Pharmacokinetics is the branch of pharmacology that explains what the body does to a drug. It describes how the drug is absorbed, distributed, metabolized, and eliminated—together known as the ADME processes. Understanding pharmacokinetics helps predict how long a drug stays in the body, how often it should be given, and whether dose adjustments are needed in special populations.
What Is Pharmacokinetics?
Pharmacokinetics studies the time course of a drug in the body. In simple terms, it explains:
- How fast the drug enters the blood
- Where it goes after absorption
- How the body breaks it down
- How quickly it is removed
This information helps design safe and effective dosing regimens.
The ADME System
1. Absorption
The movement of a drug from its site of administration into the bloodstream. Factors such as solubility, dose form, GI pH, and membrane permeability affect absorption rate and extent.
2. Distribution
Once absorbed, the drug spreads into tissues like muscles, fat, liver, kidneys, and brain. Distribution depends on blood flow, protein binding, and lipid solubility.
3. Metabolism
Drug metabolism (mostly in the liver) converts drugs into more water-soluble forms for easier elimination. Phase I and Phase II reactions play important roles.
4. Excretion
Removal of drugs from the body, mainly through the kidneys (urine) or liver (bile). Lungs, sweat, and breast milk also contribute.
Plasma Drug Concentration–Time Curve
This curve visually represents how drug levels in the blood change over time after administration. Key parameters include:
1. Cmax
The highest (peak) drug concentration in plasma.
2. Tmax
The time taken to reach peak concentration. Indicates absorption rate.
3. AUC (Area Under the Curve)
Represents the total drug exposure over time and is used to measure bioavailability.
4. Onset, Duration, and Intensity
- Onset: how quickly the drug starts working
- Duration: how long the effect lasts
- Intensity: how strong the effect is
Half-Life (t1/2)
Half-life is the time required for the plasma drug concentration to reduce by 50%. It helps determine how frequently the drug should be administered.
Drugs with long half-lives require less frequent dosing, while drugs with short half-lives require more frequent dosing.
Clearance (Cl)
Clearance represents the efficiency with which the body eliminates the drug. It is the volume of plasma cleared of drug per unit time.
Higher clearance → drug is removed quickly. Lower clearance → drug stays in the body longer.
Steady State
Steady state occurs when the amount of drug administered equals the amount eliminated. It usually takes about 4–5 half-lives to reach steady state, regardless of the dose.
Importance of Steady State
- Helps maintain consistent therapeutic levels
- Prevents peaks (toxicity) and troughs (ineffective levels)
Bioavailability
Bioavailability is the fraction of the drug that reaches systemic circulation unchanged. It is influenced by absorption and first-pass metabolism.
Factors Affecting Pharmacokinetics
- Age (children and elderly require dose adjustments)
- Body weight and composition
- Genetics (enzyme variations)
- Liver and kidney diseases
- Drug interactions (inducers or inhibitors of enzymes)
Clinical Applications of Pharmacokinetics
- Designing dosing regimens
- Adjusting doses in organ impairment
- Managing drug toxicity
- Understanding drug–drug interactions
- Predicting therapeutic response
Detailed Notes:
For PDF style full-color notes, open the complete study material below:
PATH: PHARMD/ PHARMD NOTES/ PHARMD FOURTH YEAR NOTES/ BIOPHARMACEUTICS AND PHARMACOKINETICS/ INTRODUCTION TO PHARMACOKINETICS..