Drug selectivity describes how well a drug molecule is able to bind to its target. Highly selective drugs bind to no / very few other molecules in the organism, reducing the potential for adverse reactions.
Off-target binding can cause serious issues when developing a drug. If a drug interacts with vital receptors on cardiac tissue, the function of the heart can be disrupted – potentially with fatal consequences. Binding of a drug molecule to human serum albumin (HSA) can cause issues when other drugs compete for the same binding sites – therefore, clinicians must take into account any other drugs a patient may be taking when prescribing to reduce the risk of changing the concentration of free drug in circulation.
A low off-target Kd value indicates a drug with a high selectivity. This would be ideal, reducing the potential for adverse reactions from occurring.