Pharmacodynamic Models: Linear Concentration–Effect Model

The linear concentration-effect model assumes that pharmacological effect (E) is directly proportional to plasma drug concentration (Cp). This model simplifies the Emax model when drug concentrations are significantly below the EC50 threshold. The concentration-effect curve appears approximately linear in this range, with drug effect increasing continuously until reaching maximum pharmacological effect (Emax).

The linear concentration-effect model applies when plasma drug concentrations are markedly below the EC50 threshold, where the relationship between concentration and effect remains approximately linear. However, this linearity does not hold across wide concentration ranges, limiting the model's applicability to comprehensive pharmacodynamic modeling. It is most useful for evaluating specific drug effects within restricted concentration windows.

The linear concentration-effect model is widely used to evaluate drug effects on cardiac repolarization, measured by the QT interval on an electrocardiogram (ECG). For example, the plot between moxifloxacin concentration and QTc interval prolongation yields a linear curve. This concentration-QTc relationship plays a key role in the FDA's regulatory evaluation of new drugs for assessing proarrhythmic risk.

The linear concentration-effect model assumes continuous effect augmentation with increasing drug concentrations, but this assumption does not hold across extensive concentration ranges. The model's linearity is restricted to conditions where concentrations remain significantly below EC50. These inherent limitations confine its applicability to specific pharmacodynamic scenarios rather than comprehensive drug effect modeling.

The linear concentration-effect model emerges as a pivotal simplification of the Emax model for conditions where plasma drug concentration is significantly less than EC50. Under these restricted conditions, the nonlinear Emax relationship approximates a linear trajectory. This simplification allows for straightforward analysis while maintaining accuracy within the applicable concentration range.

The concentration-QTc relationship facilitates the US FDA's regulatory review processes, particularly in assessing proarrhythmic risks associated with novel therapeutic agents. This analysis has been extensively applied in characterizing moxifloxacin and developing new pharmacological entities. The linear model's utility in this context underscores its significance in advancing drug safety evaluations during regulatory approval.

EC50 represents the plasma drug concentration at which 50% of the maximum pharmacological effect is achieved. The linear concentration-effect model applies when actual drug concentrations remain significantly below this EC50 threshold. Above EC50, the linear relationship breaks down and the model's assumptions no longer hold, requiring alternative pharmacodynamic models for accurate predictions.