Penetracion Ocular

8/11/2019 Penetracion Ocular

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Molecular Design for Enhancement of Ocular Penetration

YOSHIHISA SHIRASAKISenju Pharmaceutical Co., Ltd., 1-5-4 Murotani, Nishi-ku, Kobe, Hyogo 651-2241, Japan

Received 22 June 2007; revised 21 August 2007; accepted 23 August 2007

Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jps.21200

ABSTRACT: Over the past two decades, many oral drugs have been designed inconsideration of physicochemical properties to attain optimal pharmacokinetic proper-

ties. This strategy significantly reduced attrition in drug development owing to inade-

quate pharmacokinetics during the last decade. On the other hand, most ophthalmic

drugs are generated from reformulation of other therapeutic dosage forms. Therefore,the modification of formulations has been used mainly as the approach to improve ocular

pharmacokinetics. However, to maximize ocular pharmacokinetic properties, a specific

molecular design for ocular drug is preferable. Passive diffusion of drugs across the

cornea membranes requires appropriate lipophilicity and aqueous solubility. Improve-

ment of such physicochemical properties has been achieved by structure optimization or

prodrug approaches. This review discusses the current knowledge about ophthalmic

drugs adapted from systemic drugs and molecular design for ocular drugs. I propose the

approaches for molecular design to obtain the optimal ocular penetration into anterior

segment based on published studies to date. 2007 Wiley-Liss, Inc. and the American

Pharmacists Association J Pharm Sci 97:24622496, 2008

Keywords: tissue partition; drug design; permeability; solubility; structure-property

relationship (SPR); epithelial delivery/permeability

INTRODUCTION

Over the past two decades, oral drugs have been

designed in consideration of physicochemical

properties to maximize their pharmacokinetic

properties. Numerous papers and reviews

describe the design of molecules to improve the

pharmacokinetic properties such as oral absorp-tion, bioavailability and the duration of action.19

At present, in silico screening is widely used forthe selection of drug-like compounds from combi-

natorial libraries and is based on physicochemical

parameters such as the rule-of-five,10 which is a

qualitative absorption/permeability predictor.1113

Physicochemical property-based drug design

can reduce attrition due to inappropriate phar-

macokinetics in the drug development process.

Inappropriate pharmacokinetics accounted for

Abbreviations: 17-Ph-PGF2a

, 17-phenyl-18,19,20-trinorprostaglandin F2

a; 17-Ph-PGF2

a-IE, isopropyl ester of 17-Ph-

PGF2a

; ACD logD7, distribution coefficient in pH 7 was calcu-lated using ACD/Labs software; API, active pharmaceuticalingredient; AUC, area under the tissue concentration-timecurve; BBB, blood brain barrier; BCRP, breast cancer resistantprotein; Caco-2, human colon adenocarcinoma cells; CAI, car-bonic anhydrase inhibitor;Cmax, maximum observed concentra-tion in tissues after instillation; CNS, central nervous system;EGTA, ethylene glycol-bis(2-aminoethylether)-N,N,N0,N0-tetra-acetic acid; FDA, U.S. Food and Drug Administration; ICB, iris-

ciliary body; IOP, intraocular pressure; LAT, large neutralamino acid transporter; log Dxx, logarithm of n-octanol/bufferdistribution coefficient at pH xx; logP, logarithm ofn-octanol/water partition coefficient; MDCK, MardinDarby caninekidney cells; MDR, multidrug resistance protein; MP, meltingpoint; MRP, multidrug resistance-associated protein; NSAID,nonsteroidal anti-inflammatory drug; Papp, apparent cornealpermeability coefficient; PepT1, intestinal peptide transporter1; PGF2

a, prostaglandin F2

a; P-gp, P-glycoprotein.

Correspondence to: Yoshihisa Shirasaki (Telephone: 81-78-997-1010; Fax: 81-78-997-1016;E-mail: [email protected])

Journal of Pharmaceutical Sciences, Vol. 97, 24622496 (2008)

2007 Wiley-Liss, Inc. and the American Pharmacists Association

2462 JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 97, NO. 7, JULY 2008


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about 40% of the reasons for attrition in 1992, but

decreased to about 10% in 2000.14,15 Thus, in oral

drugs, physicochemical-based drug design can

successfully decrease attrition caused by pharma-

cokinetic reasons. On the other hand, in ophthal-

mology, only a few drugs are designed exclusively

for ocular use.1620 Systemic administration oftenresults in insufficient ocular drug concentration

because the drug penetration from the blood

stream to the eye tissues is limited by a blood-

aqueous and a blood-retinal barrier located in iris

and retina-choroid, respectively.21,22 To maximize

drug penetration to the target tissues in parti-

cular the anterior segment of the eye, topical

instillation of eye drops are used mainly in clinical

therapy. The periocular injection methods such as

sub-conjunctival, sub-tenon and intravitreal

injection can attain high levels of drugs in

intraocular tissues, but they are invasive andinconvenient.23 Therefore topical instillation is

the most useful method because it delivers the

drug easily and noninvasively to the external and

intra ocular tissues.

Ophthalmic drugs in current use originate from

oral drugs mainly because the adaptation of oral

drugs to ophthalmic drugs is very efficient in drug

developments. However, most oral drugs gener-

ally have low aqueous solubility for ophthalmic

solutions. The ocular bioavailability of eye drops is

generally low,21 so high aqueous solubility is

desirable for eye drops to attain the high drug

concentration in the formulation, unless it pro-

duces toxicity such as ocular irritation and

hyperemia. An alternative option is suspension

formulation but this is accompanied with pro-

blems such as inconvenience and technical

difficulties in manufacturing processes. Ophthal-

mic suspensions need to be resuspended when

they are administered. The active pharmaceutical

ingredients (API) of an ophthalmic suspension

have to be sterilized. Since most APIs for injection

have enough aqueous solubility as formulate eye

drops and are sterilized, such difficulties would

not be problematic. However, the majority of themis unstable in an aqueous solution for a long time

and often has low lipophilicity, which may lead

to low corneal permeability. Because ocular bio-

availability depends mainly upon pharmaceutical

formulation, the modification of formulations has

so far been used as the major approach to improve

the ocular pharmacokinetics of eye drops.24

However, if the drug only has poor corneal

permeability and aqueous solubility, it is difficult

to deliver sufficient amounts of drugs to intrao-

cular target tissues using the modification of

formulation. Therefore, the molecular design with

consideration of ocular pharmacokinetic and phy-

sicochemical properties is desirable for ophthal-

mic drugs to obtain optimal ocular bioavailability

and efficacies.

This review summarizes the current state ofknowledge about molecular design for ocular

drugs and compounds for ophthalmic use origi-

nating from systemic drugs. I will also consider

the molecular design to maximize the penetration

into the anterior segment based on published

studies to date.

ABSORPTION ROUTE OF DRUG AFTERTOPICAL OCULAR ADMINISTRATION

Various anatomic and physiologic barriers limit

the drug absorption to the anterior segment of the

eye (cornea, aqueous humor, iris, ciliary body and

lens). The structure of the eye is shown in

Figure 1. In general, only 17% of the dose of

the drugs after topical instillation is able to attain

the aqueous humor.24 The instilled drug is diluted

by tear fluid and rapidly removed from the ocular

surface by tear turnover and blinking. These

resulted in only a short contact time on the ocular

surface. The large fraction of the instilled drug

will be transferred to systemic circulation via thenasolacrimal duct in a few minutes. In the case of

lipophilic drugs (logP >0), more than 5080% ofinstilled doses are absorbed into the systemic

circulation.25

Figure 1. Cross sectional view of the eye.

DOI 10.1002/jps JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 97, NO. 7, JULY 2008

DESIGN FOR ENHANCEMENT OF OCULAR PENETRATION 2463


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The drug in tear fluid is absorbed via two routes:corneal route and noncorneal route (conjunctiva-

sclera route).24 Most drugs penetrate to the cornea

via transcellular absorption as a major route,

because the corneal epithelium cells form tight

junctions that limit paracellular drug permea-

tion.21 The drugs pass through the conjunctivaand the sclera via not only transcellular absorp-tion, but also paracellular absorption, because

these tissues are more leaky than the cornea. The

permeability of drugsviatranscellular absorptiondepends mainly on lipophilicity. Most ophthalmic

drugs with modest lipophilicity and low-molecular

weight are predominately absorbed via thecorneal route. Animal studies of topical instilla-

tion have shown that corneal route/noncorneal

route ratio is 70:1, 12:1, and 5:1 in the case of

hydrocortisone, timolol and pilocarpine, respec-

tively.26

Therefore, it is considered that the corneais the most principle route for ocular drug

penetration from tear fluid to the anterior

segment.

The cornea is very tight tissue, more than the

intestine, lung, bronchus, and nasal mucosa, and

the drug penetration is difficult.27 The cornea is

composed of five membranes: epithelium, Bow-

mans membrane, stroma, Desmets membrane,

and endothelium. Among these layers, epithe-

lium, stroma, and endothelium are the substan-

tial barriers. The corneal epithelium is a lipophilic

membrane, which forms tight and gap junctions,

and the most prominent barrier for corneal

absorption.28 Therefore, most of the lipophilic

compounds can pass through the corneal epithe-

liumvia transcellular absorption. Recent studiesshow that various uptake and efflux trans-

porters such as oligopeptide transporters, amino

acid transporters, monocarboxylate transporters,

nucleoside transporters, P-glycoprotein (P-gp,

MDR1), MRP1MRP6, and BCRP are expressed

in cornea epithelium and actively uptake and

efflux their substrates.21,2934 The stroma is in

hydrophilic environment and limits the pene-

tration of highly lipophilic or large molecularweight compounds. The endothelium is a leaky

lipophilic barrier and partially resists the pene-

tration of lipophilic compounds, but not hydro-

philic compounds.

