Ocular Toxicity Tests

The toxicity of a few boric acid esters has been summarized (30). In general the toxicities are directiy related to the toxicity of the alcohol or phenol produced on hydrolysis. Methyl borate has an oral rat LD q of 6.14 mL/kg in a range finding test (31) and the percutaneous LD q for the rabbit of 1.98 mL/kg. In eadier work (32), the oral LD q for the rat was 2.82 mL/kg the intraperitoneal LD q was 3.2 mL/kg. It has been shown that the mouse is more susceptible to these compounds than the rat. Methyl borate was found to be moderately irritating in an ocular toxicity test using rabbits (31,32) but only mildly irritating to skin (31). 

For further description of the method see chapters I.N (Ocular Toxicity Tests) and I.P (Safety Assays in Skin Pharmacology). 

Further description of the test see Chapters I.H (Peripheral Nervous System) and 1.0 (Ocular Toxicity Tests). 

ICCVAM Test Method Evaluation Report In Vitro Ocular Toxicity Test Methods for Identifying Severe Irritants and Corrosives. National Institute of Environmental Health Sciences. National Institutes of Health Publication 07-4517, 2007. 

Wilson SL, Aheame M, Hopkinson A (2015) An overview of current techniques for ocular toxicity testing. Toxicology 327C 32-46. 

Pasternak, A.S. and MiUer, W.M. (1996). Measurement of trans-epitheial electrical resistance in perfusion potential application for in vitro ocular toxicity testing. Biotechnol. Bioeng. 50 568. 

Hockwin, 0., Green, K. and Rubin, L.F., 1992, Manual of Ocular Toxicity Testing of Dmgs, Gustav Fischer Verlag, Stuttgart, Jena, New York. 

Albino rabbits have been the primary species used to test ocular toxicity and irritation of ophthalmic... 

Various parts of the ophthalmological examination are shown in Table 20.14. The most important common ophthalmological test to evaluate patients for the occurrence of chronic drug-induced toxicity is slit-lamp examination. Specific types of drugs with known potential for ocular toxicity may require that special attention be directed to other evaluations shown in Table 20.14. Most drugs that are to be taken... 

These tests are of minimal value in determining ocular toxicity and are not recommended for routine use in ophthalmological examination to detect drug toxicities. 

There are many immortalized human corneal epithelial cell models that gained high acceptance for various ocular studies (e.g., toxicity testing and corneal epithelial transport Table 12.1). 

Thus far, a wide array of useful cell culture models of the corneal epithelium has been established. Many of these cell culture models focus on toxicity testing and ocular irritation, but some cell layer models for drug permeation studies are also available. Indispensable for successful drug penetration testing is a cell layer that exhibits a tight epithelial barrier. This latter requirement of tight barrier properties disqualifies some of the models that were established as substitutes for the Draize test. At least two cell lines are available for pharmaceutical studies and some newer models may qualify as a useful tool, once they are characterized for their barrier properties. 

There are some other acute toxicity tests in which non-lethal outcome are sought. These include studies of the amount of chemical needed to cause skin or eye irritation or more serious damage. Test systems developed by J. H. Draize and his associates at the Food and Drug Administration in the early 1940s were used to study ocular effects. Warning labels on consumer products were typically based on the outcome of the Draize test. 

ICCVAM has evaluated alternative test methods for acute oral toxicity, genetic toxicity, biologies, immunotoxicity, dermal corrosion and irritation, ocular toxicity, developmental toxicity, pyrogeni-city, and endocrine disrupter effects (ICCVAM 2007). As examples, alternative test systems for dermal corrosion and irritation are described in the following text. 

Its most important adverse effects are visual disturbances. This ocular toxicity is dose dependent and has an incidence of lower than 1 % at low doses but can reach 5% at high dose regimens. Ocular toxicity manifests itself as retrobulbar neuritis usually after the second month of use. If therapy is discontinued immediately it is mostly reversible but not always. During the treatment visual function should periodically be tested. Age under 8 years is a relative contraindication as visual symptoms are difficult to monitor. 

Ocular damaging and irritant agents can be identified and evaluated by the Draize rabbit test [114]. However, more recently this test has been criticized on the basis of ethical considerations and unreliable prognosis of human response. Alternative methods such as the evaluation of toxicity on ocular cell cultures have been recommended and are being indicated as promising prognostic tools [115-120]. Direct confocal microscopic analysis [121], hydration level of isolated corneas [122], and various other tests on isolated corneas or animal eyes have also been proposed for evaluation of ocular toxic effects. 

Eye Irritation. Because of the prospect of permanent blindness, ocular toxicity has long been a subject of both interest and concern. Although all regions of the eye are subject to systemic toxicity, usually chronic but sometimes acute, the tests of concern in this section are tests for irritancy of compounds applied topically to the eye. The tests used are all variations of the Draize test, and the preferred experimental animal is the albino rabbit. 

The eye irritation test is probably the most criticized by advocates of animal rights and animal welfare, primarily because it is inhumane. It has also been criticized on narrower scientific grounds in that both concentration and volumes used are unrealistically high, and that the results, because of high variability and the greater sensitivity of the rabbit eye, may not be applicable to humans. It is clear, however, that because of great significance of visual impairment, tests for ocular toxicity will continue. 

PURPOSE AND RATIONALE This test uses an ex vivo model of corneal organ culture, preferentially porcine, to obtain information of the possible ocular toxicity of various chemicals. This test is used as an alternative to the Draize Test to minimize or replace the use of live animal testing of ocular irritancy (Symposium, Proceeding 1996). The test allows for determination of reversibility of corneal injury following exposure to chemicals, drugs or cosmetics (Xu etal. 2004). 

PURPOSE AND RATIONALE This test uses the ex vivo model of comeal organ culture previously discussed combined with surface biotinylation technique. The comeal epithelium functions as a barrier that separates the internal ocular tissues from the external environment and is therefore vulnerable to chemical insult (Gipson and Sugrue 1994). The joint assessment of comeal organ culture and surface biotinylation is a measure of ocular toxicity that is intended to reveal disrupted tight junction barriers following chemical exposure. 

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