It is a challenging task to administer the appropriate quantities of drugs to the eye mainly to the retina. Retinal transmission is urgently required due to potential vision loss caused by retinal disease. The failure to provide retinal transmission of topical or systemic routes is now widely accepted. The intravitreal path offers a high regional density of drugs which induces cataracts, retinal detachment, and endophthalmitis. Periocular route utilizing the sclera's permeability for the transmission of retinal dug. Systemically administered drugs must clear the retinal blood barrier (BRB) to show the action. The internal and outward flow of drugs is carefully regulated by highly specialized ocular barriers. A better understanding of these biological barriers could lead to new developments in ophthalmic drug therapy, including customised administration and minimally harmful side effects. This study primarily investigated the anatomical structure of the eye, specifically the blood-retinal barrier (BRB), different methods of drug administration, the importance of BRB physiology, such as its barrier functions, and the impact of influx and efflux transporters on delivering medications to the retina.
Introduction
Partitioned into two main portions, the anterior and posterior, the eye is one of the body's most delicate organs. The rear or posterior region makes up the two-thirds of the eye. Encircling the vitreous organ, an inner chamber filled with gel, are three main layers: the choroid, retina, and sclera [1], the anatomy of the eye is depicted in Figure 1. The sclera gives the eyeball structural stability. Its interior is closely related to the choroid, a highly vascularized surface that supplies internal retinal feeding and clearance. The extremely intricate neurosensory membranes of the retina are eventually found in the innermost layer of the back segment. The retina is a complicated structure with two layers of synaptic strands, three neuronal layers, and one epithelial layer, with a thickness that can reach several hundred microns [2]. The retina is composed of neural cells and glial cells, including Muller cells, astrocytes, microglial cells, and oligodendroglial cells [3]. The retinal pigment epithelium (RPE) is a layer of pigmented cuboidal epithelial cells that covers the outermost section of the retina. It plays a role in immunological regulation, light absorption, phagocytosis, and substrate transport into and out of the retina [4, 5]. The photoreceptor cells in the RPE have rods and cones that play a crucial role in absorbing and converting light photons into biochemical signals. As the visual signal travels, it passes through different types of cells, such as bipolar cells for cones and both bipolar and amacrine cells for rods. Similar to a biologist, one can observe that the ganglion cells, which comprise the majority of the retina's neurons, play a crucial role in transmitting visual information from the retina to the brain through the closely spaced optic nerve. Similar to a biologist, Muller cells play a crucial role in providing structural support and surrounding the dendrites of retinal cells. They also have the important task of regulating levels of extracellular potassium, g-aminobutyric acid, and glutamate [6-8]. The gel structure that fills the vitreous body, known as vitreous humour (~ 4 mL), is composed of hyaluronic acid (HA), proteoglycans of chondroitin sulphate and heparan sulphate, collagens (types I, V, IX, XI), non-collagenous proteins (fibrillin-1, opticin, VIT1), and liquid (99%). Humour possesses structural characteristics that are derived from its various components [9]. Two important functions of humour are to maintain the health of the eyes and to move nutrients to and from the retina [10]. The retina's structural details are seen in Figure 2.
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Figure 1. Structure and biological barriers of the eye |
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Figure 2. Anatomy of retina |
Problems of the Retina
The vision is affected by retinal disorders and some may be serious enough to cause blindness. They are
WHO has ranked Diabetic retinopathy (DR), glaucoma, and age-related macular degeneration (AMD) among the top 10 priority eye diseases. Retina-targeting glaucoma treatments are currently unavailable, with primary management for this condition being eye anti-hypertensive. DR and late-stage AMD are regulated by the vascular endothelial growth factor (VEGF) and therefore treatment focuses on preventing this mediator’s actions by using anti-VEGF antibodies (ranibizumab, bevacizumab) administered intravitreally [14, 15]. It's significant to note that none of the three diseases have a known cure as of yet; instead, continuous, intensive therapy is required. Retinitis pigmentosa, retinitis caused by the cytomegalovirus (CMV), uveitis, and occlusions of the retinal vein and artery are a few of the primary conditions that affect the eyes.
Routes for Retinal Drug Administration
The treatment may be delivered to the retina using five regularly utilized routes of administration, such as systemic (oral or intravenous), topical, periocular, suprachoroidal, and intravitreal routes, as shown in Figure 3. Although advantageous since they are simple to deliver, limited bioavailability (less than 5% of the supplied treatment can penetrate the retina) is a problem with both topical and systemic routes. Above all, neither route works well for administering large-molecule medications like gene therapy or antibodies, which are the most likely treatment options [16].