In the case of the conjunctiva-sclera route,

drugs can be directly accessed to iris and ciliary

body through conjunctiva and sclera without

diffusion to aqueous humor. The conjunctiva is

a mucous membrane and has many capillary blood

vessels. The area of human conjunctiva is

approximately 17-fold larger than that of the

cornea.26 The conjunctival epithelium forms tight

junction and limits drug penetration. In conjunc-

tival epithelium, expression of efflux transporters

such as P-gp has been reported.26 The sclera,

which is constructed of collagen bundle and elastic

fiber, is also a leaky tissue. The scleral perme-ability of polyethylene oligomer is 2-fold less than

that of conjunctiva and 10-fold more than that of

the cornea. The conjunctiva and sclera are ever

leakier than the cornea and permeate the drugs

through paracellular absorption in addition to

transcellular absorption.35 The conjunctiva-sclera

route is generally considered as nonproductive

route because the vessels in conjunctiva rapidly

absorbed most of the instilled drug into the

systemic circulation. However, several reports

suggest that the conjunctiva-sclera route is the

main route of penetration to the anterior segmentfor carbonic anhydrase inhibitors (CAIs), hydro-

philic compounds and large molecules.24

EVALUATION OF OCULAR PENETRATIONAND REQUIRED PHYSICOCHEMICALPROPERTIES

Corneal Permeability

For the sufficient drug penetration into aqueous

humor, both high corneal permeability and

aqueous solubility are generally required.36 The

corneal permeability correlates with lipophilicity

like Caco-2 and MDCK cell permeability. It is

reported that the optimal lipophilicity for corneal

permeation is logP23 in the case of steroids andb-blockers.37,38 Excess lipophilicity would lead to

a reduction in permeability of corneal stroma,

which is hydrophilic tissue and limits the pene-

tration of highly lipophilic compounds. The

corneal permeability has been evaluated gener-

ally by using rabbit corneas.39 The corneal

thickness of rabbits is similar to that of humans.

Good correlation has been observed betweenrabbit and human corneal permeability in the

case of both cyclophosphamide40 and CAIs.41 The

Ussing chamber usually has been used in the

assessment of corneal permeability. In this

system, the apparent corneal permeability coeffi-

cient (Papp) is calculated.36 ThePappvalue can be

used to evaluate the relative permeability.

Recently, the in vitro cell systems for predictingcorneal permeability are by determination of

permeability of compounds through rabbit, bovine

JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 97, NO. 7, JULY 200 8 DOI 10.1002/jps

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and human corneal epithelium cells.4247 The

permeability of the corneal epithelium cell sys-

tems is positively correlated with the corneal

permeability. In addition to passive diffusion, the

active transport and metabolism may be partially

evaluated by these systems. The expression of

various transporters4851 and the enzymes forester hydrolysis on corneal cell lines46,52 has been

reported. These systems may be useful for screen-

ing of compound libraries to predict the corneal

permeability and metabolism.

Aqueous Solubility

The high aqueous solubility is also important

because only the dissolved drug is capable of

permeating cornea membrane. The instilled drug

is diluted by tear fluid and contacts with the

cornea in a very short time.22,24,27,36 The ocular

bioavailability is generally quite low (17%) and

high drug concentration is important for ophthal-

mic solution. Therefore, ophthalmic drugs need

high aqueous solubility in tears at neutral pH

(pH 6.57.6) to permeate across the cornea.

On the other hand, the enhancement of aqueous

solubility may reduce drug lipophilicity and

corneal permeability. Although the incorporation

of a strong ionizable center, such as sulfonate,

phosphate, and guanidine moieties, into a struc-

tural template can provide highly soluble mole-

cules, excessive aqueous solubility would cause amarked reduction in corneal permeability. The

compounds having ionizable centers should be

designed not to have excess ionic strength at

physiological pH. After all, it is desirable to

possess adequate aqueous solubility without loss

of lipophilicity for corneal penetration. An excep-

tion to this is ampholyte CAI, which show higher

permeability at the higher rate of the ionic species.

The ionic form more readily sequestered in the

cornea, which led to the higher drug levels in

cornea, aqueous humor and iris-ciliary body than

the unionized form.

53

Assessment of Intraocular Penetration

In vivo ocular pharmacokinetic studies for asses-sing intraocular penetration have most commonly

been performed with rabbits, because the rabbits

have relatively large eyeballs in spite of their

small bodies.39 The corneal permeability for

lipophilic compounds in rabbits is approximately

equivalent to that in humans, but when the same

dosage is topically administrated, intraocular

drug concentration in rabbits is often higher than

that in humans. The faster precorneal loss of

instilled ophthalmic drugs in human ascribed to

blinking with about three times more frequency.

The drug penetration across the cornea is higher

in rabbits than in humans, but it is probable thatrabbits can be used for comparison of intraocular

penetration for a set of compounds. The aqueous

humor drug concentrations and area under the

curve (AUC) can be used as an indicator of

intraocular tissues drug concentration. The drug

concentration in aqueous humor would be easy to

determine due to fluid. Aqueous humor contains

only low protein content (0.2 mg/mL) compared to

plasma (50 mg/mL)54 and is approximated to the

free fraction of drugs in iris and ciliary body

located around the anterior chamber, which parti-

cipates in pharmacodynamic effects in intraoculartissues. The intraocular drug levels after instilla-

tion would be overestimated when the levels are

obtained in rabbit experiment. Therefore, it is

suggested that several-times higher ocular levels

in rabbit are required for the sufficient efficacy in

human.55

REPRESENTATIVE EXAMPLES

Representative examples of corneal permeability

and ocular penetration of sets of compounds and

molecular designs for improved ocular pharma-

cokinetics are described below.

Anti-Glaucoma Agents

Carbonic Anhydrase Inhibitors (CAIs)

Oral CAIs such as acetazolamide and ethoxzola-

mide have been used for the treatment of

glaucoma and hyper intraocular pressure (IOP),

but their systemically adverse effects resulted in

the discontinuation of the drugs in about 50% of

patients.56 Therefore, topical instillation of CAIshas been investigated. It is expected that topical

instillation of CAIs is capable of providing the IOP

lowering effect without systemically adverse

effects. However, topical instillation of oral CAIs

such as acetazolamide, methazolamide and ethox-

zolamide, did not show sufficient efficacy. For the

IOP lowering effect, CAIs have to reach the ciliary

process where aqueous humor is produced by

carbonic anhydrase. To deliver CAIs to the ciliary

process, ocular CAIs need to possess the high




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corneal permeability and aqueous solubility,

which lead to high intraocular drug concentration.

Two major approaches have been used for the

design of topical CAIs for eye drops. The

approaches, categorized into ring approach and

tail approach for enhancing ocular penetration,

have been tried to obtain the topically effectiveCAIs.5759 The ring approach implies that a

modification of ring system of lipophilic oral CAIs

such as ethoxzolamide will improve the aqueous

solubility. Two topical CAIs presently available

(dorzolamide18 and brinzolamide)60 were design-

ed by this approach. On the other hand, in the tail

approach, the functionalities, which can enhance

corneal permeability and aqueous solubility, are

attached to the ring of oral CAIs such as

acetazolamide and methazolamide without mod-

ification of the ring.

Ethoxzolamide, an oral CAI, shows high cornealpermeability, but its aqueous solubility is very

low.61 The low solubility resulted in insufficient

intraocular tissue drug concentration for the

efficacy after topical instillation of aqueous

suspension. To improve its poor properties, Merck

Research Laboratories group has designed and

synthesized many CAI inhibitor for topical instil-

lation based on ethoxzolamide using the ring

approach.57 Finally, they identified a topical CAI,

dorzolamide, which is approved as the first topical

CAI from FDA. The physicochemical and ocular

pharmacokinetic properties of several examples of

these CAIs are shown in Table 1.

