Novel Ocular Drug Delivery Systems: An Update on Microemulsions

Abstract

Sufficient ophthalmic drug delivery is still challenging for pharmaceutical technologists, despite various scientific efforts. Several ocular drug carriers have been designed to enhance bioavailability by prolonging the drug retention time. One of the current encouraging approaches is the utilization of colloidal carriers with the characteristic submicron-nanometer size. Microemulsions (MEs) are such colloid systems that present sizes between 5 and 200 nm with significant thermodynamic stability and low surface tension. In addition, MEs as topical ocular carriers can lead to great ocular drug adsorption due to their enhanced retention time. Furthermore, considering that MEs are stable for long time and various temperatures, their ocular application is of great interest. The aim of this study is to cover basic physicochemical principals of ocular MEs such as their possible size, stability, and therapeutic efficacy against various eye disorders. Thus, a comprehensive review for ocular drug delivery systems in the form of MEs that show promising characteristics as their stability and therapeutic efficiency is performed.

Introduction

Ocular drug delivery belongs to the most exciting and difficult activities that formulators are facing.¹ Thus, the effective ophthalmic drug delivery has stayed an unmet challenge till date.² The inimitable physiology, anatomy, and biochemistry of eye induce it as resistant to some active molecules; hence, efficient eye drug delivery systems should be designed by technologists aiming to overcome the ocular barriers with negligible tissue damage.³ The main physiological barriers such as nasolacrimal drainage, tear dilution, and tear turnover of the eye are responsible for poor ocular bioavailability of conventional eye formulations.⁴

More specifically, the human eye is such a complex organ due to its anatomy, physiology, and biochemistry that it is almost resistant to external molecules, including drugs.⁵ Eye unique anatomy and physiology contribute in its great protection, which restricts drug entry at the target site of action.⁶ In detail, human eyeball has spherical shape with almost 1 in of diameter. It is covered by various layers and internal complexes, each of which carries out different functions and all together facilitate the sight.⁷ Three main layers of the eye can be noted: the outer region comprised sclera and the cornea⁸; the middle layer responsible for nourishment, known as vascular tunic, which consists of the iris, the choroid, and the ciliary body; and the inner layer of photoreceptors and neurons called the nervous tunic, which consists of the retina. The conjunctiva, which comprised the outer epithelium and its underlying stroma, provides the composition of the tear film by discharging electrolytes, fluid, and mucins.⁷

Cornea is the principal path for the intraocular absorption¹ and consists of 5 layers; the epithelium, Bowman's membrane, the lamellar stroma, Descemet's membrane, and the endothelium.⁸ The retina, the tissue that lines the inner eye surface, surrounding the vitreous cavity, is protected and held in the appropriate position by the surrounding sclera and cornea.⁸ Moreover, the aqueous humor is a jelly-like substance located in the outer chamber of eye, which fills the “anterior chamber of the eye” that is placed behind the cornea and in front of the lens.⁹ The iris is visible through the cornea and is responsible for the “eye color.” All irises present dark pigmented posterior layer, whereas the amount of pigment in the anterior or stromal layer is accountable of several colors. The main duty of the iris is to adjust the size of the pupil.¹⁰

The gel-like complex, which fills the posterior portion of the globe, is known as vitreous body. Vitreous humor is the clear gel that is composed of collagen fibrils in hyaluronic acid network.⁸

Ophthalmic delivery offers several potential routes of administration and the selection of the promising route is mainly dependent on the target tissue.^10,11 Topical, local ocular (ie, subconjunctival, intravitreal, retrobulbar, and intracameral), and systemic delivery are the most frequently used approaches of ocular drug delivery (Fig. 1).¹² Topical instillation of drug to the eye is the most desirable method of administration, considering that it is easy handled and cost-effective. Topical drug instillation is handful when it comes in the management of disorders affecting the anterior segment of the eye.^13,14

FIG. 1.

The schematic eye structure and the possible ocular administration routes.

Compared to the other routes, intravitreal injection leads to higher drug concentrations in vitreous and retina, where the drug elimination is based on the molecular weight of the active ingredient.¹⁵ When hydrophilic and larger molecules should be delivered to the eye, noncorneal or conjunctival/scleral route is preferred since these molecules are not able to easily diffuse through the corneal epithelium.¹⁶ Finally, the periocular route, which involves peribulbar, posterior juxta scleral, retrobulbar, subtenon, and subconjunctival routes, has been considered the most desirable and efficient route for drug administration to posterior eye segment.¹²

It has been already reported that eye unique anatomy and physiology restrict drug entry at the target site of action.^6,17,18 In addition, due to the rapid precorneal secretion, most of the drugs present a half-life of about 1–3 min. Thus, only 1%–3% of the applied total dose can penetrate through the cornea and reach the intraocular tissues.^18,19 These factors along with the ocular barriers, lead to low ocular absorption and poor bioavailability of the conventional ophthalmic formulations such as solutions and suspensions. Thus, ocular drug delivery systems sufficient to support the maximum precorneal residence time, overcome ocular barriers, maximize ocular bioavailability, and sustain drug delivery following topical administration are of high importance.²⁰

The existing eye drops that present low bioavailability and pulsed drug release are still the most frequently used dosage forms.²¹ During the recent times, various novel nanosized carrier systems²² as ocular drug delivery systems, such as liposomes,²³ microemulsions and nanoemulsions,^24,25 and nanoparticles,^17,26 in situ gels¹⁸ have emerged as novel strategies. Microemulsions (MEs) as ocular applications, offer ease application compared to eye solutions, with additional benefits such as the friendly procedure to the patient, as a result of less frequent instillation, better retaining, and extended drug action. Numerous drugs, for example, antibiotics,²⁵ antifungals,²⁷ anti-inflammatory,²⁸ and immunosuppressive²⁹ molecules, have been impregnated into MEs for the management of ophthalmic disorders because of their structure. In this review, it is summarized the utilization of MEs as ocular drug delivery systems against various diseases. In every application, except the used components and excipients, their size and stability duration are reported.

Physicochemical Principles of MEs

MEs belong to the most promising submicron carriers of drug delivery, especially for poorly water-soluble drugs.^25,30–35 MEs are composed of basically 4 different phases, which are oil phase, aqueous phase, surfactants, and co-surfactants.^25,30,31 MEs are thermodynamically stable, inexpensive, and relatively easy to produce.²⁵ Isopropyl myristate^36–38 and nature oils³⁹ such as olive oil,⁴⁰ castor oil,^39,41 and coconut oil,³⁹oleic acid, and triacetin have been widely applied for the development of ocular MEs. Nonionic surfactants such as sugar ester surfactants and polysorbates such as Tween 60 and Tween 80^37,38,40,42 are widely used in ocular delivery since they lack toxicity and irritation. Zwitterionic surfactants such as phospholipids are also applied in MEs.^43,44 The most common used co-surfactants in ocular MEs are ethanol and glycerol.^21,37 Pentanol and hexanol are not frequently used due to their irritation.⁴³

According to the type and the amount of the surfactants in formulations, ME can be water-in-oil (W/O) or oil-in-water (O/W), or bi-continuous or liquid crystalline. Furthermore, the selection of water and oil phase as well as surfactant/co-surfactant systems should be done carefully, since these components could affect stability and the toxicity of the system.⁴⁵ They present droplet size ranges between 10 and 100 nm and do not have tendency to coalescence.²⁵ While preparing the MEs, the usage of high concentration and few physiologically optimal surfactants and co-surfactants generates the main problem in terms of application of these drug delivery systems.^37,46

Phase diagrams are created to acquire the optimal ingredients and their concentration, which can lead to greater existence area of ME. A large sum (Fig. 2) of oil, water, and co-surfactant/surfactants blends are used, to produce the pseudoternary phase diagrams. Afterward, visual inspection is applied to determine the creation of monophasic or biphasic system. To conclude when the formulations are biphasic, turbidity appears followed by phase separation.

FIG. 2.

A general schematic illustration of phase diagram construction.

Stability—stabilization of MEs

It has been reported that the stability of MEs is one of the most important characteristics that should be evaluated since drug nature could affect this property. Normally, MEs are thermodynamically stable; however, their microstructure is continuously changing in the bicontinuous region.⁴⁷ Stability is tested according to International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use (ICH) guidelines,⁴⁸ which propose to store MEs in different temperature conditions (4°C, 25°C, and 37°C and 75% ± 5% Relative humidity [RH]) and time frame. As follows, MEs are inspected for any physicochemical differentiation, including phase separation, drug entrapment, precipitation, and particle size changes.^14,31,47,49 Figure 3 shows representative illustration for possible instabilities in ocular MEs.

