Content
Pharmacokinetics and pharmacodynamics are different concepts, which are related to the action of drugs on the organism and vice versa.
Pharmacokinetics is the study of the path that the drug takes in the body since it is ingested until it is excreted, while the pharmacodynamics consists of the study of the interaction of this drug with the binding site, which will occur during this path.
Pharmacokinetics
Pharmacokinetics consists of studying the path the drug will take from the moment it is administered until it is eliminated, going through absorption, distribution, metabolism and excretion processes. In this way, the medicine will find a connection site.
1. Absorption
Absorption consists of the passage of the medicine from the place where it is administered, to the blood circulation. Administration can be done enterally, which means that the medicine is ingested through oral, sublingual or rectally, or parenterally, which means that the medicine is administered intravenously, subcutaneously, intradermally or intramuscularly.
2. Distribution
The distribution consists of the path that the drug takes after crossing the barrier of the intestinal epithelium into the blood stream, which can be in free form, or linked to plasma proteins, and can then reach several locations:
- Place of therapeutic action, where it will exert the intended effect;
- Tissue reservoirs, where it will be accumulated without exerting a therapeutic effect;
- Place of unexpected action, where you will exert an unwanted action, causing side effects;
- Place where they are metabolized, which may increase their action or be inactivated;
- Places where they are excreted.
When a drug binds to plasma proteins, it cannot cross the barrier to reach the tissue and exert a therapeutic action, so a drug that has a high affinity for these proteins will have less distribution and metabolism. However, the time spent in the body will be longer, because the active substance takes longer to reach the site of action and to be eliminated.
3. Metabolism
Metabolism occurs largely in the liver, and the following may happen:
- Inactivate a substance, which is the most common;
- Facilitate excretion, forming more polar and more water-soluble metabolites in order to be eliminated more easily;
- Activate originally inactive compounds, changing their pharmacokinetic profile and forming active metabolites.
Drug metabolism can also occur less frequently in the lungs, kidneys and adrenal glands.
4. Excretion
Excretion consists of the elimination of the compound through various structures, mainly in the kidney, in which the elimination is done through the urine. In addition, metabolites can also be eliminated through other structures such as the intestine, through feces, the lung if they are volatile, and the skin through sweat, breast milk or tears.
Several factors can interfere with pharmacokinetics such as age, sex, body weight, diseases and dysfunction of certain organs or habits such as smoking and drinking alcohol, for example.
Pharmacodynamics
Pharmacodynamics consists of studying the interaction of drugs with their receptors, where they exercise their mechanism of action, producing a therapeutic effect.
1. Place of action
The action sites are the places where the endogenous substances, which are substances produced by the organism, or exogenous, which is the case of drugs, interact to produce a pharmacological response. The main targets for the action of active substances are the receptors where it is customary to bind endogenous substances, ion channels, transporters, enzymes and structural proteins.
2. Mechanism of action
The mechanism of action is the chemical interaction that a given active substance has with the receptor, producing a therapeutic response.
3. Therapeutic effect
The therapeutic effect is the beneficial and desired effect that the drug has on the body when administered.