Pharmacodynamics is the study of biochemical, physiologic, and molecular effects of drugs on the body. It involves receptor binding, post-receptor effects, and chemical interactions. Pharmacodynamics helps to explain the relationship between the dose of a certain substance and the physical response it elicits.
Pharmacokinetics refers to the movement of a drug into, through, and out of the body through four stages; absorption, distribution, metabolism, and excretion.
The easiest way to explain the difference between these two closely related disciplines is that pharmacodynamics is what the drug does to the body, and pharmacokinetics is what the body does to the drug.
Pharmacokinetics is a great tool to help treat those suffering from a substance use disorder. The process can be divided into four simple stages as the substance works through the body.
Absorption is how a substance is absorbed through the stomach and intestines, while distribution refers to how the drug is distributed through bodily fluids and tissues. Metabolism is the process and amount that the liver and other organs can break down the substance, and excretion is how the drug leaves the body.
Absorption in pharmacology is the process of a drug moving from the delivery site into the bloodstream. The chemical composition of a drug, as well as the environment into which a drug is placed, work together to determine the rate and extent of drug absorption. Several factors can affect drug absorption. These factors will be detailed below.3
Several mechanisms of drug absorption have been identified, including passive, active, and facilitated diffusion. Passive diffusion is the most common mechanism of absorption. This process can be explained through the Fick law of diffusion, in which the drug molecules move from areas of higher concentration to a lower concentration until a balance is reached.
Active diffusion is an energy-consuming system essential for normal gastrointestinal absorption. In contrast to passive diffusion, active diffusion enables the movement of drugs from regions of lower concentration to higher concentration. The drug molecules and carrier molecules bind to form a complex which aids in the transportation and then breaks down on the other side.
Lastly, facilitated diffusion is another transporter system that appears to play a minor role in terms of drug absorption. It is similar to the active diffusion system, but it does not require energy and does not enable the movement against a concentration gradient.
Distribution in pharmacokinetics describes how a substance is spread throughout the body. This varies based on the biochemical properties of the drug as well as the physiology of the individual taking that medication. The goal of the distribution is to achieve what is known as the effective drug concentration. To be effective, a medication must reach its designated destination, at the correct amount, and not be protein-bound.[5 ]
This process is important because it can affect how much of a drug ends up in the active sites, and thus affects drug efficacy and toxicity. A drug will move from the absorption site to tissues around the body, such as brain tissue, fat, and muscle. Many factors could influence this, such as blood flow, lipophilicity, molecular size, and how the drug interacts with the components of blood, like plasma proteins.
Additionally, there are anatomical barriers found in certain organs like the blood-brain barrier, preventing certain drugs from going into brain tissue. Drugs with certain characteristics, like high lipophilicity, small size, and molecular weight will be better able to cross the blood-brain barrier.
Metabolism in pharmacokinetics describes the process by which the body breaks down the drug into compounds that are easier to eliminate. There are several factors that can impact drug metabolism, including genetics, age, and drug interactions.
The process of metabolism in pharmacokinetics can be divided into two phases:
Excretion is the final stage of pharmacokinetics. Elimination involves both the metabolism and the excretion of the drug through the kidneys, and to a much smaller degree, into the bile. Excretion into the urine through the kidneys is one of the most important mechanisms of drug removal.
Many factors can affect excretion. Direct renal dysfunction can prolong the half-life of certain drugs and make dose adjustments necessary.
The age of the patient can contribute to differing rates of excretion and impact dosing as well. Some patients will have pathologies that impact renal blood flow and make drug excretion less efficient. Examples of such disorders are congestive heart failure, liver disease, and pathologies affecting antidiuretic hormone release.
Pharmacokinetic studies are important tools to help doctors better combat addiction, especially in regard to withdrawal symptoms. People suffering from active substance use disorders may have multiple chemicals in their symptoms in combination with underlying health concerns. These variables can make it challenging to administer the proper dosage of anti-withdrawal drugs.