I was going through GPCR binding database. I was confused to see that a agonist can have abolished effect on E169A mutation for a ligand CGS21680 using kd determination experiment (paper PMID: 8609897) and for the same ligand and the same mutation the pEC50 is reduced by just 1.3-fold using pEC50 determination experiment (paper, DOI: 10.1124/mol.111.075937) . Could anybody explain to me what it means? If there is no binding how can we get ec50 values?