Exp. Eye Res. (1992) 54, 211-218

Steroid Glaucoma" Corticosteroid-induced Ocular Hypertension in Cats GUI-LIN ZHAN, OLIVIA CARII~O MIRANDA

AND L A S Z L O Z. BITO*

Columbia University, Department of Ophthalmology, 630 West 168th Street, New York, NY 10032, U.S.A. (Received Chicago 28 February 1991 and accepted in revised form 22 March 1991) This study was undertaken to develop a feline model of corticosteroid-induced ocular hypertension. In the first experiment, eight cats were selected whose intraocular pressure (17-t-0-4 mmHg) was consistently below the mean baseline intraocular pressure of our colony (24_ 0"5) during the preceding 2 months. Unilateral twice or thrice daily topical application of 10 #l 1% dexamethasone sodium phosphate caused a gradual intraocular pressure increase that became significant (P < 0"05) within 2-3 weeks. There was no significant change in body weight, but several eyes developed cataracts. Similar results were obtained with treatment of normotensive cat eyes with dexamethasone, or with 1"0% prednisolone acetate (PredForte) twice a day. Topical application of PGF2~-l-isopropyl ester (0.1 or 0'25 #g PG equivalent) to such steroid-treated eyes yielded significant intraocular pressure reduction and pupillary miosis, similar in magnitude to those exhibited by normal eyes. When dexamethasone treatment was reduced to once daily, after prolonged twice daily treatment, intraocular pressure decreased only slightly within 10 days. When dexamethasone treatment was stopped, intraocular pressure declined to normal levels within 6-7 days. These findings show that adult cat eyes develop steroid-induced ocular hypertension that is maintained and reversible. As opposed to previous findings on rabbits, steroid-induced feline ocular hypertension appears to be a good model for this clinical condition and may be suitable for the testing of potential glaucoma drugs. Key words: corticosteroid glaucoma; dexamethasone; prednisolone; intraocular pressure; ocular hypertension; glaucoma; cat; feline eye; prostaglandin.

1. Introduction Corticosteroids have been shown to produce an increase in intraocular pressure (IOP) in h u m a n s (Linner, 1959; Goldmann, 1962; Armaly, 1963b; Becker, 1965). Although the rapidity and extent of IOP elevation was found to exhibit considerable person-to-person differences, the fact that a very high percentage of glaucoma patients, or suspects, exhibited more pronounced or more rapid steroid response than normotensive subjects prompted some investigators (Armaly, 1963a; Becker and Hahn, 1964) to consider topical steroid application as a provocative test for glaucoma. Attempts to elevate IOP in rabbits by topical corticosteroids have not been uniformly successful (Jackson and Waitzman, 1965; Touvinen, Liesmaa and Esila, 1966; Levene, Rothberger and Rosenberg, 1974; Podos, 1976). More recently, some authors have reported achieving significant IOP elevations in albino rabbits (Lorenzetti, 1970; Knepper et al., 1978), especially when dexamethasone (DEX) treatment was combined with one of its metabolites, 5fl-dihydrocortisol (Southren et al., 1985). Attempts by some other investigators to produce similar responses in rabbits did not always prove to be successful (P. S. Kulkarni, also L. L. DeSantis, pers. comm.). Moreover, even successful

investigators have reported that corticosteroid-treated rabbits exhibited severe systemic effects such as prevention of weight gain, or loss of weight, and high mortality, and elevated IOP could not be maintained in rabbits even with continuous treatment (Levene et aL, 1974; Knepper et al., 1978). Furthermore, FOP elevation was shown to occur only when treatment began during the developmental period ( 8 - 1 0 weeks of age), while even relatively young, mature rabbits (3 years of age) were shown to be refractory to this effect (Knepper et al., 1978) even when dexamethasone was applied every 6 hr for 4 weeks. These findings suggest that corticosteroid-induced ocular hypertension m a y involve a different mechanism in rabbits than in h u m a n s since steroid glaucoma does develop in adult and even in aged humans. Furthermore, rabbits seem unsuitable for studying the effects, especially the long-term effects, of some glaucoma drugs (Bito, 1984; Bito et al., 1989). We have, therefore, undertaken this study to develop an adult feline model of steroid glaucoma. Different regimens of corticosteroid application were used in order to define dosing schedules that are required to achieve induction and maintenance of corticosteroid-induced ocular hypertension in ocular hypotensive and normotensive adult cats.

* For reprint requests. 0014-4835/92/020211 + 08 $03.00/0

© 1992 Academic Press Limited 14-2

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2. Materials and Methods

Experimental Animals Adult cats of mixed breeds and of either sex (2.2-4.5 kg) were maintained in our animal care facility for at least 1 yr before the onset of any of the studies reported here. Most of the cats had been in our colony for 2 - 3 yr and were adults, estimated age 2 - 4 years, at the time we received them. Thus, some of the cats used in this study were at least 4 - 5 years old. One of the cats was born in our colony and was 22 months of age when entered into prednisolone treatment. All of these animals were fully accustomed to periodic restraint in plexiglass animal boxes and had been trained to accept tonometry as they had been used in our studies on the ocular effects of prostaglandins (PGs) before they were entered into these studies. Consequently, we have long-term records on both their normal IOPs and their responses to various PGs. A wash-out period of at least 4 weeks was allowed between the use of any drug (except the topical anesthetic, Alcaine) before the onset of corticosteroid treatment. The effects of DEX was first tested on four ocular hypotensive cats that showed, over a period of several months, IOPs consistently lower (17+0.4mmHg, mean + S.E.M.) than the IOP of 24 _+0.5 mmHg, which characterized our colony of 40 cats of mixed breeds. The low IOP of these four cats could have occurred naturally or could have been due to their periodic treatment with various PGs and, occasionally, with clinically used glaucoma drugs over a period of several months prior to their use in these experiments. However, all four of these cats showed consistently low IOPs in the absence of any medication for at least 1 month prior to the onset of the DEX treatment, while other cats, which were used in similar experiments prior to these studies, did not develop such lowered IOP. During the baseline period before the first corticosteroid treatment there was no significant difference between the IOP of the left and right eyes, and all eyes were free of any signs of ocular irritation or inflammation. Careful slit-lamp examination did not reveal even minimal flare that could have suggested any abnormality of blood-ocular barrier function.