The Merck group focused on conversion of 6-

position functionalities on ethoxzolamide. The

conversion of the ethoxy group into the hydroxy

group led to an increase in aqueous solubility, and

this phenol L-643,799 demonstrated a weak IOP

lowering effect in a rabbit model.61 Since this

conversion lowered the corneal permeability,

L-643,799 was esterified to the corresponding

O-pivaloyl derivative L-645,151 to increase cor-neal permeability like dipivefrine62 (dipivaloyl

ester prodrug of epinephrine).63 This compound is

efficacious in rabbits, but its repeated instillationfor 3 months induced ocular irritation. This may

be due to a drug mediated allergic reaction caused

by the formation of an allergen generated by a

reaction between the reactive sulfamoyl group at

2-position on the benzothiazole ring and biomo-

lecular nucleophiles. Thus, less reactive ben-

zothiophene analogs have been synthesized.64

The lipophilicity of the benzothiophene derivative

L-650,719 is similar to the corresponding ben-

zothiazole derivative L-643,799. The area under

the curve of the time-drug concentration profile

in aqueous humor (AUC) after instillation for

benzothiophene phenol L-650,719 is approxi-

mately equal to that for L-643,799.65 The acetyl

analog of L-650,719 (L-651,465) showed roughly

twofold higher AUC than the pivaloyl analog of

L-643,799 (L-645,151). Even though the benzo-thiophenes and benzothiazoles show relatively

high aqueous solubility, they were only formu-

lated as suspension at 12%, not ophthalmic

solution. Thus, the discovery of compounds with

aqueous solubility exceeding 1% was continued

and the efforts led to the thienothiopyran

derivative L-654,230 with more than 1% solubility

at pH 7.4.66 The 4-hydroxy-thienothiopyran L-

654,230 showed the 8.1 mg/g ofCmaxafter topicalinstillation in iris-ciliary body (ICB) of pigmented

rabbits far exceeded the values of benzothiazole

L-645,151 and benzothiophene L-651,465. Theconversion of the benzothiophene ring into

the 5,6-dihydro-4H-thieno[2,3-b]thiopyran 7,7-dioxide ring greatly increased aqueous solubility

without the loss of adequate lipophilicity. The

substitution of 4-OH functionality in L-654,230

with isobutylamine moiety provided MK-927 with

higher Cmax value in ICB than alcohol L-654,230.67 This increase in Cmax value in ICB is

most likely ascribed to its basic functionality,

which increases the binding affinity for melanin.

The further modification of MK-927 identified

dorzolamide, which demonstrated the similar

drug levels in ICB after instillation and was

approved for the treatment of glaucoma by FDA in

1994.57,58 Thereafter, brinzolamide, which is a

6-(methoxypropyl)aza analog and is more lipo-

philic structure than dorzolamide, also received

FDA approval in 1999 as an anti-glaucoma

agent.60 The compound shows lower solubility

at neutral pH than dorzolamide and formulated

as an aqueous suspension. The drug levels of

brinzolamide are lower than that of dorzolamide,

but showed the longer duration of the action with

the approximately same efficacy as dorzolamide.

This longer duration may be attributed to thesuspension formulation and the increase in

lipophilicity caused by the introduction of the

methoxypropyl group.

The tail approach consisted of modifying the

side chains of well-known oral aromatic/

heterocyclic sulfonamide derivatives, such as

acetazolamide and itsN-methyl derivative metha-zolamide, to improve the physicochemical and

ocular pharmacokinetic properties.58,59,68 These

examples are shown in Table 2. Scozzafava et al.69


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obtained the topical effective CAIs with highintraocular penetration better than dorzolamide

by attaching tails such as protonable nitrogen

atom (EGTA, 2,3-pyridinedicarboxyimido,70 gua-

nidino,71 and pycolinoyl72) and perfluoroalkyl/

aryl moieties,73 which provided the molecules

adequate lipophilicity and aqueous solubility. The

condensation of 2,6-pyridinedicarboxy acid with

the C4-amino ethyl group in dorzolamide yielded

the ocularly permeable CAI with higher intrao-

cular drug levels than dorzolamide.70 Sharir

et al.74 reported that the acetyl group of acet-azolamide was substituted with dicarboxylic acids

such as oxalate, succuinate, adipate and succeed

in increasing corneal permeability and demon-

strating the IOP lowering effect (Tab. 3).

b-Blocker

All ophthalmic b-blocker drugs on the market

have been generated by the reformulation of oral

b-blocker drugs. This class contains various drugs

Table 1. Physicochemical and Ocular Pharmacokinetic Properties of CAI Inhibitors (Ring Approach)

Compound log D7.4a Solubility (mg/mL) Cmax (mg/mL or g)

b Refs.

Acetazolamide

0.85 0.71 (pH 7.4) 57,60

Ethoxzolamide RC2H5, XN 2.06 (pH 7.2) 0.024 (pH 7.4) 61,66

L-643,799 RH, XN 1.11 7.92 (pH 7.65) 2.67/2.21 (AHc/ICBd)e 57,61,65

L-645,151 RCOC(CH3)3, XN 2.45 0.058 (pH 7.4) 3.91/4.45 (AHc/ICBd)e 57,63,65

L-650,719 RH, XCH 1.28 1.16/1.44 (AHc/ICBd)f 57,64,65

L-651,465 RCOCH3, XCH 1.28 3.05/2.76 (AHc/ICBd)f 57,64,65

L-654,230 ROH 0.35 12.5 (pH 7.4) 8.1 (ICBd)g 57,66,67

MK-927 RNHCH2CH(CH3)2 0.90 >20 (pH 7.4) 27.8 (ICBd)g 57,67

Dorzolamide (L-671,152)

0.18 6.7 (pH 7.4) 7.8/27.0 (AHc/ICBd)g 18,57,60

Brinzolamide (AL-4623A)

0.50 (pH 7.4) 3.85 (ICBd)h 60

aDistribution coefficient in pH 7.4.bTheCmaxafter topical instillation.cAqueous h umor.dIris-ciliary body.e2% suspension.f0.5% suspension.g2% solution.h1% suspension.




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with diverse physicochemical properties. Among

these drugs, the major ophthalmic drugs aretimolol, levobunolol, betaxolol, metipranolol, car-

teolol, and so on.75 Corneal permeability and

ocular pharmacokinetics of various oral b-block-

ers has been evaluated. The lipophilicity and

pharmacokinetic parameters of these drugs are

included in Table 4. Schoenwald et al. investi-

gated the correlations between lipophilicity and

corneal permeability of 12 b-blockers.38 This

study revealed that the corneal permeability

and lipophilicity (logD7.65) exhibited a parabolic

relationship having the optimal logD7.65at about

2.5 (Fig. 2). An increase in lipophilicity of thecompound enhances the permeability to the

cornea epithelium, whereas it does not increase

the permeability to the corneal stroma due to its

hydrophilic environment.76 Wang et al.77 also

investigated the relationship between corneal

permeability and lipophilicity of 13 b-blockers.

This study demonstrated that the corneal perme-

ability varied with lipophilicity according to a

sigmoidal relationship and was saturated at about

log P3. However, acebutolol was an outlier in

Table 2. Physicochemical and Pharmacokinetic Parameters After Topical Instillation of CAI Inhibitors

(Tail Approach)

Compound

logD7.4(CHCl3)

aSolb (mM)

(pH 7.4) kinc (103 h1) C1h/C2h

d (mM)c Refs.

Acetazolamide (COCH3) 0.001 3.2 0.37 70

1.45 52 4.6268/53 (AH),

59/37 (CP) 69

0.449 81 (HCl) 3.8280/42 (AH),

50/12 (CP) 70

1.620 73 4.1308/50 (AH),

54/18 (CP) 71

1.944 75 4.7 325/45 (AH), 69/21 (CP) 71

0.589 78 (HCl) 2.7283/39 (AH),

51/10 (CP) 72

2.113 62 4.5324/42 (AH),

45/13 (CP) 73

Dorzolamide 2.0 60 (HCl; pH 5.8) 3.0 32/21 (AH), 15/6 (CP) 73

aDistribution coefficient between chloroform and buffer in pH 7.4.bAqueous solubility.cThe rate constant of transfer across the cornea.dDrug concentration in aqueous humor and ciliary process at 1 and 2 h following instillation of 2% solution.


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Table 4. Physicochemical and Ocular Pharmacokinetic Properties ofb-Blockers

Compound log P

ACD

logD7a

Pappb

(106 cm/s)

Cmaxc

(mg/mL)

AUCc

(mg h/mL) Refs.

Penbutolol 4.15 2.05 45 38,76

Bufuranol 3.65 1.43 57 22.86

(Ref. 80)

10.1

(Ref. 80)

38,76,80

Betaxolol 3.44 0.56 27 77

Propranolol 3.21 1.00 48 5.32

(Ref. 81)

3.80

(Ref. 81)

38,76,81

Alprenolol 2.37 0.77 29 38,77

Levobunolol 2.40 0.77 16

(Ref. 76),

23

(Ref. 77)

5.92

(Bunolol)

(Ref. 83)

4.56

(Bunolol)

(Ref. 83)

38,76,

77,83

Oxprenolol 2.37 0.17 25 (Ref. 76),

32 (Ref. 77)

38,76,77

Metoprolol 1.88 0.33 22

(Ref. 76),28

(Ref. 77)

2.89

(Ref. 81)

2.01

(Ref. 81)

38,76,

77,81

Timolol 1.91 1.77 12

(Ref. 76),

12

(Ref. 77)

30.65

(Ref. 80),

5.86

(Ref. 81)

25.4

(Ref. 80),

3.73

(Ref. 81)

38,76,77,

80,81

(Continued)


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prodrugs might be superior in pharmacological

activity to nonprodrugs ascribed to the presence of

much free fraction. Conversion to aliphatic acid

ester prodrugs of tilisolol, a hydrophilic b-blocker,

also provided an improved corneal permeability

and intraocular penetration. The propionyl and

butyryl ester prodrugs demonstrated approxi-

mately sixfold higher tilisolol concentration at1 h after instillation in aqueous humor than the

parent compound.87

Oxime/methoxime analogs of b-blocker have

been reported as classes of prodrugs, except for

esters. Bodor et al. synthesized oxime/methoxime

analogs of alprenolol, betaxolol, carteolol, propra-

nolol, timolol, etc. Some of these compounds

demonstrated higher and longer IOP reducing

efficacy than the parent drugs.20 For instance,

after instillation the oxime analog of propranolol

Table 4. (Continued )

Compound log P

ACD

logD7a

Pappb

(106 cm/s)

Cmaxc

(mg/mL)

AUCc

(mg h/mL) Refs.

