Drug Delivery to the Back of the Eye Following Topical Administration: an Update on Research and Patenting Activity

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Recent Patents on Drug Delivery & Formulation 2014, 8, 27-36 27 Drug Delivery to the Back of the Eye Following Topical Administration: An Update on Research and Patenting Activity

Sai H.S. Boddu*, Himanshu Gupta and Soohi Patel

Department of Pharmacy Practice, College of Pharmacy and Pharmaceutical Sciences, The University of Toledo, Ohio, USA – 43614

Received: December 31, 2013; Revised: January 26, 2014; Accepted: January 29, 2014

Abstract: Drug delivery to the back of the eye following topical administration remains an unmet need for the scientific community. Treatment of posterior segment diseases requires localized and long-term drug delivery to the retina, choroid, and Bruch’s membrane. Until the last decade, there was limited evidence from large clinical trials that demonstrated the usefulness of pharmacotherapy compared to laser therapy or other vitreoretinal surgical techniques for the treatment of retinal diseases. This paradigm has shifted in recent years, with strong evidence demonstrating superior efficacy of ophthalmic drugs compared to previous gold standards. However, ophthalmologists are left with no options other than administering the therapeutics via implants and intravitreal injections, which are highly invasive and associated with patient non-compliance. A non-invasive topical therapy would enhance patient compliance and minimize the side-effects associated with intraocular implants and intravitreal injections. In an attempt to enhance patient compliance, the focus of research has shifted to the development of novel small molecule-based eye drop formulations. This review article discusses the relevant patents and summarizes the resurgence in the treatment of posterior segment eye diseases through topical drug administration. Keywords: Drug delivery, eye drops, posterior segment, retinal diseases.

INTRODUCTION reaches the retina and choroid [6]. Common approaches to the treatment of posterior segment diseases include, but are Efficient drug delivery to the back of the eye remains a not limited to, systemic and intraocular injections and im- challenging task due to its unique anatomical and physio- plants. Systemic drug delivery results in inadequate retinal logical barriers [1]. The external and internal structures of concentrations and severe systemic adverse effects. Though the eye are protected by different barriers that prevent the intravitreal administration delivers a high concentration of penetration of drugs in required concentrations. The structure drugs to the retina, the inherent potential side effects like of the eye is divided into two segments: anterior and poste- increased intraocular pressure [7], hemorrhage [8], cataract rior. The anterior segment consists of the cornea, iris/ciliary [9], and endophthalmitis [10] lead to complications limiting body, and lens [2]. The posterior segment mainly consists of long-term therapy [11]. Moreover, the chronic nature of reti- the vitreous humor, retina, choroid, and optic nerve Fig. ( ). 1 nal diseases requires multiple injections, which are associ- Diseases affecting the posterior segment such as age-related ated with risk of vitreous hemorrhage, retinal detachment, macular degeneration [AMD], diabetic retinopathy [DR], and cataract progression. Implants have overcome many of diabetic macular edema [DME], proliferative vitreoretinopa- the disadvantages associated with intravitreal injections; thy [PVR], uveitis, and cytomegalovirus [CMV] require im- however, the surgical procedure and risk of drug precipita- mediate attention in order to prevent the loss of vision [3-5]. tion may result in undesirable effects [12]. A non-invasive Posterior segment eye diseases present unique anatomical, topical drug delivery system in the form of eye drops would physiological and biochemical barriers resulting in the fail- circumvent most of these problems and enhance patient ure of conventional dosage forms such as eye drops, oint- compliance. Administration of drugs in the form of eye ments and suspensions. The retina and choroid are the target drops for retinal diseases has several advantages; it allows sites for most of the posterior segment diseases. Following self-administration, localized therapeutic effect, a non- topical administration, static barriers (corneal layers, blood invasive and painless mode of drug administration, and high aqueous and blood-retinal barriers) and dynamic barriers patient compliance. (choroidal and conjunctival blood flow, lymphatic clearance, and tear dilution) prevent the drug from reaching the retina. New treatment modalities to facilitate the topical delivery Only about 1/100,000th of the drug observed in tear fluids of drugs are under development. There are currently no marketed topical formulations for posterior segment diseases, with only a few compounds that are undergoing testing in *Address correspondence to this author at the Department of Pharmacy clinical trials. Topical eye drops have the potential to reduce Practice, The University of Toledo HSC, College of Pharmacy and Pharma- ceutical Sciences, 3000 Arlington Ave. (MS1013), Toledo, OH 43614; Tel: the side-effects and improve patients’ quality of life and pa- 419-383-1959; Fax: 419-383-1950; E-mail: [email protected] tient compliance.

