1. Nano-Based DDS for Glaucoma

Glaucoma is a group of eye diseases that damage the optic nerve, leading to progressive vision loss and blindness. It is the leading cause of irreversible blindness globally. To this day, intraocular pressure (IOP) management is the cornerstone of treatment for glaucoma. With the aim of reducing intraocular pressure (IOP), there has been a recent surge in the use of nanocarriers to enhance ocular drug delivery. This development is particularly significant as the long-term use of conventional IOP-lowering agents, which are based on poorly penetrating molecules, can cause frequent ocular toxicity and intolerance due to their adverse effects on the corneal epithelium ^[1].

2. Nano-Based DDS Based on Regulating Intraocular Pressure (IOP)

2.1. Preclinical Studies

Nanoparticles can be modified with biodegradable polymers to enhance the bioavailability of drugs, which is particularly useful for drugs with low solubility, such as brinzolamide for glaucoma. Studies have formulated PLGA-modified (Polylactic-co-glycolic acid) nanoparticles for sustained release of brinzolamide, resulting in a significant reduction in IOP in animal models with minimal toxicity ^[2][3][4]. To improve entrapment efficiency and corneal permeation with topical delivery of brinzolamide, Song et al. (2020) coated PLGA nanoparticles with phosphatidylserine (PS) using a coaxial electrospray technique. However, systemic absorption was observed where there was an IOP reduction in the untreated eye as well ^[3]. Salama et al. (2017) used subconjunctival injection to enhance prolonged release of PLGA nanoparticles and provide targeted drug delivery, finding that efficacy depended on nanoparticle size. PLGA nanoparticles were also used to encapsulate SA-2, a nitric oxide (NO) donor and superoxide dismutase (SOD) activator ^[5]. A single-dose slow release of SA-2 significantly lowered IOP for up to 72 h, and increasing SOD enzyme activity provided cytoprotection in human trabecular meshwork (TM) cells.

Recent studies have explored the use of various polymers to form nanoparticles for ocular drug delivery. Cyclodextrin nanoparticles have been used to administer candesartan and irbesartan by LorenzoSoler et al. (2020), effectively lowering IOP comparable to timolol eye drops while preventing side effects observed with oral administration. Ultra-small chitosan nanoparticles were used to deliver nanobrimodine for open-angle glaucoma by Barwar et al. (2019), while galactomannan-based nanoparticles were used for the delivery of dorzolamide hydrochloride by Mittal et al. (2019), resulting in a prolonged IOP-lowering effect compared to conventional eye drops. Furthermore, Tan et al. (2021) delivered miRNA using polydopamine-polyethylenimine nanoparticles through intracameral injection, which showed comparable transfection efficacy with lower cytotoxicity and effectively lowered IOP in vivo.

Recent studies have modified Mesoporous Silica Nanoparticles (MSNs) to achieve biodegradation for clinical application, despite their poor degradability due to a pure silica framework. For instance, Fan et al. (2021) used biodegradable hollow mesoporous organosilica (HOS) nanocapsules to deliver NO in a stimulus-responsive manner, leading to improved penetration and bioavailability. However, prolonged use of NO-MSNs was previously found to increase IOP elevation following the initial decrease, likely caused by outflow tissue damage through protein nitration. To ameliorate these side effects, co-delivery of antioxidants such as MnTMPyP has been suggested. Meanwhile, alternative methods, such as developing biodegradable hollow polymeric nanocapsules, are currently under extensive research due to their high drug-loading capacity and ability to better control drug release.

Niosomes, a type of lipid-based vesicular system, are an effective drug delivery system (DDS) that can improve residence time and corneal permeation. Zafar et al. (2021) developed a chitosan-coated niosome that improved biodegradability and bioadhesion, enabling the delivery of carteolol (CT). Pilocarpine and latanoprost were also successfully delivered using niosomal gels to reduce IOP in rabbit models ^[6][7]. However, niosomes are known to have a limited shelf life and poor drug entrapment efficiency. To overcome these limitations, proniosomes (gel or granular) have been developed, which can rapidly convert to niosomes upon hydration, providing higher physical stability and minimizing niosomal dispersions ^[8][9].

