selectivity

Selectivity is a drug's good aim. Imagine a key cut so precisely that it opens your front door but jams in every neighbour's lock. A selective drug acts strongly on the one protein it is meant to hit and leaves closely related proteins largely untouched. That fussiness is usually what separates a clean medicine from one riddled with side effects.

It is measured as a ratio: how much more potently the drug acts on its intended target versus other targets. A compound with a 1000-fold selectivity affects the off-targets only at a thousand times higher concentration than it affects the real one, so over the useful dose range the off-targets stay quiet. Selectivity is often hardest to achieve when the intended and unintended proteins are structurally similar — different members of the same kinase family, or several subtypes of one receptor.

Perfect selectivity is rare and not always even desirable. Some valuable drugs deliberately hit several targets (so-called multi-target or 'dirty' drugs), and some toxicity comes not from related proteins but from completely unrelated ones the molecule happens to touch. So selectivity is best thought of as a property to be tuned toward a therapeutic goal, not maximized for its own sake. A drug only needs to be selective enough that, across the doses people actually take, the intended action dominates.