**4. Topical nanocarriers for drug delivery into the posterior segment of the eyeball**

To date, different nanocarriers have proven to be capable technologies to circumvent most of the ocular barriers and effectively deliver drugs to the posterior segment of the eye. Their favorable physicochemical properties, such as the small size and adjustable surface, render them advantageous in targeting drugs to the retina. Although different administration routes have been analyzed, we consider that the topical route is the one of greatest interest because of its favorable characteristics such as its ease for the patient, a potentially better compliance, and lower incidence of serious complications than injected methods. The main topically proven nanocarriers are polymeric NPs, lipid nanoparticles, micelles, dendrimers, and liposomes.

#### **4.1 Polymeric nanoparticles**

Polymeric nanoparticles (PNPs) are colloidal particles (10–1000 nm) where the drug is uniformly distributed throughout the particle matrix (nanospheres) or encapsulated inside a polymer shell (nanocapsules) [82]. Synthetic and natural biocompatible polymers can be used to develop PNPs. The polymers used to generate PNPs for ocular applications have been made from poly(alkyl cyanoacrylates) [83, 84], polycaprolactone, chitosan (CS), hyaluronic acid (HA), Eudragit (RS100 and RL100), Carbopol, gelatin, poly(butyl cyanoacrylate), polylactic acid (PLA), and poly(lactic-co-glycolic acid) (PLGA) [57, 85].

In regard to topical PNPs for posterior segment drug delivery, we would like to highlight the study performed by Tahara K et al., where submicron-sized poly(D,Llactide-co-glycolide) (PLGA) nanoparticles were used to improve the efficiency of drug delivery to the retina through topically administered drugs. Chitosan and glycol chitosan (GCS), which are mucoadhesive polymers, and polysorbate 80

*Role of Novel Drug Delivery Vehicles in Nanobiomedicine*

intravitreal injection is used as a delivery pathway.

**3.4 Intravitreal injection**

ocular diseases.

**3.3 Periocular administration**

Although topical and systemic routes are convenient, the lack of adequate bioavailability and the failure to deliver therapeutic amounts of drugs into the retina prompted vision scientists to search for alternative routes of administration. Hence, specific oral or intravenous targeting systems are needed to transport molecules through the choroid and into deeper layers of the retina [43]. An example of a systemic drug for the treatment of retinal diseases is Visudyne, which is used in photodynamic therapy for the treatment of wet age-related macular degeneration (AMD) and central serous chorioretinopathy (CSC). However, owing to toxicity and delivery concerns, intravenous administration is not very common in treating

Although not very patient compliant, these routes are partly used to overcome

Nowadays, intravitreal injection is the preferred route for ocular delivery for the treatment of diseases of the retina. It offers important advantages over periocular injections as drug molecules are directly inserted into the vitreous chamber. This method involves the injection of a drug solution directly into the vitreous, via the pars plana, using a 30-Ga needle. Unlike other routes, intravitreal injection offers higher drug concentrations in the vitreous and the retina. The advantage of IVT injection is the circumvention of the blood-retinal barrier, which keeps most drugs out of the eye in the case of oral and systemic administrations [43]. Therefore, IVT administration is able to maximize the intraocular level of drugs in the vitreous and the retina while avoiding toxicities associated with systemic treatment [66–69]. The benefits of intravitreal therapies in retinal diseases have been well documented, and the current standard of care is the injection of a bolus of anti-VEGF into the vitreous cavity on a frequent basis. Such a large bolus of a drug has been associated with some side effects [70]. In addition, this administration procedure creates a typical curve with a large peak of drug concentrations and a rapid decay. Literature supports the fact that the half-life of anti-VEGFs, such as ranibizumab, is indeed very short (halflife = 2.6–4.0 days) [71], making it necessary to use a higher initial dose in order to exceed therapeutic levels so as to allow a longer 28-day treatment interval. Moreover, mathematical modeling demonstrates that the binding activity of 0.5 mg of ranibizumab is fivefold higher if given every 14 days instead of every 28 days [72].

