Efficacy or Intrinsic Activity

In the preceding discussion, we assumed that receptor occupancy is the sole determinant of the size of the effect a drug would elicit. Thus, all drugs acting on a certain type of receptor should have a similar maximal effect, and only the dose required to achieve this would vary. This is true only when comparing certain drugs. Otherwise, one can easily notice that drugs binding to a common receptor may have very different maximal effects. Compare the effects of drugs in Fig. A.3: With some drugs that have a specific receptor-mediated effect, the maximal achievable effect remains much lower than with others. Therefore we have to add to the concept of affinity another basic feature of drugs: efficacy or intrinsic activity. Intrinsic activity is a measure of the biological effectiveness of a drug-receptor complex to elicit further cellular changes of physiological importance. For illustrative purposes, one can imagine efficacy to depend upon how closely the drug molecule and the receptor binding site fit together.

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