Drug transport by the blood-aqueous humor barrier of...

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Title Page

Drug transport by the blood-aqueous humor barrier of the eye

Jonghwa Lee and Ryan M. Pelis

Department of Pharmacology, Dalhousie University, Halifax, Nova Scotia, Canada

This article has not been copyedited and formatted. The final version may differ from this version.DMD Fast Forward. Published on February 19, 2016 as DOI: 10.1124/dmd.116.069369

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Running Title Page

Running Title: Drug transport by the blood-aqueous humor barrier

Corresponding Author:

Ryan M. Pelis, Ph.D.

Department of Pharmacology

Dalhousie University

Sir Charles Tupper Medical Building

5850 College Street, PO Box 15000

Halifax, Nova Scotia, Canada B3H 4R2

#Text pages: 16

#Tables: 2

#Figures: 2

#References: 65

#Words in Abstract: 240

Abbreviations: Apical sodium dependent bile salt transporter, ASBT; Breast cancer resistant

protein, BCRP; Multidrug and toxin extrusion transporter, MATE; Multidrug-resistance

associated protein, MRP; Organic anion transporter (OAT); Organic anion transporting

polypeptide, OATP; Organic cation transporter, OCT; Organic cation transporter novel, OCTN;

Peptide transporter, PEPT; P-glycoprotein, Pgp; Sodium dicarboxylate cotransporter, NaDC;

Sodium taurocholate cotransporter, NTCP


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Abstract

The ocular barriers (cornea, blood-retinal barrier and blood-aqueous humor barrier) make

treating eye diseases with therapeutic drugs challenging. The tight capillary endothelium of the

iris and the ciliary body epithelium form the blood-aqueous humor barrier. The iris and ciliary

body (iris-ciliary body) express a variety of drug transporters in the ATP-binding cassette (ABC)

and solute carrier (SLC) families. Drug transporters in the ABC family that are present in the

iris-ciliary body include P-glycoprotein (Pgp), breast cancer resistant protein (BCRP) and several

multidrug resistance-associated proteins (MRPs). Drug transporters in the SLC family that are

present include organic anion transporters (OATs), organic anion transporting polypeptides

(OATPs), bile acid transporters (ASBT and NTCP), organic cation transporters (OCTs, OCTNs

and MATEs) and peptide transporters (PEPTs). Freshly dissected iris-ciliary body preparations

actively accumulate a variety of substrates of SLC drug transporters that are expressed in the

tissue. The ciliary body in vitro supports active transport in the aqueous humor-to-blood

direction of several substrates of OATs and MRPs, consistent with the subcellular localization of

these transporters in the ciliary body epithelium. In vivo data suggest that drug transporters in

the iris-ciliary body reduce the permeation of drugs in the direction of blood-to-aqueous humor,

thereby reducing ocular drug bioavailability, but also, are involved in active drug elimination

from the aqueous humor. A better understanding of the influence on pharmacokinetics of drug

transporters in the blood-aqueous humor barrier should help improve drug delivery and efficacy

in the eye.


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Introduction

The eye is a pharmacological sanctuary, making it challenging to treat many ocular

diseases with therapeutic drugs. This is in part due to the presence of the ocular barriers –

cornea, blood-retinal barrier (inner and outer) and blood-aqueous humor barrier (iris-ciliary

body). The resilience of the eye to drug exposure is evident from the number of ocular drug

delivery methods in development and use. Topical, systemic, periocular and intraocular are the

main routes for drug delivery to the eye (Geroski and Edelhauser, 2000). Therapeutic

concentrations can be achieved with both topical and systemic administration, but often requires

the use of relatively high doses and/or drugs with sufficient hydrophobicity to efficiently cross

the ocular barriers and cornea (Barar, et al., 2008;Gaudana, et al., 2010). Although topical eye

drop administration is the least invasive, only a small fraction of the administered dose reaches

the eye interior (Maurice DM and Mishima S, 1984). Systemic administration can be an

effective delivery method, but the high doses often needed to achieve effective intraocular

concentrations can cause systemic toxicity. For example, intravenous administration of

melphalan can be effective at treating retinoblastoma, but the doses required to achieve a

therapeutic response are associated with severe bone marrow toxicity (Shields and Shields,

2010). Periocular and intraocular administration are used to avoid poor drug penetration across

the ocular barriers. However, once inside the elimination rate of select drugs is rapid, often

necessitating the use of high doses, repeated administration or extended release systems to

counteract short drug half-lives (Geroski and Edelhauser, 2000;Yasukawa, et al., 2005). The

poor intraocular penetration of drugs administered outside of the eye and their rapid elimination

once inside suggests that active mechanisms contribute, along with passive processes (passive

diffusion across epithelia and aqueous outflow), to poor ocular drug delivery. Indeed, there is


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considerable evidence that drug transporters expressed in the blood-aqueous humor barrier,

blood-retinal barrier and cornea function to reduce the intraocular bioavailability of systemically

and topically administered drugs, and can facilitate their elimination from both the aqueous and

vitreous humor. Several previous reviews have discussed in detail drug transport by the blood-

retinal barrier and cornea (Mannermaa, et al., 2006;Nakano, et al., 2014;Tomi and Hosoya,

2010;Chemuturi and Yanez, 2013;Jordan and Ruiz-Moreno, 2013;Hosoya and Tachikawa,

2009), whereas little attention has been given to the blood-aqueous humor barrier (i.e., iris-

ciliary body).

Although there are numerous drug transporters that interact with organic electrolytes that

display a net negative charge (organic anions), a net positive charge (organic cations), or both

negative and positive charges (zwitterions) at physiological pH, most of our understanding of

drug transport by the iris-ciliary body come from studies using organic anions as ligands – i.e.,

substrates and/or inhibitors. Research in the 1960’s and 1970’s by Becker (Becker, 1960),

Forbes and Becker (Forbes and Becker, 1960), and Bárány (Barany, 1972;Barany,

1973b;Barany, 1973a;Barany, 1974;Barany, 1975;Barany, 1976) show that organic anion

transport by the iris-ciliary body has properties similar to those found in liver and kidney – the

two major excretory organs for drugs, and determinants of systemic pharmacokinetics. Even

back then, before the molecular identity of drug transporters were known, it was speculated that

these active transport mechanisms in the iris-ciliary body were important for ‘scavenging’ from

the aqueous and vitreous humor potential xenobiotic and endobiotic toxins that could be

deleterious to vision (Barany, 1976). This review covers 1) the tissue-, cellular and subcellular

expression of drug transporters in the iris-ciliary body, 2) in vitro evidence for drug transport

activity in the iris-ciliary body (with a focus on organic anions), and 3) in vivo evidence for the


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involvement of active transport in influencing intraocular drug concentrations, with a likely

contribution from the blood-aqueous humor barrier.

