Ocular Drug Delivery

Brian C. Gilger, DVM, MS, Dipl. ACVO, Dipl. ABT Professor, Ophthalmology

North Carolina State University College of Veterinary Medicine 1060 William Moore Drive, Raleigh, NC 27609

Office phone: 919-513-6659 Office fax: 919-513-6711 Email: Brian_Gilger@ncsu.edu

Introduction Traditionally, medication is delivered to the eye by three main mechanisms: Topical ocular administration, systemic administration, and intraocular or periocular injection. Each of these methods has significant disadvantages. Topical ocular solutions or ointments have less than a 1% penetration, have rapid dilution and tear washout, and rely substantially on owner compliance to administer the medication. Systemically administered medications, in general, have limited ocular penetration and may require high peripheral drug levels with the potential of toxicity. Ocular or periocular injection of medication is traumatic and invasive, is rapidly diluted, and may require repeat procedures for adequate drug levels. Because of these limitations, especially for chronic internal ocular diseases of large animals, we have studied methods to deliver medications to the eye through the use of constant-release drug delivery devices. The practice of ophthalmology can be reduced to the simple goal of getting the right pharmacologic agent at the appropriate therapeutic dose to the target ocular tissue by a method that does not damage healthy tissue (Weiner & Gilger, 2010). In ocular disease, however, this simple goal becomes more challenging because of the highly sensitive ocular tissues (e.g., the uveal tract and retina) and the presence of tissue barriers to drug penetration, namely the lipophilic corneal epithelium, the hydrophilic corneal and scleral stroma, the conjunctival lymphatics, choroidal vasculature, and the blood-ocular barriers. When considering drug delivery in ocular therapeutics, there are three important aspects:

1) Duration of delivery 2) Intended tissue target 3) Owner or patient compliance.

Duration of drug delivery varies from minutes, in the case of topical eye drops, to years, in the case of some ocular implants. The route of drug delivery may determine whether or not the drug can reach the targeted tissue. For example, topical ocular medications are likely to reach the cornea and conjunctiva in therapeutic concentrations, but are unlikely to reach the retina and choroid. Finally, the issue of compliance must be considered in ocular drug delivery. If a drug must be given every hour for a year, for example, to reach therapeutic tissue concentrations when treating a chronic disease, it is very unlikely to be given consistently, if at all, by the animal’s owner. Therefore, the

method of ocular drug delivery must correlate to the intended disease in terms of site of drug target and duration of effect to ensure appropriate compliance by the animal’s owner (Figure 1) (Weiner & Gilger, 2010).

Figure 1. Kinetic profiles of standard dose forms (blue line), sustained release systems (red line), and controlled release systems (green line). Modified from Weiner A.L. Drug Delivery Systems I Ophthalmic Applications. Yorio T, Clark A, Wax M, eds. Ocular Therapeutics, Eye on New Discoveries, Elsevier Press/Academic Press, New York, Pgs 7 – 43.

The issue of treatment compliance is particularly important in veterinary medicine because of the difficulty in consistently treating an animal in pain, which is commonly left to an untrained owner to perform. Therefore, sustained release ocular drug delivery technology that may eliminate or diminish this owner compliance concern has begun to be addressed, but needs more emphasis (Davis et al, 2004; Gilger et al 2006).

General Features of Ocular Drug Delivery Drug formulations Ophthalmic formulations must fulfill the essential requirements of safety, stability, manufacturability, and bioavailability. In addition to fulfilling these common requirements, ophthalmic dosage forms must be designed with special attention to formulation factors that may affect ocular tolerability and safety, such as the pH, buffer type, buffer capacity, excipient type and levels, osmolarity, sterility, endotoxin content,preservatives, and particulate matter (see table). The chemistry, manufacturing, and controls (CMC) of ophthalmic formulations is especially constrained for ophthalmic formulation by issues

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Table. Important features of ophthalmic drug formulations for ocular tolerability Appearance Describe color, clarity, particulate, and precipitate

tests pH pH between 4–8 (most marketed products 5-8)

Osmolality (mOsm/kg)

160 to 480 mOsm/kg (0.5–1.5 % sodium chloride concentration; however, the formulation should be as close as possible to isotonic (equivalent to 0.9 % NaCL) to assure comfort and tolerability)

Active ingredients 95-105% of label claim

Particulates Solutions: 10um size: <50 per ml; 25um size: < 5 per ml; 50um size: < 2 per ml Suspensions: 95 % <10 μm