G.-L. ZHAN ET AL.

In the first study, one IOP measurement was performed on most days, or at least every other day, with the exception of weekends. However, treatment regimens continued uninterrupted in all studies during weekends and holidays.

Other Observations Slit-lamp examinations of all eyes were performed at least twice, but in most cases three or four times each week, by a dark-adapted observer to be able to note even the lowest level of flare. Pupil diameters (horizontal) were measured in dim light using a millimeter ruler. The weight of each animal was measured repeatedly and recorded before the beginning of each treatment regimen and weekly, or at least bi-weekly, thereafter. The animals were observed periodically to note any behavioral changes. Photographic records of the cats' general appearance were made at the beginning of the first treatment regimen. Since no effects were noted on fur quality, body proportions or any other parameter that could be documented by photography, further photographs were not taken.

Preparation and Administration of the Dexamethasone (DEX) Solution A solution manufactured for i.v. use (Dexamethasone Sodium Phosphate Injection USP; Quad Pharmaceuticals, Indianapolis, IN) was diluted to the desired concentration of 0.5 or 1.0% with Tears Naturale I2 (Alcon Laboratories). In all cases the DEX solution was applied to the corneal surface in a volume of 10 #1 (to minimize systemic effect) with an Eppendorf automatic micropipette, either unilaterally or bilaterally one, two or three times daily. The times of daily corticosteroid application were as follows: once daily application between 0900 and 1000 hr; twice daily application, 0900 to 0930 hr and 2100 to 2130 hr; three times daily application, 0900 to 0 9 3 0 h r , 1500 to 1 5 3 0 h r , and 2100 to 2130 hr. For each experiment, the frequency and dose of treatment is indicated on the figures between arrows that mark the beginning and end of each type of treatment regimen.

Administration of the Prednisolone (PRED) Solution lOP Measurements These were made prior to, and during all treatments using an Alcon ' Applanation P n e u m o t o n o g r a p h ' after corneal anesthesia was obtained by the topical application of one drop of proparacaine hydrochloride 0-5 % (Alcaine; Alcon Laboratories, Fort Worth, TX). IOP was measured typically at 0900 hr and on some days also at 1, 3, and 6 hr thereafter, at least once or twice a week during the course of long-term treatments and daily for a few days after a treatment regimen was started, changed or terminated.

A clinical preparation of 1% prednisolone acetate solution (1% PredForte ophthalmic suspensions; AIlergan Pharmaceuticals, Irvine, CA) was used. Each treatment consisted of the topical application of 10 #1 of this solution performed twice, 3-5 min apart. This method of applying a 10-#1 volume twice was used to approximate the 3 0 - 4 0 #1 drop size applied clinically, and also to enhance drug delivery into the eye since a single larger volume is more rapidly drained into the lacrimal canal. Treatment times were as described above.

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FIe. 1. The effects of topically applied dexamethasone (DEX) on the IOP of four ocular hypotensive cats. The regimen of DEX application for each time period is indicated between arrows at the top of the panels. Mean IOP values of the right eyes (OD; open circles) treated from day 0 to day 30 (A) that are significantly higher (P < 0-05) than pro-treatment baseline values are indicated by asterisks above, and for control eyes below the corresponding error bars. Asterisks between error bars indicate pairs of means (OD versus OS) that are significantly different from each other (P < 0"05). After a further 7 days of no treatment (A), the left eyes (OS ; closed circles) only were treated for I month (B) followed by treatment of both eyes (OU). Here, and in all other figures, the values represent means and the limits, if shown, indicate 4- 1 S.E.M.

The Effects of PGF2~-l-isopropgl Ester on Dexametbasone Treated Eyes The ocular hypotensive effect of PGF-~-IE was tested at a dose of 2.5 #g per eye on the left eyes of four cats on day 62 of the second experiment, and at 0.1 #g dose on day 65. The [OPs and pupil diameters were measured 1, 3, and 6 hr after the PG application. Slitlamp examination of the anterior chamber for flare was done at 6 hr, and for cells and flare at 24 hr.

3. Results After about 10 days of thrice daily unilateral treatment with 10 #1 of 1% DEX, there was a gradual increase in the IOP of the DEX-treated eyes of a group of four ocular hypotensive cats [Fig. I(A)]. Beginning with the 18th day of treatment, the mean IOP of the

treated eyes was consistently and significantly higher (P < 0'05) than the mean fOP of these eyes just before the first treatment. The contralateral untreated eyes also showed an increase in IOP as compared to baseline values; however, this increase was much smaller and never became consistently significant during the 30 days of treatment. The difference between the mean fOPs of the two eyes of each cat became consistently statistically significant (P < 0"05) after 23 days of treatment. When DEX treatment was stopped after 30 days, the IOP of the previously treated eye began to decline and the significant IOP elevation, as compared to baseline or the control eye, was abolished within 7 days [Fig. I(A)]. One week after the last unilateral DEX treatment of the right eye of these four cats, unilateral treatment of the left eyes began using the same concentration of DEX (1%), but with only two treatments each day. Again, after a 10- to 12-day delay, a significant (P