Drug transporters

Drug transporters support flux across plasma membranes of relatively small (<1000 mol

wt) organic molecules (organic anions, cations and zwitterions) that are structurally diverse.

Included within this class of chemicals are pharmacologically and toxicologically relevant

xenobiotics along with endogenous chemicals of physiological importance (endobiotics)

(Klaassen and Aleksunes, 2010). Due to their charge at physiological pH, efficient movement of

organic electrolytes across plasma membranes requires facilitated transport. Drug transporters

are expressed in a variety of barrier epithelia/endothelia, such as the intestinal epithelium, liver

hepatocytes, kidney tubules and brain capillary endothelium, where they influence tissue drug

concentrations, i.e., pharmacokinetics (Klaassen and Aleksunes, 2010). Drug transporters are

contained within two distinct families, the solute carrier (SLC) family and the ATP-binding

cassette (ABC) family. Transporters in the ABC family hydrolyze ATP to facilitate efflux of

their substrates out of cells. The multidrug resistance associated proteins (MRPs), breast cancer

resistant protein (BCRP) and P-glycoprotein (Pgp) are examples of drug transporters contained

within the ABC family. SLC drug transporters use a variety of energetic mechanisms to support

solute flux, including Na+-dependent co-transport, exchange and electrogenic facilitated

diffusion, and can facilitate cellular uptake or efflux depending on the electrochemical driving

forces. However, most of the SLC drug transporters discussed here support cellular

accumulation of their substrates in other tissues – the MATEs are an exception. Notable

examples of SLC drug transporters are the organic anion transporting polypeptides (OATPs),

organic anion transporters (OATs), organic cation transporters (OCTs) and multidrug and toxin


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extrusion transporters (MATEs) – although there are others. A key feature of drug transporters is

their broad and sometimes overlapping ligand selectivity. SLC and ABC drug transporters likely

evolved with increasing physiological complexity to provide protection from exposure to an

increasing diversity of xenobiotics (Eraly, et al., 2004). Indeed, many of the SLC and ABC drug

transporters have arisen by gene duplication and display unique but also overlapping ligand

selectivity (Eraly, et al., 2004;Moitra and Dean, 2011). Within the ocular barriers drug

transporters likely function to protect tissues that are important for vision from exposure to

potentially toxic xenobiotics and endobiotics, especially the avascular lens epithelium and

cornea. Importantly, many of the drug classes used to treat ocular disease contain small

molecule drugs that are substrates of drug transporters, such as antibiotics, antivirals, anti-

inflammatory drugs, α-adrenergic receptor agonists, prostaglandin analogs and antimetabolites.

The blood-aqueous humor barrier

The blood-aqueous humor barrier consists of the ciliary body epithelium and the tight

capillary endothelium of the iris (Cunha-Vaz, 1979). The ciliary body epithelium is responsible

for secretion of aqueous humor into the posterior chamber of the eye. The ciliary body extends

posteriorly from the iris root to the retina, forming a ring around the globe. The ciliary body has

two structurally distinct segments, the pars plicata and pars plana. The pars plicata is located

anteriorly and is characterized by extensive villi-like foldings called ciliary processes. The pars

plana is scalloped and is positioned between the pars plicata and the retina. The ciliary body is

highly vascularized by the choroidal capillaries, which are fenestrated and leaky (Friedenwald

and Stiehler, 1938;Stewart and Tuor, 1994). The iridial capillaries are continuous and contain

tight junctions, but have a higher permeability than inner retinal capillaries (i.e., the inner blood-

retinal barrier) (Alm, 1992). The ciliary body is a bilayer comprised of two distinct cell layers –


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a pigmented cell layer and a non-pigmented cell layer. The pigmented and non-pigmented cells

are coupled via gap junctions (Shin, et al., 1996;Coffey, et al., 2002). The pigmented cells

contact the choroidal blood supply, whereas the basolateral membrane of the non-pigmented

epithelium contacts aqueous humor. Tight junctions are present in the non-pigmented epithelial

cells, but not in the pigmented cells (Sonsino, et al., 2002). Thus, the physical barrier to drug

movement across the ciliary body is the non-pigmented epithelium.

Expression of ABC transporters in iris-ciliary body

Several studies have examined mRNA (RT-PCR, qPCR or microarray) expression of

ABC drug transporters in isolated human iris-ciliary body (Zhang, et al., 2008;Chen, et al.,

2013), or in preparations containing only human iris or ciliary body that were isolated separately

by microdissection (Pelis, et al., 2009;Dahlin, et al., 2013;Lee, et al., 2015) – Table 1

summarizes the findings from these studies. In general, the studies are in good agreement with

each other. The majority of the studies observed mRNA for MRP1, MRP2, MRP3, MRP4,

MRP5, MRP6, MRP7, BCRP and Pgp in either ciliary body or iris-ciliary body preparations,

with only a few discrepancies. For example, Chen et al. (Chen, et al., 2013) failed to observe

mRNA for MRP3 in iris-ciliary body while Zhang et al. (Zhang, et al., 2008) did not detect

MRP2 mRNA in iris-ciliary body. Of the ABC transporter genes examined by Dahlin et al.

