The water-loading test in rabbits. A method to detect potential ocular hypotensive drugs

Translational Preclinical Pharmacologic Disease Models for Ophthalmic Drug Development

Preclinical models of human diseases are critical to our understanding of disease etiology, pathology, and progression and enable the development of effective treatments. An ideal model of human disease should capture anatomical features and pathophysiological mechanisms, mimic the progression pattern, and should be amenable to evaluating translational endpoints and treatment approaches. Preclinical animal models have been developed for a variety of human ophthalmological diseases to mirror disease mechanisms, location of the affected region in the eye and severity. These models offer clues to aid in our fundamental understanding of disease pathogenesis and enable progression of new therapies to clinical development by providing an opportunity to gain proof of concept (POC). Here, we review preclinical animal models associated with development of new therapies for diseases of the ocular surface, glaucoma, presbyopia, and retinal diseases, including diabetic retinopathy and age-related macular degeneration (AMD). We have focused on summarizing the models critical to new drug development and described the translational features of the models that contributed to our understanding of disease pathogenesis and establishment of preclinical POC.

Animal models of glaucoma

Glaucoma is a heterogeneous group of disorders that progressively lead to blindness due to loss of retinal ganglion cells and damage to the optic nerve. It is a leading cause of blindness and visual impairment worldwide. Although research in the field of glaucoma is substantial, the pathophysiologic mechanisms causing the disease are not completely understood. A wide variety of animal models have been used to study glaucoma. These include monkeys, dogs, cats, rodents, and several other species. Although these models have provided valuable information about the disease, there is still no ideal model for studying glaucoma due to its complexity. In this paper we present a summary of most of the animal models that have been developed and used for the study of the different types of glaucoma, the strengths and limitations associated with each species use, and some potential criteria to develop a suitable model.

The effect of the Rho-associated protein kinase inhibitor, HA-1077, in the rabbit ocular hypertension model induced by water loading

PURPOSE

The aim of this study was to investigate the intraocular pressure (IOP)-lowering effect of a new anti-glaucoma drug, the Rho-associated protein kinase (ROCK) inhibitor, HA-1077, in a rabbit ocular hypertension model.

METHODS

Experiments were carried out in 18 male New Zealand white rabbits, with ocular hypertension induced by water loading. Animals were divided into three groups followed by topical administration of 1 mM, 2 mM, and 3 mM HA-1077 in the left eye. As a control, phosphate buffered saline was administered in the opposite eye.

RESULTS

After administration of HA-1077 eye drops, there was a significant time- and dose-dependent decrease of the IOP. While minor conjunctival injection was seen in a few cases, no abnormalities of the anterior chamber or fundus were observed.

CONCLUSIONS

This is the first report of the effect of the ROCK inhibitor, HA-1077, on the IOP in an ocular hypertension model. Study results indicated that HA-1077 has a strong IOP-lowering effect.

Comparative effects of alpha-2 and DA-2 agonists on intraocular pressure in pigmented and nonpigmented rabbits

Alpha-2 and DA2 adrenoceptor agonists produce ocular hypotension in cats, rabbits, monkeys and humans. In this study, the effects of topical, unilateral administration of medetomidine (MED), an alpha-2 agonist, and Ha-118 (HA), a DA2 agonist, were compared on intraocular pressure (IOP) in normal, unilaterally sympathectomized (SX), and ocular hypertensive (oral water loaded) Dutch Belted (DB) and New Zealand White (NZW) rabbits. MED (75 micrograms) produced bilateral ocular hypotensive effects in both DB and NZW normal rabbits; however, HA (250 micrograms) lowered IOP unilaterally in NZW rabbits only. MED (25 micrograms) inhibited the rise in IOP caused by oral water loading in both DB and NZW rabbits; HA, on the other hand, was effective only in NZW rabbits. Topical bilateral pretreatment with metoclopramide, a DA2 antagonist, inhibited the ocular antihypertensive effect of HA in NZW rabbits. The IOP lowering effects of MED were absent in the SX eyes of DB and NZW rabbits, but hypotensive responses to MED were present in normal (contralateral) eyes of both strains. In contrast, HA was effective only in the treated, normal eyes of SX NZW rabbits. These data support pharmacodynamic roles for alpha-2 and DA2 receptors in modulating IOP. The lack of activity by HA in DB rabbit eyes suggests a possible absence of DA2 receptors or excessive binding of HA to pigment in the anterior segment of DB rabbit eyes.

Transitory models of experimentally induced intraocular pressure changes in the rabbit. A reappraisal

The purpose of this study was to evaluate the relationship between inducer dosage, animal weight, and kinetics intraocular pressure (IOP) changes in three transitory-induced models for antiglaucoma drugs screening: oral water-loading, 5% glucose intravenous administration, and 20% NaCl infusion. For these models, a dose-dependent elevation of IOP was observed in three weight groups of rabbits (1, 2.2, and 3.8 kg). Dose-effect relationships were established, considering for each weight group two relevant parameters: maximal IOP changes and areas under or upper the kinetic curves of IOP changes. Among the three models studied, our results led us to consider water-loading as a weight-independent model, allowing us to compare results obtained by this model in New Zealand rabbits with different ages (40-150 days) or weights (0.9-3.8 kg).

