Selectivity
The ability of a drug to preferentially bind to one receptor subtype or target over others. High selectivity reduces off-target effects and improves safety. Difelikefalin's selectivity for kappa over mu opioid receptors, for example, avoids addiction potential while providing antipruritic effects.
Technical Context
Selectivity is quantified as the ratio of binding affinities or functional potencies at the target receptor versus off-target receptors. Difelikefalin has >500-fold selectivity for KOR over MOR (Ki ratio). Setmelanotide is approximately 20-fold selective for MC4R over MC3R. Highly selective peptides tend to have cleaner side effect profiles. Selectivity can be engineered through amino acid substitutions, conformational constraint, and systematic structure-activity relationship studies. For receptor families with closely related subtypes (like the 5 melanocortin receptors or 5 somatostatin receptors), achieving selectivity is a significant design challenge — pasireotide's broader SSTR binding profile compared to octreotide illustrates how different selectivity profiles lead to different clinical applications.