Journal of Biomolecular Research & Therapeutics

Ocular drug delivery has remained one of the most challenging tasks for pharmaceutical researchers. The unique and complex structure of the eye restricts the entry of drug molecules at the required site of action. The eye is anatomically divided into the anterior and posterior segments with the lens–iris barrier roughly demarcating the two segments. The anterior segment consists of the front one-third of the eye that mainly includes pupil, cornea, iris, ciliary body, aqueous humor, and lens while the posterior segment consists of the back two-thirds of the eye that includes vitreous humor, retina, choroid, macula, and optic nerve (Figure 1) [1,2]. Further, the anatomy of the eye presents a unique system with two barriers that prevents penetration of substances from the blood, namely, the bloodaqueous and blood-retinal barrier [3].

Figure 1: Schematic representation of major structure of the eye. The anterior segment includes pupil, cornea, iris, ciliary body, and aqueous humor. The lens separates anterior portion from posterior, which includes vitreous humor, retina, choroid, macular, and optic nerve. Sclera encasing the whole eye protects it impedes penetration of majority external agents. Reprinted with permission from reference [1]

There are a large number of major diseases affecting the eye including Age-Related Macular Degeneration (AMD), Diabetic Macular Edema (DME), cataract, Proliferative Vitreoretinopathy (PVR), uveitis, Cytomegalovirus (CMV), and glaucoma. Table 1 highlights the classification, signs and symptoms and current treatment options for these diseases [4-38].

Table 1: Major ocular diseases, signs and symptoms and current treatment

Ocular drug delivery methods to treat anterior segment disease include topical (i.e. eye drops, ointment), systemic, and periocular (i.e. subconjunctival injections, implants) administration routes.

Table 2 provides a general overview of various delivery routes. Systemically administered drugs reach the retino-choroidal tissue via blood circulation; this is sufficient for many drugs to achieve therapeutic levels in the eye. However, because the outer and inner blood-retinal barriers limit influx of drug into the retina (and vitreous), large doses of the drug are required in order to achieve and maintain therapeutic concentrations [4]. This usually leads to unwanted side effects, as the drug also accumulates in other tissues of the body. Periocular injections avoid the side effects associated with systemic delivery, but carry increased risks of retinal detachment, cataract formation, and other ocular maladies.

Table 2: Summary of routes of Administration, benefits and challenges in ocular delivery.

Topical administration is the least invasive of the methods, and numerous topically applied drugs may enter the eye through the cornea, conjunctiva, and sclera [39,40]. However, only a small amount of a given drug penetrates the cornea, conjunctiva, and sclera to ultimately reach the target sites [41-43]. Some of the factors that affect this penetration of topically applied drugs include, but are not limited to, lachrymation, aqueous production, blood flow, and barriers imposed by the corneal epithelium and endothelium and by the stromal tissues of the cornea and sclera [43-45].

Consequently, topically applied drugs must be of relatively high concentration with frequent instillations, to achieve and/or maintain therapeutic concentrations, even for tissues in the anterior segment. In addition to the above mentioned factors, delivery of drugs to the posterior segment is even more challenging, due to the longer diffusion distance and the a cellular nature of the vitreous body. Thus, very often, methods alternative to topical administration are used to deliver drugs to the posterior vitreous, retina, or choroid (Figure 2).

Figure 2: Drug delivery alternative for treating posterior eye diseases. (a)Topical drops must diffuse across the precorneal milieu, cornea, iris, ciliary body, and vitreous before reaching the back of the eye, severely diluting the fraction of drug reaching the retina and macula area; (b) Systemic (oral or intravenous) drug delivery, also has a poor dose–response profile for vitreoretinal targets; (c) Intravitreal injection or implant and (d) transscleral diffusion tend to increase drug proportions reaching the retina and macula area. Reprinted with permission from reference [135].