FIG. 3.

An illustration of the instability phases of ocular MEs, after their long-term storage. MEs, microemulsions.

Characterization of MEs

MEs can be characterized by physical, electrochemical evaluation, and microscopic evaluation. Osmolarity is significant for the formulation acceptance from the ocular tissues. Microscopic methodologies such as electron microscopy in cryogenic mode (SEM)⁵⁰ and transmission electron microscope (TEM) are applied for the evaluation of clarity, appearance, and size of MEs.¹⁴ Moreover, light scattering techniques can also be used for the determination of particle size and zeta potential.^43,51,52 Differential scanning calorimetry (DSC) is normally utilized to determine the structure and type of ME.^47,50,53

Applications of ocular MEs

Ocular MEs are interesting formulations due to their various features such as stability, ocular bioavailability, and prolonged drug effect in eye. The topical delivery of various drugs into the eye can be achieved by using ocular MEs. Considerably, Table 1 summarizes various applications of MEs in ocular delivery. It should be said that among the most important parameters when ocular MEs are studied, are droplet size and their possible stability. The droplet size demonstrates a direct relationship with the permeation process, whereas stability of the product should be prolonged. Hence, MEs can easily retain within the cul-de-sac of the eye and can exhibit its therapeutic effects in better manner. Hence, in this review, in every application, the droplet size and stability of the MEs are reported.

Table 1.

Various Examples of Ocular Microemulsions, the Used Oil Phase, Surfactant, and Co-Surfactant and Their Therapeutic Target

Microemulsion	Drug	Oil	Surfactants	Co-surfactants	Treatment	References
W/O	Timolol	Ethyl oleate	Tween 80 and Span 20	—	Glaucoma	⁵¹
O/W	Timolol	IPM-Transcutol P	Tween 80 and Labrasol	Propylene glycol	Glaucoma	⁵⁴
O/W	Pilocarpine HCl	Ethyl Oleate	Sorbitan mono laurate, polyoxyethylene sorbitan mono-oleate	—	Model treatment	⁵⁵
W/O	Pilocarpine	Soybean oil	Brij 35P and Span 80	l-Butanol	Glaucoma	⁵⁶
O/W	Cyclosporine A	Castor oil	Solutol HS 15	Glycerol	Immunosuppressant	⁶²
O/W	Cyclosporine A	Ethyl butyrate	Brij 97	—	Immunosuppressant	⁶³
O/W, W/O	Diclofenac sodium	IPM	Tween 80	Glycerin	Anti-inflammatory	^21,37
O/W	Diclofenac sodium	Ethyl butyrate	Brij 97 or Tween 80 cetalkonium chloride	—	Anti-inflammatory	⁴²
O/W	Docosahexaenoic acid	TG-DHA	Tween 80 and Cremophor EL	Glycerol	Dry eye syndrome	⁶⁷
O/W	Dexamethasone	IPM	Cremophor EL	Propylene glycol	Uveitis	²⁸
O/W	Dexamethasone	IPM	Tween 80	Propylene glycol	Uveitis	^64,65
O/W	Riboflavin phosphate	Soybean lecithin	Sucrose ester monolaurate, glycolipids	Glycerol, decaglycerol monolaurate	Keratoconus	^67,69,70
O/W	Fluocinolone acetonide	Lauroglycol	Labrasol	Transcutol	Anti-inflammatory	⁷²
O/W	Ofloxacin	Oleic acid	Tween 80	Ethanol	Keratitis	²⁵
O/W	Chloramphenicol	IPM	Span20, Span80 Tween20, Tween 80	n-Butanol	Trachoma and keratitis	⁷³
O/W	Gatifloxacin	IPM	Tween 80, Transcutol P		Bacterial ocular infections	³⁸
O/W	Lauric, tridecanoic, myristoleic, palmitoleic and α-linolenic acid	α-Linolenic acid	Tween 80	Polyethylene glycol 400 (PEG 400)	Bacterial ocular infections	^74,75
O/W	Azithromycin	Transcutol P, oleic Acid	Tween 80, Span20	Propylene glycol	Bacterial ocular infections	⁷⁶
W/O	Moxifloxacin	IPM	Tween 80, Span20	—	Bacterial keratitis	⁷⁷
W/O	Moxifloxacin	Ethyl oleate	Tween 80, Soluphor P	—	Bacterial keratitis	⁷⁸
O/W, W/O	Vancomycin	Oleic acid	Tween 80	Propylene glycol	Bacterial ocular infections	⁷⁹
W/O	Dexamethasone-Tobramycin	Ricinoleic acid	Cremophor EL	1-Butanol	Bacterial ocular infections	⁸⁰
O/W	Manuka Honey	Caprylic/capric triglyceride MI12	Polysorbate 80	Glycerol	Bacterial ocular infections	⁸¹
O/W	Voriconazole	Oleic acid, isopropyl myristate	Tween 80	Propylene glycol	Fungal ocular infections	³⁵
O/W	Voriconazole	Oleic acid, isopropyl myristate and isopropyl palmitate	Tween 80	Propylene glycol	Fungal ocular infections	²⁷
O/W, W/O	Retinol and its esters	IPM, Miglyol 812	Tween 60 and Tween 80, Epicurone 135	n-Butanol, triacetin, propylene glycol	Antioxidant	⁸⁸
O/W	Lutein	Coconut oil, caprylic/capric triglyceride	Tween 80	—	Antioxidant	⁸⁹

DHA, docosahexaenoic acid; IPM, isopropyl palmitate; O/W, oil-in-water; W/O, water-in-oil.

Ocular MEs for glaucoma treatment

Glaucoma is the second notable cause of optic nerve eventual damage and vision loss worldwide. Various drugs are applied for glaucoma management, but topical drops of β-adrenergic blockers, such as timolol maleate,¹⁷ or miotics, such as pilocarpine, are in extensive use.

In a promising study, authors prepare W/O ocular ME involving timolol maleate to prolong time of reduced intraocular pressure. They applied the MEs onto glaucomatous rabbit's eye and measured the pressure by using a Schiotz tonometer. To prepare ME, ethyl oleate, Tween 80, Span 20, and water were used. According to the results, monodisperse distribution behavior and a uniformed size distribution of 123.6 nm were observed. MEs of timolol were screened for their stability under freeze-thawing at temperature from 0°C to 40°C and ultracentrifugation at 3,000 rpm. Instability signs did not depict. In vitro drug release studies exhibited Higuchi's pattern and zero-order drug release, while ex vivo studies showed delayed release of drug. A higher drug release was seen by increasing water content, whereas the highest permeation was revealed when the O/W emulsion consisted of droplet size of higher order. Intraocular pressure of ME lasted for 12 h when eye drops lasted for 5 h. It can be concluded that due to the oily nature of ME, the formulation adhered onto the lipophilic corneal epithelium, and thus, permeation of drug was processed by the lipophilic association through the corneal membrane and thus an enhanced retention time is depicted. Because of the proper outcomes of the study, authors suggested that this formulation is promising to reduce systemic side effects.⁵¹

Moghimipour et al. developed timolol-loaded ME of ocular delivery to treat glaucoma. In this regard, they used isopropyl myristate-transcutol P (10:1) mixture as oil phase, Tween 80 and Labrasol as surfactants, and propylene glycol as co-surfactant. It was demonstrated that the mean droplet size of the MEs was between 2.48 and 46.2 nm. In first 8 h, only 13.5% of drug was released from the formulation with particle size of 9.86 nm and high water content. The highest permeation was also revealed for the specific ME. It can be concluded that as the particle size was increased, the release was decreased. In addition, micelle structure was seen in the SEM photographs. Stability screening test was confirmed by visually observing MEs for 3 months. The visual inspection did not show phase separation, flocculation, or precipitation signs. Similarly, no evidence of phase separation was depict under stress due to centrifugation at 10,000 rpm for 30 min. Authors concluded that timolol MEs compared to the tested timolol eye drops.⁵⁴

