How do scientists measure whether a drug binds to a receptor?

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Gleevec; imatinib — drug Docking of the anticancer drug Gleevec (imatinib) in the abl domain of the bcr-abl tyrosine kinase. Abnormalities in bcr-abl stimulate the continuous proliferation of bone marrow stem cells, causing an increase in myelogenous cells (granulocytes and macrophages) in the body and leading to chronic myelogenous leukemia (CML).

Scientists measure the binding of a drug to a receptor using two primary approaches:

Direct Measurement: This involves the use of radioactively labeled drugs to track and quantify the formation of the drug-receptor complex.
Indirect Inference: Binding can be inferred by measuring the biological effects produced by agonists and antagonists.

These measurements demonstrate that the interaction generally follows the law of mass action, establishing a predictable relationship between the concentration of a drug and the amount of drug-receptor complex formed. While affinity defines a drug's tendency to bind to a receptor, the drug's ultimate effect on cell function is determined by its efficacy, or intrinsic activity.1