- Agonists, partial agonists, and inverse agonists are drug ligands that interact with receptors to elicit different cellular responses. Agonists mimic the effects of endogenous ligands, partial agonists produce submaximal effects, and inverse agonists stabilize receptors in their inactive state. - The two-state receptor model describes receptors existing in two conformational states (active and inactive) that ligands differentially stabilize. Biased agonism occurs when ligands preferentially activate different intracellular signaling pathways. - Key concepts include efficacy, intrinsic activity, and constitutive receptor activity. Partial agonists have efficacy below full agonists and produce submaximal responses even at full receptor occupancy. Inverse agonists suppress constitutive receptor activity.