Recent advances in ocular drug delivery systems and targeting VEGF receptors for management of ocular angiogenesis: A comprehensive review

Angiogenic ocular diseases address the main source of vision impairment or irreversible vision loss. The angiogenesis process depends on the balance between the pro-angiogenic and anti-angiogenic factors. An imbalance between these factors leads to pathological conditions in the body. The vascular endothelial growth factor is the main cause of pathological conditions in the ocular region. Intravitreal injections of anti-angiogenic drugs are selective, safe, specific and revolutionized treatment for ocular angiogenesis. But intravitreal injections are invasive techniques with other severe complications. The area of targeting vascular endothelial growth factor receptors progresses with novel approaches and therapeutically based hope for best clinical outcomes for patients through the developments in anti-angiogenic therapy. The present review article gathers prior knowledge about the vascular endothelial growth factor and associated receptors with other angiogenic and anti-angiogenic factors involved in ocular angiogenesis. A focus on the brief mechanism of vascular endothelial growth factor inhibitors in the treatment of ocular angiogenesis is elaborated. The review also covers various recent novel approaches available for ocular drug delivery by comprising a substantial amount of research works. Besides this, we have also discussed in detail the adoption of nanotechnology-based drug delivery systems in ocular angiogenesis by comprising literature having recent advancements. The clinical applications of nanotechnology in terms of ocular drug delivery, risk analysis and future perspectives relating to the treatment approaches for ocular angiogenesis have also been presented. The novel ocular drug delivery systems involving nanotechnologies are of great importance in the ophthalmological sector to overcome traditional treatments with many drawbacks. This article gives a detailed insight into the various approaches that are currently available to be a road map for future research in the field of ocular angiogenesis disease management.

million people with blindness [1,2]. In India, more than 30 percent of people become blind before they cross 17 years of age and most of them are of less than 5 years [3]. Impairment in vision is also widespread among elderly individuals in various other forms [4]. According to a study conducted in Al-Madinah Al-Munawarah, Saudi Arabia, among diabetic patients (n = 690), 36.1% were found to be suffering from diabetic retinopathy (DR) of which 6.4% had proliferative disease [5]. An additional cross-sectional study conducted in Al Ain, United Arab Emirates reported DR in 19% of diabetic patients (n = 513). Almost all the patients were completely unaware of the condition of their retina [6,7]. Approximately 8.7% of worldwide blindness is occurred due to age-related macular degeneration (AMD) especially in aged patients [8]. Angiogenesis accounts for the formation of new blood vessels from the existing vasculature. The physiological angiogenesis process in the human body is the balance between anti-angiogenic and proangiogenic factors [9]. Disturbance of such balance leads to a pathological condition in the human body. During the conditions such as wound healing and peripheral arterial disease ischemic heart disease, the stimulation of angiogenesis will cure the disease. Wherein case of diseases such as rheumatoid arthritis, cancer and ophthalmic conditions, the inhibition of angiogenesis is the cure [10]. Global ocular morbidity is the main reason behind severe ocular angiogenesis. In ocular angiogenesis conditions, the angiogenic switch must be turned "on" for neovascularization progression [11]. It may lead to diseases like retinal vein occlusions, diabetic retinopathy, corneal neovascularization, age-related macular degeneration, retinopathy of prematurity, choroidal and retinal neovascularization, etc. Pro-angiogenic growth factors implicated in the development of pathological vessels in ocular diseases include endothelial growth factor (EGF), fibroblast growth factor (FGF), platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), etc. [12]. The present review gives clear-cut knowledge on VEGF and their respective receptors with various regulations, affecting factors and available treatments with recent literature. It majorly focuses on numerous novel nanotechnology-based approaches in ocular drug delivery to treat many ocular conditions specifically angiogenesis to overcome traditional injection treatments that affect bioavailability and patient compliance.

Regulation of angiogenesis
In the human body, angiogenesis is involved in various processes [13]. In healthy adults, angiogenesis is a rare phenomenon, involved only locally and transiently under distinctive physiological and pathological conditions in the body. Angiogenesis is regulated by endogenous proangiogenic and anti-angiogenic factors (Table 1) [12,[14][15][16]. Among all angiogenic and anti-angiogenic factors, VEGF is marked to be a highly critical regulator of ocular angiogenesis. Regulation of angiogenesis involved five steps, initially, angiogenic factors bind to endothelial cells leading to the degradation of basement membrane with the proliferation of endothelial cell, further, migration and also tube formation, elongation finally vessel stabilization (Fig. 1).

VEGF-A
VEGF-A is one of the well characterized and highly investigated of the VEGF family members. Mainly it enhances the endothelium's permeability by forming the intercellular gaps and fenestrations. Hence, it was originally known as a vascular permeability factor (VPF). Most commonly VEGF-A isoforms have been identified from six transcripts: VEGF 111 , VEGF 121 , VEGF 145 , VEGF 165 , VEGF 189 and VEGF 206 [19].
in vitro and in vivo. VEGF-E is also a potent angiogenic. The binding of VEGF-E to VEGFR-2 with greater affinity results in angiogenesis stimulation and vascular permeability thus increasing in viral infection [20,36].