Intravitreal injections have gained considerable momentum during the past two decades. This method involves injection of drug solution directly into vitreous via pars plana using a 30 G needle. Unlike other routes, intravitreal injection offers higher drug concentrations in vitreous and retina. Due to the uniqueness of the vitreous environment, the pharmacokinetics of the drug need to be considered to ensure adequate tissue drug levels and safe elimination, which is closely related to the molecular weight of the substance. Linear and globular shaped molecules (especially protein and peptide drugs) with molecular weight greater than 40 and 70 kDa respectively tend to cause longer retention in vitreous humor [46]. Though intravitreal administration offers high concentrations of drugs in retina, it is associated with various short term complications such as retinal detachment, endophthalmitis and intravitreal hemorrhages [47]. Moreover, patients need to be carefully monitored for specific time periods following intravitreal injections to ensure minimal procedure complications.

Periocular route has been considered as the most promising and efficient route for administering drugs to the posterior eye segment. Periocular refers to the region surrounding the eye and includes peribulbar, posterior juxtascleral, retrobulbar, subtenon and subconjunctival routes (Figure 3).

Figure 3: Routes of ocular drug delivery. Least invasive methods are topical administration. Peribulbar, subtendon, and retrobulbar methods all involve surrounding tissues of the eye and are commonly referred to as periocular routes. Subconjunctival and intravitreal injections involve the eye tissues. The approaches to the delivery indicated in italics font.

Drug solutions are placed in close proximity to the sclera which results in high retinal and vitreal concentrations, as the sclera is made up of fibrous tissue and offers less resistance to the permeation of Active Pharmacological Agents (APIs) [48].

Although intravitreal injections via the pars plana place the drug directly into the posterior segment, bypassing the corneo-scleral barriers, many of the vitreoretinal diseases (i.e. Age-Related Macular Degeneration (AMD) and diabetic retinopathy) for which intravitreal injections targeted are not controlled by a single injection, and the need for multiple injections increases the potential for cataract, retinal detachment, vitreous hemorrhage, and endophthalmitis. One of the alternative routes of drug delivery to the posterior segment is via the sclera. In humans, the sclera is relatively thick near the limbus, thin at the equator, and much thicker near the optic nerve. The total scleral area is 16-17 cm² [45,46,49,50]. Surgical thinning of the sclera demonstrates that scleral permeability is inversely proportional to the thickness [51]. Thus, the ideal location for transscleral drug delivery is near the equator at 12–17 mm posterior to the corneoscleral limbus. However, the thinness of the area would also make this site more susceptible to needle or instrument perforation.

Traditionally, subconjunctival, subtenon and retrobulbar injections have been used clinically to administer corticosteroids and local anesthetics, and it is known that higher concentrations in the posterior tissues can be achieved by these injections as opposed to topical eye drops [52,53]. Relatively high permeability of sclera to macromolecules has revived interest in this route of drug administration [50,51]. Delivery of macromolecules is important therapeutically, since anti-angiogenic antibodies, oligonucleotides, growth factors, and trans-gene expression products are all large molecules. Development of controlled release materials and dosage forms may provide means to achieve long duration of action and less frequent drug administration. This article gives an update on selected aspects of transscleral drug delivery. Particular emphasis is directed to the nanoparticulate systems of transscleral drug delivery for various ocular major diseases.

Periocular penetration routes

The periocular (transscleral) route of delivery is gaining increased interest as a promising alternative to the intravitreal administration for drug delivery to the choroid, retina, and vitreous [48]. In transscleral drug delivery to the posterior eye, the drug delivery system (e.g. solution or implant) is placed into the periocular space. Several possible sites are available, including subconjunctival, sub-tenon, peribulbar, posterior juxtascleral and retrobulbar spaces (Figure 4) [48].

Figure 4: Schematic representation of drug delivery from the periocular spaces into the vitreous via direct penetration pathway. Depending on the drug and disease state the target tissue can be vitreous, neural retina, Retinal Pigment Epithelium (RPE) or choroid. Reprinted with permission from reference [136].

It has been demonstrated that the transscleral route can deliver therapeutically effective drug levels for the treatment of choroidal neovascularization associated with age-related macular degeneration [54,55]. It has also been demonstrated that the periocular (transscleral) route can be used for delivery of large molecules to the choroid and the retina in vivo [56,57].