Insoluble particulate material No more than one particle >300um per ml

Sterility Sterile Bacterial endotoxin <0.5 EU/ml

Packaging No interactions with packaging material Leachates should be justifiable Packaging integrity must be demonstrated

related to patient safety and tolerability, compendia requirement, and regulatory guidelines (Ghosh 2014). In addition, characterization of the physicochemical properties of the drug substance or active pharmaceutical ingredient (API) is critical in developing a successful ophthalmic product. Solutions Most of the currently marketed ophthalmic products are solutions. The solution dosage form has several advantages including dose uniformity, ease of manufacturability and often provides better bioavailability. The limitations with solution dosage forms is rapid clearance and a short precorneal residence time after instillation. Additionally, solutions of hydrolytically labile compound may have a limited shelf life. Suspensions A suspension formulation is a coarse dispersion of insoluble solid particles of a drug substance in an aqueous vehicle containing a suitable amount of surfactant, preservative, buffering, and tonicity agents. The particle size of the suspension may vary from nanometer to micrometer range. Although development of suspension formulation is more complex and challenging than solution, ophthalmic suspension formulations can provide higher bioavailability by prolonging residence time of formulations in precorneal area and therefore may be desirable. The particle size 95 % <10 of the suspended drug particles in an ophthalmic suspension will avoid foreign body sensation and ocular tolerability issue. Ointments

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Ointments for ophthalmic use are sterile semisolid preparations and have several advantages in terms of formulation. They can offer better product stability with hydrolytically unstable and pH-sensitive API. In addition, ointments may offer better bioavailability due to longer residence time of the formulation, and dilution effect due to tear is marginal and low nasolacrimal clearance. Ophthalmic ointment preparation typically involves a combination of a suitable amount of mineral oil and white petrolatum. Basic ocular pharmacology Depending on site of the target tissue, the main problems to address when ocular drug delivery is attempted are how to localize drug action at this site and maintain therapeutic drug levels while minimizing systemic effects. Topical ophthalmic applications Topical eye drops may be solution, emulsion, or suspension consisting of water, active pharmaceutical ingredient, excipients, and preservatives. It is noninvasive, avoids first-pass metabolism, and allows selective delivery of drugs to anterior ocular tissues.

Therefore, topical eye drops remain the mainstay for the treatment of anterior ocular pathologies. The main route of topical drug entry to the anterior chamber is penetration through the cornea. The time it takes for most drugs to penetrate into the aqueous humor in peak concentration is 20 to 60 minutes. The time between topical administration of a drug and its appearance in the aqueous humor is called that drug’s lag time. Lag time is

equal to the rate of diffusion of a drug across the cornea. The amount of drug penetrating the cornea is linearly related to its concentration in the tear film, unless the drug has other physiochemical properties that alter its penetration (i.e., interaction with other molecules, adherence to proteins, limited solubility of the drug, metabolism by enzymes in tears). The decline of drug concentration in the tears (hence the concentration of drug penetrating the cornea) follows first order kinetics and the rate depends on the rate of dilution by fresh tears. Commercially available topical medications dispense a range from 25.1 to 70 μL with an average drop size of 39 μL. In a healthy human, the tear volume is 7–9 μL with a turnover rate of 0.5–2.2 μL/min. Application of topical drop causes increase in tear volume and rapid reflex blinking. Of the dose instilled ~50 % is retained in the cul-de-sac and the remainder is drained into systemic circulation via nasolacrimal duct or spilled onto the

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cheeks. Of the administered dose, only 1–7 % of drug reaches aqueous humor. The remainder exits with the tear film through the nasolacrimal system, is deposited on the eyelids, or metabolized by enzymes in the tears and surface tissue. Systemic absorption of some drugs can be significant. Constant rate infusion or solid drug implants generally have Zero order kinetics.

Topically applied drugs penetrate through the cornea by passive diffusion usually proceed via either transcellular or paracellular pathway. Partition coefficient (log P) of the drug molecule has significant impact on drug penetration through cornea. Compounds with log P between 1 and 3 show maximum corneal penetration. Molecular weight and size of the drug molecule also plays a role in penetration through

