Intraocular Drug DelIvery - part 6 ppsx

The degradation rate of other biodegradable devices such asmicrospheres and intrascleral implants is also controlled in a similar manner.high-Drug Delivery Systems General Overview Sever

weight: 5000) at weight ratios of 80/20 These plugs included 25% GCV The molecular-weight PLA may play a substantial role in the framework of the deviceand restrict the degradation rate of the low-molecular-weight PLA Also the low-molecular-weight PLA may regulate drug release by slowing pore formation duringthe diffusional phase The degradation rate of other biodegradable devices such asmicrospheres and intrascleral implants is also controlled in a similar manner.

high-Drug Delivery Systems

General Overview

Several different intraocular drug delivery systems using biodegradable polymerssuch as microspheres (5–9), intraocular implants (10–13), scleral plugs (3,4,14–22),and intrascleral implants (23) have been developed

Moritera et al (5) first reported an intravitreal drug delivery system using degradable polymer microspheres PLA microspheres containing doxorubicin hydro-chloride (6) and PLGA microspheres containing retinoic acid (7) have been reportedfor the treatment of proliferative vitreoretinopathy (PVR) GCV-loaded PLGAmicrospheres have been developed using a new oil-in-oil emulsion technique withfluorosilicone (8) Interestingly, after intravitreal injection of PLA nanoparticleswith the mean size of 310 nm, nanoparticles transversed the retina and reachedthe retinal pigment epithelium (9) Targeted drug delivery to the retina and retinalpigment epithelium could be feasible using PLA nanoparticles

bio-Surodex1 (Oculex Pharmaceuticals, Inc.) is a PLGA rod containing methasone, which is implanted at cataract surgery for treatment of postsurgicalinflammation (10) In a multicenter, randomized, double-masked, parallel group

dexa-Figure 3 Cumulative release GCV from the scleral plugs of PLA-70,000 and PLA-5000(whose content ratio was 80:20) containing 25% of GCV The values shown are mean
SD.The duration of GCV release was prolonged further compared with the plug made of PLGA(75/25)-121,000 Abbreviations: GCV, ganciclovir; PLA, polylactic acid; PLGA, polyglycolicacid Source: From Ref 4

study, Surodex1safely and effectively suppressed postoperative inflammation afteruncomplicated cataract surgery (11) Posurdex1(Allergan, Inc.), which has a similardesign, is implanted in the vitreous cavity to deliver dexamethasone to the posteriorsegment of the eye Clinical trials for Posurdex1 are ongoing for the treatment ofmacular edema associated with diabetes and other conditions (see Chapter 19) Forthe treatment of PVR, two intravitreal implants have been reported; a PLGA rodcontaining 5-fluorouracil (12) and a multiple drug delivery implant consisted of threecylindrical segments, each of which contained one of the following drugs: 5-fluoro-uridine, triamcinolone, or human recombinant tissue plasminogen activator (13).The scleral plug is a device that is implanted through a sclerotomy at the parsplana; it releases the drug intravitreally (Fig 4) Its shape is similar to that of ametallic scleral plug, which is used temporarily during pars plana vitrectomy Con-trolled release of doxorubicin hydrochloride [adriamycin (ADR)] (15,16), GCV(3,4,17,19,21), fluconazole (18), 5-fluorouracil (20), and tacrolimus (FK506) (22)have been reported.

