Figure 11.1 Eye drops demonstrated as a primary route for ocular drug delivery. (Courtesy of Robert Myles, CRA.)
Small-animal model systems, such as the mouse or rat, have the unique advantage of a well-defined genotype with defined disease processes. These small animals are also less expensive and easier to maintain. While small animals are ideal for studying a drug’s role in the disease, the pharmacokinetics of such small eyes is quite different from those of larger animals. Since the thickness of the sclera, distance for diffusion, and vitreous and lens dimensions are very different from in humans, interpreting the pharmacokinetics from small-animal studies directly to humans is limited.
Larger animals that are commonly employed to study pharmacokinetics in preclinical human studies include the rabbit, primate, pig, cat, or dog. These animal models give a more accurate representation of drug delivery pharmacokinetics that would apply to delivery in the human eye. The rabbit model is commonly employed for such studies. Rabbits are less expensive, easy to handle, and have a relatively large eye. Limitations with the rabbit model include thinner sclera than in humans, a larger lens that alters vitreous diffusional kinetics compared to humans, absence of a macula, and a merangiotic retinal vasculature (essentially an avascular retina).28
The pig eye is slightly smaller than a human eye, but has some unique advantages that directly apply to the human eye. For example, pigs have a scleral thickness that is very similar to that in humans.23 The pig also has a holangiotic retina (vascular pattern very similar to humans), a cone-dense region (area centralis) simulating a macula, a smaller lens than the rabbit, and retinal pigment epithelium with choroid very similar to that in humans.29 The disadvantage is that pigs are more expensive and more difficult to handle in a lab setting than smaller animals.
Nonhuman primates seem to be the most ideal model system of pharmacokinetics because they have a true macula and anatomy nearly identical to humans; however, there are significant limitations. First, there are the ethical concerns30 of using large-scale nonhuman primate studies for pharmacokinetics requiring significant animal sacrifice in order to obtain tissue levels. Second, the nonhuman primate sclera is significantly thinner than that in humans.31 And finally, nonhuman primates are very expensive, require special housing and handling, and are more dangerous to handle.
DRUG DELIVERY MODALITIES
TOPICAL DRUG DELIVERY
The primary barrier to topical drug delivery targeting the retina is the diffusional distance that the drug must overcome in order to achieve therapeutic levels in the retina. The classic example of topical drug
delivery for retinal disease is for the treatment of cystoid macular edema (CME). While the center of the macula is the target area of the disease, many times the origin of the pathology is from anterior-seg- ment tissues. Postcataract CME results from inflammation originating during the postoperative period. Inflammatory mediators released from the anterior segment diffuse posteriorly to the fovea, leading to edema and photoreceptor dysfunction.32 It seems natural that topical therapy, with advantageous pharmacokinetics to treat anteriorsegment tissues such as the iris and ciliary body, will lead to resolution of the CME. Therefore, treatment of the CME is achieved primarily by treatment of anterior-segment tissues (indirectly), rather than treating the macula directly. Topical delivery of medications to treat retinal disease could also be effective when the amount of drug required is extremely low. Very potent agents may not require high tissue levels; therefore, the topical route could conceivably be utilized for direct therapy to the macula or optic nerve. Cautious interpretation of data should be considered when examining results of topical therapies for treating macular disease. If pharmacokinetics are done in small-animal models, or in models such as the rabbit with the aforementioned limitations, the data may not be directly applicable to the human eye. Specifically, the thinner sclera and variations in blood flow patterns, globe size, and other parameters of the animal models should be interpreted cautiously to the human condition. Data may be misleading for the translation to posterior-segment tissues of the human eye.
The corneal epithelium, as well as the endothelium, serves as a significant barrier for drug diffusion into the anterior chamber.33 Some of the epithelial barrier is broken down partially by the preservative benzalkonium chloride, thereby providing increased penetration of various compounds through the epithelium.34,35 Drugs may be metabolized or even selectively transported36,37 by cellular mechanisms that affect the overall penetration of drugs via the topical route.38 The degree of lipophilicity of an agent will also affect the penetration through the corneal epithelium.39 The crystalline lens, or in some cases, a pseudophakic lens, also serves as a physical barrier for the bulk diffusion of drugs applied topically, for delivery to the retina.
TRANSSCLERAL DRUG DELIVERY
TSDD offers a relatively safe route for posterior-segment drug delivery.19 This route avoids intraocular manipulation and takes advantage of simple diffusion kinetics through the sclera. Previous work on human scleral permeability19 suggests that smaller, hydrophilic molecules delivered in thinner equatorial regions of the eye22 may be the optimal choice for transscleral systems. Adequate quantitative delivery of macromolecules or less hydrophilic compounds may be limited by suboptimal drug diffusion kinetics to target tissue. Such parameters may limit the clinical feasibility of the transscleral route for such molecules.