VEGFR-1
Vascular endothelial growth factor receptor-1 (VEGFR-1) is also termed as fms-like tyrosine kinase-1 (Flt-1) which is having 180 kDa and is also seemingly linked to receptor tyrosine kinase (RTK). The VEGFR-2 and Flt-1 are majorly expressed on vascular endothelium, Even though some of the mRNA remains in the stroma of human placenta, monocytes and renal mesangial cells. With high affinity, VEGF-A 165 binds to VEGFR1 when compared with VEGF-A 121 [37,38]. Table 1 Pro-angiogenic and anti-angiogenic factors involved in the angiogenesis process VEGF vascular endothelial growth factor, PGF placental growth factor, KDR kinase-insert-domain receptor, EGF epidermal growth factor, TGF transforming growth factor, FGF fibroblast growth factor, MMP matrix metalloproteinase, PEDF pigment epithelium-derived factor, EC endothelial cells PEDF Anti-angiogenic Promotes endothelial cell apoptosis Also known as serpin F1 Secreted by retinal epithelial cells First factor is described for its neurotrophic properties in vitro [28,29] Prolactin Anti-angiogenic Suppresses cell proliferation process also stimulates expression of plasminogen activator inhibitor N-terminal 16-kDa fragment 16 K-PRL is a natural inhibitor of ocular angiogenesis [30,31] Angiostatin Anti-angiogenic Inhibits ATP synthesis and that slows down migration of EC and proliferation It is a fragment of protein plasminogen Plasminogen and angiostatin can be synthesized by corneal layers [19] Vaso inhibins Anti-angiogenic It blocks inducers of angiogenesis (VEGF, FGF, etc.) through in vitro and in vivo It can be generated by various proteases (MMP's) [32] Endostatin

Anti-angiogenic Suppresses migration and proliferation of endothelial cells and increases apoptosis
Derived from collagen XVIII [19] Thrombospondin Anti-angiogenic Assists in tumor death First discovered in activated platelets Contains five multifunctional proteins that can bind extracellular calcium that is TSP-1 to TSP-5 [33]

VEGFR-2
VEGFR-2 is the second VEGF tyrosine-kinase receptor that is present on chromosome 4q12. It is also called a kinase-insert-domain containing receptor (KDR). This KDR is majorly expressed in the region where endothelial cells are abundantly present and were also replicated from a human endothelial cell cDNA library. Due to the ligands of VEGF family, the VEGFR-1 and VEGFR-2 convert the signals for endothelial cells [39].

VEGFR-3
VEGFR-3 is also called fms insert-like tyrosine kinase 4 (Flt-4). They have got the extracellular domain which is almost 80% homologue to the other VEGFRs. The VEGF-C and VEGF-D that belong to the family of VEGF are also associated with VEGFR-3. The Flt-4 is majorly expressed in lymphatic endothelium specifically in adult tissues which are usually not seen for VEGFR-1 and VEGFR-2 [40].

Neuropilins
The neuropilin-1 (NRP-1) and neuropilin-2 (NRP-2) are expressed in endothelial cells just like VEGFR-1 and VEGFR-2. It is particularly with less affinity binds to the VEGF-A 165 . The neuropilin-1 balances the development of blood vessels during the angiogenesis embryonic stage, conveying a major role for VEGFR-2 as a coreceptor [41].

Regulation of VEGF
VEGF is a crucial regulator of angiogenesis, in a condition of uncontrolled regulation of VEGF leads to pathological angiogenesis. As we have already discussed earlier, angiogenesis is a balance between the pro-angiogenic and anti-angiogenic factors if is there any imbalance between these factors that leads to pathological angiogenesis [42]. Although, many growth factors and cytokines are released in response to any damage in the tissue which leads to stimulation of angiogenesis either directly or indirectly through VEGF that is crucial in tissue repairing. The elevated pathological angiogenesis is occurred due to the stimulation of VEGF expression during pathophysiological conditions like diabetes mellitus and cancer. The literature data was supported this hypothesis by demonstrating suppression of neovascularization by the inhibition of VEGF or its effects. But, during the conditions like atherosclerosis, the elevated concentration of plasma VEGF might be an attempt to make up for the damage of tissue [43,44]. In all these pathological conditions, angiogenesis is stimulated by local tissue hypoxia. The ocular cells like muller cells, astrocytes, retinal pigmented epithelium, endothelial cells (EC) and ganglion cells secret and produce VEGF. In vitro studies showed  that under hypoxic condition muller cells and astrocytes produces larger amounts of VEGF [45].