The intrascleral implant is a device that is implanted in the sclera; it deliversthe drug through the sclera to the intraocular tissues (Fig 5) Transscleral deliverymay be an effective method of achieving therapeutic concentrations of drugs in the pos-terior segment (24–27) The intrascleral implant that incorporated betamethasone phos-phate (BP) successfully delivered the drug to the retina/choroid and vitreous (28) Theconcentration of BP was maintained at a level that should suppress inflammation inthe retina–choroid for more than eight weeks, and did not produce any ocular toxicity.Relative Advantages and Disadvantages of Different Biodegradable Systems

Microspheres can be administered into the vitreous cavity by injection as a sion Although this is an advantage of this system, it can be disadvantageous as

suspen-a lsuspen-arge qususpen-antity of microspheres csuspen-annot be given by intrsuspen-avitresuspen-al injection suspen-andmicrospheres may cause a temporary disturbance in vitreous transparency In

Figure 4 Scleral plug made of biodegradable polymers The plug weighs 8.5 mg and is5.0 mm long Source: From Ref 14

contrast, relatively large amounts of the drug can be loaded into scleral plugs,intrascleral implants, and intravitreal devices without decreasing vitreous transpar-ency Furthermore, scleral plugs can be applied at the sclerotomy sites at the end

of pars plana vitrectomy as an adjunctive therapy Intrascleral implants are less sive than microspheres and scleral plugs, as complications such as endophthalimitis,vitreous hemorrhages, retinal detachment, and potential risks of intraocular systems,are virtually eliminated In intravitreal drug delivery systems such as microspheres andscleral plugs, the drug is released intravitreally, reaches the surface of the retina,and diffuses into the retina–choroid Transvitreal permeation into the retina islimited for relatively large molecules, such as tissue plasminogen activator(70 kDa), because the inner limiting membrane is a barrier to penetration (28) Incontrast, large molecules such as immunoglobulin (150 kDa) have been reported

inva-to penetrate the retina through a transscleral route (26) Accordingly, we speculatethat intrascleral implants may be more useful for site-specific treatment in theretina–choroid and for intraocular delivery of large molecular compounds, such asbioactive protein and antibody, than intravitreal systems

SPECTRUM OF DISEASES FOR WHICH BIODEGRADABLE

SYSTEMS MAY BE USEFUL

All intraocular disorders that require systemic administration or frequent localadministration of the drug may be appropriate for these biodegradable systems.Uveitis is a chronic disorder that requires long-term medical therapy Topical drugtreatment is not effective in the treatment of posterior uveitis because of limitedintraocular penetration Systemic administration of corticosteroid or immuno-suppressive agents may be effective but are associated with systemic side effects.Sustained drug delivery systems may be effective in the treatment of uveitis In

Figure 5 Intrascleral implant made of biodegradable polymers The implant weighs 7 mgand is 0.5 mm thick and 4 mm in diameter Source: From Ref 23

endophthalmitis may be treated with sustained delivery systems of antiviral agents,antibiotics, or antifungal agents Especially, for very chronic inflammation, repeatadministration would likely be necessary even with sustained drug delivery systems.The exudative type of age-related macular degeneration (AMD), which is asso-ciated with choroidal neovascularization, also may be a good target for biodegrad-able drug delivery systems Numerous anti-angiogenic agents have been investigated

in the treatment of AMD In addition, AMD is a chronic disease and it can beexpected that any pharmacological therapy will likely require long-term treatment.Therefore, sustained drug delivery may be beneficial

Recently, macular edema associated with uveitis (29), diabetic retinopathy (30),and central retinal vein occlusion (31,32) has been treated with intravitreal injection

of triamcinolone acetonide Macular edema decreased after treatment but recurredthree to six months after injection A sustained-release steroid delivery system may

be more attractive than a simple injection of triamcinolone as it could reduce or inate the need for multiple intravitreal injections

elim-PVR is a serious complication of retinal detachment surgery Inhibition ofcellular proliferation and postoperative inflammation may reduce the development

of PVR Inhibition of postoperative inflammation would eliminate one of thecomponents of PVR and biodegradable sustained delivery systems that containanti-inflammatory agents may be useful