In another study, ME-based phase transition systems were investigated for ocular delivery of pilocarpine hydrochloride by Chan et al. The preparation of the MEs was based on 2 nonionic surfactants: sorbitan mono laurate and polyoxyethylene sorbitan mono-oleate. Ethyl oleate was used as the oil component and water as the aqueous phase. When the water content and the viscosity change, ME phase was reversed from ME to liquid crystalline. The physical appearance of the investigated systems depended on the water content. Thus, the ME 5% and ME 10% aqueous components were characterized microscopically as MEs. In vitro release profile showed that the ME with 5% reveals better release characteristics. Moreover, the MEs showed the greatest miotic response and duration of action compared to the solution, indicating high ocular bioavailability.⁴⁶

One year later, same research group studied the prepared systems in New Zealand rabbits so as to examine their effects on the outer lipid layer of precorneal tear film. It was revealed that the MEs present longer recovery time compared to solution and improved tear evaporation.⁵⁵

Similarly, MEs of pilocarpine were developed and studied as ocular antiglaucoma agents. In this regard, physicochemical properties as well as intraocular pressure and ocular tolerance were investigated. Soybean oil was used as the oil phase, Brij 35P and Span 80 were the surfactants, and 1-butanol was used as the co-surfactant. New Zealand White rabbits were used to perform ocular irritation test and intraocular lowering activity of MEs. The average particle size of pure ME was 0.709 nm with polydispersity index (PDI) of 0.219. However, the addition of pilocarpine led to an average droplet size of 0.695 nm with PDI of 0.432. It was demonstrated that the formulations showed efficient stability for 6 months after storage at 4°C ± 1°C, 25°C ± 2°C, and 40°C ± 2°C in a dark setting. A significant decrease of intraocular pressure was depicted after ME instillation to the rabbit eye. Also, the irritation test showed that formulation caused no significant allergies to the eye.⁵⁶

Brinzolamide is an intraocular pressure reducing agent with low bioavailability. O/W nanoemulsions based on Triacetin or Capryol 90 as oil phase, Brij 35, Cremophor RH40, Labrasol, or Tyloxapol as surfactants, and Transcutol as co-surfactant were prepared and evaluated for their efficacy to deliver brinzolamide. Their droplet size ranged between 7.53 and 42.38 nm. In this work, in vitro release behavior was associated with the lower thermodynamic activity of drug and not the droplet size. Furthermore, the MEs exhibited high formulation stability under different conditions such as Heating-cooling (4°C and 40°C) and Freeze-thaw (−21°C and +25°C) cycles, as well as centrifugation (13,000 rpm, 30 min). Finally, it was confirmed that brinzolamide (0.4%) penetration into the corneal tissue was achieved and resulted in higher therapeutic efficacy in vivo than the commercial product (1% brinzolamide).⁵⁷ Similarly, other brinzolamide MEs consisted of isopropyl myristate, and Tween-80 and Transcutol-P as surfactant and co-surfactant, respectively, and water showed a droplet size of 41.69 nm, while a prolonged release pattern was achieved. Finally, the MEs were nonirritant with great safety.⁵⁸ Nonetheless, in vivo therapeutic efficacy studies were not performed.

Ocular MEs for anti-inflammatory ocular conditions

Inflammatory ocular diseases compose a wide spectrum of disorders and are a growing public health matter. Dry eye disorder and allergic conjunctivitis are such disorders, which are in high prevalence. Uveitis is the inflammation of the uvea and can be caused from eye injuries (eg, surgery) or inflammatory diseases. Various active ingredient-loaded eye drops have been used as anti-inflammatory agents, which lack of bioavailability.⁵⁹

Cyclosporine A (CysA) is an immunosuppressant drug used for the alleviation of dry eye syndrome. In vivo studies on human individuals have proved that cyclosporine MEs are superior than the conventional formulations in terms of efficacy and safety.^60,61 Gan et al. aimed to design a new type of ME based on in situ electrolyte-responsive gel system for ocular administration of CysA. To prepare the ME, castor oil as oil, Solutol HS 15 as surfactant and glycerol as co-surfactant, and water were used. The ME-based in situ electrolyte-triggered gelling system was prepared by dispersing the ME in a Kelcogel solution. MEs present an average particle size at 54.5 ± 31.6 nm according to DLS and TEM studies. From in vitro studies, the viscosity of the system was increased when the ME system contacted with artificial tear fluid and exhibited pseudo-plastic rheology. Furthermore, in vivo results showed that the area under the curve of corneal cyclosporine-loaded ME Kelcogel system was ∼3-fold greater than the cyclosporine emulsion. Moreover, it was also seen that cyclosporine concentration at 32 h after administration remained at therapeutic level at cornea. According to the obtained results, scientists suggest that this system might be a promising alternative approach to provide prolonged precorneal residence time of CsA.⁶²

Similarly, other researchers studied the release of CsA from ME-based hydrogels and micelles. Kapoor and Chauhan focused on developing poly (2-hydroxyethyl methacrylate) nanohydrogels containing MEs or micelles of Brij 97 [C₁₈H₃₅(OCH₂CH₂)₁₀] as sustained-release carriers. It was demonstrated that the developed formulations were able to extend the release period of the loaded drug. In addition, increased concentrations of surfactant ease the diffusion of the drug. The mean particle size of MEs was the typical, ranging from 10 to 13 nm and increased with a reduction in surfactant loading.⁶³

Habib et al., developed diclofenac-entrapped ME for topical management of various inflammatory ocular conditions. Diclofenac MEs were carried out using isopropyl myristate as oil phase, Tween 80 as surfactant, glycerin as co-surfactant, and isotonic phosphate buffer pH 7.4 as aqueous phase. The characterization properties of MEs were suitable for ocular applications, since small particle size (220–480 nm) and optimal pH values (6.8–7.4) were obtained. ME regions were changed slightly in size with the increasing ratio of Tween 80: glycerin. The ME region increased in size due to the increase of the surfactant concentration, which in turn increased the spontaneity and the area of the self-emulsification region. The ME depicted a sustained release up to 24 h.³⁷ The following year, Habib et al., developed similar ocular MEs loaded with diclofenac and they checked their irritation potential by using Draize tests. According to their study, the developed formulations showed no irritation and shorter recovery time compared to the marketed product.²¹ An interesting ME containing diclofenac was developed by Torres-Luna et al. In fact, diclofenac was embedded in oil (ethyl butyrate)-in-water ME systems, which are prepared with 2 nonionic surfactants, Brij 97 or Tween 80, together with a long alkyl chain cationic surfactant, cetalkonium chloride (CKC). The ME was further incorporated into contact lenses based on poly-2-hydroxyethyl methacrylate hydrogels. It was revealed that the average particle size ranged from 2.4 to 18.1 nm. The obtained reduction in droplet size was associated with the decrease in the surface tension of the surfactant solution due to the addition of CKC, which improves the dispersion of the oil phase in the surfactant solution. Furthermore, the uniform drug loading was achieved due to the improved enhanced dispersion of oil globules with smaller size in the ME-laden contact lenses. The CKC-ME contact lenses extended diclofenac release for a long time. The optimal formulation consists of 1.8% CKC and B97. However, by increasing the CKC concentration, ME exhibited lower drug release. Finally, the prepared ME contact lenses are found stable within the time frame in which the release studies were performed due to the utilization of surfactants with long carbon chain lengths, as the CKC (Fig. 4).⁴²

FIG. 4.

In vitro release studies of diclofenac from MEs based on CKC and Brij 97 (B97) PHEMA hydrogel contact lenses. Control represents the PHEMA contact lenses loaded with diclofenac, whereas B97-CKC-1.8% represents PHEMA contact lenses loaded with diclofenac MEs. Reprinted from Torres-Luna et al.⁴² CKC, cetalkonium chloride.

