Drug Delivery Systems for Infectious Eye Diseases: Advancements and Prospects

Abstract

Infectious ocular diseases like keratitis, conjunctivitis, and endophthalmitis pose significant clinical challenges due to the complexities of delivering drugs to the eye. Recent advancements in drug delivery systems offer promising improvements for treating these conditions. Key strategies include targeted delivery through physicochemical modifications, magnetic nanoparticles, and ligand-receptor interactions. This review explores the safety and biocompatibility of ocular drug delivery systems through in vivo ocular toxicity studies, in vitro cytotoxicity assays, hemocompatibility studies, ocular tolerance tests, and genotoxicity assays. It also examines combination therapies and stimuli-responsive delivery systems for their potential to enhance therapeutic efficacy. Furthermore, we discuss tailored and optimized drug delivery approaches for infectious ocular diseases, outlining current challenges and future directions for developing effective ocular drug delivery systems.

1. Introduction

Infectious eye diseases caused by microorganisms such as bacteria, viruses, fungi, and parasites, affect different structures of the eye. These infections, if left untreated, can result in severe inflammation, tissue damage, and vision impairment [1,2]. Common examples include conjunctivitis, keratitis, and endophthalmitis [1]. Nonetheless, they remain a substantial concern, particularly in regions with limited access to prompt and effective treatment options. Conventional treatments for infectious eye diseases, typically involving antimicrobial drugs administered topically or systemically, are often limited by low ocular bioavailability, rapid drug clearance, and the development of antimicrobial resistance [1,3]. Drug delivery systems (DDSs) have emerged as a promising solution to address these limitations by enhancing drug stability, bioavailability, and therapeutic efficacy [3,4,5,6,7]. Various DDS technologies, including nanoparticles, liposomes, hydrogels, and in-situ gelling systems, have shown potential in overcoming ocular barriers and optimizing drug delivery to infected tissues [8,9,10]. Targeted DDSs are designed to deliver therapeutic agents directly to specific sites within the eye, further enhancing treatment efficacy [11,12,13,14]. Combination therapies, such as antibiotics with anti-inflammatory or antifungal agents, offer synergistic effects against infections and resistance [8,15,16,17,18]. Moreover, stimuli-responsive DDSs, which react to physiological changes, allow for customizable, controlled drug release, representing a new frontier in ocular disease treatment [19,20,21,22,23]. Multiple parameters can be used to assess toxicity of these DDSs both in vivo and in vitro. Additionally, market products such as DuraSite^® and Vigamox^® exemplify the clinical translation of these advancements, providing more effective treatments for infectious ocular diseases while minimizing toxicity and enhancing patient compliance [24,25]. In this review, we provide a comprehensive overview of infectious eye diseases, advancements in DDSs including targeted and stimuli-responsive DDSs, combination therapies, and their safety and efficacy assessments.

2. Infectious Eye Diseases

Infectious eye diseases refer to a range of conditions that are caused by microorganisms such as bacteria, viruses, fungi, or parasites, and affect the various structures of the eye [1]. These diseases can lead to inflammation, tissue damage, and visual impairment if not promptly diagnosed and treated [2]. Common examples of infectious ocular diseases include conjunctivitis (pink eye), keratitis (corneal infection), endophthalmitis (infection inside the eye), and ocular manifestations of systemic infectious diseases like herpes simplex virus and toxoplasmosis [1]. From an epidemiological standpoint, infectious diseases rank significantly lower than the leading global causes of avoidable moderate-to-severe distance visual impairment (MSVI) and avoidable blindness. According to a comprehensive meta-analysis, in 2020, these conditions were estimated to affect 553 million and 43 million individuals, respectively [26].

2.1. Infectious Keratitis

Infectious keratitis is the inflammation of the cornea, which is the transparent, dome-shaped outer layer of the eye that covers the iris and the pupil, by the microorganisms. It is a medical emergency as it progresses very fast. Keratitis is the most common cause of corneal blindness and its prevalence in the USA is 30,000 cases per year. Infectious keratitis can be further classified into bacterial keratitis, viral keratitis, fungal keratitis, or parasitic keratitis. The symptoms include eye pain, redness, blurred vision, tearing, photophobia, foreign body sensations, discharge, and sometimes decreased visual acuity [1,27].

Bacterial keratitis is the most common cause of infectious keratitis. This can result from the invasion of the cornea by various bacterial species, with common culprits including Staphylococcus aureus, Streptococcus pneumoniae, Pseudomonas aeruginosa, and Haemophilus influenzae. Bacterial keratitis occurs when bacteria gain access to the cornea through an injury or a compromised ocular surface, such as in cases of contact lens wear, corneal trauma, or by a blocked nasolacrimal duct. The accumulation of bacterial toxins and cellular debris induces an inflammatory response by the host, involving the corneal epithelium, stromal keratocytes, resident immune cells, and infiltrating leukocytes, predominantly polymorphonuclear cells (PMNs). This local inflammasome activation is a double-edged sword: while it can effectively eliminate bacteria, the indiscriminate release of reactive oxygen species (ROS), lysosomal enzymes, and autolytic proteins can cause collateral damage to the surrounding stroma. This damage may result in permanent structural changes that impair vision (Figure 1A). The primary approach to address this is the administration of topical antibiotic eye drops or ointments [28,29]. In severe cases, oral antibiotics or even hospitalization may be required as treatment modalities [27].

Fungal keratitis occurs when fungi gain access to the cornea through an injury, contact lens use, or exposure to fungal-contaminated environments. It is most commonly caused by filamentous fungi, such as Fusarium, Aspergillus, and Candida species. The incidence of fungal keratitis is only 6–20% and is majorly prevalent in developing countries, tropical and sub-tropical regions [30,31]. During fungal infections, neutrophils make up 95% of the cellular infiltrate. Infected corneas exhibit progressive erosion and necrosis of corneal tissue, a thinning of the corneal epithelium, and a disordered corneal stroma. Corneal ulcers caused by filamentous fungi usually present with dry-looking infiltrates with feathery margins. These ulcers can also feature satellite lesions, a hypopyon, and endo-exudates (Figure 1B) [29,32]. Hypopyon is a condition where severe inflammation leads to pus accumulation in the anterior chamber of the eye due to infection. Endo-exudates are retinal exudates in the form of lipid- or protein-rich fluid deposition in the retina during infection. Antifungal medications in the form of topical eye drops or ointments are typically prescribed for relief [31].

Viral keratitis is a type of corneal infection caused by viruses, and it is characterized by inflammation of the cornea, which can lead to significant visual impairment if not promptly diagnosed and managed. The most common virus associated with viral keratitis is the herpes simplex virus (HSV), specifically HSV-1 [1,33]. The global incidence of herpes keratitis is about 1.5 million, including 40,000 new cases of severe monocular visual impairment or blindness each year [34]. Based on the part of cornea that is affected, herpes keratitis can be classified as epithelial, stromal, endothelial, or mixed, and based on occurrence, it can be primary or secondary. The primary clinical manifestations can include corneal opacity, edema, corneal scarring, and neovascularization, which can result in irreversible vision impairment and blindness. The infection by HSV-1 starts with the interaction of virus capsid glycoproteins such as gB, gC, gD, gH, and gL with the cell receptors such as nectin-1 and -2, herpes virus entry mediator (HVEM), and 3-O sulfated heparan sulfate (3-OS HS) on corneal epithelial cells [13]. Epithelial cell swelling due to HSV infection can cause the breakdown of the outer corneal layers and subsequent cell death [35]. In an infected cornea, HSV initiates viral replication in the corneal epithelial cells, and binds to Toll-like receptors on the cell surface, stimulating the influx of inflammatory cells and the release of cytokines and chemokines that gradually infiltrate the stroma. These inflammatory cells, including neutrophils, dendritic cells (DCs), natural killer cells, and macrophages, reportedly aid in clearing HSV from the cornea during the initial infection (Figure 1C) [36]. Stromal keratitis is likely driven by CD4+ T cells as part of the inflammatory response to stromal infection, alongside the direct effects of the virus [37]. The major treatment strategies involve oral or topical antiviral and topical steroids [1].

Parasitic keratitis is a rare but serious condition characterized by corneal infection caused by parasitic organisms such as Acanthamoeba species and microsporidia. Risk factors for parasitic keratitis include contact lens use, ocular trauma, exposure to contaminated environments, and a compromised immune system. Parasitic keratitis causes intense eye pain, inflammation, and damage to the epithelium and stroma, which can lead to vision loss if not diagnosed and treated promptly. It starts when the Acanthamoeba adheres to the corneal epithelium through a mannose-binding protein. Acanthamoeba trophozoites causes epithelial barrier breakdown through mechanisms including direct cytolysis, phagocytosis, and the induction of apoptosis. Following corneal epithelial cell barrier break down, it gradually invades the stroma. Once the infection reaches the stroma, it causes extensive damage to the collagen matrix, triggering intense inflammation [38]. The innate immune cells, specifically neutrophils and macrophages, play a crucial role in resolving parasitic keratitis. Notably, both neutrophils and macrophages are capable of killing trophozoites. Eosinophils are also present in the cornea of rabbits affected by Acanthamoeba, though the function is not fully known (Figure 1D) [39,40]. The treatment for parasitic keratitis is challenging and may involve multiple approaches. Antiparasitic medications in the form of eye drops, ointments, or oral medications are commonly used [41]. However, the complications associated with keratitis are corneal damage, vision loss, corneal scarring, and, in some cases, the need for corneal transplantation. Successful treatment for keratitis requires multiple challenges to be addressed to design an effective drug delivery system (DDS). A basic requirement is to concede the drug dosage along with maintaining therapeutic concentrations for prolonged periods.

2.2. Infectious Conjunctivitis

Conjunctivitis, commonly known as pink eye, is an inflammation associated with the dilation of blood vessels of the conjunctiva, the thin, transparent tissue that covers the white part of the eye and lines the inner surface of the eyelids. Infectious conjunctivitis is caused by bacteria and viruses [1]. It is estimated that acute conjunctivitis affects 6 million people annually in the United States alone. The cost of treating bacterial conjunctivitis is estimated to be $377 million to $857 million per year [20]. Both bacterial and viral conjunctivitis are highly contagious and can spread through direct contact with infected individuals [1].

Bacterial conjunctivitis is caused by the colonization of bacteria, most commonly by Staphylococcus aureus, Streptococcus pneumoniae, Moraxella catarrhalis, or Haemophilus influenzae, on the ocular surface, disrupting the epithelial barrier. It is a common form of conjunctivitis characterized by redness, discharge, and discomfort in the affected eye. Inflammatory cytokines (e.g., IL-1, IL-6, TNF-α) and chemokines (e.g., IL-8) secreted by infected epithelial cells play significant roles in recruiting and activating immune cells in the conjunctiva during bacterial infection. In bacterial conjunctivitis, the infiltration of neutrophils and macrophages into the conjunctiva is a key feature of the body’s response to infection (Figure 2A) [42,43]. Bacterial conjunctivitis is accountable for increased morbidity and provides a more challenging clinical scenario for physicians [44]. Children are most affected by bacterial conjunctivitis [20]. Bacterial conjunctivitis is typically treated with antibiotic eye drops or ointments. The specific antibiotic prescribed may depend on the suspected or identified bacteria. With appropriate treatment, bacterial conjunctivitis tends to resolve without complications. However, in rare cases, complications such as corneal involvement or the spread of infection to other parts of the eye may occur.