Acebutolol 1.77 0.11 0.85

(Ref. 76),1.1

(Ref. 77)

1.26

(Ref. 80)

2.93

(Ref. 80)

38,76,

77,80

Pindolol 1.75 0.18 10 77

Nadolol 0.93 0.83 1.0

(Ref. 76)

38,76

Atenolol 0.16 2.02 0.67

(Ref. 76)

2.22

(Ref. 81)

0.93

(Ref. 81)

38,76,81

Sotalol 0.62 1.82 1.6

(Ref. 76)

38,76

aDistribution coefficient in pH 7 was calculated using ACD/Labs software.bCorneal permeability.cThe values in aqueous humor after single81,83 or triple80 instillation normalized to 1% solution dosing.

Figure 2. Relationship between logarithm of cor-

neal permeability coefficient (Papp) and lipophilicity

(logD7.65) in the rabbit intact cornea. This graph was

reconstructed from the graph in Ref. 38.




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Table 5. Physicochemical and Ocular Pharmacokinetic Properties of Timolol86 and Tilisolol Prodrugs87

Compound log D7.4a Papp (10

6 cm/s)b C20minc or C1h

d (mM)

Timolol prodrugs

RH 0.04 8.1 7.7c

RCOCH3 1.1 23 23c

RCOCH2CH3 1.6 29 19c

RCO(CH2)2CH3 2.1 32 22c

RCOC(CH3)3 2.7 13 17c

RCO(CH2)3CH3 2.7 31 26c

RCO(CH2)4CH3 3.3 21 21c

RCO(CH2)6CH3 4.4 8.9 12c

Tilisolol prodrugs

RH 0.27 2.7 3.1d

RCOCH3 1.02 8.9 4.0 (vs. parent)d

RCOCH2CH3 1.56 8.4 5.1 (vs. parent)d

RCO(CH2)2CH3 2.02 15 6.2 (vs. parent)d

RCO(CH2)3CH3 2.47 13 5.9 (vs. parent)d

aDistribution coefficient in pH 7.4.bCorneal permeability. The data were constructed from the graph in Ref. 86.cDrug concentration in aqueous humor at 20 min after instillation. The data were constructed from the graph in Ref. 86.

dDrug concentration in aqueous humor at 1 h after instillation.

Figure 3. Relationship between corneal permeability

coefficient (Papp) and prodrug lipophilicity (logD7.4) in

the rabbit intact cornea. This graph was reconstructed

from the graph in Ref. 86.

Figure 4. Relationship between aqueous humor

timolol concentration at 20 min and prodrug lipophili-

city (logD7.4) in the rabbit intact cornea. This graph was

reconstructed from the graph in Ref. 86.


2472 SHIRASAKI


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showed higher intraocular penetration and longer

duration of action by sustainable release of the

parent drug (Tab. 6).88 Since release of parent

compound for oxime/methoxime analogs is slower

than that of ester prodrugs, conversion to oxime/

methoxime is useful for continuation of the

duration of action.

In the case ofb-blockers that possess relatively

low lipophilicity and hydrophilic functionalities,

such as hydroxyl and secondary amine group,

increasing lipophilicity of the molecule would

improve the ocular penetration. However, exces-

sive lipophilicity would cause an increase in

ocular tissue binding, leading to a decrease in

unbound fraction and an increase in resistance

to hydrophilic corneal stroma. For this reason,

the highly lipophilic molecule is unsuitable for

ocular drugs as ophthalmic solution. It is most

likely to be effective for ocular drugs to in-

troduce a heterocycle, which allows the moleculesto increase lipophilicity without loss of water

solubility. The ester prodrug approach is also

desirable because the molecule will be converted

into the less lipophilic parent compound in corneal

epithelium.

a2-Agonist

a2-Agonist showed an IOP lowering effect and is

used for the treatment of glaucoma. Clonidine, a

representative selective a2-agonist, has been used

as an ocular hypertensive agent via topicaladministration. However, it can also penetrate

to the central nervous system (CNS) through the

blood-brain barrier (BBB) due to its high lipo-

philicity and causes centrally mediated cardio-

vascular side effects.89 Therefore, to reduce the

adverse effect, p-aminoclonidine (apraclonidine),which possess higher polarity than clonidine, was

synthesized and is used for reducing the IOP. The

introduction of the p-amino group resulted in areduction of adverse effects, but also a decrease of

corneal permeability because of an increase in

polarity.

Brimonidine, a quinoxaline derivative, is more

lipophilic than apraclonidine but less lipophilic

than clonidine.90 Conversion of 2,6-dicloro-p-aminobenzene into 5-bromoquinoxaline provided

more than 20-fold higher corneal permeability

and 10-fold higher drug levels in aqueous humorafter instillation (Tab. 7). Although this drug

passes BBB, its side effects are tolerable.91 This

may be due to less penetration to the CNS than

clonidine in addition to the receptor subtype

selectivity. On the other hand, to minimize access

the CNS without a loss of ocular penetration,

derivation of brimonidine has been reported.

Munk et al. synthesized brimonidine analogs that

possessed the 5-methyl group instead of the 5-

bromo group.92 The methyl derivative AGN

Table 6. Physicochemical and Ocular Pharmacokinetic Properties of Propranolol and its Oxime Prodrug88

Compound ACD log D7a

Concentration (mg/mL or g)b

0.5 h 1 h

Propranolol

3.21 1.28 (AHc)

8.05 (ICBd)

0.26 (AHc)

0.00 (ICBd)

Propranolone oxime

3.27 0.86e (0.04f) (AHc)

9.90e (2.11f) (ICBd)

1.51e (0.71f) (AHc)

1.79e (1.79f) (ICBd)

a

Distribution coefficient in pH 7 was calculated using ACD/Labs software.bDrug concentration after instillation at 0.5 or 1 h.cAqueous h umor.dIris-ciliary body.eTotal concentration (prodrugpropranolol).fPropranolol concentration.




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191103 and its corresponding benzodioxane deri-

vative AGN 192836 crossed the BBB, but the

structurally related benzoxazin derivative AGN

193080 did not. This benzoxazin derivative had

higher IOP lowering efficacy than clonidine. This

suggests that AGN 193080 shows good corneal

penetration with low BBB penetration. The

tetrahydroquinoxaline derivative AGN 192172

demonstrated neither IOP lowering efficacy nor

BBB penetration. These data indicated that the

introduction of morpholine ring may be useful for

decreasing BBB penetration without corneal

penetration.

Prostaglandin (PG) F2aDerivatives

Topical administration of PGF2a lowered the

elevated IOP of glaucoma patients.89,93 Ocular

Table 7. Physicochemical and Ocular Pharmacokinetic Properties ofa2 Agonists

Compound log D7.4a Papp

b (106 cm/s) C1h (mg/mL or g)c (Aqueous Humor/Iris) Refs.

Chlonidine

0.52 36 11.53/13.87 90

Aprachlonidine

0.96 0.44 0.59/0.87 90

Brimonidine (AGN190432)

0.17 9.8 7.67/7.84 90

AGN 191103

3.40 91

AGN 192836

91

AGN 193080

0.80 92

AGN 192172

3.90 92

aDistribution coefficient values in pH 7.4.bCorneal permeability.cDrug concentration in aqueous humor at 1 h after instillation.


2474 SHIRASAKI


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penetration of PGF2a is limited because it will

exhibit an ionized form in tears at neutral pH due

to the presence of the carboxylic acid function-

ality.94 Esterification of the carboxyl moiety at

1-position significantly increased the corneal

permeability and the IOP lowering activity

(Tab. 8).9598 Since esters of PGF2a are metabo-lized to the free acid in the cornea, they act as

prodrugs. Esterifications of alcohol moiety at

11- and/or 15-position of PGF2a also provided

prodrugs, which showed an enhanced corneal

permeability.98 However, 11,15-diester had lower

corneal permeability than 11- or 15-monoester.

Di-esterification may cause the decline of perme-

ability to the corneal epithelium and hydrophilic

stroma because it leads to an increase inmolecular weight and an excessive lipophilicity

causing strong interaction between the molecule

Table 8. Corneal Permeability and ACD logD7 of PGF2a and Related Compounds

Compound

Papp (106 cm/s)a

ACD logD7bRabbit Human Porcine

PGF2a derivatives

PGF2a R1R

11OH, R15OH 0.2

c/0.13d 1.7e 0.13f 0.09

R1R11OH, R

15OCOCH3 1.0c

R1R11OH, R

15OCOC(CH3)3 5.2c

R1OCH3, R11H, R

15OH 8.9g 2.70

PGF2a isopropyl ester (PGF2a) R1OCH(CH3)2, R

11OH, R15OH 19

c 3.3e 29f 3.58

R1OCH2Ph, R11OH, R

15OH 26g 4.45

R1

R11

1,11-Lactone, R15

OH 18c 2.80

R1OH, R11COC(CH3)3, R

15OH 7.7e 2.27

R1OH R11R

15OCOC(CH3)3 4.0c 2.6e 4.33

S-1033 (15-deoxy PGF2a) R1R

11R15OH 0.58

d 1.77

S-1033 methyl ester R1OCH3, R11R

15OH 1.3d 4.55

Isopropyl unoprostone R1OCH(CH3)2 0.95h 4.63

Unoprostone R1OH NDh,i 0.97

aCorneal permeability.bDistribution coefficient was calculated using ACD/Labs software.cRef. 97.dRef. 102.eRef. 98.fRef. 96.gRef. 95.hRef. 104.iThe drug was not detected in the receiver cell.




