The human eye is a complex organ with intricate anatomical and physiological barriers. The anterior segment of the eye consists of the cornea, conjunctiva, aqueous humor, iris, ciliary body, and lens. The posterior segment mainly consists of the vitreous humor, retina, choroid, and optic nerve (Figure 1) [1,2]. Common diseases affecting the anterior segment of the eye are dry eye syndrome, glaucoma, allergic conjunctivitis, anterior uveitis, and cataract. Prominent diseases affecting the posterior segment of the eye include age-related macular degeneration (AMD), diabetic retinopathy macular edema (DME), proliferative vitreoretinopathy (PVR), posterior uveitis, and cytomegalovirus (CMV) . The global population includes 258 million visually impaired people, with 39 million among them being completely blind . These concerning statistics validate an increased need for exploring improved drug delivery strategies for ocular therapies.
Drug delivery to the anterior segment of the eye via the topical route typically involves the conventional dosage forms, such as solutions (62.4%), suspensions (8.7%), and ointments (17.4%), which compose an estimated 90% of marketed ophthalmic formulations. However, topical drug administration demonstrates poor ocular bioavailability (<5%) due to lacrimal secretions that lead to poor retention time and decreased permeability across the corneal epithelium. Tear turnover from lacrimal secretions contributes to a majority of the drug loss. A healthy eye with a tear volume of ~7–9 µL has a turnover rate of 0.5–2.2 µL/min . During topical administration, the average volume of major formulations is ~35–56 µL, and excess volume drains via the nasolacrimal duct into systemic circulation. In addition, conjunctival blood circulation affects the topical drug absorption. All of the barriers combined result in a drug loss of ~95% from topical administration. The rest of the drug encounters the corneal epithelial barrier. The significant role of the cornea as a barrier is discussed in detail in subsequent sections.
2. Anterior Segment Drug Delivery Barriers
2.1. Epithelial Tight Junction (ZO)
The corneal epithelium forms the primary barrier to drug absorption via topical administration. The stratified corneal epithelium consists of a basal layer of columnar cells, two to three layers of wing cells and one or two outer layers of squamous cells . Superficial cells are surrounded by the intercellular tight junctions (zonula occludens). The tight junctions act as barriers for permeation of drug molecules via the paracellular route. Tight junctions are composed of anastomotic strands that confer resistance to the paracellular drug absorption . There are four tight junction proteins, ZO-1, cingulin, ZO-2 , and occluden , with occluden being the most important. Extracellular and intracellular calcium levels in tight junctions influence the permeability . If tight junction membrane integrity is disrupted or extracellular calcium ions are removed by EDTA, drug permeability increases throughout the tight junctions [10,11]. The pores of the corneal epithelium are negatively charged at physiological pH, hence negatively charged molecules permeate slowly compared to positively charged molecules . Also, the cellular calcium levels and actin filaments present on the cytoskeleton play an important role in maintaining the integrity of tight junctions [6,9,10].
2.2. Reflex Blinking
A normal eyedropper delivers 25–56 µL of the topical formulation with an average volume of 39 µL. However, an eye can transiently hold up to 30 µL, and the rest is lost either by nasolacrimal drainage or reflex blinking (5–7 blinks/min), significantly decreasing the overall drug available for therapeutic action .
2.3. Metabolism in Ocular Tissues
Drugs containing aromatic hydrocarbons are metabolized in the pigmented epithelium and ciliary body to their corresponding epoxides and phenols, or further metabolized by other enzymes present in the eye . Hayakawa et al. demonstrated that poor absorption of peptide drugs and insulin is due to extensive metabolism during conjunctival permeation in albino rabbits . Schoenwald et al. demonstrated that clearance via aqueous humor turnover is significantly lower when compared to the rest of the clearance pathways, indicating that a majority of the drugs are eliminated via metabolic pathways [16,17].
2.4. Tear Turnover
A significant impediment to topical ocular delivery is tear turnover. Following topical administration, an increase in the volume of cul-de-sac occurs that leads to reflex blinking and increased tear secretion, eventually resulting in rapid drug loss from the precorneal area . Loss of the solution occurs due to tear turnover and nasolacrimal drainage until the tear volume in the conjunctiva cul-de-sac returns to a normal range (7–9 µL) . The initial first order drainage rate of eye drops from the ocular surface is 1.2 µL/min in humans [17,20] and 0.5–0.7 µL/min in rabbits .