Effect of oxygen, nitric oxide, glucose and other growth factors on VEGF regulation
In various diseases like atherosclerosis, solid tumors, ocular diseases, etc., the stimulation of VEGF results in neovascularization and it is mainly due to the hypoxic condition. The major protein named hypoxia-inducible protein complex (HIPC) or hypoxia-inducible factor (HIF) is produced by hypoxia. The up-regulation of transcription of VEGF mRNA was occurred due to the activation of basic heteromeric helix-loop-helix transcriptional regulator. The production and stability of some VEGF isoforms are majorly due to the hypoxia condition. In terms of stability, the VEGF-A isoforms are highly sensitive to hypoxia, wherein the case of VEGF-B and VEGF-C mRNA has little or no effect on hypoxia [46,47]. The vascular endothelium and endothelial cells can release nitric oxide (NO) in response to VEGF. Additionally, during the VEGF-induced angiogenesis, the production of nitric oxide synthase (NOS) is also increased. A demonstration showed the part of NO in VEGF-induced angiogenesis on NOS knock-out mice as well as after inhibition of NOS, leads to angiogenesis depletion [48,49]. In the beginning, the increased expression of VEGF was believed to be from the hypoxic condition but it was the hypoglycemic condition. However, in later stages, it was reported that the cells exposed to hypoglycemia without HIF (hypoxia) elevated the expression and upregulation of VEGF. The production of VEGF came back to the pre-experimental level in response to a balanced concentration of glucose suggesting the acute hypoglycemia is the sole responsible to activate angiogenesis mediated via VEGF. Also, pro-angiogenic growth factors such as fibroblast growth factor 4 (FGF 4), tumor necrosis factor-α (TNF-α), transforming growth factor-β (TGFβ), insulin-like growth factor I, PDGF, angiotensin-2, keratinocyte growth factor, interleukin 1 (IL-1) and IL-6 can alter the production of VEGF. It was also found that some of the anti-angiogenic growth factors such as the cytokines IL-10 and IL-13 can deduce the production of VEGF [50,51].

Angiogenesis inhibitors or vascular endothelial growth factor inhibitors (anti-VEGF)
The inhibition of VEGF-VEGFRs protein factors unlocked new prospects in medicine since they are the sole ones responsible for many pathological conditions such as angiogenesis and ocular vascular diseases. In many ocular neovascularization conditions, the hindrance of VEGF activity plays a major role in treating such conditions. In recent years, research to develop anti-VEGF has transformed the treatment of ocular angiogenic conditions. These are the most considered treatments for many conditions such as vein occlusions, myopic neovascularization of the choroid, retinopathy of prematurity (ROP), diabetic macular edema and choroidal neovascularization [52,53]. The FDA approved the ranibizumab for many treatments such as macular edema along with branch retinal vein occlusion (BRVO) and also for treating all angiographic subtypes of the subfoveal neovascularization of AMD [54,55]. Many literatures indicated short-term effects of ranibizumab on foveal thickness (FT) and visual acuity for about 1 week and 1 month, respectively, after injecting the ranibizumab. The short-term effects ranging from few minutes to hours, after injection of anti-VEGF drugs for BRVO have also been evaluated [56]. Bevacizumab is a full-length humanized recombinant monoclonal Immunoglobulin-G (IgG) anti-VEGF-A antibody [57] that is approved to treat many tumors by hindering all the VEGF-A isoforms.
Since this therapy is economical compared to other treatments, it is the most widely used anti-VEGF medicine in ophthalmology [58,59]. Intravitreous bevacizumab, ranibizumab and aflibercept were potent and also safe in the treatment of diabetic macular edema that causes vision impairment. The mild loss of initial visual acuity was able to manage with help of all three agents with a slight difference between each other. Where, in case of severe loss of initial visual acuity, aflibercept played a better role in improving the vision [60]. In India, a high alert puts ophthalmologists in a legal and ethical dilemma. Commercial entities must not be allowed to dictate which drug should be used for which disorder. The safety of the patient must be the paramount concern and physicians and governmental agencies must ensure this by fair drug compounding practices. Strong leadership of national and international ophthalmological societies is needed to represent the scientific facts regarding bevacizumab to drug regulatory agencies globally [61]. The research initiatives continue at organizations and pharmaceutical companies globally to find a safe and effective medicine for AMD. The currently available anti-VEGF drugs in the market furnish a slight hope but the drawbacks associated with them are such as repeated intravitreal injections that lead to patient incompliance [62]. Many patients living in developing countries like India face an economic crisis due to these expensive treatments. Even though the bevacizumab is not officially approved to treat a wide range of ocular conditions, it has shown better results at a relatively low cost. Since extensive research is still going on. Thus, we may expect the many novel and potent drugs that can treat many ocular neovascular diseases (Table 2) [63,64].

Novel approaches in ocular drug delivery
The novel approach-based dosage forms in ophthalmic delivery may ray of hope for better therapies for the future, in the treatment of ocular angiogenesis (Table 3).