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and membrane lipids of corneal epithelium. The

9-pivaloyl ester and 9,11-lactone of PGF2a were

unsuitable for prodrugs, because they were not

substantially hydrolyzed by ocular tissue homo-

genates.99

S-1033, a 15-deoxy derivative of PGF2a, showed

higher permeability than PGF2a. Removal of thepolar hydroxyl group led to the increased perme-

ability. Additionally, esterification of the 1-car-

boxyl moiety resulted in a further increase in

corneal permeability (Tab. 8).100

PGF2a 1-ester analogs show high corneal

permeability and IOP-lowering activity, but are

not suitable for clinical use due to side effects such

as irritation and conjunctival hyperemia. There-

fore, to reduce the side effects, several v-side

chain (1320 position) analogs of PGF2a have been

synthesized. The addition of two carbons at the

20-position of a PGF2ametabolite (15-keto-13,14-dihydro PGF2a) provided a compound (unopros-

tone) with an improved side-effect profile in the

eye and without a loss of IOP-lowering activ-

ity.101103 Unoprostone is a carboxylic acid and

showed quite low corneal permeability (Tab. 8).

Therefore, its isopropyl ester (isopropyl unopros-

tone, UF-021) is marketed as an anti-glaucoma

agent.104 Substitution of carbons at positions

1820 of PGF2a with phenyl or at positions

1720 with phenoxy moiety also improved the

side-effect profile.105107 17-phenyl-18,19,20-tri-

nor PGF2a

(17-Ph-PGF2a

) and 16-(3-chlorophe-

noxy)-17,18,19,20-tetranor PGF2a (cloprostenol)

demonstrated a comparable corneal permeability

to PGF2a(Tab. 9).108 The isopropyl ester of 17-Ph-

PGF2a (17-Ph-PGF2a-IE) exhibited lower corneal

permeability than the isopropyl ester of PGF2a(PGF2a-IE). Latanoprost, the 13,14-dyhydro

analog of 17-Ph-PGF2a-IE, showed improved

pharmacological profiles and is widely used as

anti-glaucoma agent.10 Amidation of 17-Ph-

PGF2a is also effective for increasing corneal

permeability, but showed lower impact on corneal

permeability than esterification.109 This may be

because an amide group is a hydrogen bonddonor. Since the hydrolysis rate of amides is

generally slower than that of esters, amide

derivation is not suitable for prodrugs except for

nonsubstituted amide (CONH2), which showed a

somewhat higher hydrolysis rate than mono-

substituted amides (CONHR).110 Bimatoprost,

an ethyl amide derivative of 17-Ph-PGF2a, is used

for the treatment of glaucoma and is most likely to

show an ocular hypotensive effect without meta-

bolism to the corresponding free acid.109 Con-

version of the hydroxyl group at position 15 of

17-Ph-PGF2a-IE into a carbonyl group resulted in

an increase in corneal permeability. Since the con-

version did not significantly change the lipophi-

licity, the reduction of the number of hydrogen

bond donor would enhance the permeability.108

To enhance the corneal permeability of PGF2aand its analogs, esterification and amidation of

the C1-carboxyl functionality is the most desirable

derivation. Esterification of the alcohol moiety

can also increase the corneal permeability due to

an increase in lipophilicity and a reduction in the

number of hydrogen bond donors. All ocular

PGF2aanalog prodrugs on the market (isopropyl

unoprostone, latanoprost and travoprost) are

isopropyl esters, which would show suitable stabi-

lity in aqueous solution due to its bulky structural

nature.104,107,108 Because amide derivatives are

also generally stable for hydrolysis, they are sup-erior to ester derivatives in the self-life.

Anti-Infective Agents

Fluoroquinolones

Fluoroquinolone is a class of anti-bacterial agents,

which are widely used for the treatment of ocular

infection as eye drops.111115 Increasing the

lipophilicity of fluoroquinolones has a tendency

to increase corneal penetration (Tab. 10). Robert-

son et al. reported that the corneal permeability ofseven fluoroquinolones (which are used as

eye drops) is positively correlated with their

MardinDarby canine kidney (MDCK) cell perme-

ability.112 Moxifloxacin demonstrates the highest

ocular penetration in commercial products used as

eye drops.112115

Antiviral Agents (Acyclovir/Ganciclovir)

The acyclic guanosine analogs acyclovir (ACV)

and ganciclovir (GCV) are clinically used in the

treatment of various infections caused by the

herpes family of viruses.116,117 However, thesedrugs have low ocular permeability due to their

hydrophilic nature and did not show sufficient

efficacy for intraocular infection via topical

instillation.118,119 Therefore, the lipophilic pro-

drug approach of these drugs was investigated.

Esterification of ACV and GCV with fatty acids

has improved the corneal permeability (Tabs. 11

and 12).120125 These esters are rapidly hydro-

lyzed to the parent drugs in the cornea. It is shown

that increasing the length of an alkyl side chain of


2476 SHIRASAKI


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acyclovir ester prodrug from propionyl to hex-

anoyl leads to the gradually enhancement of

ocular penetration in vivo in addition to in vitrocorneal permeability.126

The conversion of ACV and GCV into their L-

valine ester (valacyclovir127 and valganciclovir,128

respectively) or into dipeptide esters having

specific amino acid sequences remarkably

enhance the corneal permeability.129,130 These

derivatives possess a free amino group and show

high aqueous solubility but low lipophilicity.

However, their corneal permeability is higher

than that expected from their lipophilicity. This

may arise from an active uptake by amino acid or

Table 9. Corneal Permeability and ACD logD7 of 17-Ph PGF2a and Related Compounds

Compound

Papp (106 cm/s)a

ACD logD7bHuman Porcine

17-Ph PGF2a derivatives

17-Ph PGF2a(17-Ph-18,19,20-trinor PGF2a)

R1OH

0.696c 0.10

Bimatoprost

R1NHC2H5 3.24d 1.98

R1

OCH(CH3)2 5.9e

3.56

15-Keto-17-Ph PGF2a 11.0e 3.34

Latanoprost (13,14-dihydro-17-Ph PGF2a) 6.8e 3.65

Cloprostenolol 1.49c 0.09

aCorneal permeability.bDistribution coefficient was calculated using ACD/Labs software.cRef. 98.dRef. 110.eRef. 108.




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oligopeptide transporters in corneal epithelium

such as LAT 1, LAT 2, and PepT1.131 Therefore,

such amino acid and dipeptide prodrug appro-

aches are useful for increasing ocular penetration.

The corneal permeability of these derivatives is

higher than that of aliphatic ester prodrugs. The

rank order of ocular penetration was Val-Gly >

Val>Tyr-Val>Val-Val in the case of acyclo-

vir amino acid or dipeptide esters.132 In the case of

ganciclovir amino acid or dipeptide esters, the

rank order of ocular penetration was Val-Tyr >

Val-Val>Val>Gly-Val.122

Table 10. Physicochemical and Ocular Pharmacokinetic Properties of Fluoroquinolone Derivatives

Compound Sol. (%)a

Papp (107 cm/s)

logD7.4

AQCmax (mg/mL)d

AUC01 (mg h/mL)e

MDCKb Corneac Ref. 115 Ref. 114 Ref. 113

Norfloxacin

0.05 3.3 1.63 1.60 0.21,

0.92

0.22,

0.92

Ciprofloxacin

0.02 4.5 2.46 1.52 0.64,

3.18

Lomefloxacin

0.13 6.6 3.58 1.45 1.25,

4.22

Ofloxacin

0.35 15.1 6.78 0.45 1.15,

3.20

1.59,

5.08

0.87,

2.24

Gatifloxacin

0.21 10.3 4.6 0.97 2.30,

5.90

1.26,

2.52

Moxifloxacin

>6.43 35.2 15.8 0.23 5.42f,

7.34f

aAqueous solubility.bMDCK cell permeability (Ref. 112).cCorneal permeability (Ref. 112).dDrug concentration in aqueous humor after three times topical dosing to rabbit at 15 min interval.eThe area under the curves for 0 h to infinity.fAQCmaxand AUC values of moxifloxacin (0.5% solution) were normalized to 0.3% solution.


