Exp. I,.,~/e Res. (1987) 44, '2 ! 7-2:2:6 The Ocular Phar~acoki~etics of Eicosanoids and their D i "" s er vatlve. 1. Co parison of Ocular Eicosa~oid Penetration 1" a~d Distribution Following the Topical App ication of PGF2=, PGF2=-I- ethyl ester, and PGF2~-l-isopropyl ester L. Z. BITO AND R. A. BAROODY Deparlme,t of Ophthalmology, Research Division, Colleffe of Physicians and Surgeons, Columbia University, NY 10032, U.S.A. (Received 11 July 1986 and in revised Jbrm 18 August 1986) here experiments were undertaken to determine whether the increased ocular hypotensive potency of topically applied prostaglandin (PG) PGF2a esters, as compared with that of PGF2~ acid, n be accounted for by incre d netrafion of the eicosanoid moiety of the esterified PG into the e. One hour a r the topical application of [aH] Fa:-l-methyl ester (ME) in peanut oil, the aH activities in the cornea, aqueous humor, and ciliary body of the rabbit eye werefl2-, 22-~ and 8-fo]tt higher, respectively, than they were following the topical application of [aH]PGF2~ free acid. aH activity during the flint 3 hr declined rapidly in the cornea and more slowly in the aqueous humor, but remained essentially constant in the ciliary body for up to 6 Ilr, declining ratfidly only between 6- and 24 hr. aH activity in eyes that received [aH]POF2~ ME was also several-fold higher in the anterior sclera and iris than in eyes that were treated with [aH]PGF~a e acid, but this difference was much smaller in the eonjunctiva. At 1 hr, most of the aH activity in the aqueous Ilumor was a~sociated with PGF2a, as determined by chromatography, but at 2- anti 3 hr other peaks, umably reflecting metabolites of PGF2a, became appar~ant. The penetration and int cular distribution of all activity w similar when [aH]PGF2~ ME was applied to the eye in normal saline rather than in anut oil or when the isopro I rather than the methyl ter of PG~a us~. e studi indica~ that esterification of the carbaxyl group of PGF~a greatly enhances the penetration of the PGF2a moiety into the eye and sug~sts that e etive de- terification of the PGE2a ester occurs in the cornea, suiting in the deliver, of F2~ acid into the aqueous humor. It is ncluded that topically a i~ PG esters act as pro-drugs and that the increased ocular penetration of these es~m may account for the l)reviously reported increase in their o~ular hypotensive potency a~s compared to that of PG acid or salts. Key words: eye; prostaglandin, pharmacokinetics; PGF~a; prostaglandin estem; glaucoma; corneal l)ermeahility. 1. Introduction Based on reports from many laboratories, it has become generally accepted that prost landins (PGs) have adve e e cts on the e, such as breakdown of the blood-aqueous barrier and a sharp increase ill intraocular p ssure (IOP), and tl~ercfore u~t be regarded as mediators of ocular inflamInation (see Eakins, 1977). However, in a recent review, it was concluded that early results on the ocular e cts of PGs, which were obtained mostly from rabbits, should not be generalized; that these reports tended to overstate the adverse effects of PGs on the eye; and that the primary ocular e ct of PGs in most species, especially primates, is a reduction, rather than an increase in IOP (Bito, 1984a). In recent studies, significant reductions of lOP, produced as a result of daily or twice daily PG application, were maintained in both ca~ and rhesus onk s ~ although not in rabbits- for as long as the t at ent was continued {Bi~, D a, Blanco and Cam s, 1983). This long-~rm treat ent was not associated with progressively, clinically significant~ local or systemic side e cts (Bito, Srinivasan, Baroody, and Schubert, 1983). It has been suggested, therefore, that PGs and eicosanoids in general 4 " ~ °0' (~)1 --48.L)/87/0~ 217 + 10 $03.(~)/0 (~) 1987 Academic Press lnc. (London) Limited