Credit: National Eye Institute, National Institutes of Health Ref #: NEA09 Fig. (1). Structure of the eye.

Pharmacokinetic studies have proven repeatedly that the The tear volume of a normal eye is ~7–9 μL, with a turnover transfer of a drug from an eye drop into the retina or vitreous rate of 0.5–2.2 μL/min [22]. Upon administration of drugs in humor is limited. This is mainly because of the long diffu- the form of an eye drop (~35–56 μL), the tear volume in- sional distance from the application site to the retina and the creases, resulting in a rise of blinking reflex rate. The excess above mentioned dynamic barriers [13]. volume of eye drops is drained into the systemic circulation through the nasolacrimal duct [26]. Conjunctival blood and However, during the last decade, some investigators have lymphatic vessels also result in the systemic drainage of documented the ability of small molecules to achieve thera- topically applied drugs. Though not completely true, drug peutic concentrations in the retina/vitreous humor following absorption through the conjunctiva is considered nonproduc- topical application [14-19]. Drug delivery to the back of the tive because of the systemic drainage. The nonkeratinized eye has become the topic of intense research among oph- squamous epithelium of the cornea and the conjunctiva pro- thalmologists and pharmaceutical scientists due to its ability tect the eye against pathogens and drugs [27]. The expres- to reduce unwanted complications resulting from intravitreal sion of efflux pumps such as P-glycoprotein (P-gp) on the injections and to enhance patient compliance. The previous corneal [28, 29] and conjunctival epithelia [30] also hinder notion that drugs applied topically do not penetrate to the the absorption of drugs. These factors collectively lower the back of the eye is currently being reassessed. This article absorption of topically applied drugs from the precorneal presents a thorough review of the findings, both research region [31]. publications and patents, related to the use of eye drops in retinal delivery. Primarily, the penetration of drugs into the cornea and conjunctiva is driven by the concentration gradient, lipophil- OBSTACLES TO THE RETINAL DELIVERY OF icity and molecular weight of the drug. The epithelial layers DRUGS VIA TOPICAL EYE DROPS of the cornea and conjunctiva act as rate-limiting barriers for drug absorption. Depending on the lipophilicity, the drug The treatment of back-of-the-eye diseases is relatively will enter the conjunctival or corneal epithelium through the more complex and challenging compared to anterior- paracellular and transcellular routes. The hydrophilic drugs segment diseases [12, 20]. In addition to the longer diffu- such as atenolol and inulin enter the epithelial layers via the sional distance, the path of a topically administered drug is paracellular route, while lipophilic drugs such as timolol and hindered by many components such as the corneal epithe- propranolol enter through the transcellular pathway [32, 33]. lium and endothelium, conjunctiva, sclera and the acellular The intercellular space of corneal and conjunctival epithelia nature of the vitreous [21]. Because of these barriers, the is sealed by the junctional complexes that hinder the trans- development of topical ocular formulations for retinal dis- port of hydrophilic compounds [34]. The rate of paracellular th eases has become unpredictable. Less than 1/100,000 quan- penetration decreases with an increase in molecular size. The tity of drug administered topically reach the retina, and often conjunctiva is 15-25 times more permeable to hydrophilic times this value is far below the therapeutic concentration of compounds compared to the cornea. This is mainly because the drug. These challenges are well understood and docu- of the larger paracellular pore diameter of the conjunctiva mented in several references [1, 12, 22-25]. The anatomical (3.0 nm ± 1.6), which allows the permeation of molecules and physiological barriers that prevent the topically adminis- with a size ranging between 5-10kDa . The paracellular pore tered drug from reaching the retina are shown in Fig. (2). diameter of the corneal epithelium is 2.0 nm ± 0.2, and hence Drug Delivery to the Back of the Eye Following Topical Administration Recent Patents on Drug Delivery & Formulation, 2014, Vol. 8, No. 1 29