ANIMAL MODELS USED TO TEST BIODEGRADABLE

DRUG DELIVERY SYSTEMS

Experimental Cytomegalovirus Retinitis

Experimental cytomegalovirus retinitis was induced by intravitreal injection

of human cytomegalovirus (HCMV) solution (21) HCMV AD169 was grown onhuman fetal lung fibroblast monolayers HCMV AD169 supernatant was collectedand injected onto confluent monolayers of Hs68 cells HCMV-infected cells were har-vested, and their culture medium was collected Eyes of pigmented rabbits were inocu-lated with 0.1 mL (5 106

pfu/mL) HCMV supernatant The eyes were examined byophthalmoscopy at one, two, three, and four weeks after HCMV inoculation Poster-ior segment disease was graded on a 0þ to 4þ scale of increasing severity The retinaland choroidal diseases were scored as follows: 0þ, no abnormalities; 1þ, focal whiteretinal infiltrates; 2þ, focal-to-geographic retinal infiltrates and vascular engorge-ment; 3þ, severe retinal infiltrates, vascular engorgement, and hemorrhage; and4þ, all the foregoing, plus retinal detachment and necrosis

Experimental Uveitis

A relatively severe, nonspecific experimental uveitis model was created according

to a modification of a previously published protocol (33,34) Pigmented rabbits wereinjected subcutaneously with 10 mg of Mycobacterium tuberculosis H37RA antigensuspended in 0.5 mL of mineral oil One week later, a second injection of the sameamount of subcutaneous antigen was given A microparticulate suspension of

M tuberculosis H37RA antigen was prepared by ultrasonicating a suspension of crudeextract in sterile balanced salt solution Fifty micrograms of antigen suspended in0.1 mL of balanced salt solution was injected into the vitreous cavity (first challenge)

complete compression of the vitreous Homologous fibroblasts from Tenon’s capsulewere cultured Seven days after the second gas injection, 1 105

cultured fibroblasts wereinjected over the medullary wings The animals were then placed immediately in a supineposition for one hour to allow the cells to settle on the vascularized retina Funduschanges were observed for four weeks by indirect ophthalmoscopy The fundus findingswere graded as follows: stage 1, normal retina or wrinkling of the medullary wing; stage 2,pucker formation; and stage 3, traction retinal detachment (5)

RESULTS OF EFFICACY STUDIES

Scleral Plugs Containing GCV for Experimental

Cytomegalovirus Retinitis

Scleral plugs were prepared by dissolving PLA with an average molecular weight of70,000 and 5000 (PLA-70,000 and PLA-5000, respectively) whose content ratio was80:20 and 25% of GCV in acetic acid

Scleral plugs were prepared by dissolving PLA and GCV in acetic acid in theratio 3:1 The PLA used was a blend of two molecular weight ranges, 80% had anaverage molecular weight of 70,000 (PLA-70,000) and 20% had an average molecularweight of 5000 (PLA-5000)

The resultant solution was lyophilized to obtain a homogeneous cake The cakethen was compressed into a scleral plug on a hot plate In a rabbit study the scleralplug containing GCV was found to maintain GCV concentrations in the vitreous

in a therapeutic range adequate to treat HCMV retinitis for more than 200 days(4) The 20 eyes of 20 pigmented rabbits that were inoculated with HCMV weredivided into two groups One week after HCMV inoculation, the control group(n¼ 10) received no treatment In the treatment group (n ¼ 10), a scleral plug contain-ing GCV was implanted at the pars plana (21)

In the control eyes, whitish retinal exudates developed three days after HCMVinoculation and increased gradually until three weeks after inoculation Thereafterthe chorioretinitis decreased until four weeks after injection In the treated group,scores for vitreoretinal lesions were significantly lower than those in the controlgroup at three weeks after HCMV inoculation (Fig 6)

Sustained release of GCV into the vitreous cavity with biodegradable scleralplugs was thus effective for the treatment of experimentally induced HCMV retinitis

in rabbits

Scleral Plugs Containing Tacrolimus (FK506) for Experimental UveitisScleral plugs were prepared by dissolving a bioerodible polymer (99%) and FK506(1%) in 1,4-Dioxane We used poly(Dl-lactide-co-glycolide), with a weight-averagedmolecular weight of 63,000, whose copolymer ratio of DL-lactide to glycolide was50:50 (22) In in vitro tests, the scleral plug released FK506 for more than 35 days.