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`218 L.Z. BITO AND R. A. BAROODY should be considered as a new class of potential anti-glaucoma agents (Bito, 1984a; 1985). In fact, PGs appear to be ideally suited for the treatment of glaucoma by topical application. Because PGs are natural compounds that are normally present in the eye, they a unlikely to be toxic, and because th are not e ctively metabolized by intraocular tissues (Eakins, Atwal and Bhattacherjee, 1974; Bito and Baroody, 1974), topically applied PGs can I)e expected to reach tissues of the anterior segment in an active form. On the other hand, their accumulation in the extraeellu|ar fluids of the tina is ~inimized by tl~e PG transport, function of the ciliary epitheliu and the blood-retinal barriers (Bito and Salvador, 1972", Bito and Wallenstein, 1977). Furthermore, tol)ieally applied PGs ttlat, enter the venous drainage of the eye are unlikely to produce systemic side effects because PGs a actively oved from tlle circulation and are e ctively inactivated by the lungs and kidneys (Bito, 1986). It has been shown, however, that the corneal-presumably by virtue of the tight-~unctional surface layer of the corneal epithelium- is not readily per cable to PGs, which are highly polar compounds that do not readily penetrate the basic cell membrane (Bito and Baroody, 1975, 1982). It has been argued, therefore, that less polar PG derivatives may be more effectively delivered to intraoeular tissue compartments from topiea|ly applied solutions than naturally occurring free acids can be (Bito, 1984a). Indeed, some esters of PGF~ exhibit a 10- to 30-fold greater ocular F itself Bi , 1984b). The present investigation was hypotensive potency than PG 2~ ( to undertaken, therefore, to co pare the lative efficacy of eicosanoid delivery into the aqueous humor and other intraocular compartments after the topical application of all-labeled PGF and its methyl and isopropyl esters 2~ " 2. Materials and Methods Twenty-five microliters of peanut oil or physiological saline containing 0"2 9Ci of [all ]PGF2= (New England Nuclear Co.; specific activity 12-2 Ci mmol -l) and either [aH]PGF~ l-methyi es~r (PGF2~-ME)or isopropyl ester (PGF2~-IE)(Pharmacia, Uppsala, Sweden; specific activ-ity 1-40- and 0"69 Ci mmo1-1, respectively) was applied with a micr ipet to the cornea of either eye of conscious female New Zealand Whi~ rabbits (2"5-to 3"0 kg). All solutions contained sufficient carrier to bring the total PG concentration to 5 ~.g PGF2a equivalent per 25 ~1 of solution. For preparation of the saline solution of tile PGF~ free acid or its methy! or isopropyl esters these compounds were first dissolved in a small volume of ethyl or benzyl alcohol; the final solution contained proximately 1 ~/o alcohol. The peanut oil preparations contained no~ other solvents. Both eyes of each rabbit were given drugs in the sa e vehicle solution. However, one eye received [3H]PGF~,z, while the contrala~ral eye received either [aH] F2~-ME or [aH]PGF2~-IE. The procedure was adopted in order to reduce the number of bits that had to be killed, since it had been shown earlier th ~llowing the topical application of [aH]PGF2~ to one eye, the 3H activity in the contrala~ral e is less than 0" 1% of that in the treated eye (Bito and Baroody, 1982). The rabbits were killed with an overdose of sodium pentobarbita! (Diabutol, Butler Co., lumbus OH)at 0"25-, 0"5-, i-, 2-, 3-, 6-, or 24 hr a r the application of these solutions. The aqueous humor w withdr n and the globe and as much conjunetival tissue as possible were removed. The globes were then dissec~d. All tissues and fluids were weighed separately and then either oxidized in a Packard Tri-Carb _Model 306 sample oxidizer or elu~d 9vernight in~ 10 ml of Aquasol ( w England Nuclear). A]! samples ~ coun~d in a Packard Model 3 3 liquid scintillation counter; propria~ corrections were made i~r quenching. In experiments involving chromatographic identification of the tracer in the aqueous humor, the total amount of the PGF2~-ME was kept constant, but the amount of [aH]PGF~- ME per 25 ~.| of applied solution w increased to 1-0 ~Ci. After 1-, 2-, or 3 hr, animals were killed and samples were collect~ed above, except that only a 25-~1 aliquot of the aqueous