Viral conjunctivitis is caused by viruses, and it represents up to 80% of all cases of acute conjunctivitis. It affects both adults and children. A major cause of viral conjunctivitis (65 to 90%) is adenovirus [1,45]. The transmission of adenovirus-associated conjunctivitis occurs through direct contact with contaminated fomites, water, or fecal-oral contamination [4]. Typical symptoms of viral conjunctivitis include redness and swelling of the conjunctiva, watery discharge, hyperemia, chemosis, ipsilateral lymphadenopathy, discomfort, and a gritty sensation in the eye [45]. Adenoviral conjunctivitis begins when the adenovirus interacts with conjunctival epithelial cells. Primary cellular receptors, including CAR, CD46, and sialic acid, interact with the adenoviral fiber-knob protein to facilitate viral attachment to the host cell, while the interaction between the adenoviral penton base and integrins mediates viral internalization [4]. Both innate immune response viral conjunctivitis types are mediated by natural killer cells, neutrophiles, dendritic cells and macrophages and adoptive immune response by CD8 T cells, and T-helper 1cells (Figure 2B) [4,42,46]. The goblet cells of the conjunctiva are also involved in HSV-1 and adenovirus mediated conjunctivitis [47]. There is no effective treatment currently available for viral conjunctivitis.

2.3. Endophthalmitis

Endophthalmitis is a severe inflammation and infection of the internal structures of the eye, including the aqueous humor, vitreous humor, and surrounding tissues. Endophthalmitis can be classified as either exogenous or endogenous, depending on the source of the infection. Exogenous endophthalmitis arises when infecting organisms enter the eye through direct inoculation, such as during intraocular surgery, penetrating trauma, or contiguous spread from nearby tissues. On the other hand, endogenous endophthalmitis occurs when infectious agents are disseminated hematogenously into the eye from a remote focus of infection. Exogenous endophthalmitis is more common than the endogenous endophthalmitis. Endophthalmitis is mainly caused by gram positive bacteria like Staphylococcus aureus and Streptococcus spps. and gram-negative bacteria such as Klebsiella spp. and E. coli [8,48]. The symptoms of endophthalmitis, whether ocular or systemic, often lack specificity. Therefore, early diagnosis depends on the alertness of ophthalmologists and other physicians. It is considered a medical emergency that requires prompt diagnosis and immediate treatment to prevent vision loss and potential complications. The identification of the pathogen typically involves microbiological or molecular techniques, which entail staining and culturing specimens from the eye or bodily fluids such as blood, urine, or cerebrospinal fluid. Polymerase chain reaction (PCR) also serves as a valuable tool in confirming the diagnosis [49]. Both bacterial and fungal endophthalmitis are associated with infiltration of neutrophils and macrophages into the retina and vitreous humor [50]. The infiltration of immune cells and activated microglia are associated with the cytokine and chemokines secretion (Figure 2C,D) [51,52]. Intravitreal injections of antimicrobial agents are the principal method of treatment, along with systemic broad-spectrum antibiotics [48,49,53].

3. Optimized Drug Delivery Systems for Infectious Ocular Diseases

Infectious ocular diseases are emerging medical problems in recent years, with conjunctivitis, keratitis and endophthalmitis being the most important eye infections, which are generally treated with topical or systemic administration of conventional anti-microbial drugs [1]. However, conventional antimicrobial drugs have certain limitations such as poor ocular bioavailability, low ocular residence time, low ocular penetration, high rate of antibiotic resistance, and some allergic reactions [3]. The DDSs enable efficient delivery of antimicrobial agents to the ocular tissues affected by infectious agents by protecting drugs from degradation, increasing their solubility, and facilitating their sustained release, thereby improving drug bioavailability and therapeutic outcomes [3,5,6,8,54]. Different DDSs, such as nanoparticles, nanospheres, liposomes, micelles, niosomes, nanosponges, nanosuspensions, nanoemulsions, cubosomes, hydrogels, in-situ gelling systems, contact lenses, microneedles, and ocular implants, have been explored to optimize the delivery of antimicrobials/drugs for infectious ocular diseases [8,,9,10]. Hereby, we discuss the inherent characteristics of above DDSs in the context of infectious ocular diseases:

Nanoparticles and nanospheres are solid colloidal particles that serve as carriers for drugs. Their small size (typically 1–1000 nm) allows them to penetrate ocular barriers more effectively. Nanoparticles can be engineered to release drugs in a controlled manner, enhancing the drug’s retention on the ocular surface. This is particularly useful for treating ocular infections or chronic diseases like diabetic retinopathy [8,55,56]. Furthermore, they can be modified to be pH or enzyme-sensitive, enhancing targeted drug delivery.

Liposomes are vesicular structures composed of lipid bilayers that can encapsulate both hydrophilic and lipophilic drugs. Due to their biocompatibility and ability to fuse with cell membranes, liposomes are ideal for delivering drugs to ocular tissues. They can enhance the permeability of drugs across the cornea, making them suitable for treating anterior segment diseases like keratitis and conjunctivitis. Their versatility in surface modification allows for prolonged retention on the corneal surface [56,57,58].

Micelles are self-assembling colloidal particles formed by amphiphilic molecules. They offer enhanced solubility for hydrophobic drugs, making them useful in ocular formulations. Micelles are particularly effective in delivering drugs to the posterior segment of the eye due to their small size and ability to avoid clearance mechanisms. They can be used for delivering anti-inflammatory or anti-microbial agents with improved bioavailability [56,59].

Niosomes are non-ionic surfactant-based vesicles similar to liposomes but with enhanced stability. They are biocompatible and can encapsulate both hydrophilic and lipophilic drugs, making them versatile in ocular drug delivery. Their ability to prolong drug release and improve the retention of drugs in the eye makes them suitable for conditions requiring sustained drug action, and for diseases like keratitis, conjunctivitis and endophthalmitis [60,61,62].

Nanosponges are porous particles that can encapsulate drugs within their sponge-like structures, allowing for controlled and sustained drug release. They can deliver both hydrophilic and hydrophobic drugs efficiently. They can directly act against microorganisms. In ocular applications, nanosponges are useful in treating diseases where long-term drug exposure is required, as they reduce dosing frequency and improve patient compliance [56,63].

Nanosuspensions are colloidal dispersions of drug nanoparticles stabilized by surfactants. They improve the solubility and bioavailability of poorly water-soluble drugs and have the capability to overcome ocular barriers. In ocular applications, nanosuspensions can enhance drug penetration through the cornea and provide prolonged drug release. They are particularly useful for delivering drugs to the posterior segment of the eye, such as in the treatment of infectious retinal diseases [19,64,65].

Nanoemulsions are thermodynamically stable mixtures of oil, water, and surfactants, with droplet sizes in the nanometer range. These systems improve the solubility and bioavailability of hydrophobic drugs. In the eye, nanoemulsions enhance drug penetration and provide sustained release, making them ideal for delivering anti-inflammatory and anti-microbial drugs. Their small droplet size improves drug absorption across ocular barriers [20,56,66,67].

Cubosomes are nanostructured particles with a cubic crystalline phase formed from amphiphilic lipids. They are stable, biocompatible, and capable of carrying both hydrophilic and lipophilic drugs. Cubosomes can enhance drug delivery across ocular barriers and provide a controlled release, making them particularly useful for both anterior and posterior infectious eye diseases [68,69,70].

Hydrogels are three-dimensional networks of hydrophilic polymers that can absorb water and swell, allowing them to release drugs in a controlled manner. In situ gelling systems are liquid formulations that transition to a gel state upon contact with the eye. These systems enhance drug retention on the ocular surface and reduce the need for frequent dosing, making them ideal for treating conditions like keratitis and conjunctivitis [15,71,72,73].

Drug-eluting contact lenses offer a novel method for sustained drug delivery to the eye. By incorporating drug-loaded nanoparticles or hydrogels into the lens material, these lenses provide continuous drug release over extended periods. They improve drug bioavailability and patient compliance by eliminating the need for frequent eye drops. Contact lenses can be used to treat various anterior segment diseases, such as corneal and conjunctival infections [16,56,74].

Microneedles are tiny needles that can penetrate the ocular surface or sclera without causing significant pain or damage. These devices offer a minimally invasive method for delivering drugs directly to the anterior and posterior segment of the eye, bypassing ocular barriers [75,76,77].

Ocular implants are small, solid devices placed inside the eye to deliver drugs over an extended period. These implants provide continuous, controlled drug release, reducing the need for repeated injections or topical applications. Implants are particularly beneficial for posterior chronic and infectious diseases [78,79].

Each of these above DDSs offer distinct advantages for ocular applications, including enhanced drug stability and bioavailability, prolonged retention time, ocular penetration, and controlled release. Their selection depends on the specific disease being treated, the required duration of therapy, and the drug’s physicochemical properties as explained in Table 1, Table 2 and Table 3.

3.1. Drug Delivery Systems for Infectious Keratitis

Infectious keratitis is a condition characterized by corneal inflammation resulting from an infection, posing a potential threat to vision and constituting an urgent eye problem. Managing infectious keratitis effectively depends on identifying the specific microorganism responsible, and treatment often involves the frequent use of antibacterial, antifungal, or antiviral eye drops. However, these eye drops may face challenges such as limited bioavailability, unwanted side effects, and reduced patient compliance due to barriers presented by the cornea [80]. The corneal barriers include tight junctions formed by the corneal epithelium, reflex blinking, nasolacrimal drainage and drainage of drug to other ocular layers by aqueous barrier [11,81]. Advancements in DDSs have been put forward to overcome corneal barriers leading to enhanced drug residence time, increased permeability of the drug, enhanced drug solubility, and protection of the drug from acids and enzymatic effects, overcoming drug resistance and reductions in toxicity as described in Table 1.

Table 1. Drug delivery systems for infectious keratitis.