Cubosomes have been investigated as a potential drug delivery system for ocular delivery due to their continuous lipid bilayer structure similar to the corneal epithelium. Teba et al. (2021) and Huang et al. (2017) used cubosomes to deliver acetazolamide and timolol maleate, respectively, resulting in improved bioavailability and IOP-lowering effects. Both studies used biodegradable polymers, such as P407 and GMO, to develop the cubosomes. This suggests that cubosomes may offer a promising DDS for ocular delivery with improved corneal permeation and residence time, as well as reduced ocular irritation.

Nanoemulsions have been used to carry both hydrophilic and hydrophobic materials and have been found to enhance drug absorption and prolong IOP-lowering effects of travoprost and brinzolamide. However, the high level of surfactants required for their synthesis can result in cytotoxicity, which is further compounded by the addition of preservatives. Therefore, long-term safety tests regarding toxicity are necessary for the use of this DDS.

Ocular inserts using nano-based carriers are being researched to improve treatment adherence and decrease administration frequency. Chitosan, a highly biodegradable polymer, has been modified to enhance its solubility in developing ocular inserts and films for topical delivery. Chondroitin sulfate and hydroxyethyl cellulose have been added to chitosan to develop modified chitosan inserts that can deliver 4-aminodiphenylamine and dorzolamide to lower IOP, while also exhibiting a neuroprotective effect towards the retinal ganglion cells. Sodium alginate-ethyl cellulose inserts have been reported to carry hydrophilic drugs, while a cyclodextrin multilayer film incorporating PBAE and graphene oxide enabled a time-controlled release of brimonidine, dependent on layer thickness in vitro. Li et al. (2020) presented an eco-friendly method of producing chitosan polymer dissolved in a water-based film, overcoming the solubility issue while retaining high cornea permeability.

Researchers have used nanomicelles to deliver both latanoprost and timolol using a drug-laden contact lens developed by Xu et al. (2019) ^[10]. They utilized biodegradable mPEG-PLA nanomicelles prepared through thin-film hydration to ensure the contact lens remained transparent and transmitted light. Although this approach provided a slow, sustained release of both drugs, ocular safety was confirmed, but it was reported that this system might impact the physical properties of the lens, making it rough after drug release. Another co-delivery system was developed by Samy et al. (2019), where they utilized PCL thin-film implants to deliver timolol and brimonidine ^[11]. Although this approach provided a controlled release of both drugs, systemic absorption and related side effects were not measured. These polymeric films were also used to deliver a novel hypotensive agent, DE-117, which resulted in a sustained IOP-lowering effect. However, due to the bulky size of the implant, there was a high risk of device migration and corneal endothelium damage ^[12].

Hydrogels have been extensively studied as a promising ocular delivery system, with properties of in situ sol-to-gel formation triggered by environmental factors like pH and temperature ^[13]. However, recent studies have developed hybrid systems embedding hydrogels with biodegradable nanoparticles to ensure sustained drug release. These have led to successful DDS, including sustained release of brimonidine tartrate ^[14], timolol maleate ^[15][16], and bimatoprost ^[17], as well as co-delivery of curcumin nanoparticles and latanoprost ^[18] from a thermosensitive in situ hydrogel. Chou et al. (2017) presented a DDS based on dual functions, delivering pilocarpine loaded with antioxidants GA through a biodegradable gelatin-based thermogel ^[19]. Modulating the degradation by controlling redox radical initiation reaction temperatures (20–50 ◦C) provides sustained release (ideal at 30 ◦C). This raises the possibility of modifying biodegradability to improve sustained drug release, with Luo et al. (2019) showing that an increase in the amination degree of gelatin in biodegradable thermogels enhances resistance to biodegradation ^[20], and in another study by the same group, increasing the deacetylation degree enhances resistance to biodegradation in chitosan-based thermogels ^[21].

