Orthosteric/Allosteric Bitopic Ligands

A bitopic, orthosteric/allosteric, model of GPCR ligand action predicts novel pharmacological properties. (A) A bitopic scheme and associated equation describing fractional equilibrium occupancy (ρ_A) of orthosteric probe (circle) in the presence of bitopic ligand [square (allosteric pharmacophore) linked to circle (orthosteric pharmacophore)]. The curved arrows denote the potential for allosteric modulation. Also shown is the potential for differential signaling (S) between an orthosteric and a bitopic ligand. (B) Simulations based on the bitopic model with the following parameters: [A] = K_A =1 nM; K_B-ortho = 3 nM; K_B-allo = 30 nM; α = 4. (C) Experimental proof of concept of a bitopic binding phenomenon, as determined at the M₂ muscarinic acetylcholine receptor. The indicated ligands were each tested against the equilibrium binding of the orthosteric probe NMS. Data replotted from (41).