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`PHARMACOKINETICS OFTOPICAI.LY APPLIEI) PGF,a ESTERS 219 humor was used fbr direct isotope colznting; the remainder of each sample (about 200- to 250 .u]) m two to ffJur identically treated eyes was pooled and extracted by the procedure of Stam~rd and Unger (1972). Aliquots of the aqueous phase and the chloroform ext cts were counted as above, and sho~'ed an extraction efficiency from rabbit aqueous humor of great~:r than 97 % for both PGF and its methyl ester. Ha|f of the r~maining extracted 2~ material was chromatographed according to the A] system of Green and Samuelsson (1964), using a solvent system of chloroform - benzene ' methanol " acetone " acetic acid ~5 5). The other half of the ma~rial w chromatographed under similar (1 -100"25-o . conditions, using acetone" triethylamine (99' I) as the solvent system. in most oi'these experiments, the concentration of the aH-labe|ed eieosanoids or eieo noid derivatives in ocular tissues or fluids after the topical application of 5/,g of [aH]PGF,~ or [aH ]l GF~ esters was calculated on the basis of the aH activity in each sample and the specific activity of the PGF,a or its esters in the topically applied solution. In both the PGF~ a f~ee acid and its esters, the aH label was on C9 in the cyclopentane ring, a position at which the label remains associated with the eicosanoid moiety in all initial metabolites and even in most P i " urinary excretion products of PGI ,~. Since int ocular tissues exhibit little or no eapac t:, to metabolize PGs, it is reasonable to assume that the aH activity in all these tissues represented the concentration of an eicosanoid or its initial metabolite. On the other hand. the I~ F ,~ esters are expected to be de-esterified relatively rapidly and as will be seen later, most of the aH activity in the aqueous humor was shown by paper chromatography to be associated with PGF,~ free acid alter the topical application of PGt ,~-ME For these reasons, 'PG e *' I ,~ will be used throughout this paper to denote all 3H-labeled PGF,~ and-or F,~-ME or IE derivatives lound in eacl~ sample calculated on the basis of PGF equivalents. 3. Results Fifteen to 60 rain after the topical application of 5-~g [aH]PGF~a-ME in 25/ll of peanut oil, 2- to 3 % of the total topically applied aH activity was found in the cornea O as compared with 0-04- to 0"07 ~ of the total topically applied activity in corneas that eeived PGF~ fi-'ee acid in the same vehicle (Fig. I AI. At 1 hr after topical application, the concentration of PGF2~* was some 30-fold higher in corneas that. received P(.IF2~-ME than in those t ated with the sa e dose of PGF~ free acid (Table 1). During the first 3 hr, there was a rapid exponential decline in the PGF~* concentration of the cornea; this was followed by a much slo~er, rate of loss between 3- and 24 hr (Fig. 2). At 6 hr after the topical application of [3H]PGF~-ME, the PGF-z~* concentration in the cornea was still high (approximately 0.2 ng per mg ti ue), some 10-fo|d i~igher than the simultaneously measured concentration of PGF 2 * in the contralateral eyes, which received the same dose of [aH]PGF2~ free acid (Fig. 1 A). Up to 6 hr, the PGF~a* concentration in the aqueous humor was 10- to pc:le -ME treated'eyes than in those treated with PG_F~ free 20-fold higher in the ~ "-~'~ 2a ~oncentratlon of PGI~ e~ in tim conjunctiva of PGF2~-trea~d eyes acid. In gene 1, the ~ " ~ * was only slig|~tly lower than t tlat in the conjunctiva of PGF2~-ME-treated eyes (Fig. l, panels B vs. A). The PG F2~* concentration in the ciliary body was remarkably high at 15 rain after the topical application of PGF2~-ME. There was a s all, statistically insignificant increase at I- and 2 hr, followed by only a slight decrease at 3- and 6 tlr. The amount of PG~a* in the ciliary body at any time during the first 6 hr equalled about 0.2- to 0-3 % of the total amount of [ G~2a-~IE contained in the topi lly applied solution • concentration in the ciliary body w 8 to 10- (Fig. I D). In these eyes, the PGF~ . - fold higher at all time periods than in those treated witll PGF2~ e acid (Fig. i D; Table I). A similar time-course was observed in the concentration of PGF~* in tim iridial portion of the anterior urea (Fig. 1F).
