Selectivity versus specificity
Selectivity is how far a drug prefers one target (a receptor, enzyme, ion channel or transporter) over others, and that preference weakens as the dose rises.
Remember
Selectivity is a preference, not exclusive binding, so as the dose rises a selective drug begins to bind additional targets.
Memory aid
selective means prefers, whereas specific means only
Selective names a preference for one target, whereas specific names the strict sense of acting on one target alone.
A target is the receptor, enzyme, ion channel or transporter that a drug acts on, and no drug binds only one target. Selectivity is therefore a matter of degree, like a dimmer switch rather than an on-off switch. Specificity is the strict one-target-only term, so selective is not the same as specific.
A more selective drug needs a smaller dose to produce its therapeutic effect, and at that dose it has fewer off-target effects (effects from binding targets it was not meant to act on), but fewer does not mean none.
Atenolol is a beta-blocker, a drug that blocks beta receptors, the sites where adrenaline and noradrenaline act, and it is described as cardioselective. Its affinity (tendency to bind) is higher for beta-1 receptors, found mainly in the heart, than for beta-2 receptors, including those on the smooth muscle of the airways. At a usual dose it therefore blocks mainly cardiac receptors, so the heart rate falls with little bronchoconstriction (narrowing of the airways).
Selectivity is dose-dependent, which means that it is lost as the dose rises. With more drug present, beta-2 receptors in the airways are blocked as well, and the resulting bronchoconstriction can matter in a person with asthma, because beta-2 receptors normally relax the airway. This is why cardioselective beta-blockers still need care in asthma, and why people with asthma are advised to tell their doctor before taking atenolol.
Key facts
- Selectivity
- a degree of preference for one target, not exclusive binding.
- Specificity
- the strict one-target-only sense, which no drug reaches because no drug binds only one target.
- Dose effect
- higher doses reach the less-preferred targets, so selectivity is lost as the dose rises.
- Benefit
- a more selective drug needs a smaller dose to produce its therapeutic effect and, at that dose, has fewer off-target effects, but not none.
- Atenolol
- a cardioselective beta-blocker with a higher affinity for beta-1 receptors in the heart than for beta-2 receptors in the airways.
- Propranolol
- a non-selective beta-blocker, meaning it blocks both types of beta receptor, so it is more likely to cause bronchoconstriction.
Exam traps
- Being cardioselective does not make a beta-blocker specific for beta 1 receptors, because no drug binds only one target and beta 2 receptors in the airways begin to be blocked at higher doses.
- Selectivity and specificity are not identical terms: the words are sometimes used interchangeably, but selectivity is a matter of degree whereas specificity is the strict one-target-only term.
- Selectivity is not independent of the dose: a selective drug mainly occupies its preferred target at a low dose, and it begins to bind additional targets as the dose rises.
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