(Dahlin, et al., 2013), MRP5 and Pgp (MRP5>Pgp) showed the highest expression level in the

human ciliary body (Dahlin, et al., 2013). At the protein level (immunoblotting), MRP1, MRP2,

MRP3, MRP4, MRP6, MRP7 and Pgp were detected in human iris-ciliary body (Chen, et al.,

2013), and MRP1, MRP2, MRP4, BCRP and Pgp were detected in human ciliary body (Pelis, et

al., 2009) (Figure 1). While Pelis et al. (Pelis, et al., 2009) detected BCRP protein in human

ciliary body from a single donor by immunoblotting, another study failed to detect BCRP protein


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expression in iris-ciliary body from multiple donors (Chen, et al., 2013). MRP2 localizes to the

apical membrane of non-pigmented epithelial cells of the human pars plana and plicata (Pelis, et

al., 2009), whereas MRP4 is located in the basolateral membrane of pigmented cells (Lee, et al.,

2015) (Figure 2). Both transporters are speculated to play a role in effluxing drugs from ciliary

epithelial cells into the choroid. In two independent studies, mRNA for MRP1, MRP3, MRP4,

MRP5, MRP6, MRP7 and Pgp were detected in human iris, whereas BCRP was low to

undetectable (Dahlin, et al., 2013;Lee, et al., 2015) (Table 1). Of the ABC transporter genes

examined in the iris by Dahlin et al. (Dahlin, et al., 2013), Pgp and MRP5 (Pgp>MRP5) were the

most highly expressed. Although Dahlin et al (Dahlin, et al., 2013) detected MRP2 mRNA in

human iris, Lee et al. (Lee, et al., 2015) did not. Within the human iris, protein for MRP1,

MRP2, MRP4 and BCRP were observed (Pelis, et al., 2009) (Figure 1). Pgp has been localized

to iridial vessels by immunohistochemistry (Schlingemann, et al., 1998;Holash and Stewart,

1993) (Figure 2).

Expression of SLC transporters in the iris-ciliary body

The mRNA expression of numerous transporters in the SLC family have been examined

in the iris-ciliary body, ciliary body or iris of humans (Table 2). Most of the SLC transporters

listed are cellular uptake transporters. In some cases there is a discrepancy between the studies

regarding whether mRNA for the transporters are expressed in the ciliary body. Regardless,

from Table 2 it is apparent that both the ciliary body and iris express many of the SLC

transporters known to influence pharmacokinetics, and some transporters with important

physiological roles, such as transporters of bile acid acids (ASBT and NTCP), dicarboxylates

(NaDC3), peptides (PEPT1 and PEPT2), ergothioneine (OCTN1) and carnitine (OCTN2).


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Several of the transporters listed in Table 2 have been examined at the protein level in

human ciliary body by immunoblotting and/or immunohistochemistry. Protein for OATP1A2,

OATP1C1, OATP2B1, OATP3A1 and OATP4A1 were detected in human ciliary body extracts

by immunoblotting (Gao, et al., 2005) (Figure 1). Kraft et al. (Kraft, et al., 2010) also detected

mRNA for OATP2A1 and OATP2B1 by qPCR in human ciliary body as well as iris – albeit, the

expression of OATP2A1 in iris was near undetectable in their study. Gao et al. (Gao, et al.,

2005) demonstrated expression of OATP2B1, OATP3A1 and OATP4A1 protein in the

basolateral membrane of non-pigmented epithelial cells of the pars plicata, while OATP1A2 and

OATP1C1 were undetectable in the pars plicata (Figure 1). At odds with the study by Gao et al.

(Gao, et al., 2005), another study observed expression of OATP2B1 in basolateral membranes of

both non-pigmented cells as well as pigmented cells of the pars plicata (Kraft, et al., 2010)

(Figure 2). Thus, it is not clear based on this discrepancy whether OATP2B1 is present in the

basolateral membrane of pigmented cells of the pars plicata. OATP2A1 localizes to basolateral

membranes of pigmented cells and non-pigmented cells of the pars plicata (Kraft, et al., 2010)

(Figure 2). In the pars plana, OATP1A2 and OATP2B1 are restricted to the basolateral

membrane of non-pigmented epithelial cells, and OATP1C1, OATP3A1 and OATP4A1 are

present in basolateral membranes of non-pigmented epithelial cells as well as pigmented cells

(Gao, et al., 2005) (Figure 2). Both OATP2A1 and OATP2B1 also immunolocalize to iridial

vessels (Kraft, et al., 2010) (Figure 2). Importantly, prostaglandins are well-known substrates of

several OATPs (Hagenbuch and Stieger, 2013), and prostaglandin analogs, such as unoprostone

and latanoprost are anti-glaucoma drugs. The metabolite of unoprostone (unoprostone

carboxylate) is a substrate of OATP1A2, OATP2B1 and OATP4A1 (Gao, et al., 2005), and

latanoprost is a substrate of OATP2B1 and OATP2A1 (Kraft, et al., 2010), suggesting that


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OATPs in the ciliary body may influence the pharmacological action of prostaglandin analogs

used to treat glaucoma.

In addition to several OATPs, protein for the renal organic anion transporters OAT1 and

OAT3 were detected in ciliary body from four different human donors by immunoblotting (Lee,

et al., 2015) (Figure 1). Within the kidney, both OAT1 and OAT3 use energy in the outwardly-

directed α-ketoglutarate (Kreb’s cycle intermediate) gradient to facilitate cellular substrate

uptake via an anion exchange mechanism, and this gradient is maintained in part by the Na-

dependent dicarboxylate cotransporter 3 (NaDC3) (Pelis and Wright, 2011). Accordingly,

NaDC3 protein was detected in ciliary body from four human donors by immunoblotting (Lee, et

al., 2015) (Figure 1). Within the pars plicata of the human ciliary body both OAT1 and OAT3

are present in the basolateral membrane of non-pigmented epithelial cells, along with Na,K-

ATPase (Figure 2) (Lee, et al., 2015). Although the subcellular localization of NaDC3 in ciliary

body is unknown, Nadc3 mRNA is present in non-pigmented epithelial cells of mice, as shown

by in situ hybridization (George, et al., 2004).

While the previous discussion of drug transporter expression in the blood-aqueous humor

barrier comes from studies that primarily used human tissues, in vitro and in vivo evidence

discussed below comes mostly from studies with other species (rabbit, monkey and bovine). It is

important to point out that species differences in drug transport exist. For example, whereas

humans have a single Pgp gene (ABCB1), rodents have two genes corresponding to Pgp

(Abcb1a and Abcb1b, also referred to as Mdr1a and Mdr1b). Regardless, many of the drug

transporter orthologs discussed in this review have ligand selectivities that are conserved across

species – example, Mdr1a and Mdr1b share many of the same ligands that interact with human

Pgp.