Topical delivery

Drug wash away Nasolacrimal due to tear turnover Precorneal area drainage system

Conjunctiva Lateral Cornea diffusion Sclera

Aqueous chamber Systemic Choroid circulation Lens/iris

Retina/vitreous humor

Fig. (2). General ocular penetration routes for topically applied drugs. Black arrows-Transcorneal pathway; Dotted arrows = Transconjuncti- val/transscleral pathway; Empty arrows = Systemic return pathway; Ash gray arrows = Lateral diffusion (Transcorneal diffusion and Uvea- scleral pathway). Modified from Hughes et al. (2005). it allows the paracellular permeation of molecules with size posterior segment delivery. The same anatomical and < 500Da [34]. Though the conjunctiva covers approximately physiological properties that effectively protect the eye hin- 8% of the ocular surface, its pore density is 16 times higher der the efficient absorption of drugs. Despite extremely low than the cornea’s, and the total paracellular space in the retinal bioavailability, topical delivery for retinal diseases is conjunctiva is approximately 230 times greater than in the quite appealing due to its minimally invasive nature. Of late, cornea [35]. These properties of the conjunctival epithe- several research groups have reported the delivery of thera- lium have attracted researchers worldwide and have re- peutic concentrations of drugs to the back of the eye follow- sulted in studying the importance of the noncorneal absorp- ing topical administration. Non-corneal absorption plays a tion pathway for retinal delivery following topical admini- significant role in the absorption of hydrophilic, drugs which stration. The penetration of drugs through the tight paracel- are poorly absorbed across the cornea. lular junctions can be enhanced by the addition of chelating Less than 5-10% of the drug that overcomes the precor- agents such as EDTA and permeation enhancers such as neal clearance mechanisms enters the eye, primarily via the polyoxyethylene-20-stearyl ether [15, 32]. Regardless of the cornea [16] or the conjunctiva [37]. Following corneal ab- higher permeability of an isolated conjunctiva, drugs enter- sorption, the drug enters the anterior chamber and distributes ing through this route are rapidly cleared in a live animal due to surrounding ocular tissues such as the lens, iris-ciliary to the presence of blood and lymphatic circulation [36]. A fraction of the drug escaping conjunctival barriers will per- body, vitreous and posterior retina [16]. Studies also indicate meate through the sclera, which is then challenged by the that drugs penetrating through the cornea diffuse laterally choroidal circulation and the retinal pigment epithelium, a into the sclera and distribute among other intraocular tissues monolayer of cells with tight junctions (outer blood–retinal [38]. The penetration of drugs to the posterior tissues follow- barrier), before reaching the neural retina. The next section ing non-corneal or conjunctival absorption can occur in three discusses the fate of the small quantity of drugs which enters ways: a) diffusion of the drug through the conjunctiva, the eye after resisting the precorneal clearance mechanisms. sclera, choroid and reaching the retina, b) clearance of the drug by the conjunctival blood vessels into the systemic circulation and re-entering the eye from systemic circulation, GENERAL PATHWAYS FOR RETINAL PENETRA- and c) lateral diffusion of the drug from the conjunctiva into TION FROM AN EYE DROP the cornea or iris/ciliary body, and hence into the anterior The topical route of delivery is emerging as an alternative chamber and other intraocular tissues Fig. (2) [38-40]. Most and patient-friendly route for treating vitreoretinal diseases. of the early studies identifying the role of corneal and non- Topical administration remained the preferred route for ante- corneal routes were carried out by Ahmed and Patton [41]. rior segment diseases, and eye drops constitute more than The authors have used an in vivo mechanical blocking tech- 90% marketed ophthalmic medications. However, this well- nique that allowed the restricted drug access to the cornea established and preferred route poses considerable hurdles to and the conjunctiva using timolol and inulin as model drugs 30 Recent Patents on Drug Delivery & Formulation, 2014, Vol. 8, No. 1 Boddu et al.

[41]. These studies concluded that: (a) a predominant frac- that conjunctival absorption is responsible for up to 40% of tion (> 90%) of the drug absorbed through the cornea ends the absorbed amount of inulin in the eye, and both corneal up in the anterior chamber, while drug absorbed through the and non-corneal absorption routes resulted in vitreal concen- conjunctiva bypasses the anterior chamber and distributes trations of inulin and timolol [41]. primarily in the uveal tract and the vitreous humor, and (b) Acheampong et al. [14] studied the distribution of bri- corneal absorption increases with drug lipophilicity, and monidine (2-adrenergic agonist) into anterior and posterior drugs that are hydrophilic in nature enter the eye via the con- ocular tissues following administration of single or multiple junctival route. Most of the studies published have randomly doses of 0.2 or 0.5% brimonidine tartrate solution in one or reported the retinal concentration of drugs following topical both the eyes of monkeys or to a single eye in rabbits. application; however, the exact properties of drugs such as [14C]brimonidine was found to be rapidly absorbed in the charge, molecular weight, and lipophilicity that result in en- cornea and conjunctiva, and distributed throughout the eye, hanced ocular penetrance and therapeutic concentrations in and the vitreous humor concentration of brimonidine was posterior tissues remains unclear. As a matter of fact, nu- found to be 82 ± 45 nM. Similar results were observed in merous studies reported in the literature failed to identify rabbits, confirming the ability of brimonidine to reach the retinal drug concentrations following topical administration back of the eye at nanomolar concentrations relevant in neu- [42]. roprotection models.

