Nanomedicine is the medical application of nanotechnology, which involves the creation and use of materials to construct nanoparticles (NPs) with at least one dimension between 1 and 100 nm . The most commonly used materials in nanomedicine include lipids (liposomes), proteins (albumin NPs), cyclic oligosaccharides (cyclodextrins), synthetic polymers (polymeric micelles, dendrimers, hydrogel), and inorganic compounds (cerium oxide NPs)(Fig. 2) (Table 1) [20,21,22]. In the context of ophthalmology, NPs are of interest for their ability to increase the solubility of hydrophobic drugs, their capability to provide sustained drug release with reduced toxicity and improved efficacy, their ability to prolong drug retention time and enhancement of drug penetration through ocular barriers, and their proficiency to direct drugs to specific tissues and cells . The different routes of ocular drug administration of NPs include topical, oral/systemic, subconjunctival, subtenon, retrobulbar, intracameral, and IVT .
Some challenges in nanotechnology are (a) nanoparticles aggregate inside the tissues after IVT , (b) few in vivo studies have been accomplished, and (c) most ocular pharmacokinetics studies have been performed in animal models, mostly in rabbits. However, the rabbit eye is different from the human eye, and such differences can contribute to the outcomes of IVT drug delivery . Part of the effort to improve nanoparticle therapies focuses on the difficulties described above.
Nanomedicine Based on Liposomes
Delivery of drugs at therapeutic concentrations to the posterior segment of the ocular chamber (retina/choroid) is a very challenging task . Liposomes constitute a promising nanosystem strategy for ocular drug delivery. Liposomes are the first nano drug delivery systems that have been successfully translated into real-time clinical applications. Liposomes are spherical vesicles characterized by a bilayer of lipids with an internal aqueous cavity. Liposome structural components are phospholipids or synthetic amphiphiles incorporated with sterols, such as cholesterol, to influence membrane permeability. Thin-film hydration is the most widely used preparation method for liposomes, in which lipid components with or without a drug are dissolved in an organic solvent . Besides, liposomes are nontoxic, metabolized, biodegradable, and have low antigenicity . These characteristics make them suitable carriers of several molecules, including drugs.
It is noteworthy that an increasing number of studies have demonstrated longer retention time of drugs loaded in liposomes compared with the free drug administration, thereby reducing the number of IVT injections and their concomitant risks [7, 31,32,33,34,35]. Recently topical liposomal formulations have even been developed to avoid IVT and injection and their hazards.
For example a topical ophthalmic triamcinolone acetonide–liposome formulation (TA-LF) was developed by our group to accomplish a safer alternative for IVT corticosteroid injection. This TA-LF was used to successfully deliver TA into vitreous and retina of rabbits  and its activity was confirmed in patients with refractory macular edema. Twelve eyes with refractory pseudophakic cystoid macular edema (PCME) were treated with TA-LF for 90 days, and patients under treatment showed a significant improvement in visual acuity and in central foveal thickness with no adverse events recorded . Moreover, TA-LF showed its utility to prevent PCME and significantly improve the contrast sensitivity after femtosecond-laser-assisted cataract surgery [37, 38].
Chemical modifications to the liposomes are currently being made and evaluated to make the delivery of the drug to the posterior segment of the eye more efficient. Recently Khalil et al.  coated liposomes with chitosan with the goal to improve the encapsulation efficiency, retention time, and permeability for a topical formulation of TA. Highlights of their results are that this liposome modification enhances encapsulation efficiency, increases retention time, and sustained release when compared with the conventional liposome loaded with TA. The chitosan-coated liposomes showed successful in vivo penetration of the corneal mucosal barrier and accumulation in vitreous body after topical administration, coinciding with what was previously reported by Li et al. . Regardless of all this information, our recently published data demonstrates the only evidence of successful use in a clinical scenario .
More complex systems have been created like the nanosystem of Gu et al. , who designed novel multifunctional nanocomposites for efficient drug delivery to the posterior segment via topical instillation. Their system was generated by hybridizing dexamethasone salt-loaded liposome with glycylsarcosine-anchored layered double hydroxide, the latter as a positive carrier that promoted precorneal retention via electrostatic adsorption. The system showed sustained-release performance and prolonged precorneal retention. In vivo eye distribution assay of the nanosystem showed strong residence on the eye surface compared with commercial eye drops. Two hours after topical administration the drug concentration remained in the choroidal and retinal tissues at therapeutic concentrations, even higher, indicating that this nanosystem could effectively deliver the drug to the posterior segment of the eye.
Liposomes became the first nanomedicines in US Food and Drug Administration (FDA) clinical trials . Interventional clinical trials are summarized in Table 2 according to status (recruiting; enrolling by invitation; active, not recruiting; or completed) [37, 38, 42].
