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http://tpx.sagepub.com/content/24/5/595The online version of this article can be found at:

 DOI: 10.1177/019262339602400509

1996 24: 595Toxicol PatholJohn E. Dillberger, Robert L. Peiffer, Michael J. Dykstra, Michael O'Mara and Dipak K. Patel

The Experimental Antipsychotic Agent 1192U90 Targets Tapetum Lucidum in Canine Eyes  

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The Experimental Antipsychotic Agent 1192U90Targets Tapetum Lucidum in Canine Eyes*JOHN E. DILLBERGER,1 ROBERT L. PEIFFER,2 MICHAEL J. DYKSTRA,3

MICHAEL O’ MARA,4 AND DIPAK K. PATEL4

1Medicines Safety Evaluation Division, Glaxo Wellcome Inc.,Research Triangle Park, North Carolina 27709-2700,2Departments of Ophthalmology and Pathology, University of North Carolina,

Chapel Hill, North Carolina 27599-7040,3Microbiology, Pathology and Parasitology Department, North Carolina State University,

Raleigh, North Carolina 27606, and4Bioanalysis and Drug Metabolism Division, Glaxo Wellcome Inc.,

Research Triangle Park, North Carolina 27709-2700

* Address correspondence to: J. E. Dillberger, Medicines Safety Eval-uation Division, Glaxo Wellcome, PO. Box 12700, Research TrianglePark, North Carolina 27709-2700.

ABSTRACT

To assess the potential adverse effects in people of the antipsychotic agent 1192U90, we dosed mice, rats, beagles, and cynomolgusmonkeys for up to 3 mo. In dogs, but not the other species, 1192U90 caused ocular changes detectable ophthalmoscopically as lossof tapetal reflectivity, altered tapetal color, and the appearance of black pigmentation on the tapetal fundus. Eyes from affected dogshad atrophic tapeta lucidum due to cell loss. Rodlets in remaining tapetal cells were separated by electron-lucent spaces or finelygranular material, varied in size and shape, and often contained irregularly shaped electron-dense inclusions. Nontapetal ocular struc-tures were unaffected. Because 1192U90 caused no ocular changes in nontapetal species, we hypothesized that it targeted only tapetumlucidum and spared other ocular structures. We tried to test this hypothesis by dosing congenitally atapetal dogs; however, althoughthese dogs were ophthalmoscopically "atapetal," they had scattered tapetal cells visible by electron microscopy, and these tapetalcells had ultrastructural changes indistinguishable from those that occurred in treated normal-eyed dogs. Tapetal degeneration causedby 1192U90 resembled that described in hereditary tapetal degeneration in beagles. That 1192U90 caused no ocular changes in

nontapetal species suggests that the ocular changes in dogs do not imply a risk for humans, whose eyes also lack a tapetum lucidum.

Keywords. Oculotoxicity; atapetal dogs; beagle

INTRODUCTION

1192U90 is a novel antipsychotic agent being devel-oped to treat schizophrenia (Fig. 1). To assess the risk ofpotential adverse effects in people, we conducted a seriesof toxicity studies in rats, dogs, monkeys, and mice.Chronic dosing with 1192U90 produced changes in theocular fundus of dogs but not other species. We investi-gated the nature of these changes using light and electronmicroscopy and concluded that ocular changes occurredonly in dogs because 1192U90 targeted the tapetum lu-cidum, a structure that other species lacked. To test thishypothesis, we conducted an additional study using ata-petal dogs. Results of both studies in dogs are reportedhere.

METHODS

Dogs

Beagles were purchased from Marshall Farms USA,Inc. (North Rose, NY) for Experiment 1 and from WhiteEagle Labs (Doylestown, PA) for Experiment 2. Atapetalbeagles used in Experiment 2 were identified by oph-thalmoscopic examination at the supplier’s facility prior

to shipment, and their atapetal status was confirmed be-fore they were assigned to the study.

During the studies, dogs were housed individually incages in the Toxicology Animal Facility (BurroughsWellcome Co., Research Triangle Park, NC), fed AgwayProLab Canine 1600 (Agway, Inc., St. Marys, OH), andallowed tap water ad libitum. They were acclimated tothe laboratory environment (71 1 ± 2°F, 50 ± 10% relativehumidity, 10-15 air changes per hour, and a fixed 12-hrphotoperiod) for at least 28 days prior to dosing and werebetween 10 and 16 mo old when dosing began.Dogs were cared for and used during these studies in

accordance with the Guide for Care and Use of Labo-ratory Animals (4).