Drug Delivery System	Details of Drug Delivery System	Disease	Mechanism of Action	Applications
Nanoparticles	Gelatin-capped silver nanoparticles	Bacterial keratitis	Antibacterial and anti-angiogenic properties of the silver nanoparticles	The gelatin-capped silver nanoparticles alleviate S. aureus-induced bacterial keratitis in rabbit eyes and bacterial infection-induced corneal neovascularization [82].
	Silver nanoparticles prepared from fungus	Viral keratitis	Show antiviral activity by blocking interaction between cells and virus	Silver nanoparticles reduce viral infectivity in a size-dependent manner against herpes simplex virus and human parainfluenza virus; smaller sizes resulting in higher antiviral activity [83].
	Protease responsive TLR-4 conjugated gelatin nanoparticles	Fungal keratitis	Enhance drug residence time by anchoring to the cornea	Used for management of keratitis by sustained and stimuli responsive release of antifungal drug and inhibition of inflammation [9].
	Nigella sativa aqueous extract and chitosan nanoparticles	Parasitic keratitis	Overcome drug resistance	The combination of Nigella sativa aqueous extract and chitosan nanoparticles shows synergistic antiparasitic activity in an animal model of Acanthamoeba keratitis [84].
Nanospheres	Silver nanospheres	Keratitis	Kill S. aureus by predominance of specific particle density and high-atom-density	Shows higher biocompatibility, anti-bacterial activity, anti-angiogenic capability, and repair of infected corneal tissues as compared to rod and triangular silver nanoparticles [10].
Liposomes	Liposome-dual TLR 3/9 agonist nanoparticles	HSP-1 induced conjunctivitis, keratitis, uveitis	Immunomodulation in subconjunctival tissues and corneal epithelium and suppression of herpesvirus replication and signs of infection	Significant lessening of ocular signs of infection and significantly fewer episodes of viral shedding compared to feline herpesvirus -1 infected cats. This is achieved by induction of both type I and II interferon responses along with suppression of feline herpesvirus -1 replication [57].
	Tobramycin liposomes	Bacterial keratitis	Single topical administration of tobramycin liposomes reduced frequent dosing by sustained drug release	Single topical administration of tobramycin liposomes provides bactericidal effects in Pseudomonas keratitis rabbit model and may provide better patient compliance [85].
	Liposomal amphotericin B (Fungisome)	Fungal keratitis	Liposomal formulation slows down the ocular drug clearance and acts as a drug reservoir	Topical application of Fungisome was more effective in reduction of fungal keratitis with lower toxicity than naked amphotericin B, though not statistically significant [58].
	Combinatorial delivery of siRNA (serine phosphatase and glycogen phosphoryl)-loaded liposomes and chlorhexidine	Parasitic keratitis	Provide enhanced corneal penetration and chlorhexidine- siRNA act against Acanthamoeba.	After 15 days of eight daily administrations, the liposomal complex combined with chlorhexidine treatment is able to reverse the lesions associated with keratitis [86].
Micelles	Chtosan-poly(lactide-co-glycolide)/poloxamer mixed micelles	Bacterial keratitis	Show enhanced antibacterial activity by high cellular uptake, corneal retention, muco-adhesiveness, and antibacterial effect	Exhibiting superior therapeutic effects in P. aeruginosa and S. aureus-infected BK mouse models by reducing the corneal bacterial load and preventing corneal damage [87].
	Acyclovir-loaded Soluplus micelles	Viral keratitis	Increases its solubility, corneal permeability and sclera penetration	In vivo efficacy in viral keratitis needed to be evaluated [88].
	Itraconazole-loaded polymeric micelles	Fungal keratitis	Enhance corneal permeability of hydrophobic drug and mucoadhesivity	Shows in vitro anti-fungal activity against Candida albicans through inhibition of chitin synthesis [89,90].
Niosomes	Ciprofloxacin-loaded Solulan C24- and Bile Salts-Modified Niosome	Bacterial keratitis	Enhance drug permeability and prolong ocular residence time	Significantly improves the Pseudomonas aeruginosa induced corneal ulcer, enhanced healing rate, inhibition of inflammatory cytokines secretion in vivo in rabbit [60].
	Natamycin-loaded nanoparticle noisome	Fungal keratitis	Enhance corneal penetration, protection against acidic and enzymatic effects, reduction in toxicity	Exhibits superior anti-inflammatory and antifungal activity as compared to free natamycin in rabbit in both Candida and Aspergillus keratitis [91,92].
	Isoniazid-nanoparticles-amphotericin B	Parasitic keratitis	Yet to be tested for the eye and hence, can be explored	Shows antiamoebic effects against Acanthamoeba castellanii [93].
Nanosuspensions	Eudragit- Ganciclovir Nanosuspension	Viral keratitis	Enhance corneal penetration, provides sustained release at pH 7.4	In vitro and in vivo efficacy study needed to be performed. Ganciclovir’s main mode of action against CMV involves the inhibition of viral DNA replication through the activity of ganciclovir-5’-triphosphate (ganciclovir-TP) [94,95].
	Moxifloxacin–pamoate nanosuspensions	Bacterial keratitis	Provide a significant increase in ocular drug absorption	Improved prevention and treatment of ocular keratitis in rat model of ocular Staphylococcus aureus infection [96].
	Voriconazole-loaded Glycol chitosan nanosuspensions with 4-carboxyphenylboronic acid pinacol ester as a ROS-responsive group (GC-EV-VOR)	Fungal keratitis	Exhibits high penetration through corneal barriers, good retention in the cornea and controllable drug release under low concentrations of ROS	Improved therapeutic effects with depletion of ROS, ocular inflammation and antifungal activity. Voriconazole exhibits antifungal activity by inhibition of cytochrome P450–dependent 14α-lanosterol demethylation, which is a vital step in cell membrane ergosterol synthesis by fungi [64,97].
	Praziquantel nanosuspensions	Parasitic keratitis	These nanosuspensions greatly modify the energetic metabolism of T. cysticerci in vivo	Treating with nanosuspensions in T. crassiceps cysticerci-infected mice led to elevated levels of glycolysis related organic acids and promoted a partial reversal of the tricarboxylic acid cycle, urea cycle, and ketone body production within the parasites, in contrast to the groups treated with the Praziquantel [98].
Nanoemulsions	Acyclovir-loaded termoresponsive in situ gel nanoemulsion	Viral keratitis	Enhance ocular permeation and sustained drug release	The DDSs shows good ocular tolerance bioavailability; however, the in vitro and in vivo efficacy need to be evaluated [20].
	Ciprofloxacin-loaded nanoemulsion fabricated from sing oleic acid and Labrafa Lipophile WL 1349, Tween 80 and Poloxamer 188	Bacterial keratitis	Enhance corneal penetration and sustained drug release	The nanoemulsion system demonstrated prolonged release and a 2.1-fold increase in permeation compared to the commercial ciprofloxacin formulation. Additionally, it exhibits robust stability at room temperature and can be frozen for up to one month. These findings suggest promising potential for treating bacterial keratitis [99].
	Luliconazole-loaded nanoemulsion composed of Capryol 90, ethoxylated hydrogenated castor oil, Transcutol® P and water	Fungal keratitis	Enhance drug release and antifungal activity and ocular bioavailability	The nanoemulsion formulation displayed outstanding physicochemical properties, excellent tolerability, enhanced antifungal activity, and improved bioavailability in ocular tissues [100].
	Amphotericin-loaded nanoemulsion formulated from cholesterol and stearyl amine	Parasitic keratitis	Improve drug stability, bioactivity and sustained delivery	A single dose of nanoemulsion formulation could provide stability and sustained release of drug in vitro. However, in vivo evaluation needs to be performed [101].
Cubosomes	Gatifloxacin Loaded cubosomes	Bacterial keratitis	Enhance corneal penetration, bioactivity of the drug, and overcome bacterial resistance	Gatifloxacin-loaded cubosomal dispersions enhanced corneal permeation and a fourfold reduction in the minimum inhibitory concentration against a clinically isolated methicillin-resistant strain in a rat model of bacterial keratitis [102].
	Fluconazole-loaded cubosome	Fungal keratitis	Enhance corneal penetration and improved efficacy	Fluconazole-loaded cubosomes shows two-fold corneal penetration, and higher efficacy and safety as compared to aqueous fluconazole solution in rat model [68].
Hydrogel	Supramolecular hydrogel generated by ganciclovir and 2′-deoxyguanosine in the presence of potassium ions.	Herpes simplex keratitis	Enhance corneal retention because of high viscoelasticity, excellent spreadability, and effective deformation recovery properties	Ganciclovir and 2′-deoxyguanosine-based supra molecular hydrogel exhibit longer ocular retention higher therapeutic efficacy compared to the clinical ganciclovir gel [72].
	Norfloxacin-loaded PLGA-laden hydroxypropyl methylcellulose hydrogel	Bacterial keratitis	Enhance corneal permeation, sustained drug release, and antibacterial properties	The DDS enhances corneal permeation, ensuring sustained delivery and effectiveness against Pseudomonas aeruginosa-induced infection and inflammation in a rabbit corneal ulcer model [73].
	Econazole-cyclodextrin embedded in deacylated gellan gum and k-carrageenan hydrogel or hyaluronic acid hydrogel	Fungal keratitis	Enhance drug solubility and ocular retention	The econazole hydrogel shows superior ocular retention, sustained release, non-irritation, and a better safety profile compared to the control drug [94].
	Poly-Epsilon-Lysine (PEK) Functionalized Hydrogels	Parasitic keratitis	Offer amoebicidal activity	The PEK-hydrogels shows equivalent anti-microbial properties as compared to Chlorohexidine against A. castellanii in in vitro and ex vivo studies [103].
In-situ gelling system	Acyclovir-loaded sodium alginate based in situ gelling system	Viral keratitis	Increase the half-life of the drug, prolong corneal contact time, eradicate drug elimination and increase the bioavailability	Ophthalmic in situ gels offer the potential for sustained delivery, leading to extended ocular residence time, enhanced shelf life, and precise dosing compared to conventional dosage forms [104].
	Oxytetracycline-loaded gelatin-polyacrylic acid nanoparticles In Situ Poloxamer N-407 gel	Bacterial keratitis	Provides sustained delivery of drug	The in situ gelling system provides sustained delivery of drug, minimizing irritation and exhibiting excellent antibacterial properties. It holds promise as a potential delivery system for treating bacterial keratitis [105].
	Voriconazole-loaded poloxamer-188, poloxamer-407 and carboxymethyl cellulose based in situ gel	Fungal keratitis	Increase residence time, and the bioavailability of voriconazole in the ocular mucosa	This in situ gelling system offers sustained delivery, potent antifungal effects, and causes no eye irritation. It shows promise for development as a potential delivery system for treating fungal keratitis [106].
	Super aggregated amphotericin B with a thermoreversible in situ gel	Parasitic keratitis	Enhance biocompatibility, low toxicity, and high residence time on the ocular surface	This in situ gelling system could enhance the residence time while reducing toxicity and being active against Acanthamoeba keratitis, thus, holds promise for treating parasitic keratitis [107].
Contact lenses	Methacrylic acid-based imprinted valacyclovir-loaded contact lenses	Viral keratitis	Provide sustained release and enhance ocular penetration of drug	These contact lenses improve drug solubility, boost drug bioavailability through enhanced ocular penetration, and demonstrate excellent ocular compatibility [74].
	Melimine-coated contact lens	Bacterial keratitis	Provide antibacterial properties against Pseudomonas aeruginosa by inhibiting adhesion of bacteria.	Contact lenses coated with melimine decreased the occurrence of microbial keratitis linked to Pseudomonas aeruginosa in a rabbit model of keratitis [108].
	Hybrid hydrogel-based contact lens which comprises quaternized chitosan, silver nanoparticles, and graphene oxide with voriconazole	Fungal keratitis	Provide sustained delivery of drug and silver nanoparticles and further enhances the antifungal properties of the developed contact lens.	The hybrid hydrogel-based contact lens offers continuous dry release, exhibits excellent biocompatibility, and effectively treats fungal keratitis in a mouse model [109].
	Miltefosine coated PLGA contact lens	Acanthamoeba Keratitis	Provide sustained and local delivery of drug	The drug-eluting contact lens not only shows similar physicochemical and biocompatible properties as compared to the commercial contact lens, but also provides sustained and local delivery of drug at a therapeutic concentration for treatment of parasitic keratitis [110].
Micro-needles	Core-shell microneedles containing Ag@ZIF8 nanoparticles in the core and Rapamycin in the hydrophilic shell	Bacterial keratitis	The core provides antimicrobial properties, and shell provides anti-inflammatory and antiangiogenic properties.	A single application of the core–shell microneedles patch in a rat model of bacterial keratitis demonstrates effective antimicrobial action, along with superior anti-angiogenic and anti-inflammatory effects compared to daily topical eyedrops [77].
	Amphotericin B-loaded polyvinyl alcohol and polyvinyl pyrrolidone microneedle patch	Fungal keratitis	Enhances drug solubility, corneal penetration, and provides sustained drug delivery	The application of amphotericin B-loaded microneedles patch significantly decreased the Candida albicans presence within the cornea and enhanced the epithelial and stromal differentiation of the corneal membrane in rabbit model [75].
	polyhexamethylene biguanide (PHMB)-loaded PLGA microneedle patch	Acanthamoeba keratitis	Provide sustained delivery	A solitary application of a biodegradable microneedle can replace the repetitive use of eyedrops in treating Aacanthamoeba keratitis [76].
Ocular implants	Besifloxacin HCl-loaded nanofibrous PCL/PEG mucoadhesive ocular inserts	Bacterial keratitis	Enhance bio-adhesion and sustained delivery	The ocular inserts demonstrated effective reduction of bacterial keratitis in rabbit eyes in a single dose, outperforming multiple doses of the commercial drug [78].

Table 1. Drug delivery systems for infectious keratitis.

