Pharmacodynamics describes what the drug does to the body, and pharmacokinetics describes what the body does to the drug. Pharmacokinetics (PK) and pharmacodynamics (PD) are both components of in-vivo pharmacology. Pharmacokinetics evaluates absorption, distribution, metabolism, and excretion of drugs by the determination of systemic exposure over time. Pharmacodynamics studies the relationship between drug concentration at the site of action (receptor) and the observed pharmacological response. PK describes drug exposure, and PD describes drug effects. PK PD analysis provides the relationship between the drug’s in-vivo exposure and efficacy. PK PD studies in multiple species during drug discovery and development are often useful for predictions of initial dosing levels and regimens for safety and efficacy in the clinic.