Experimental DesignExperiment 1. Normal-eyed dogs (5/sex in control and

high-dose groups; 3/sex in low- and mid-dose groups)were given 1192U90 in gelatin capsules once a day for90 days at doses of 0 (empty capsule), 5, 20, or 80 mg/kg.Plasma samples taken just before, and at intervals after,the 2nd and 86th doses were analyzed for 1192U90 con-centration to create a toxicokinetic profile. At the end ofthe dosing period, 3 dogs/sex were euthanatized and nec-ropsied. Remaining dogs (2/sex in the control and high-dose groups) were allowed to recover for 35 days withoutbeing dosed and then euthanatized and necropsied to as-sess the reversibility of 1192U90-related effects.

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Ophthalmoscopic examinations were done before dos-ing began, near the end of the dosing period (day 86),and at the end of the recovery period (day 125). Fundusphotographs were taken on days 86 and 125. At necropsy,left and right eyes were fixed for light and transmissionelectron microscopic examination by immersion in 10%neutral-buffered formalin or McDowell’s and Trump’s4F:1 G fixative, respectively.

Experiment 2. Six normal-eyed dogs (3/sex) and 6 ata-petal dogs (4 males, 2 females) were given 1192U90 ingelatin capsules once a day for 68 days at a dose of 80mg/kg. Six other normal-eyed dogs (3/sex) given emptygelatin capsules for 68 days served as controls. Ophthal-moscopic examinations were done before dosing beganand on days 15, 29, 50, 58, and 68. Fundus photographswere taken on days 16, 33, and 58. Plasma samples takenjust before and at intervals after the 2nd and 64th doseswere analyzed for 1192U90 concentration to create a tox-icokinetic profile.

At the end of the dosing period, dogs were euthanati-zed and necropsied. To improve fixation of intraocularstructures over that achieved in Experiment 1, left and

right eyes were fixed for light and transmission electronmicroscopic examination, respectively, by perfusing theposterior chamber with fixative and then immersing theglobe in the same solution. Fixative was perfused througha 20-gauge needle inserted at the corneoscleral junction.A second 20-gauge needle was inserted at the corneos-cleral junction 180° opposite the first to relieve pressurein the posterior chamber and allow a larger volume offixative to be instilled. Left eyes were fixed in 10% neu-tral-buffered formalin and right eyes in McDowell’s andTrump’s 4F:1 G fixative.

Toxicokinetic Analysis1192U90 concentrations in plasma were determined

using a sensitive reversed-phase high-performance liquidchromatography assay with detection by fluorescence.Data were used to determine Cmax (the maximum plasmaconcentrations) by direct observation and to plot concen-tration-time curves from which AUCo-+24 (the areas underthe concentration time curves from time 0-24 hr) werecalculated by the trapezoidal rule using a proprietary pro-gram called PKCAL.

OphthalmoscopyMydriasis was induced by instilling 1.0% tropicamide

(Mydriacyllm, Alcon Laboratories) into eyes. Adnexa andthe anterior segment were examined with a slit-lampbiomicroscope and the posterior segment with an indirectophthalmoscope. Examinations were done by an ACVO-certified Veterinary Ophthalmologist (RLP). Fundus pho-tographs were taken with a Kowa fundus camera.

ER ET AL TOXICOLOGIC PATHOLOGY

Light and Transmission Electron MicroscopyMicroscopic evaluations were done by an ABT- and

ACVP-certified Toxicologic Pathologist (J.E.D.). For-malin-fixed eyes from all dogs were processed by routinehistologic methods, sectioned at 6 /-1m, stained with he-

matoxylin and eosin, and examined light microscopically.Sections were taken through the central median plane ofthe globe to include the optic disc, dorsal (tapetal) cho-roid and adjacent retina, and ventral (atapetal) choroidand adjacent retina.

Samples of tapetal choroid and adjacent retina fromcontrol dogs and those given 80 mg/kg/day in both ex-periments were processed for electron microscopy as de-scribed in Dykstra (6). The primary fixative (McDowell’sand Trump’s 4F:1 G fixative) contains 4% formaldehydeand 1 % glutaraldehyde in a sodium monophosphate buff-er, pH 7.2-7.4. Samples were rinsed in a 0.1 M phosphatebuffer at the same pH and postfixed for 1 hr in 1 % os-mium tetroxide in the same buffer. Samples then weredehydrated in an ethanolic series, passed through absoluteacetone, and infiltrated with Spurr epoxide resin. Afterthe blocks were polymerized, sections were cut with adiamond knife, picked up on copper grids, stained withmethanolic uranyl acetate and aqueous lead citrate as de-scribed by Dykstra (6), and examined with a transmissionelectron microscope.