Figure 6 Clinical disease grading (A) HCMV-inoculated rabbit eyes (control) (B) Treatedeyes with scleral plug containing ganciclovir in HCMV-inoculated rabbit eyes P < 0.01,unpaired t-test Abbreviation: HCMV, human cytomegalovirus Source: From Ref 21

those in the control eyes.

Together, the results show that biodegradable scleral plugs containing FK506are highly effective in suppressing the inflammation of experimental uveitis in therabbit

Scleral Plugs Containing ADR for Experimental PVR

Scleral plugs have been tested in a rabbit model of PVR For these experiments, scleralplugs composed of 99% PLA (average molecular weight of 20,000) and 1% of ADRwere prepared (16) The scleral plug released ADR over five weeks in vitro (Fig 8).Experimental PVR was induced in 22 eyes of 22 pigmented rabbits as describedabove At the time of fibroblast intravitreal injection, the treatment group (n¼ 11)received scleral plugs containing ADR and the control groups (n¼ 11) received notreatment Fundus changes were observed by indirect ophthalmoscopy

The scleral plug decreased the incidence of traction retinal detachment from100% to 64% at 28 days after implantation (Fig 9) The differences in traction retinaldetachment rate between control and treatment groups were significant (P¼ 0.002,two-way ANOVA)

PHARMACOKINETIC AND PHARMACODYNAMIC STUDIES

Scleral Plugs Containing GCV

For in vivo release studies, scleral plugs prepared from blends of PLA-70,000and PLA-5000 at weight ratios of 80/20 and 25% of GCV were used The scleralplugs containing GCV were implanted in pigmented rabbits Animals were killed

at days 1 and 3 and at weeks 1, 2, 3, 4, 6, 8, 10, 12, 14, 16, 18, 20, and 24 after tation, and the eyes were enucleated Five rabbits were used at each time point.The intravitreal GCV concentration was determined by high-performance liquidchromatography (HPLC)

implan-The scleral plugs maintained a constant vitreous GCV concentration withinthe ED50 range (0.1–2.75 mg/mL) for six months without any sudden burst(Fig 10) (4) However, further studies may be needed to evaluate effective GCVconcentrations clinically, as the ED50s are values determined in various conditions

in vitro

Scleral Plugs Containing ADR

To evaluate scleral plug ADR vitreous pharmacokinetics, 1% ADR-loaded PLAscleral plugs with a weight-averaged molecular weight of 20,000 were used (16).Pigmented rabbits underwent vitrectomy, and a scleral plug was implanted at thepars plana Vitreous fluid (0.2 mL) was aspirated through the pars plana with a30-gauge needle from the center of the vitreous cavity Samples of vitreous humor

Figure 7 Clinical disease grading (A) Anterior chamber cell grade (B) Anterior chamberflare grade (C) Fundus opacity grade (mean
SEM, P < 0.001, a Mann–Whitney U nonpara-metric test) Source: From Ref 22

were collected at days 1, 3, 7, 14, 21, and 28 after implantation The concentrations

of ADR in the vitreous humor were determined by HPLC

The vitreous humor ADR concentrations are shown in Figure 11 ADRwas maintained between 0.27
0.06 and 0.76
0.38 ng/mL between day 1 and day

7 and from a level 3.72
0.57 to 8.07
0.76 ng/mL between day 14 and day 21

Intrascleral Implants Containing Betamethasone Phosphate

Intrascleral implants were prepared with 25% betamethasone phophate (BP) and75% PLA with a weight-averaged molecular weight of 20,000 (23) Each intrascl-eral implant weighed approximately 7 mg and was 0.5 mm thick and 4 mm in dia-meter The in vitro BP release from the implant was evaluated The cumulativerelease of BP from the intrascleral implants is plotted in Figure 12 The data showbiphasic release profiles, with an initial burst and a second stage An initial burst(35%) was observed during the first day, and then BP was gradually released over

Figure 8 Profiles of in vitro release of adriamycin from the implant The values are shown asmean
SD Abbreviation: ADR, adriamycin Source: From Ref 16

Figure 13 shows the profile of in vivo release of BP from the implant at thesclera The profile was obtained by estimating the percentage of BP remaining versusthe initial content in the implant In contrast with the in vitro release profile, noinitial burst was observed In addition, more than 80% of BP was released at 28 days.