The drug may permeate from the periocular space into the vitreous (or another target tissue) via (1) anterior chamber, (2) systemic circulation or (3) direct penetration pathway. In the anterior chamber route, the drug diffuses into the aqueous humor either directly across the sclera and ciliary body or indirectly via the tear fluid and cornea after it has refluxed through the conjunctiva. Thereafter, the drug may permeate further to the posterior chamber and vitreous. In the systemic circulation route, the drug is absorbed into general circulation via conjunctival, episcleral, or choroidal vessels; then, it is diluted in the circulation, and later returned to the eye with blood flow.

In the direct penetration pathway, the drug permeates into the vitreous through the underlying tissues. As seen in Figure 4, there is no single pathway after the drug has passed across the sclera. In the anterior eye, the drug may diffuse through the ciliary body (pars plicata or pars plana) into the posterior chamber and vitreous. In the posterior eye, the drug has to permeate across the choroid, Retinal Pigment Epithelium (RPE), and neural retina in order to reach the vitreous.

It is also important to consider that drug delivery across the sclera is governed in part by the transient diffusion of a solute across the tissue that typically occurs over a time course of minutes, unless some type of sustained-release formulation or device is used. Sustained drug delivery systems can provide sustained drug levels to a particular tissue thereby significantly reducing the dosing frequency and the associated complications. Several delivery systems including implants, scleral plugs, and micro-and nanoparticles have been used for this purpose [58-62]. Among these systems, the particulate systems offer several advantages including ease of repeated injections and biodegradability. In addition, nanoparticles have the advantage of cellular entry, which can be used in the delivery of macromolecules like DNA and proteins inside the cells [63]. The right combination of the drug/polymer can control the rate of delivery of a particular drug from these systems and extend the duration for which the drug is delivered.

Experimental studies aimed at determining scleral permeability typically derive scleral permeability for a particular solute from steadystate flux data. One must recognize that in the absence of some type of sustained-release system, drug–sclera contact times would be too brief to permit the attainment of steady-state flux. Consequently, the in vitro flux measurements would be expected to over-estimate transscleral drug delivery [64]. Sustained drug delivery to the retina by the periocular (transscleral) route would require that the delivery system be retained at the periocular site for a prolonged time so that the drug can be released from the device and become available to the retina. In the case of gene delivery, it would be ideal if the system penetrates the sclera and enters into the choroid and retina where the cells in the choroid or the retina can be transfected. Thus, based on clinical need, the choice of a particulate delivery system can be made. The disposition of particles after periocular administration is size dependent. Specifically, particles in the range of 200–2000 nm were almost completely retained at the site of administration for at least two months while the smaller 20 nm particles were cleared rapidly from the site of administration.

Transscleral diffusion

A relatively newer method of drug delivery, transscleral delivery, is a less invasive method in which the drug permeates through ocular tissues to reach the neuroretina. Transscleral delivery includes avenues such as subconjunctival, retrobulbar, peribulbar, sub-Tenon’s and intrascleral delivery [65]. An overview of these approaches and their applications are summarized in Table 3 [65,66].

Table 3: Overview of existing transscleral drug delivery techniques [65,66].

Barriers to Transscleral Drug Delivery

Although transscleral methods eliminate some of the side effects of intravitreal delivery, they in turn have their own limitations. Because the drug molecules must cross through several layers of tissue, the bioavailability of the drug at the target site can sometimes be drastically reduced and, thus, may require very high doses to be effective. These barriers are categorized into three major groups: static, dynamic and metabolic [66]. Table 4 provides a quick overview of these three barrier types [66]. Static barriers include the tissues that must be penetrated (e.g., sclera, Bruch’s membrane-choroid and RPE).

Table 4: Barriers to transscleral drug delivery [66].

Dynamic barriers include blood flow, lymphatic drainage, transport proteins of the RPE, drug efflux pumps, organic ion transporters and bulk fluid flow from intraocular drainage systems. Metabolic barriers include enzyme systems such as cytochrome P450 and lysosomal enzymes, which have the ability to degrade or detoxify drugs. Mathematical models of posterior segment drug delivery by subconjunctival injection reveal that the dominant pathway for entry into the vitreous is direct penetration, while recirculation or movement from the aqueous to vitreous chambers is not significant [67-70].