RESULTS

OphthalmoscopySimilar ocular changes occurred in normal-eyed dogs

given 1192U90 at 80 mg/kg/day in both experiments, butnot in normal-eyed dogs given 1192U90 at lower dosesin Experiment 1, in atapetal dogs given 1192U90 at 80mg/kg/day in Experiment 2, or normal-eyed control dogsin either experiment. Changes were limited to the tapetumlucidum and included 2 patterns of alteration that werenot mutually exclusive (Fig. 2). One pattern was replace-ment of the more peripheral tapetum by geographic areasof black pigmentation, and the other, generalized loss oftapetal reflectivity with change in coloration from thenormal green to a diffuse dull red-brown. These changeswere irreversible. In Experiment 2, where we followedthe development of lesions over time, the geographic tap-etal lesions tended to begin and remain more prominenttemporally than nasally.

Light and Electron Microscopic Ocular ChangesTapetal atrophy, characterized by thinning of the ta-

petum lucidum due to cell loss, was visible by light mi-croscopy in all dogs given ~20 mg/kg/day for 90 daysin Experiment 1 and 3 of 6 normal-eyed dogs given 80mg/kg/day for 68 days in Experiment 2 (Figs. 3 and 4).In Experiment 2, tapetal atrophy also was noted in por-tions of the tapetum lucidum of 1 control dog. Atrophywas not uniform within individual tapeta; instead, tapetalthickness varied from 1 to 10 cells. Variation in the de-

gree of atrophy in different areas of the tapetum likelycontributed to the multifocal nature of the tapetal changesseen ophthalmoscopically. Atrophy was present in dogsallowed to recover for a month at the end of the dosing

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FIG. 2.-a) Normal ocular fundus from a control dog. b) Fundus of dog given 1192U90. Pattern 1: tapetal fundus has geographic areas of black

>igmentation. c) Fundus of dog given 1192U90. Pattern 2: generalized loss of tapetal reflectivity. d) Fundus of dog given 1192U90. Combined>attern with generalized loss of reflectivity ventrally and geographic areas of black pigmentation dorsally.

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FIG. 3.-Tapetal choroid and adjacent retina from a control dog. IN= inner nuclear layer; IP = inner plexiform layer; ON = outer nuclearlayer; OP = outer plexiform layer; PVC = pigmented vascular choroid;RC = rod and cone layer; TL = tapetum lucidum. H&E. X 125.

period in Experiment 1, suggesting that tapetal atrophywas irreversible.We could not confirm tapetal atrophy by electron mi-

croscopy because of its irregular distribution. Generally,the thickness of the tapetal cell layer in electron micros-copy samples did not differ between control and treatednormal-eyed dogs. Electron microscopy did reveal ultra-

FIG. 4.-Tapetal choroid and adjacent retina from a dog given1192U90. Tapetum lucidum is essentially absent. IN = inner nuclearlayer; IP = inner plexiform layer; ON = outer nuclear layer; OP =outer plexiform layer; RC = rod and cone layer; TL = tapetum lucidum.H&E. X 125.

FIG. 5.-Tapetal cells from a control dog. Cells contain closelypacked, round, uniformly sized tapetal rodlets with homogenous matri-ces. X6,930.

structural changes in tapetal cells from dogs given1192U90, which included the following (Figs. 5 and 6):. Variation in tapetal rodlet size. Normal rodlets were

uniform in size, but most rodlets from treated dogswere several times larger or smaller than normal.

0 Variation in tapetal rodlet shape. Normal rodlets wereuniformly round in cross-section, but rodlets fromtreated dogs often were irregular in shape, especiallythe larger ones.

0 Altered rodlet contents. Normal rodlets contained ho-

mogenous, finely granular material that was moderate-ly electron dense, but rodlets from treated dogs oftencontained irregularly shaped, electron-dense inclu-sions. When inclusions did not fill the rodlet, the re-maining space inside the rodlet was electron-lucent.

0 Increased interrodlet space. Normal rodlets were close-

ly packed and filled the tapetal cell, but rodlets fromtreated dogs were separated by electron-lucent spacesand irregular patches of finely granular material.

