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Toxicology course
Toxicological Testing in vivo In vivo testing, Use of laboratory animals, Animal testing, Animal experimentation, Animal research.
Benefits Scientists use whole living systems, animal models, in experiments to understand the relation-ships between exposure and effect. Data from these studies provide the essential foundation to distinguish and recommend which level of exposure is safe and which is harmful to people, animals and the environment.
Toxicologists perform research in whole animals to learn how various chemicals and dosages interact with living systems and to ensure the short-term and long-term safety of such products before they are brought to market.
Guiding Principles in the use of animals in toxicology: 1. The use, care and transportation of animals for toxicological research and testing for the purpose of protecting human and animal health and the environment must comply with all applicable animal welfare laws. 2. When scientifically appropriate, alternative procedures that reduce the number of animals used, refine the use of whole animals or replace whole animals (e. g. , in vitro models, invertebrate organisms) should be considered.
3. The species should be carefully selected and the number of animals kept to the minimum required to achieve scientifically valid results. 4. All reasonable steps should be taken to avoid or minimize discomfort, distress or pain of animals.
5. Appropriate aseptic technique, anesthesia and postoperative analgesia should be provided if a surgical procedure is required. 6. Care and handling of all animals must be directed by veterinarians or other individuals trained and experienced in the proper care, handling and use of the species being maintained or studied.
7. Investigators and other personnel shall be qualified and trained appropriately for conducting procedures on living animals, including training in the proper and humane care and use of laboratory animals.
Laboratory Animals Rat Mice
Animal house
Estimation of lethality LD 50 (The median lethal dose)
Definition: The LD 50 (mean or average lethal dose) is the dose of a chemical needed to produce death in 50% of treated animals and statistically calculated. The lower the LD 50, the more toxic the chemical.
Importance: It is an initial step in the assessment and evaluation of the toxicity of a substance (new substance). To determine therapeutic index (TI), i. e. ratio between the mean lethal dose and the effective dose in the same strain and species (LD 50/ED 50). The greater the index, safer is the compound. The test is a measure of acute systemic toxicity.
Test procedure
Initial Considerations: All available information on the test substance should be considered : ØChemical structure of the substance. ØPhysical and chemical properties. ØThe results of any other in vitro or in vivo toxicity tests on the substance or mixtures. ØToxicological data on structurally related substances or similar mixtures. ØThe anticipated use (s) of the substance.
All factors should be fixed. Animal species, sex, age, average weight and environmental conditions (temperature, bedding, cages, water and food supply……. , )
Animals 1 - Species: the animals used most often in the LD 50 Test are rats and mice; others include rabbits, guinea pigs, dogs, monkeys, birds, and fish. 2 - Age: between 8 and 12 weeks old and its weight should fall in an interval ± 20 % of the mean initial weight of all previously dosed animals. 3 - Sex. 4 - Females should be nulliparous and non-pregnant
Housing: Animals were group-caged by sex, but the number of animals per cage must not interfere with clear observation of each animal. The biological properties of the test substance or toxic effects (e. g. morbidity, excitability, etc. ) may indicate the need for individual caging.
Preparation of doses: Solvent : When necessary, the test substance was dissolved or suspended in a suitable solvent. Dosing preparations must be prepared shortly prior to administration. Doses are typically measured in terms of milligrams of the chemical per kilogram of the animals’ body weight (mg/kg b. w. ).
Volume: In rodents, the volume should not exceed 1 ml/100 g of body weight, maximum of 5 ml/100 g.
Administration of doses: Oral– gavage: dosing is typically through force-feeding. Inhalation: forcing the animal to breathe substance. Dermal : applying the substance directly to the animals shaved skin. Intraperitoneal (IP) : injecting the substance into the animal’s abdomen. Intravenous (IV): injection.
IP injection
Observation period: 1– 2 h, and thereafter at intervals. Maximum for 14 days.