Drug Delivery System	Details of Drug Delivery System	Disease	Mechanism of Action	Applications
Nanoparticles	Gelatin-capped silver nanoparticles	Bacterial keratitis	Antibacterial and anti-angiogenic properties of the silver nanoparticles	The gelatin-capped silver nanoparticles alleviate S. aureus-induced bacterial keratitis in rabbit eyes and bacterial infection-induced corneal neovascularization [82].
	Silver nanoparticles prepared from fungus	Viral keratitis	Show antiviral activity by blocking interaction between cells and virus	Silver nanoparticles reduce viral infectivity in a size-dependent manner against herpes simplex virus and human parainfluenza virus; smaller sizes resulting in higher antiviral activity [83].
	Protease responsive TLR-4 conjugated gelatin nanoparticles	Fungal keratitis	Enhance drug residence time by anchoring to the cornea	Used for management of keratitis by sustained and stimuli responsive release of antifungal drug and inhibition of inflammation [9].
	Nigella sativa aqueous extract and chitosan nanoparticles	Parasitic keratitis	Overcome drug resistance	The combination of Nigella sativa aqueous extract and chitosan nanoparticles shows synergistic antiparasitic activity in an animal model of Acanthamoeba keratitis [84].
Nanospheres	Silver nanospheres	Keratitis	Kill S. aureus by predominance of specific particle density and high-atom-density	Shows higher biocompatibility, anti-bacterial activity, anti-angiogenic capability, and repair of infected corneal tissues as compared to rod and triangular silver nanoparticles [10].
Liposomes	Liposome-dual TLR 3/9 agonist nanoparticles	HSP-1 induced conjunctivitis, keratitis, uveitis	Immunomodulation in subconjunctival tissues and corneal epithelium and suppression of herpesvirus replication and signs of infection	Significant lessening of ocular signs of infection and significantly fewer episodes of viral shedding compared to feline herpesvirus -1 infected cats. This is achieved by induction of both type I and II interferon responses along with suppression of feline herpesvirus -1 replication [57].
	Tobramycin liposomes	Bacterial keratitis	Single topical administration of tobramycin liposomes reduced frequent dosing by sustained drug release	Single topical administration of tobramycin liposomes provides bactericidal effects in Pseudomonas keratitis rabbit model and may provide better patient compliance [85].
	Liposomal amphotericin B (Fungisome)	Fungal keratitis	Liposomal formulation slows down the ocular drug clearance and acts as a drug reservoir	Topical application of Fungisome was more effective in reduction of fungal keratitis with lower toxicity than naked amphotericin B, though not statistically significant [58].
	Combinatorial delivery of siRNA (serine phosphatase and glycogen phosphoryl)-loaded liposomes and chlorhexidine	Parasitic keratitis	Provide enhanced corneal penetration and chlorhexidine- siRNA act against Acanthamoeba.	After 15 days of eight daily administrations, the liposomal complex combined with chlorhexidine treatment is able to reverse the lesions associated with keratitis [86].
Micelles	Chtosan-poly(lactide-co-glycolide)/poloxamer mixed micelles	Bacterial keratitis	Show enhanced antibacterial activity by high cellular uptake, corneal retention, muco-adhesiveness, and antibacterial effect	Exhibiting superior therapeutic effects in P. aeruginosa and S. aureus-infected BK mouse models by reducing the corneal bacterial load and preventing corneal damage [87].
	Acyclovir-loaded Soluplus micelles	Viral keratitis	Increases its solubility, corneal permeability and sclera penetration	In vivo efficacy in viral keratitis needed to be evaluated [88].
	Itraconazole-loaded polymeric micelles	Fungal keratitis	Enhance corneal permeability of hydrophobic drug and mucoadhesivity	Shows in vitro anti-fungal activity against Candida albicans through inhibition of chitin synthesis [89,90].
Niosomes	Ciprofloxacin-loaded Solulan C24- and Bile Salts-Modified Niosome	Bacterial keratitis	Enhance drug permeability and prolong ocular residence time	Significantly improves the Pseudomonas aeruginosa induced corneal ulcer, enhanced healing rate, inhibition of inflammatory cytokines secretion in vivo in rabbit [60].
	Natamycin-loaded nanoparticle noisome	Fungal keratitis	Enhance corneal penetration, protection against acidic and enzymatic effects, reduction in toxicity	Exhibits superior anti-inflammatory and antifungal activity as compared to free natamycin in rabbit in both Candida and Aspergillus keratitis [91,92].
	Isoniazid-nanoparticles-amphotericin B	Parasitic keratitis	Yet to be tested for the eye and hence, can be explored	Shows antiamoebic effects against Acanthamoeba castellanii [93].
Nanosuspensions	Eudragit- Ganciclovir Nanosuspension	Viral keratitis	Enhance corneal penetration, provides sustained release at pH 7.4	In vitro and in vivo efficacy study needed to be performed. Ganciclovir’s main mode of action against CMV involves the inhibition of viral DNA replication through the activity of ganciclovir-5’-triphosphate (ganciclovir-TP) [94,95].
	Moxifloxacin–pamoate nanosuspensions	Bacterial keratitis	Provide a significant increase in ocular drug absorption	Improved prevention and treatment of ocular keratitis in rat model of ocular Staphylococcus aureus infection [96].
	Voriconazole-loaded Glycol chitosan nanosuspensions with 4-carboxyphenylboronic acid pinacol ester as a ROS-responsive group (GC-EV-VOR)	Fungal keratitis	Exhibits high penetration through corneal barriers, good retention in the cornea and controllable drug release under low concentrations of ROS	Improved therapeutic effects with depletion of ROS, ocular inflammation and antifungal activity. Voriconazole exhibits antifungal activity by inhibition of cytochrome P450–dependent 14α-lanosterol demethylation, which is a vital step in cell membrane ergosterol synthesis by fungi [64,97].
	Praziquantel nanosuspensions	Parasitic keratitis	These nanosuspensions greatly modify the energetic metabolism of T. cysticerci in vivo	Treating with nanosuspensions in T. crassiceps cysticerci-infected mice led to elevated levels of glycolysis related organic acids and promoted a partial reversal of the tricarboxylic acid cycle, urea cycle, and ketone body production within the parasites, in contrast to the groups treated with the Praziquantel [98].
Nanoemulsions	Acyclovir-loaded termoresponsive in situ gel nanoemulsion	Viral keratitis	Enhance ocular permeation and sustained drug release	The DDSs shows good ocular tolerance bioavailability; however, the in vitro and in vivo efficacy need to be evaluated [20].
	Ciprofloxacin-loaded nanoemulsion fabricated from sing oleic acid and Labrafa Lipophile WL 1349, Tween 80 and Poloxamer 188	Bacterial keratitis	Enhance corneal penetration and sustained drug release	The nanoemulsion system demonstrated prolonged release and a 2.1-fold increase in permeation compared to the commercial ciprofloxacin formulation. Additionally, it exhibits robust stability at room temperature and can be frozen for up to one month. These findings suggest promising potential for treating bacterial keratitis [99].
	Luliconazole-loaded nanoemulsion composed of Capryol 90, ethoxylated hydrogenated castor oil, Transcutol® P and water	Fungal keratitis	Enhance drug release and antifungal activity and ocular bioavailability	The nanoemulsion formulation displayed outstanding physicochemical properties, excellent tolerability, enhanced antifungal activity, and improved bioavailability in ocular tissues [100].
	Amphotericin-loaded nanoemulsion formulated from cholesterol and stearyl amine	Parasitic keratitis	Improve drug stability, bioactivity and sustained delivery	A single dose of nanoemulsion formulation could provide stability and sustained release of drug in vitro. However, in vivo evaluation needs to be performed [101].
Cubosomes	Gatifloxacin Loaded cubosomes	Bacterial keratitis	Enhance corneal penetration, bioactivity of the drug, and overcome bacterial resistance	Gatifloxacin-loaded cubosomal dispersions enhanced corneal permeation and a fourfold reduction in the minimum inhibitory concentration against a clinically isolated methicillin-resistant strain in a rat model of bacterial keratitis [102].
	Fluconazole-loaded cubosome	Fungal keratitis	Enhance corneal penetration and improved efficacy	Fluconazole-loaded cubosomes shows two-fold corneal penetration, and higher efficacy and safety as compared to aqueous fluconazole solution in rat model [68].
Hydrogel	Supramolecular hydrogel generated by ganciclovir and 2′-deoxyguanosine in the presence of potassium ions.	Herpes simplex keratitis	Enhance corneal retention because of high viscoelasticity, excellent spreadability, and effective deformation recovery properties	Ganciclovir and 2′-deoxyguanosine-based supra molecular hydrogel exhibit longer ocular retention higher therapeutic efficacy compared to the clinical ganciclovir gel [72].
	Norfloxacin-loaded PLGA-laden hydroxypropyl methylcellulose hydrogel	Bacterial keratitis	Enhance corneal permeation, sustained drug release, and antibacterial properties	The DDS enhances corneal permeation, ensuring sustained delivery and effectiveness against Pseudomonas aeruginosa-induced infection and inflammation in a rabbit corneal ulcer model [73].
	Econazole-cyclodextrin embedded in deacylated gellan gum and k-carrageenan hydrogel or hyaluronic acid hydrogel	Fungal keratitis	Enhance drug solubility and ocular retention	The econazole hydrogel shows superior ocular retention, sustained release, non-irritation, and a better safety profile compared to the control drug [94].
	Poly-Epsilon-Lysine (PEK) Functionalized Hydrogels	Parasitic keratitis	Offer amoebicidal activity	The PEK-hydrogels shows equivalent anti-microbial properties as compared to Chlorohexidine against A. castellanii in in vitro and ex vivo studies [103].
In-situ gelling system	Acyclovir-loaded sodium alginate based in situ gelling system	Viral keratitis	Increase the half-life of the drug, prolong corneal contact time, eradicate drug elimination and increase the bioavailability	Ophthalmic in situ gels offer the potential for sustained delivery, leading to extended ocular residence time, enhanced shelf life, and precise dosing compared to conventional dosage forms [104].
	Oxytetracycline-loaded gelatin-polyacrylic acid nanoparticles In Situ Poloxamer N-407 gel	Bacterial keratitis	Provides sustained delivery of drug	The in situ gelling system provides sustained delivery of drug, minimizing irritation and exhibiting excellent antibacterial properties. It holds promise as a potential delivery system for treating bacterial keratitis [105].
	Voriconazole-loaded poloxamer-188, poloxamer-407 and carboxymethyl cellulose based in situ gel	Fungal keratitis	Increase residence time, and the bioavailability of voriconazole in the ocular mucosa	This in situ gelling system offers sustained delivery, potent antifungal effects, and causes no eye irritation. It shows promise for development as a potential delivery system for treating fungal keratitis [106].
	Super aggregated amphotericin B with a thermoreversible in situ gel	Parasitic keratitis	Enhance biocompatibility, low toxicity, and high residence time on the ocular surface	This in situ gelling system could enhance the residence time while reducing toxicity and being active against Acanthamoeba keratitis, thus, holds promise for treating parasitic keratitis [107].
Contact lenses	Methacrylic acid-based imprinted valacyclovir-loaded contact lenses	Viral keratitis	Provide sustained release and enhance ocular penetration of drug	These contact lenses improve drug solubility, boost drug bioavailability through enhanced ocular penetration, and demonstrate excellent ocular compatibility [74].
	Melimine-coated contact lens	Bacterial keratitis	Provide antibacterial properties against Pseudomonas aeruginosa by inhibiting adhesion of bacteria.	Contact lenses coated with melimine decreased the occurrence of microbial keratitis linked to Pseudomonas aeruginosa in a rabbit model of keratitis [108].
	Hybrid hydrogel-based contact lens which comprises quaternized chitosan, silver nanoparticles, and graphene oxide with voriconazole	Fungal keratitis	Provide sustained delivery of drug and silver nanoparticles and further enhances the antifungal properties of the developed contact lens.	The hybrid hydrogel-based contact lens offers continuous dry release, exhibits excellent biocompatibility, and effectively treats fungal keratitis in a mouse model [109].
	Miltefosine coated PLGA contact lens	Acanthamoeba Keratitis	Provide sustained and local delivery of drug	The drug-eluting contact lens not only shows similar physicochemical and biocompatible properties as compared to the commercial contact lens, but also provides sustained and local delivery of drug at a therapeutic concentration for treatment of parasitic keratitis [110].
Micro-needles	Core-shell microneedles containing Ag@ZIF8 nanoparticles in the core and Rapamycin in the hydrophilic shell	Bacterial keratitis	The core provides antimicrobial properties, and shell provides anti-inflammatory and antiangiogenic properties.	A single application of the core–shell microneedles patch in a rat model of bacterial keratitis demonstrates effective antimicrobial action, along with superior anti-angiogenic and anti-inflammatory effects compared to daily topical eyedrops [77].
	Amphotericin B-loaded polyvinyl alcohol and polyvinyl pyrrolidone microneedle patch	Fungal keratitis	Enhances drug solubility, corneal penetration, and provides sustained drug delivery	The application of amphotericin B-loaded microneedles patch significantly decreased the Candida albicans presence within the cornea and enhanced the epithelial and stromal differentiation of the corneal membrane in rabbit model [75].
	polyhexamethylene biguanide (PHMB)-loaded PLGA microneedle patch	Acanthamoeba keratitis	Provide sustained delivery	A solitary application of a biodegradable microneedle can replace the repetitive use of eyedrops in treating Aacanthamoeba keratitis [76].
Ocular implants	Besifloxacin HCl-loaded nanofibrous PCL/PEG mucoadhesive ocular inserts	Bacterial keratitis	Enhance bio-adhesion and sustained delivery	The ocular inserts demonstrated effective reduction of bacterial keratitis in rabbit eyes in a single dose, outperforming multiple doses of the commercial drug [78].