Dexamethasone is widely used for the reduction of inflammation and the management of acute and chronic eye disorders such as uveitis. Hence, dexamethasone-loaded O/W MEs based on oily phase (isopropyl myristate), the surfactant (Cremophor EL), and the water phase were developed and characterized. The obtained formulation had desirable physicochemical properties such as desirable droplet size (50.85 ± 1.24 nm) and convenient stability for 3 months at 4°C, 25°C, and 37°C. Furthermore, there was no significant histological differentiation to the eyelids, conjunctiva, cornea, and iris. In addition, it was seen that penetration and release features were better compared to the conventional formulations. The specific MEs were criticized as advantageous due to their high bioavailability and well toleration in the eye.²⁸ Kesavan et al. investigate the potentiality of dexamethasone-entrapped mucoadhesive chitosan-coated positively charged MEs (CH-MEs) to overcome the frequent instillation of eye drops and prolong the therapeutic activity. The isopropyl myristate as oil, Tween 80 as a surfactant, propylene glycol as a co-surfactant, and distilled water as aqueous phase were used for the development of MEs. Afterward, the MEs were coated with chitosan solution. For CH-ME preparation, water titration method was selected. MEs were evaluated for their in vivo efficacy by topically applying them to endotoxin-induced uveitis rabbit model. It was shown that the prepared MEs indicated desirable physicochemical properties, good mucoadhesion, and optimal stability for 3 months, as well as sustained release. It was seen that the globule size of all formulations was between 85 and 187 nm with positive surface charge. The ideal chitosan-coated MEs depicted higher penetration of drug molecule in the anterior eye segment compared to the uncoated ME formulation and suspension, respectively. Furthermore, in vivo studies exhibited significant increment on the anti-inflammatory activity of chitosan-coated MEs compared to the marketed dexamethasone solution.^64,65

Moreover, Baspinar et al. investigated the topical instillation of everolimus, which is an immunosuppresive drug, and its potential to prevent corneal-graft rejections. For this reason, researchers developed MEs consisting of triacetin, poloxamer, propylene glycol, and water under continuous stirring. Ex vivo permeation rate of the drug was evaluated through the fresh pig cornea. It was revealed that everolimus was successfully loaded in ME, which was relatively stable at 25°C for over a 12-month period. Furthermore, it was seen that, in half an hour, drug concentration in the cornea was measured at 8.64 ng/mL.⁶⁶

Lidich et al. proposed an interesting W/O (Tween 80/Cremophor EL/glycerol) ME of docosahexaenoic acid (DHA), which supports the synthesis of anti-inflammatory prostaglandins to the eye to relieve dry eye symptoms. The W/O ME when was diluted with water, demonstrated a more structured bicontinuous phase with high viscosity. By further dilution, mesophases were inverted to oil-in-water droplets with a size of 8 nm. In addition, the poly(propyleneimine) dendrimers were also added in the MEs as solubilization agent. These dendritic molecules, when they were located near surfactant headgroups, increased the size of nanodroplets and decreased the diffusivity of surfactants. Finally, it was concluded that by solubilizing dendrimers along with TG-DHA into water-dilutable MEs, the stability and dilutability of the formulation can be retained.⁶⁷ Same research group developed and studied Riboflavin phosphate-loaded MEs to cross the eye epithelium. Riboflavin phosphate is an essential molecule used in the management of the degenerative disease Keratoconus. Although some studies demonstrate that Keratoconus is a noninflammatory disease, others support that it could be, at least in part, an inflammatory condition.⁶⁸ At the present time, the only way an efficient amount of riboflavin delivery to the cornea can be achieved is by the mechanical removal of epithelium. Hence, researchers developed water-dilutable MEs based on blends of glycerol/decaglycerol monolaurate with mixtures of sucrose ester monolaurate, soybean lecithin, and medium chain triglycerides. According to the DSC and SD-NMR results, the MEs with up to 40 wt% water exhibited hydrophilic surfactant headgroups, whereas glycerol strongly binds with water molecules. Furthermore, MEs with above 60 wt% water form globular O/W nanodroplets with a diameter of 14 nm. Moreover, MEs with 10–80 wt% water hindered the mobility of riboflavin in the ME due to strong interactions between surfactants and co-surfactant. Thus, riboflavin was able to be delivered using MEs with higher (>80 wt%) water dilutions.⁶⁹ A more recent study of Lidich et al., showed that the already developed riboflavin-loaded MEs can actually deliver the drug through the intact epithelium, according to biomechanical properties of the cornea and fluorescent microscopy.⁷⁰

Ma et al. prepared ocular vitamin A palmitate (VAP)-loaded cationic ME-in situ gel aiming to study its corneal retention behavior and corneal irritation. Researchers prepared VAP-loaded MEs by using supplying energy process. VAP-cationic ME was then incorporated into in situ gel, which was prepared by using Poloxamer 407. Before-and-after gelation rheology, in vitro VAP release, gel dissolution was evaluated. According to TEM images, it was seen that VAP-loaded MEs were homogenous with no significant differences between droplet sizes in ME formulation itself and in situ gel. In addition, rheological studies showed that VAP-loaded MEs reduced the transition temperature of Poloxamer 407 gel. Also, the prepared formulation showed longer retention time and smaller contact angle compared to Oculotect Gel. Irritation test exhibited desirable ocular compatibility. As conclusion, the present VAP-loaded ME in situ gel is a promising way of treat dry eye ocular disease.⁷¹

Finally, in a recent study Gupta et al., designed and developed an ME system comprising Lauroglycol as oil, Labrasol as surfactant, and Transcutol as co-surfactant for Fluocinolone acetonide administration in the posterior eye. Furthermore, a permeation enhancer known as cow ghee was also added. The droplet size ranged between 63.92 and 83.56 nm and probably is associated with the use of co-surfactants that decrease the interfacial tension. Moreover, the MEs displayed a 3-month stability at 3 different temperatures (4°C, 25°C, and 40°C). Ex vivo permeation studies demonstrated great permeation and nonirritancy. Finally, in vivo pharmacokinetic study on Sprague Dawley rats exhibited retention of the ME to the posterior eye.⁷²

Ocular MEs against bacterial keratitis and infections

The effective management of ocular infections is of high clinical importance, given that ocular infections, if left untreated, can lead to the damage of the eye structure with possible blindness and visual impairments. Gram-positive bacteria are the most frequent species that contribute to ocular infections worldwide. Various antimicrobial drugs have been approved as eye drops for the management of such bacterial infections, which present a major disadvantage, the fast and high precorneal loss due to drainage and tear fluid turnover. In general, up to 5% of drug instilled to the eye penetrates the cornea/sclera and goes through the intraocular tissue.

Chloramphenicol is applied for trachoma and keratitis management. However, this drug is being hydrolyzed easily when it is in the eye drops. In a significant study, chloramphenicol was loaded in O/W ocular ME containing Span 20, Span 80, Tween 20, Tween 80, n-butanol, water, and isopropyl palmitate, as well as isopropyl myristate. The globule size ranged from 38.5 to 59.5 nm due to the addition of the drug, which increased the droplet size. The encapsulation of the drug in the MEs, as well as the addition of normal saline and sodium hyaluronate, affected the electrical conductivity of the formulation. The stability data obtained by HPLC revealed that the drug is stable for 3 months, since glycol concentration (confirms the hydrolysis) in the ME was much lower than the commercial formulations.⁷³ In another recent study, authors prepared novel ofloxacin (OFX)-loaded O/W ME for topical ophthalmic application. To prepare microemulsions, various combinations of oleic acid (oil phase), Tween 80 as surfactant, ethanol as co-surfactant, and NaOH water solution as aqueous phase were used. The ideal formulation was modified with chitosan oligosaccharide lactate (COL). It was revealed that the ME showed increased preocular residence time in comparison with the commercial solution, whereas the COL modification did not cause any significant difference. The droplet size of the produced MEs was around 4.5 and 150 nm, while stability studies indicate good stability for 12 months. Given the above, OFX-loaded ME could be an alternative strategy for ocular drug delivery.²⁵

Kalam et al., aimed to prepare gatifloxacin-loaded MEs and evaluate them as ophthalmic carriers by comparing them to the conventional eye drops. In this regard, oil-in-water type MEs were developed by using different concentrations of oil [isopropyl palmitate (IPM)], surfactant (Tween 80), co-surfactant, and water. Titration method was used to prepare formulations. Results showed that formulations had circular shaped droplets with a size between 51 and 74 nm, and zeta potential ranged from 15 to 24 mV. The ideal formulation showed effective stability and improved adherence to the corneal surface, as well as good permeation of the drug in the anterior chamber of the eye. It was seen that gatifloxacin concentration was 2-fold higher in the anterior chamber compared to conventional formulations.³⁸