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`220 L.Z. BITO AND R. A. BAROODY A Cornea i .............. ~~6F2a melhyl eSf~ 0 O.S 0.4 ~o ~,o 1.0 !~.o ~ v ~_~_.._o=: ~ .... ............... = T ...... • _~ L C. ~ueous Numor 02 E An~erlor Sclero % w 0 l 2 3 4 5 6 24 Hours B Conjunctivo , ~o ~.o ,4.0 ~.0 ~ 0 ~ ......................... ; ................ ............ ; ............. = ............... ..... ~:~ ~ a cmo~ e~ ~- ! 0.I ~. °' E Iris o.oe ~t ~04 ~-~ 0 ~--__~ ~ ....... 7 ...... ____~. ~ ~~ o I 2 3 4 5 6 24 Hours Fla. I. The distribution of all-labeled eicosanoids (PGF~=*) in ocular tissues at various intervals after the topical application of [aH)-PGF2a or |aH]PGF2a-ME to the ~es of rabbits. The le~ abscissa shows G N the aH activity expressed ng of PGF~a equivalent per mg of tissue. The right abscissa indicates P Y 2~ equivalents ~r total tissue weight expressed ~ rcent of the total PGF2a equivalent applied to the ~e at time zero. Poin~ repr ent means obtained on 4 to 10 and the limits rep t ± l S.E.(:,I.). TABLE 1. ncentration of 2a* in all ocular tissues at 1 hr after tr i 1 lication of 25 ~l of peanut oil or saline containing 5/~g of F~ equivalent of either F.z~-Me or [aH] F2~ free acid ng of PGF2~* per g of tissue ....................... ~ :~ .......... ........................... ~ ......................... PGF2~ PGF2~ (A) (B) (c) Peanut oil Saline Oil _-~_ __==-_ : ............ ~=~ ........................... ...... ...................... ~ ............. ........................................................ -- __ .... ........ -~: ........ Aqueous 222 ± 223 ± 37 12 ± 6 Vitreous 11 ± 4 4 ± I 3± i Cornea 1470 ± 3 973 ± 71 47 ~ 9 Anterior sclera 228 ± 41 27"7 ' 37 38 ~ 8 sterior sclera 89 ~ 14 75 ~ 9 27 ± 3 Iris 349 ± 72 21'2 ' 28 62 + 23 Ciliary body 242 ± 35 i ~ 7 <: ~ 5 2I ± 3 ~ns 42~17 : " i 12±I tina 69± 12 10± 2 19±3 Choroid 144 ± 27 38 =I= 7 31 + 7 Total globe 128 ± 22 74 ± 6 14 ~ 1 Conjunctiva 398 ± 49 266 ~ 56 138 ± 8 PGF~a-ME :PG F2a (D) (A:C) (B:D) Saline Oil Saline 1! ~3 19 20 1~1 4 4 35~ 8 31 28 39 4- 7 6 7 25___4 3 3 13~4. 6 16 I l ~ i 12 13 I+_i 4 1 6~1 3 2 13_1 5 3 6± l 9 i2 lo5d: 16 3 3
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`PHARMACOKINETICS OF TOPICALLY APPLIED PGF~a ESTERS 221 2.0 ~ ~ 1.0- ~ 0, 5 O! ~ 0,05 ~ .~ 0.02 0.0t ~ 0.~5 o 0.002 0.001 ' Cornea Aqueous Humor ...................... ' ~-~ .................. i .................. ....... ~ ......... ................... ~ .... ........................... ....... .............. 0 2 4 6 8 I0 12 14 16 18 20 22 24 Time (hou~) F~o. 2. Semilogarithmic plot of the time course of eicosanoid concentration, given in PGF~ equivalents, in the cornea and aqueous humor after the topical application of 5 #g of [aH]PGF~a-ME to bbit ~ . See also ]~end to Fig. 1. ~4. A [,2" .~ I.O- O ~ .4' ~ .2" ~ 0" o Conjunctiva [~] PGF2e [~ PGF2e-methyl ester PGF2a- isopropyt ester Cornea Aqueous Iris Ciliary Amerior Sc Fro. 3. Comparison of PGF~a* concentrations in various tissues I hr (Panel A} and 6 hr (Panel B) after the topical apl)lication ofl)eanut oil containing [aH]PGF~, [aH]PGF2a-ME, or |aH]PGF2a-IE. Them was no significant di~rence in tim l~F2a* concentration of tissu obtained from ~ that we~ t~_~ated with the methyl ester or the isol)ropyl ester. After the administration ofPGF2a-ME, the time cour of the PGF2a* concentration ij1 the anterior portion of tile sclera was similar to that in tl~e conjunctiva, although at the peak concentration (30 rain) and at ost other time periods, the PGF2~* concentration ill tile anterior sclera was some 30 % lower than tlmt.in the conjunctiva (Fig. 1 E vs. 1 B; Table 1). However, in the anterior sclera, unlike the conjtlnctiva, * concentration between eyes that ceived there was a large difference in the PGF2a PGF2a-ME and those that received PGF2a- e acid. The penetration of PGF2~* into the globe and its distribution within ocular tissues following tile topical application of the isopropyl ester of [all] F~ was not significantly diffe nt m that observed after the topical application of the methyl ester of [3H]PGF~ (Fig. 3). In all of the experiments presented above, peanut oil was used as the vehicle solution. However, a similar picture was obtained when PGF2a-ME was applied to the
