When drugs are administered to patients, there is no single characteristic relationship between the drug concentration in plasma and the measured effect; the concentration-effect curve may be concave upward, concave downward, linear, sigmoid, or an inverted-U shape. Moreover, the concentration-effect relationship may be distorted if the response being measured is a composite of several effects, such as the change in blood pressure produced by a combination of cardiac, vascular, and reflex effects. However, such a composite concentration-effect curve often can be resolved into simpler curves for each of its components. These simplified concentration-effect relationships, regardless of their exact shape, can be viewed as having four characteristic variables: potency, maximal efficacy, slope, and individual variation. These concepts are discussed in more detail in Chapter 1.
Potency can be viewed as the location of the concentration-effect curve along the concentration axis. Potency can be useful in the design of dosage forms, but more potent drugs are not always superior therapeutic agents. Maximal efficacy is the maximal effect that can be produced by a drug, as determined principally by the properties of the drug and its receptor-effector system and is reflected in the plateau of the concentration-effect curve. In clinical use, undesired effects may limit a drug's dosage such that its true maximal efficacy may not be achievable and clinical efficacy is then seen at a lower concentration. The slope of the concentration-effect curve reflects the mechanism of action of a drug, including the shape of the curve that describes drug binding to its receptor. The steepness of the curve dictates the range of doses that are useful for achieving a clinical effect (Figure 5-3).
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