Conjuctiva is a thin and transperant membrane, having rich supply of capillaries and lymphatics (Figure 5), therefore drugs administered through conjuctival space need to be cleared through blood and lymph.

Figure 5: Schematic representations of subconjunctival or episcleral blood vessels, and lymphatics network. Schematic representations of subconjunctival or episcleral blood vessels, and lymphatics network. (Adapted from Robinson MR, Lee SS, Kim H, et al. A rabbit model for assessing the ocular barriers to the transscleral delivery of triamcinolone acetonide, Exp. Eye Res. 2006, 82, 479-487, with permission from Elsevier Inc., and The Human Eye with permission from Sinauer Associates, Inc., copyright 1999) [138].

Due to loose junction barrier of conjuctival blood vessels, drug molecules can transport into blood circulation by pinocytosis and/or convective approach through paracellular pores in the vascular endothelial layer.

The conjuctival lymphatics also act as an efflux system for the efficient elimination of drug. Therefore, drugs transported by lymphatics alongwith elimination by blood circulation can contribute to systemic circulation after filtered through lymphatic systems [71-74].

Sclera mainly consists of collagen fibers and proteoglycans embedded in an extracellular matrix. Permeability through scleral is strongly dependent on the molecular radius, as it decreases roughly exponentially with molecular radius. As mentioned earlier, surgical thinning of the sclera demonstrates that scleral permeability is inversely proportional to the thickness [51]. Thus, the ideal location for trans scleral drug delivery is near the equator at 12–17 mm posterior to the corneoscleral limbus. In addition to this, scleral permeability is also dependent on hydrophobicity of drugs. Hydrophilic drugs may diffuse through the aqueous medium of proteoglycans in the fiber matrix pores more easily than lipophilic drugs. Also, the charge of drug molecule affects the permeability. Positively charged compounds may exhibit poor permeability due to their binding to the negatively charged proteoglycan matrix [74].

Bruch’s membrane-choroid is the highly vascularized tissues of the body to supply the blood to retina. Also the choroidal capillary endothelial cells are fenestrated and large in diameter (Figure 6).

Figure 6: Schematic of blood-retinal barrier, and capillary wall in the retina and the choroid. Reprinted with permission from reference [138].

In addition to this, histological studies have shown choroidal thickness changes from 200 μm at birth to about 80 μm by age 90 and chorioretinal diseases also affect choroidal thickness. In contrast, Bruch’s membrane causes thickening with age. These changes cause calcification of elastic fibers, cross-linkzge of collage fibers and increased turnover of glycosaminoglycans. Thickness changes of choroid and BM might affect drug permeability from subconjunctiva or episcleral space into the retina and the vitreous [74].

Blood-Retinal Barrier (BRB) is composed of tight junctions of retinal capillary endothelial cells and RPE, called iBRB for the inner and oBRB for the outer BRB, respectively, supported by muller cells and astrocytes. The function of BRB is to restrict drug transport from blood into retina. The muller cells and retinal capillary vessels are responsible to maintain the proper functioning of iBRB, in the uptake of nutrients and in the disposal of metabolites in the normal conditions. Dysfunction of Müller cells may contribute to a breakdown of the iBRB in many pathological conditions, such as diabetes. Astrocytes are known to increase the barrier properties of the retinal vascular endothelium by enhancing the expression of the tight junction and modifying endothelial morphology [74].

After systemic administration, drugs can easily enter into the choroid as choroid is highly vascularized tissue than retina. The choriocapillaris are fenestrated resulting in rapid equilibration of drug molecules will be achieved in the bloodstream with the extravascular space of the choroid. Therefore, oBRB (RPE) restricts further entry of drugs from the choroid into the retina. RPE is a monolayer of highly specialized hexagonal-shaped cells, located between the sensory retina and the choroid. The tight junctions of the RPE efficiently restrict intercellular permeation into the sensory retina.