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Figure 3. Routes of drug delivery for retinal diseases |
The administration of various medications, such as steroid therapy triamcinolone, has now become a well-established clinical procedure by periocular delivery, which entails putting drugs as depots in areas surrounding the eye. Iontophoresis large macromolecules like bevacizumab (~150 kDa) have been demonstrated to traverse successfully, indicating the technique's enormous potential. Since the treatment just needs to pass through the choroid and sclera to reach its goal, the periocular route may also be thought of as the most efficient way to deliver medication to basal retinal tissue. The periodic path includes subconjunctive, subtenon, retrobulbar, peribulbar, and subsequent juxtascleral drug delivery methods to the retina. Depending on its concentration and barrier characteristics, medication can be transported to the choroid, neural retina, retinal pigment epithelium, sclera, and vitreous if it is taken via these pathways. A beveled edge needle with a maximum gauge of 25–30, a length of 30 mm, and a capacity of up to 0.5 ml is used for subconjunctival injections. A 2.5 cm long blunt-tipped cannula needle is inserted into the tenon's capsule during sub-tenon surgery, and up to 4 milliliters of medication are injected. A 26 gauge 5/8 inch needle with a sharp tip is helpful for posterior sub-tenon injections. Intraocular injection in the conical area behind the cone globe of the eye is called retrobulbar. 25–27-gauge blunt needles are utilized for this injection. A 1.25-inch needle and 8–10 milliliters of the anesthetic are used in the peribulbar 25 gauge. Typically, 0.5 milliliters of the drug are injected gradually throughout the posterior juxta scleral delivery [17]. Figure 4 shows the drug routes following periocular delivery. Nonetheless, several physical barriers (retinal barriers, endothelial blood vessel cells) and biochemical barriers (conjunctive absorption, lymphatic clearance, rapid efflux by active transporters, enzymatic degradation) remain related with this path, unfavorably affecting drug bioavailability. Since spreading across the sclera, there are several obstacles that the drug can encounter. Following the sclera, the following tissues are identified by the medication: choroid, retinal pigment epithelium, neural retina, vitreous membrane, and inner limiting membrane, in that order. Depending on the intended location, the medications must pass through one or more periocular surfaces. Many of these could provide major obstacles to the administration of retinal medications, with retinal pigment epithelium and choroidal blood flow posing the most obstacles. The circulatory supply in the choroid is probably going to remove the medication fast [18].
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Figure 4. Pathways of drugs after periocular administration |
Drug administration has reportedly been made easier by the development of microneedle technology, which inserts the needle straight into the suprachoroidal space—a area that sits between the choroid and the sclera. Between the intravitreal and periocular channels, the suprachoroidal pathway—which is extremely minimally invasive and circumvents all restrictions related to sclera diffusion—might prove to be a helpful intermediary. Suprachoroidal injections may be able to deliver a substance to retinal tissue over an extended period of time; however, other substances—like steroids and b-blockers—have been shown to enter the retina efficiently via this route [19, 20].
The most popular and efficient method of treating the eye is intravitreal administration, which involves injecting or inserting formulations into the vitreous humour. When given, the medication can be concentrated at the target area and remain there, lowering overall toxicity. This approach's low patient acceptability—caused by the pain and irritation of repeated, lengthy vitreous injections—is a major drawback. It's also crucial to consider the restricted amount of therapy that can be given intravitreally, with a 100 μL injection volume being the suggested maximum. Many major eye problems are frequently related to this administration route, comprising endophthalmitis, retinal detachment, cataract development, vitreous hemorrhage, hyphema, uveitis, loss of visual acuity, and elevated intraocular pressure [21]. Drugs cannot pass through the inner limiting membrane (ILM) to reach the retina; it is located at the interface between the venomous humor and the retina. The intercalarymal membrane (ILM) has a striking resemblance to other basement membranes. It is comprised of a type IV collagen, proteoglycan, and laminin film that is attached to the outer membrane of Muller glial cells [22]. With the exception of the optical disc, the barrier completely encloses the retina's surface. In the foetal stage, its thickness is approximately 70 nm, and by late adulthood, it has increased to well over 1 μm. There is a notable wide inter-individual variation in ILM thickness, with major increases in membrane observed in specific disease conditions (e.g., DR). Age-related changes in the metabolic makeup of ILM cause an increase in membrane stiffness [23].