Liposomes are highly biodegradable nanocarriers capable of carrying both hydrophilic and hydrophobic drugs. However, their low stability, low entrapment efficiency, and rapid release of hydrophilic drugs have been a common drawback. To address these issues, Hathout et al. (2018) developed gelatinized core liposomes for the sustained release of timolol maleate, which significantly improved entrapment efficiency and did not cause ocular irritation. TPGS-modified liposomes were also presented by Jin et al. (2018) as a carrier for brinzolamide, showing greater sustained release, maintained IOP reduction, and no significant side effects.

Liposomes, which are highly biodegradable nanocarriers, have the potential to carry both hydrophilic and hydrophobic drugs. Jin et al. (2018) modified liposomes with TPGS for brinzolamide, resulting in greater sustained release and maintained IOP reduction without significant side effects. However, their low stability, low entrapment efficiency, and rapid release of hydrophilic drugs are common drawbacks. To address these issues, Hathout et al. (2018) developed gelatinized core liposomes for the sustained release of the hydrophilic drug, timolol maleate. This method significantly increased entrapment efficiency and showed no signs of ocular irritation.

PAMAM (polyamidoamine) dendrimers have shown promise in sustaining the release of timolol with no observed cytotoxicity or ocular irritation in vitro. Further research involving in vivo models is necessary to optimize drug loading and investigate the effects of chronic application ^[22]. Lancina et al. (2017) utilized electrospun dendrimer-based nanofiber mats to deliver brimonidine tartrate (BT), which improved the effectiveness of IOP lowering over three weeks with daily dosing, but not with a single dosage.

Several DDS systems have been investigated for sustained ocular drug delivery. PAMAM dendrimers have been utilized to deliver timolol, showing no signs of cytotoxicity or ocular irritation in vitro, but further studies are needed to optimize drug loading and assess the effects of chronic use. Electrospun dendrimer-based nanofiber mats were used to deliver brimonidine tartrate, leading to improved efficacy of IOP lowering with daily dosing over 3 weeks. Self-assembly drug nanostructures (SADN) and phase transition microemulsions (PMEs) have also shown promise for sustained IOP lowering but require further investigation for cytotoxicity and systemic effects. Modified micelles delivering ligands targeting FLT-4/VEGFR3 receptors have shown receptor targeting and IOP lowering, but improvements in corneal permeability and sustained release are needed. Nanosuspensions have been used to deliver acetazolamide, demonstrating sustained drug release, improved solubility, and bioavailability, but the stability of nanosuspensions remains a limitation to be investigated. Finally, hyaluronic acid was used to stabilize the nanosuspension but was only able to maintain dispersion characteristics for up to 6 months.

Chae et al. (2020) proposed a drug-free, non-surgical method to reduce IOP using a hyaluronic acid hydrogel microneedle injection to expand the suprachoroidal space ^[23]. This method facilitates aqueous humor drainage through the uveoscleral outflow pathway and extends the lowering of IOP without the associated loss of endothelial cells seen in suprachoroidal MIGS such as Cypass and iStent Supra. The efficacy of this approach needs further mechanistic studies, and the prevention of fibrosis with multiple injection treatments needs to be addressed.

Recent studies have modified existing DDS, including microspheres ^[24][25], solid lipid nanoparticles (SLNs) ^[26], and chitosan nanoparticles, by embedding montmorillonite (Mt) in the biodegradable hybrid polymer. Mt is a biocompatible silicate with a negative surface charge that forms an ion complex with cationic drugs such as betaxolol hydrochloride (BH) and brimonidine, allowing for controlled and sustained drug release. These hybrid nanocarriers have been shown to cause a prolonged decrease in IOP.

Recent studies propose the use of electrospinning biodegradable polymers as an alternative to solid formulations for ocular delivery. While the latter provide prolonged ocular residence, they can interfere with vision and comfort. Andreadis et al. (2022) developed an in situ electrospun film gel for the delivery of timolol maleate, and Morais et al. (2021) created electrospun ocular implants for acetazolamide delivery. Both methods showed a significant sustained IOP-lowering effect and increased local delivery. However, further optimization of the implant sterilization methods is required to avoid adverse events and infection, without compromising efficacy.