Figure 9 Effect of scleral plug containing adriamycin on experimental proliferative retinopathy The plugs significantly reduced the incidence of traction retinal detachment(P¼ 0.002) Abbreviation: PVR, prolitrative vitreoretinopathy Source: From Ref 16

vitreo-Figure 10 GCV concentrations in the vitreous after implantation of the scleral plugprepared from the blend of PLA-70,000 and PLA-5000 with a ratio of 80/20 The valuesare shown as mean
SD The shaded area indicates the ED50range of GCV for CMV replica-tion Abbreviations: CMV, cytomegalo virus; GCV, ganciclovir; PLA, ploylactic acid; PLGA,polyglycolic acid Source: From Ref 4.

Figure 11 ADR concentrations in the vitreous after implantation of the scleral plug ing ADR The values are shown as mean
SD Abbreviation: ADR, adriamycin Source: FromRef 16

contain-The BP concentrations in the vitreous and the retina–choroid after tion are shown in Figure 14 The level of BP in the retina–choroid was significantlyhigher than in the vitreous at all times Both in the vitreous and in the retina–choroid, maximum concentrations were observed at two weeks after implantation.Thereafter, the levels of BP gradually decreased The BP concentrations in thevitreous and retina–choroid remained within the concentration range capable ofsuppressing inflammatory responses (0.15–4.0 mg/mL) for more than eight weeks(35–40) In the aqueous humor, BP was below the detection limit during theobservation period.

implanta-Figure 12 Profiles of in vitro release of BP from the implant The values are shown asmean
SD Abbreviation: BP, betamethasone phosphate Source: From Ref 23

Figure 13 Profiles of in vivo release of BP from the implant The values are shown asmean
SD; Abbreviation: BP, betamethasone phosphate Source: From Ref 23

SUMMARY AND FUTURE HORIZONS

In this chapter, we have described a variety of biodegradable polymeric devices, andhave presented data on two systems, scleral plugs and intrascleral implants, in moredetail The scleral plugs are illustrative of intravitreal drug delivery systems in whichthe released drug diffuses in the vitreous and reaches the retina The intrascleralimplant is a transscleral drug delivery system in which the released drug diffuses intothe retina–choroid A drug with low molecular weight can easily diffuse into intra-ocular tissues after intravitreal administration The ocular penetration of macro-molecules such as antibodies is, however, generally poor Especially in intravitrealdelivery of the drug, macromolecules cannot penetrate through the internal limitingmembrane of the retina (28) In contrast, macromolecules can reach the retina bytransscleral delivery (25,26) Therefore, drug delivery systems may be selectedaccording to drug characteristics or to target specific disorders

The pathogenesis of several ocular disorders has been recently revealed by usingmolecular biological techniques Experimentally, specific agents such as genes, anti-sense oligonucleotide therapy, antibodies, and growth factors have been reported to

be effective for the treatment of ocular diseases However, efficient delivery systems ofthose agents to target tissues are not available at present Intraocular drug delivery sys-tems using biodegradable polymers represent one promising method for that purpose

Figure 14 BP concentrations in the vitreous and the retina–choroid after intrascleral tation of the BP-loaded implant The values are shown as mean
SD Abbreviation: BP,betamethasone phosphate Source: From Ref 23

implan-biodegradable scleral implant for the treatment of cytomegalovirus retinitis J ControlRelease 2000; 68:263–271.

