When we talk about ADME, that means four processes taken place when the drug molecule was administered to humans or animals bodies. They are Absorption, Distribution, Metabolism, Excretion, and ADME is the acronym that has been used for decades to describe such studies.
In fact, the ADME affects the drug levels in plasma and tissues, as well as the kinetics of its exposure to the tissues, which influence the performance of the drug.
As we see this word, we can know it’s meaning literally. Absorption refers to a processes that a drug entering into the blood ( the reason why a drug must pass into the bloodstream, is that it aims to reach its target tissue in the body, unless there is a direct injection to the site of action).
Typically, before it can reach the target cells, a drug has to get through surfaces such as the digestive tract. However, absorption can be affected by many factors, such as blood flow, pain stress, intestinal transit time, chemical instability in the stomach, poor compound solubility, and inability to permeate the intestinal wall, etc. All of these can reduce the absorption effect of medicine when it is taken.
Take some medicine as a example. For Acidic drugs, such as asprin, they will be better absorbed in the stomach, while some basic drug, like morphine, they will be absorbed better in the intestine. Actually, most of the absorption of the drug takes place in the small intestine, because the surface area of the stomach is much smaller than that of the intestine.
When we talk about the absorption, we actually know about distribution. Usually, before a dug is being absorbed, it has to be transported to tissues and organs via the blood stream. Distribution is kind like how a drug moving throughout the body, and it is regarded as the drug transfer between one tissue or organ to another.
However, the drug distribution can also be affected by some factors, such as blood flow rates, polarity and binding to serum proteins, or molecular size,etc. Besides, when a drug meet some natural barriers like the blood–brain barrier, the distribution can be a serious problem.
Usually, a drug begins to be metabolized since it enters the body. This is because our bodies recognize those drugs and chemicals as foreign substances, and thus metabolism has to be proceed so that it can make these “foreign substances” more hydrophilic and successfully reach the next level—Excretion, otherwise those drugs and chemicals will be reabsorbed and remain in the body for prolonged periods.
In terms of the metabolism process, generally speaking, if we take the medicine orally, there will be enzymes in the GI tract and lumen which will naturally start the metabolism. In fact, most of the metabolism is taken place by the liver where the drug goes immediately after being absorbed through the GI.
Moreover, some enzymes are highly specific and will only breakdown “foreign substances” that they recognize, such as glucose dehydrogenase. While some other enzymes such as pepsin, they are not so specific and will breakdown most soluble proteins into smaller polypeptides or amino acids.
After distribution and metabolism, here comes the next level— Excretion. It means the “foreign substances” can finally get out of our bodies, which frequently occurs in the urine and feces, or through the lungs as gases. In addition, excretion can also be realized by some natural routes, such as tears, sweat, breath and saliva.
For patients who have kidney or liver problems, they have to be more cautious and careful about the medicine they take, because they can have a higher level of drug in the system compare to other normal people, and a high dose in the blood can lead to drug toxicity.
As we all know, the process to develop a new drug and bring it into market usually takes a long time, and is both a high cost and high risk endeavor. —it is estimated the drug discovery process can cost in the range of $1–2 billion dollars and typically takes 12 years to complete.
After knowing what is ADME, you can have a idea how important it is to know the rules of ADME, and what aspects should a drug developer should pay attention to, so as the drug can successfully pass though the four processes— Absorption, Distribution, Metabolism, and Excretion.
Obviously, this also has made the ADME and toxicology stages of drug discovery very important status, and it is quite necessary for the whole pharmaceutical industry to improve the ADME and toxicology studies.
As a matter of fact, advances in ADME studies have the potential to increase the chance of success, lower production costs and shorten the time to market. Thus, drug developers and researchers have devote themselves to know more about human ADME. In the future, more results will be achieved, we can definitely know more about the ADME.