Pharmacodynamic principles and the time course of immediate drug effects

Abstract

This tutorial defines the principles of the concentration - effect relationship which are the basis of pharmacodynamics. The two key parameters of pharmacodynamics are the maximum response (Emax) and the concentration producing 50% of Emax (C₅₀). The time course of effect is illustrated under the assumption that drug effects are immediately related to concentration in the central compartment e.g. plasma. The related idea of duration of drug action and its relationship to dose is shown to have a simple relationship with drug half-life.

Keywords: C50; Emax; duration of drug action; emax model; time course of drug effect.

Figure 1. Clinical pharmacology is defined by the relaionship between dose, concentration and effect.

Figure 2. Concentration, effect and the Emax model.

Figure 3. Log Concentration and Effect.

Figure 4. Sigmoid Emax model curves with different values of the Hill exponent.

Figure 5. Time course of effect when initial concentration is 10 times the C₅₀.

Figure 6. Time course of effect when initial concentration is equal to the C₅₀.

Figure 7. Time course of effect when initial concentration is 100 times the C₅₀.

Figure 8. Schematic diagram of the 3 regions of the time course of effect.

Figure 9. Duration of response and half-life.