POSTERIOR PENETRATION OF TOPICALLY IN- A study by Osborne et al. [17] concluded that topically STILLED DRUGS applied Betoptic(R) (0.5% betaxolol) in rabbit or rat eyes reach the retina and can counteract the detrimental effects This section provides an overview of the therapeutic de- caused by ischaemia/reperfusion or N -methyl- d -aspartate velopments based on the peer-reviewed literature, patents (NMDA)-induced insults. In a different study, the same and patent applications. group reported the neuroprotective activity of flunarizine in rabbit and rat retinas [44]. Kent et al. [45] reported the vitre- Research Articles ous levels of brimonidine (< 2 nM concentration) following Despite the efforts of pharmaceutical scientists world- topical application of Purite 0.15% (b.i.d. or t.i.d.) for 2 wide, drug delivery to the back of the eye remains challeng- weeks in patients scheduled for pars plana vitrectomy. ing due to the unfavorable anatomy, physiology and bio- The penetration into the ipsilateral posterior retina– chemistry of the eye. Much of the current research is directed choroid of nipradilol, an 1, -blocker with a nitric oxide towards the development of effective drugs that penetrate donative action, was studied in rabbits, and its effect on N- into the posterior tissues following topical administration. methyl-D-aspartate (NMDA)–induced retinal damage was There has been a limited number of studies published in the studied in rats. [14C]-nipradilol (1%, 100μL, 1.5 MBq [41 literature supporting the penetration of drugs into the vitre- μCi]/dose) was topically instilled twice daily for a week and ous and retina after topical instillation in rabbits [18]. How- resulted in effective concentrations of nipradilol in the poste- ever, the mechanism of penetration still remains contentious. rior segment (vitreous: 4.3 ± 0.4 and retina-choroid: 318.6 ± Some studies claim the direct penetration of drugs via cor- 42.9 ng.equivalent/gm). Also, nipradilol effectively sup- neal or non-corneal pathways, while several others support pressed the NMDA-induced retinal damage in rats [46]. The the drug being absorbed into the blood stream across the same group later studied the route of penetration of topically conjunctiva or naso-lacrimal duct and then penetrating back instilled nipradilol into the ipsilateral posterior retina [47]. into the ocular tissues of both eyes by crossing the blood- retinal barrier. In this section, we have attempted to summa- Palanki et al. [48] have developed and tested several po- rize the research articles supporting the retinal penetration of tent benzotriazine inhibitors for targeting VEGFr2, Src, and drugs following topical administration. YES kinases. These ester analogs exhibited excellent ocular pharmacokinetics and poor systemic circulation and showed Ahmed and Patton [43] studied the ocular distribution good efficacy in the laser-induced choroidal neovasculariza- and disposition of topically applied timolol and inulin fol- tion model following topical administration. The ocular dis- lowing corneal and non-corneal absorption in New Zealand tribution and excretion of tritium-labeled difluprednate Albino male rabbits. For timolol, in the presence of corneal ((3)H-DFBA) ophthalmic emulsion 0.05% after single or access, the rank-order in terms of concentration was conjunc- repeated topical instillation was studied in pigmented rabbit tiva> cornea>sclera> iris-ciliary body >aqueous humor> lens eyes. (3)H-DFBAconcentrations were obsereved in various > vitreous humor, from greatest to lowest concentration. anterior segment and posterior segment tissues, including With corneal access blocked, the aqueous humor and corneal anterior retina/choroid (273 ng.eq/g) and posterior ret- drug levels dropped drastically, and concentrations in in- ina/choroid (59 ng.eq/g), suggesting the effectiveness of traocular tissues such as iris-ciliary body, lens and vitreous topical therapy in posterior segment diseases [49]. were low. The concentration of timolol in the sclera and conjunctiva remained similar for the corneal and non-corneal Tissue distribution and epithelial penetration of radiola- routes. A similar pattern was observed with inulin. This belled moxaverine, a papaverine–hydrochloride derivative, study demonstrated the absorption of drugs into posterior were studied in rabbits following systemic and topical ad- tissues via the corneal and non-corneal routes. The same ministration. High concentrations of moxaverine were found group also evaluated the ocular distribution and disposition in the cornea, conjunctiva (anterior part) and retina (posterior of topically applied timolol and inulin following the corneal part) with topical administration. Low plasma levels of mox- and non-corneal absorption with time. The authors identified averine were attributed to the lipophilic nature of moxaver- Drug Delivery to the Back of the Eye Following Topical Administration Recent Patents on Drug Delivery & Formulation, 2014, Vol. 8, No. 1 31 ine [50]. Similarly, Baklayan et al. studied the ocular distri- rior retina, anterior vitreous, and optic nerve was 2-, 7-, 2.6-, bution of 14C-bromfenac in rabbits following topical appli- 1.4-, 1.9-, 1.2-, and 9-fold higher than those of brinzolamide. cation. Peak concentrations in the aqueous humor and most Cmax of dorzolamide was 2-5 times higher than brinzola- ocular tissues including cornea, conjunctiva and sclera were mide’s. A similar trend was observed after multiple dosing, observed in the first 2 hours. However, a much lower con- however; statistically higher concentrations of dorzolamide centration was found in the posterior segment (retina) [51]. were only obtained in the aqueous humor, vitreous humor, and optic nerve [56]. The log D values of dorzolamide and Eye drop formulations consisting of cyclodextrin-drug brinzolamide at pH 7.4 are 1.72 and 6.6, respectively. For complexes are widely studied for retinal delivery following corneal penetration, the optimum log D value should range topical