Pharmacodynamics explores the relationship between drug concentration and its effect. In a quantal response drug, the duration of action better correlates with drug concentration, while for graded effect drugs, the intensity of response is more relevant. This intensity depends on the dose, drug removal rate, and the region of the concentration–response curve.
The concentration–response curve can be divided into three regions. Region 3 (80–100% maximum response) demonstrates that even as drug concentration rapidly falls, the intensity of response remains maximal and almost constant.
In Region 2 (20–80% maximum response), the relationship between the intensity of response and the logarithm of concentration is nearly linear, allowing for predictable changes in effect with small concentration changes. In this region, as drug concentration declines in a first-order process, the intensity of response decreases non-linearly over time, though some drugs may exhibit an approximately linear phase.. However, the drug concentration itself reduces logarithmically or exponentially.
Region 1 (0–20% maximum response) shows that the intensity of the effect is generally proportional to drug concentration (where C is much less than EC50), meaning the effect decreases exponentially over time, similar to the fall in concentration.
These relationships highlight the complex interplay between drug concentration and pharmacological effect, crucial for understanding drug efficacy and safety.
For most drugs, the response or effect intensity increases with the drug concentration.
When the IV-injected drug distributes rapidly, the plot of percentage effect versus log drug concentration yields a sigmoidal curve.
A plot of the plasma drug concentration, time, and percent maximum response depicts the fall in response intensity in three regions.
Region 3 marks 80–100% maximum response from the initial IV bolus. Despite a rapid concentration drop, the response remains constant.
Region 2 indicates a maximum response of 20–80%. Here, the intensity is proportional to the logarithm of the drug concentration. Substituting for log C and rearranging yields the effect intensity.
Intensity declines linearly with time, while concentration falls exponentially, following first-order kinetics.
Lastly, region 1 denotes 0–20% response with direct proportionality between the effect intensity and drug concentration. Here, drug effect intensity declines exponentially with time.
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