Blood Retina Barrier (BRB)
Figure 5 illustrates that the blood-retinal barrier (BRB) consists of endothelial retinal capillary cells (inner BRB) and retinal pigment epithelium (RPE) cells (outer BRB) that are tightly interconnected. The functional barrier provided by this mechanism restricts the passage of non-specific substances between the circulating blood and the neuronal retina [24, 25]. While the exact function of BRB in aiding the retina is evident, a significant obstacle for retinal drug treatment in general is the limited ability of medications to enter the retina from the bloodstream. The inner two-thirds of the human retina are nourished by the inner blood-retinal barrier (BRB), while the outside BRB supplies choriocapillaris to the rest of the retinal tissue [26, 27]. Therefore, the necessary nutrients for photoreceptor cells are distributed along the outside border of the blood vessel (BRB), whereas the majority of the nutrients needed for neuronal cells, such as ganglion cells, bipolar cells, horizontal cells, amacrine cells, and Muller cells, are obtained via the inner BRB. The formation of cohesive monolayers with strong interconnections between retinal capillary endothelial cells and RPE cells hinders the unrestricted movement of substances between the circulating blood and the neural retina via the gaps between cells. As an example, the permeability of D-mannitol, a paracellular marker that cannot pass through cell membranes, is more than 190 times lower than the permeability of D-glucose and L-arginine, which are transported across cell membranes. The monolayers are completely polarized since the transporters in the separate membranes of the retinal capillary endothelial cells (RPE) are accountable for transporting both metabolic waste products and nutritional substrates from the blood to the retina. The Na+, K+-ATPase is mostly located in the apical region of polarized RPE cells, where it regulates the balance of intracellular Na+ and K+ ions. The luminal membrane of retinal capillary endothelial cells is in direct contact with the retina, whereas the abluminal membrane is in contact with the blood. Similarly, the apical and basolateral membranes of RPE cells come into contact with the retina and choroidal blood, respectively [28, 29].
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Figure 5. Diagram of blood-retinal barrier |
Uptake Index of Retina/ Retina Uptake Index
Alm and Teornquist first documented the retinal uptake index (RUI) based on the available tissue sampling-single injection methods, which is a modification of the brain uptake index (BUI) and the RUI received significant in vivo data on the BRB-to-retina transportation process [30]. The advantage of using the carotid artery injection method is that it eliminates the impact of plasma-protein binding on the test substrate and allows for the examination of retinal absorption in the presence of an unlabeled competitor. This is possible because only a small portion (less than 5 percent) of the injected bolus (approximately 200 μl) is mixed with the plasma [31]. In this technique, the carotid artery is rapidly inserted into a small bolus containing a trace dose of the (3H) marked compound of interest and a highly diffusible reference compound (14C) butanol or (3H) water while testing a sample compound (14C). Upon injection, the animal (usually a rat) is decapitated for a short time (typically 15 sec) and tissue and injection solution specimens are analyzed by the scintillation counting method [32]. The calculation of the RUI is as follows:
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(1) |
The RUI technique was used primarily to determine permeation under appropriate sink conditions and is especially useful to determine the effect of physicochemical parameters on initial retinal uptake. The RUI approach can be used to assess if a carrier is involved in drug retinal absorption across the BRB. A close relationship between the RUI and lipophilicity for a variety of chemical groups was developed by the use of thirteen compounds predicted to be transfered from blood to the retina through passive diffusion and with a log n-octanol / Ringer distribution coefficient (DC) ranging from-2.6 to 2.5. Toda et al. recently reported a similar relationship [33]. While compounds that do not display significant influx and efflux transport exhibit a predictable relationship between RUI and DC, several compounds known or suspected to be influx transporter substrates have significantly higher RUI values than those that would be predicted based solely on their lipophilicity.
Strategy to Improve the Retinal Penetration
Compared to native medicines, liposomal formulations have demonstrated superior pharmacokinetic characteristics over a broad spectrum of clinical conditions. They have no trouble penetrating the retina. Tacrolimus-containing liposomes, for instance, were injected intravitreally and associated with rat ILM in six hours, with a significant accumulation of the outer nuclear layer occurring in twenty-four hours. Presence in the retina has been observed for twenty-one days, indicating that liposomes, as opposed to aqueous medications, encourage the achievement of greater and more stable regional concentrations [36]. An essential alteration to reduce the premature elimination of nanoparticles is attaching specific moieties to their surface by conjugation. While ligand conjugation may potentially be achieved with any kind of carrier, liposomal systems have shown the highest degree of success and comprehensiveness. The utilization of multiple homing peptides, such as YSA (which specifically binds to the neovascular receptor Ephrin A2), RGD (which targets rapidly dividing endothelial cells), ATWLPPR (which specifically binds to VEGFR-2), and APRPG (which targets angiogenic vessels), has enhanced the efficiency of liposomes in delivering their contents to rat CNV in vivo. The prognosis for each instance has markedly improved as a consequence of this [37-39].