Signs recorded during acute toxicity studies: increased motor activity, tremors, arching and rolling, colonic convulsions, tonic extension, lacrimation, salivation, muscle spasm, opisthotonus, hyperesthesia, depression, ataxia, stimulation, sedation, hypnosis, cyanosis and analgesia.
Experimental design : 1. Preliminary test: Before the actual LD 50 determination. A-pilot study : conducted on a small group of mice mainly to select the dose ranges for the subsequent study. B- Up and down or the ‘staircase’ method: The subsequent doses were then increased or decreased.
ØOnce the maximum non-lethal dose (kill none of the exposed animals) and the minimum lethal dose (kill all the animals) were thus determined using only about ten mice. Ø Final and more reliable LD 50 assay was planned using at least three or four dose levels within this range.
2 - The test proper: From the previous responses we can select some doses where: - There is a fixed range between these doses - At least three or four dose levels - The obtained responses from these doses must be clearly different. The data should be sufficient to produce a dose-response curve and permit an acceptable estimation of LD 50.
Record all results in a table. Prepare the dose effect curve on a logarithmic paper. From the curve detect the average dose which causes 50% lethality. LD 50 will be calculated statistically from the table.
ØDetection of the toxicity degree: Compare the obtained value of LD 50 with the Hodge and Sterner table to detect either the tested compound is toxic or not. If it is toxic, detect the degree of toxicity. Note: -A chemical with a small LD 50 is highly toxic than a chemical with large LD 50.
Hodge and Sterner table Oral LD 50 in male albino rat Degree of toxicity Up to 5 mg/kg b. w. Super toxic More than 5 - 50 mg/kg b. w. Extremely toxic More than 50 – 500 mg/kg b. w. Very toxic More than 500 – 5000 mg/kg b. w. Moderately toxic More than 5000 – 15000 mg/kg b. w. Slightly toxic More than 15 g/kg b. w. Practically non toxic
Example: Preliminary toxicity studies ( IP - LD 50) of the methanol extract of the stem bark of Garcinia kola (Heckel) in mice.
1 - Preliminary test Ø 4 mice per pilot dose level, Ø With the pilot tests it was possible to establish the highest dose of the extract that killed none of the exposed animals (200 mg/kg) and the lowest dose that killed all the animals (500 mg/kg). Ø The dose levels used in the acute toxicity study ranged between these two dose extremes 200 and 500 mg/kg.
2 - Proper test Ø 30 mice weighing between 20 - 25 g were kept in 5 cages (6 per cage) and handled according to standard guidelines for the use and care of laboratory animals. Ø The animals were maintained on standard animal diet and water. Ø food was withdrawn 18 h before the start of the experiment.
Ø The five groups of mice were administered intraperitoneally with 250 mg/kg, 300 mg/kg, 350 mg/kg, 400 mg/kg & 450 mg/kg of the extract respectively. Ø Animals were observed for mortality for 24 hours.
Results 850
LC 50 (median lethal concentration) § The concentration of a chemical in the air or water that causes death to 50% of the animals. This concentration is usually quoted as parts per million (ppm) or milligrams per cubic metre (mg/m 3).
v Number of used animals “Classical” LD 50 Test : approximately 100 animals. The Organization for Economic Co-operation and Development (OECD) called for at least 30 animals divided into 3 dose groups of 10 animals. The OECD calling for fewer animals per group, resulting in 20 animals used per test.
The 3 Rs of humane experimental technique: Replacement, some involve taking tissues from animals euthanized for this purpose, so these are not ideal. Reduction : use cell-based tests to better estimate the starting doses for the animal-based tests Refinement: less animal suffering Each uses fewer animals and refined the endpoint from death to “evident signs of toxicity. ”
use cell-based tests (tissue culture) to better estimate the starting doses for the animal-based tests. Consequently, the number of animals used in these LD 50 substitutes should drop. Tissue culture
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