Tapetal cells were roughly the same size in normal andtreated dogs, and tapetal rodlets in treated dogs were as

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FIG. 6.-Tapetal cells from a dog given 1192U90. Tapetal rodletsvary in size and shape and are separated by electron-lucent areas andfinely granular material. Many rodlets are swollen and contain irregu-larly shaped, electron-dense inclusions; others are shrunken with elec-tron-dense cores. X6,930.

often swollen as shrunken. Together, these 2 observationssuggest that the increased interrodlet space in treated

dogs was a consequence of rodlet loss. The ultrastructuralchanges observed in tapetal cells might be described bestby the term &dquo;tapetal cell degeneration&dquo; or &dquo;degenerativetapetopathy. &dquo;A tapetum lucidum could not be visualized ophthal-

moscopically or with the light microscope in eyes fromatapetal dogs in Experiment 2, but electron microscopyrevealed that eyes of &dquo;atapetal&dquo; dogs did have tapetalcells, albeit fewer than normal-eyed dogs. Tapetal cellsin these &dquo;atapetal&dquo; dogs exhibited ultrastructural changesindistinguishable from those noted in tapetal cells oftreated normal-eyed dogs.The ultrastructure of the retina and nontapetal choroid

were unaffected by treatment with 1192U90.

Toxicokinetics

In Experiment 1, systemic exposure to 1192U90 (ex-pressed as Cmax or AUCo--724) increased disproportionally

TABLE L-Systemic exposure to 1192U90 in Experiment 1.

with dose between 20 and 80 mg/kg/day (Table I). Thedisproportionality was greater in females than in males(14-fold increase and 8-fold increase in AUCo~24’ respec-tively, on day 2 with 4-fold increase in dose), so thatfemales given 80 mg/kg/day were exposed to more

1192U90 than males given the same dose. AUCs gen-erally were similar on days 2 and 86, indicating that1192U90 neither induced its own metabolism nor accu-mulated with chronic dosing.

In Experiment 2, dogs given 80 mg/kg/day had Coaxvalues similar to those in dogs given the same dose inExperiment 1; however, AUCs were greater in Experi-ment 2 than in Experiment 1. Systemic exposure to

1192U90 was similar in normal-eyed and atapetal dogs(Table II). Individual exposures varied widely, but as inExperiment 1 mean exposure was greater in females thanmales, especially early in the dosing period. Exposureswere similar on days 2 and 64, indicating that 1192U90did not accumulate with chronic dosing in dogs.

DISCUSSION

In dogs, chronic dosing with 1192U90 at high dosescaused loss of tapetal reflectivity, altered tapetal color,and the appearance of black pigmentation on the tapetalfundus. These changes reflected degeneration of individ-ual tapetal cells with consequent atrophy of the tapetumlucidum as a tissue, presumably due to loss of damagedtapetal cells. Pigmented areas in the tapetal fundus mostlikely were areas where the tapetum had thinned enoughto make visible the underlying pigmented vascular cho-roid.

Tapetal degeneration in beagles can be induced by sev-eral experimental compounds and also occurs spon-taeously as a recessive heritable condition. The patho-genesis of tapetal toxicity with some compounds has beenattributed to their ability to chelate zinc, which forms upto 7% of the tapetum lucidum (by weight) in dogs (8,19). Zinc-chelating compounds that target tapetum luci-dum include diphenylthiocarbazone, also called dithizone(1, 5, 8, 19); diethyldithiocarbomate (16); 1-hydroxy-2-( 1 H) pyridinethione (5) and its zinc-containing form, zinc

TABLE IL-Systemic exposure to 1192U90 in Experiment 2.

&dquo;’&dquo;T , ~ ~ ... , , ..

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pyridinethione (3, 17); and a related series of ethylene-diamines (10), the best studied of which is ethambutol,the dextro isomer of 2,2’-(ethylenediimino)-di-l-butanol(8, 11, 18). These compounds fall into 2 groups basedon the tapetal changes they cause.

Diphenylthiocarbazone, diethyldithiocarbomate, andthe 2 pyridinethiones cause ocular changes that can besummed up as a necrotizing tapetopathy. Tapetal changesbegin within hours of the first dose as &dquo;bleaching&dquo; or&dquo;blanching&dquo; of the tapetal fundus that progresses rapidlyto blindness. The earliest light microscopic changes areincreased tapetal cell eosinophilia and choroidal conges-tion and edema, which progresses to tapetal cell necrosiswith infitration of neutrophils and macrophages, subretin-al edema and hemorrhage, and retinal detachment.Changes are irreversible.One other compound, 7-chloro-2-methyl-5-phenylimi-

dazolidino(5,1-b)-SH-quinazoline, also causes necrotiz-

ing tapetopathy in dogs (13). This imidazo quinazolinebleaches the tapetal fundus and causes subretinal hem-orrhage, retinal detachment, and blindness in normal-

eyed dogs but not atapetal dogs. The single publishedreport does not describe the histopathologic lesions inmuch detail and does not say if the compound chelateszinc.