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`222 L. Z. BITO AND R. A. BAROODY 2.0, .~ I.o- O" C ctivo o Aqu~s o¢ ~] Peo,ut C] Normal Saline Ciliary iris Anterior ~lere le G FIo. 4. The comparison ofthe PGF2a* concentration in ocular tissues I hra r the topical application of Fea-ME in peanut oil or saline. In both eases, the topically lied dose was 5pg of PGF2a equivalent r 25 .,l of solution applied to each ~e; the saline solution also contained approximately ! % ethyl or benzyl alcohol. Values are b ed on five or six ex~riments d the limits represent ± 1 s.~.(~.). A! System PGF2 a ~ ~ Acetone: triethylomine ~9:1) (I PGF2a PGF2a POF2a t t methyl ester methy! ester t i 4 Topically Applied ~otution Aque~s Humor +t Hour It +3 Hours FIG. 5. Thin-layer chromatograms of extracts of the topically applied solutiQn containing [aH]PGF2~- ME and of aqueous humor obtained from samples removed from the rabbit eyes at various times a r the application of [a~]PG -ME and their corn Hson to chromat s of a solution containing a mixture of [aH]PGF2~ and [sHIPGF2=-ME. eye in 25/LI of saline containing approximately 1% ethyl or benzyl alcohol to solu bilize pGF2=-ME (Fig. 4). in so e tissues, especially in the co ea, the PGF2a* concen~ation was lower when the PGF2~-ME was delivered to the eye in saline rather than in peanut oil, but even in the cornea the difference was not statistically significant. The concen~ ion of PGF2~* w uch lower in all other tissues of the eye than
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`PHARMACOK!NETICS OF TOPICALLY APPLIED PGF~a ESTERS 223 in the tissues included in Fig. 1. In fact, especially at the longer time periods, the radioactivity in some of these tissues was insufficient to allow reliable measurements. Thus, only values obtained at 1 hr are presented for all intraocular tissues (Table I). When chromatographed on the A1 system, the solution of PGF~a-ME that was applied to ttle eye contained a single peak of radioactivity that corresponded to the ~ PGF2a-I~ E standard There was no indication that PGF~a free acid was p sent (Fig. 5). In contrast, aqueous humor taken l- and 2 hr a r the topi I application of [sH]PGF2a showed several peaks; the largest peak corresponded to PGF2a. At 3 hr, the PGF2~* peak in the aqueous was smaller than that at 1- or 2 hr and approximately half of the activity was associa~d with Rf values g ater than that of PGF . So e of this activity corresponded to the PGF2~-ME peak, while some, as in the 1- and 2-hr samples, t~ad Rf values greater than the PGFea-ME value. It should be noted, however, that in the AI system, the Rf value of at least one of the initial ~etaboliVes of PGF~a, 15-keto-PGF~a, is indistinguishable fro that of PGF~a-ME, and the separation between PGF~a and PGF~a-ME is not complete. For these reasons, samples of ME were a.lso run in acetone" triethylamine (99" 1) that gives an Rf value of 0"06 for PGF~ ~nd 0-58 for PGF2~-Mo (F g. 5) On this sys~m the was a hint of some PGF~ activity in tim ME input edium and also an indication of some other labeled compound at an Ftf value slightly higher than that of PG~a-~ E In the aqueous i~umor sample obtained 1 hr after the topical application of PGF~a-ME, approximately 80 ~/o of the activity was associated with the PGF~a peak on this ~stem. At 2- and 3 hr a double peak was present in the PGF~a