5 Moritera T, Ogura Y, Yoshimura N, et al Biodegradable microspheres containingadriamycin in the treatment of proliferative vitreoretinopathy Invest Ophthalmol VisSci 1992; 33:3125–3130

6 Moritera T, Ogura Y, Honda Y, Wada R, Hyon SH, Ikada Y Microspheres of degradable polymers as a drug-delivery system in the vitreous Invest Ophthalmol VisSci 1991; 32:1785–1790

bio-7 Giordano GG, Refojo MF, Arroyo MH Sustained delivery of retinoic acid frommicrospheres of biodegradable polymer in PVR Invest Ophthalmol Vis Sci 1993; 34:2743–2751

8 Veloso AA Jr, Zhu Q, Herrero-Vanrell R, Refojo MF Ganciclovir-loaded polymermicrospheres in rabbit eyes inoculated with human cytomegalovirus Invest OphthalmolVis Sci 1997; 38:665–675

9 Bourges JL, Gautier SE, Delie F, et al Ocular drug delivery targeting the retina andretinal pigment epithelium using polylactide nanoparticles Invest Ophthalmol Vis Sci2003; 44:3562–3569

10 Tan DT, Chee SP, Lim L, Lim AS Randomized clinical trial of a new dexamethasonedelivery system (Surodex) for treatment of post-cataract surgery inflammation Ophthal-mology 1999; 106:223–231

11 Chang DF, Garcia IH, Hunkeler JD, Minas T Phase II results of an intraocular steroiddelivery system for cataract surgery Ophthalmology 1999; 106:1172–1177

12 Rubsamen PE, Davis PA, Hernandez E, O’Grady GE, Cousins SW Prevention ofexperimental proliferative vitreoretinopathy with a biodegradable intravitreal implantfor the sustained release of fluorouracil Arch Ophthalmol 1994; 112:407–413

13 Zhou T, Lewis H, Foster RE, Schwendeman SP Development of a multiple-drug ery implant for intraocular management of proliferative vitreoretinopathy J ControlRelease 1998; 55:281–295

deliv-14 Kimura H, Ogura Y, Hashizoe M, Nishiwaki H, Honda Y, Ikada Y A new vitreal drugdelivery system using an implantable biodegradable polymeric device Invest OphthalmolVis Sci 1994; 35:2815–2819

15 Hashizoe M, Ogura Y, Kimura H, et al Scleral plug of biodegradable polymers forcontrolled drug release in the vitreous Arch Ophthalmol 1994; 112:1380–1384

16 Hashizoe M, Ogura Y, Takanashi T, Kunou N, Honda Y, Ikada Y Implantable degradable polymeric device in the treatment of experimental proliferative vitreoretino-pathy Curr Eye Res 1995; 14:473–477

bio-17 Hashizoe M, Ogura Y, Takanashi T, Kunou N, Honda Y, Ikada Y Biodegradable meric device for sustained intravitreal release of ganciclovir in rabbits Curr Eye Res1997; 16:633–639

poly-18 Miyamoto H, Ogura Y, Hashizoe M, Kunou N, Honda Y, Ikada Y Biodegradablescleral implant for intravitreal controlled release of fluconazole Curr Eye Res 1997;16:930–935

19 Yasukawa T, Kimura H, Kunou N, et al Biodegradable scleral implant for intravitrealcontrolled release of ganciclovir Graefes Arch Clin Exp Ophthalmol 2000; 238:186–190

20 Yasukawa T, Kimura H, Tabata Y, Ogura Y Biodegradable scleral plugs for retinal drug delivery Adv Drug Deliv Rev 2001; 52:25–36

vitreo-21 Sakurai E, Matsuda Y, Ozeki H, Kunou N, Nakajima K, Ogura Y Scleral plug of degradable polymers containing ganciclovir for experimental cytomegalovirus retinitis.Invest Ophthalmol Vis Sci 2001; 42:2043–2048

22 Sakurai E, Nozaki M, Okabe K, Kunou N, Kimura H, Ogura Y Scleral plug of degradable polymers containing tacrolimus (FK506) for experimental uveitis InvestOphthalmol Vis Sci 2003; 44:4845–4852

32 Ip MS, Kumar KS Intravitreous triamcinolone acetonide as treatment for macularedema from central retinal vein occlusion Arch Ophthalmol 2002; 120:1217–1219.

