Pharmacokinetics: Understanding ADME and Half-Life

Understanding Pharmacokinetics: Absorption, Distribution, Metabolism, Excretion, and Half-Life

Pharmacokinetics (PK) is the science of how drugs move through the body, encompassing their absorption, distribution, metabolism, and excretion. This intricate process determines the drug's efficacy and safety, as well as its optimal dosing regimen.

1. Absorption

Absorption is the process by which a drug enters the bloodstream, typically occurring in the gastrointestinal tract (GI) for orally administered drugs. Factors that influence absorption include the drug's solubility, lipophilicity, and formulation (e.g., tablets, capsules, liquids). Absorption also depends on the drug's site and mechanism of administration, such as intravenous (IV) injection or transdermal patches.

2. Distribution

Distribution refers to the movement of a drug throughout the body after absorption. It is influenced by factors such as the drug's lipophilicity, protein binding, and the concentration gradient between the blood and tissues. The drug's distribution can impact its efficacy, as it determines the amount of drug that reaches target tissues and the concentration at which the drug is available.

3. Metabolism

Metabolism, or drug biotransformation, is the process by which the body breaks down drugs to form more polar, water-soluble compounds that can be excreted. Metabolism occurs primarily in the liver, but also takes place in other organs such as the kidneys and lungs. Enzyme systems like the cytochrome P450 (CYP) family and phase II conjugation enzymes are responsible for the majority of drug metabolism.

4. Excretion

Excretion is the process by which the body eliminates drugs and their metabolites. Renal excretion (via the kidneys) and biliary excretion (via the liver) are the primary routes of drug elimination in the body. Drug excretion is influenced by factors such as the drug's molecular structure, lipophilicity, and the presence of ionizable groups.

5. Half-life

Half-life (t½) is the time it takes for the concentration of a drug in the body to decrease by half. It is an important PK parameter that helps to predict a drug's duration of action, as well as its dosing frequency and regimen. Half-life is influenced by factors such as the drug's absorption, distribution, metabolism, and excretion rates.

Understanding these fundamental PK principles is essential for optimizing drug therapy, minimizing side effects, and improving patient outcomes. It also enables the development of new drugs with improved pharmacokinetic profiles, leading to more effective and safer treatments. "Pharmacokinetics: Absorption, Distribution, Metabolism, and Excretion," National Center for Biotechnology Information, PubChem, 2022, https://pubchem.ncbi.nlm.nih.gov/compound/Pharmacokinetics-Absorption-Distribution-Metabolism-and-Excretion. Accessed 15 February 2024.