Definition of Structure–Activity Relationships
Once a lead compound has been identified, a series of analogs is synthesized and tested to determine how structural variations affect the pharmacological activity, this is known as structure–activity relationships. In vitro tests can be carried out to test the affinity of a range of analogs for a target binding site and will indicate the groups that are important for binding. If a group is modified and the binding affinity drops, then this indicates that the original group is important for binding. In vitro tests that measure a cellular or pharmacological effect are more relevant since the results obtained indicate the important groups required, not only for target binding but also for efficacy. However, it cannot be assumed that the structural features identified as important for activity in vitro will also be important for activity in vivo. In vivo tests will demonstrate whether structural variations are important to physiological activity in a test animal or human.