application. The relative efficiencies of topical and between 2.0 and 3.0. The higher concentrations of dorzola- systemic absorption of dexamethasone-cyclodextrin complex were examined in rabbits. The systemic absorption following mide in various ocular tissues can be attributed to the better penetration and higher concentration-dependent flux of the topical application to the eye was compared with intravenous drug across the cornea. In a different study, Jansook et al. and intranasal administrations. With topical administration, developed dorzolamide self-assembled microparticle suspen- dexamethasone concentration in the retina of treated rabbits sion using 18%w/v -cyclodextrin and 0.5%w/v of hydroxyl was found to be 33 ± 7 ng/g, while 14 ± 3 ng/g was observed propyl methyl cellulose. Maximum drug concentration in in the control eye. Topical, intranasal and intravenous routes resulted in similar systemic levels. The authors concluded aqueous humor was reached in 4 hours, along with its maximum availability in the posterior segment [57]. that the topical absorption plays a significant role in deliver- ing dexamethasone to the retina. Approximately 60% of the The ocular bioavailability and retinal concentrations of dexamethasone concentration in the retina resulted from hesperidin and hesperetin were compared following intrave- topical administration, with the rest coming from systemic nous and topical administration in rabbits. Hesperidin is an administration [52]. aglycone form of hesperetin. Intravenous administration failed to demonstrate any detectable levels of hesperidin/ Loftsson et al. [53] studied the retinal penetration of dex- hesperetin in ocular tissues, while topical administration amethasone upon topical application of dexametha- produced significant concentrations of hesperidin/hesperetin sone/cyclodextrin microparticles with a mean particle di- in ocular tissues 1 and 3 hours postdosing, with hesperetin ameter of 20.4±10.3 m. After 2 hours, the vitreal concentra- concentrations higher than hesperidin concentrations. Fur- tion of dexamethasone was found to be 29 ± 16 ng/g, with 86% of drug reaching the vitreous via direct penetration. The thermore, benzalkonium chloride improved the hesperetin levels in the posterior section of the eye [58]. In situ gelling retinal concentration was found to be 57 ± 22 ng/g (49% via polymers such as gellan gum also play a significant role in direct penetration). Microparticles made of randomly methy- enhancing the permeation of drugs into the retina. The ocular lated b-cyclodextrin also resulted in similar vitreal (22.6 ± 9 pharmacokinetics and tissue distribution of aesculin, a cou- ng/g) and retinal (66 ± 49 ng/g) concentrations. The same marin glucoside, after topical administration in rabbit eye, group also studied the safety and efficacy of cyclodextrin microparticles in nineteen diabetic macular edema (DME) were found to be significantly higher in the presence of deacetylated gellan gum. This is due to longer precorneal patients. Dexamethasone-cyclodextrin microparticles in the contact times of in situ gels compared to conventional eye form of eye drops were administered three or six times/day drops [59]. for 4 weeks and then observed for 4 weeks without treatment. The eye drop formulation was well tolerated and an Nanonosystems, such as nanoparticles, mixed micelles, improvement in the visual acuity was observed, with a de- animations, nano suspensions, liposomes, and dendrimers, crease in central macular thickness [54]. are slowly making their presence felt in the complex area of ocular drug delivery. Nanocarriers have the potential to im- The physicochemical properties of drug substances such prove the efficacy and ocular bioavailability of drugs by as lipophilicity, water solubility, and molecular size play an overcoming diffusion barriers. Recently, Velagaleti et al. important role in crossing the eye’s biological barriers and [60] have developed nanomicellar formulations of vitamin E reaching the retina. For example, dorzolamide (324 Da) and brinzolamide (383 Da) are carbonic anhydrase inhibitors TPGS and octoxynol-40 for voclosporin, a next generation calcineurin inhibitor. Ocular distribution studies following used in the treatment of primary open angle glaucoma. Dor- topical application of nanomicellar formulations in rabbits zolamide and brinzolamide have similar molecular weights; revealed a significantly higher concentration of voclosporin however, they differ in lipophilicity and water solubility. in the posterior segment tissues such as retina and choroid, Topical application of dorzolamide resulted in a significant with vitreous humor concentration below detectable limits. improvement in blood flow to the retina and optic nerve head compared to brinzolamide [55]. The variation in the pharma- The same mixed nanomicellar formulation also resulted in therapeutic concentrations of dexamethasone in the rabbit cological effect is mainly due to the ability of dorzolamide to choroid/retina (~50 ng/g tissue) following topical application better penetrate the ocular barriers and reach the posterior [61]. However, the mechanism by which nanomicelles de- segment of the eye in a higher concentration. The ocular liver the drug to the retina following topical administration is pharmacokinetics of Trusopt (dorzolamide hydrochloride not well understood. As stated by Mitra, entrapping the hy- ophthalmic solution, 2%) and Azopt (brinzolamide ophthalmic suspension, 1%) were compared in rabbits upon single drophobic drug within the hydrophilic mixed micelles helps the drug to evade the clearance mechanisms of the eye. and multiple topical dosing. After a single dose, the area under the curve (AUC0-24h) for dorzolamide in the aqueous More recently, there has been a surge in the use of large humor, anterior sclera, posterior sclera, anterior retina, poste- protein and peptide molecules in treating retinal diseases. 32 Recent Patents on Drug Delivery & Formulation, 2014, Vol. 8, No. 1 Boddu et al.