different ocular tissues due to different cutoff molecular weight of ocular tissues. For example, molecular weight cutoff for cornea is at approximately 500 Da, whereas for conjunctiva and sclera are approximately 40 and 150 kDa, respectively. Therefore, higher molecular weight molecules would more likely be absorbed in conjunctiva and sclera compared to cornea. The ionization constant is an important parameter in ocular absorption of acidic and basic drugs. Although both the ionizable and the unionized forms of the drug may diffuse across ocular membranes, it is predominantly the unionized form that determines the extent of ocular drug absorption (bioavailability). Subconjunctival injection The conjunctiva is a thin, semitransparent, mucous-secreting tissue which forms the loose inner lining of the eye. It is continuous with cornea and forms a thin membranous layer (bulbar conjunctiva) above the white part of the eye called “sclera.” Administration into the space between conjunctiva and sclera, i.e., beneath the conjunctiva, is called as “subconjunctival injection.” This mode of drug administration is considered minimally invasive. Usually, a 25–30 gauge needle is used for drug administration. Depending on the species, 250 - 500 μL of drug solution may be administered by this method. The advantage of this approach is that it avoids a major barrier to drug penetration in the eye, namely the corneal and conjunctival epithelium allowing rapid penetration of highly soluble drugs into the eye. Similarly, a subtenon injection implies administration of solutions into tenon’s capsule, a fibrous membrane that envelopes the eye from limbus to the posterior optic nerve. Subtenon’s space is a virtual cavity that is bound by tenon’s capsule and sclera. To

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administer drug solution into subtenon’s space, a 25-30 gauge needle or cannula (with conjunctival incision) is directed deep to the conjunctival into the fibrous connective tissue layer. The injection is directed posteriorly to deliver up to 4 mL of drug solution around the ocular muscles behind the equator. Subconjunctival hemorrhage may be observed following this procedure in addition to chemosis. Suprachoroidal space Suprachoroidal injection has been described for several years and appears as a promising, semi-invasive technique for treatment of ocular posterior segment diseases. The suprachoroidal space is a potential space between the sclera and choroid that surrounds nearly the entire posterior segment. Using specific microneedles or a surgical approach, an implant or drug is injected directly into or near the suprachoroidal space. Injection into this space results in rapid infiltration into the ciliary body and choroid. A single injection in the suprachoroidal space in cadaver eyes (canine and porcine) has resulted in distribution to over 50% of the posterior segment of the eye (Seiler et al. 2011). The advantages are that there is direct interaction between the drug and choroid without the need to do an intravitreal injection with the associated complications (see below). Furthermore, unlike intravitreal injections, the drug does not diffuse into the ocular anterior segment resulting in complications (e.g. glaucoma, infectious keratitis). Recent studies in a pig uveitis model have demonstrated excellent efficacy in inflammation control with administration of a steroid into the SCS (comparable or better than intravitreal steroid). With increasing availability of the appropriate needles, this route will be come more popular. Intravitreal Intravitreal injection is considered an invasive method of drug delivery involving direct administration of drug solutions into the vitreous humor via pars plana using a 30 gauge needle. High drug concentrations are achieved in the vitreous/retina following intravitreal injections. To perform an intravitreal injection, the dilated and usually the animal is anesthetized or deeply tranquilized. Topical anesthetics such as proparacaine 0.5 % and an antibiotic (moxifloxacin) are applied to the eye. The outer ocular skin, eye lashes, caruncle, and upper and lower eyelids are swabbed with 5% betadine and irrigated with sterile eyewash. The injection site is superior temporally needle is directed towards the center of the vitreous humor to a depth of 4–6 mm. A volume of 50 to 200 μL (depending on the species) may be administered with a 30 or a 32 gauge needle. Post injection the needle is slowly retracted and removed. The ocular posterior segment should be examined immediately after injection to verify drug delivery and to determine if there are any injection complications (hemorrhage, cataract, retinal detachment etc.). Intravitreal elimination of drugs after injection is dependent on the molecular weight of drug candidates; macromolecules such as proteins (between 40 and 70 kDa) may tend to cause longer retention in vitreous body leading to slower elimination relative to small molecules, such as most non-proteinacious drugs. Other routes of therapy

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Other routes of therapy, such as intrastromal corneal injections, intrascleral injections, retrobulbar injections and subretinal injections are increasingly being done in veterinary medicine, but the pharmacokinetics are not yet well studied for these techniques.

Anatomic and Physiologic Barriers to Ocular Drug Penetration The eye has several features that challenge drug delivery and drug penetration: the lipophilic corneal epithelium, the hydrophilic corneal and scleral stroma, conjunctival lymphatics, choroidal vasculature, and the blood-ocular barriers. The vasculature of the eye is similar in nature to the blood–brain barrier (BBB). The blood–retinal barrier (BRB) constitutes a microvascular unit in the eye, which is selectively permeable, therefore restricting access of organic compounds to the retina. The vasculature of the retina has several tight junctions that largely prevent paracellular uptake of compounds. Furthermore, local administration of medications to the eye is challenging because the eye has unique functional and structural protective mechanisms, such as blinking, permanent lacrimation, and lacrimal drainage, which are necessary to preserve visual acuity, but favor rapid removal of drugs topically applied to the eye. For drugs periocularly or systemically administered, the sclera and the blood ocular barriers, act as major obstacles for the access of drugs to intraocular target tissues. The cornea is essentially a fat (epithelium)-water (stroma)-fat (endothelium) multilayered sandwich. The epithelium is the major barrier to absorption, especially for hydrophilic medications. The corneal stroma is a major barrier for lipophillic drugs. Therefore, the drug with the optimum ratio of hydrophilocity and lipophilocity provides best corneal transfer.

