Pharmacokinetic–Pharmacodynamic Relationship: Intensity of Dose-Effect Relationship

When IV-injected drugs distribute rapidly, plotting percentage effect versus log drug concentration produces a sigmoidal curve because drug response intensity increases with concentration in a non-linear manner. This curve reflects how the relationship between intensity and logarithm of concentration becomes nearly linear in the middle range, while flattening at both low and high concentration extremes.

Region 3 represents 80–100% maximum response where the intensity of response remains constant and maximal despite rapid drug concentration decline. Even as plasma concentration drops significantly following first-order kinetics, the pharmacological effect persists at near-maximum levels, demonstrating a plateau effect at high drug concentrations.

In Region 2 (20–80% maximum response), the intensity of response is proportional to the logarithm of drug concentration, creating a nearly linear relationship. As drug concentration declines through first-order kinetics, response intensity decreases non-linearly over time, allowing predictable changes in effect with small concentration adjustments in this clinically relevant range.

Region 1 (0–20% maximum response) shows direct proportionality between effect intensity and drug concentration when concentration is much less than EC50. In this low-concentration range, drug effect intensity declines exponentially with time, mirroring the exponential fall in plasma concentration during first-order elimination.

Drug concentration falls exponentially following first-order kinetics, while effect intensity declines differently depending on the region. In Region 3, response remains constant despite concentration drop; in Region 2, intensity decreases non-linearly; and in Region 1, intensity declines exponentially, matching the concentration decline pattern.

For graded effect drugs, the intensity of response depends on dose, drug removal rate, and the region of the concentration–response curve. Since graded responses vary continuously with concentration, measuring intensity provides clinically meaningful information about drug efficacy, whereas duration of action is more relevant for quantal responses where effects are all-or-nothing.

The three regions highlight the complex interplay between drug concentration and pharmacological effect, crucial for understanding drug efficacy and safety. Region 1 shows minimal effects at low doses; Region 2 provides predictable dose-response relationships for therapeutic dosing; and Region 3 demonstrates maximum effect plateau, helping clinicians optimize dosing while minimizing toxicity risk.