· Donepezil (Aricept®) is a potent acetylcholinesterase inhibitor that is used in treating Alzheimer's disease. The compound is also a potent sigma-1 receptor ligand with an affinity of 14.6 nM
Probably a decent idea to read the above link imho.
“Usually, the *lower* the Ki number, the more affinity; which is measured by the displacement of another (radiolabled for example) exogenous ligand.”
“For binding affinity people generally are referring to either the dissociation constant (the Kd value) or the association constant, Ka, which has no units. For your question why does it seem backwards, well Ka = 1/Kd. If units are given it is usually referring to Kd, the equilibrium dissociation constant (equilibrium referring to the binding and unbinding reactions).
Ki, or the inhibition constant, has to do with competitive binding assays and relates the ligand with the amount of competitor that displaces half of it (the IC50). “
“The affinity is measured by the concentration required to get 50% binding. If it is 5 nM for one compared to 50 uM for another, then it takes 10,000 times more ligand to get the same binding for the 50 uM compound as the 5 nM compound. However, the binding tells you nothing about the efficacy. The strongly bound compound could be a partial agonist and the weakly binding compound could be a full agonist, or vice versa. So, the 5 nM compound is stronger on a weight basis, but may not have stronger effects at equivalent doses.”
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