteOcular9

A B

C D

E F

Figure 9.6  (A) Aspect of the microspheres immediately after injection in human eye and (B) 30 days postinjection (note that the visual axis is not obstructed by the system). In detail, the aspect of the microsphere in the vitreous cavity of the same patient (C) 24 hours and (D) 30 days postinjection injection. In high magnification, the spheres (E) 24 hours and (F) 30 days postinjection injection.