2.2. Clinical Studies

Rubiao et al. (2021) conducted a Phase 2 controlled study to compare the efficacy and safety of a chitosan-based bimatoprost insert to conventional LumiganTM eyedrops in patients with POAG and ocular hypertension. The study enrolled 16 and 13 patients, respectively, with a small control group of 5 patients. The biodegradable nature of the insert eliminated the need for removal. The insert was well-tolerated with no significant side effects, changes in visual acuity, or central corneal thickness. IOP reduction was 30% by the third week, compared to 35% with eyedrops. Using inserts in 3-week intervals improved patient compliance and offers a better therapeutic regime. Rubiao et al. plans to conduct Phase 3 confirmatory studies.

In 2020, DurystaTM, an FDA-approved bimatoprost implant, was developed with poly-lactic acid and poly-lactic-co-glycolic polymers, similar to biodegradable sutures. This non-pulsatile, sustained release implant provides 10 µg bimatoprost and is currently being tested in multiple Phase 3 trials for long-term safety and efficacy in open-angle glaucoma or ocular hypertension patients. It is found to be non-inferior to timolol eye drops and reduces the treatment burden associated with glaucoma while improving adherence. Although there is a potential risk of adverse events, including corneal events, intraocular inflammation, or endophthalmitis, implant biodegradation was observed within 12 months while maintaining the IOP-lowering effect. The implant serves as a viable option for patients with glaucoma who are unreliable or have dementia, and those who are not suitable for incisional glaucoma surgery.

Brandt et al. (2017) investigated the sustained release of bimatoprost using an ocular ring in double-masked randomized Phase 2 clinical trials ^[27]. The safety and efficacy of the ocular ring were studied in 65 and 63 patients for 7 and 13 months, respectively. The ocular ring was found to be safe and well tolerated, but caused mucus discharge as a side effect. The study suggested that clinically significant IOP reduction can be achieved with applications at 6-month intervals. However, since the ocular ring is made of a silicone matrix over a polypropylene structure, further studies on its biodegradability are necessary to improve its biocompatibility.

Kouchak et al. (2017) conducted a randomized controlled trial on 20 patients with OAG and OH to compare the efficacy and safety of dorzolamide nanoliposomes (DRZ-nanoliposome) with a control group using dorzolamide eye drops (BiosoptTM) ^[28]. DRZ-nanoliposomes demonstrated greater reduction in IOP at days 14 and 28, with increased adhesion and corneal permeation attributed to the liposome delivery system. Reports of irritation and redness were similar between the groups, likely due to dorzolamide itself. This liposomal delivery system may provide extended release and higher intensity of the drug, potentially reducing the frequency of administration without compromising efficacy.

3. Nano-Based DDS for Neuroprotection

Conventional therapies for glaucoma focus on reducing IOP, but this is insufficient for addressing permanent damage to RGCs and the optic nerve, particularly in advanced diseases. Normotensive glaucoma and cases of controlled IOP can also lead to visual loss progression, highlighting the need for neuroprotective strategies. Nano-based biodegradable DDS may be a promising approach for delivering these drugs to the posterior segment of the eye, with increased drug permeation and long-lasting effects.

3.1. Preclinical Studies

Brimonidine, a drug previously discussed for its ability to reduce IOP, also exhibits neuroprotective effects by regulating excitatory NMDA receptors in RGCs. Two studies investigated the delivery of brimonidine to the posterior eye for its neuroprotective effects in a glaucoma model. Lou et al. (2021) developed a dual-function PDA biodegradable nanoparticle loaded with brimonidine, where PDA aids in ROS scavenging and anti-inflammation effects, achieving sustained and increased permeation of brimonidine and promoting RGC survival. Rodrigo et al. (2020) proposed using a biodegradable LAPONITETM synthetic clay, capable of controlled release of drugs, which also achieved sustained and increased permeation of brimonidine, promoting RGC survival. However, the LAPONITE system had some adverse side effects in their in vivo model, including systemic absorption of brimonidine and CNS depressant effects leading to early deaths among rats.