the inefficiency of topical and systemic dosing in order to deliver therapeutic drug concentrations to the posterior segment of the eye globe. Moreover, systemic administration may lead to side effects, making it a less desirable delivery route for geriatric patients. Periocular routes include subconjunctival, subtenon, retrobulbar, and peribulbar administrations, and they are comparatively less invasive than intravitreal routes. Drugs administered by periocular injections can reach the posterior segment through three different pathways: the transscleral pathway, systemic circulation through the choroid, and the anterior pathway through the tear film, cornea, aqueous humor, and vitreous humor [63]. The administration of a drug via subtenon injection resulted in the highest and more sustained vitreous concentration of drug molecules compared to other periocular injections [64]. However, anterior segment complications have been described in some patients following periocular injections. The complications include ocular hypertension, cataract, hyphema, strabismus, and corneal decompensation [65]. Additionally, the concentration of many drugs, such as that of anti-angiogenic drugs, is better if an

**30**

(P80) were used as surface modifiers of PGLA nanoparticles since this substance increases the interaction of NPs with cells. Coumarin-6 was used as a model drug and a fluorescent marker. The in vivo fluorescence image analysis showed detectable fluorescence intensity of coumarin-6 in the retina of mice after its topical administration [86]. The author proposed that the delivery of coumarin-6 into the posterior segment was associated with ocular surface modifications induced by CS, GCS, or P80. However, they did not verify this hypothesis.

Another interesting work concerning topically administered PNPs for retinal drug delivery was the study performed by Binstock et al., where the penetration of charged fluorescent nanoparticles into rabbit eyes using hydrogel iontophoresis was evaluated. Polyacrylic hydrogels were loaded with charged nanoparticles. NP suspensions (20–45 nm) were administered using cathodal and anodal iontophoresis and applying a current intensity of 1.5 mA for 5 min at the central cornea and the sclera (pars plana). The mean fluorescence intensity in outer ocular tissues (cornea, conjunctiva, and sclera) was 2–3 times higher for positively charged NPs with anodal iontophoresis. In deeper ocular tissues (retina, choroid, ciliary body, and iris), the mean fluorescence intensity was 5–15 times higher for the negatively charged NPs with cathodal iontophoresis. The positively charged particles demonstrated better penetration into inner ocular tissues than negatively charged particles [87]. This work showed that hydrogel iontophoresis combined with charged nanoparticles could be a potential technique for effective retinal drug delivery, but the investigation will have to be broadened and the potential adverse events explored.

## **4.2 Lipid nanoparticles**

Three different types of lipid nanoparticles (LNPs) have been used for ophthalmic formulations: solid lipid nanoparticles (SLNPs), nanostructured lipid carriers (NLCs), and hybrid lipid nanoparticles. SLNPs are aqueous colloidal dispersions composed of solid lipids as a matrix material with the capacity to encapsulate hydrophilic and lipophilic drugs. SLNPs are provided with multiple advantages such as physical stability, protection of the encapsulated drug, controlled release, biocompatibility, feasible production, and the possibility of being sterilized by autoclaving [88–91]. On the other hand, SLNs have limited drug loading and expulsion for both hydrophilic and lipophilic drugs. Loaded drugs are usually located between fatty acid chains, in lipid layers, or in crystal imperfections [92]. NLCs consist of a mixture of solid and liquid lipids, and they exhibit a higher drug-loading capacity and longer stability than SLNPs [93]. Finally, hybrid lipid nanoparticles modified by multifunctional polymers [94] combine the merits of polymeric nanoparticles and lipid-based systems, which improve the pharmacokinetics and biodistribution of the loaded drug [95].

Experimentally, LNPs have been used for the delivery of substances to the posterior segment of the eye via corneal and non-corneal pathways. Araujo et al. [96] developed a nanometric (∼200 nm), unimodal, and negatively charged NLC loaded with the fluorescent lipid, Nile red (NR-NLC) marker and a drug, triamcinolone acetonide (TA). The method used for the construction of the NLC was high-pressure homogenization. After NR-NLC eyedrop instillation in mice, lipophilic actives were released into the posterior segment of the eye. Retinal fluorescence of Nile red gradually increased over time, peaking 40 min after administration and almost disappearing at 160 min. The authors suggested that the fluorescence component of NR-NLC reached the retina through the BRB via non-corneal routes (sclera and conjunctiva).

