PK/PD modeling
PK/PD modeling builds a mathematical bridge between two stories: what the body does to the drug over time (pharmacokinetics — how concentration rises and falls) and what the drug does to the body (pharmacodynamics — the size of its biological effect). Stitching them together lets you predict not just how much drug is in the blood at a given hour, but how strong the effect will be at that moment.
The pharmacokinetic half describes the concentration-time curve from dose, clearance and volume of distribution. The pharmacodynamic half links concentration to effect, often through a saturating curve where effect rises with concentration up to a maximum. Joining them turns a dosing schedule into a prediction of the effect-versus-time profile — sometimes with a delay, since effect can lag behind concentration.
PK/PD modeling is a workhorse of modern drug development. It helps choose a first human dose, design dosing regimens, explain why two drugs with the same blood levels differ in effect, and reason about whether efficacy tracks peak concentration, total exposure, or time above a threshold. Done well, it replaces guesswork with quantitative, testable predictions.
For many antibiotics, PK/PD modeling shows efficacy depends on the time the concentration stays above the bacteria's minimum inhibitory concentration, guiding whether to dose frequently or use a sustained level.
Modeling reveals which PK feature drives the effect.
Effect often lags concentration: a drug's peak blood level and its peak effect need not coincide, because the response may build through downstream biology after the molecule binds. Good models capture this delay.