Ocular gene therapy
Gene therapy is a novel technique in the field of medicine which delivers the nucleic acids into the cells of a patient as the drug to treat diseases. It is the supplementation of an ineffective gene with a healthy working gene in defective target cells. Depending on the types of cells treated, the gene therapy can be divided into two types called germline and somatic gene therapy. Some of the techniques to achieve gene therapy are said to be inhibition gene therapy, gene augmentation therapy and scavenging specific cells. Ocular gene therapy is the introduction of an exogenous gene product into a host's cell. The delivery of drugs to the ocular region is a hurdle due to the presence of many cellular barriers. These techniques can clear all the hurdles and challenges. To target a tissue, further development in this field with novel strategies may necessary [78,79].

Ocular inserts
Ocular inserts are tiny, thin, sterile, stratified solid pieces of a device placed into the conjunctival sac to deliver various drugs. Erodible and non-erodible are two types of ocular inserts. Ocular inserts also offer the advantages of increasing the contact time, better bioavailability of drugs and reducing the dosing frequency. Drug release profile from the ocular inserts depends on the following mechanism: diffusion, osmosis and bio-erosion. Within 24 h, the inserts can dissolve completely. The erosion of the inserts is majorly dependent on the type and concentration of polymer used. The pattern of drug release from the ocular inserts varies between individuals depending upon their physiological conditions. The non-erodible inserts consist of either matrix structure or reservoir that helps in sustaining the drug release [80,81].

Ocular implants
Ocular implant is an artificial material that is surgically implanted in the position of the eye, to improve impaired vision. For the delivery of drugs into a posterior region, the implants are surgically inserted anteriorly to the retinal region and posteriorly to the lens. There are two types of ocular implants biodegradable and non-biodegradable.    Drugs are encapsulated by using biodegradable polymers like polylactic acid (PLA) and polylactic-co-glycolic acid (PLGA) are in a particular system of nanoparticles or microparticles. The polymers are usually viscous materials that help in releasing the drug for a prolonged period. But, the nanoparticles or microparticles can distribute unusually in the ocular conditions upon injection via needles due to their compact size and composition [82].

Microneedle technology
These are micron-sized needle configurations, designed using microelectronics industry devices. Besides, microneedles were designed for the delivery of drugs for transdermal delivery. Glass microneedles are made of borosilicate material. Generally, the microneedles were made by using stainless steel called solid microneedles (75-1000 μm in length). It is a minimally invasive technology where it can deliver the drug into the posterior region and also overcomes complications associated with intraocular injections. Mainly, microneedle technology in ophthalmic delivery can provide localized and target delivery of drug into the posterior region [83,84].

Iontophoresis
This is a novel technique to deliver various drugs to the target site of action. The drugs, specifically charged macromolecules can be delivered into the anterior and posterior segments. The delivery is mainly based on a basic principle of attraction/binding between opposite charges whereas repelling between same charges. The iontophoretic device consists of a continuous DC source with two electrodes. The mechanism of this device is placing of an ionized drug in the compartment of an electrode that bears the same charge and the ground electrode is placed at any region around the eye [85].

In situ gelling system
In situ gel is a novel approach to deliver the drug to the ocular region that is solution form before administration and converts into gel form to release the drug that is triggered by an external stimulus like pH, temperature, etc. [86]. Several mechanisms are involved in triggering the conversion of the solution to in situ gel such as a physical change in biomaterials such as exchange of solvent and cross-linking between solvent/swelling. Trigger due to physiological stimuli such as pH of body fluids and temperature of the body and chemical reactions. Trigger due to various chemical reactions such as photopolymerization, oxidation and reduction. In a temperature triggered system, when the temperature rises, the formulation converts into gel. The polymers like poloxamers are used to formulate these gels. In pH triggered in situ gel formation of gel is induced by a change in pH. Most of the anionic pH-sensitive polymers are based on polyacrylic acid (PAA) (carbopol, carbomer) or its derivatives. In the diffusion method, the solvent present in the solution of polymer will diffuse and enter the nearby tissue leading to the polymer precipitation. The ionic strength can also promote the formation of gel from the introduced polymeric solution. Gelrite is the best example of an ionsensitive polymer. In the enzyme responsive process, gel formation can be occurred due to specific enzymes present in physiological conditions even in the absence of any chemicals like monomers and initiators. In photopolymerization process, the polymeric solution injected to the desired site will swell with help of a photo via fiber optic cables to sustain the drug release for a longer period [87,88].

Contact lens
Contact lenses are a thin type of plastic-shaped twisted type of cover that protects the eye. The contact lens can deliver the drug efficiently when compared with eye drops. Due to the longer contact time, the dosage frequency can be lowered with less systematic toxicity. The topical drug delivery to the ocular region became a major hurdle for scientists due to the various barriers like corneal barriers, conjunctival barriers, blood-retinal barriers and blood-aqueous barriers. The fabrication of polymeric nanoparticle embedded contact lens may prolong the delivery of drug leading to reduced frequency of administration which will improve patient compliance [89,90].

