selectivity optimization

Selectivity optimization is the work of teaching a molecule to hit the one protein you want and ignore the close relatives you do not. Imagine a master key that opens dozens of similar locks: useful for a janitor, dangerous for a drug. Optimization files down the molecule until it fits its intended lock and jams in the others.

Most drug targets belong to families with very similar binding sites, such as the hundreds of kinases or the many subtypes of a receptor. A molecule that also blocks the cousins causes off-target effects and side effects. Chemists improve selectivity by exploiting the small structural differences between the target and its relatives, adding groups that reach a unique pocket or clash with the off-targets.

Selectivity is usually expressed as a ratio of potencies, for example a compound being 100-fold more potent on the target than on a related protein. Perfect selectivity is rarely needed or achievable; the goal is a window wide enough that, at therapeutic doses, off-target binding stays low. Sometimes deliberate multi-target activity is even desirable, so 'more selective' is not automatically 'better'.