In another study, researchers studied a quantification method of 5 fatty acids (lauric, tridecanoic, myristoleic, palmitoleic, and α-linolenic acid) as alternatives to antibiotics for the neonatal eye infections. Fatty acids present great safety profile and broad-spectrum activity against virulent bacteria. For the ME preparation, α-linolenic acid, Tween 80 as surfactant, polyethylene glycol 400 (PEG 400) as co-surfactant, and water as aqueous phase were used. A newly developed gas chromatography method was applied to determine the quantification of drug content in ME and was criticized as efficient. The antimicrobial efficacy of fatty acid-based MEs was examined against Staphylococcus aureus and exhibited strong antimicrobial effect. Their droplet size ranged between 281.90 and 350.50 nm, which is slightly higher than the usual ME droplet size due to the particle aggregation.⁷⁴ More recently, the same research group studied similar α-linolenic acid-based MEs to manage ophthalmia neonatorum infections from Neisseria gonorrhoeae and S. aureus. Tween 80 and Cremophor EL were chosen as surfactants and Transcutol P and PEG 400 were selected as co-surfactants. The mean droplet size of the MEs was in the range of 190.4 to 350.5 nm and stability upon storage was for at least 8 weeks. The higher than normal droplet size could be due to the addition of Tween 80 and Cremophor EL.⁷⁵

Salimi et al. aimed to design and characterized a novel azithromycin ME for ocular delivery. From the physicochemical characterization as well as corneal rabbit permeability, it was indicated that average droplet size of ME ranged from 6.78 to 26.65 nm. Viscosity was between 115 and 361 cps, whereas pH ranged from 5.1 to 5.7. It was further depicted that almost 79% of azithromycin released in 24-h drug permeation through rabbit cornea was 12.87% and 0.909%. The stability of the MEs were tested in various temperature conditions (4°C, 25°C, 37°C, and 75% ± 5% RH) for 6 months, indicating that the MEs were stable.⁷⁶

A W/O ME comprised Tween 80 and Span 20, isopropyl myristate, and acetate buffer loaded with Moxifloxacin was developed for bacterial keratitis treatment. In vivo antibacterial efficacy test of ideal formulation (average globule size of <40 nm) was performed on infected rabbit eye and compared to commercial eye drops. Results showed that MEs exhibit proper physicochemical parameters, good stability for 3 months, and sustained release rate. In vivo studies exhibited greater efficacy of developed formulation compared to the marketed eye drops.⁷⁷ Similarly, nanoemulsions consisting of Moxifloxacin, deionized water, ethyl oleate, Tween 80, and Soluphor P were developed and evaluated by Shah et al. (Fig. 5). The particle size of the MEs ranged from 28.78 to 81.04, whereas stability assessment for 3 months did not exhibit any signs of instability. The optimal formulation (average size = 28.78 ± 10.34) was further evaluated for ex vivo permeation, antimicrobial activity, ocular irritation, and stability, indicating its safety, great aqueous humor concentration, and antimicrobial properties.⁷⁸

FIG. 5.

(a) TEM image of the optimized moxifloxacin nanoemulsion (MM3) and (b) ex vivo permeation studies on the isolated rabbit cornea membrane from the optimized nanoemulsion (MM3) and control (commercial eye drops). MM3 comprised the optimized formulation of the nanoemulsion that consists of ethyl oleate (4%), Tween 80 (12%), Soluphor P (24%), and water (60%). Moxifloxacin was included at a concentration of 0.5% w/w. Reproduced by Shah et al.⁷⁸ TEM, transmission electron microscope.

Vancomycin (VM)-loaded MEs for ocular applications were studied by Nair et al. For their preparation, oleic acid, ethyl oleate, linseed oil, olive oil, castor oil, and soybean oil were chosen as oil phases and span and Tween 80 as surfactants. It was found out that that MEs showed acceptable physicochemical behavior mostly for pH value, refractive index, and viscosity. The optimal MEs were kept at 3 different temperatures 40°C, 25°C, and 45°C and observed for phase separation and particle size for 45 days, indicating good stability. The droplet size was measured at 36.41 ± 0.2, 44.15 ± 0.1, and 41.35 ± 0.3 nm, which are normal values. Furthermore, in vitro release studies showed prolonged VM release. Given that sol-to-gel transition improves the viscosity and therefore ocular retention of formulation enhances, VM-ME seems a promising way of treating ocular diseases.⁷⁹

A novel dexamethasone- and tobramycin-loaded ME for anterior segment eye infections was produced by Bachu et al. MEs comprised ricinoleic acid, Cremophor EL, 1-butanol, and deionized water. EDTA disodium dehydrate was added to the formulation as an antioxidant, and BAK was added for its preservative property. A droplet size of <20 nm was displayed, whereas toxicity was not detected. The anti-inflammatory activity as well as antimicrobial property were significantly higher than the marketed product. Given the above, the prepared ME could be a suitable strategy instead of tobramycin marketed product.⁸⁰ A very interesting approach for blepharitis management was proposed by Craig et al. More specifically, authors prepared cyclodextrin complexes with methylglyoxal Manuka Honey ME and examined their in vitro antimicrobial effects on bacteria blepharitis. In vivo studies on rabbit eyes exhibited the tolerability and safety of the MEs, indicating that the cyclodextrin-ME can be further subjected for human application.⁸¹

Ocular MEs against fungal infections

Eye fungal infections are extremely rare, but, if it happens, can lead to serious damage of the eye. Trauma is the most predominant cause of ocular fungal infections, whereas the most common isolated species are Fusarium, Aspergillus, and Candida.^18,82,83 Few eye drops are available for fungal infection treatment; thus, researchers aimed to develop voriconazole-loaded (O/W) type MEs for efficient drug delivery. For example, O/W MEs of voriconazole were produced based on oleic acid and isopropyl myristate as the oil phase and Tween 80 and propylene glycol as surfactant and co-surfactant. The physicochemical parameters of formulations were checked and no significant interactions were found between drug and excipients according to H NMR and FTIR studies. The drug content was detected at 53% and 72%. The optimal formulations, which were chosen for further studies, showed acceptable viscosity and droplet size (160–280 nm). It was revealed that the droplet size enhanced by enhancing oil concentration (keeping surfactant concentration constant), while size reduced by increasing surfactant concentration (keeping oil concentration constant). In vitro release studies showed sustained release of drug pattern, with almost 70% of the drug being dissolved in 12 h. In addition, ex vivo permeation exhibited improved drug flux through cornea from ME.³⁰

A more recent research studied voriconazole loaded MEs and investigated its potential therapeutic effect by performing irritation tests. Ex vivo and in vivo studies are used to ascertain the permeation efficacy of drug-loaded MEs. Moreover, irritation tests involving Hen's Egg Test Chorio Allantoic Membrane assay, corneal histopathology, and Draize test were performed to determine the irritation of MEs. It was concluded that the MEs had a nonirritant nature. Furthermore, according to ex vivo studies, MEs showed significant enhancement of permeation/penetration rate of the drug compared to drug suspension. In addition, in vivo study showed higher availability of drug molecule in the aqueous humor.²⁷ Similarly, Bhosale et al. prepared and characterized a novel voriconazole ME consisting of oleic acid, isopropyl myristate and isopropyl palmitate (oil phases), Tween 80 (surfactant), propylene glycol (co-surfactant), and distilled water (aqueous phase), as well as modified chitosan as mucoadhesive polymer. The promising chitosan MEs exhibited droplet size under 250 nm, whereas the in vitro drug release study depicted sustained release rate. In vitro transcorneal permeation studies revealed great permeation, while ME antifungal activity against Candida albicans species was of great importance. Finally, in vivo studies on rabbit eyes demonstrated that voriconazole was detected in aqueous humor and the corneal bioavailability was improved significantly.⁸⁴

Da Silveira et al. aimed to develop amphotericin B-loaded ocular MEs by titration method. The toxicity of the MEs was assessed by in vitro toxicity assay against red blood cells and it was found relatively low. It was revealed that the amphotericin B loading did not affect the physicochemical properties of formulations compared to the blank formulations. The HPLC studies demonstrated that the drug loading was in high concentration. The drug-loaded MEs showed great antifungal activity against Candida strains, compared to the marketed product. It can be suggested that the developed MEs is a promising way of delivering amphotericin B to the eye.⁸⁵