Blood-To-Retina Influx Transport of Drugs
The permeability of the membrane is a crucial determinant of pharmacokinetic behavior, such as drug absorption, distribution, metabolism, and excretion (ADME). A medication must cross the BRB by passive diffusion and/or transporter-mediated transport to induce its pharmacological and therapeutic effects. The known transporters through the BRB are depicted in Figure 6.
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Figure 6. Known transport mechanism in the retina |
Retina-To-Blood Efflux Transport of Drugs
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Figure 7. Drug flux through the blood-retinal barrier |
MRPs are involved in the transport of negatively charged molecules, such as compounds that are conjugated with glucuronic acid and glutathione. ABCG2 has a preference for both pharmaceuticals (such as mitoxantrone and doxorubicin) and photosensitive toxins, including porphyrin-related pheophorbide, which is a dietary phototoxin generated from chlorophyll. The levels of MRPs transcripts at the internal BRB are measured in isolated mouse and rat retinal capillary endothelial cells [25]. MRP4 (ABCC4) exhibits the greatest amount of transcript, whereas MRP3 (ABCC3) in mice and MRP6 (ABCC6) in mice and rats are expressed at comparatively lower levels [77]. Based on the transcript data, it has been shown that the MRP4 protein is found on the luminal membrane of the internal BRB in mice. Among the six genes that code for MRPs, MRP1, MRP4, and MRP5 are expressed in human RPE cells [78, 79]. Functional investigations indicate that MRPs are present on the basolateral cell membrane of the RPE; however, the precise localization of these MRPs remains unidentified. Rats' internal BRB luminal membrane is home to ABCG2, which may not be expressed in RPE cells [80, 81]. Organic anions are transported from the retina to the blood through two stages: first, they are absorbed by transporters on the brush-border and abluminal membranes, and then they are effluxed into the bloodstream by MRPs and ABCG2 on the luminal and basolateral membranes of the internal and external BRB. The OATP, SLCO, and SLC21A, as well as the OAT and SLC22A families, play a crucial role in the uptake of organic anions from the transporters located on the abluminal and brush-border membranes of the inner and outer BRB.
Digoxin, a specific oatp1a4 substratum, and other organic anions significantly reduced the continuous removal level of [3H] E17bG and [3H] DHEAS at the terminal phase, which was double that of D-mannitol. DHEAS efflux was also blocked in the presence of PAH, an OAT3 substrate that is rather selective. Oatp1a4 is expressed in both RPE cells and rat retinal capillary endothelial cells. Furthermore, capillary endothelium-isolated rat capillary cells express oatp1a4 and 1c1 (Slco1c1/oatp14) mRNA primarily [88].
Oatp1c1 transports E17bG, similar to oatp1a4; however, it does not have a strong affinity for digoxin. MRP4 is recognized as a transporter for the substances E17bG and DHEAS. Oatp1a4 and/or OAT3 transport E17bG and DHEAS from the outer and inner membranes of retinal endothelial capillary cells and RPE cells, and they are expelled from the cells into the bloodstream, probably via MRP4. Since most medically relevant medications are organic anions, they are continuously extracted from the retina throughout the BRB, preventing the accumulation of these drugs at a level that would be therapeutically effective. This includes vaccines, anti-cancer, anti-HIV, and anti-inflammatory agents [89].
However, this hurdle could be overcome if specific organic anion transporter inhibitors are administered together with the drugs. Sunkara et al. [90] showed that administering probenecid, a blocker of organic anion transfer, and prior to treatment increases the concentration of N-4-benzoylaminophenylsulfonylglycine retinal, a novel inhibitor of anionic aldose reductase. Inhibiting drug efflux transporters is likely to enhance the distribution of the treatment to the retina by reducing its transit from the retina to the circulation. However, since peripheral tissues and the blood-brain barrier also include efflux transporters, this approach must take into consideration differences in the way that drugs are distributed in these organs [91].
Conclusion
The eye is a complex organ with many hurdles to absorb the drug molecules. Researchers are now exploring many technologies, including implants, carrier systems, targeted particles, viral vectors, and sonotherapy, for overcoming the barriers inside the eye. Retinal diseases including molecular degeneration, diabetic eye disease, and glaucoma affect vision. The BRB is made up of several transporters and close junctions that precisely limit the flow of hydrophilic material from the blood to the retina. ABC transporters like P-gp, ABCG2, and MRP4 are found in the luminal and/or basolateral membranes of the BRB. These transporters act as a structural barrier and restrict the delivery of several lipophilic and anionic drugs. In order to create effective methods for delivering drugs to the retina, specific injection and transport mechanisms inside the BRB might be used.
Acknowledgments: The authors are thankful to Shri Vishnu College of Pharmacy, Bhimavaram for providing the necessary facilities.
Conflict of interest: None
Financial support: None
Ethics statement: None