Table 1. List of drugs that were successfully delivered to the back of the eye following topical application.

Drug/ formulation Animal model Result Reference

Brimonidine eye drops Monkey and rabbits Vitreous humor concentration of brimonidine in monkeys was found to [14] be 82 ± 45 nM. Similar results were observed in rabbits, confirming the ability of brimonidine to reach the back of the eye following topical application

Betoptic (R) (0.5% betaxolol) Rabbit Betaxolol was observed in the retina and could counteract the detri- [17] mental effects caused by ischemia/reperfusion

Flunarizine Rabbit and rat retina Flunarizine was detected in the retina after topical administration [44]

Dexamethasone eye drops Rabbit Approximately 60% of the dexamethasone concentration in the retina [52] resulted from topical administration, with the rest coming from systemic administration

Dexamethasone/ Humans Dexamethasone/cyclodextrin complex was well tolerated and an im- [53] cyclodextrin complex provement in the visual acuity with a decrease in central macular thickness was observed

Trusopt (dorzolamide hydrochloride oph- Rabbits Higher concentrations of dorzolamide were obtained in the aqueous [56] thalmic solution, 2%) and Azopt (brinzo- humor, vitreous humor, and optic nerve compared to brinzolamide lamide ophthalmic suspension, 1%)

Hesperidin and hesperetin Rabbits Benzalkonium chloride improved the hesperetin levels in the posterior [58] section of eye

Aesculin in situ gel Rabbit Aesculin concentration was found to be significantly higher in the [59] presence of deacetylated gellan gum in situ gel

Voclosporin nanomicellar Rabbit Higher concentration of voclosporin was detected in the posterior [60] segment tissues (retina and choroid)

Dexamethasone nanomicellar Rabbit Dexamethasone was detected in the choroid / retina (~50 ng/g tissue) [61]

scFv, antibody fragment (~28 kDa) Rabbit scFv was detectable in the vitreous of rabbit eyes after 4-12 h and use [62] of sodium caprate enhances its permeability

