New receptor protein must be synthe- sized if agonist receptor complexing is to occur. As with ago- nist drugs, not all receptors are bound by antagonists. Antagonists cause a rightward shift in the drug dose-response curve. The extent of rightward shift reflects the number of available receptors occu- pied by the antagonist drug (Figure 5-9). Comparison of the ED 50 in Figure 5-9 shows a reduced affinity of the agonist for its receptor when the antagonist is present.