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Anti-Inflammatory Agents

Nonsteroidal Anti-Inflammatory Drugs (NSAIDS)

Most of NSAIDS for eye drops have arisen from

oral NSAIDS such as diclofenac,133 bromfenac,134

flurbiprofen,135 pranoprofen,136 and ketorolac

tromethamine.137 Ocular penetration of these

drugs is not very high due to the incorporationof a carboxyl functionality that will completely

ionize in tears at a neutral pH. In addition to these

NSAIDS adapted from oral drugs, nepafenac, the

NSAID used only for eye drops, was recently

approved by FDA in 2005. Nepafenac is nonsub-

stituted amide prodrug of amfenac,138 which is

used as an oral drugs for the treatment of

rheumatoid in Japan, and was designed to

improve the corneal permeability and tissue

distribution profile.139 Nepafenac showed about

4- to 30-fold higher corneal permeability than

conventional NSAIDS such as diclofenac, bromfe-

nac and ketorolac (Tab. 13).140 The permeability

data of amfenac were not given, but amidation of

carboxylic acid in amfenac would result in at least

a 28-fold increase in permeability than the parent

drug, based on the estimation from permeability

of ketorolac and ACD logD7 value of ketorolac andamfenac. Since nepafenac showed low aqueous

solubility, it was formulated as an aqueous

suspension. Its high permeability enables it to

deliver to posterior segment of the eye in addition

to the anterior segment. Moreover, its duration of

action is longer than that of diclofenac.141 Con-

version of carboxylic acid into nonsubstituted

amide is useful for the enhancement of corneal

permeability, although it leads to a decrease in

aqueous solubility. Amides are most likely to be

Table 11. Physicochemical and Pharmacokinetic Properties of Ganciclovir Ester Derivatives

Compound

Solba

(mM)

logD7.4b or

ACD logPcPapp

(106 cm/s)dCmax

(mM)e,fAUCinf

(mM min)e,g Refs.

Aliphatic fatty acid esters

RH 15.7 1.55b 3.8 119

RCOCH3 15.2 1.08b 4.9 119

RCOCH2CH3 11.9 0.92b 5.7 119

RCO(CH2)2CH3 8.4 0.30b 7.7 119

RCO(CH2)3CH3 4.1 0.07b 24 119

a-amino acid or dipeptide esters

RH 3.4 2.07c 4.1 201 42259 130,132

RVal 92 1.28c 32 647 82112 130,132

RValVal 82 0.73c 31 943 301370 130,132

RValTyr 74 0.55c 1458 536278 130,132

RTyrVal 68 0.54c 130

RValGly 63 1.95c 130

RGlyVal 66 1.95c 109 34460 130,132

RTyrGly 68 1.77c 130

RGlyTyr 74 1.78c 130

aSolubility in pH 4.2 phthalate buffer.bDistribution coefficient values in pH 7.4.cPartition coefficient was calculated using ACD/Labs software.

dCorneal permeability.eAfter 2 h of corneal infusion (0.43%, 200mL) via topical well.fThe maximum concentration of GCV in aqueous humor.gThe area under the curve for 0 h to infinity in aqueous humor.




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hydrolyzed slower than ester and are metabolized

in iris-ciliary body and choroid/retina rather than

the cornea. However, in the case of a more

lipophilic NSAID such as flurbiprofen, amidation

did not change the corneal permeability

(Tab. 13).142

Steroids

Prednisolone and dexamethasone derivatives areused for the treatment of ocular inflammation.

Their acetyl and phosphate esters at the 21-

position of prednisolone are widely used as eye

drops. Both esters of prednisolone rapidly repro-

duced the parent drug in the cornea (Tab. 14). The

purpose of acetylation is to enhance the corneal

permeability, while the purpose of phosphoryla-

tion is to increase the aqueous solubility. The

corneal permeability of the acetate and phosphate

are 30-fold higher and 10-fold lower than that of

their parents, respectively.143 Although the phos-

phorylation greatly decreased its corneal perme-

ability, the AUC06h values in aqueous humor

after instillation of phosphate 0.5% solution to

rabbits were approximately equal to that after

instillation of acetate 0.5% suspension possibly

due to the great increase in aqueous solubility.144

Esterification of dexamethasone at the 21-

position with fatty acids also increases the corneal

permeability (Tab. 15).145 Increasing the alkylchain from acetyl to butyryl gradually enhanced

the corneal permeability. The valeryl ester is

slightly resistant to hydrolysis, but its flux of

dexamethasone through the cornea is similar to

that of butyryl ester. The palmitoyl ester did not

permeate substantially to the cornea. Since the

lipophilicity of palmitoyl ester is too high (log

P12.4), it may be trapped in the cornealmembrane. The phosphate and m-sulfobenzoateprodrugs of dexamethasone have been marketed

Table 12. Physicochemical and Pharmacokinetic Properties of Acyclovir Ester Derivatives

Compound

Solba

(mM)

logD7.4b/

ACD logD7c

Papp(106 cm/s)d

C25 mine or

Cmaxf,g (mM)

AUCinff,h

(mM min) Refs.

Aliphatic fatty acid esters

RH 11.2 1.22b/1.76c 3.7 38e 118,126

RCOCH2CH3 5.0 0.85b/0.57c 4.3 118,126

RCO(CH2)2CH3 4.6 0.08b/0.04c 5.1 42e 118,126

RCOCH2CH(CH3)2 4.8 0.06b/0.22c 3.9 118,126

RCO(CH2)3CH3 1.5 0.30b/0.49c 6.5 59e 118,126

RCOC(CH3)3 1.6 0.37b/0.13c 118,126

RCO(CH2)4CH3 0.7 0.93b/1.02c 8.5 79e 118,126

a-amino acid or dipeptide esters

RH >30 1.76c 4.2 129

RVal >30 1.71c 12 124g 7247 122,129

RValVal >30 1.52c 9.9 34g 2063 122,129

RValGly >30 2.27c 12 200g 12007 122,129

RValTyr >30 0.84c 7.2 129

RTyrVal >30 1.33c 8.3 80g 13930 122,129

aSolubility in pH 7.4 phosphate buffer.bDistribution coefficient values in pH 7.4.cDistribution coefficient in pH 7 was calculated using ACD/Labs Software.dCorneal permeability.eACV concentration in aqueous humor at 25 min after instillation of 50mL of a 1 mMsolution. The datawereconstructedfromthe

graph in Ref. 126.f

After 2 h of corneal infusion (200mL)via topical well.gThe maximum concentration of ACV in aqueous humor.hThe area under the curve for 0 h to infinity in aqueous humor.


2480 SHIRASAKI


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as water-soluble derivatives. The corneal perme-

ability of phosphate andm-sulfobenzoate is about1.3 and 10 times less, respectively, than that of the

parent. The 2.5 mM solution of the phosphate

partly penetrates into aqueous humor after

instillation. The AUC of the phosphate was

twofold less than that of the 2.5 mM dexametha-

sone suspension and threefold less than that of the

butyrate suspension. In contrast, the m-sulfo-benzoate solution was not detectable in aqueous

humor probably due to its low corneal perme-

ability.

Table 13. Physicochemical and Pharmacokinetic Properties of NSAIDs

Compound ACD log D7a Corneal permeability Papp (10

6 cm/s) Refs.

Nepafenac

1.17 64 140

Amfenac

0.62

Bromfenac

0.30 3.4 140

Diclofenac

1.28 15 140

Ketorolac

0.60 2.3 140

Flurbiprofen

1.31 21 142

Flurbiprofen amide

3.06 22 142

aDistribution coefficient in pH 7 was calculated using ACD/Labs Software.




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Table 14. Ocular Pharmacokinetic Properties of Prednisolone Derivatives143,144

Compound Papp (106 cm/s)a Cmax (mg/mL)

b AUC06h (mg h/mL)c

Prednisolone

RH

2.7

Prednisolone disodium phosphate

RP(O)(ONa)2 (1% solution)

0.26 1.5 (2.1)d 3.7 (4.5)d

Prednisolone acetate

RCOCH3 (1% suspension)

83 1.6 4.2

aCorneal permeability.bPrednisolone concentration in aqueous humor after instillation of prednisolone prodrugs.c

The area under the curve for 06 h in aqueous humor.dTotal concentration (prodrugprednisolone).

Table 15. Physicochemical Ocular Pharmacokinetic Properties of Dexamethasone Derivatives145

Compound log Pa Papp (106 cm/s)b Cmax (ng/mL)

c AUC03h (ngh/mL)d

Dexamethasone

RH

2.12 5.1 75 104

Dexamethasone

disodium phosphate

RP(O)ONa2

0.54 3.9 39 54

Dexamethasone

sodium m-sulfobenzoate

R

1.65 0.51


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Fatty acid esterification of steroids is extremely

effective for enhancing of corneal permeability.

Phosphorylation of steroids at the 21-position can

provide highly water-soluble prodrugs. Although

the phosphorylation led to a great decrease in the

corneal permeability, its drug penetration into

aqueous humor is similar or slightly lower thanthe parent drug due to compensation by the

increased aqueous solubility. It is expected that

the increasing drug concentration in formulation

will produce large concentration gradients and

high ocular penetration. Esterification with m-sulfobenzoic acid also resulted in an increase in

aqueous solubility, but its ocular penetration

would be reduced greatly because of a decrease

in corneal permeability.

OthersCalpain Inhibitor

Calpains, a family of cysteine endoproteases,

degrade lens proteins such as crystalline. There-

fore, calpain inhibitors are studied as a potential

anti-cataract agent.146148 SJA6017, a dipeptidyl

aldehyde, shows very high potency against two

major calpain isoforms.149 However, instillation

of SJA6017 aqueous suspension showed only low

concentration in aqueous humor (Tab. 16).150

The low ocular penetration of SJA6017 may be

in part due to the presence of the reactive

aldehyde group, which may form reversible co-

valent adducts with nucleophiles in biomolecules.