Ethambutol-induced ocular changes contrast sharplywith those induced by other zinc-chelating compoundsand can be summed up as a degenerative tapetopathy.Like other zinc chelators, the first dose of ethambutolcauses rapid &dquo;bleaching&dquo; or &dquo;decoloration&dquo; of the tapetalfundus, but this tapetal color change is unaccompaniedby tapetal cell necrosis or its sequelea (edema, hemor-rhage, and cell infiltration). Instead, nontapetal ocularstructures are entirely spared and tapetal changes arecompletely reversible, even after prolonged dosing (18).Electron microscopy reveals 3 ultrastructural changes intapetal cells:

1. Rodlets become twisted and haphazardly arrangedwithin the cell instead of being arranged in regularparallel rows.

2. Rodlet matrix becomes electron-lucent instead of elec-tron-dense.

3. Small hollow areas normally present in rodlets be-come so enlarged that they resembled vacuoles. These&dquo;vacuoles&dquo; contain an electron-dense core that is pre-sumed to be remnants of rodlet matrix.

Three other compounds cause degenerative tapetopa-thies in dogs that resemble ethambutol-induced tapeto-pathy : the macrolide antibiotic rosaramicin (12); the azal-ide antibiotic CP-62,993, which is structurally related tothe macrolide antibiotics (9); and the aromatase inhibitorCGS 14796C (14).

Rosaramicin causes brown-tan discoloration and gen-eral pallor or loss of reflectivity of the tapetum lucidumin beagles dosed intravenously for a month. Fundus

changes begin within 10 days and resolve within 10 wkafter dosing stops. No lesions are visible by light micro-scopic examination of routine eye sections stained withhematoxylin and eosin or l-/-1m-thick sections stainedwith Toluidine blue; eyes have not been examined elec-

tron microscopically. The published report says that ro-saramicin is &dquo;not known to be a chelating agent&dquo; butdoes not say whether or not the compound was tested forzinc chelating ability.

CP-62,993 causes widespread phospholipidosis accom-panied by &dquo;decoloration&dquo; and reduced reflectivity of thetapetal fundus in beagles dosed orally for a month. Fun-dus changes are present at the end of the dosing period(eyes have not been examined earlier); their potential re-versibility has not been investigated. Light microscopi-cally, tapetal cells are swollen. Electron microscopy re-veals that they are devoid of tapetal rodlets and filled withvacuoles that contain amorphous, electron-dense debris.Inflammation is absent but retinal cells contain lysosomallamellar bodies typical of phospholipidosis. According tothe published report, SCH 19927 is &dquo;not known to have

zinc-chelating properties,&dquo; but whether or not it has beentested for chelating ability is unclear.CGS 14796C causes areas of &dquo;brownish peppered or

mottled or uniform brown&dquo; pigmentation and loss of tap-etal reflectivity in beagles dosed orally for 3 mo. Funduschanges begin between 8 and 13 wk and do not resolvewithin a month after dosing stops. Light microscopically,the tapetum lucidum is atrophic (thinned or absent dueto cell loss). Electron microscopy reveals that remainingtapetal cells contain autophagic vacuoles but are other-wise unremarkable. Inflammation is absent and adjacentocular structures are unaffected. CGS 14796C is not azinc chelator.

’

The (3-adrenergic blocker SCH 19927 causes a tape-topathy that differs from the degenerative and necrotizingtapetopathies already described in the extent of tapetalinvolvement and the type of inflammatory response in-duced. In beagles dosed orally for 4 mo, SCH 19927causes discoloration and pigmentation of the tapetal fun-dus that is multifocal instead of diffuse (15). Focal serousretinal detachment can occur but is rare. Fundus changesbegin within a week and do not resolve within a monthafter dosing stops. Light microscopically, tapetal cells de-generate and die, and a few macrophages, lymphocytes,and occasional plasma cells focally infiltrate the tapetumand adjacent choroid. Based on electron microscopic ex-amination, the initial change is described as &dquo;loss of theelectron-dense core of cytoplasmic rodlets followed bytheir lysis.&dquo; Affected but still viable tapetal cells haveelectron-lucent cytoplasm, fewer rodlets, and swollen mi-tochondria. These changes progress to necrosis. Accord-ing to the published report, SCH 19927 is &dquo;not knownto be a zinc chelator,&dquo; but whether or not it has beentested for chelating ability is unclear.