Other applications of ocular MEs

Alany et al. studied the phase transition of W/O ME to lamellar liquid crystals or bicontinuous MEs to check their prolonged corneal retention. For this reason, they prepared MEs with Crodamol EO, Crill 1, and Crillet 4. An alkanol or alkanediol was used as co-surfactants. The modified hen's egg chorioallantoic membrane test was used to check the possible irritation of the system. For the preocular retention of formulations, rabbit eye was tested by using gamma scintigraphy. According to the results, Crill 1, Crillet 4, and Crodamol EO were nonirritant compared to the other co-surfactants, which present irritation due to the carbon chain length. Furthermore, the formulation that was prepared without co-surfactants showed an eye-protective effect. In accordance with the precorneal clearance studies, it was seen that the colloidal and coarse dispersions showed higher retention than water suspension, but no alteration was detected between MEs containing less water amount and O/W emulsion of 85% water content. In contrast, liquid crystal network containing no co-surfactant showed greater retention time compared to other formulations.⁸⁶

Another researcher group aimed to study how cyclodextrin addition into ME would effect the phase behavior of MEs. In this purpose, 3 different O/W types of MEs were prepared by increasing the concentration of various cyclodextrins into these formulations. Depending on the concentration and type of cyclodextrin, the developed MEs showed either phase separation or transition into a liquid crystalline state. It was seen that by adding a cyclodextrin, the formulations were transiting from ME to liquid crystalline. Finally, when the concentration of the cyclodextrin increased, a prolonged release of drug and tissue retention was depicted.⁸⁷

Formulations based on O/W and W/O MEs loaded with retinol and its esters were developed by Radomska et al. for ocular delivery. The components chosen to form MEs were as follows: IPM and Miglyol 812 as oil phase, Tween 60, Tween 80, and Epicurone 135 (soybean lecithin) as surfactants, n-butanol, triacetin, and propylene glycol as co-surfactants, and bidistilled water as aqueous phase. It was found that formulations were well tolerated and physically stable. As a result, the prepared ME formulations were found convenient for ocular usage.⁸⁸ Finally, an interesting ME was formulated by Poorani et al. for the protection of eye oxidative damage by diseases such as cancer, cataracts, and age-related macular degeneration. Authors prepared an O/W lutein-loaded ME that exhibited controlled release rate. Coconut oil was chosen as oil phase and water as aqueous phase. The droplet size was measured at 0.41, 1.37, and 0.813 μm, whereas the zeta potential indicated the stability of ME droplets. It can be said that the ME presents significant antioxidant activity and other physicochemical properties.⁸⁹

Conclusions

Ocular drug carriers are of great importance in the pharmaceutical technology and ophthalmology area. The unique characteristics of the eye and the ocular barriers hinder ocular bioavailability since tear fluids wash off the topically applied solution of drugs. Thus, the design and development of novel and sufficient drug delivery systems for ocular diseases management are mandatory. MEs of submicron and nanosize belong to such systems due to their unique characteristics. It can be said that, although MEs are easily handled and cost-effective, applications found in literature are limited. However, MEs could significantly play a major role for ocular disease treatments and thus, technologists and clinicians should focus on ME research area.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Funding Information

The authors received no specific funding for this work.

References

Sharma

U.K.

, Verma

, Prajapati

S.K.

, and Pandey

Ocular drug delivery: assorted obstructions and contemporary progresses. Int. J. Res. Dev. Pharm. Life Sci. 2:464–473, 2013.

Gagandeep

Garg

, T., Malik

, Rath

, and Goyal

A.K.

Development and characterization of nano-fiber patch for the treatment of glaucoma. Eur. J. Pharm. Sci. 53:10–16, 2014.

Üstündağ-Okur

, Gökçe

E.H.

, Bozbıyık

.İ., Eğrilmez

, Ertan

, and Özer, Ö. Novel nanostructured lipid carrier-based inserts for controlled ocular drug delivery: evaluation of corneal bioavailability and treatment efficacy in bacterial keratitis. Expert Opin. Drug Deliv. 12:1791–1807, 2015.

Arul Kumaran

K.S.G.

, Karthika

, and Padmapreetha

Comparative review on conventional and advanced ocular drug delivery formulations. Int. J. Pharm. Pharm. Sci. 2:1–5, 2010.

Gote

, Sikder

, Sicotte

, and Pal

Ocular drug delivery: present innovations and future challenges. J. Pharmacol. Exp. Ther. 370:602–624, 2019.

Nagarwal

R.C.

, Kant

, Singh

P.N.

, Maiti

, and Pandit

J.K.

Polymeric nanoparticulate system: a potential approach for ocular drug delivery. J. Control. Release. 136:2–13, 2009.

Souto

E.B.

, Dias-Ferreira

, López-Machado

, et al. Advanced formulation approaches for ocular drug delivery: state-of-the-art and recent patents. Pharmaceutics. 11:460, 2019.

Willoughby

C.E.

, Ponzin

, Ferrari

, Lobo

, Landau

, and Omidi

Anatomy and physiology of the human eye: effects of mucopolysaccharidoses disease on structure and function - a review. Clin. Exp. Ophthalmol. 38:2–11, 2010.

Jtirvinena

, Jävinenb

, and Urttiasc

Ocular absorption following topical delivery. Adv. Drug Deliv. Rev. 16:3–19, 1995.

10.

Bhujbal

S.S.

, Wale

K.K.

, Mahale

N.B.

, Landage

A.D.

, Sayyed

S.F.

, and Chaudhari

S.R.

Ocular drug delivery system: a review. World Journal of Pharmaceutical Research, Ramaiyan Dhanapal. 3:320–343, 2014.

11.

Srinivas

S.P.

, Chaiyasan

, Niamprem

, et al. Penetration of fluorescent silica nanoparticles into the cornea. Mater. Today Proc. 5:11106–11113, 2018.

12.

Macwan

J.S.

, Hirani

, and Pathak

Challenges in ocular pharmacokinetics and drug delivery. In: Nano-Biomaterials for Ophthalmic Drug Delivery. Cham: Springer International Publishing; 2016; p. 593–611. DOI: 10.1007/978-3-319-29346-2_26

13.

Gaudana

, Jwala

, Boddu

S.H.S.

, and Mitra

A.K.

Recent perspectives in ocular drug delivery. Pharm. Res. 26:1197–1216, 2009.

14.

Üstündaě-Okur

, Gökçe

E.H.

, Bozbiyik

D.I.

, Eěrilmez

, Özer, Ö., and Ertan

Preparation and in vitro-in vivo evaluation of ofloxacin loaded ophthalmic nano structured lipid carriers modified with chitosan oligosaccharide lactate for the treatment of bacterial keratitis. Eur. J. Pharm. Sci. 63:204–215, 2014.

15.

Yorston

Intravitreal injection technique. Community Eye Health. 27:47, 2014.

16.

Urtti

Challenges and obstacles of ocular pharmacokinetics and drug delivery. Adv. Drug Deliv. Rev. 58:1131–1135, 2006.

17.

Siafaka

P.I.

, Titopoulou

, Koukaras

E.N.

, et al. Chitosan derivatives as effective nanocarriers for ocular release of timolol drug. Int. J. Pharm. 495:249–264, 2015.

18.

Üstündağ Okur

, Yozgatlı

, Okur

M.E.

, Yoltaş

, and Siafaka

P.I.

Improving therapeutic efficacy of voriconazole against fungal keratitis: thermo-sensitive in situ gels as ophthalmic drug carriers. J. Drug Deliv. Sci. Technol. 49:323–333, 2019.

19.

Üstündağ Okur

, Çağlar

.Ş., and Yozgatlı

Development and validation of an HPLC method for voriconazole active substance in bulk and its pharmaceutical formulation. Marmara Pharm. J. 20, 2016. DOI: 10.12991/mpj.20162076793

20.

Yang

, Patel

, Vadlapudi

D.A.

, and Mitra

A.K.

Application of nanotechnology in ocular drug delivery. In: Yang, X., Patel, A., Vadlapudi, D.A., and Mitra, A.K., eds. Treatise on Ocular Drug Delivery. Bentham Science Publishers; 2013; p. 253–284. DOI: 10.2174/9781608051755113010013

21.

Habib

, El-Mahdy

, Abdel-Hafez

, and Maher

Microemulsion for ocular delivery: ocular irritancy test and in vivo studies of anti-inflammatory action. J. Drug Deliv. Sci. Technol. 22:541–544, 2012.

22.

Souto

E.B.

