Ki is the equilibrium inhibitor dissociation constant, the concentration of the competing inhibitor that results in binding to half the enzymes sites at equilibrium in the absence of substrate or other competitors. The Cheng and Prusoff equation: Ki - (IC50) / ( 1+ ([L]/Kd)) relates IC50 to Ki under conditions of competitive binding; Kd equilibrium dissociation constant of the (labeled) ligand, [L] ligand concentration. The Cheng and Prusoff equation: Ki - (IC50) / ( 1+ ([S]/Km)) relates IC50 to Ki under conditions of competitive inhibition; Ki equilibrium enzyme inhibitor dissociation constant; Km Michaelis-Menton constant, [S] substrate concentration.