Photothermal therapy
Photothermal therapy (PTT) is a novel treatment of choice for various medical complications; it is a minimally invasive, local treatment with less toxicity [91]. The mechanism involves the activation of a photosensitizing agent by using electromagnetic radiation to convert energy into heat to kill cancerous cells [92]. PTT has several advantages when compared with radiotherapy or chemotherapy; it specifically targets the unhealthy cell by its deep penetrating power without affecting surrounded healthy cells or tissues [93].
In photothermal therapy, cell death occurs due to the denaturation of proteins, lysis of membrane and evaporation of cytosol [94]. The ideal photosensitizing agent should have several characteristics like low toxicity to the cells, high solubility in biocompatible solution and ease in functionalization. Phosphorous photothermal agents exist in three allotropic forms, i.e., white phosphorous, red phosphorous and black phosphorous (BP) [95]. Among three allotropes of phosphorus, BP is the most stable at high temperature and high pressure. The most interesting properties of BP are its photothermal property, narrow band gaps, large specific surface area, high carrier mobility, good biodegradability and biocompatibility properties [96]. Studies have been shown that BP offers promising and better applications in the nano-field of technologies like bio-imaging, photothermal therapy and drug delivery fields. BP is a direct bandgap semiconductor in which band topology remains the same and also it has high carrier mobility [97]. Bulk silicon is another photothermal agent that has an indirect bandgap semiconductor, which means its valence band maximum and conduction band minimum have different momentum vectors. The photoluminescence effect of BP increases exponentially as the layer thickness decreases from 5 to 2 layers [97,98]. In silicon nanocrystals, the wavelength of photoluminescence is dependent on the diameter of nanocrystals. The photothermal effect is mainly the conversion of optical energy to thermal energy. In ocular phototherapy, the laser is the source of light [99]. Ocular phototherapy has wide application in the treatment of ocular tumors [100]. The study has been conducted to assess the safety and clinical efficacy of non-damaging photothermal therapy for the treatment of the retina. The study included 16 patients suffering from persistent central serous retinopathy who were treated with the PASCAL streamline at 577 nm wavelength, using 200 mm retinal spot sizes. They concluded that photothermal therapy was safe and it was improved visual acuity and resolution of subretinal fluid in patients suffering from chronic central serous retinopathy [101].
Another study was conducted to evaluate the effect of near-infrared (NIR) on photothermal therapy agents by using Ag@Oxides nanoprisms for uveal melanoma therapy. Silver oxide nanoparticles were prepared by a simple sol-gel route and irradiated with an 808 nm NIR laser. They concluded that Ag@oxides nanoparticles were demonstrated to be an efficient photothermal therapy agent for solid cancers by local delivery [102].

Artificial intelligence
For the last two decades, pharmaceutical scientists are developing novel techniques for targeted drug delivery with maximum efficacy by minimizing the side effects. Artificial intelligence (AI) is the branch of computer science also and it is the intelligence demonstrated by machines. Generation of new information, automated working system and prediction, continuous performance, monitoring various diseases is the main advantage of AI. AI technique enables prediction of pharmacokinetic responses including quantitative structural activity relationships, in vivo responses, etc. So, the incorporation of AI technology into the ophthalmic sector may be a ray of hope for the ocular drug delivery system [103].

Nanotechnology in ophthalmic drug delivery
Many researchers are facing huge problems in the sector of ocular drug delivery. The bioavailability of a drug is not up to the mark due to several ocular barriers [104,105]. Many literatures revealed that the particle size of the drug should be appropriate and narrow. It should also possess less irritation, more biocompatible and possess appropriate bioavailability to achieve ocular drug delivery [106]. Hence, the ideal ocular drug delivery system must be in the form of eye drops without inducing any irritation or blurred vision [107]. The topical delivery is the only efficient way to deliver the drug into the anterior segment of an eye but the only minute concentration of the drug will reach into the posterior segment of the eye. But the systemic administration will help to achieve a small quantity of drug in the target site of ocular tissues. However, the dose needed to obtain therapeutic efficacy may induce several drug-related side effects. Thus, the adoption of nanotechnology-based drug delivery such as liposomes, niosomes, solid lipid nanoparticles, nanosuspensions, nanoemulsions, nanomicelles and biodegradable microspheres could help in overcoming various toxicity and bioavailability issues of many drugs. The drugs that are intended to deliver to a specific target site for treating many ocular conditions could be achieved by surpassing the ocular barriers. These nanotechnologybased drug delivery systems can also help in sustaining the release of drugs by crossing several ocular barriers such as the blood-retinal barrier in the eye [108]. This can further improve the bioavailability of many drugs thereby increasing the therapeutic efficacy [109].

Nanoemulsions
They are fine dispersions of infinitesimal droplets of two immiscible liquids. They contain the dispersed phase, where the particles dispersed are in the nanoor submicron range. Generally, the nanoemulsions are made of one or more surfactants containing both hydrophilic and hydrophobic parts. The high-pressure homogenization is adapted to obtain the dispersed globules in a size range of below 100 nm with a translucent look. Since the nanoemulsions are globule-sized, the dispersions are thermodynamically unstable which requires more surfactants to stabilize. This can be the major reason behind the stickiness of the formulation. The phospholipids are one class, which is also commonly used in stabilizing the nanoemulsion formulation. The four major types of nanoemulsions are o/w, w/o, w/o/w, o/w/o type emulsion [110][111][112].