, Doktorovova

, Gonzalez-Mira

, Egea

M.A.

, and Garcia

M.L.

Feasibility of lipid nanoparticles for ocular delivery of anti-inflammatory drugs. Curr. Eye Res. 35:537–552, 2010.

23.

Phua

, Hou

, Lui

, et al. Topical delivery of senicapoc nanoliposomal formulation for ocular surface treatments. Int. J. Mol. Sci. 19:2977, 2018.

24.

Ammar

H.O.

, Salama

H.A.

, Ghorab

, and Mahmoud

A.A.

Nanoemulsion as a potential ophthalmic delivery system for dorzolamide hydrochloride. AAPS PharmSciTech. 10:808–819, 2009.

25.

Üstündag-Okur

, Gökçe

E.H.

, Eğrilmez

, Özer, Ö., and Ertan

Novel ofloxacin-loaded microemulsion formulations for ocular delivery. J. Ocul. Pharmacol. Ther. 30:319–332, 2014.

26.

Singh

, Guzman-Aranguez

, Hussain

, Srinivas

C.S.

, and Kaur

I.P.

Solid lipid nanoparticles for ocular delivery of isoniazid: evaluation, proof of concept and in vivo safety & kinetics. Nanomedicine. 14:465–491, 2019.

27.

Kumar

, and Sinha

V.R.

Evaluation of ocular irritation and bioavailability of voriconazole loaded microemulsion. Curr. Drug Deliv. 14:718–724, 2017.

28.

Ligório Fialho

, and da Silva-Cunha

New vehicle based on a microemulsion for topical ocular administration of dexamethasone. Clin. Exp. Ophthalmol. 32:626–632, 2004.

29.

Silva-Cunha

, da Silva

G.R.

, de Castro

W.V.

, and Fialho

S.L.

Evaluation of the pharmacokinetics and ocular tolerance of a microemulsion containing tacrolimus. J. Ocul. Pharmacol. Ther. 30:59–65, 2014.

30.

Kumar

, and Sinha

V.R.

Preparation and optimization of voriconazole microemulsion for ocular delivery. Colloids Surf. B Biointerfaces. 117:82–88, 2014.

31.

Üstündaě-Okur

, Ege

M.A.

, and Karasulu

H.Y.

Preparation and characterization of naproxen loaded microemulsion formulations for dermal application. Int. J. Pharm. 4:33–42, 2014.

32.

Üstündağ Okur

, Apaydın

Ş.

, Karabay Yavaşoğlu

N.Ü.

, Yavaşoğlu

, and Karasulu

H.Y.

Evaluation of skin permeation and anti-inflammatory and analgesic effects of new naproxen microemulsion formulations. Int. J. Pharm. 416:136–144, 2011.

33.

Karasulu

H.Y.

, Oruç

, Üstündağ-Okur

, et al. Aprotinin revisited: formulation, characterization, biodistribution and therapeutic potential of new aprotinin microemulsion in acute pancreatitis. J. Drug Target. 23:525–537, 2015.

34.

Okur

.Ü., Er

, Çağlar

.Ş., Ekmen

T.Z.

, and Sala

Formulation of microemulsions for dermal delivery of cephalexin. ACTA Pharm. Sci. 55:27, 2017.

35.

Ustündağ Okur

, Yavaşoğlu

, and Karasulu

H.Y.

Preparation and evaluation of microemulsion formulations of naproxen for dermal delivery. Chem. Pharm. Bull (Tokyo). 62:135–143, 2014.

36.

Bharti

S.K.

, and Kesavan

Phase-transition W/O microemulsions for ocular delivery: evaluation of antibacterial activity in the treatment of bacterial keratitis. Ocul. Immunol. Inflamm. 25:463–474, 2017.

37.

Habib

, El-Mahdy

, and Maher

Microemulsions for ocular delivery: evaluation and characterization. J. Drug Deliv. Sci. Technol. 21:485–489, 2011.

38.

Kalam

M.A.

, Alshamsan

, Aljuffali

I.A.

, Mishra

A.K.

, and Sultana

Delivery of gatifloxacin using microemulsion as vehicle: formulation, evaluation, transcorneal permeation and aqueous humor drug determination. Drug Deliv. 23:886–897, 2016.

39.

Xavier-Junior

F.H.

, Vauthier

, Morais

A.R.V.

, Alencar

E.N.

, and Egito

E.S.T.

Microemulsion systems containing bioactive natural oils: an overview on the state of the art. Drug Dev. Ind. Pharm. 43:700–714, 2017.

40.

El Agamy

H.I.

, and El Maghraby

G.M.

Natural and synthetic oil phase transition microemulsions for ocular delivery of tropicamide: efficacy and safety. J. Appl. Pharm. Sci. 5(Suppl 2):67–75, 2015.

41.

Pakkang

, Uraki

, Koda

, Nithitanakul

, and Charoensaeng

Preparation of water-in-oil microemulsion from the mixtures of castor oil and sunflower oil as makeup remover. J. Surfactants Deterg. 21:809–816, 2018.

42.

Torres-Luna

, Hu

, Koolivand

, et al. Effect of a cationic surfactant on microemulsion globules and drug release from hydrogel contact lenses. Pharmaceutics. 11:262, 2019.

43.

Hegde

R.R.

, Verma

, and Ghosh

Microemulsion: new insights into the ocular drug delivery. ISRN Pharm. 2013:1–11, 2013.

44.

Tiwari

, Pandey

, Asati

, Soni

, and Jain

Therapeutic challenges in ocular delivery of lipid based emulsion. Egypt J. Basic Appl. Sci. 5:121–129, 2018.

45.

Hopkins Hatzopoulos

, Eastoe

, Dowding

P.J.

, and Grillo

Cylinder to sphere transition in reverse microemulsions: the effect of hydrotropes. J. Colloid Interface Sci. 392:304–310, 2013.

46.

Chan

, El Maghraby

G.M.M.

, Craig

J.P.

, and Alany

R.G.

Phase transition water-in-oil microemulsions as ocular drug delivery systems: in vitro and in vivo evaluation. Int. J. Pharm. 328(1 SPEC. ISS.):65–71, 2007.

47.

Moghimipour

, Salimi

, and Changizi

Preparation and microstructural characterization of griseofulvin microemulsions using different experimental methods: SAXS and DSC. Adv. Pharm. Bull. 7:281–289, 2017.

48.

EMEA. ICH topic Q 2 (R1) Validation of analytical procedures: text and methodology. 2006:1–15. DOI: 10.1136/bmj.333.7574.873-a

49.

Çağlar

.Ş., Pekcan

A.N.

, Okur

M.E.

, Ayla, Ş., and Üstündağ-Okur

Preparation, optimization and in vivo anti-inflammatory evaluation of hydroquinone loaded microemulsion formulations for melasma treatment. J. Res. Pharm. 23:662–670, 2019.

50.

Fukumoto

, Dalmazzone

, Frot

, Barré

, and Noïk

Characterization of complex crude oil microemulsions-DSC contribution. Dalmazzone, C., ed. Oil Gas Sci Technol—Rev d'IFP Energies Nouv. 73:1–10, 2018.

51.

Hegde

R.R.

, Bhattacharya

S.S.

, Verma

, and Ghosh

Physicochemical and pharmacological investigation of water/oil microemulsion of non-selective beta blocker for treatment of glaucoma. Lat. Am. J. Pharm. 39:155–163, 2014.

52.

Rahdar

Effect of tocopherol on microemulsions: turbidity studies and Dynamic light scattering and dynamic surface tension measurements. Nanomed. Res. J. 3:206–218, 2018.

53.

Szumała

Structure of microemulsion formulated with monoacylglycerols in the presence of polyols and ethanol. J. Surfactants Deterg. 18:97–106, 2015.

54.

Moghimipour

, Salimi

, and Rad

A.S.

A microemulsion system for controlled corneal delivery of Timolol. Int. Res. J. Pharm. Appl. Sci. 3:32–39, 2013.

55.

Chan

, El Maghraby

G.M.

, Craig

J.P.

, and Alany

R.G.

Effect of water-in-oil microemulsions and lamellar liquid crystalline systems on the precorneal tear film of albino New Zealand rabbits. Clin. Ophthalmol. 2:129–138, 2008.

56.

