Differences between pharmacokinetics and pharmacodynamics

Pharmacokinetics and pharmacodynamics are different concepts. Pharmacokinetics is the study of the path that the drug takes in the body since it is ingested until it is excreted and the pharmacodynamics is 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 is the passage of the medicine from the place where it is administered, to the bloodstream. 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 bloodstream, which may be in free form, or linked to plasma proteins, and can then reach several locations:

Place of therapeutic action, where it will exert the desired effect; Tissue reservoirs, where it will be accumulated without exerting a therapeutic effect; Place of unexpected action, where it will exert an unwanted action causing side effects; inactivated; places where they are excreted.

When a drug binds to plasma proteins, it cannot cross the barrier to reach the tissue and exert 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 can 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

The 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 body, 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.