3.2. Drug Delivery Systems for Infectious Conjunctivitis

Infectious conjunctivitis is associated with inflammation due to infection by bacteria or virus [1]. These are generally treated with antimicrobial eyedrops or ointments [45]. However, the major shortcomings associated with the current treatment strategies are- poor solubility of drugs, low bioavailability of drugs due to conjunctival barrier properties, and development of resistance for current antimicrobial agents [111,112]. The conjunctiva’s protective functions, including guarding the ocular surface, producing the tear film, and aiding in drug clearance into systemic circulation or transportation to deeper eye tissues, represent significant hurdles in conjunctival drug delivery [113]. The advancements in DDSs have been put forward to overcome conjunctival barriers by enhancing drug residence time, drug solubility, drug stability; increasing permeability of the drug, providing sustained drug delivery, and overcoming drug resistance and reductions in toxicity as explained in Table 2.

Table 2. Drug delivery systems for infectious conjunctivitis.

Drug Delivery System	Details of Drug Delivery System	Disease	Mechanism of Action	Applications
Nanoparticles	Biological adhesion reticulate structure (BNP/CA-PEG)	Conjunctivitis	Enhances ocular surface retention by mucoadhesive properties	BNP/CA-PEG showed significantly higher adhesion properties, sustained drug release and better treatment efficacy in an ocular rat model of conjunctivitis [111].
Nanospheres	Levofloxacin-loaded chitosan-cyclodextrin nanospheres	Conjunctivitis	May increase ocular retention by promoting the interaction of positively charged nanospheres with the negatively charged ocular tissue	In vitro, antibacterial activity against Gram-positive and Gram-negative bacteria showed double the activity in levofloxacin-loaded chitosan-cyclodextrin nanospheres compared to the free drug [114].
Liposomes	Liposome containing gaseous ozone or ozonated oil	Conjunctivitis	Ozone generates free oxygen radicals that promote the formation of hydrogen peroxide and lipo-peroxide, contributing to bacterial lysis and eventual death	Shows antibacterial properties against broad spectrum bacteria like Pseudomonas aeruginosa, and is more evident against Staphylococcus and Streptococcus spp. [115,116].
Micelles	Erythromycin-loaded polymeric micelles	Bacterial conjunctivitis	Prolongs the drug release and permeation profile.	Even after ocular permeation, the formulation retains antibacterial properties, which is a promising strategy against ocular infection [112].
Dendrimers	Aminoterminated-poly(amidoamine) (PAMAM) dendrimer	Bacterial conjunctivitis	Effective against gram negative bacteria by disrupting bacterial cell membrane	Effective against gram-negative bacteria but does not cause cell death to corneal cells, thus expanding its application towards conjunctivitis [117].
Niosomes	Lomefloxacin Hcl (LXN)-loaded niosomes	Ketaritis and bacterial conjunctivitis	Enhances corneal penetration of LXN	Effective against Staphylococcus aureus induced conjunctivitis in vivo without having any ocular toxicity in rabbit model [61].
Nanosuspension	Ion-paired moxifloxacin–pamoate nanosuspension	Keratitis and conjuctivitis	Increases intraocular antibiotic absorption	Single dosing per day of moxifloxacin–pamoate nanosuspension is as effective as three doses per day of bare moxifloxacin [19].
Nanoemulsion	Moxifloxacin hydrochloride-loaded nanoemulsion-based in situ gel	Bacterial conjunctivitis	Provides sustained delivery and reduced toxicity	The nanosuspension improves ocular bioavailability of drugs through enhanced penetration into the eye and sustained delivery [66].
Cubosomes	Cubosomes formed by 60 mg of ciprofloxacin, 100 mg phytantriol, 25 mg Lutrol, and hydration media pH equal 5.8	Conjunctivitis and corneal ulcer	Improves eye permeation, prolonged the ocular retention time, and enhanced the antimicrobial activity	Single administration of cubosomal DDSs can maintain drug levels above the minimum inhibitory concentration (MIC), contrasting with commercial drops that necessitate administration four times daily [69].
Hydrogel	Poloxamer 407 and chitosan hydrogel containing combination of neomycin sulphate and betamethasone sodium phosphate	Conjunctivitis	Enhances permeation and without any irritation	It can inhibit infection and inflammation simultaneously while providing sustained release of drugs [15].
In situ gelling system	levofloxacin hemihydrate containing ion-sensitive gellan gum based in situ gelling system	Bacterial conjunctivitis	Provides prolonged drug release (24 h), enhanced stability and shows good ocular tolerability	The gellan gum-based in situ gelling system provides higher bioavailability of drug in conjunctiva as compared to the commercial product Levotop PF® with a good ocular tolerability, indicating potential use for treatment of bacterial conjunctivitis [118].
Contact lens	Contact lenses loaded with moxifloxacin HCl and hyaluronic acid	Bacterial conjunctivitis	Provides sustained drug release for 96 h, enhanced retention in eye	It shows better efficacy against Staphylococcus aureus induced conjunctivitis as compared to eye drops [16].

Table 2. Drug delivery systems for infectious conjunctivitis.

Drug Delivery System	Details of Drug Delivery System	Disease	Mechanism of Action	Applications
Nanoparticles	Biological adhesion reticulate structure (BNP/CA-PEG)	Conjunctivitis	Enhances ocular surface retention by mucoadhesive properties	BNP/CA-PEG showed significantly higher adhesion properties, sustained drug release and better treatment efficacy in an ocular rat model of conjunctivitis [111].
Nanospheres	Levofloxacin-loaded chitosan-cyclodextrin nanospheres	Conjunctivitis	May increase ocular retention by promoting the interaction of positively charged nanospheres with the negatively charged ocular tissue	In vitro, antibacterial activity against Gram-positive and Gram-negative bacteria showed double the activity in levofloxacin-loaded chitosan-cyclodextrin nanospheres compared to the free drug [114].
Liposomes	Liposome containing gaseous ozone or ozonated oil	Conjunctivitis	Ozone generates free oxygen radicals that promote the formation of hydrogen peroxide and lipo-peroxide, contributing to bacterial lysis and eventual death	Shows antibacterial properties against broad spectrum bacteria like Pseudomonas aeruginosa, and is more evident against Staphylococcus and Streptococcus spp. [115,116].
Micelles	Erythromycin-loaded polymeric micelles	Bacterial conjunctivitis	Prolongs the drug release and permeation profile.	Even after ocular permeation, the formulation retains antibacterial properties, which is a promising strategy against ocular infection [112].
Dendrimers	Aminoterminated-poly(amidoamine) (PAMAM) dendrimer	Bacterial conjunctivitis	Effective against gram negative bacteria by disrupting bacterial cell membrane	Effective against gram-negative bacteria but does not cause cell death to corneal cells, thus expanding its application towards conjunctivitis [117].
Niosomes	Lomefloxacin Hcl (LXN)-loaded niosomes	Ketaritis and bacterial conjunctivitis	Enhances corneal penetration of LXN	Effective against Staphylococcus aureus induced conjunctivitis in vivo without having any ocular toxicity in rabbit model [61].
Nanosuspension	Ion-paired moxifloxacin–pamoate nanosuspension	Keratitis and conjuctivitis	Increases intraocular antibiotic absorption	Single dosing per day of moxifloxacin–pamoate nanosuspension is as effective as three doses per day of bare moxifloxacin [19].
Nanoemulsion	Moxifloxacin hydrochloride-loaded nanoemulsion-based in situ gel	Bacterial conjunctivitis	Provides sustained delivery and reduced toxicity	The nanosuspension improves ocular bioavailability of drugs through enhanced penetration into the eye and sustained delivery [66].
Cubosomes	Cubosomes formed by 60 mg of ciprofloxacin, 100 mg phytantriol, 25 mg Lutrol, and hydration media pH equal 5.8	Conjunctivitis and corneal ulcer	Improves eye permeation, prolonged the ocular retention time, and enhanced the antimicrobial activity	Single administration of cubosomal DDSs can maintain drug levels above the minimum inhibitory concentration (MIC), contrasting with commercial drops that necessitate administration four times daily [69].
Hydrogel	Poloxamer 407 and chitosan hydrogel containing combination of neomycin sulphate and betamethasone sodium phosphate	Conjunctivitis	Enhances permeation and without any irritation	It can inhibit infection and inflammation simultaneously while providing sustained release of drugs [15].
In situ gelling system	levofloxacin hemihydrate containing ion-sensitive gellan gum based in situ gelling system	Bacterial conjunctivitis	Provides prolonged drug release (24 h), enhanced stability and shows good ocular tolerability	The gellan gum-based in situ gelling system provides higher bioavailability of drug in conjunctiva as compared to the commercial product Levotop PF® with a good ocular tolerability, indicating potential use for treatment of bacterial conjunctivitis [118].
Contact lens	Contact lenses loaded with moxifloxacin HCl and hyaluronic acid	Bacterial conjunctivitis	Provides sustained drug release for 96 h, enhanced retention in eye	It shows better efficacy against Staphylococcus aureus induced conjunctivitis as compared to eye drops [16].

3.3. Drug Delivery Systems for Endophthalmitis

Endophthalmitis is a devastating disorder characterized by ocular inflammation due to infection of the intraocular cavity that can lead to irreversible visual loss. Current treatment strategies are majorly based on systemic or intravitreal antimicrobial agents. Intravitreal injection of antimicrobial agents along with anti-inflammatory drugs is sometimes utilized in treatment protocols [49,53]. Repeated intravitreal injections of antimicrobial agents represent the prevailing treatment approach; however, they may be linked to discomfort, ocular pain, elevated intraocular pressure, intraocular bleeding, heightened risk of retinal detachment, and retinal toxicity, all of which can worsen complications and frequently result in blindness [8,56,119]. The advancements in DDSs have been put forward to overcome static and dynamic barriers of the eye such as corneal and conjunctival barriers, blood ocular barriers, blood retinal barriers, and barriers associated with conjunctival and choroidal blood flow, enhancing ocular drug adsorption, drug residence time, permeability of the drug, and drug solubility, while overcoming drug resistance and reducing toxicity as explained in the Table 3.

Table 3. Drug delivery systems for endophthalmitis.

