ADME is an abbreviation for Absorption, Distribution, Metabolism, and Excretion. It describes the disposition of a pharmaceutical compound within an organism. The ADME data is obtained from an ensemble of studies where the availability of the drug after administration is investigated. The criteria influence the drug levels and kinetics of drug exposure to the tissues and hence impact the performance and pharmacological activity of the compound as a drug. Because of its importance, ADME studies are usually conducted since the early stage of drug development.
Absorption is the movement of drug molecules from site of administration to the bloodstream.
Once the drug is administered, the pharmacokinetic phase or absorption begins. The route of administration, the solubility of the drug, and the presence of inflammation influence the rate of absorption. Intravenous administration is the fastest, while oral injection is the slower. Water soluble drugs are absorbed more quickly.
The compound needs to be carried to its effective site, most often via the bloodstream. From there, the compound may distribute into muscle and organs, usually to differing extents. After entry into the systemic circulation, either by intravascular injection or by absorption from any of the various extracellular sites, the drug is subjected to numerous distribution processes that tend to lower its plasma concentration.
Distribution is defined as the reversible transfer of a drug between one compartment to another. Some factors affecting drug distribution include regional blood flow rates, molecular size, polarity and binding to serum proteins, forming a complex. Distribution can be a serious problem at some natural barriers like the blood-brain barrier.
Compounds begin to break down as soon as they enter the body. The majority of small-molecule drug metabolism is carried out in the liver by redox enzymes, termed cytochrome P450 enzymes. As metabolism occurs, the initial (parent) compound is converted to new compounds called metabolites.
When metabolites are pharmacologically inert, metabolism deactivates the administered dose of parent drug and this usually reduces the effects on the body. Metabolites may also be pharmacologically active, sometimes more so than the parent drug.
Compounds and their metabolites need to be removed from the body via excretion, usually through the kidneys (urine) or in the feces. Unless excretion is complete, accumulation of foreign substances can adversely affect normal metabolism.
There are three main sites where drug excretion occurs. The kidney is the most important site and it is where products are excreted through urine. Biliary excretion or fecal excretion is the process that initiates in the liver and passes through to the gut until the products are finally excreted along with waste products or feces. The last main method of excretion is through the lungs.