SUSTAINED OCULAR DRUG DELIVERY Because the eye is highly compartmentalized, it offers unique locations for delivery system placement. There are non-invasive topical strategies and more invasive implant technologies.

Non-Invasive Strategies for Delivery While standard topical ophthalmic solutions are usually sufficient to elicit an efficacy response in the majority of ocular surface or anterior segment disorders, there are several factors that usually limit the ability of drops to achieve above a minimally effective concentration for posterior segment therapy. Those factors include the partition and diffusion coefficients of the drug in tissues, hydraulic conductivity of the ocular tissues, concentration boundary conditions surrounding the instilled dose (i.e. drug solubility limits), conjunctival clearance, and intraocular and episcleral venous pressures, to name a few. Two main approaches have been used to improve drug levels posteriorly - increasing topical residence time of the drug and enhancing drug permeation through tissue. Topical residence time can be extended using gels and solid inserts. Drug permeation can be increased using appropriate pro-drugs, solubilization vehicles or iontophoretic methods (Weiner & Gilger, 2010).

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Implant Strategies for Delivery

Eroding implants Ocular implants have many advantages including the ability to deliver constant therapeutic levels of drug directly to the site of ocular disease while minimizing systemic side effects. These devices for controlled, sustained drug release are classified as biodegradable (i.e., eroding) and nonbiodegradable (i.e., non-eroding). Biodegradable implants have the advantage of being able to be fashioned into various shapes and they do not require removal. Nonbiodegradable implants have the advantage of steady, controlled release of a drug for potentially long periods of time (i.e., years) and the disadvantage of removal and/or replacement when the drug is depleted.

Non-eroding diffusion based implants Non-eroding implants release drug from a nonbiodegradable device that either contains a central reservoir or a solid central device containing a drug coating. Each type, once depleted of drug, can be removed from the eye and replaced. Reservoir implants are

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typically made with a pelleted drug core surrounded by nonreactive substances such as silicon, ethylene vinyl acetate (EVA), or polyvinyl alcohol (PVA). A major advantage of these implants is that they can deliver continuous amounts of a drug for years.

Non-eroding active pump implants Research on small pumps to study effects of constant infusion of drugs has been historically limited to evaluations using the Alza mini-osmotic pumps. These commercially available pumps deliver pre-defined rates, are typically implanted in the subcutaneous space, and provide about a month of delivery. Their application to ophthalmic infusions has been studied in horses (Herring

Intraocular lens Use of an intraocular lens to deliver drugs to the eyes after cataract surgery has been studied (Davis et al, 2012). Either the IOL itself is used as a drug reservoir or a coating or separate reservoir is attached to the IOL. The more common use of higher water content IOLs, such as hydrogels and soft acrylics, has recently allowed simple “soaking” of the IOL to be used to load drug (Davis et al, 2012). The goals of this approach are to reduce post-operative endophthalmitis by the delivery of antibiotics,7 prevent inflammation by delivery of dexamethasone or NSAIDS, and/or prevent posterior capsular fibrosis (Davis et al, 2012). In our study, acrylic IOLs that were incubated in celecoxib solution were shown to be able to release celecoxib at sufficient drug levels to reduce inflammation and inhibit PCO formation for 7 days in vitro (Davis et al, 2012). Although no studies have been reported to date in veterinary patients, this approach appears practical and further development is warranted. Hydrogels Many types of ocular sustained drug delivery systems have been investigated, but most often require surgical implantation/removal or do not have a predictable/controlled rate of delivery, limiting their clinical use. A product that can provide sustained release of ocular therapeutics with controlled release and without the need for surgery would be highly beneficial. Hydrogels are an aqueous solution and when exposed to certain external stimuli such as pH or temperature form a deformable gel depot from which a high concentration of drug is slowly released into the surrounding tissue. One commonly used hydrogel, poly(D,L-lactic-co-glycolic acid (PLGA), has been shown to be easily injectable and has a controlled and predictable