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In vitro evidence for drug transporter activity in iris-ciliary body

In vitro data using fresh preparations of iris-ciliary body, or ciliary body devoid of iris

indicate that drug transporters, in particular, organic anion transporters, are active in the iris-

ciliary body. The renocystographic agent iodohippurate and the cholecystographic agent

iodipamide, which actively accumulate in kidney tubules (Barany, 1972) and liver hepatocytes

(Joppen, et al., 1985), respectively, are actively taken up by the ciliary body (ciliary processes)

from a variety of species (Barany, 1972). Freshly excised segments of rabbit iris-ciliary body

accumulate iodopyracet to ~15-fold above the bath iodopyracet concentration, and accumulation

is saturable, dependent on oxidative metabolism, temperature-sensitive, and inhibited by several

organic anions that are ligands of organic anion transporters, including para-aminohippurate,

chlorophenol red, penicillin and probenecid (Becker, 1960). Interestingly, iodopyracet is

actively secreted by renal tubules in a probenecid-sensitive manner, an active process likely

mediated by the classical renal organic anion secretory system, which involves the uptake

transporters OAT1 and/or OAT3 (Russel, et al., 1989). Chlorophenol red, which is secreted by

organic anion transporters in renal tubules, actively accumulates in rabbit ciliary body segments

that are devoid of iris (Sugiki, et al., 1961). That is, accumulation is metabolic- and temperature-

dependent, and inhibited by iodopyracet, probenecid and penicillin (Sugiki, et al., 1961). Also,

the well-established substrate of the renal organic anion secretory system and OAT1 substrate

para-aminohippurate accumulates in the monkey ciliary body in vitro, and independently in the

iris as well (Stone, 1979). Para-aminohippurate accumulation by the monkey ciliary body and

iris is saturable, and active transport is inhibited by metabolic poisons, Na,K-ATPase inhibition

(ouabain), iodopyracet and probenecid (Stone, 1979). Several bile acids, including cholate,

glycocholate, deoxycholate and chenodeoxycholate show significant temperature-dependent


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accumulation by rabbit iris-ciliary body preparations, and accumulation is sensitive to

iodipamide (Barany, 1975). Consistent with these data, the ciliary body and iris express mRNA

for several bile acid uptake transporters, including ASBT, NTCP and several OATPs (Table 2).

Prostaglandin F2α accumulation by rabbit iris-ciliary body is saturable, and is inhibited by

metabolic poisons and a variety of organic anions, including probenecid, bromocresol green,

iodipamide and indomethacin, but is insensitive to the prototypic OAT1 substrate para-

aminohippurate (Bito, et al., 1976). As indicated previously, prostaglandins are well-established

substrates of several OATP uptake transporters (Hagenbuch and Stieger, 2013). The studies with

fresh preparations of iris-ciliary body show that the tissue(s) support active drug transport.

However, given the heterogeneity of cell types (hematopoietic, non-pigmented cells and

pigmented cells) expressed in this preparation, it is not possible from these studies to distinguish

the cellular source of the activity.

The aforementioned studies indicate that the iris-ciliary body expresses a variety of drug

transporters in the ABC and SLC families, and that the tissue can actively accumulate known

substrates of several SLC drug uptake transporters, but can the tissue support active

transepithelial transport of drugs? Two studies have in fact shown that the ciliary body mounted

in Ussing chambers can preferentially transport organic anions in the aqueous humor-to-blood

direction. The strength of the Ussing chamber technique is that by eliminating the chemical

(identical saline on both sides of the tissue) and electrical (voltage-clamped conditions) gradient

across the tissue preparation, it allows for the determination of ‘active’ transepithelial transport

of solutes. The organic anions tested were 6-carboxyfluorescein, para-aminohippurate and

estrone-3-sulfate, all of which are substrates of OAT1 and/or OAT3, and that are secreted by

renal proximal tubules (Pelis and Renfro, 2004). Kondo et al. (Kondo and Araie, 1994) showed


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a transport flux ratio (aqueous humor-to-blood flux/blood-to-aqueous humor flux) for 6-

carboxyfluorescein (10 μM) of ~5 across the rabbit ciliary body. The blood-to-aqueous humor

flux of 6-carboxyfluorescein occurs via passive diffusion, whereas the aqueous humor-to-blood

flux is saturable, with an apparent Michaelis constant of 28 μM (Kondo and Araie, 1994).

Active aqueous humor-to-blood flux of 6-carboxyfluorescein is inhibited by ouabain (Na+,K+-

ATPase inhibition), low bath [Na+], metabolic inhibition (2,4-dinitrophenol), low temperature,

probenecid, iodipamide and hippurate (Kondo and Araie, 1994). OAT1 and OAT3 are Na+-

independent anion exchangers but cellular substrate uptake by the transporters is indirectly

dependent on the Na+-gradient for generation of the α-ketoglutarate gradient across the plasma

membrane, which in part is established by Na+,K+-ATPase and by Na+-dependent dicarboxylate

cotransporters. The Na+-dependence of active 6-carboxyfluorescein transport by the ciliary body

suggests coordination of Na+-dicarboxylate cotransport activity (likely NaDC3) and OATs in

active organic anion transport by the ciliary body epithelium. Consistent with the active

transport of 6-carboxyfluorescein in the aqueous humor-to-blood direction across the ciliary

body in Ussing chambers, the elimination rate of 6-carboxyfluorescein from the perfused bovine

eye in vitro is slowed ~5-fold by the OAT1 and OAT3 inhibitor, novobiocin, when it is co-

administered to the aqueous humor (Lee, et al., 2015). The perfused bovine eye preparation

allows for administration of drugs directly into the aqueous humor, and for measurement of drug

appearance in the venous effluent due to the combined influence of passive aqueous humor

outflow and transport across the blood-aqueous humor barrier.

The flux ratio for para-aminohippurate transport (para-aminohippurate bath

concentration, 1 mM) and estrone-3-sulfate transport (estrone-3-sulfate bath concentration, 5

μM) across the bovine ciliary body in Ussing chambers was 5.3-fold and 3.5-fold, respectively –


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indicating preferential transport in the blood-to-aqueous humor direction (Lee, et al., 2015). Net

active transport of para-aminohippurate across the bovine ciliary body in the aqueous humor-to-

blood direction is inhibited by probenecid and novobiocin, but also by the MRP inhibitor MK571

(Lee, et al., 2015). The sensitivity of transepithelial organic anion transport to a variety of

inhibitors, the expression of OAT1 and OAT3 in the basolateral membrane of non-pigmented

epithelial cells, MRP2 in apical membrane of non-pigmented epithelial cells, and MRP4 in

basolateral membrane of pigmented cells (Figure 2), is in agreement with the active transport by

the ciliary body of para-aminohippurate, 6-carboxyfluorescein and estrone-3-sulfate in the

direction of elimination from the aqueous humor.