Nanosuspension
They usually consist of hydrophobic drugs that are suspended in the specific dispersion medium. The nanosuspensions can also be prepared using various polymers and resins to sustain the drug release and to achieve therapeutic efficacy by increasing bioavailability [113]. The polymers that are inert and biocompatible without causing any irritation to the iris, cornea, conjunctiva, etc. will be most suitable for ocular drug delivery. The various formulation techniques such as high-pressure homogenization, milling, emulsificationsolvent, precipitation, supercritical fluid process, melt emulsification method, lipid emulsion/microemulsion template and solvent evaporation are used to design the nanoformulation [114].

Nanoparticles
They are defined as nano-sized particles, whose diameter ranges from 10 to 1000 nm. They are usually made using several types of biodegradable and biocompatible polymers, resins, phospholipids, etc., either occurred naturally (albumin, sodium alginate, chitosan, guar gum, xanthan gum, gelatine, etc.) or synthesized in the laboratory [polycyanoacrylate, poly(D,L-lactides), polylactides, etc.]. These can be used for delivering the drug to ocular tissues efficiently. The nanoparticles consist of three major properties such as larger surface area, highly mobile in the dispersed state and can exhibit what is known as quantum effects. Based on dimension nanoparticles can be classified as one-, two-and threedimensional nanoparticles. The various techniques like emulsion solvent evaporation, double emulsion solvent evaporation, salting out, emulsions-diffusion method and solvent displacement/precipitation method are used to design the nanoparticles [115,116].

Nanomicelles
The nanomicelles are nano-sized (10-1000 nm), micellarshaped, self-assembling and highly mobile colloidal-like dispersions consisting of a hydrophilic shell and hydrophobic core. They are made of lipids by arranging themselves in a circular form in the solution. The amphiphilic characteristic of fatty acids is responsible for forming a micellar structure since they contain both hydrophilic (polar) and hydrophobic (non-polar) sections. The core of the nanomicelles consists of hydrophilic chains that extend outward, leading to the formation of the clear formulation. Nanomicelles are classified as polymeric nanomicelles, surfactant nanomicelles and polyion complex nanomicelle. The main advantages of nanomicelles are can be prepared easily and finally yields very small size particles which lead to a larger surface area with higher absorption automatically increases the bioavailability of drugs also encapsulates a large number of drugs [117].

Liposomes
They are spherical vesicles containing a minimum of one hydrophobic (Lipid) bilayer. They are made of many nontoxic lipids and cholesterols such as phosphatidylcholine, phosphatidylethanolamine as far as they are compatible with each other. The liposome vesicles size under 10 to 100 nm can be named unilamellar vesicles and huge measured vesicles 100 to 300 nm. Liposomes are promising systems for drug delivery due to their size, amphiphilic properties and biocompatibility. The properties of the liposomes significantly vary upon their size, surface charge, preparing method and composition of lipid/cholesterol. Since they possess a specific surface charge, they can be used in delivering various drugs into the ocular tissues. For example, the negative charge bearing corneal surface can attract the positive surface charged liposomes [118].

Niosomes
The structure of niosomes is quite similar to liposomes except for the composition that is present in liposomes. They are mainly constituted of non-ionic surfactants. They tend to incorporate both hydrophobic and hydrophilic drugs. Unlike liposomes, they are chemically stable and less toxic due to the absence of phospholipids. This makes niosomes select over liposomes in drug delivery. They also exhibit flexibility in structural characterization due to their size, composition and fluidity. This can increase the sustained action of drugs with better bioavailability. The preparation and storage are quite simple in the case of liposomes compared to niosomes due to their composition of non-ionic surfactants over phospholipids [119,120].

Dendrimers
They are one more novel drug delivery system for the ocular region. They are the macromolecular compounds composed of symmetric branches surrounding a central core (like a tree). These are the nano-sized polymeric system. The hydrophilic and lipophilic drugs in the central core and are entrapped with polymers. The drug can be either encapsulated inside the dendrimers or bonded to the surface functional groups to achieve drug loading. The preparation and functionalization of dendrimers are easy up to the generation 2 (G2) level, beyond that it will be difficult to fabricate since they are in the nano-size range. But, most of the drugs can be incorporated into the dendrimers of G2 level. Thus, it could be an efficient way to deliver drugs to the ocular region [121].