Consequently, to investigate calpain inhibitors

that have the superior corneal permeability,

Nakamura et al.150 modified the structure of

SJA6017 using two approaches. One approach

was to introduce into a pyridine ring as a water-

solubilizing group. Another is to convert the

reactive aldehyde moiety with hemiacetal moiety

that is a masked aldehyde with less reactivity.

Both approaches increased the ocular penetra-

tion. 3-Pyridlyacetomide analog (SNJ-1664) and

hemiacetal analog (SNJ-1709) of SJA6017 showeda six- and eight-fold higher ocular penetration,

respectively, following instillation of their suspen-

sions than SJA6017. Furthermore, the conversion

of a (4-fluorophenyl)sulfonamide moiety of SNJ-

1709 into a phenylthiourea moiety provided a

compound (SNJ-1715) with about 30-fold higher

increase of AUC in aqueous humor. SNJ-1709 and

SNJ-1715 have the similar aqueous solubility and

log P values, but have significantly differentmelting points (about 162 and 628C, respectively).

The superior ocular penetration of the thiourea

derivative is possibly due to the difference in

melting point. The low melting point may be

capable of enhancing corneal permeability

through an increase in the dissolution rate. A

good positive linear correlation between disso-

lution rate and oral bioavailability has beenreported.151 It was also reported that transdermal

absorption of drugs correlated with their melting

points.152 In the case of suspension formulation,

the difference in dissolution rate greatly affects

the ocular penetration because of the short con-

tact time between the drugs and the corneal

surface.153

Moreover, Nakamura et al.154 reported the

cyclic hemiacetal analogs (Tab. 16). A cyclic

hemiacetal SNJ-1757 in this series showed a

3.5-fold increase in corneal permeability in vitro

than did the corresponding linear aldehyde(dehydroxy analog SNJ-1770). Both compounds

were evaluated for reactivity with semicarbazide

(H2NNHCONH2) hydrochloride, which simulates

biomolecules containing nucleophiles such as

the NH2 and SH groups. The cyclic hemiacetal

SNJ-1757 did not significantly react with semi-

carbazide, whereas the corresponding linear

aldehyde SNJ-1770 immediately reacted and

formed a semicarbazide adduct. These results

suggested that the presence of aldehyde moiety is

a limiting factor for corneal permeability.

Conversion of the aldehyde into the hemiacetal

provided an increase in corneal permeability,

which may be ascribed to a reduction of the

electrophilicity. A decline of electrophilicity by

structure modification or formation of prodrug

is desirable for enhancement of the corneal

permeability.

Quinidine

Quinidine is not an ocular drug, but has been

investigated for the corneal permeability as a

model P-gp substrate. Jain et al.155 attempted

the transporter-targeted prodrug derivatization.Esterification of the hydroxyl group of quinidine

with L-valine or L-valyl-L-valine showed a 1.5- and

3-fold increase in the corneal permeability com-

pared to quinidine (Tab. 17).156 The corneal

permeation of quinidine is most likely to be

limited by efflux, via P-gp on the corneal epi-

thelium. Attaching L-valine or L-valyl-L-valine to

quinidine not only significantly decreases the

affinity to P-gp but also may increase the affi-

nity to oligopeptide transporters. Such prodrug




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approaches are useful for the improvement ofcorneal permeability of P-gp substrates.

APPROACHES FOR MOLECULAR DESIGN TOENHANCE OCULAR PENETRATION

As mentioned above, to enhance the ocular

penetration of drugs, aqueous solubility and

corneal permeability can be important factors.

Approaches for molecular design to improve these

factors are described below. There are roughly two

classifications of these approaches: the structuremodification of molecules themselves and the

prodrug approach. These strategies are summar-

ized in Table 18.

Structure Modification Approach(Nonprodrug Approach)

Ophthalmic drug compounds require not only

high aqueous solubility but also adequate lipo-

philicity to penetrate to the membranes. Although

Table 16. Physicochemical and Ocular Pharmacokinetic Properties of Calpain Inhibitors

Compound

Solb

(mg/mL)a logD7b

Papp(106 cm/s)c

Cmax(mg/mL)d

AUC

(mg h/mL)e Refs.

SJA6017

0.10 1.7 ND 0.037 0.051 150

SNJ-1664

>100 ND 0.21 0.31 150

SNJ-1709

1.5 0.60 ND 0.21 0.39 150

SNJ-1715

1.5 0.70 1.1 1.0 1.7 150

SNJ-1757

2.0 0.38 19 ND ND 154

SNJ-1770

0.91 0.54 5.4 ND ND 154

aSolubility in pH 7 phosphate buffer.bDistribution coefficient in pH 7.cCorneal permeability.dAqueous humor concentration after instillation of 50mL of 0.5% suspension.eThe area under the curve for 03 h.


2484 SHIRASAKI


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an introduction of strong ionizable centers, such

as phosphate, sulfonate, carboxylate and guani-

dino groups increases the aqueous solubility, it

will also simultaneously decrease the membrane

permeability because of the increase of the

charged form. In contrast, incorporation of lipo-

philic moieties to enhance the corneal perme-

ability led to a decline in the aqueous solubility.

Moreover, the excess lipophilicity would reduce

the unbound fraction in the target tissues and the

pharmacological activities. It is desirable that the

molecules for eye drops should be designed to have

high aqueous solubility without a loss of lipophi-

licity for absorption. These examples of molecular

design are summarized in Table 19.

An example of the molecular design is an

incorporation of heterocycles including more than

two heteroatoms, which generally show high

solubility for both aqueous solution and organic

solvent, and can be considered to be amphipathic

molecules (amphiphiles). Introduction of hetero-

cycles led to the ophthalmic drugs, such as

dorzolamide, timolol and brimonidine, having

excellent ocular pharmacokinetic properties.

Another approach is to introduce a nonionic

amphiphile like an oligoethylene glycol methyl

ether chain, which is able to enhance aqueous

solubility without a large loss of lipophilicity. In

the case of oral absorption, incorporation of such

functionality produced the highly oral bioavail-

able compounds.157,158 Introduction of a basic

moiety like an amino or pyridine moiety as anionizable center is also useful for increasing

aqueous solubility. In this case, one problem is

the pKavalue of the basic groups. The compoundsthat extensively ionize around neutral pH may

decrease the corneal permeability. Since ampho-

lytes like fluoroquinolones and dorzolamide would

exist as zwitterions, which is an apparent non-

ionic species, it is expected that the compounds

will show high aqueous solubility and the corneal

permeability to some extent.

Table 17. Partition Coefficient and Corneal Permeability of Quinidine and its Val and ValVal Derivatives156

Compound log Pa

Papp (106 cm/s)b

None Verapamilc GlySard

Quinidine

RH 19 71

ValQuinidine

RVal 3.52 31 35 18

ValValQuinidine

RValVal 4.62 52 50 35

aPartition coefficient.bCorneal permeability.cCorneal permeability with verapamil (P-gp inhibitor).dCorneal permeability with Glycylsarcosine (oligopeptide transporter substrate).

Table 18. Summary of Strategies for Improvement of

Ocular Penetration

Structure modification (nonprodrug) approaches

Optimization of lipophilicity

Enhancement of aqueous solubility

Reduction of hydrogen bond donors

Reduction of molecular weight

Removal of highly reactive moieties

Decline in melting point

Prodrug approaches

Optimization of lipophilicity

Enhancement of aqueous solubility

Reduction of hydrogen bond donors

Enhancement of affinity for uptake transporters




25/36

In addition to lipophilicity, hydrogen-bond

ability also involves membrane permeability. A

decline in the number of hydrogen bond donors

can enhance the membrane permeability.

Removal of hydrogen bond donor is more effective

for improvement of membrane permeability than

introduction of additional hydrocarbon.159 The

corneal permeability of the 15-keto-17-Ph PGF2a-

IE is twofold higher than 17-Ph PGF2a-IE (the

corresponding 15-alcohol analog; Tab. 9).108

Removal of the 15-alcohol group of PGF2a also

provided about 10-fold increase in corneal perme-

ability.102 In the case ofb-blockers, the drugs with

hydrogen bond donors in the substituents showed

lower permeability than the drug without hydro-

gen bond donors (Tab. 4).79 Additionally, it has

been reported that P-gp, an efflux transporter,

recognize hydrogen bond donors. An increase in

Table 19. Examples of Structure Modification (Nonprodrug) Approaches for Improvement of Ocular Penetration

Strategy Approach and examples

Lipophilicity and aqueous solubility Incorporate heterocycles including more than two heteroatoms

Preferable logD7.4 is about 23

Incorporate nonionic amphiphiles

Incorporate amino groups

Avoid strong ionizable centers

Hydrogen bonding Reduce hydrogen-bond donors

Molecular weight Reduce molecular weight MW


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number of hydrogen bond donors in molecules

may raise the possibility of substrate recognition

by efflux transporters including P-gp. Therefore,

to enhance the corneal permeability, reduction of

number of hydrogen bond donor is important.