In vivo evidence for drug transporter function in the eye

In vivo studies using either systemic (intravenous or oral) or intraocular drug

administration provide evidence that drug transporters can reduce drug concentrations in the eye

either by limiting drug penetration across the ocular barriers (i.e., a functional barrier) and/or by

facilitating drug elimination once inside. The Pgp substrate cyclosporine A administered orally

to humans (BenEzra, et al., 1990), rabbits (BenEzra and Maftzir, 1990a) and rats (BenEzra and

Maftzir, 1990b) at therapeutic doses is not detected in the aqueous humor or vitreous, except

when the ocular barriers are disrupted by inflammation, suggesting that Pgp-mediated efflux

plays an important role in drug absorption across the ocular barriers following systemic

administration. Indeed, following intravenous administration of the Pgp substrates quinidine,

digoxin and verapamil, their aqueous humor uptake index values (concentration of drug in

aqueous humor/concentration of drug in blood) are lower than predicted based on their log D7.4

values, but are increased in the presence of Pgp inhibitors (Toda, et al., 2011)(Fujii, et al., 2014).

Kajikawa et al (Kajikawa, et al., 2000) showed that the aqueous humor clearance of the P-gp


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substrate rhodamine-123 following its intracameral injection in rabbits is ~4-fold greater than

clearance due to passive aqueous humor outflow. Additionally, they found a 50% reduction in

the aqueous humor clearance of rhodamine-123 following co-administration of the Pgp inhibitor

quinidine (Kajikawa, et al., 2000). These results are similar to those of Senthikumari et al.

(Senthilkumari, et al., 2008), who showed following intravitreal injection a 2.6-fold increase in

vitreous rhodamine-123 levels with co-administration of the Pgp inhibitor, elacridar. Probably

the most definitive evidence for a role for Pgp in blood-aqueous humor barrier function in ocular

drug disposition comes from a study done by Fujii et al (Fujii, et al., 2014) using Mdr1a

knockout (Pgp deficient) rats. Following intravenous administration in vivo of quinidine,

digoxin and verapamil, the aqueous humor uptake index values and the in vivo blood-to-tissue

influx permeability clearances of these Pgp substrates across the blood-aqueous humor barrier

were higher in Mdr1a knockout rats compared to wild-type rats (Fujii, et al., 2014).

In addition to Pgp substrates, several organic anions are actively eliminated from the eye

in vivo. The elimination of carbenecillin (Barza, et al., 1982), fluorescein and fluorescein

monoglucuronide (Kitano and Nagataki, 1986), and iodopyracet (Forbes and Becker, 1960) from

the rabbit eye, and cefazolin and carbenecillin from the monkey eye (Barza, et al., 1983)

following intravitreal administration are reduced following concomitant administration of

probenecid, a prototypical inhibitor of many drug transporters that interact with organic anions

(e.g., OATs, OATPs and MRPs). Furthermore, the elimination of iodopyracet from the rabbit

eye is a saturable process with a secretory maximum, consistent with the presence of active

transport (Becker and Forbes, 1961).

Although these in vivo data do not directly implicate a contribution of the iris-ciliary

body in limiting drug exposure in the eye, the expression of a variety of drug transporters in this


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tissue, and the presence of their activity in in vitro preparations, strongly suggest that the tissue

performs this function. It is important to note that the blood-retinal barrier and cornea also

express many drug transporters, and are most certainly important for ocular drug disposition.

The question remains, which blood ocular barrier is most important to ocular drug disposition?

The barrier most important is likely dependent on a variety of factors, including the site of drug

administration (topical, systemic, intravitreal or intracameral), the surface area of the tissues,

barriers to drug diffusion (e.g., unstirred fluid layers), and the abundance and activity of drug

transporters in these tissues.

Perspectives

The data presented indicate that the iris-ciliary body expresses a variety of drug

transporters that are likely involved in limiting drug exposure in the eye. However, it may be

possible to ‘high jack’ select drug transporters in order to enhance ocular drug bioavailability.

Indeed, drug transporters in other tissues have been targeted to improve the pharmacokinetic

profile of drugs. For example, PEPT1 has been targeted to increase the oral bioavailability of the

acyclovir pro-drug, valacyclovir (MacDougall and Guglielmo, 2004). Could PEPT1 in the iris-

ciliary body be used to increase the ocular bioavailability of acyclovir for treatment of ocular

herpes virus infections? To take advantage of drug transporters in order to enhance ocular drug

bioavailability will require a better understanding of their cellular and subcellular distribution in

the iris-ciliary body, and the predominant direction in which they transport their drug substrates.

Although the focus of this review is on drug transporters and their influence on ocular

pharmacokinetics, many of the transporters present in the iris-ciliary body play important

physiological roles, yet we lack an understanding of their importance in ocular physiology. For

example, what purpose do ergothioneine (OCTN1), carnitine (OCTN2), peptide (PEPT1 and


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PEPT2) and cyclic nucleotide (MRP4 and MRP5) transporters serve in the iris-ciliary body?

Ergothioneine is a naturally occurring amino acid derivative that has antioxidant properties.

Although it is not made in the human body, some human tissues contain high concentrations of

ergothioneine. Relatively high levels of ergothioneine are present in bovine and porcine ocular

tissues, including aqueous humor, where it likely protects ocular tissues from oxidative damage

(Shires, et al., 1997). Carnitine is important for conversion of fatty acids into metabolic energy,

and as an antioxidant. Within the eye of animals levels of �-carnitine are highest in the iris,

ciliary body, and choroid-retina, and the use of �-carnitine and its derivatives in humans may

potentially slow age-related ocular pathologies, such as age-related macular degeneration

(Pescosolido, et al., 2008). Future work should examine the influence of OCTN1 and OCTN2

on levels of ergothioneine and carnitine in ocular tissues. Aqueous humor contains a variety of

components that absorb ultraviolet radiation (UV), including ascorbate, uric acid, proteins and

the amino acids tyrosine and tryptophan (Ringvold, et al., 2003). Perhaps peptide transporters in

the iris-ciliary body contribute to the UV-absorbing potential of aqueous humor. The cAMP-

adenosine signaling pathway modulates aqueous humor inflow (Farahbakhsh, 2003). The ability

of MRP4 and MRP5 to modulate intracellular and extracellular cAMP levels may make these

transporters novel targets for glaucoma therapy. A better understanding of the role drug

transporters in the blood-aqueous humor barrier play in ocular pharmacokinetics and physiology

should lead to improved treatment of ocular diseases.


