Before testing candidate molecules on humans in clinical trials, scientists must show that the candidates do not present an unacceptable level of risk, given the expected therapeutic benefit.

The regulatory authorities require pharmaceutical companies to demonstrate the safety of the drug for humans and to prove that the therapeutic advantages of the compound greatly outweighs any associated undesirable side-effects, such as migraine or high blood pressure in the case of cancer treatment. These studies are conducted in conformity with the rules of the regulatory bodies. Thus, the American and Canadian authorities are particularly stringent in regards to drug safety. The protocols from clinical trials must be subjected to strict monitoring and evaluation.

Duration: a minimum of 4 to 6 months The twenty or so perfected compounds are evaluated both in vitro and in vivo. Of these, between one and five will have the characteristics required for phase I clinical studies.

Chemists, biochemists, pharmacologists, toxicologists and histologists continue to evaluate the pharmacokinetic,
pharmacodynamic and toxicological properties
of the compound in vitro and in vivo (on animals).

The bioanalytic support for these studies is supplied by chemists, who, using chromatography, spectrometry, spectrophotometry and immunochemistry, develop the methodologies required to identify and quantify the molecule and its metabolites in various biological matrices.

Depending on the results obtained in the preceding experiments, chemists and pharmacists develop different dosages and pharmaceutical formulations, taking particular account of the physico-chemical and metabolic properties of the molecule and the biomedical characteristics of the targeted therapeutic application. For example, for pathologies affecting young children, it may be preferable to use a syrup, which is more easily swallowed than a tablet.