Drug Delivery Systems	Details of Drug Delivery Systems	Disease	Mechanism of Action	Applications
Nanoparticles	Chitosan coated poly-l-lactide nanoparticles	Endophthalmitis	Overcome the blood ocular and retinal barrier by enhanced penetration	The azithromycin- and triamcinolone acetonide-loaded dual drug DDSs showed antibacterial properties against both gram positive and gram negative bacteria [8].
Nanospheres	Vancomycine-loaded lipid nano- capsules/spheres	Endophthalmitis	Show higher bioavailability of drug in the vitreous region by enhanced corneal penetration	Antimicrobial activity is comparable to intravitreal injection of vancomycin. Further, the optimized formula was found to be nonirritating and safe for ophthalmic administration [120].
Liposomes	Liposomes-loaded Amphotericin-B or Fluconazole	Endophthalmitis	Show lower efficacy against Candida albicans induced fungal endophthalmitis	The liposomal antifungal drug shows lower toxicity and lower intravitreal clearance; however, it shows lower efficacy. Amphotericin B acts on disruption of fungal cell wall synthesis, and fluconazole inhibits the synthesis of ergosterol to increase cellular permeability [121,122,123,124].
Micelles	Miconazole nitrate-loaded micelles of tri-block copolymers Pf 127 and Pf 68	Fungal endophthalmitis	Enhance corneal penetration	Sustained drug delivery and achieved the therapeutic level of drug at 8 h of release [59].
Dendrimers	Dendrimeric polyguanidilyated translocators-gatifloxacin complex	Endophthalmitis	Enhance gatifloxacin solubility, enhanced epithelial permeability and antimicrobial activity against Staphylococcus aureus	Shows promising effects against S. aureus induced endophthalmitis by enhancing drug solubility, permeability, antimicrobial activity, and in vivo delivery, potentially allowing a once-daily dose regimen [125].
Niosomes	Niosome made up of Tween 60, cholesterol and dicetyl phosphate	Ocular infection	Provides sustained delivery of Gentamicin with no ocular irritation	May be useful for both gram positive and gram-negative bacteria and can be explored for ocular infections like endophthalmitis [62].
Nanosponges	RBC-PLGA nanosponges	Endophthalmitis	Effective against Enterococcus faecalis	The biomimetic nanosponges neutralize cytolysin, protect the retina, preserve vision, and may provide an adjunct detoxification therapy for bacterial infections [63].
nanosuspensions	Acyclovir containing polymeric nanosuspension	May be useful for ocular infection	Enhance drug solubility, ocular bioavailability, and sustained drug release	The nanosuspensions exhibit promising potential due to their ability to enhance drug solubility, ocular bioavailability, sustained drug release, and antifungal properties. However, additional in vitro and in vivo assessments are required to evaluate their efficacy further [65].
Nanoemulsion	Moxifloxacin Mucoadhesive Nanoemulsion	Endophthalmitis	Effective against both gram positive and negative bacteria	Provides sustained drug release for enhanced corneal penetration and equivalent antibacterial properties as compared to commercial Vigamox® eyedrops [67].
Cubosomes	In situ gel containing Natamycin-loaded cubosomes	Endophthalmitis	Enhance drug solubility and ocular penetration with less ocular irritation	May be effective against fungal endopthalmitis, though in vivo evaluation in a disease model is needed [70].
In situ gelling system	Ciprofloxacin containing micro emulsion-based in situ gelling systems	Endophthalmitis	Enhances the drug’s absorption, penetration, and retention, thereby boosting its bioavailability	Given that the drug concentration in the vitreous is approximately 0.4 µg/mL, surpassing the therapeutic threshold required for gram-negative bacteria, it presents a potential avenue for investigation in the context of endophthalmitis [126].
Intraocular implants	Moxifloxacin releasing hyaluronic acid	Endophthalmitis	Maintain therapeutic concentration of drug for Pseudomonas aeruginosa and Staphylococcus aureus for more than 5 days after implantation	Can be utilized as a potential drug delivery method for the prevention and treatment of bacterial infections like endophthalmitis after ophthalmic surgery [79].
Intra-ocular lens	Intraocular lens containing antibiotic solutions of 0.3% and 0.5% gatifloxacin and 0.5% and 1.5% levofloxacin	Prevention of post cataract endophthalmitis	Effective against Enterococcus faecalis	Provide sustained release of antibiotics and prevent bacterial proliferation [127].

Table 3. Drug delivery systems for endophthalmitis.

Drug Delivery Systems	Details of Drug Delivery Systems	Disease	Mechanism of Action	Applications
Nanoparticles	Chitosan coated poly-l-lactide nanoparticles	Endophthalmitis	Overcome the blood ocular and retinal barrier by enhanced penetration	The azithromycin- and triamcinolone acetonide-loaded dual drug DDSs showed antibacterial properties against both gram positive and gram negative bacteria [8].
Nanospheres	Vancomycine-loaded lipid nano- capsules/spheres	Endophthalmitis	Show higher bioavailability of drug in the vitreous region by enhanced corneal penetration	Antimicrobial activity is comparable to intravitreal injection of vancomycin. Further, the optimized formula was found to be nonirritating and safe for ophthalmic administration [120].
Liposomes	Liposomes-loaded Amphotericin-B or Fluconazole	Endophthalmitis	Show lower efficacy against Candida albicans induced fungal endophthalmitis	The liposomal antifungal drug shows lower toxicity and lower intravitreal clearance; however, it shows lower efficacy. Amphotericin B acts on disruption of fungal cell wall synthesis, and fluconazole inhibits the synthesis of ergosterol to increase cellular permeability [121,122,123,124].
Micelles	Miconazole nitrate-loaded micelles of tri-block copolymers Pf 127 and Pf 68	Fungal endophthalmitis	Enhance corneal penetration	Sustained drug delivery and achieved the therapeutic level of drug at 8 h of release [59].
Dendrimers	Dendrimeric polyguanidilyated translocators-gatifloxacin complex	Endophthalmitis	Enhance gatifloxacin solubility, enhanced epithelial permeability and antimicrobial activity against Staphylococcus aureus	Shows promising effects against S. aureus induced endophthalmitis by enhancing drug solubility, permeability, antimicrobial activity, and in vivo delivery, potentially allowing a once-daily dose regimen [125].
Niosomes	Niosome made up of Tween 60, cholesterol and dicetyl phosphate	Ocular infection	Provides sustained delivery of Gentamicin with no ocular irritation	May be useful for both gram positive and gram-negative bacteria and can be explored for ocular infections like endophthalmitis [62].
Nanosponges	RBC-PLGA nanosponges	Endophthalmitis	Effective against Enterococcus faecalis	The biomimetic nanosponges neutralize cytolysin, protect the retina, preserve vision, and may provide an adjunct detoxification therapy for bacterial infections [63].
nanosuspensions	Acyclovir containing polymeric nanosuspension	May be useful for ocular infection	Enhance drug solubility, ocular bioavailability, and sustained drug release	The nanosuspensions exhibit promising potential due to their ability to enhance drug solubility, ocular bioavailability, sustained drug release, and antifungal properties. However, additional in vitro and in vivo assessments are required to evaluate their efficacy further [65].
Nanoemulsion	Moxifloxacin Mucoadhesive Nanoemulsion	Endophthalmitis	Effective against both gram positive and negative bacteria	Provides sustained drug release for enhanced corneal penetration and equivalent antibacterial properties as compared to commercial Vigamox® eyedrops [67].
Cubosomes	In situ gel containing Natamycin-loaded cubosomes	Endophthalmitis	Enhance drug solubility and ocular penetration with less ocular irritation	May be effective against fungal endopthalmitis, though in vivo evaluation in a disease model is needed [70].
In situ gelling system	Ciprofloxacin containing micro emulsion-based in situ gelling systems	Endophthalmitis	Enhances the drug’s absorption, penetration, and retention, thereby boosting its bioavailability	Given that the drug concentration in the vitreous is approximately 0.4 µg/mL, surpassing the therapeutic threshold required for gram-negative bacteria, it presents a potential avenue for investigation in the context of endophthalmitis [126].
Intraocular implants	Moxifloxacin releasing hyaluronic acid	Endophthalmitis	Maintain therapeutic concentration of drug for Pseudomonas aeruginosa and Staphylococcus aureus for more than 5 days after implantation	Can be utilized as a potential drug delivery method for the prevention and treatment of bacterial infections like endophthalmitis after ophthalmic surgery [79].
Intra-ocular lens	Intraocular lens containing antibiotic solutions of 0.3% and 0.5% gatifloxacin and 0.5% and 1.5% levofloxacin	Prevention of post cataract endophthalmitis	Effective against Enterococcus faecalis	Provide sustained release of antibiotics and prevent bacterial proliferation [127].

4. Targeted Drug Delivery Systems for Infectious Ocular Diseases

Targeted drug delivery for infectious ocular diseases involves the development of advanced DDSs that can precisely deliver therapeutic agents to specific sites within the eye. This approach enhances the efficacy of treatments while minimizing systemic side effects. The various strategies of causing DDSs to target different specific ocular tissues include controlling the physicochemical properties of the nanoparticles, magnetic nanoparticles, and ligand receptor-based targeting [11,12,13,14].

4.1. Ocular Tissue Targeting by Varying the Physicochemical Properties of the Drug Delivery Systems

Due to the inherent barrier properties of eye tissues, a major challenge in treating eye diseases is delivering therapeutic agents to the desired tissue in effective quantities and durations. In our previous study, we aimed to understand how the physicochemical properties of nanoparticles influence their spatiotemporal biodistribution in the mouse eye. For this purpose, core-shell nanoparticles with varying properties such as hydrophilicity, surface charge, and mucoadhesivity were designed by altering either the core or the shell, and were administered as eye drops to mice. The results demonstrated that all nanoparticles, regardless of the core or shell type, followed the conjunctival-scleral pathway. However, their spatiotemporal distribution varied on the basis of their physicochemical properties. For instance: polycaprolactone (PCL)-Gelatin showed the highest biodistribution in the cornea, PCL-Pluronic F68 in the conjunctiva, choroid, and retina, PCL-Chitosan in the conjunctiva and iris, and PCL in the lens and sclera compared to other nanoparticles [11,12]. These nanoparticles can be explored to target infectious diseases in these specific tissues of the eye.

4.2. Ocular Tissue Targeting by Magnetic Nanoparticles (MNPs)

MNPs stand out from other nanocarriers due to their unique magnetic properties, which make them particularly suitable for targeted drug delivery. Drug molecules can be attached to the surface of MNPs and injected into the body. An external magnetic field can then be used to concentrate these drug-laden nanoparticles in a specific area, minimizing damage to surrounding tissue. MNPs specifically localize in the retinal pigment epithelium (RPE), where they remain for several days after being injected intravitreally or near the lens in the anterior eye. This localization specificity is independent of the MNPs’ particle size and surface properties [13]. However, functionalizing the MNP surface with vascular endothelial growth factor (VEGF), a bioactive molecule capable of transcytosis from the RPE to posterior layers, successfully targeted the MNPs to the choroid. In contrast, MNPs functionalized with a control polypeptide, poly-L-lysine, exhibited the same localization pattern as the unmodified MNP particles [14]. These magnetic nanoparticles can be a promising treatment modality for endophthalmitis.

4.3. Ocular Tissue Targeting by Ligand-Receptor Interactions

Ligand-receptor-based targeting is a promising strategy for the treatment of ocular infectious diseases, leveraging the specific interactions between ligands and receptors to enhance drug delivery efficacy. Various ligands, including antibodies, proteins, peptides, fatty acids, the carbohydrate mitis, and aptamers are used to target specific receptors expressed on diseased cells, or microorganisms improving drug delivery efficiency and therapeutic outcomes in infectious ocular diseases [128,129,130,131]. There are multiple ligands that are used to modify DDSs explored for corneal targeting. For example: (i) nanoemulsions have been modified with stearoyl L-carnitine to target carnitine transporter 2 and amino acid transporter B in human corneal epithelial cells, (ii) VEGF receptors, including VEGFR-1, VEGFR-2, and VEGFR-3, have been explored for targeted delivery in ocular therapies to manage conditions like ocular angiogenesis, (iii) LyP-1 peptide-modified alginate-based nanoparticles for drug delivery targeting the cornea, (iv) lectin-decorated nanoparticles to enhance binding to inflamed tissues specifically due to infection [128,129,130,132].