In addition to the static, dynamic and metabolic barriers, other factors that must be considered in transscleral delivery include the individual pharmacokinetic properties of the drug. The pharmacokinetics of drug diffusion across these barriers is dependent on the molecular dimensions, molecular weight, atomic charge and chemical components of the drug. In vitro studies demonstrated that the human sclera is permeable to 70 kDa dextran [50]. The radius of the molecule is considered to be a more important predictor of permeability than the weight and charge [50]. Finally, the solubility of the drug compounds is impacted by the water and lipid interactions. Hydrophilic compounds tend to permeate through the sclera more rapidly than lipophilic (hydrophobic) molecules, making the delivery of lipid-dominant molecules such as corticosteroids via transscleral routes more challenging [51]. However, a balance may be critical since many lipophilic compounds can easily penetrate the RPE; problems such as toxicity can arise due to lack of drug elimination. The delivery of drugs via the transscleral route continues to undergo investigation owing to the potential benefits over systemic and intravitreal delivery.

Nanoparticulate Transscleral Drug Delivery Systems

Colloidal carriers have been widely exploited in the field of drug delivery science. It provides a more selective targeting along with sustained release of molecules at the desired site. Applications of nanotechnology can be very exciting in the treatment of a gamut of diseases affecting the anterior as well as the posterior segment of the eye. One has to understand the physiological and biochemical factors involved in normal and pathological conditions for designing a successful ocular drug delivery system. Delivery of a drug via nanotechnology-based product fulfills mainly three objectives as follows:

• Enhancement of drug permeation;

• Controlled release of the API; and

• Targeting of the delivery [71].

Colloidal systems consisting of micro/nanoparticles, micro/ nanoemulsions, nanosuspensions, liposomes, dendrimers, niosomes, and cyclodextrins have also been exploited to achieve optimal drug delivery [46,58,59,61,63,71].

Many sustained intraocular drug delivery methods have been developed as alternatives to implantation. Ocular drug delivery systems using particulates have been developed, which provide sustained release with high target specificity in the form of microspheres and microcapsules with diameters of 1–1000 μm, as well as nanospheres and nanocapsules with diameters of less than 1 μm [75-76]. Drugs can be incorporated into biodegradable polymers to form either a matrix system or a reservoir system [62]. In a matrix system, the drugs and polymer are combined and the drug is released through diffusion from the polymer matrix with simultaneous polymer degradation. This system is used for micro- and nanospheres. The reservoir-type system involves encapsulating drugs within polymeric shells and the system is used for biodegradable micro- and nanocapsules [62]. Some of the commonly used synthetic biodegradable polymers are the aliphatic polyesters such as Polylactic Acid (PLA), Polyglycolic Acid (PGA), Poly (Lactic-Co-Glycolic Acid) (PLGA) and poly (caprolactone) [4,75,76]. These polymers are appropriate for controlled-release applications because they are nontoxic, nonimmunogenic and degrade through enzymatic reactions and hydrolysis to natural metabolic products over a period of months to years [77-78]. Drug-release profiles can be modified through variations in polymer molecular weights and copolymer formulations [76]. An emulsion–diffusion process is used to encapsulate drugs in micro- and nanocapsules, while solvent evaporation is used to prepare micro- and nanospheres using an oil-in-water emulsion for hydrophobic drugs or an oil-in-oil emulsion for improved encapsulation efficiency for hydrophilic drugs [62,79-81]. The particulates are suspended in a carrier solution to enable ocular injection. Intravitreal injections can potentially impair vision due to clouding. However, microspheres larger than 2 μm tend to settle out owing to gravity, whereas nanoparticulates diffuse quickly and localize within ocular tissues [82]. Table 5 summarizes major research contributions in the area of nanoparticle-transscleral delivery [80,81,85-90,92,101,82-107,109-111].

Table 5: List of research done on Nanoparticulate Transscleral Drug Delivery System

Applications of Nanoparticulate Transscleral Drug Delivery System

Nanotechnology for advanced drug delivery: The era of molecular medicine, which has been accelerated by the human genome project, has led to early disease detection through diagnostics and targeted drug and gene therapy. With the development of nanoparticles, barriers to drug delivery may be overcome at the organ, tissue, cellular, and subcellular levels. Nanoparticle drug delivery may be accomplished through gold nanospheres and rods, nanowires, nanotriangles, nanostars, nanocubes, and nanorice. The size of these nano configurations varies from 1 to 100 nm. Nanoplatforms include organic nanostructures, polymeric nanoparticles, lipid systemsliposomes, self-assembling micelles, dendrimers, and carbon nanostructures (e.g. nanotubes). Inorganic nanostructures include metal nanoparticles and nanoshells, silicon nanostructures, nanocrystals, and quantum dots. Hybrid nanostructures, combining two to three of those previous listed can also be produced.