Nanomedicine Based on Other Nanoparticles
Other types of lipids have been used for eye drug delivery. Solid lipid nanoparticles (SLN) are being developed to overcome disadvantages of other colloidal carriers like liposomes. Lipid components of SLN are solid at body and ambient temperature. SLN are composed of 0.1–30% (w/w) solid fat which is dispersed in an aqueous phase. The lipids used in SLN include fatty acids, steroids, waxes, monoglycerides, diglycerides, and triglycerides. Compared with liposomes, SLN have drug stability and prolonged release and they are safer because no organic solvents are used in their production; others benefits are easy preparation, low cost, high-scale production, chemical versatility, among others . Promising results have been achieved with the use of SLN as a nanosystem eye drug delivery. Abrishami et al.  administered SLN containing diclofenac into eyes of rabbits via IVT injection; comparing with the free drug form, the drug concentration in eyes injected with SLN was significantly higher than the eyes treated with the free form, a characteristic allowing one to increase the injection intervals and improve the patient compliance. On the other hand, Singh et al.  delivered isoniazid in SLN; isoniazid is a potential bactericidal agent used to treat all forms of tuberculosis. The in vivo aqueous humor pharmacokinetics showed an isoniazid–SLN extended release, enhanced corneal permeability, increased ocular bioavailability, among other characteristics.
Other types of NPs have been provided. Kashiwagi et al.  fabricated a multilayered biodegradable nanosheet composed of chitosan and sodium alginate and latanoprost (antiglaucoma ophthalmic drug) loaded on the nanosheet to reduce intraocular pressure. The nanosheet was applied into the center of the cornea of Sprague–Dawley rats. After nanosystem administration intraocular pressure was monitored until 9 days after. The authors observed a significant intraocular pressure reduction after treatment with no local adverse effects. Using the same approach, Wang et al.  combined two drugs (latanoprost and timolol) in a nanosheet made of alginate and chitosan to reduce intraocular pressure in a slow and sustained manner for possible therapeutic strategy in glaucoma disease. In an in vivo study, the administration of the nanosheet reduced intraocular pressure 9 days after application. These results concurred with those of Kashiwagi et al. .
For the first time Silva et al.  designed a new erythropoietin formula for topical ocular administration. They developed a mucoadhesive hydrogel of chitosan and hyaluronic acid as a nanosystem for topical delivery. The ex vivo permeation evaluation using conjunctivas, corneas, and scleras from pig eyes showed that the nanosystem formulation permeated more rapidly through the conjunctiva, followed by the sclera and the cornea. Thus, this carrier form could increase erythropoietin ocular bioavailability through the enhanced retention time and permeation in the different ocular membranes. Hamcerencu et al.  generated a thermoreversible hydrogel as ocular insert using gellan maleate and N-isopropylacrylamide (alkyl acrylamide). The resultant hydrogel showed a temperature-responsive behavior, releasing the biological active compound (adrenaline) when the polymer conformation changes. Animals included in the in vivo biocompatibility test showed good tolerance to the implanted hydrogel. With the in vivo release assay, the authors found no changes in anatomical structures after 24 h and the effect of the drug was demonstrated after 60–90 s, suggesting efficiency of the inserts without complications.
On the other hand, polymeric micelles are core/shell-structured NPs formed by self-assembly. The core/shell structure enables encapsulation of hydrophobic drugs . In 2017 Mandal et al.  published a review about the technical/chemical aspects and preclinical studies of the polymeric micelles in the field of ocular drug delivery. Recently Xu et al.  used nanomicelles of chitosan for topical eye delivery of dexamethasone. Nanomicelles were prepared with chitosan oligosaccharide–valyvaline–stearic acid, in which valyvaline functioned as a targeting ligand of eye peptide transporter 1. The in vivo ocular assays exhibited sustained release, biocompatibility, bioavailability, good retention time, and penetration-enhancing properties. In similar way, Yu et al.  utilized chitosan nanomicelles to carry dexamethasone with similar achieved results, suggesting that self-assembled chitosan nanoparticles might be a promising candidate for ophthalmic drug delivery.
In another study, Sai et al.  formulated an in situ gel-forming system with micelles incorporated for curcumin eye delivery. The system was based on 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (PEG-DSPE)/polyoxyethylene esters of 12-hydroxystearic acid (Solutol HS 15) (PEG-DSPE/Solutol HS 15) mixed micelles and gellan gum. This approach significantly increased the solubility of curcumin. In vivo assays showed that the nanosystem increased the eye retention time of curcumin, possibly owing to the formation of a gel network under the influence of cations from tears.
Novel nanocarriers have been proven, e.g., the sweetener constituent of Stevia rebaudiana, rebaudioside A (RA). The chemical structure of RA allows one to generate ultrasmall micelles in aqueous solution ; on the basis of that characteristic, Song et al.  encapsulated pterostilbene, a natural antioxidant, in self-assembled ultrasmall micelles of RA. The authors administered RA–pterostilbene into the eyes of different animals (rabbit, mouse, rats); the results showed a significant improvement of intraocular permeation of RA–pterostilbene, compared with the free drug, and in consequence enhanced efficacy of the drug. Another natural molecule is ginsenoside Rb1 (Rb1), a principal bioactive molecule in the roots of Panax ginseng with intrinsic functions including antioxidative, anti-inflammatory, anti-angiogenic, and neuroprotective effects. In addition, the chemical structure of Rb1 has a hydrophobic side chain and a hydrophilic sugar side chain which allow formation of self-assembled micelles in aqueous solutions . Li et al.  formulated diclofenac-loaded Rb1 micelles. Animal permeation tests showed increased concentrations of diclofenac in corneas of rabbits administered with Rb1 nanomicelles compared with commercial diclofenac eye drops, but the concentration in the aqueous humor was undetectable, the formulation showed no additional viscosity above that of water, and drained away from the eye surface.