nge, probably repre nting PG:F~a and one of its initial metabolites. At 1-, 2-, and 3 hr, peaks of increasing height correspondi~g in Rf value to PGF~a-ME were found. However, it cannot be concluded that some PG E~a-ME entered unchanged into the aqueous humor, since on this syste some metabolites of PGF~a may have an l~f value similar to float of PGF~a-ME. 4. Discussion ~[!" ~e results presented here clearly demonstrate that esterification of the carboxyl group of PGF. 2~ greatly eIlhances its llptake by the cornea from topically applied solutions and facilities the entry of the PGF~ moiety into the aqueous humor and sur unding tissues. F urthertl o , our preliminary ell atog phic studies show that most of the 3H activity found in the aqueous }~umor after the topical application of p~-~li' -ME is associated wittl PGt,'~ rather than its methyl ester. Ttlese obser- vations substantiate the hypothesis that PGF~= esters applied topically to the eye act as intraocular pro-drugs (Bito, 1984a). At least during the first couple of hours a r the topical application of PGF~a or one of its esters, the effective concentration of PGFy~ in the aqueous humor can be assumed to be equal, to the calcula~d PGFza* concentration, since most of the aH activity in the aqueous humor was associated with PGF fi~ee acid Furthermore, there is no reason to believe that significant amounts of PGF~ would be present in firmly bound ~r in this essentially protein- e fluid. Although it is possible tlmt PGF2~-IE is bydrolyzed ~re slowly than PGF~-ME, the same argument should l~old l TM for botl~ esters, since both will yield PGF2~ free acid upon hydrolysis, urthermo~e, since PG esters are highly lipid-soluble and are quite hydrophobic, we can assume that the esterified compound tends to st in the lipidic environ ent of the corneal epithelium and that the ore l~ydrophilie hydrolysis product will en~r tim corneal stro a and tim aqueous humor.
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`224 I,. Z. BITO AND R. A. BAROO1)Y In tile ciliary body and anterior sclera, both local metabolic activity and binding to tissue co ponents must, however, be considered. Although in the present ~ tud 5 no attempt was made go identify the chemical nature of the PGF2~* found in the anterior urea and selera, previous studies have shown that PGs are not effectively metabolized by intraocular tissues (Eakins et al., 1974; Bito and Baroody, 1974). In fact, the aH activity accumulated by tl~e anterior urea froln a medium containing [~H]PGF2~ was found by chromatography or i munoaclsorption to be associated a|most enti ly with PGF2~ (Bito and Baroody, 1974). Although PGs a actively accumulated by the anterior urea, presumably by tile ciliary processes, the accum- ulated PG was readily elutable, indicating that PGs are not firmly bound in this tissue (Bito, 1972). ~.,hre can assume, thereL-~re, tllat the higher concentration of PGF2~* found in the anterior urea and sclera after the topical application of PGF2~ esters than after tile application of PraF free acid does reflect the increased availability of biologically active PGli'2~ in these tissues. A quantitative evaluation of the relationship between enhanced delivery and increased hypotensive potency is complicated by two facts" (I) we do not yet know which intraoeular tissues edia~ this hypotensivee et, and (2} the measured extent of the ine ase in hypotensive potency is dependent on the gi e after topiea| app|ieation (Bito, I984a). TIlerefo , on the basis of presently available information we can only conclude that esterifieation of the P(IF~ increases both the delivery of the PGF2~ moiety into intraoeular compartments and the ocular hypotensive potency of tills autaeoid. Thus, the enhanced delivery of the PGF2~ moiety to target sites within the an~rior seg ent