To investigate the delivery of neurotrophic factors (NFs) for preventing retina damage in glaucoma models, studies have been conducted using nanoparticles. Yang et al. (2021) successfully delivered CTNF and oncostatin M (OSM) with nanoparticles, improving RGC survival and photoreceptor preservation. Similarly, Giannaccini et al. (2017) delivered NGF and BDNF using biodegradable magnetic nanoparticles, which prevented RGC loss with lower dosages of NFs. However, further studies are required to determine the optimal dosage using in vivo models that closely resemble human eye anatomy. Garcia-Caballero et al. (2017) delivered GDNF/Vitamin E through biodegradable PLGA microspheres, allowing for effective neuroprotection for up to 6 months. Additionally, multitherapy with dexamethasone, melatonin, and coenzymeQ10 reduced RGC loss and retinal stress in an in vivo model. Cubosomes were also used for targeted delivery of LM22A-4, a small NF mimetic, and were found to prevent RGC loss and improve functional outcomes through a gradual release.

Hydrogels have been utilized for sustained release and improved bioavailability of neuroprotective drugs. Chitosan thermogel, a biodegradable hydrogel, was used to deliver pilocarpine and RGFP966, an HDAC inhibitor, which protects RGCs and optic nerves from damage, in a study ^[29]. The thermogel had antioxidant effects, promoting myelin growth, and reducing RGC loss. Nguyen et al. (2019) co-delivered pilocarpine and ascorbic acid, an anti-inflammatory agent, through a PAMAM dendrimer thermogel ^[30], achieving sustained release of both agents for over 80 days, suppressing inflammation, and aiding in the regeneration of stromal collagen and retinal laminin.

Cannabinoids have potential as a neuroprotective treatment for glaucoma, but limited delivery to the posterior eye has hindered their effectiveness. Kabiri et al. (2018) used an HA-MC thermosensitive hydrogel loaded with CBGA nanoparticles to improve bioavailability, corneal permeation, and reduce ocular irritancy. Similarly, the derivative ∆9-Tetrahydrocannabinol-valine-hemisuccinate was delivered using SLNs, providing prolonged residence time and neuroprotective and IOP-lowering effects. However, further in vivo studies are necessary to evaluate the safety and cytotoxicity of these methods.

Nanoparticles have been used as drug delivery systems (DDS) for neuroprotective agents in various studies. For instance, biodegradable PEGylated nanoparticles were used to load memantine, an NMDA antagonist, which improved drug delivery and reduced RGC loss in an in vivo rodent model. Additionally, Gemini (PGL) Nanoparticles were used for a non-invasive gene delivery system by Narsineni et al. (2022), who delivered peptide-modified CAPgemini surfactants, a potential Aβ40 aggregation inhibitor. These surfactants showed a 10-fold improvement in Aβ40 aggregation inhibition, which is associated with RGC neurodegeneration. Further in vivo studies are required to evaluate the therapeutic efficacy of glaucoma neurodegeneration.

Li et al. (2020) utilized biodegradable PEG-based nanoparticles to co-deliver brinzolamide and miR-124, providing prevention to RGC damage and IOP-lowering effects through sustained release without ocular toxicity [131]. Zhao et al. (2017) employed a PEG-based nanoparticle system conjugated with cholera toxin B domain (CTB) to improve targeted delivery of DHEA, an FDA-approved S1R agonist, to RGCs, showing effective RGC protection ^[31]. However, entrapment improvement is required for a more efficient, sustained release of nanoparticles. Silva et al. (2022) used a chitosan and HA nanoparticle system to deliver Epoetin Beta, which has tissue-protective properties, showing sustained release for up to 21 days with no local or systemic adverse effects ^[32]. Further investigation is necessary to assess the therapeutic efficacy of these methods on glaucoma models.