In the work of Balguri et al., SLNPs and NLCs loaded with indomethacin (IN) were prepared through a hot homogenization method. Additionally, CS was used to modify the surface of SLNPs in order to increase penetration. After the topical application of formulations in rabbits, the concentration of IN was evaluated in

**33**

**4.4 Dendrimers**

*Breaking down the Barrier: Topical Liposomes as Nanocarriers for Drug Delivery…*

are the corneal pathway and the conjunctival-scleral pathway [97].

Finally, SLNPs comprising Compritol® 888 ATO and a polyethylene glycol (PEG) core, into which KTZ is dissolved, were topically used in rabbits. These SLNPs were generated by using a hot high-pressure homogenization method. Ocular pharmacokinetic studies indicated 2.5-fold and 1.6-fold increases in the aqueous humor and the vitreous humor via the topical administration compared to KTZ suspensions, respectively. Additionally, SLNPs were safe for the ocular surfaces of the study rabbits, and cell viability assays showed nontoxicity [98].

Micelles (10–100 nm) comprise amphiphilic block copolymers above the critical micellar concentration with a hydrophobic core and a hydrophilic shell [99]. The preparation process of colloidal dispersions consists of the exposition of self-assembly oriented amphiphilic molecules in order to form core-shell monomer nanomicelles in a special solvent. These nanodispersions solubilize hydrophobic drugs by entrapping them within a mixed micellar hydrophobic core, resulting in clear aqueous formulations [100]. Micelles prevent or minimize drug degradation, improve drug permeation, increase the solubility and stability of drugs, and

Lately, nanomicelles/mixed nanomicelles loaded with different drugs such as dexamethasone [103, 104], voclosporin [105], and rapamycin [106] have been designed to reach the posterior segment of the eye. Liaw et al. [107] also demonstrated that micelles could be effective for intraocular gene delivery. In this work, pDNA with the lacZ gene was coupled to poly(ethylene oxide)-poly(propylene oxide) poly(ethylene oxide) (PEO-PPO-PEO) nonionic copolymeric micelles. Following topical administration, gene expression was detected around the rabbit iris, sclera, and conjunctiva, and it was also seen in the intraocular tissues of nude mice after topical application for 48 h. In another study, Cholkar et al. [108] optimized an aqueous micellar solution of isopropyl ester carrying the resolvin (RX-10045) prodrug. Ocular tissue distribution studies demonstrated significant drug concentrations in anterior ocular tissues. Moreover, RX-10008 (the active metabolite of RX-10045) was detected in the retina/choroid upon topical drop instillation in rabbits. Micelles appeared to follow the conjunctival-scleral pathway to reach the retina, and no evidence of ocular

damage after multiple dosing of this micellar solution was observed [108].

Dendrimers are highly branched treelike structures with a core and many side chain [109] moieties. The size of dendrimer nanoparticles is usually under 100 nm, and peripheral functional groups (neutral, negative, or positive) may suffer secondary surface modifications in order to improve their characteristics for

ocular tissues by using high-performance liquid chromatography (HPLC)-UV method. In vivo bioavailability studies demonstrated that the IN of the formulations (IN-SLNPs, IN-NLC, and IN-CS-NLC) reached the posterior ocular tissues 2 h posttopical application. Moreover, the IN-NLC showed higher drug-loading capability, higher entrapment efficiency, less drug expulsion, and, especially, the highest distribution concentrations of IN in deeper ocular tissues (vitreous humor, choroidretina, and sclera). In vitro corneal penetration studies showed that CS increased corneal penetration by twofold (IN-CS-NLC compound), possibly thanks to the enhanced interaction with the mucus of the eye, the increased cellular uptake, and even its internalization. On the basis of their results, the authors proposed that the main transport routes of IN-NLC to successfully deliver IN to the posterior segment

*DOI: http://dx.doi.org/10.5772/intechopen.86601*

**4.3 Polymeric micelles**

improve ocular bioavailability [101, 102].