The kinetics of TSDD have been defined in vitro, and proposed to offer a safe route for delivery, namely by avoiding intraocular manipulation and taking advantage of simple diffusion kinetics through the sclera.19 Data suggest that smaller, more hydrophilic molecules delivered in thinner equatorial regions of the eye22 may be optimal choices for transscleral systems. Key anatomic scleral parameters have been defined for both the human and the pig model of transscleral delivery.22,23 An important limitation of TSDD may be adequate quantitative delivery of macromolecules or less hydrophilic compounds due to suboptimal drug diffusion kinetics to the target tissue. These physical restrictions limit the feasibility of the transscleral route for select macromolecules. Drug potency of the specific pharmacologic or biologic agent is an important factor for the relevance of the particular methodology of delivery for a given disease process.
There are some unique advantages of TSDD. First, the sclera is similar to the cornea, without the epithelial or endothelial barriers (assuming one delivers a drug under the conjunctival epithelium). The sclera may serve as a barrier for infectious agents to gain access to the inner coats of the eye as endophthalmitis remains a rare, but potentially dangerous, potential complication of intravitreal injections.40 In addition, the sclera may also serve as a repository of drug as well as dispersing the drug
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in various directions by lateral diffusion.41 Similar to the cornea, the size of the molecule as well as the lipophilicity will also affect overall permeability using the transscleral route.42,43,44
SUPRACHOROIDAL DRUG DELIVERY
The suprachoroidal space (SCS) is a potential space (Figure 11.2), limited anteriorly in the region of the scleral spur, and posteriorly by the transscleral connections of the short posterior ciliary vessels to the choroid.45 There are focal, equatorial connections at the vortex ampulae where venous blood exits the globe. Krohn and Bertelsen have examined the detailed anatomy of the SCS46,47 and drainage channels from this space.48 Ophthalmologists and clinicians are well aware of the SCS. The clinical appearance of a choroidal detachment, either serous (fluidfilled) or hemorrhagic (blood-filled), results from a variety of pathologic states, especially following trauma, surgery, and hypotony.
Taking advantage of this space for pharmacologic drug delivery has recently been explored.49,50 Initial studies used smooth metal cannulae to access the SCS while more recent technologies have been developed to optimize the safety and visualization. Disruption of the retinal pigment epithelium may occur with aggressive intervention in the SCS. Therefore, utilization of small, flexible, illuminated cannulae may offer a safer and more effective methodology to access the SCS for drug delivery in humans50 (Figure 11.3).
The pharmacokinetics of the SCS are unique. Basically, suprachoroidal drug delivery could be considered an extension of TSDD, with the choroid and retina as the target tissue. Directly accessing the SCS using various methodologies allows for higher therapeutic levels of pharmacologic agents at the level of the outer choroid. Larger molecules, such as antibodies, proteins, and perhaps lipophilic molecules, would not be required to diffuse across the sclera in order to target uveal tissue. The target tissues, namely the choroid and neurosensory retina, receive higher levels than would be expected from an external transscleral system. Drug levels from the depot site in the SCS would decline based on the diffusional pathways as represented from the drug diffusion schematic (Figure 11.4). Drug would also leave the site through choroidal blood flow, aqueous diffusion pathways (uveoscleral diffusional pathways), and into the orbit and surrounding vasculature. Local drug metabolism may also play a role, with drug and drug breakdown productseventuallydiffusingintothesystemiccirculation.Theoretically, some drug could also diffuse into the optic nerve and nerve sheath tissues into the central nervous system.51
Figure 11.2 A cast of the suprachoroidal space, taken from a pig eye. The cast was formed by injection into the suprachoroidal space (SCS), and digesting away the tissue. Scleral surface designates the outer surface of the SCS. The gap in the cast is the site where the optic nerve exits the globe.
The volume of the SCS, as a potential space for drug delivery, is relatively large, and somewhat expandable (Figure 11.5). Obviously, the space could not expand to the point of interference with visual function, as may occur in pathologic states. Clinicians are well aware of the risk associated with this space, namely the development of acute supra choroidal hemorrhage, a highly undesirable potential complication of
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Figure 11.3 Schematic representation of the microcannula (9) entering the suprachoroidal space (7) through a small anterior sclerotomy (8), and extending toward posterior-segment tissues. Other labeled structures include the cornea (1), sclera (2), iris (3), lens (4), ciliary body (5), and retina (6). (Reproduced from Am J Ophthalmol 2006;142:777–787 by permission of Elsevier.)