Hsueh et al. (2021) proposed a microcrystal DDS that delivers sunitinib, an FDA-approved multi-kinase inhibitor that promotes RGC survival ^[33]. They achieved a sustained release of therapeutically relevant concentrations in a pig model and provided neuroprotection for at least 20 weeks in a rat optic nerve crush model.

3.2. Barriers to Clinical Translation

Current neuroprotective trials for glaucoma are facing difficulties in replicating in vivo efficacy in humans due to the use of acute damage animal models, while human glaucoma is a chronic and prolonged disease. Moreover, current neuroprotective agents suffer from low bioavailability, chemical instability, and adverse effects. For example, brimonidine monotherapy has side effects like hyperemia, hypersensitivity, and ocular discomfort. Although stem cell therapy, NMDA antagonist-drugs, and neurotrophic factors show promise, they may cause undesired systemic absorption and low bioavailability to the posterior eye. To overcome these limitations, biodegradable nano-based DDS can improve bioavailability, targeted delivery, and sustained release, increasing patient adherence. However, many DDS approaches are still in vitro or preliminary in vivo stages, and animal models are needed to confirm these neuroprotective findings before clinical translation.

4. Nano-Based DDS after Laser and Surgical Treatment of Glaucoma

4.1. Preclinical Studies

Nano-based DDS can also enhance anti-inflammatory and anti-fibrotic effects and provide sustained IOP-lowering after glaucoma filtration surgeries and SLT. In a recent study by Ghosn et al. (2022), the FDA-approved bimatoprost implant, DurystaTM, was tested in a beagle in vivo model to evaluate its effectiveness in reducing IOP post-selective laser trabeculoplasty (SLT) ^[34]. The results showed a sustained IOP lowering for up to 42 weeks after the implant was placed. However, the study had a small sample size, and translating these results to human patients with chronic glaucoma medication may pose challenges due to greater variability in IOP levels.

Andres-Guerrero et al. (2021) developed a collagen matrix implant loaded with bevacizumab and sodium hyaluronate to promote wound healing and reduce bleb failure post-conventional trabeculectomy ^[35]. The anti-VEGF properties of bevacizumab combined with mechanical support from sodium hyaluronate provided anti-scarring and improved tissue repair. However, there is a risk of collagen matrix-induced inflammation after degradation. While this system improved tissue repair in vitro, further improvement in DDS for controlled, sustained release may be required to achieve clinical significance.

Swann et al. (2019) formulated a PLGA film containing MMC and 5-FU, providing sustained anti-fibrotic treatment post-trabeculectomy. Vildanova et al. (2022) developed a biodegradable modified chitosan and HA hydrogel to improve sustained delivery of MMC and 5-FU, with in vitro studies reporting sustained release of both drugs, with MMC having a longer release compared to 5-FU. Qiao et al. (2017) used a chistosan-modified hydrogel to deliver heparin post-glaucoma surgery, maintaining filtration bleb and lowering IOP for a prolonged time. However, the cytotoxicity profiles for these studies, particularly the long-term effect on corneal limbal stem cells, need further investigation.

Chun et al. (2021) used a gelatin-based hydrogel to deliver siSPARC for reducing subconjunctival scarring post-trabeculectomy ^[36]. The hydrogel DDS was non-toxic, highly biocompatible, and effective in reducing scarring. Similarly, LbL (layer-by-layer) nanoparticles were used to deliver siSPARC by Seet et al. (2018) ^[37]. Although siSPARC showed no cellular toxicity, it is easily degradable, and a hydrogel may help improve its ocular delivery. Further optimization of the dosage to ensure sustained release is necessary for clinical applications.

4.2. Clinical Studies

In a recent clinical study, Johannesson et al. (2020) used dexamethasone nanoparticles (DexNP) to deliver MMC post-trabeculectomy in a randomized, single-masked clinical trial. DexNP proved non-inferior to conventional MMC MaxidexTM eye drops in a small sample of 20 patients. Despite limitations such as a small sample size and unmasked patients, this nanoparticle system offers a potentially safer alternative to MMC administration, which is associated with risks and complications.