Karasulu

, Ince

, Ates

, Yavasoglu

, and Levent

A novel pilocarpine microemulsion as an ocular delivery system: in vitro and in vivo studies. J. Clin. Exp. Ophthalmol. 06:2–6, 2015.

57.

Mahboobian

M.M.

, Seyfoddin

, Rupenthal

I.D.

, Aboofazeli

, and Foroutan

S.M.

Formulation development and evaluation of the therapeutic efficacy of brinzolamide containing nanoemulsions. Iran. J. Pharm. Res. 16:847–857, 2017.

58.

Gohil

, Patel

, Pandya

, and Dharamsi

Optimization of brinzolamide loaded microemulsion using formulation by design approach: characterization and in-vitro evaluation. Curr. Drug Ther. 14, 2019. DOI: 10.2174/1574885514666190104115802

59.

Jeng

B.H.

Recent updates in inflammatory ocular diseases. US Ophthalmic Rev. 11:19, 2018.

60.

Goel

, Desai

, Lavingia

, Gokhale

, Jadhav

, and Gogtay

Efficacy and safety of 0.05% cyclosporine ophthalmic microemulsion for the treatment of moderate to severe dry eye disease—an Indian Experience. Delhi J. Ophthalmol. 25:238–241, 2015.

61.

Shin

E.H.

, Lim

D.H.

, Yang

C.M.

, and Chung

T.-Y.

Comparison of efficacy and sensation of instillation between 0.05% cyclosporine nanoemulsion and microemulsion type. J. Korean Ophthalmol. Soc. 60:239–245, 2019.

62.

Gan

, Gan

, Zhu

, Zhang

, and Zhu

Novel microemulsion in situ electrolyte-triggered gelling system for ophthalmic delivery of lipophilic cyclosporine A: in vitro and in vivo results. Int. J. Pharm. 365:143–149, 2009.

63.

Kapoor

, and Chauhan

Ophthalmic delivery of Cyclosporine A from Brij-97 microemulsion and surfactant-laden p-HEMA hydrogels. Int. J. Pharm. 361:222–229, 2008.

64.

Kesavan

, Pandit

J.K.

, Kant

, and Muthu

M.S.

Positively charged microemulsions of dexamethasone: comparative effects of two cosurfactants on ocular drug delivery and bioavailability. Ther. Deliv. 4:1385–1395, 2013.

65.

Kesavan

, Kant

, Nath Singh

, and Kumar Pandit

Mucoadhesive chitosan-coated cationic microemulsion of dexamethasone for ocular delivery: in vitro and in vivo evaluation. Curr. Eye Res. 38:342–352, 2013.

66.

Baspinar

, Bertelmann

, Pleyer

, Buech

, Siebenbrodt

, and Borchert

H.-H.

Corneal permeation studies of everolimus microemulsion. J. Ocul. Pharmacol. Ther. 24:399–402, 2008.

67.

Lidich

, Aserin

, and Garti

Structural characteristics of oil-poor dilutable fish oil omega-3 microemulsions for ophthalmic applications. J. Colloid Interface Sci. 463:83–92, 2016.

68.

Galvis

, Sherwin

, Tello

, Merayo

, Barrera

, and Acera

Keratoconus: an inflammatory disorder?. Eye. 29:843–859, 2015.

69.

Lidich

, Wachtel

E.J.

, Aserin

, and Garti

Water-dilutable microemulsions for transepithelial ocular delivery of riboflavin phosphate. J. Colloid Interface Sci. 463:342–348, 2016.

70.

Lidich

, Garti-Levy

, Aserin

, and Garti

Potentiality of microemulsion systems in treatment of ophthalmic disorders: keratoconus and dry eye syndrome—in vivo study. Colloids Surf. B Biointerfaces. 173:226–232, 2019.

71.

S.W.

, Gan

, Zhu

C.L.

, and Zhu

J.B.

Preparation and in vitro corneal retention behavior of novel cationic microemulsion/in situ gel system. Yaoxue Xuebao. 43:749–755, 2008.

72.

Gupta

, Nayak

, and Misra

Cow ghee fortified ocular topical microemulsion; in vitro, ex vivo, and in vivo evaluation. J. Microencapsul. 36:603–621, 2019.

73.

F.F.

, Zheng

L.Q.

, and Tung

C.H.

Phase behavior of the microemulsions and the stability of the chloramphenicol in the microemulsion-based ocular drug delivery system. Int. J. Pharm. 301:237–246, 2005.

74.

Butt

, ElShaer

, Snyder

L.A.S.

, Chaidemenou

, and Alany

R.G.

Fatty acid microemulsion for the treatment of neonatal conjunctivitis: quantification, characterisation and evaluation of antimicrobial activity. Drug Deliv. Transl. Res. 6:722–734, 2016.

75.

Butt

, Elshaer

, Snyder

L.A.S.

, and Alany

R.G.

Fatty acid based microemulsions to combat ophthalmia neonatorum caused by Neisseria gonorrhoeae and Staphylococcus aureus. Nanomaterials. 8:1–22, 2018.

76.

Salimi

, Panahi-Bazaz

M.-R.

, and Panahi-Bazaz

A Novel Microemulsion system for ocular delivery of azithromycin: design, characterization and ex-vivo rabbit corneal permeability. Jundishapur J. Nat. Pharm. Prod. 12:1–11, 2017.

77.

Kant Bharti

, and Kesavan

Ocular Immunology and inflammation phase-transition W/O microemulsions for ocular delivery: evaluation of antibacterial activity in the treatment of bacterial keratitis. Ocul. Immunol. Inflamm. 25:463–474, 2017.

78.

Shah

, Nair

A.B.

, Jacob

, et al. Nanoemulsion based vehicle for effective ocular delivery of moxifloxacin using experimental design and pharmacokinetic study in rabbits. Pharmaceutics. 11:230, 2019.

79.

Nair

, Chakrapani

, and Kaza

Preparation and evaluation of vancomycin microemulsion for ocular drug delivery. Drug Deliv. Lett. 2:26–34, 2012.

80.

Bachu

R.D.

, Stepanski

, Alzhrani

R.M.

, Jung

, and Boddu

S.H.S.

Development and evaluation of a novel microemulsion of dexamethasone and tobramycin for topical ocular administration. J. Ocul. Pharmacol. Ther. 34:312–324, 2018.

81.

Craig

J.P.

, Rupenthal

I.D.

, Seyfoddin

, et al. Preclinical development of MGO Manuka Honey microemulsion for blepharitis management. BMJ Open Ophthalmol. 1:e000065, 2017.

82.

Kalkanci

, and Ozdek

Ocular fungal infections. Curr. Eye Res. 36:179–189, 2011.

83.

Roche

, Lannoy

, Bourdon

, et al. Stability of frozen 1% voriconazole eye-drops in both glass and innovative containers. Eur. J. Pharm. Sci. 141:105102, 2020.

84.

Bhosale

, Bhandwalkar

, Duduskar

, Jadhav

, and Pawar

Water soluble chitosan mediated voriconazole microemulsion as sustained carrier for ophthalmic application: in vitro/ex vivo/in vivo evaluations. Open Pharm. Sci. J. 3:215–234, 2016.

85.

da Silveira

W.L.

, Damasceno

B.P.

, Ferreira

L.F.

, et al. Development and characterization of a microemulsion system containing amphotericin B with potential ocular applications. Curr. Neuropharmacol. 13:982–993, 2015.

86.

Alany

R.G.

, Rades

, Nicoll

, Tucker

I.G.

, and Davies

N.M.

W/O microemulsions for ocular delivery: evaluation of ocular irritation and precorneal retention. J. Control. Release. 111:145–152, 2006.

87.

Thakur

S.S.

, Solloway

, Stikkelman

, Seyfoddin

, and Rupenthal

I.D.

Phase transition of a microemulsion upon addition of cyclodextrin—applications in drug delivery. Pharm. Dev. Technol. 23:167–175, 2018.

88.

Radomska

, and Dobrucki

The use of some ingredients for microemulsion preparation containing retinol and its esters. Int. J. Pharm. 196:131–134, 2000.

89.

Poorani

T.R.

, Vellingiria

, Deepa

V.S.

, and Abdula

A.N.

Formulation of Eichhornia crassippes derived lutein: coconut oil micro-emulsion for sustained ophthalmic drug delivery. Int. J. Pharm. Sci. Res. 8:4159–4171, 2017.