may account for most, and possibly ai| of the increased hypotensive potency of PGl~ 2~-N ~-~ and PGF2~-IE and, presumabl ,~ 5 , that of other PGF2~ esters. Our experiments also show that the cornea of the PGF2a-ester-treated eye must act as a slow-release depot for PGF2~*. In the absence of such a slow release, we would expect a drug to ach a aximum concentration in the aqueous hu or within a few minutes after its topical application, and then to clear ttle anterior chamber with the flow of aqueous humor- i.e. with a half-time of approximately 40 rain. In contrast, the concentration of PGF * in the aqueous humor did not reach its maximum until I t~r 2~ after the topical application of [aH]PGF2~-ME; the half-time of decrease was considerably slower, between 3- to 4 hr. The aqueous" cornea PGF~* concentration ratio increased more rapidly in eyes treated witl~ PGF~ free acid than in eyes treated with PGI?~-ME where this ratio reached a higher maximum at 3 hr, and did not decline sigtiifleantly within 6 hr. ~ r ~ Because PGI~ ~-es~ s are highly lipop!~ilie, we can assu e that ueh of the PGF~* found in the cornea is contained in he t corneal epithelium in the esterified for and that most of the de-es~rification tal~es place within the epithelium, while the product of the t~ydrolysis, the hydrophilic e acid, is partitioned into the stroma. The cornea itse.lfmust therefore be regarded as a t, wo-cornpartment system. Furthermore, the rate of transfer of:PGF~* the epithelial to the stro al compart ent after the topical plication of a PGF~= ester must depend on i~ rate of enzy atic de-e rification. The corneal epithelium has been shown to have high esterase activity (Lee, Morimoto, and Stratford, 1982) and the removal of the eithelium has been reported to reduce significantly the rate of hydrolysis of dipivolyl epinephrine by the rabbit cornea (Anderson, Davis and Wei, 1980). The analysis of the pharmacokinetics a r the topical application of an es~rified hydrophobic drug such as PGF~a-ME, which is de-esterified within the cornea, must
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`PHARMACOKINETICS OF TOPICALLY APPLIED PGF2a ESTERS 2~ take into consideration the following rates" (a) the loss of ester from the solution applied to the eye; (b) transfer of the ester from this solution into the corneal epithelium ; (c) the flux of the PGF2~ ester from the epithelium back into the tear film and (d)to the corneal stroma; (e) the hydrolysis of the ester within the corneal epitheliu ; (f) the ftux of the e acid back into the t r fil and (g) to the stroraa: (h) the net flux of free acid from the cornea into the anterior chamber; (i) aqueous humor turnover; and (j) the net, flux of free acid into the anterior urea. Experiments to determine some of these rates are currently under way. A quantitative analysis of the ocular pharr_nacokinetics of topically applied PG esters will be v' pro lded following completion of these studies. ACKNOWLEI)GMENTS We wish to thank |)rs Johan Stjernschantz and Bahrain Resul of Pharmacia AB, Uppsala. Sweden for providing the radioactive PGF2~-l-rnethyi and PGF~-I-isopropyl esters, as well as for many llelpful discussions of the analytical techniques and ottler aspects of our research. We would also like to acknowled~ the a istan of Dr Olivia Miranda, Ms Ann ga goza. and Mr Santos Rodriguez in these studies and the preparation of this manuscript. These studies were supported in part by Research Grant EY 00333 fi~orn the National Eye Institute (USPHS) and in part by Pharmacia A B of Sweden. R E F E R E N C E S o, i S ~¸ Anderson J. A., Davis, W. L. and Weig, C P. (1980). :.lte of ocular hydrolysis ot' a t~rodrug. dipivefrin, and a comparison of its ocular metabolism with that of tile parent compound epinephrine. I,~vest. Ophthalmol. Vie. &i. 19, 8! 