33 Cheng CK, Berger AS, Pearson PA, Ashton P, Jaffe GJ Intravitreal sustained-releasedexamethasone device in the treatment of experimental uveitis Ophthalmology 1998;105:46–56

34 Jaffe GJ, Yang CS, Wang XC, Cousins SW, Gallemore RP, Ashton P Intravitreal tained-release cyclosporine in the treatment of experimental uveitis Ophthalmology1998; 105:46–56

sus-35 Arya SK, Wong-Staal F, Gallo RC Dexamethasone-mediated inhibition of human T cellgrowth factor and gamma-interferon messenger RNA J Immunol 1984; 133:272–276

36 Culpepper JA, Lee F Regulation of IL 3 expression by glucocorticoids in cloned murine

inter-39 Knudsen PJ, Dinarello CA, Strom TB Glucocorticoids inhibit transcriptional and transcriptional expression of interleukin 1 in U937 cells J Immunol 1987; 139:4129–4134

post-40 Lee SW, Tsou AP, Chan H, et al Glucocorticoids selectively inhibit the transcription ofthe interleukin 1b gene and decrease the stability of interleukin 1b mRNA Immunology1988; 85:1204–1208

As discussed in other chapters, drugs can be delivered locally to the retina and roid by intravitreous injection, or intravitreous biodegradable or nonbiodegradablesustained delivery devices However, with these methods, potential complicationssuch as retinal detachment, posterior dislocation, endophthalmitis, vitreous hemor-rhage, and cataract formation are significant Further, polymer implants easilyencase drugs with molecular weight less than about 1 kDa, but not antibodies againstgrowth factors and cytokines, which are much larger Even with potential advances inpolymer technology that may accommodate larger molecules, a fundamental prob-lem persists The internal limiting membrane of the retina prevents diffusion of sub-stances larger than about 4.5 nm in molecular radius (1,2), i.e., molecules (depending

cho-on their shape) larger than 40–70 kDa cannot diffuse into the retina from the vitreous,whereas antibodies (IgG) are about 150 kDa in size

We and others have shown that transscleral delivery may be a viable modality

of delivering drugs to the posterior segment (3–6) The sclera has a large and sible surface area, and a high degree of hydration that renders it conducive to water-soluble substances It is also hypocellular and thus has few proteolytic enzymes orprotein-binding sites that can degrade or sequester drugs The fact that scleral per-meability does not appreciably decline with age (4) is serendipitous for the treatment

acces-of chronic diseases such as diabetic retinopathy and age-related macular tion, which affect older persons

degenera-SCLERAL ANATOMY

The sclera is composed of collagen fibrils embedded in a glycosaminoglycan (GAG)matrix Scleral collagen is predominantly type I (7) Collagen types III, V, and VI,VIII, and XII are also found in human sclera (8–12), while the lamina cribrosa

193

collagen (13).

The collagen fibrils are quite heterogeneous in diameter (25–300 nm) and woven into bundles of 500–600 nm diameter (14) On the external surface of the humansclera they are arranged in a reticular configuration with diameters of 80–140 nm,while the internal fibrils are arranged in an irregular rhombic pattern (15) Collagenbundles have a macroperiodicity of 35–75 nm and a microperiodicity of about 11 nm.Elastic fibers and fibroblast processes are interposed between these bundles.There are regional differences in the orientation of scleral collagen fibrils In theequatorial region, collagen fibrils are heterogeneous in diameter (25–300 nm) andare arranged in lamellar bundles with random orientations

inter-The microscopic architecture of bovine sclera is similar inter-The inner collagenlayers demonstrate criss-crossed laminae while outer layers are more heterogeneousand larger in diameter (16) The axial periodicity of the fibrils as revealed by atomicforce microscopy is 67 nm (17)