On the topically effective ocular hypotensive properties of ICI 147,798, a natriuretic beta-adrenoceptor antagonist, in rabbits

The ocular antihypertensive effects of ICI 147,798, a natriuretic, non-selective beta-blocker lacking local anaesthetic and intrinsic sympathomimetic activities, were evaluated in unanaesthetized rabbits with water-load-induced ocular hypertension. The racemate was approximately 20 times more potent as a beta-blocker than its d-isomer. The magnitude of reductions in intraocular pressure (IOP) elicited by both compounds was equal following topical administration of an equal dose-concentration (100-300 microliters, 0.5% or 0.75%) to one or both eyes, and the activities were similar to those obtained with timolol. A five-hour duration of the IOP lowering effect was achieved by ICI 147,798 and timolol in this rabbit model. ICI d-147,798 exhibited a shorter duration, which could be prolonged by increasing the concentration of the ophthalmic solution. Timolol caused a pronounced reduction of isoproterenol-induced tachycardia after a single dose (200 microliters, 0.5%) or by repeating the dose twice a day for four consecutive days. Following the same dose regimen, neither ICI 147,798 nor its d-isomer significantly altered the heart rate response produced by isoproterenol. These findings indicate that ICI 147,798, while effective as a potential antiglaucoma agent, does not display the characteristics of beta-blockers on heart rate effects noted with timolol. The results also cast some doubt on the contention that the IOP lowering effect of certain beta-blockers is predominantly determined by specific blockade of beta-adrenoceptors.

Effects of adrenergic agents on alpha-chymotrypsin-induced ocular hypertension in albino and pigmented rabbits: a comparative study

Alpha-chymotrypsin-induced ocular hypertension in albino rabbits is widely used as an experimental model to screen potential antiglaucoma drugs. The present study compares the intraocular pressure (IOP) response following the ocular application of single or repeated adrenergic agents in conscious albino and pigmented rabbits. A single instillation of clonidine was not as effective in lowering the IOP in pigmented hypertensive rabbit eyes as in albino hypertensive eyes. Similarly, betaxolol moderately lowered the IOP in albino rabbits but induced a slight response when pigmented rabbits were used as an experimental model. Twice-a-day applications of betaxolol in pigmented hypertensive eyes permitted an identical level of IOP decrease to be reached, as observed in a one-day study in albino rabbits, after at least 6 days of treatment. It has been suggested that the pigmented layers of the iris-ciliary body may act as sites for topically applied antiglaucoma drugs. Non-specific binding could explain in part the frequent discrepancy observed between the preclinical results obtained in albino hypertensive rabbit eyes and clinical results obtained in glaucomatous human eyes.

Ocular hypertensive responses in pigmented rabbits following different methods of waterloading

Waterloading tests are used in rabbits to screen potentially useful ocular hypotensive drugs. The present study examines the ocular hypertensive response following oral, intravenous and intraperitoneal administration of water in conscious Dutch belt pigmented rabbits. All methods of waterloading were well tolerated by rabbits. However, intraperitoneal waterloading provided an ocular hypertensive effect of longer duration than either oral or intravenous waterloading.

Local ocular hypotensive effect of topically applied acetazolamide

Acetazolamide's usefulness in the treatment of the glaucomas is limited by the systemic side effects that often accompany its oral administration, and topical administration was initially thought to have no effect upon the intraocular pressures of human and rabbit eyes. Recent studies, however, have shown the usefulness of water-loading tests for screening drugs with potential antiglaucomatous activity. We found evidence that topical acetazolamide has the ability to lessen the increase in intraocular pressure after water-loading in pigmented rabbits and correlated this observation with low levels (0.0 to 0.7 microgram/ml) of plasma acetazolamide. Further, a separate study showed that 10% topical acetazolamide can enhance the ocular hypotensive effects of systemically administered acetazolamide in normal pigmented rabbits, suggesting that topically applied acetazolamide can have a local effect on intraocular pressure.

Some aspects of waterloadings in rabbits

The effect of various amounts of waterload and the differences caused by oral and parenteral administration of fluids on the intraocular pressure (IOP), pupil diameter and blood osmolality were investigated. Trained rabbits were given loadings of fluids by different routes. An oral waterload of 100 ml/kg bodyweight gave an elevation of the IOP of about 12 mm Hg. Smaller amounts give a less pronounced IOP elevation. Intraperitoneal administration does not seem to increase IOP markedly. Rapid infusion of glucose 5% 15 ml/kg produces a transient and modest increase of the IOP. The mechanisms by which oral waterloading increases the IOP are discussed.

The effect of hypotensive drugs on the intraocular pressure after waterloading in rabbits

The waterloading test in rabbits, using 100 ml of waterloads per kg body weight, without general anaesthesia and with applanation tonometry to measure intraocular pressure (IOP), deserves a place in glaucoma research. Using this model we found adrenaline not to have an effect on the IOP with or without waterloading. Isoprenaline and to a lesser physostigmine reduced the pressure in the normal eye markedly, but these drugs did not have any effect on the pressure after waterloading. delta 9-THC (Tetrahydrocannabinol) and pilocarpine increased the IOP in the normal rabbit eye. After waterloading delta 9-THC decreased the pressure, but its value was still significantly higher than the control pressure. Pilocarpine did not reduce the pressure after waterloading. Acetazolamide and timolol did not reduce the pressure in the normal eye, but after waterloading a significant reduction was observed.