Polymeric micelles
Polymeric micelles are the novel drug delivery system used to target the drug into the specific site and release it in a controlled manner [122]. Polymeric micelles can be defined as nano-sized molecules of core-shell structure that are formed by the self-association of amphiphilic block copolymers when they are added to an aqueous solvent. Usually, polymeric micelles are spherical in shape and size in the range of 10-100 nm. These are widely used in drug delivery systems due to their low toxicity, nano-size, good biocompatibility and mainly high stability [123]. The release of drug from polymeric micelles depends on (i) physicochemical properties of the drug and copolymer (ii) method of preparation (iii) structure of micelle forming copolymer and drug (iv) localization of drug in the micelle [124]. The methods like drug dissolution, dialysis, oil in water emulsion, solvent evaporation, co-solvent evaporation and freeze-drying are commonly used to encapsulate the drug into micelles [125]. Due to their small size, it is easily penetrated through the ocular tissues and automatically increases the bioavailability of the drugs. The unique core-shell structure of polymeric micelles, hydrophobic drugs can incorporate within the micelle core will lead to increase the aqueous drug solubility [126,127].

Biodegradable microspheres
Microspheres are spherical microparticles with a size range between 1 and 1000 µm [128]. Biodegradable microspheres were prepared by using synthetic and natural biodegradable polymers [129]. Microspheres are mainly of two types, matrix and capsular. The microspheres were fabricated by using biodegradable natural and synthetic polymers [130]. Natural origin biodegradable polymers are sub-classified as polysaccharides and proteins. Polysaccharides are mainly derived from a plant (dextran, starch, pectin), animal (hyaluronic acid), microbial (xanthan, pullulan, alginic acid) and marine source (chitosan) [131]. Proteins are mainly from plant (gluten) animal (gelatin, collagen and albumin) and microbial (polyhydroxyalkanoates) origin. Synthetic biodegradable polymers are classified as polyesters [PLA, polylacticco-glycolic acid (PLGA), polyglycolic acid (PGA), polycaprolactone (PCL) and polyphenylene ether (PPE)], polyorthoesters and polyanhydrides [132]. Diffusion, dissolution and surface erosion are the major mechanism by which drug release from biodegradable microspheres [133]. The biodegradable microspheres prepared by using various techniques like interfacial polymerization [134], in situ polymerization [135], phase separation [136], ionotropic gelation [137], emulsion solvent evaporation [138], double emulsion [139], spray drying [140,141], spray congealing and air suspension method [142]. In ocular drug delivery, biodegradable microsphere concept has been used to deliver the drug in a controlled manner and to the specific site. Biodegradable microspheres for the intravitreal delivery of acyclovir were formulated and characterized for various characteristics. These microspheres were prepared by spray drying technique which showed good encapsulation efficiency and in vitro dissolution mainly dependent on the molecular weight of the polymer. Also in vivo evaluation evidenced that prepared formulation shown sustained release of acyclovir [143]. A study has been carried out for the controlled and extended release of bioactive aflibercept hydrogel for the treatment of ocular neovascular diseases and studied in vitro release of the drug. They fabricated afliberceptloaded microspheres by using biodegradable synthetic polymers and concluded that the prepared microsphere hydrogel was safe and delivers aflibercept in a controlled and extended manner for the period of 6 months [144].

Advantages of nanotechnology-based anti-angiogenic therapy
Angiogenesis inhibitors are the revolutionized drug molecules to target existing tumor infiltrating blood vessels and to inhibit the formation of new blood vessels [178]. These agents mainly act on vascular endothelial growth factors and thereby inhibit the angiogenesis process. Currently, intravitreal injections are the treatment of choice but it is associated with several complications [179]. So, here alternative therapy for the treatment of pathological angiogenesis is the nanotechnology-based drug delivery to overcome several complications. Nanoapproach-based drug delivery techniques play an important role to overcome the drawbacks of present therapy due to their interesting physicochemical properties like nano-sized particles, prolonged half-life, high targeting efficiency, high surface area and the small size of the particle may cross ocular barriers [180]. The study has been conducted for the topical delivery of anti-VEGF drugs for the treatment of choroidal neovascularization using cell penetrating peptides. They evaluated the biological efficacy of the topical anti-VEGF using cell penetrating peptide that is compared with the intravitreal anti-VEGF injections. They have shown that cell penetrating peptides have high penetrating capabilities and non-toxic to the eye. In this study, they delivered bevacizumab and ranibizumab to the posterior segment of mouse, rat and pig eyes. They concluded that topical delivery of anti-VEGF with cell penetrating peptide was efficacious as a single intravitreal injection. A study has been highlighted that within 24 h the cell penetrating peptide and anti-VEGF drug complexes were cleared from the retinal region [181]. Seah et al. reviewed on use of biomaterials for sustained delivery of anti-VEGF to treat retinal These studies showed that novel approach-based topical delivery of anti-VEGF drugs is the choice of a treatment system for pathological angiogenesis [144,186].