Tumor cell targeted drug delivery: Studies were described in which polymeric nanoparticles were used for tumor-targeted delivery to block tumor blood vessels and to mediate in vitro drug delivery of tamoxifen and paclitaxel in human cancer xenograph models [108-110].

Gene therapy: Nanoparticles present the potential to be used in the future for gene therapy, as a safer and more effective alternative to viral vectors to deliver full genes or gene-suppression agents to the eye. For example, gelatin based engineered nanoparticles have been used for gene delivery [111]. Along with tissue engineering, gene therapy approaches stand on the front line of advanced biomedical research to treat blindness arising from corneal diseases, which are second only to cataract as the leading cause of vision loss [112]. Several kinds of viruses including adenovirus, retrovirus, adeno-associated virus, and herpes simplex virus, have been manipulated for use in gene transfer and gene therapy applications [113]. Topical delivery to the eye is the most expedient way of ocular gene delivery. However, the challenge of obtaining substantial gene expression following topical administration has led to the prevalence of invasive ocular administration [112]. Retroviral vectors have been widely used due to their high efficacy; however, they do not have the ability to transduce nondividing cells, leading to restriction of their clinical use [113]. The advanced delivery systems that prolong the contact time of the vector with the surface of the eye may enhance transgene expression, thereby facilitating noninvasive administration [112].

Multifunctional nanoemulsions for oral and intravenous delivery: Gadolinium-loaded nanoemulsion has been utilized for brain imaging in animal studies [114-133,134]. This imaging technology could be utilized for imaging within the eye to observe the results of various drug delivery modalities. In addition, gold nanostructures have been developed for OCT imaging along with superparamagnetics from oxide-gold-core–shelled nanoparticles (60 nm iron oxide nanoparticles with 5 nm gold shells) for MRI imaging.

Stem cell Therapy: Emerging cell therapies for the restoration of sight have focused on two areas of the eye that are critical for visual function, the cornea and the retina [111,135-137]. Current strategy for management of ocular conditions consists of eliminating the injurious agent or attempting to minimize its effects. The most successful ocular application has been the use of limbal stem cells, transplanted from a source other than the patient for the renewal of corneal epithelium. The sources of limbal cells include donors, autografts, cadaver eyes, and recently, cells grown in culture. Stem-cell Therapy has demonstrated great success for certain maladies of the anterior and posterior segment [115].

Protein and Peptide therapy: Delivery of therapeutic proteins/ peptides has received a great attention over the last few years [116]. The intravitreous injection of ranibizumab is one such example. In order to develop design of optimized methods for sustained delivery of proteins and prediction of clinical effects of new compounds administered in the eye, basic knowledge of proteins and peptides is required [56]. However, several limitations such as membrane permeability, large size, metabolism and solubility restrict their efficient delivery. A number of approaches have been used to overcome these limitations. Poor membrane permeability of hydrophilic peptides may be improved by structurally modifying the compound, thus increasing their membrane permeability. Ocular route is not preferred route for systemic delivery of such large molecules. Immunoglobulin G has been effectively delivered to retina by transscleral route with insignificant systemic absorption [56].

Oligonucleotide therapy: Oligonucleotide (ON) therapy is based on the principle of blocking the synthesis of cellular proteins by interfering with either the transcription of DNA to mRNA or the translation of mRNA to proteins. Among several mechanisms by which antisense molecules disrupt gene expression and inhibit protein synthesis, the ribonuclease H mechanisms are the most important. A number of factors have been determined to contribute to the efficacy of antisense ON. One primary consideration is the length of the ON species. Lengths of 17– 25 bases have been shown to be optimal, as longer ONs have the potential to partially hybridize with nontarget RNA species. Biological stability is the major barrier to consider when delivering both DNA and RNA oligonucleotides to cells. Protection from nuclease action has been achieved by modification of phosphate backbones, sugar moiety, and bases [117].