2.5. Nasolacrimal Drainage
As mentioned above, a majority of the instilled drug is lost due to tear turnover or nasolacrimal drainage. About 95% of the dose administered is eliminated systemically via the conjunctiva and nasolacrimal duct . The lacrimal drainage system in human adults serves as a conduit for tear flow from the eye to the nasal cavity. The pathway consists of the puncta, canaliculi, lacrimal sac, and nasolacrimal duct. Histologically, the walls of the lacrimal sac and the nasolacrimal duct are vascularized, and hence are potential sites for systemic drug absorption. After topical application, the eye drop solution initially mixes with lacrimal fluid. The contact time of the drug with ocular tissues is approximately 1–2 min due to the constant production of lacrimal fluid. Approximately half of the drug flows into the upper canaliculus and the rest into the lower canaliculus of the lacrimal sac. The flow further opens into the nasolacrimal duct, and, from there, drains into the nose . A few factors that determine the topically applied drug concentration are the volume of the instilled drug solution, reflex blinking by the patient and the patient’s age. Larger instilled volumes easily pass into the nose from the nasolacrimal sac , and smaller volumes are easily eliminated from the lacrimal sac . The loss of drugs from the nasolacrimal duct via transconjunctival absorption or transnasal absorption is unwanted because of direct exposure to systemic circulation .
2.6. Efflux Pumps
The efflux proteins are located either on the apical or basolateral cell membranes. These proteins either restrict or enhance the drug absorption, depending on their cellular localization . The ATP-binding cassette, commonly known ABC proteins, are a superfamily of proteins that are encoded by an MDR1 gene responsible for the efflux of various substrates across the plasma membrane and cytoplasm into the extracellular fluid. There are primarily two major efflux pumps that are responsible for drug resistance: (a) P-glycoprotein, which restricts entry of amphipathic compounds, both in normal and cancer tissue, and (b) multidrug resistant protein (MRP) (ABCC1), which is known to efflux organic anions and conjugated compounds [27,28].
P-glycoprotein 1 (P-gp), also known as MDR1 or ABCB1, is a ~170 kDa ATP dependent efflux pump. It is located on the apical surface of polarized cells  and is responsible for decreasing drug accumulation in multidrug-resistant cells. Further, it confers resistance to the absorption of anticancer drugs by tumor cells. P-gp was shown to be present on the ocular conjunctival epithelial cells , ciliary non-pigmented epithelium , human and rabbit cornea , iris and ciliary muscle cells, and retinal capillary endothelial cells . P-gp has been detected at the mRNA level in the human cornea, rabbit corneal epithelium, and primary cultures of rabbit corneal epithelial cells . According to Constable et al. the presence of P-gp on three human RPE cells (ARPE19, D407, and h1RPE) have been studied. It was demonstrated that only D407 cells express P-gp and can be utilized for in vitro drug transport studies without any modifications .
MRP is a ~190 kDa membrane-bound efflux protein encoded by the ABCC1 gene. It is generally found on the basolateral surface of the intestine, hepatocytes, and kidney cells [35,36]. It acts as an organic anionic transporter with glutathione, cysteinyl leukotrienes, glucuronides, sulfate conjugates and bile salts . MRP expression has been detected in the human corneal epithelium at the RNA level . MRP5 was expressed at a higher level than MDR1, MRP1–MRP4, MRP6, and BCRP . Chen et al. investigated the expression sites of various efflux transporters in different ocular tissues. The study reported that in human cornea efflux transporters including MRP1–4, MRP6 were localized in the basal layer of corneal epithelium, whereas MRP7 and MDR1 were expressed in the entire corneal epithelium. In human conjunctiva, MRP2–4, MRP6, MDR1, and BCRP were expressed in basal cell layer while MRP1, MRP7 were detected in the entire conjunctival epithelium. In human iris ciliary body, MRP1–2, MDR1 were detected in stromal cells . Zhang et al. studied drug transporter and cytochrome P450 mRNA expression in ocular drug disposition. They concluded that both BCRP and MRP2 have very low expression levels in the human cornea, while MRP1 was moderate and MRP3 had low expression levels in the human cornea. Thus, designing drugs that could efficiently evade MRP1 efflux can play an important role in enhancing the ocular absorption .