In hereditary tapetal degeneration, tapetal rodlets failto accumulate electron-dense material or zinc and instead

degenerate into spherical inclusion bodies of varyingelectron density that eventually become electron-lucent(2). Tapetum lucidum is normal at birth in affected dogs,but by the time dogs are 3 wk old ultrastructural changesappear in tapetal rodlets, and by the time dogs are 2 moold the tapetum lucidum has begun to atrophy, presum-ably due to death and subsequent loss of degenerate cells.The underlying genetic defect is unknown but presumedto involve a gene responsible for synthesis of a rodlet

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matrix protein or the zinc-complexing protein within tap-etal cells.

That several zinc-chelating compounds cause tapeto-pathies in mature dogs suggests that removal of zinc fromtapetal cells damages them. On the other hand, as Massaet al (12) pointed out in 1984, zinc removal alone doesnot determine the pathogenesis of the resulting tapeto-pathy, because single doses of ethambutol or dithizoneremove almost identical amounts of zinc from canine ta-

peta lucidum (8) yet cause distinctly different tapetopa-thies. Moreover, it is clear that compounds can damagetapetal cells by mechanisms other than interaction withzinc. 1192U90 presumably does so, as it does not chelatezinc (M. Norman, personal communication).1192U90 has not caused ocular changes in mice, rats,

or cynomolgus monkeys dosed daily for up to 3 mo (re-port in progress). Like human eyes, the eyes of mice, rats,and monkeys lack tapeta lucidum. That 1192U90 causesno ocular changes in nontapetal species suggests that theocular changes in dogs are clinically irrelevant and donot imply a risk for human volunteers or patients. More-over, the doses planned for clinical trials, up to 0.8

mg/kg/day, will yield patient exposures well below thoseassociated with tapetal degeneration in dogs.The gender-related difference in systemic exposure to

1192U90 at higher doses suggested that absorption, me-tabolism, or excretion of 1192U90 differed between gen-ders. Further studies are underway to investigate thesepossibilities.

The difference in 1192U90 exposure, as reflected inAUC values, between Experiments 1 and 2 may reflect

underlying genetic differences between the 2 populationsfrom which dogs were drawn. Dogs in Experiment 1 werefrom Marshall Farms in New York, and those in Exper-iment 2 from White Eagle Labs in Pennsylvania. Furtherstudies are planned to investigate possible differences inhow dogs from these 2 sources metabolize 1192U90.

ACKNOWLEDGMENTS

We thank Ms. Alice Ladd for her histotechnical ser-vices and Ms. Jacqueline Lee for preparing TEM speci-mens.

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4. Committee on Care and Use of Laboratory Animals (1985). Guidefor the Care and Use of Laboratory Animals. NIH Publication No.86-23, U.S. Government Printing Office, Washington, D.C.

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11. Kaiser JA (1964). A one-year study of the toxicity of ethambutolin dogs: Results during life. Toxicol. Appl. Pharmacol. 6: 557-567.

12. Massa T, Davis GJ, Schiavo D, Sinha DP, Szot RJ, Black HE, andSchwartz E (1984). Tapetal changes in beagle dogs II. Ocular

changes after intravenous administration of a macrolide antibioticRosaramicin. Toxiol. Appl. Pharmacol. 72: 195-200.

13. Schiavo DM (1972). Retinopathy from administration of an imi-dazo quinazoline to beagles. Toxicol. Appl. Pharmacol. 23: 782-783.

14. Schiavo DM, Green JD, Traina VM, Spaet R, and Zaidi I (1988).Tapetal changes in beagle dogs following oral administration ofCGS 14796C, a potential aromatase inhibitor. Fund. Appl. Toxicol.10: 329-334.

15. Schiavo DM, Sinha DP, Black HE, Arthaud L, Massa T, MurphyBF, Szot FJ, and Schwartz E (1984). Tapetal changes in beagle dogsI. Ocular changes after oral administration of a beta-adrenergicblocking agent SCH 19927. Toxicol. Appl. Pharmacol. 72: 187-194.

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