7-23. Bito, L. Z. (1972). Accumulation and apparent active transport of prostaglandins by some tissues in vitro. J. Physiol. 221, 371-87 Bito, L.Z. (1984a). Prostaglandins, other eicosanoids, and their derivatives as potential antiglaucoma agents. In e dical Treatment of ~aucomas (Eds Drance, S. M. and Neufeld, A. H.) Pp. 477-505. Grune and Stratton" New York. Bito, L. Z. (1984b). Comparison of the ocular llypo~nsive efficacy of eicosanoids and related compounds. . Eye Res. 38, 181-94. Bito, L. Z. (1986). Eicosanoid transport sys~ms" their ptlysiological roles and inhibitors. In ~dbook of ~cosanoids" ostaglandins and Related Lipids, Vol. I" Chemical and B 1 A ect8 (Eds Willis, Vieke~ and ce-Asciak). CRC Press. Bito. L.Z. (1985). Prostaglandins and related compounds as pot,ential ocular therapeutic agents. In Biological Protection with Proslaglandins, Vol. I (Ed. Cohen, M. M.). Pp. 231-52. CRC P ss. Bito, L. Z. and Ba ody, R. A. (1974). Concentrative accumulation of[aH]prostaglandins by some rabbit tissues in vitro: the chemical nature of the accumula~d all-labeled sub- stances, ostafflandins 7, 131 ~0. • . i ( ~ Bito, L Z and Baroody, R A. (19~5). Impermeability of rabbit erytl~rocytes to prosta- glandins. Am. J. l~hysio!. 229 158 . Bito, L.Z. and Baroody, R.. A. (!982). ~he t)enetration of exogenous prostaglandin and arachidonic acid into, and their distribution within, the mammalian eve. Curt. Eye Res. 1,659-69. 9 ' , Bito, L.Z., I)raga, A., Blanco, j. and Camras, C. B. (1, 83) Long-term maintenance of reduced int ocular p ssure by daily or twice daily topical application ofprostaglandins to cat and rl~esus monk eyes. Invest. Ophthahnol. Vis. Sci. 24, 312-19. Bito, L.Z. and Salvador, E.V. (1972). Intraocular fluid dynamics. III. The site and mechanism of prostaglandin trans£er across the blood intraocular fluid barrie . . Eye Res. 14, 233~ 1. l~ito, L.Z., Srinivasal~. B.D., Baroody, R.A. and Schubert, H. (1983). Noninvasive observations on eyes of cats after long-term main~nance of reduced in traocular pressure by topical application of prostaglandin Fa. In v t Ophthalmol. Vis. ~i. 24, i~80.
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`226 L, Z. BITO AND R, A, BAROODY Bito, L. Z. and Wallenstein, M. C. (1977). Transport of prostaglandins ac s ~he blood-brain and blood-aqueous barriers and the physiological significance of these absorptive transport processes. . Eye Re,. 25, (Suppl), 229-43. Camras, C. B., Bito, L. Z. and Eakins, K. E. (i977). Reduction of intraocuiar pressure by prostaglandins applied topically tv tt~e eyes of conscious rabbits. Invest. Ophthalmol. 15, 112,5--34. Crawlord, K.. True, B. and Kaufinan, P. L. (!985). Topical prostaglandin effects on aqueous humor dynamics in cynomolgus monkeys. Inve~st Ophthalmol. Vis. 8ci. 26, (ARVO Suppl.), 233. Eakins, K.E. (1977}. Prostaglandin and non-prostaglandin mediated breakdown of the blood-aqueous barrier. Exp. Eye Res. 25, (Suppl.) 483-98. Eakins, K E Atwal M and Bhattacherjee, P (t ~4) Inactivation of prostaglandin E l by ocul tissues invit . .EyeR .19, 141-6. Green, K. and Samuelsson, B. (1964). P stagiandins and related factor. XIX. Thin-Jayev chromatography of prostaglandins. J. Lipid Res. 5, I 17-20. Lee, P.-Y., Podos, S. M., Severin, C. and Camr , C. (1984). Effect of prostaglandin F~ on aqueous humor dynamics of bbit, cat, and monkey, lnve~. Ophthatmol. [!is. Sci. 25, (A RVO Suppl.) 96. Lee, V. H.-L., Morimoto, K. W. and Stratford, R .... E., Jr (1982). Esterase distribution in the rabbit cornea and its implications in ocular drug bioavailabilit.y. Biopharm. Drug Disposition 3, 291-300. Stamfold, I. F. and Unger, W. G. (J972). Improved purification and chromatography of extracts containing prostaglandins. J. siol. 225, 4P-5P.
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