A biological target is anything within a living organism to which some other entity (like an endogenous ligand or a drug) is directed and/or binds, resulting in a change in its behavior or function. Examples of common classes of biological targets are proteins and nucleic acids. The definition is context-dependent, and can refer to the biological target of a pharmacologically active drug compound, the receptor target of a hormone (like insulin), or some other target of an external stimulus. Biological targets are most commonly proteins such as enzymes, ion channels, and receptors.
Biological target identification and validation are among the most important steps in developing a new drug. Drug target validation involves proving that either DNA, RNA, or a protein molecule is directly involved in a disease process and can be a suitable target for development of a new therapeutic drug. To qualify as ‘druggable’, a target must be accessible to the proposed drug molecule, and a measurable biological reaction must be provoked as a consequence of the drug interacting with the target. This reaction may be measured both in vitro and in vivo.
Once the target has been identified, for example by genomic or proteomic investigations, the target is then validated in functional studies. The main strategies involved in target validation are gene knockout studies and direct inhibition of the target by small molecules, peptides, antibodies or any other class of inhibitors. The knockout approach genetically inhibits the target, thereby mimicking the action of the drug and allows a drug to be modeled before it is developed.