For development of receptor-targeted DDSs for conjunctivitis, certain receptors can be explored, for example- (i) hyaluronic acid is often used to target CD44 receptors, which are overexpressed in inflamed or diseased conjunctival tissues. HA-modified drugs can enhance drug delivery to the conjunctiva by binding specifically to CD44 receptors [133]. (ii) LHRH and transferrin receptor can be targeted by nanoparticles conjugated with deslorelin or transferrin significantly enhancing conjunctival uptake of nanoparticles and hence, can be explored for conjunctivitis for enhanced efficacy and lower toxicity [134]. (iii) Lectins can specifically bind to carbohydrate moieties on the surface of conjunctival cells. Lectin-functionalized nanoparticles can enhance drug delivery by targeting these glycan structures [135]. Receptor-targeted DDSs offer a promising approach to improve the efficacy of treatments by specifically targeting infected cells or tissues. An example of receptor-targeted drug DDSs that could be applied for the treatment of endophthalmitis is as follows: Nanoparticles conjugated with ligands for TLRs enhance the delivery of antimicrobial agents to the infected sites by targeting immune cells that express TLRs, thereby improving the immune response against pathogens causing endophthalmitis [136]. These receptor-targeted DDSs aim to improve the specificity and efficacy of treatments for ocular infection by leveraging the interactions between ligands and receptors expressed on immune cells and infected tissues.

5. Combination Therapies for Infectious Ocular Diseases

The efficacy of current drug therapies for treating infectious ocular diseases is at risk of becoming obsolete due to inadequate physical, chemical, biological, and pharmacokinetic properties of drugs, along with low bioavailability caused by ocular barriers, inflammation associated with infection, and the elevated risk of developing resistance [8,137]. To tackle this problem, researchers have investigated development of DDSs and combinatorial therapy approaches. In combinatorial therapies, the major approaches investigated are (i) utilizing combinations of two or more antibiotics, (ii) pairing antifungal agents with antibiotics, (iii) combinations of two or more antifungal agents, (iv) merging anti-infective agents with anti-inflammatory drugs, (v) combinatorial therapy, including antimicrobial agents and photodynamic therapy, and (vi) antibacterial and comfort agents for eye-related conditions [8,15,16,17,18].

In treating bacterial eye infections, the choice of drugs can vary. Typically, antibiotics target specific aspects of bacterial physiology: interfering with cell wall formation for gram-positive bacteria and disrupting ribosomal protein synthesis for gram-negative bacteria, thus enhancing drug specificity. For acute-onset bacterial endophthalmitis, current therapy involves a combination of vancomycin for gram-positive coverage, and amikacin and ceftazidime for gram-negative coverage. This combination therapy approach may yield improved outcomes, particularly in cases involving antibiotic-resistant bacteria [17]. Moreover, combining antifungal agents with antibiotics demonstrates an appealing potential to reduce the duration, stress, and fatigue linked with topical treatment regimens against B-hemolytic Streptococcus spp., Aspergillus spp., and Fusarium spp. [138].

Combination antifungal therapy has been used for fungal eye infections to increase efficacy, reduce toxicity and improve the prognosis of the patient in infections that are difficult to treat. The concurrent administration of natamycin and voriconazole for treating fungal keratitis patients effectively eliminates ocular fungi, enhances ocular symptoms and visual acuity improvement, and maintains good safety with no significant increase in adverse reactions as compared to natamycin alone [139]. In posterior infectious diseases such as endophthalmitis, inflammation linked to infection represents a significant challenge. To tackle this issue, researchers have investigated combination therapy for disorders affecting the posterior segment of the eye.

A combination of anti-inflammatory agents like triamcinolone acetonide, prednisolone acetate, dexamethasone, and ketorolac, has been employed alongside anti-infective agents like antibiotics or antifungal agents [8,53,140,141,142]. By incorporating anti-inflammatory and anti-microbial agents into a single delivery system, researchers could achieve synergistic effects on both anti-inflammatory and antimicrobial properties, leading to improved treatment outcomes [8,143]. The sequential release of antibacterial followed by anti-inflammatory drugs in infectious diseases like endophthalmitis is essential. To achieve this, Ailinmg Yu et al. developed a supramolecular hydrogel formed by co-assembly of a sparingly soluble antibiotic (levofloxacin, Lev) and dexamethasone-peptide amphiphile (Dex-SA-RGD). In the in vitro drug release study, it was observed that Lev was swiftly released from the supramolecular hydrogel within 6 hours. Conversely, Dex-SA-RGD escaped from the hydrogel due to matrix erosion, subsequently functioning as a prodrug to facilitate the sequential slow release of active dexamethasone via hydrolysis. The Lev/Dex-SA-RGD supramolecular hydrogel exhibited potent antibacterial activity against both gram-negative and gram-positive strains [143].

In efforts to combat bacterial and fungal resistance, researchers have explored combining antimicrobial agents with photodynamic therapy, yielding synergistic effects [18,144,145]. Furthermore, to make the drug formulation more patient compliant and offer relief for microbial infections, a contact lens is designed with moxifloxacin, an antibacterial agent, and hyaluronic acid for added comfort during conjunctivitis treatment. This specialized lens ensures sustained delivery of the medication for up to 96 h, providing both therapeutic effectiveness and comfort for the eye [16]. Despite the potential effectiveness of various combination of therapeutic agents, conventional dosage forms face challenges, including limited access to infected ocular tissues/cells, poor solubility, short half-life, and toxicity, highlighting the pressing need for innovative scientific research to address emerging rapid resistance to these drugs. DDSs can overcome ocular barriers and target specific ocular tissues by varying physicochemical properties, while increasing drug solubility and enhancing the half-life of the drug with reductions in ocular toxicity [8,11,12,56,88].

6. Stimuli Responsive Drug Delivery Systems for Infectious Ocular Diseases

In recent years, there has been a significant surge of interest in stimuli-responsive DDSs in infectious ocular diseases. This heightened attention stems from their ability to precisely regulate the release of drugs in both space and time, allowing for customizable drug release tailored to the patient’s physiological or pathological state [146]. These DDSs can react to various stimuli, such as changes in temperature, pH levels, presence of ions, enzymatic activity, changes in ROS, exposure to light, magnetic fields, and ultrasound (Figure 3) [20,21,22,23,64].

Temperature-sensitive ocular DDSs utilize changes in temperature, which can be higher in inflamed or infected areas, to trigger drug release. Thermoresponsive gels are materials that undergo a reversible sol-gel transition in response to changes in temperature (Figure 3A). Thermoresponsive gels or nanoparticle-loaded thermoresponsive gels have been explored for all the infectious ocular diseases like keratitis, conjunctivitis and endophthalmitis [20,105,106]. Acyclovir-loaded thermosensitive in situ gel nanoemulsions showed a 2.8 time higher corneal penetration than the control drug, and also provided sustained acyclovir release without having corneal irritation, which sounds to be a promising treatment strategy for viral keratitis [20]. In the oxytetracycline-loaded gelatin-polyacrylic acid nanoparticles-loaded in poloxamer-N407 thermo- responsive hydrogel for the treatment of bacterial keratitis, the residence time of the drug is enhanced, which shows antibacterial activity against Pseudomonas aeruginosa in vitro and in vivo, which is comparable with the commercial counterpart. A Voriconazole-loaded thermos-responsive in situ gelling system was prepared using poloxamer 188, poloxamer 407 and carboxymethylcellulose. This DDS increased residence time and the bioavailability of voriconazole in the ocular mucosa, provided sustained drug release, and protected the cornea and conjunctiva from damage due to fungal infection [98]. Vancomycin-loaded poly (ethylene glycol) diacrylate and poly(N-isopropylacrylamide)-based thermo-responsive hydrogel DDS showed a lower infection score than the control drug in Staphylococcus aureus induced rat endophthalmitis model [105].

Enzyme-responsive ocular DDSs react to specific enzymes that are overexpressed in healthy and infected tissues. In the eye, various enzymes are present under both physiological and pathological conditions, such as matrix metalloproteinases (MMP) in tissues, hyaluronidase in the vitreous, and lysozyme and esterase in tears (Figure 3B) [147]. These enzymes can be targeted for development of enzyme-responsive DDSs. MMP-sensitive supramolecular nanoparticles (MMP-S NPs) were developed to enhance the photodynamic antibacterial effect against biofilm-associated bacterial keratitis. These nanoparticles were prepared via host-guest self-assembly between hydrophobic Ce6-conjugated β-cyclodextrin (β-CD) and hydrophilic MMP-9-sensitive peptide-modified adamantane. The MMP-9-sensitive peptide included three functional moieties: a cationic AMP sequence (YGRKKKRRQRRR), an MMP-9-responsive sequence (GPLGVRG), and a negatively charged peptide sequence (EEEEEE). The negatively charged EEEEEE acted as an antifouling layer to prevent adhesion to normal ocular surfaces or healthy corneal cells, but was stripped off by overexpressed MMP-9 at the infection site. This exposure of the cationic AMPs allowed the nanoparticles to penetrate deeply into the biofilm and bind to bacterial surfaces through electrostatic interactions, thereby enhancing the antimicrobial efficacy of photodynamic therapy (PDT) against Pseudomonas aeruginosa [148]. Chitosan-coated liposomes containing levofloxacin were developed to use lysozyme in tears as an intelligent “switch” for precise antibiotic delivery through the catalytic degradation of chitosan. The release rate of levofloxacin depends on the lysozyme concentration and the degree of chitosan deacetylation. This drug carrier shows significant potential to enhance targeted antibiotic delivery, achieve on-demand controlled release, increase drug delivery efficiency, reduce side effects, and improve anti-infective therapy for bacterial keratitis [23].

pH-sensitive ocular DDSs exploit the differences in pH between healthy and infected tissues to release drugs precisely where needed. The change in pH value triggers different nanocarriers to release drugs at specific sites using two main strategies (Figure 3C). Polymers with many acidic groups experience increased electrostatic repulsion and swelling at high pH. Conversely, polymers with basic groups ionize and exhibit electrostatic repulsion at low pH [147]. However, the eye can tolerate a pH range of 4 to 10, with the pH of tears being approximately 7.4. Natural or synthetic pH-responsive polymers are designed to respond effectively within this environment and explored for keratitis, conjunctivitis, and endophthalmitis [21,22,147]. Ciprofloxacin-loaded Poly (N-isopropylacrylamide-methacrylic acid-vinylpyrrolidone) nanogels demonstrated enhanced antibacterial effects in vitro and showed reasonable efficacy in treating severe keratitis in vivo. This developed system has the potential for localized application in the treatment of keratitis [22]. A pH-sensitive metal oxide framework was developed for chemotherapy and photodynamic therapy against bacterial endophthalmitis. This hybrid DDS utilizes a pH-responsive zeolitic imidazolate framework-8-polyacrylic acid material, loaded with ammonium methylbenzene blue, and further modified with AgNO₃/dopamine for in situ reduction to silver nanoparticles. The system undergoes a secondary modification with vancomycin/NH2-polyethylene glycol, forming a composite nanomaterial. This system demonstrates antibacterial activity against Escherichia coli, Staphylococcus aureus, and methicillin-resistant S. aureus, and exhibits cytocompatibility with both corneal and retinal pigmented epithelial cells, along with in vivo biocompatibility after ocular injection. The DDS shows synergistic and long-term antibacterial activity [21].

Ion-responsive ocular DDSs take advantage of the presence of Na⁺, K⁺, Mg²⁺ and Ca²⁺ in tears. These systems offer controlled and sustained drug release, which is particularly beneficial given the anatomical and physiological barriers of the eye [147]. The ion-activated in situ gels can respond to the ionic composition of tear fluid, allowing the gel to form on the ocular surface and release the drug over an extended period. This approach helps in maintaining effective drug concentrations at the site of infection, reducing the frequency of administration, and improving patient compliance. For example, Kesavan et al. developed ion-responsive hydrogels using gellan gum or sodium alginate combined with a sodium methylcellulose sulfate to treat experimental bacterial keratitis. The gatifloxacin-loaded hydrogel demonstrated significantly stronger adhesion compared to commercially available gatifloxacin formulations, thereby effectively prolonging its retention time on the ocular surface. On the 4th day of treatment, there were notable differences in keratitis scores between the hydrogel group and the control group. Specifically, the hydrogel group showed significant improvements in symptoms such as chemosis, eye irritation, tear secretion, mucus removal, and eyelid swelling [149]. Díaz-Tomé et al. developed ion-activated in situ gels using gellan gum, κ-carrageenan, or hyaluronic acid for treating fungal keratitis. Their study demonstrated that, compared to a free drug solution, the residual amount of econazole in two hydrogel formulations was approximately double that of the free drug solution two hours after administration, as evidenced by PET-CT imaging without having any ocular irritation [150].