Figure 11.4 An example of a drug diffusion modeling schematic, demonstrating drug distribution from a drug patch at postapplication day 4. (Courtesy of Victor Barocas PhD and Ram Balachandran, University of Minnesota.)
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Figure 11.5 Histology of the suprachoroidal space (SCS) in a pig eye. The asterisk (*) indicates the site of separation between the inner sclera and the choroid. In vivo, this space is a potential space. In this specimen, the space has been created by injection of Healon into the SCS.
delivery into the SCS. In order to minimize such complications, accessing the space in a delicate and deliberate manner is essential. All efforts to minimize stress and tension in the tissues of the SCS in order to avoid vessel rupture would be important during surgical maneuvers in this space. Indeed, forceful impact of a cannulae into the optic nerve in the pig model has been shown to result in local inflammation (Figure 11.6).50 In these studies, successful cannulization of the SCS using a novel microcannula technology for delivery of drug to the SCS has been demonstrated to be safe in the pig model with excellent, sustained local tissue levels and low systemic levels (triamcinolone). The technique has been shown to be relatively safe with minimal impact on the choroidal and retinal blood flow as documented with the high-speed confocal scanning laser ophthalmoscopic (cSLO) video angiography, fundus exam, and using wide-field fundus photography, along with shortand long-term histopathologic tissue analysis.
Drug delivery via the SCS using the microcannula technology and methodology may be useful for the treatment of posterior-segment disease, including macular, optic nerve, or even panretinal disease. Proper drug selection and target disease will be critical to determine the clinical role for this route of delivery. Further clinical studies will be necessary to define the role of this technology further in the management of specific posterior-segment disease. Currently, human studies are under way to assess the safety and efficacy in human disease states.
INTRAVITREAL GAS-PHASE NANOPARTICLE DRUG DELIVERY
Intraocular air or gas is commonly deployed in vitreoretinal surgery, due largely to its mechanical properties. Essentially, the intraocular gas bubble serves as a mechanical aid that will tamponade the neurosensory retina, holding the retina in place during reattachment surgery.52
Figure 11.6 Histology demonstrating areas of the posterior pole of an eye in which the cannulae impacted the optic nerve region. Note that there are inflammatory cells and vacuolization of the choroidal tissue, plus inflammation around the vessels outside the globe.
The surface tension of the gas bubble that surrounds a retinal break may prevent the fluid from disrupting the development of a strong chorioretinal adhesion.53 By generating nanoparticles and suspending these particles in the gas phase of the intraocular tamponade, a novel methodology for drug delivery is possible.54 An example of a target disease is proliferative vitreoretinopathy (PVR); however, there are numerous potential applications of this technology. Use of neuroprotectants during retinal detachment surgery, antiproliferative agents to inhibit PVR, antiviral agents or antibiotics for infectious retinitis, or even pneumatic techniques for localized delivery, is also possible.
PVR is the leading cause of retinal redetachment, and requires aggressive surgical means to correct the anatomic abnormalities.55–60 Some have postulated that mechanical intervention (surgical treatment) will be required until we can control the cellular healing processes with pharmacological intervention.56 To date, limited therapeutic options are available to modify the healing response that occurs following retinal detachment.
Pharmacologic management of retinal disorders, such as PVR, requires a measured and reliable methodology of drug delivery that will allow for better management of the healing response. Utilizing drug delivery in the gas phase of surgery, through delivery of specific agents, creates a unique opportunity for addressing the healing response at a critical stage. Intravitreal injections of drugs are becoming one of the more common routes of delivery in the clinical setting, especially with the advent of anti-VEGF agents. Delivering an intraocular gas with suspended nanoparticle drug could combine the mechanical aspects of gas tamponade with therapeutic effects of antiproliferative agents for use in pneumatic retinopexy. These approaches require knowledge of the pharmacokinetics and quantifiable delivery amounts. Intravitreal gas-phase nanoparticle technology would also have applications for treating other retinal disorders during vitrectomy, including infectious retinitis, proliferative disorders such as diabetic retinopathy with traction detachments, and immune modulation such as the treatment of uveitis, or even CME by using aerosolized corticosteroids.