3. Nanocarriers for Anterior Segment Drug Delivery
Despite extensive research efforts, drug delivery to the eye remains a challenge. The anatomical position of the eye confers a unique advantage for site-specific drug delivery and non-invasive clinical assessment of a disease state. For optimal therapeutic activity, drug molecules should circumvent the protective physiological barriers without causing permanent tissue damage. Anterior segment drug delivery comprises the conventional dosage forms, such as solutions, suspensions, ointments and novel dosage forms, such as liposomes, nanoparticles, and implants [41,42]. However, >90% of the marketed formulations are conventional dosage forms, with limited bioavailability due to precorneal clearance and less duration of action, thus requiring frequent administration . Major research is directed towards the development of sustained release nanocarrier systems with higher precorneal retention. Such systems can improve the ocular bioavailability of drugs and provide high patient compliance. For example, patient adherence to eye drops plays a key role in the management of glaucoma and is frequently low (<50%) . Administration of drugs to the eye by means of a droptainer bottle is challenging in elderly patients, due to the lack of physical acuity and inability to aim adequately . The adherence to antiglaucoma therapy using eye drops deteriorates with age . Nanocarriers, such as liposomes, micelles, microemulsions, biodegradable nanoparticles, nanosponges, punctal plugs, and dendrimers hydrogels have been investigated as carriers for antiglaucoma drugs for their ability to deliver drugs in a sustained manner . Further, precorneal retention of drugs loaded into nanocarriers has been improved by coating them with mucoadhesive polymers such as polyethylene glycol (PEG), chitosan and hyaluronic acid, and by dispersing nanocarriers in stimuli-responsive hydrogel, such as pH-, thermo-, and ion-sensitive hydrogels. Nanocarriers were found to effective in the prevention and treatment of cataract, where the nanodrug reached higher lens concentrations; while, the free drug was washed away by tears . More recent research efforts are focused on identifying enhanced drug permeability across the cornea via nanocarrier-mediated tight junction reorganization effect . The most widely employed nanoformulations in treating anterior segment diseases will be discussed in detail in the subsequent sections.
A microemulsion is a dispersion of water and oil stabilized by surfactants or co-surfactants to reduce the interfacial tension. Microemulsions are clear in appearance and thermodynamically stable with a small droplet size (~100 nm). Microemulsion formulations are shown to increase the solubility of drugs. An oil-in-water type of microemulsion in the presence of surfactant and co-surfactant is able to increase corneal membrane permeability . Increased permeability and sustained release of drugs makes microemulsions an attractive vehicle for ophthalmic drugs. Microemulsification improved solubility of poorly soluble drugs, such as indomethacin and chloramphenicol . Microemulsions have low surface tension and high spreading coefficient, allowing for the drug to spread and mix well with the precorneal fluid. This improves the corneal contact time of drugs . They can be sterilized by filtration for formulation as eye drops. Many studies have reported the occurrence of electrostatic attraction between the emulsified cationic droplets and anionic cellular charges of ocular tissues. Incorporation of a positively charged lipid might therefore increase the binding of cationic droplets to the negatively charged corneal surface . Microemulsion formulations of the ocular drugs, indomethacin, delta-8-tetrahydrocannabinol, pilocarpine, and timolol were tested extensively . In vivo rabbit studies using microemulsions demonstrated a sustained release effect and improved bioavailability . The pilocarpine microemulsion demonstrated increased absorption and reduced dosing frequency to twice a day, as compared to four times a day with conventional eye drops . Pilocarpine microemulsion systems exhibited different morphological forms (crystalline liquid and emulsion) with changes in aqueous content. This altered rheological behavior contributed to higher viscosity and longer retention of the formulation on the corneal surface . The moxifloxacin-loaded water-in-oil microemulsion demonstrated sustained drug release with higher in vivo antimicrobial activity as compared to the conventional solution . Gan et al. developed a cyclosporine-loaded microemulsion with in situ gelling capacity. The developed formulation demonstrated prolonged residence time, with three times higher AUC, compared to the conventional emulsion. Further, the formulation resulted in a sustained cyclosporine delivery for 32 h, preventing the corneal allograft rejection (Figure 2) .
Despite advantages, a narrow range of surfactants and oils that are non-toxic and biocompatible limits the success of microemulsions in ocular drug delivery . A review by Hedge et al. provided detailed information on microemulsions for ocular drug delivery .
Nanosuspensions are sub-micron colloidal dispersions of poorly water-soluble drugs in a dispersion medium stabilized by surfactants or polymers. These formulations usually consist of a colloidal carrier, such as a polymeric resin, which is inert in nature, for enhancing drug solubility and bioavailability. Unlike microemulsions, they are non-irritant and are regarded a desirable ocular drug delivery vehicle . The inert carriers employed in nanosuspensions are non-irritating to the cornea, iris, and conjunctiva . Nanosuspensions increase the precorneal residence time and enhance solubility and ocular bioavailability of drugs. Glucocorticoids, such as dexamethasone, prednisolone, and hydrocortisone are widely used in treating anterior segment inflammatory diseases . Repeated administration of glucocorticoid doses was clinically shown to induce cataract formation and cause damage to the optic nerve. Nanosuspension formulations of corticosteroids resulted in sustained drug release and increased ocular bioavailability [
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