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Authorship Contributions

Wrote or contributed to the writing of the manuscript: JL and RMP


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References

Alm A (1992) Ocular Circulation, in Adler's Physiology of the Eye (Hart WJ ed) pp 198-227, Mosby-Year Book, Inc, St. Louis.

Barany EH (1972) Inhibition by hippurate and probenecid of in vitro uptake of iodipamide and o-iodohippurate. A composite uptake system for iodipamide in choroid plexus, kidney cortex and anterior uvea of several species. Acta Physiol Scand 86:12-27.

Barany EH (1973a) The liver-like anion transport system in rabbit kidney, uvea and choroid plexus. I. Selectivity of some inhibitors, direction of transport, possible physiological substrates. Acta Physiol Scand 88:412-429.

Barany EH (1973b) The liver-like anion transport system in rabbit kidney, uvea and choroid plexus. II. Efficiency of acidic drugs and other anions as inhibitors. Acta Physiol Scand 88:491-504.

Barany EH (1974) Bile acids as inhibitors of the liver-like anion transport system in the rabbit kidney, uvea and choroid plexus. Acta Physiol Scand 92:195-203.

Barany EH (1975) In vitro uptake of bile acids by choroid plexus, kidney cortex and anterior uvea. I. The iodipamide-sensitive transport systems in the rabbit. Acta Physiol Scand 93:250-268.

Barany EH (1976) Organic cation uptake in vitro by the rabbit iris-ciliary body, renal cortex, and choroid plexus. Invest Ophthalmol 15:341-348.

Barar J, Javadzadeh AR and Omidi Y (2008) Ocular novel drug delivery: impacts of membranes and barriers. Expert Opin Drug Deliv 5:567-581.

Barza M, Kane A and Baum J (1982) The effects of infection and probenecid on the transport of carbenicillin from the rabbit vitreous humor. Invest Ophthalmol Vis Sci 22:720-726.

Barza M, Kane A and Baum J (1983) Pharmacokinetics of intravitreal carbenicillin, cefazolin, and gentamicin in rhesus monkeys. Invest Ophthalmol Vis Sci 24:1602-1606.

Becker B (1960) The transport of organic anions by the rabbit eye. I. In vitro iodopyracet (Diodrast) accumulation by ciliary body-iris preparations. Am J Ophthalmol 50:862-7.:862-867.

Becker B and Forbes M (1961) Iodopyracet (diodrast) transport by the rabbit eye. Am J Physiol 200:461-464.

BenEzra D and Maftzir G (1990a) Ocular penetration of cyclosporin A. The rabbit eye. Invest Ophthalmol Vis Sci 31:1362-1366.

BenEzra D and Maftzir G (1990b) Ocular penetration of cyclosporine A in the rat eye. Arch Ophthalmol 108:584-587.


at ASPE



Dow

nloaded from


DMD # 69369

21

BenEzra D, Maftzir G, de Court and Timonen P (1990) Ocular penetration of cyclosporin A. III: The human eye. Br J Ophthalmol 74:350-352.

Bito LZ, Davson H and Salvador EV (1976) Inhibition of in vitro concentrative prostaglandin accumulation by prostaglandins, prostaglandin analogues and by some inhibitors of organic anion transport. J Physiol 256:257-271.

Chemuturi N and Yanez JA (2013) The role of xenobiotic transporters in ophthalmic drug delivery. J Pharm Pharm Sci 16:683-707.

Chen P, Chen H, Zang X, Chen M, Jiang H, Han S and Wu X (2013) Expression of efflux transporters in human ocular tissues. Drug Metab Dispos 41:1934-1948.

Coffey KL, Krushinsky A, Green CR and Donaldson PJ (2002) Molecular profiling and cellular localization of connexin isoforms in the rat ciliary epithelium. Exp Eye Res 75:9-21.

Cunha-Vaz J (1979) The blood-ocular barriers. Surv Ophthalmol 23:279-296.

Dahlin A, Geier E, Stocker SL, Cropp CD, Grigorenko E, Bloomer M, Siegenthaler J, Xu L, Basile AS, Tang-Liu DD and Giacomini KM (2013) Gene expression profiling of transporters in the solute carrier and ATP-binding cassette superfamilies in human eye substructures. Mol Pharm 10:650-663.

Eraly SA, Monte JC and Nigam SK (2004) Novel slc22 transporter homologs in fly, worm, and human clarify the phylogeny of organic anion and cation transporters. Physiol Genomics.

Farahbakhsh NA (2003) Purinergic signaling in the rabbit ciliary body epithelium. J Exp Zool A Comp Exp Biol 300:14-24.

Forbes M and Becker B (1960) The transport of organic anions by the rabbit eye. II. In vivo transport of iodopyracet (Diodrast). Am J Ophthalmol 50:867-875.

Friedenwald JS and Stielher RD (1938) Circulation of the Aqueous: VII. A mechanism of secretion of the intraocular fluid. Arch Ophthal 20:761-786.

Fujii S, Setoguchi C, Kawazu K and Hosoya K (2014) Impact of P-glycoprotein on blood-retinal barrier permeability: comparison of blood-aqueous humor and blood-brain barrier using mdr1a knockout rats. Invest Ophthalmol Vis Sci 55:4650-4658.

Gao B, Huber RD, Wenzel A, Vavricka SR, Ismair MG, Reme C and Meier PJ (2005) Localization of organic anion transporting polypeptides in the rat and human ciliary body epithelium. Exp Eye Res 80:61-72.