Quantification of the aerosol concentration of a given drug, expressed as the mass of drug in a given volume of air as well as the total mass, represents a key parameter for each possible therapeutic agent. Characterization of these parameters allows for more predictable delivery of drug during the gas delivery phase. Other methodologies are available for the fluid phase of delivery and include intravitreal injec-
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Figure 11.7 Schematic representation of delivering aerosolized nanoparticles in the gas phase of vitrectomy. (A) The flow-through method demonstrates a continuous flow of gas and drug with the exit site filtering out drug before entering room air. (B) The single-fill method shows the globe with aerosolized drug; once full, the sclerotomies are closed. (Reproduced from Zhang G, Feng X, Wabner K, et al. Intraocular nanoparticle drug delivery: a pilot study using an aerosol during pars plana vitrectomy. Invest Ophthalmol Vis Sci 2007;48:5243–5249, with permission; original adapted from National Eye Institute, NEA05.)
tions, implants, biodegradable polymers, or pellets. However, the pharmacokinetics from these methods are unpredictable in gas-filled eyes.
A methodology and pharmacokinetics for the delivery of aerosolized nanoparticles using a tracer dye in the pig model have been characterized.54 There are two distinct methods of aerosol drug deposition (Figure 11.7). First, the flow-through system utilizes an aerosolized drug passing through the eye continuously with entry through the primary sclerotomy and exit through the secondary sclerotomy. The exit site has a filter to prevent drug from entering room air. This method provides maximal drug deposition because it maintains a higher concentration of intraocular drug to drive diffusion into the retinal tissue. The efficiency of delivery is low, because much of the drug will exit the eye, being filtered at the sclerotomy.
The second method of aerosol delivery is the single-fill method. This involves simply filling the eye or simply injecting the eye with a known concentration of drug, and allowing diffusion of the aerosolized nanoparticle suspension into the retinal tissue. This method results in lower tissue levels because the concentration of the drug within the gas decreases as it diffuses into the retinal tissue. The efficiency of delivery is higher because the drug is essentially trapped in the eye, and will eventually reach a steady state with the tissues.
There are three main mechanisms by which aerosol particles can deposit at the retinal surface: inertial impaction, sedimentation and diffusion.61 Inertial impaction and sedimentation are more efficient with larger particle size. Diffusion is more efficient with smaller particle size. Both of these principles of transfer lead to a nonuniform deposition pattern. In experimental studies using either the flow-through or singlefill modes of delivery, there does not appear to be a difference in the mass of drug on either the inflow or outflow sides of the retina at any time point.54 Additionally, evidence suggests that there is greater diffusion into the uveal tissues than into the lens, presumably because the lens is less metabolically active and lacks blood flow. Drug delivery in the gas phase of vitrectomy may increase cataractogenesis, especially dependent upon the pharmacologic agent delivered. Overall, drug would be more selectively deposited into the neurosensory retina and uveal tissues. Logically, increased diffusion of drug removal from the uveal tissue also occurs, as well as diffusion into the sclera and orbital tissues. Time dependence of drug distribution is also important in
delivery efficiency. The use of larger molecules or peptides will likely slow the known diffusional pharmacokinetics.
In order to optimize the use of these two methodologies for drug transfer of aerosolized nanoparticles, it becomes clear that each method has unique advantages. The single-fill delivery mode creates a steady state in which the rate of mass deposition in the uvea becomes smaller than the rate of mass transfer from the retina and choroid to the sclera. The use of the single-fill method would be used primarily during pneumatic retinopexy, gas-phase intravitreal injections, or at the conclusion of vitreous surgery. During surgery, the single-fill method would supplement higher tissue drug levels achieved with the flowthrough method.
Deposition efficiency is the percentage of drug deposited relative to the total drug available. In the flow-through mode, the amount of drug diffusing into the target tissue increases rapidly because there is a constantly high concentration of drug within the gas phase. Realistic application of this methodology to optimize delivery would utilize the less-efficient flow-through method to get higher tissue levels and then leaving the eye “filled” with aerosolized nanoparticles gas for continued local delivery after the eye is closed. Other technical options include an increase in the particle number concentration that could enhance the delivery capability or a reduced particle size.
Calculating the pharmacokinetics for the delivery of drugs to the retina and choroid in the gas phase may be achieved through modeling systems. For the flow-through method, a model of aerosol particles of uniform concentration, with the assumption that the deposition occurs by diffusion through a boundary layer (stagnant layer), may be assumed. The rate of mass deposition, ∆m/∆t, would be given by:
m
t = DAC
where D is the diffusion coefficient of the aerosol particle, A is the surface area of the chamber, C is the aerosol concentration, and h is the boundary layer thickness.
For single-fill delivery, two models estimate the deposited mass.54 A pure diffusion model is mathematically represented. The diffusion model has an expression for the mass deposited with time that provides a solution for the diffusion equation with spherical coordinates:
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