Study in animals or pre-clinical testing
The pre-clinical testing evaluates the efficacy and the toxicity of the product before its possible administration to human beings.
Whatever the drug selected, an analgesic, an antibiotic…, it is necessary to determine thoroughly its principal property, to specify by a systematic study any other possible parallel effect on the other systems: cardiovascular, respiratory, renal, etc. A systematic exploration of all the possible effects of a drug is always necessary.
These studies, which it is not possible to quote here, are carried out in live animals, on isolated organs, isolated cells, isolated cellular fractions, enzymes, receptors. They specify the properties and the mechanisms of action.
Generally, pharmacokinetic studies are carried out in parallel to the preceding ones, particularly to find the main metabolites.
Study of mortality after a single administration of the product to an animal species allows the determination of the lethal dose 50, or LD50, which is the dose which kills 50% of the treated animals in a determined time, for example eight days. The study is conducted on different animal species, mouse, rats… which receive different doses of the product studied, administered under well defined conditions. One notes mortality but also all other modifications which appear. LD50 of the same product depends on the animal species and the route of administration; it is generally lower (i.e. toxicity is higher) by parenteral route than by oral route.
Chronic toxicity consists in studying the consequences of repeated administrations of the investigated product. The product is given daily, one or twice a day, for a more or less long length of time, three to six months, in general, according to the duration of administration envisaged in humans.
The experimentation is carried out in two or three adult different animal species, mouse, rats, rabbits, receiving each one generally three different doses (low, medium, high) of the product.
When the drug is intended for a pediatric use, a complementary experimentation on young animals, a few days old for example, can be useful to detect a possible particular toxicity in children.
The signs of toxicity are examined clinically: aspect, weight, intake of food and drink, behaviour and biologically: hematologic, biochemical, pathological parameters.
These studies are very expensive and are undertaken only when the product is supposed to become a “drug”.
More and more, in vitro tests supply information on the potential toxicity of compounds, but they are not sufficient to avoid the experimentation on the whole animal before the administration to humans.
Any molecule, studied as a potential drug can be suspected to modify sexual activity, fertility and offspring if it is taken during pregnancy.
The modifications of the sexual activity after administration of the product can be detected by studying couplings and fertility (frequency of gestations). A study of the spermatozoa can also be undertaken.
A compound can have toxic effects on the offspring whatever the moment of gestation where it is taken and more particularly during embryogenesis. The terms of teratogen, embryotoxic and foetotoxic are used to characterize the toxicity of a compound. Restricted to the etymological meaning, a compound is teratogenic when taken by the mother during gestation, it elicits visible malformations in the offspring, as those caused by thalidomide. But if, for example, the toxic effect results in deafness, the term teratogen can still be employed in an extended sense: any morphological or functional damage or growth retardation elicited in the offspring by the intake of a drug by the mother during gestation. To indicate the toxic effects elicited during the period of embryogenesis (the first two months of the pregnancy in mankind) the word embryotoxic is used and to indicate the alterations induced during the second phase of the pregnancy (starting from the beginning of the third month) the term foetotoxic is used. In this last case, it can be a question, for example of a growth delay.
The teratogenic activity of a product is demonstrated by the advent of morphologic or functional abnormalities in the offspring of females treated during gestation. The experimentation is carried out on three animal species, in repeated administrations and at several doses. It involves the examination of the animals at birth and possibly later on. If important or frequent anomalies are observed, the product is contraindicated in pregnant women. If the anomalies are not more frequent than those occurring spontaneously, the teratogenic risk is low. The absence of teratogenic effect of a product in two animal species, rat and rabbit, is an essential data which however does not guarantee its total safety in pregnant women; often the pharmaceutical laboratory can, in spite of this absence of teratogenic effect in animals, disadvise the use of the drug by pregnant women.
Moreover, one can study the possible repercussion of a product administered to the female in gestation on the postnatal development of his offspring of the first-generation and possibly to the following ones.
The accidents of perinatality are disorders which occur in the new-born baby in the majority of the cases following the intake of a drug by the mother little before the labor and of its diffusion through the placenta (for example drowsiness of the neonate after the intake of a sedative by the mother). More rarely the new-born baby can show a withdrawal syndrome consecutive to the discontinuation of the supply of a drug by the mother through the placenta.
The mutagen risk of a drug consists of the damage of the genome, i.e. deoxyribonucleic acid or DNA. A mutation consists of a change in a sequence of nucleotides in the genome. This change can be with or without consequences. If the change affects the genome of the germ cells, it is transmissible to the following generations.
The search for mutations, part of genetic toxicology, is performed using in vitro tests on mutant strains, for example Salmonella Typhymurium.
To know if a product could increase the risk of cancers, it should be administered daily for a long time, from one to three years, to mice or rats. The experiment must be performed on animals of the two sexes.
This research is especially important for the drugs used for long lengths of time. Anticancer drugs, immunosuppressive agents can induce cancers.