A different novel approach was developed by Göttel et al.  in which a drug delivery system is applied in dry form and forms a gel immediately after administration. The system innovation is based on gellan gum/pullulan electrospun nanofibers. After dye-loaded fibers were sprayed onto porcine corneas, the fibers showed a homogeneous distribution over the cornea surface and a prolonged residence time compared to conventional eye drops. To ensure a good fit to the eye anatomy, the authors prepared fibers shaped into curved geometries for future applications.
Wang et al.  designed a system in which NPs are injected intravenously and are converted to a tissue-targeting state only upon irradiation in the eye. The general design was based in photo-targeted NPs that are formed by self-assembly of a chemically modified poly(ethylene oxide)–poly(d,l-lactic acid) (PEG-PLA) block copolymer; this core presents other surface elements including the “peptide cargo” and a targeting molecule that allows local drug delivery and in situ activation upon irradiation with blue light. The NPs were intravenously administered in mice with laser-induced choroidal neovascularization. Immediately after injection, NPs were observed in the mouse fundus in the retinal blood vessels, visible in vivo after 8 h after administration. After the proof of concept, to evaluate the therapeutic effect of the phototriggered activation of NPs in choroidal neovascularization, the authors loaded NPs with doxorubicin which inhibits choroidal neovascularization. NPs were injected via tail vein and 30 s thereafter, the mouse eyes were irradiated with blue light. Animals treated with NPs loaded with doxorubicin showed a 46.1% reduction in neovessel area compared to the control group.
Liu et al.  modified β-cyclodextrin with ethylene diamine as a nanocarrier of curcumin for eye delivery. In vitro assays of the modified β-cyclodextrin displayed enhanced corneal penetration with improved biocompatibility. Ethylene diamine–β-cyclodextrin encapsulation could hugely promote corneal curcumin delivery. On the other hand, Mazet et al.  explored the effect of mixing dexamethasone with cyclodextrin to improve the aqueous solubility of curcumin. Hydroxypropyl-β-cyclodextrins and hydroxypropyl-γ-cyclodextrins were combined with a marketed mucoadhesive gel for topical administration. The in vitro studies showed that the inclusion of hydroxypropyl-γ-cyclodextrin increased 1500-fold the drug concentration in a water solution and an appropriate release factor. Lorenzo-Veiga et al.  improved the cyclodextrin characteristics by mixing it with different hydrophilic polymers to generate nanoaggreagtes with nepafenac (potent non-steroidal anti-inflammatory drug) and to promote solubility. In contrast with Mazet et al., the best results of solubility were accomplished with the use of hydroxypropyl-β-cyclodextrin, for this particular drug, while hydroxypropyl-γ-cyclodextrin performed best in terms of enhancing nanoaggregate formation. They observed that addition of hydrophilic polymers carboxymethylcellulose, polyvinylpyrrolidone, and tyloxapol to the formulation with cyclodextrins led to higher nepafenac solubility.
Jóhannesson et al.  hypothesized that high concentration of drug in eye drops and adhesion of nanoparticles to the mucous membrane enhances the drug bioavailability in the tear film. To prove their hypothesis, they evaluated the concentration of nanoparticles of dexamethasone γ-cyclodextrin and dorzolamide γ-cyclodextrin in the tear film of human volunteers. Twelve volunteers were randomized in two groups, six eyes received one drop of dexamethasone γ-cyclodextrin and the control eyes the regular dexamethasone. Tear fluid was sampled at seven points after treatment administration and the drug concentration was measured by mass spectrometry. The same was performed for the dorzolamide γ-cyclodextrin. The results showed that dexamethasone γ-cyclodextrin nanoparticles had higher concentration and with longer duration in tear fluid than the regular dexamethasone; the effect was not replicated with dorzolamide γ-cyclodextrin, which may be explained by the particle sizes achieved for each drop of preparation: some of the particles of the dexamethasone γ-cyclodextrin had nanoparticle diameters of 100 nm, whereas dorzolamide γ-cyclodextrin had microparticles with particle sizes higher than 100 nm that could have modified the drug kinetics.
Ohira et al.  described the results of a prospective randomized controlled trial in which 22 patients were included with diabetic macular edema and randomized in two arms: (a) topical treatment with dexamethasone γ-cyclodextrin nanoparticle eye drops or (b) one posterior subtenon injection of triamcinolone acetonide. The patients were followed for 16 weeks. The treatment with dexamethasone γ-cyclodextrin nanoparticle eye drops significantly improved vision and macular thickness. The therapeutic outcome was similar to that achieved with subtenon triamcinolone injections. The clinical trial outcomes showed that this dexamethasone γ-cyclodextrin nanoparticle eye drop (topical administration) had a significant effect in the posterior segment of the ocular globe comparable with invasive techniques.