However, they fail to penetrate the inner eye following topi- The studies discussed above clearly demonstrate the abil- cal application, largely due to their large molecular weight of ity of topically administered eye drop formulations to deliver 50-150 kDa. Use of penetration enhancers (e.g. sodium ca- drugs to the retina and exert pharmacological effects. The prate) aided in the permeation of topically applied antibody above-mentioned studies have been summarized in Table 1. fragments such as scFv, a protein of ~28 kDa [62]. However, It is noteworthy that most of these studies have suggested the use of penetration enhancers to enhance the transcorneal drug absorption via the transconjunctival/transcleral route transport have resulted in ocular toxicity and loss in the in- (conjunctiva sclera choroid retina). tegrity of inter-epithelial tight junctions. Currently, ophthalmologists are left with no options other than administering PATENTS ON THE USE OF EYE DROPS FOR RETI- protein molecules via intravitreal injections, which are NAL DISEASES highly invasive and associated with patient non-compliance. Topical delivery of antibody-based therapeutics would Patents related to the use of topical eye drop formulations minimize the side-effects resulting from intravitreal injec- for retinal diseases are limited. Unlike the peer-reviewed tions, and this would be a major innovation in ophthalmol- research articles, patents and patent applications disclose the ogy [63]. Furrer et al. studied the pharmacokinetics of intentions of leading pharmaceutical companies in conveying ESBA105 (26 kDa), a potent scFv directed against tumor ocular drugs to the posterior segment and how these systems necrosis factor. Pharmacokinetics following topical and in- could be commercialized. travenous administration were compared. Systemic exposure In 2003, Matier et al. (US 7,442,711) filed a patent on the after topical administration is 25000-fold lower than the in- use of disubstituted hydroxylamine small molecules in order travenous administration, whereas exposure to vitreous hu- to reduce the oxidative stress and inflammation involved in mor (half life 16-24h) is higher via topical route than through age-related macular degeneration and retinal disorders. This intravenous injection. ESBA105 is absorbed and distributed patent describes several new chemical entities, including to all compartments of the eye with low systemic drug expo- OT-551 (1-hydroxy-4-cyclopropanecarbonyloxy-2,2,6,6- sure [64]. tetramethylpiperidine hydrochloride), a topically dosed small Drug Delivery to the Back of the Eye Following Topical Administration Recent Patents on Drug Delivery & Formulation, 2014, Vol. 8, No. 1 33 molecule that inhibits oxidative stress and disease-induced the drug substance used in the eye drops should be highly inflammation. OT-551 is converted into TEMPOL-H (1,4- lipophilic, with solubility of less than 0.1% at physiological dihydroxy-2,2,6,6-tetramethylpiperidine) in the body by ocu- pH; (2) the pKa of the drug substance should be close to lar and systemic esterases [65, 66]. TEMPOL-H is known to physiological pH; (3) the salt form of the drug substance in suppresses photo-oxidative cell damage in lipofuscin fluoro- eye drops should have higher solubility than that of the free phore-laden RPE cells in vitro [67]. However, OT-551 has base and hydrochloride salt crystals; (4) the effective drug shown greater protection of the RPE cell layer morphology concentration in the composition should be preferably compared to TEMPOL-H at equivalent doses. More recently, greater than 0.25%; (5) the final formulation should be non- Othera Pharmaceuticals conducted a single-center, open- stinging, with a pH between 4.0-7.0; and (6) the pKa of the label phase II trial of OT-551 eye drops in 10 patients with buffer used should be outside of the formulation pH range bilateral geographic atrophy associated with age-related and the physiological pH of 7.4. These generalized rules macular degeneration. The eye drop formulation of OT-551 were based on findings that the citrate form of tandospirone (0.45%w/v), administered 3 times a day, was able to main- stings upon administration into the eye, and the stinging sen- tain visual acuity with no apparent serious adverse effects, sation is aggravated with the acidic formulation pH (pH < 5). but this deserves further substantiation. However, contrasting Based on the fact that tandospirone has two pKas, 2.17 and results were obtained in a concurrent phase II clinical trial 7.54, its solubility dramatically increases as pH decreases (randomized, parallel-group, placebo-controlled trial) involv- below its pKa of 7.54. Hence, the inventors concluded that ing 137 participants. No significant benefits were observed tandospirone in a free base or as a hydrochloride salt is more in geographic atrophyareal progression between treated and comfortable than a composition containing its citrate salt placebo groups after 18 months of treatment. The failure to when administered topically. Promising treatment results replicate the visual acuity effect in a larger population has were obtained in rat photo-oxidative induced retinopathy been attributed to the current strength of the eye drop formu- model [72]. lation and mode of delivery [68]. Nanocarriers have the potential