Small molecular weight tends to increase both

aqueous solubility and membrane permeability.For instance, CNS drugs on the market, which

need to pass BBB that is a very tight barrier, have

smaller molecular weight than nonCNS drugs.160

In ophthalmology, the P3 truncated calpain

inhibitors can pass the corneal membrane more

easily than calpain inhibitors having the P3

moiety (Tab. 16).154 Small inhibitors will possess

both high aqueous solubility and corneal perme-

ability, possibly leading to a high ocular bioavail-

ability.

Reactive functionalities often limit the corneal

permeability. Of course, the functionalities thatform irreversible covalent adducts should be

avoided. Functionalities like aldehydes, which

can react with amino or thiol groups in biomole-

cules and produce the reversible covalent adduct,

and possibly reduce the corneal permeability.154

Replacement of an aldehyde moiety with a

hemiacetal moiety, which is a masked aldehyde,

increases the corneal permeability possibly due to

decline of the electrophilicity in the molecule

(Tab. 16).150,154 The lower reactivity would

diminish the reaction between the compound

and nucleophiles in membrane substances. Both

replacement of the reactive moieties with less or

nonreactive ones, and designing prodrugs with

their reactive sites masked are favorable for

enhancement of the corneal permeability.

In the case of suspension formulation, not only

aqueous solubility but also the dissolution rate

will affect ocular penetration. The rapid dissolu-

tion rate of the compound is preferable to

suspension formulations. A tentative index to

predict the dissolution rate is the meting point.

The thiourea calpain inhibitor with lower melting

point (MP 628C) demonstrated superior ocular

penetration to the sulfonamide derivative (MP1628C).150

Prodrug Approach

Prodrug design is a useful method to improve

physicochemical and pharmacokinetic properties.

Prodrugs are drugs with attached functionalities

in order to obtain favorable structural natures and

will regenerate their active parent form by

enzymatic or chemical reactions.161 In ophthal-

mology, this approach is mainly used to improve

the corneal permeability and aqueous solubi-

lity.17,162,163 Additionally, application for the

site-specific chemical delivery system (CDS) for

iris-ciliary targeting has also been investigated.20

These prodrug approaches are capable of signifi-cantly improving the physicochemical properties

without a decreasing pharmacological activity.

However, the functional groups that can connect

the pro-moiety are limited to several groups, such

as hydroxyl, carboxyl and so on. These examples of

this approach are summarized in Table 20.

Prodrug derivatization is very effective for

enhancing not only the lipophilicity but also the

corneal permeability in circumstances where

incorporation of polar functionalities like a

carboxylic acid and an alcohol moiety lowered

the corneal permeability of the molecules. Thecompounds including carboxylic functionalities

generally showed low corneal permeability due to

their ionization in tears at a neutral pH. Ester

derivatization of carboxylic acids can increase

lipophilicity and lead to the significantly improved

corneal permeability. Since the cornea has high

esterase activity, eater prodrugs can easily

regenerate the parent drugs.17 The major pro-

blems of ester prodrug derivatization are that

ester prodrugs would show decreased aqueous

solubility and increased susceptibility for hydro-

lysis. Considering their stability in an aqueous

solution, the bulky esters are desirable for pro-

moieties. For instance, all marketed ophthalmic

PGF2a prodrugs are bulky isopropyl

esters.104,107,108 However, more bulky tert-butylesters are inadequate for pro-moieties because

they resist enzymatic hydrolysis.164,165 Conver-

sion of carboxylic acids into amides is also effective

for improving the corneal permeability, but its

effect is less than that of esters (Tab. 9). Although

the nonsubstituted amides are partly hydrolyzed

by ocular tissues (Tab. 13),140 the reactions of

mono-substituted amides are generally slower

and di-substituted amides are not substantiallyhydrolyzed.109 Thus, mono- and di-substituted

amides are not suitable for a pro-moiety.

The presence of hydrogen bond donors like an

alcohol or a phenol group causes low corneal

permeability. The conversion of them into fatty

acid esters increases the corneal permeability

(Tabs. 5, 8, 11, 12, 14 and 15). Taking into

consideration their stability in an aqueous solu-

tion, pivaloyl esters are favorable as pro-moiety.

The dipivaloyl ester of epinephrine is marketed as




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the ophthalmic solution for the treatment of

glaucoma.62 Ester prodrug derivatization is also

used in steroid drugs. In the case of compounds

having multiple alcohol functionalities such as

prostaglandins, each ester prodrugs having pro-

moiety at various positions should be compared,

because the impact on corneal permeability

depends on the position of esterification. The

permeability of multiple esters of a compound

with more than two alcohol groups is not alwayshigher than that of monoesters (Tab. 8).

Replacement of alcohol moiety by ketone leads

to an increase in corneal permeability through a

decline in the number of the hydrogen bond

donors. Since ketones are most likely to regener-

ate alcohols by ketone reductase in the corneal

epithelium and the iris-ciliary body,20 ketones can

often be a pro-moiety. Furthermore, conversion of

ketones into oximes or methyloximes provides the

prodrug with increased stability in an aqueous

solution and sustained release of the parent

compound, because the oximes and methoximes

are generally more chemically and enzymatically

stable than the corresponding ketones. Since the

oximes and methoximes are hydrolyzed to the

corresponding ketones by enzymes that exist in

the iris-ciliary body, these can be considered as

site-specific enzyme activated delivery systems.

The several oximes or methoximes analogs ofb-

blocking agents showed a higher and moresustainable IOP reducing effect than the parent

compounds (Tab. 6).20 These compounds did not

induce the transient bradycardia, a major side

effect ofb-blockers, due to their nonactivation in

plasma.

On the other hand, the introduction of a strong

ionic moiety like a phosphate as pro-moiety easily

enhances aqueous solubility. Phosphate and m-sulfobenzoate ester prodrug approaches are use-

ful to improve the aqueous solubility. Although

Table 20. Examples of Prodrug Approaches for Improvement of Ocular Penetration

Strategy Approach and examples

Lipophilicity Convert carboxylic acid into ester

Convert carboxylic acid into amide (slow hydrolysis)

Convert alcohol into ester fatty acid esters

Aqueous solubility Convert alcohol to phosphoric or m-sulfobenzoic acid ester

Hydrogen bonding Convert alcohol into ketone

Transporter Convert alcohol to a-amino acid or di-peptidyl esters (transporter-mediated)

Others Convert alcohol into oxime or methoximes via ketone (slow hydrolysis)


2488 SHIRASAKI


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such strong ionic moieties significantly increase

the aqueous solubility, they decrease corneal

permeability. Therefore, these approaches may

be beneficial for lipophilic parent drugs like

steroids (Tabs. 14 and 15). In this case, the

increased solubility may decrease the corneal

permeability. The phosphate prodrug of predni-solone shows approximately equal ocular pene-

tration after instillation to the corresponding

acetate ester, even though the phosphate had a

320-fold lower corneal permeability than the

acetate.

Conversion of a hydroxyl group into a specific a-

amino acid ester or a dipeptide ester with a

specific sequence results in enhanced corneal

permeability most likely due to transportation by

amino acid transporters (such as LAT 1 and 2) or

oligopeptide transporters (such as PepT1) in

corneal epithelium. Incorporation of a free aminogroup also contributes to the increased ocular

penetration through an increase in aqueous

solubility. The compounds with (1) a specific a-

amino acid ester or (2) dipeptide esters with a

specific sequence recognized as a substrate by

uptake transporters, both showed higher ocular

penetration than that expected by their lipophi-

licity (Tabs. 11 and 12). This is a very effective

approach for enhancement of ocular penetration,

but the use will be limited by the parent compound

structure, its molecular weight, and physicochem-

ical properties.

So far, ester and amide prodrugs have been

reported as marketed eye drops. Since the linkage

between pro-moiety and the parent drug is most

likely to be chemically labile, the stability of the

formulation is often problematic.

CONCLUSION

As the many previous reports mentioned, a

balance between lipophilicity and hydrophilicity

is the most important factor for ocular penetra-

tion. The compounds for ophthalmic solution arerequired a higher aqueous solubility than oral

drugs. The amphipathic structure incorporating

heterocycles or nonionic amphiphiles would be

favorable for the enhancement of ocular penetra-

tion due to addition of an appropriate lipophilicity

and hydrophilicity. Prodrug approaches for car-

boxylic acids and alcohols are also useful for the

enhancement in corneal permeability. In this

case, we should pay attention to the decline in

aqueous solubility and stability in an aqueous

solution. In the case of formulating the compounds

as an aqueous suspension, a compound having a

lower melting point is preferable. P-gp substrates

should be avoided to enhance corneal permeabil-

ity. On the other hand, the substrates of uptake

transporters may demonstrate high permeability.

Thus, in an ophthalmic drug, the molecular designconsidering pharmacokinetic properties can pro-

vide more effective and less adverse drugs.

Moreover, such physicochemical property-based

drug design may be able to provide ocular drugs

that can reach the posterior segment of the eye via

topical instillation, in addition to the anterior

segment.

FUTURE DIRECTIONS

Currently, the conformation and pharmacophoreof many transporters and metabolic enzymes are

not fully understood. In future studies, the

understanding of crystal structure and the site

of binding for compounds will be developed and

will result in the capability of designing molecules

with good pharmacokinetic properties by in silicoscreening. I expect that further development in

the field of ocular pharmacokinetics and an

understanding of transporters and metabolic

enzymes will produce excellent drugs in the near

feature.

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