Define pharmacokinetics and pharmacodynamics

Pharmacokinetics versus Pharmacodynamics

Pharmacology is the study of the interactions between drugs and the body. The two broad divisions of pharmacology are pharmacokinetics and pharmacodynamics. Pharmacokinetics (PK) refers to the movement of drugs through the body, whereas pharmacodynamics (PD) refers to the body’s biological response to drugs.

PK describes a drug’s exposure by characterizing absorption, distribution, bioavailability, metabolism, and excretion as a function of time. PD describes drug response in terms of biochemical or molecular interactions.

Understanding the exposure-response relationship (PK/PD) is key to the development and approval of every drug. PK and PD data contribute to much of what is on a drug package insert. Strategic planning of the overall program for a drug and intelligent pharmacokinetic study design can speed the development process.

PK and PD Analyses are used to:

• Characterize drug exposure: With the exception of drugs delivered intravenously, only a fraction of a drug’s dose is absorbed and pharmacologically active. Quantifying the rate and magnitude of exposure to a drug is critical for determining how best to guide its use in the clinic.
• Predict dosage requirements: PK/PD modeling can help predict dosing requirements early in the development process, making the first dose-range finding studies more informative and consequential.
• Assess changes in dosage requirements: Predicting the biological effect of small dosing changes is important early in the development process, when alterations and formulation changes are common.
• Estimate rate of elimination and rate of absorption: Knowing how quickly a drug is absorbed and eliminated can help make decisions regarding formulation design and dosing regimens.
• Assess relative bioavailability / bioequivalence: Comparing the extent of a new formulation’s absorption to an existing formulation can often help demonstrate therapeutic advantages.
• Characterize intra- and inter-subject variability: High variability can quickly derail clinical development programs. Understanding how a drug’s PK and PD change within and between individuals can help design clinical trials in ways that reduce variability and make the results more robust.
• Understand concentration-effect relationships: The concentration-effect relationship is the cornerstone of pharmacodynamics. Identifying the variables that affect the relationship is critical for a successful development program.
• Establish safety margins and efficacy characteristics: Successful drugs have clearly defined therapeutic windows. PK/PD modeling can help determine dosing thresholds. Sola dosis facit venenum… “The dose makes the poison.”