to improve the solubility Cooke and colleagues (US 20070167526 A1) disclosed a of drugs and increase ocular bioavailability by overcoming patent concerning a topical mecamylamine formulation for diffusion barriers [22, 73]. Recently, Mitra et al., (US ocular administration for the treatment and/or prevention of 2010/0310642 A1) developed a mixed nanomicellar formu- conditions mediated by vascular permeability, neovasculari- lation of water-insoluble drugs such as voclosporin, a potent zation, and abnormal angiogenesis, including conditions like calcineurin inhibitor, for topical delivery. The formulation proliferative retinopathies and macular edema [69]. Feinstein mainly includes drug-loaded (0.01-1%) nanomicelles in a et al. (US 8168607 B2) disclosed novel siRNA molecules physiological acceptable buffer (pH of 5.0 to 8.0). Nanomi- that inhibit the RTP801 gene inhibition for treating micelles were prepared with vitamin E TPGS (3.0-5.0% w/v) crovascular disorders, eye diseases and respiratory condi- and stabilized with octoxynol-40 (1.0%-3.0% w/v). Vitamin tions. The compounds disclosed in this patent can be admin- E TPGS is an amphipathic non-ionic surfactant with an HLB istered either intravitreally or topically for treating various index of 13, and octoxynol-40 has an HLB index of 18. Ex- retinal diseases [70]. amples of rapamycin nanomicelles and dexamethasone nanomicelles were also included. Administration of a single Alpha adrenergic agonists, or beta-blockers are known to dose of eye drop formulation containing dexamethasone in enhance the transfer of drugs to the choriod-retina upon topi- rabbits resulted in therapeutic drug concentrations of 30-50 cal administration. This has been attributed to drug absorp- ng/g in the posterior choroid and retina. The measured con- tion via the transconjunctival/transcleral route (conjunctiva sclera choroid retina) involving a two-step centration of voclosporin was ~50 ng/g, which is substan- tially higher than the required therapeutic concentration of mechanism. First is the confinement of drugs in the orbit due 30 eq ng/g [74]. Kompella et al. (US 20090087494) patented to their vasoconstricting properties. Second, the oncotic compositions of nanocarrier systems made of poly(lactide- pressure helps the movement of drug to the choroid. Rekik co-glycolide), poly(lactide), poly -caprolactone, albumin, (WO 2009112878) has investigated the delivery of several and chitosan for targeted delivery of anti-VEGF agents to the drugs (dexamethasone, indomethacin, bevacizumab and ramipril), in combination with alpha adrenergic agonists and eye. Further, nanocarrier systems were surface-modified for targeting specific receptors and increasing the overall uptake -blockers, to the posterior segment of the eye. This study via endocytosis. The free carboxylate groups on the nanopar- was conducted on six patients with non-proliferative diabetic ticle surface were modified in order to target LHRH and retinopathy or proliferative diabetic retinopathy. After 2-3 transferrin receptors. The expression of LHRH receptor months of treatment, all patients showed a decrease in macu- (LHRH-R), transferrin receptor 1 (TfR1) and transferrin re- lar edema and an improvement in visual activity. Similar results were observed in patients with retinal vein occlusion, ceptor 2 (TfR2) was confirmed by Western blot and real- time polymerase chain reaction on bovine conjunctiva and age related macular degeneration (ARMD), and glaucoma corneal epithelia. The nanocarrier systems were loaded with neuropathy [71]. therapeutic agents such as budesonide, celecoxib, and US 20100160342, issued to Chowhan et al., describes a Flt23K plasmid and tested in a newborn mouse animal topical delivery system to deliver tandospirone for treating model of neovascularization or a combination of any or all of retinal disorders. This patent mentions a generalized set of the foregoing agents. Animals treated with surface-modified pharmaceutical requirements for ophthalmic eye drops for nanoparticles (both deslorelin and LHRH analog) showed a achieving unexpectedly high drug levels in the retinal tis- significant decrease in neovascular nuclei due to a decrease sues, mainly for hydrophobic drugs. These include that: (1) in the activity of VEGF in the retina [75]. 34 Recent Patents on Drug Delivery & Formulation, 2014, Vol. 8, No. 1 Boddu et al.

Cyclodextrins are also widely used in ophthalmic formu- CONFLICT OF INTEREST lations for increasing solubility and permeability of drugs. The authors confirm that this article content has no con- Cyclodextrins have been exploited for back-of-the-eye deliv- flicts of interest. ery following topical application. Thorsteinn and Einar [76] proposed a suspension comprised of dexamethasone and cyclodextrin complex for both anterior and posterior dis- ACKNOWLEDGEMENTS eases. Ocular bioavailability in rabbits after topical admini- The author would like to thank Charisse Montgomery for stration was found to be 60%. An aqueous isotonic solution her assistance in preparation of this manuscript. This work containing dexamethasone (0.5%), randomly methylated - was supported by the research start-up funds from The Uni- cyclodextrin (5.3%), benzalkonium chloride (0.02%), EDTA versity of Toledo. 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