Scleral Plug therapy: Scleral plug can be implanted using a simple procedure at the pars plana region of eye, made of biodegradable polymers and drugs, and it gradually releases effective doses of drugs for several months upon biodegradation. The release profiles vary with the kind of polymers used, their molecular weights, and the amount of drug in the plug. The plugs are effective for treating vitreoretinal diseases such as proliferative vitreoretinopathy, cytomegalovirus retinitis responds to repeated intravitreal injections and for vitreoretinal disorders that require vitrectomy [60].

Ribozyme therapy: RNA enzymes or ribozymes are a relatively new class of single-stranded RNA molecules capable of assuming threedimensional conformations and exhibiting catalytic activity that induces site-specific cleavage, ligation, and polymerization of nucleotides involving RNA or DNA. They function by binding to the target RNA moiety through Watson-Crick base pairing and inactivate it by cleaving the phosphodiester backbone at a specific cutting site. Successful implementation of this principle has been demonstrated in the work of Lewin and Hauswirth in the delivery of ribozymes in Autosomal dominated retinitis pigmentosa (ADRP). ADRP is caused by mutations in genes that produce mutated proteins and lead to the apoptotic death of photoreceptor cells [117]. The results of this study in rats may pave the way for ribozyme therapy in many other autosomal dominant eye diseases, including glaucoma.

Aptamer: Aptamers are oligonucleotide ligands that are used for high-affinity binding to molecular targets. They are isolated from complex libraries of synthetic nucleic acid by an iterative process of adsorption, recovery, and reamplification. They bind with the target molecules at a very low level with high specificity. One of the earliest aptamers studied structurally was the 15-mer DNA aptamer against thrombin, d(GGTTGGTGTGGTTGG) [118]. For ocular neovascularization and vascular permeability, Pegaptanib sodium is an RNA aptamer available from Eyetech Pharaceuticals/Pfizer to target VEGFb165 [117].

Transscleral Iontophoresis: Iontophoresis is a noninvasive technique in which a weak electric current is used to drive the penetration of charged molecules into and across percutaneous tissue. Iontophoresis has been used in several areas of medicine, and iontophoresis via the cornea has been in use in ophthalmologic practice and research for many years. However, with the development of potent drugs that can be applied topically, corneal iontophoresis is rarely used today. Maurice and Barza et al. performed transscleral iontophoresis by placing the iontophoretic probe over the pars plana and were able to deliver high concentrations of drugs to the vitreous humor. Placing the iontophoretic probe over the pars plana enabled them to bypass the lens–iris barrier, which prevented significant levels of drug from moving from the anterior to the posterior segment. These early studies on the use of iontophoresis for ocular drug delivery were conducted with devices that were clumsy or inconvenient; as the charged drug reservoir in solution had to be placed over the iontophoretic site. Transscleral iontophoresis therapy is a promising field, although at present, the full potential for its use in ophthalmology has not been realized.

Effective treatment of ocular diseases is a formidable challenge for scientists in the field especially because of the nature of diseases and presence of the ocular barriers especially in posterior ocular segments. An ideal therapy should maintain effective levels of drug for the longer duration following a single application. Drug delivery by topical and intravitreal routes cannot be considered safe, effective and patient friendly. Drug delivery by periocular route can potentially overcome many of these limitations and also can provide sustained drug levels in ocular pathologies affecting both segments. Transporter targeted delivery can be a promising strategy for many drug molecules. Nanoparticulate systems can substantially improve the current therapy and may emerge as an alternative following their periocular administration. In recent years, scientists have focused on designing a strategy with a multidisciplinary approach e.g. microneedle, microemulsion, nanosuspension, iontophoresis and MRI. Also, continued innovation in gene delivery appears to be very exciting for a gamut of diseases. Development of noninvasive delivery techniques will revolutionize ocular drug delivery. The potential for the growth of sustained drug delivery systems involving polymeric systems is limitless, and newer polymers would serve the purpose of controlled and sustained delivery for treating vision-threatening diseases. Advances in nanotechnology and noninvasive drug delivery techniques will remain in the forefront of new and novel ophthalmic drug delivery systems.