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.[1]
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.[2]
Understanding Pharmacokinetics
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.
Drug Absorption Stage
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
Formulation of a drug: The majority of absorption happens in the small intestines. Acidic substances are absorbed in the acidic areas, and basic substances will be better absorbed in more alkaline areas.
Drug and food interactions: These interactions happen because the presence of food or other medications in the stomach may impact drug absorption. Sometimes it enhances the rate of absorption and sometimes it mixes to form insoluble complexes that cannot be absorbed.
Route of administration: This can have an impact on the speed at which the substance is absorbed by the body.
The Mechanism of Drug Absorption
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
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.
Facilitated Diffusion
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.[4]
Drug Distribution Stage
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 ]
Importance of Distribution
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.[6]
Drug Metabolism Stages
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.[7]
Genetics: Genetics can impact the speed at which someone metabolizes drugs.
Age: Age can impact liver function. Elderly people often have reduced liver function and may metabolize drugs more slowly, increasing the risk of intolerability, and newborns or infants have immature liver function and may require special dosing considerations.
Drug Interactions: These can also decrease the speed of metabolism by enzyme inhibition or increase the speed by enzyme induction.
Phrases of Pharmacokinetics
The process of metabolism in pharmacokinetics can be divided into two phases:[8]
Phase 1: Phase 1 involves chemical reactions in the body such as oxidation, reduction, or hydrolysis to inactivate the drug. This results in the drug becoming completely inactive or partially inactive.
Phase 2: Phase 2 involves reactions that chemically change the drug into compounds that are soluble enough to be easily excreted. Some drugs only undergo one phase or the other, but most go through the first phase and then move on to the second.
Drug Excretion Stage
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.
Attributing Factors
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.
Importance of Pharmacokinetics
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.
The team of professionals at Genesis Recovery is well equipped to overcome these challenges and help you receive the care you deserve. Get help now and start recovery today.
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