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release of protein. However, the degradation products of PLGA leads to production of lactic and glycolic acid, resulting in lowering of local pH and hence degradation of fragile drugs. Thermosensitive hydrogels, which are liquid at room temperature and solid at body temperature (see Figure), have been developed using different block co-polymers such as Poly (ethylene glycol) (PEG), PLGA, poly (L-lactic acid) (PLA), polycaprolactone (PCL), poly (propylene oxide) (PPO) and many others. Advantages of thermosensitive gels include improved protein stability due to lack of necessity of an organic solvent, ability to sterilize the solution by filtration, and ease of injection through small gauge needles. In our laboratory, we have shown promising results for the sustained release of a model therapeutic protein, IgG, using pentablock copolymers (Schaefer 2016). Pentablock co-polymers are thermosensitive hydrogels (PTSgel) made up of m-PEG, PCL, and PLA biodegradable polymer blocks. PTSgels can be easily injected through a small gauge needle and form a hydrogel depot in situ; they are biocompatible and provide sustained release of IgG in vitro and in vivo while maintaining a stable biologic protein. In addition, PTSgels have the added benefit of allowing high drug loading which ensures a therapeutic concentration over an extended period of time. Evaluation of these drug delivery polymers are currently being done for topical, intracameral, suprachoroidal, and intravitreal drug delivery.

Suprachoroidal drug delivery Sustained release cyclosporine implants placed in the suprachoroidal space have resulted in long-term control of uveitis in horses (Gilger et al, 2010). Use of specific microneedles to access the suprachoroidal space may allow this technique to be applicable to a wide variety of pharmacotherapies, and allows access to the macula, optic nerve, and posterior pole of the eye (Jiang et al, 2009).

CONCLUSIONS AND SUMMARY Efforts to develop ophthalmic devices and products designed to improve compliance; targeting and duration of ocular drug delivery have been accelerating within the past decade. This comes from greater understanding of ocular kinetics and drug distribution, experimentation into new administration sites within the eye, and improvements in technology leading to better biomaterials and mechanisms for drug release. Furthermore, the development of new drug categories in areas of glaucoma, uveitis, and retinopathy, has given rise to unique systems that are designed to overcome deficiencies observed with classical therapy of these new compound types. This oncoming wave of new devices under clinical evaluation will offer patients and clinicians a variety of highly desirable and more effective treatments and therapies.

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REFERENCES 1. Wiener AL, Gilger BC. Advancements in Ocular Drug Delivery. Vet Ophthalmol 2010;13(6):395-406. 2. Davis JL, Gilger BC, Robinson MR. Novel approaches to ocular drug delivery. Curr Opin Mol Ther 2004;6:195-205. 3. Gilger BC, Salmon JH, Wilkie DA, et al. A novel bioerodible deep scleral lamellar cyclosporine implant for uveitis. Invest Ophthalmol Vis Sci 2006;47:2596-2605. 4. Blair MJ, Gionfriddo JR, Polazzi LM, et al. Subconjunctivally implanted micro-osmotic pumps for continuous ocular treatment in horses. Am J Vet Res 1999;60:1102-1105. 5. Davis DL, Yi NY, Salmon JH, Charlton AN, Colitz CMH, Gilger BC. Sustained-release celecoxib from incubated acrylic intraocular lenses suppress lens epithelial cell growth in an ex vivo model of posterior capsule opacity (PCO). J Ophthalmol Pharm Therapeutics (In press) 6. Seiler GS, Salmon JH, Mantua R, Feingold S, Dayton PA, Gilger BC. Distribution of contrast after injection into the anterior suprachoroidal space using 2D and 3D ultrasound in pig eyes. Invest Ophthalmol Vis Sci 2011 52(8):5730-5736 7. Gilger BC, Wilkie DA, Clode AB, McMullen RJ, Utter M, Komaromy A, Brooks DE, Salmon JH. Long-term outcome after implantation of a suprachoroidal cyclosporine drug delivery device in horses with recurrent uveitis. Vet Ophthalmol 2010;13(5):294-300. 8. Jiang J, Moore JS, Edelhauser HF, et al. Intrascleral drug delivery to the eye using hollow microneedles. Pharm Res 2009;26:395-403. 9. Schaefer E, Abbaraju S, Walsh M, et al. Sustained release protein therapeutics from subcutaneous thermosensitivie biocompatible and biodegradable pentablock copolymers. J Control Release. 2016.

10. Patel S, Vaishya R, Mishra G, Tamboli V, Pal D, Mitra A. Tailor-made pentablock copolymer based formulation for sustained ocular delivery of protein therapeutics. J Drug Deliv. 2014;2014:401747

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