There is spatial variation of GAG composition within the sclera (18) illary sclera is rich in dermatan sulfate The post-equatorial region is rich in chon-droitin sulfate, while the equatorial sclera contains higher amounts of hyaluronicacid In the thin myopic sclera, there is a reduced concentration of GAGs (19) Diffu-sion across sclera can occur through perivascular spaces, the aqueous media of thegel-like mucopolysaccharides, and across the scleral matrix itself

Peripap-There is topographic variation in the density of scleral emissaries, with the poral sclera being most free of these vascular conduits (20) The landscape of scleralthickness is quite varied The mean thickness of human sclera is 0.53 mm at thelimbus, 0.39 mm at the equator, and 0.9–1.0 mm near the optic nerve (21) However,even these figures are subject to great variation, with equatorial thickness frequentlybelow 0.1 mm These factors would be important considerations in the placement of

tem-a trtem-anssclertem-al drug delivery device With tem-a metem-an tottem-al surftem-ace tem-aretem-a of 16.3 cm2, thesclera is an inviting portal for intraocular drug delivery

IN VITRO STUDIES OF SCLERAL PERMEABILITY

The ease with which a molecule diffuses across a tissue can be characterized by itspermeability, measured in cm/sec This value can be conceived of as the velocity

of the molecule across the tissue

Several investigators have used a two-chamber diffusion cell apparatus tocharacterize in vitro scleral permeability (3–5,22–25) to radioactively—or fluores-cently—labeled compounds

The common element of most such studies is the dissection and isolation ofscleral tissue, followed by placement of the sclera between two chambers represent-ing the episcleral surface and the uveal surface One chamber contains the labeledcompound, and the other chamber is sampled periodically after steady-state flux isattained Some studies have used an apparatus where the chambers are constantlystirred This may yield a higher apparent permeability by utilizing an unmixed depot

on the tissue surface where static boundary layers may exist However the impact ofboundary layers on high molecular weight tracers is not expected to be significant,especially when temperature fluctuations are minimal (26,27)

Bovine sclera is permeable to molecules as large as albumin (69 kDa) (3).Human sclera is permeable to 70 kDa dextran (4), while rabbit sclera is permeable

by electron microscopy and demonstrating similar permeability characteristics offresh sclera and stored sclera.

Scleral permeability declines exponentially with increasing molecular weight(3–5,28) However, molecular radius is a much better predictor of scleral permeabi-lity than molecular weight (5) For example, the globular protein albumin (69 kDa;3.62 nm) has higher scleral permeability than the linear 40 kDa dextran (4.5 nm) Log-linear regression analysis demonstrated that molecular radius was a better predictor

of permeability (r2¼ 0.87, P ¼ 0.001) than molecular weight (r2¼ 0.31, P ¼ 0.16) In

an ideal aqueous medium the Stokes–Einstein equation predicts that permeabilitydeclines as a linear function of molecular radius However, in porous diffusion through

a fiber matrix such as the sclera, permeability declines roughly exponentially withmolecular radius (29,30), as observed in our experiments

We found that rabbit sclera was more permeable to bovine serum albumin andIgG than to dextrans of comparable molecular weight This disparity may stem fromdifferential binding of proteins and dextrans to collagen fibers in the hypocellularsclera (30) It is also interesting that proteins, despite having greater numbers ofnegative charges than dextrans, diffuse faster through sclera This suggests that

Table 1 The Permeability of Sclera to Tracers of Varying Molecular Weight and MolecularRadius

Tracer

Molecularweight (Da)

Molecularradius (nm)

Permeability coefficient(
106cm/sec) (mean SD)Sodium fluorescein 376 0.5 84.5 16.1