ROS sensitive ocular systems have shown significant promise in the treatment of infectious diseases, particularly due to their ability to target and release drugs in response to the oxidative stress associated with infections (Figure 3D) [64,151]. A ROS-sensitive nanoparticle-based eye drop has been developed using glycol chitosan (GC) as the nanocarrier and 4-carboxyphenylboronic acid pinacol ester (EB) as the ROS-responsive component. This innovative DDS is loaded with voriconazole (VOR). The developed GC-EB-VOR nanoparticle-based eye drop demonstrates high corneal penetration, controlled drug release under low ROS concentrations, effective ROS depletion, and promising antifungal efficacy, making it a potential therapy for fungal keratitis [50]. Another innovative approach involves using nanoparticles modified with ROS-sensitive polymers. For instance, researchers developed a system where poly (ethylene glycol)-thioketal-coated nanoparticles encapsulated with moxifloxacin and functionalized with a bacterial-targeting peptide were used to treat bacterial biofilms and endophthalmitis. The ROS-responsive degradation of the polymer coating ensures that the drug is released precisely at the site of infection, where ROS levels are high, thereby enhancing the antibacterial effects and reducing systemic toxicity [151].

The other stimuli responses, such as light, magnetic fields, and ultrasound responsive DDSs are reported for other ocular diseases such as glaucoma, age related macular degeneration, diabetic retinopathy, and uveitis, and can be extrapolated for infectious ocular diseases such as keratitis, conjunctivitis and endophthalmitis [147].

7. Safety and Biocompatibility of Ocular Drug Delivery Systems

The eye is an essential sensory organ that plays an important role in visualization of surroundings. Ocular infections can stem from bacteria, fungi, parasites, or viruses, each capable of inducing a range of diseases. At present, a variety of ophthalmic DDSs are accessible in the market, with others progressing through preclinical and clinical phases. Nonetheless, there is a scarcity of data regarding the safety, efficacy, and toxicology of these advanced ocular DDSs. Multiple observable and quantifiable parameters exist for assessing the toxicity of ocular DDSs both in vivo and in vitro (Figure 4).

7.1. In Vivo Ocular Toxicity Study

The globally accepted standard assay for evaluating acute ocular toxicity is known as the eye irritation test, or in vivo Draize eye test, developed by the FDA [54,105,126,154]. This method serves as an endorsed means to assess the safety and toxicity of materials employed in ophthalmic formulations. The DDSs are administered into the conjunctival sac, and signs of irritation such as redness, swelling, cloudiness, edema, hemorrhage, discharge, and blindness are observed in this test (Figure 4A) [7,61,105,106,118,126]. An eye irritation study can also be conducted by applying the DDSs to one eye while using the other eye as a control. Histological analysis can then be performed to assess inflammation and cell death. DDSs for ophthalmic applications are quickly advancing, aiming to enhance the safety and efficacy of targeted therapeutics while improving patient compliance (Figure 4A) [7,88]. Ocular organotypic models, or the Enucleated Eye Test (EET), use isolated rabbit eyes from animals utilized for other research or commercial purposes. DDSs can be applied at relevant concentrations, and observations are made for corneal opacity and swelling. Corneal opacity indicates protein denaturation, swelling, vacuolation, and damage to the epithelium and stroma. Bovine and porcine corneas, as well as enucleated chicken eyes, are widely accepted as reliable and accurate tissues for assessing eye irritation. The EET is considered a scientifically sound method for evaluating and identifying substances that are either non-irritants or cause irreversible eye damage [155,156]. The in vivo toxicity of DDSs can be assessed by measuring changes in intraocular pressure and evaluating retinal function using an electroretinogram after intravitreal injection. This evaluation is crucial for DDSs developed for treating posterior infectious eye diseases, such as endophthalmitis [55,155,157]. DDSs that penetrate ocular tissues or are administered intraocularly may impact retinal morphology and circulation. These effects can be experimentally assessed using optical coherence tomography (OCT) for retinal morphology, ultrasound Doppler measurements for circulation, and retinal fluorescein angiography for vascular analysis (Figure 4B) [155].

7.2. In Vitro Cytotoxicity Assays

A cytotoxicity assay for ocular applications is designed to evaluate the potential toxic effects of DDSs on ocular cells. This is crucial for ensuring the safety and efficacy of ocular DDSs (Figure 4C) [11,12]. The different cell line or cells explored for cytotoxicity study for ocular applications are majorly based on the targeted tissues in the eye. Commonly used cell lines/cells include human corneal epithelial cells (HCE), rabbit corneal epithelial cell line (SIRC), rabbit corneal keratocytes, retinal pigment epithelial cells (ARPE-19), human conjunctival epithelial cells (WKD), Muller glia (MIO-M1), Microglia (BV-2), and macrophages (RAW-267) [8,64,82,86,117]. The cytotoxicity studies were performed by MTT assay, Cell Titer-Glo Luminescent Cell Viability Assay, Cell Counting Kit-8 assay, LDH release assay, LIVE/DEAD assay, and morphological analysis after treatment with ocular DDSs [8,9,64,82,158,159]. The cells under oxidative stress after exposure can be evaluated by DCF (2′,7′-dichlorofluorescein)/DHE (Dihydroethidium) staining, lipid peroxidation, and Nytro tyrosine evaluation [52,82,158]. Maintaining barrier function by epithelial and endothelial cells in the eye is crucial. Changes in barrier integrity and function in the presence of DDSs can be evaluated using electron microscopy, transepithelial electrical resistance (TEER) measurements, and permeability studies [159].

7.3. In Vitro Hemocompatibility Study

The hemocompatibility study of DDSs for ocular applications are essential to ensure the safety and efficacy of these delivery systems [8,55]. This study evaluates how DDSs interact with blood components, which is critical for preventing adverse reactions such as blood clotting, hemolysis, and immune responses when the DDSs come into contact with the bloodstream, either directly or indirectly [8,55]. Understanding the hemocompatibility of nanoparticles helps in optimizing their design for ocular drug delivery, ensuring they are safe for use in medical treatments involving the eye. A hemolysis assay is a laboratory test used to evaluate the compatibility of DDS with red blood cells (RBCs). This assay measures the extent to which nanoparticles cause the rupture or lysis of RBCs, leading to the release of hemoglobin into the surrounding plasma [8,55,93,160]. Furthermore, the plasma recalcification assay and blood-clotting assay measures the time it takes for plasma to clot after the addition of calcium chloride, which triggers the coagulation cascade. It is essential to ensure that ocular DDSs do not induce unwanted coagulation or thrombotic events when they come into contact with blood or ocular tissues. By incorporating the plasma recalcification assay into the development process, researchers can ensure that ocular DDSs are both effective and safe, minimizing the risk of coagulation-related complications [8,55,160] (Figure 4D).

7.4. Ocular Tolerance Test (HET-CAM)

The Hen’s Egg Test on the Chorioallantoic Membrane (HET-CAM) is an in vitro assay used to assess the irritation potential of substances, including those intended for ocular applications (Figure 4E). This test serves as an alternative to the Draize rabbit eye test, reducing the need for animal testing. The HET-CAM test evaluates the irritant effects of a test substance on the highly vascularized chorioallantoic membrane of a fertilized chicken egg. This is employed to assess the predictive model for in vitro irritation by observing vascular damage, hemorrhage, hyperemia, and coagulation, and by computing an irritation score. The CAM contains arteries, veins, and capillaries, which provide reactions similar to inflammation-induced responses in the corneal tissue of rabbits during the Draize eye irritancy test [20,88,96,153].

7.5. In Vitro Genotoxicity Assay

A genotoxicity study for an ocular DDS is essential to evaluate whether the system causes genetic damage that could lead to mutations or cell death due to DNA strand break. These studies typically involve in vitro assays designed to detect various types of genetic damage, such as gene mutations, chromosomal aberrations, and DNA strand breaks (Figure 4C) [82,158]. The DNA strand break can be studied by comet assay, DNA gel electrophoresis, and DAPI staining [82,158]. Gene mutation assays detect changes in the DNA base sequence that result from DNA misrepair or replication errors. Mutations in critical genes can increase the risk of tumor formation. One of the classical methods for detecting such mutations is the bacterial mutation assay, known as the Ames test. This test helps ensure that the materials and drugs designed for ocular use do not induce genetic mutations that could lead to adverse effects such as tumor formation [96]. Assays for chromosomal damage detect alterations in chromosome structure, resulting in chromatid-type and chromosome-type breaks and rearrangements, or in chromosome number, leading to the production of aneuploid or polyploid cells. By incorporating assays that detect chromosomal damage, researchers can ensure that ocular DDSs are not only effective in treating eye conditions, but are also safe for long-term use, minimizing the risk of genetic alterations and associated health issues [161].

8. Marketed Products for Infectious Eye Diseases Based on Drug Delivery Systems

The market offers a range of innovative DDSs specifically designed to treat infectious eye diseases. These formulations enhance drug retention, bioavailability, and therapeutic efficacy, overcoming the challenges posed by ocular barriers. For instance, Azithromycin in DuraSite^® is a 1% azithromycin formulation that utilizes a low-viscosity, gel-forming delivery system. This system is soluble in tear fluid, allowing for increased drug retention time on the ocular surface, making it effective for the treatment of bacterial conjunctivitis [24]. Similarly, Moxifloxacin hydrochloride ophthalmic solution 0.5% (Vigamox^®) is an ocular adaptation of moxifloxacin, designed to enhance penetration into ocular tissues and fluids. It is commonly used for the treatment of bacterial eye infections [25]. Natamycin ophthalmic suspension (Natacyn^®) is the only commercially available antifungal agent FDA approved for the treatment of superficial fungal eye infections, including fungal keratitis, blepharitis, and conjunctivitis [162]. Zirgan^®, a 0.15% aqueous gel formulation of ganciclovir, is approved as a topical antiviral agent for the treatment of acute herpes simplex epithelial dendritic ulcerative keratitis [163]. In addition to treating infections, various formulations such as AzaSite Plus™, DexaSite^TM, Lotemax^®, and FML^® have been developed to address ocular inflammation related to eye surgery, infections, and post-infectious inflammatory conditions [164,165,166]. These products leverage various DDS technologies to enhance drug efficacy, retention, and safety in treating infectious eye diseases.

9. Conclusions and Future Directions

The landscape of DDSs for infectious ocular diseases has significantly advanced, offering new avenues for effective treatment of conditions such as keratitis, conjunctivitis, and endophthalmitis. Enhanced drug delivery methods, including targeted approaches and stimuli-responsive systems, have demonstrated the potential to improve therapeutic outcomes by increasing drug bioavailability and precision targeting within ocular tissues. Safety and biocompatibility remain critical considerations in the development of these advanced systems. In vivo and in vitro studies, including cytotoxicity assays, hemocompatibility tests, and genotoxicity evaluations are essential to ensure that new drug delivery technologies are safe for clinical applications. The HET-CAM ocular tolerance test in particular provides valuable insights into the irritation potential of these systems.

While significant progress has been made in developing innovative DDSs for ocular infections, several major challenges persist, including antimicrobial resistance, systemic side effects, ocular toxicity, chronic and recurrent infections, and the need for invasive procedures. In addition, issues such as drug stability, delivery efficiency, and patient compliance require ongoing attention. Overcoming these obstacles will necessitate the development of personalized therapies, rapid diagnostic tools, and a deeper understanding of antimicrobial peptides and host defense mechanisms, alongside advancements in drug stability and delivery technologies.