1. The aim is to predict the toxicity in humans by using animals or cell cultures as surrogates, mimicking the possible exposure scenarios that a human being would face using the most appropiate animal model (or cell cultures).
1.1. ** The oral route is the most common route of exposure used in this kind of studies.
2. Acute Toxicity Testing
2.1. Objective: Investigate the potential adverse effects arising from exposure to a given chemical over a short period of time.
2.1.1. Endpoint = Lethality. The results are represented as a dose-response curve.
2.1.1.1. LD50 Test: describe the acute oral/dermal median letal dose (which will kill 50% of the test population). ** For acute inhalation the equivalent value used is LC50.
2.1.1.1.1. Three groups of test animals (10 animals per group usually rodents) are administered with increasing graduated doses of the test chemical one group per dose. After 14 days, where all mortalities are noted, the animals are autopsied and the percentage lethality vs dosage administered is plotted.
2.1.1.1.2. This test provides infomation on the magnitude of the acute toxic dose (labelling and classification purposes) and can be used to predict what might happen as a result of large amounts of chemical contact, inhalation or ingestión. ** The results will depend on the test specie and the route of exposure chosen.
2.1.1.1.3. The lower the LD50 value is the more toxic the test compound is and viceversa.
2.1.2. Alternative methods: The fixed dose method (OECD 420), identify a dose that produces clear signs of evident toxicity but not lethality, therefore the number of animals used are smaller and only four fixed dose levels are used. ** This method is not intended to generate data for the estimation of an LD50.
2.1.2.1. The up and down procedure (OECD 425), this method permits estimation of an LD50. Is a sequential test that uses a maximum of 5 animals. It consists of a single ordered dose progression in which animals are dosed, one at a time, at a minimum of 48-hour intervals. The first animal receives a dose a step below the level of the best estimate of the LD50. If the animal survives, the dose for the next animal is increased by [a factor of] 3.2 times the original dose; if it dies, the dose for the next animal is decreased by a similar dose progression until the limit dose has been reached or the 5 animals have been dosed.
2.1.2.2. Acute toxic class method (OECD 423),is a stepwise procedure based on lethality that uses 3 animals of a single sex per step dosed with one of the three levels that corresponds to the acute oral LD50 classification limits. Depending on the mortality and/or the moribund status of the animals, on average 2-4 steps may be necessary to allow judgement on the acute toxicity of the test substance. The method allows for the determination of an LD50 value only when at least two doses result in mortality higher than 0% and lower than 100%.
3. Short-term Testing
3.1. Objective: Involves the subacute and subchronic studies designed to investigate the adverse effects resulting from repeated exposures to smaller levels of chemicals over part of the organism's lifetime (usually under 10% of their lifespan). These test aim to mimic the exposure pattern of humans who may daily work with or are exposed to low levels of chemicals.
3.1.1. The lethality is not the endpoint of this kind of tests.
3.1.1.1. LOEL (Lowest observable effect level): The smallest or lowest dose which produces any kind of detectable adverse effect. NOEL (Non observable effect level): the highest dose level at which there's not a detestable adverse effect.
3.1.1.1.1. Group of test animals (usually rodents) are daily administered with graduated doses of the test chemical over one-tenth of their total lifetime. ** A dose-response curve can be constructed. Sub-acute studies usually last for 14-28 days using at least 10 rodents per group, while subchronic studies last 90 days and use at least 20 test animals per group.
3.1.1.1.2. They can also help to pinpoint any potential target-organ effects, i.e those organs where the chemical produces adverse effects and help to determine wheter there is any possible accumulation effects. ** The last affirmation is importante because "delayed toxicity" can't be detected by this kind of study, but the results can be used for the selection of dose levels in further tests.
4. Long-term Testing
4.1. Objective: Investigate the adverse effects arising from prolonged or repeated exposure to low levels of a chemical over the whole or the greater part of the organism's lifetime. This tests have a long latent period for any adverse effect to show up along with cumulative effects.
4.1.1. They provide information on adverse effects resulting from prolongated or repeated exposures and also effects of chemicals that have a long latency perro or are cumulative.
4.1.1.1. These studies are similar to short-term testing, the major differences rely on the test duration, dosage levels and the number of animals used. ** The oral route of exposure is the most common route used. **Normally they're combined with carcinogenicity tets. **Can be used for the determinaron of LOEL and NOEL values.
5. Tests for skin irritancy and corrosion
5.1. Irritancy Draize test: it's objective is to predict potencial skin irritants. Involves the administering of the test chemical either directly onto shaved skin or onto gauze patches in albino rabbits (4 hours of exposure under semi-occluded conditions). Then, observations are made for any signs of irritation at specific time intervals (72-hours after exposure) and the results are grande based on the scheme provided in the test guidelines. **This test only detects primary irritants.
5.2. Alternative corrosion tests: In vitro skin corrosion (TER), is a validated in vitro test used to identify corrosive materials by their hability to produce the loss of normal stratum corneum integrity and barrier function, measured as a reduction in the transcutaneous electrical resistence.
5.2.1. Human skin model test: Uses an in vitro re-constructed human epidermis model with a functional stratum corneum to which the test chemical is topically applied.
5.2.2. CORROSITEX test: In vitro test based on the time it takes for the chemical to pass through a special bio-membrane (reconstitued collagen mix) into a chemical detection system that becomes colored when exposed to corrosive substances.
5.3. Tests for eye irritancy and corrosion
5.3.1. Chemical that have been shown to be either severely irritating or corrosive in dermal studies do not have to be tested.
5.3.1.1. If a previous result indicates that the chemical may not be corrosive, then further in vitro/vivo tests must be carried out to validate non corrosive effects to the eye. ** The substance must be applied on the albino rabbit one, having the other one as a control. If the results are positive then no further tests are needed however if is not, two additional animals are tested with the chemical with 72 hours post-examination.
5.4. Tests for respiratory irritation
5.4.1. There is not an specific animal test but OECD 403, is used. One dose level is usually employed and upon termination the tissues are examinated for any signs of inflammation.
5.5. Allergies
5.5.1. Skin Sensitation the test methods most commonly used are the the Buehler and the Guinea Pig Maximisation Test. The only difference between these two tests is that GMPT involves the subcutaneous injection of the test compound and the use of an adjuvant, whereas the Buehler test involves the tropical application of the quemical without the adjuvant. At the end of the test and being exponed to a challenge dose, the animals are compared to a control group and graded according to their severity.
5.5.1.1. The Murine Local Lymph Node Assay (LLNA), is an in vivo esas that reduces and refine the GMPT test because the sensitation potential is quantitatively determined by measuring the lymphocyte proliferation nodes.
5.5.2. The classification for respiratory sensitisation comes mostly from human experience and in some cases, the use of structure-activity relationships.
5.6. Genetic toxicity testing
5.6.1. This kind of tests are part from the short-term tests. they have been developed with the aim to investigate whether somatic cells mutations can be produced leading to the development of clínica cancer. Some mutations tests are assays for gene mutations, chromosomal aberrations and DNA effects.