*Breaking down the Barrier: Topical Liposomes as Nanocarriers for Drug Delivery… DOI: http://dx.doi.org/10.5772/intechopen.86601*

ocular tissues by using high-performance liquid chromatography (HPLC)-UV method. In vivo bioavailability studies demonstrated that the IN of the formulations (IN-SLNPs, IN-NLC, and IN-CS-NLC) reached the posterior ocular tissues 2 h posttopical application. Moreover, the IN-NLC showed higher drug-loading capability, higher entrapment efficiency, less drug expulsion, and, especially, the highest distribution concentrations of IN in deeper ocular tissues (vitreous humor, choroidretina, and sclera). In vitro corneal penetration studies showed that CS increased corneal penetration by twofold (IN-CS-NLC compound), possibly thanks to the enhanced interaction with the mucus of the eye, the increased cellular uptake, and even its internalization. On the basis of their results, the authors proposed that the main transport routes of IN-NLC to successfully deliver IN to the posterior segment are the corneal pathway and the conjunctival-scleral pathway [97].

Finally, SLNPs comprising Compritol® 888 ATO and a polyethylene glycol (PEG) core, into which KTZ is dissolved, were topically used in rabbits. These SLNPs were generated by using a hot high-pressure homogenization method. Ocular pharmacokinetic studies indicated 2.5-fold and 1.6-fold increases in the aqueous humor and the vitreous humor via the topical administration compared to KTZ suspensions, respectively. Additionally, SLNPs were safe for the ocular surfaces of the study rabbits, and cell viability assays showed nontoxicity [98].

## **4.3 Polymeric micelles**

*Role of Novel Drug Delivery Vehicles in Nanobiomedicine*

GCS, or P80. However, they did not verify this hypothesis.

(P80) were used as surface modifiers of PGLA nanoparticles since this substance increases the interaction of NPs with cells. Coumarin-6 was used as a model drug and a fluorescent marker. The in vivo fluorescence image analysis showed detectable fluorescence intensity of coumarin-6 in the retina of mice after its topical administration [86]. The author proposed that the delivery of coumarin-6 into the posterior segment was associated with ocular surface modifications induced by CS,

Another interesting work concerning topically administered PNPs for retinal drug delivery was the study performed by Binstock et al., where the penetration of charged fluorescent nanoparticles into rabbit eyes using hydrogel iontophoresis was evaluated. Polyacrylic hydrogels were loaded with charged nanoparticles. NP suspensions (20–45 nm) were administered using cathodal and anodal iontophoresis and applying a current intensity of 1.5 mA for 5 min at the central cornea and the sclera (pars plana). The mean fluorescence intensity in outer ocular tissues (cornea, conjunctiva, and sclera) was 2–3 times higher for positively charged NPs with anodal iontophoresis. In deeper ocular tissues (retina, choroid, ciliary body, and iris), the mean fluorescence intensity was 5–15 times higher for the negatively charged NPs with cathodal iontophoresis. The positively charged particles demonstrated better penetration into inner ocular tissues than negatively charged particles [87]. This work showed that hydrogel iontophoresis combined with charged nanoparticles could be a potential technique for effective retinal drug delivery, but the investiga-

tion will have to be broadened and the potential adverse events explored.

improve the pharmacokinetics and biodistribution of the loaded drug [95]. Experimentally, LNPs have been used for the delivery of substances to the posterior segment of the eye via corneal and non-corneal pathways. Araujo et al. [96] developed a nanometric (∼200 nm), unimodal, and negatively charged NLC loaded with the fluorescent lipid, Nile red (NR-NLC) marker and a drug, triamcinolone acetonide (TA). The method used for the construction of the NLC was high-pressure homogenization. After NR-NLC eyedrop instillation in mice, lipophilic actives were released into the posterior segment of the eye. Retinal fluorescence of Nile red gradually increased over time, peaking 40 min after administration and almost disappearing at 160 min. The authors suggested that the fluorescence component of NR-NLC reached the retina through the BRB via non-corneal routes (sclera and conjunctiva). In the work of Balguri et al., SLNPs and NLCs loaded with indomethacin (IN) were prepared through a hot homogenization method. Additionally, CS was used to modify the surface of SLNPs in order to increase penetration. After the topical application of formulations in rabbits, the concentration of IN was evaluated in