Potentials of nanotechnology-based ocular drug delivery systems for clinical applications
Currently in the pharmaceutical field, ocular drug delivery has become the most challenging area. To overcome this limitation targeted drug delivery system came into existence [187]. Nanotechnology emerged as a promising drug delivery system in the field of ocular therapy. Various nanotechnology-based products have been under investigation and few products have already been clinically approved by the United States Food and Drug Administration (USFDA) and are available for the treatment of medical conditions like autoimmune disorders, cancer, age-related macular degeneration, etc. Currently, many ocular delivery systems are in clinical trials and some products have already been introduced into the market [188]. The development of nanotechnology seems to be a ray of hope for the currently facing challenges. Pre-clinical/clinical/approved formulations (nano/micro) in ocular drug delivery system are listed in Table 4.

Risk analysis
In the current review, we have presented the applications of novel approaches for the treatment of pathological ocular angiogenesis [209]. The literature survey represented the main aim to formulate these novel approaches and nanotechnology-based formulations to improve the uptake and for the better entrapment efficiency of drug which ultimately improves the therapeutic effect. But, stability was the major problem associated with the nanobased formulations [210]. The reason behind the low therapeutic efficacy of the nanoformulation is their ability to self-aggregate at low drug concentration, affecting the drug entrapment and ultimately leads to poor stability of the formulations. For example, it has been reported that self-aggregation of doxorubicin nanoformulation due to their high ionic strength ultimately leads to the high particle size and affects their drug entrapment efficiency [211]. The same problem was reported for the liposome formulation which increases in size due to their high ionic strength [212]. Another challenge is the swelling mechanism of nanoformulation. When swelling occurs, the size of the particles increases and this limitation can be fixed by controlling the swelling mechanism by using pH-sensitive coatings or the capping agents over the formulation. Finally, certain nanoformulations were failed to meet FDA quality profile and difficulties associated with formulation manufacturing, make nano-approach-based drugs formulation unfit for large-scale production. Thus, the upcoming research should focus on above mentioned challenges and concentrate on large-scale manufacturing of nanoformulations abiding by the guidelines of USFDA to resolve all the associated hurdles and inexpensively [188,213].

Future perspectives
The vision impairment and irreversible vision loss can be decreased or completely prevented by enormous research and developments necessitate the application of novel approaches and strategies. Novel approaches in the development of ocular dosage forms have very wide applications in the treatment of various ocular diseases. Topical delivery of drugs into the ocular region is one of the best approaches in ocular drug delivery in terms of patient compliance. Despite its seemingly easy way, they possess several limitations such as tear turnover, nasolacrimal drainage, blinking, induced lacrimation that leads to quick elimination of drug particles from the surface of the eye. This results in sub-therapeutic drug levels in the target tissue, particularly at the retinal level. Thus, novel approaches can assist in the manufacture of nano-based formulations. A very less amount of research was carried out on the above discussed approaches for ocular angiogenesis particularly. The application of nanomedicine in the ocular drug delivery area has shown great potential in the pharmaceutical field of research [104]. Nanomedicine also has its potential in improving the pharmacokinetic and pharmacodynamic properties of few therapeutic agents. Many nanoformulations are under the pre-clinical and clinical stage of development for the treatment of ocular diseases. Unfortunately, this nanotechnologybased drug delivery has few limitations when it enters the large-scale process. The reason behind the low therapeutic efficacy of the nanoformulation is their ability to self-aggregate at low drug concentration, affecting the drug entrapment and ultimately leads to poor stability of the formulations it again leads to the safety and toxicity issue of the prepared formulation. Also, other factors like size, shape, administration dose can influence toxicity in nanomedicine. So advanced nanofabrication technologies like particle replication in non-wetting templates (PRINT) [214] and the hydrogel template method have been introduced to create ocular nanomedicine. This technology can create uniform nanoparticles and microparticles with controlled shape, size and surface modification at a large scale. Another study conducted using the principle of PRINT technology is AR13503 implant, which was manufactured to provide sustained release of drugs for more than 2 months [214,215]. Considering the immense amount of past and present research on these techniques, there is potential and a ray of hope for better therapies for the future in the treatment of diseases and is with the eventual goal of stopping ocular angiogenesis. There are several areas to be explored for future research for the benefit of a novel therapy for ocular angiogenesis. Furthermore, the ongoing development of novel therapy or modification to current dosage forms in the context of ocular angiogenesis will be helpful to focus on the new strategy of medication for the treatment of angiogenesis.

Conclusion
As we discussed earlier, drug delivery to the ocular region became challenging to pharmaceutical scientists. The barriers which are present in the eye hinder ocular bioavailability mainly when the drugs are applied topically. So, novel approach-based drug delivery systems are of great importance in the pharmaceutical technology as well as the ophthalmological sector. Thus, for ocular disease management, the design and development of new techniques are mandatory. Only a few works can be seen specifically for ocular angiogenesis. Currently to treat ocular angiogenesis, intravitreal injections of anti-VEGF drugs are the revolutionized treatment. But, repeated intravitreal injections to the eyes lead to several serious complications. So, novel approach-based drug delivery techniques may be a ray of hope for better therapies for the future in the treatment of ocular angiogenesis. Also, the inclusion of novel techniques may be a road map for future research in the field of ocular angiogenesis disease management.