Pharmacodynamics (PD)

Pharmacodynamics (PD) designates the study of biochemical, physiological, and molecular effects drugs produce in the body and mechanisms through which effects occur, encompassing drug-receptor interactions, dose-response relationships, time course of effects, and therapeutic and toxic actions, fundamentally addressing what drugs do to the body. This essential pharmacology discipline provides mechanistic understanding underlying rational drug use, dose selection, and combination therapy design.

Fundamental concepts include receptor theory describing drug-receptor binding producing conformational changes transducing signals, dose-response relationships characterised by parameters including EC50 and Emax, drug selectivity referring to preferential actions on specific targets, and therapeutic index representing the safety margin. Agonists produce effects by activating receptors, antagonists block receptor activation, and inverse agonists reduce constitutive receptor activity below baseline. Pharmacodynamic biomarkers prove valuable throughout development, with target engagement biomarkers confirming drugs reach targets, pharmacodynamic biomarkers measuring pathway modulation, and clinical efficacy biomarkers predicting therapeutic outcomes. Pharmacokinetic-pharmacodynamic modelling mathematically relates drug exposure to effects. As biomarker technologies advance enabling precise pharmacodynamic measurements and modelling approaches integrate systems pharmacology, pharmacodynamic knowledge continues guiding rational drug development.