Gaudana R, Ananthula HK, Parenky A and Mitra AK (2010) Ocular drug delivery. AAPS J 12:348-360.


at ASPE



Dow

nloaded from


DMD # 69369

22

George RL, Huang W, Naggar HA, Smith SB and Ganapathy V (2004) Transport of N-acetylaspartate via murine sodium/dicarboxylate cotransporter NaDC3 and expression of this transporter and aspartoacylase II in ocular tissues in mouse. Biochim Biophys Acta 1690:63-69.

Geroski DH and Edelhauser HF (2000) Drug delivery for posterior segment eye disease. Invest Ophthalmol Vis Sci 41:961-964.

Hagenbuch B and Stieger B (2013) The SLCO (former SLC21) superfamily of transporters. Mol Aspects Med 34:396-412.

Holash JA and Stewart PA (1993) The relationship of astrocyte-like cells to the vessels that contribute to the blood-ocular barriers. Brain Res 629:218-224.

Hosoya K and Tachikawa M (2009) Inner blood-retinal barrier transporters: role of retinal drug delivery. Pharm Res 26:2055-2065.

Joppen C, Petzinger E and Frimmer M (1985) Properties of iodipamide uptake by isolated rat hepatocytes. Naunyn Schmiedebergs Arch Pharmacol 331:393-397.

Jordan J and Ruiz-Moreno JM (2013) Advances in the understanding of retinal drug disposition and the role of blood-ocular barrier transporters. Expert Opin Drug Metab Toxicol 9:1181-1192.

Kajikawa T, Mishima HK, Murakami T and Takano M (2000) Role of P-glycoprotein in ocular clearance of rhodamine 123 in rabbits. Pharm Res 17:479-481.

Kitano S and Nagataki S (1986) Transport of fluorescein monoglucuronide out of the vitreous. Invest Ophthalmol Vis Sci 27:998-1001.

Klaassen CD and Aleksunes LM (2010) Xenobiotic, bile acid, and cholesterol transporters: function and regulation. Pharmacol Rev 62:1-96.

Kondo M and Araie M (1994) Movement of carboxyfluorescein across the isolated rabbit iris-ciliary body. Curr Eye Res 13:251-255.

Kraft ME, Glaeser H, Mandery K, Konig J, Auge D, Fromm MF, Schlotzer-Schrehardt U, Welge-Lussen U, Kruse FE and Zolk O (2010) The prostaglandin transporter OATP2A1 is expressed in human ocular tissues and transports the antiglaucoma prostanoid latanoprost. Invest Ophthalmol Vis Sci 51:2504-2511.

Lee J, Shahidullah M, Hotchkiss A, Coca-Prados M, Delamere NA and Pelis RM (2015) A renal-like organic anion transport system in the ciliary epithelium of the bovine and human eye. Mol Pharmacol 87:697-705.

MacDougall C and Guglielmo BJ (2004) Pharmacokinetics of valaciclovir. J Antimicrob Chemother 53:899-901.


at ASPE



Dow

nloaded from


DMD # 69369

23

Mannermaa E, Vellonen KS and Urtti A (2006) Drug transport in corneal epithelium and blood-retina barrier: emerging role of transporters in ocular pharmacokinetics. Adv Drug Deliv Rev 58:1136-1163.

Maurice DM and Mishima S (1984) Ocular pharmacokinetics, in Handbook of Pharmacology pp 19-116, Springer-Verlag, Berline.

Moitra K and Dean M (2011) Evolution of ABC transporters by gene duplication and their role in human disease. Biol Chem 392:29-37.

Nakano M, Lockhart CM, Kelly EJ and Rettie AE (2014) Ocular cytochrome P450s and transporters: roles in disease and endobiotic and xenobiotic disposition. Drug Metab Rev 46:247-260.

Pelis RM, Shahidullah M, Ghosh S, Coca-Prados M, Wright SH and Delamere NA (2009) Localization of multidrug resistance-associated protein 2 in the nonpigmented ciliary epithelium of the eye. J Pharmacol Exp Ther 329:479-485.

Pelis RM and Renfro JL (2004) Role of tubular secretion and carbonic anhydrase in vertebrate renal sulfate excretion. Am J Physiol Regul Integr Comp Physiol 287:R491-R501.

Pelis RM and Wright SH (2011) Renal transport of organic anions and cations, in Comprehensive Physiology pp 1795-1835, John Wiley & Sons, Inc..

Pescosolido N, Imperatrice B and Karavitis P (2008) The aging eye and the role of L-carnitine and its derivatives. Drugs R D 9:3-14.

Ringvold A, Anderssen E, Jellum E, Bjerkas E, Sonerud GA, Haaland PJ, Devor TP and Kjonniksen I (2003) UV-Absorbing compounds in the aqueous humor from aquatic mammals and various non-mammalian vertebrates. Ophthalmic Res 35:208-216.

Russel FG, Wouterse AC and Van Ginneken CA (1989) Physiologically based pharmacokinetic model for the renal clearance of iodopyracet and the interaction with probenecid in the dog. Biopharm Drug Dispos 10:137-152.

Schlingemann RO, Hofman P, Klooster J, Blaauwgeers HGT, Van Der Gaag R and Vrensen GF (1998) Ciliary muscle capillaries have blood-tissue barrier characteristics. Exp Eye Res 66, 747-754.

Senthilkumari S, Velpandian T, Biswas NR, Saxena R and Ghose S (2008) Evaluation of the modulation of P-glycoprotein (P-gp) on the intraocular disposition of its substrate in rabbits. Curr Eye Res 33:333-343.

Shields CL and Shields JA (2010) Intra-arterial chemotherapy for retinoblastoma: the beginning of a long journey. Clin Experiment Ophthalmol 38:638-643.

Shin BC, Suzuki T, Tanaka S, Kuraoka A, Shibata Y and Takata K (1996) Connexin 43 and the glucose transporter, GLUT1, in the ciliary body of the rat. Histochem Cell Biol 106:209-214.


at ASPE



Dow

nloaded from


DMD # 69369

24

Shires TK, Brummel MC, Pulido JS and Stegink LD (1997) Ergothioneine distribution in bovine and porcine ocular tissues. Comp Biochem Physiol C Pharmacol Toxicol Endocrinol 117:117-120.

Sonsino J, Gong H, Wu P and Freddo TF (2002) Co-localization of junction-associated proteins of the human blood--aqueous barrier: occludin, ZO-1 and F-actin. Exp Eye Res 74:123-129.