Three different types of lipid nanoparticles (LNPs) have been used for ophthalmic formulations: solid lipid nanoparticles (SLNPs), nanostructured lipid carriers (NLCs), and hybrid lipid nanoparticles. SLNPs are aqueous colloidal dispersions composed of solid lipids as a matrix material with the capacity to encapsulate hydrophilic and lipophilic drugs. SLNPs are provided with multiple advantages such as physical stability, protection of the encapsulated drug, controlled release, biocompatibility, feasible production, and the possibility of being sterilized by autoclaving [88–91]. On the other hand, SLNs have limited drug loading and expulsion for both hydrophilic and lipophilic drugs. Loaded drugs are usually located between fatty acid chains, in lipid layers, or in crystal imperfections [92]. NLCs consist of a mixture of solid and liquid lipids, and they exhibit a higher drug-loading capacity and longer stability than SLNPs [93]. Finally, hybrid lipid nanoparticles modified by multifunctional polymers [94] combine the merits of polymeric nanoparticles and lipid-based systems, which

**4.2 Lipid nanoparticles**

**32**

Micelles (10–100 nm) comprise amphiphilic block copolymers above the critical micellar concentration with a hydrophobic core and a hydrophilic shell [99]. The preparation process of colloidal dispersions consists of the exposition of self-assembly oriented amphiphilic molecules in order to form core-shell monomer nanomicelles in a special solvent. These nanodispersions solubilize hydrophobic drugs by entrapping them within a mixed micellar hydrophobic core, resulting in clear aqueous formulations [100]. Micelles prevent or minimize drug degradation, improve drug permeation, increase the solubility and stability of drugs, and improve ocular bioavailability [101, 102].

Lately, nanomicelles/mixed nanomicelles loaded with different drugs such as dexamethasone [103, 104], voclosporin [105], and rapamycin [106] have been designed to reach the posterior segment of the eye. Liaw et al. [107] also demonstrated that micelles could be effective for intraocular gene delivery. In this work, pDNA with the lacZ gene was coupled to poly(ethylene oxide)-poly(propylene oxide) poly(ethylene oxide) (PEO-PPO-PEO) nonionic copolymeric micelles. Following topical administration, gene expression was detected around the rabbit iris, sclera, and conjunctiva, and it was also seen in the intraocular tissues of nude mice after topical application for 48 h. In another study, Cholkar et al. [108] optimized an aqueous micellar solution of isopropyl ester carrying the resolvin (RX-10045) prodrug. Ocular tissue distribution studies demonstrated significant drug concentrations in anterior ocular tissues. Moreover, RX-10008 (the active metabolite of RX-10045) was detected in the retina/choroid upon topical drop instillation in rabbits. Micelles appeared to follow the conjunctival-scleral pathway to reach the retina, and no evidence of ocular damage after multiple dosing of this micellar solution was observed [108].

#### **4.4 Dendrimers**

Dendrimers are highly branched treelike structures with a core and many side chain [109] moieties. The size of dendrimer nanoparticles is usually under 100 nm, and peripheral functional groups (neutral, negative, or positive) may suffer secondary surface modifications in order to improve their characteristics for ophthalmic use [110]. Due to their enhanced aqueous solubility, a large variety of surface groups, and their nontoxic nature, polyamidoamine (PAMAM) dendrimers have been investigated the most in regard to ocular drug delivery [111, 112].

Yavuz et al. [113] generated various anionic dexamethasone (DEX)-PAMAM complexes and evaluated the delivery of DEX to the posterior segment following topical administration in rats. The ocular tissue distribution study in rabbit eyes showed that anionic DEX-PANAM dendrimers reach the highest concentration of DEX in the vitreous body, the retina-choroid, and the sclera compared to topical DEX suspension. In addition, in vitro studies demonstrated that dendrimers with peripheral carboxyl or hydroxyl functional groups rarely showed cytotoxicity. Transport study results showed that dendrimer complexation increases DEX transport across both corneal and scleral tissues. Additionally, dendrimers have proven their efficiency to release genetic material when topically administered [114].
