Study of the physicochemical characteristics
Initially, a drug was a complex mixture, of animal or vegetable origin, badly defined and containing a very great number of compounds. Later a drug was a purified compound obtained by extraction either of vegetable origin, like digitalin, or of animal origin, like insulin.
Today, in about all cases, a drug is a well defined molecule obtained by synthesis according to the usual processes of chemistry, or by recombinant DNA technique, which is particularly adapted to the polypeptide molecules.
The chemists synthesized an innumerable quantity of molecules whose pharmacologic properties were systematically analyzed by screening or sometimes found fortuitously. When a new active molecule was identified, the chemists “copied” this molecule by modifying its structure in order to find other molecules more active or better tolerated. In general, these copies give interesting results but not a frank innovation.
Currently, the synthesis directed according to the target to reach is possible because of a better knowledge of the mechanisms of actions at molecular level.
The physicochemical characteristics of the molecule must be perfectly defined: developed chemical formula specifying, if necessary, the existence of an isomerism, solubility in water or other solvents, constants of dissociation, stability for determinating the conditions of preservation (temperature, light). The knowledge of the light absorption and the mass spectra of the molecule is useful for its identification and its determination.
The molecule, called active principle, can be marketed under different presentations, solid forms (tablets, capsules) or fluid forms (injectable or drinkable vials). A tablet, for example, contains the active ingredient and generally an excipient.