Stewart PA and Tuor UI (1994) Blood-eye barriers in the rat: correlation of ultrastructure with function. J Comp Neurol 340:566-576.

Stone RA (1979) The transport of para-aminohippuric acid by the ciliary body and by the iris of the primate eye. Invest Ophthalmol Vis Sci 18:807-818.

Sugiki S, Constant MA and Becker B (1961) In vitro accumulation of chlorphenol red by rabbit ciliary body. J Cell Comp Physiol 58:181-183.

Toda R, Kawazu K, Oyabu M, Miyazaki T and Kiuchi Y (2011) Comparison of drug permeabilities across the blood-retinal barrier, blood-aqueous humor barrier, and blood-brain barrier. J Pharm Sci 100:3904-3911.

Tomi M and Hosoya K (2010) The role of blood-ocular barrier transporters in retinal drug disposition: an overview. Expert Opin Drug Metab Toxicol 6:1111-1124.

Yasukawa T, Ogura Y, Sakurai E, Tabata Y and Kimura H (2005) Intraocular sustained drug delivery using implantable polymeric devices. Adv Drug Deliv Rev 57:2033-2046.

Zhang T, Xiang CD, Gale D, Carreiro S, Wu EY and Zhang EY (2008) Drug transporter and cytochrome P450 mRNA expression in human ocular barriers: implications for ocular drug disposition. Drug Metab Dispos 36:1300-1307.


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Figure Legends

Figure 1. Schematic diagram of the anterior globe highlighting ABC and SLC drug transporters

shown to be expressed in dissections of human iris-ciliary body, ciliary body or iris by

immunoblotting. See text for references to the published studies.

Figure 2. Cellular and subcellular expression of protein for ABC and SLC drug transporters in

the human ciliary body (left panel) and iris (right panel) determined by immunohistochemistry.

*, indicates discrepancies between studies (see text). BLM, basolateral membrane; NPE, non-

pigmented epithelium; PC, pigmented cell. See text for references to the published studies.


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Table 1. Expression of mRNA for ABC drug transporters in iris-ciliary body (ICB), ciliary body

(CB) or iris (I) from human.

Transporter Chen ICB

Zhang ICB

Dahlin CB

Lee CB

Dahlin I

Lee I

MRP1 + + + + MRP2 + - + + + - MRP3 - + + + + + MRP4 + + + + + MRP5 + + + + + MRP6 + +/- + + MRP7 + + + BCRP + + + +/- +/- - P-gp + + + + + +

The studies by Chen et al. (Chen, et al., 2013), Dahlin et al. (Dahlin et al., 2013), Lee et al. (Lee

et al., 2015), Pelis et al. (Pelis et al., 2009) and Zhang et al. (Zhang et al., 2008) used qPCR,

microarray, microarray, RT-PCR and qPCR for mRNA detection, respectively. -, not detected.

+, detected. +/-, indicates that mRNA expression was low to negligible. Blank areas indicate

that the study did not examine expression of the respective ABC drug transporter. The number

of individual donor eyes used in these studies are as follows: Chen et al. (n=6), Dahlin et al.

(n=15), Lee et al. (n=5-6), Pelis et al. (n=1) and Zhang et al. (n=3).


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Table 2. Expression of mRNA for SLC drug transporters in iris-ciliary body (ICB), ciliary body

(CB) or iris (I) from human.

Transporter

Zhang ICB

Dahlin CB

Lee CB

Dahlin I

Lee I

Organic anion transporting polypeptides

OATP1A2 - + +/- + +/- OATP1B1 - + - + - OATP1B3 - + - +/- - OATP2B1 + + + + + OATP1C1 +/- +/- OATP2A1 + +/- OATP3A1 + + OATP4A1 + + OATP4C1 - - OATP5A1 +/- +/- OATP6A1 - -

Organic anion transporters

OAT1 + + + + + OAT2 - + - + - OAT3 + + + + + OAT4 +/- +

Bile acid transporters

ASBT - + + NTCP + +

Dicarboxylate transporters

NaDC1 - +/- NaDC3 + +

Organic cation transporters

OCT1 + + + OCT2 + + + OCT3 + + + OCTN1 + OCTN2 + + + MATE1 + + MATE2 + +

Peptide transporters

PEPT1 +/- + + PEPT2 +

The studies by Zhang et al. (Zhang et al., 2008), Dahlin et al. (Dahlin et al., 2013) and Lee et al.

(Lee et al., 2015) used qPCR, microarray and microarray for mRNA detection, respectively. -,

not detected. +, detected. +/-, indicates that mRNA expression was low to negligible. Blank areas

indicate that the study did not examine expression of the respective SLC drug transporter. The


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number of individual donor eyes used in these studies are as follows: Dahlin et al. (n=15), Lee et

al. (n=5-6) and Zhang et al. (n=3).


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leciliarybody

cor

aqueous humor

vitrhum

pars plicata

pars plana

: tight end

: fenestra

: retinal p

: ciliary ep

MRP1, MRP2, MRP4, Pgp, BCRP, OATP1A2, OATP1C1, OATP2B1, OATP3A1, OATP4A1, OAT1, OAT3, NaDC3

Fig

ens

rnea iris

eous mor

Retinal pigmented epithelium

dothelial cells

ated endothelial cells

pigmented epithelium

pithelium (bilayer)

MRP1, MRP2, MRP4, BCRP

iris-ciliary body

MRP1, MRP2, MRP3, MRP4, MRP6, MRP7, Pgp

igure 1

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atted. The final version m

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ciliary epithelium

aqueous humochoroid

nonpigmented cellpigmented celltight

junction

CILIARY BODY

MRP2 (apical, NPEgap

junction

OAT1OAT3 OATP2B1OATP2A1OATP3A1OATP4A1

MRP4(BLM, PC)

OATP1A2OATP1C1OATP2B1OATP3A1OATP4A1

OATP2A1OATP2B1*

BLM, PC pars plana

OATP1C1OATP3A1OATP4A1

BLM, PC pars plicata

Figur

mor

PE)

BLM, NPE pars plicata

BLM, NPE pars plana

IRIS

tight junction

iridialcapillary

Subcellular localization unknown

OATP2A1OATP2B1Pgp

gure 2

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atted. The final version m

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