Pharmacodynamics examines the relationship between drug exposure and biological response, characterizing what a drug does to the body rather than what the body does to the drug. This discipline encompasses the drug's mechanism of action at the molecular level, its effects on cellular and organ function, and the relationship between concentration and response. While pharmacokinetics asks how much drug reaches the site of action, pharmacodynamics asks what happens when it gets there.

Pharmacodynamic assessments employ biomarkers to measure drug effects at various biological levels. Molecular biomarkers might include receptor occupancy, enzyme inhibition, or gene expression changes. Cellular biomarkers could measure changes in cell signaling, proliferation, or death. Physiological biomarkers assess functional effects such as blood pressure reduction, glycemic control, or tumor shrinkage. These measurements establish proof of mechanism, demonstrating that the drug engages its intended target, and proof of concept, showing that target engagement produces the desired biological effect.

The integration of pharmacokinetics and pharmacodynamics through PK/PD modeling provides powerful tools for dose optimization. By mathematically relating drug exposure to biological response, these models can predict the effects of different dosing regimens, identify the exposure levels associated with efficacy and toxicity, and support dose selection for subsequent trials. This quantitative approach to dose-response characterization has become increasingly important in modern drug development, enabling more efficient clinical programs and more rational dose selection than traditional empirical approaches.