I’ve Heard of Pharmacokinetics Being Used: What is It and Why Should I Care?

Pharmacokinetics is one of the more commonly used, and one of the more important, terms that you will see in journals and online. Learn more about it here. 

Pharmacokinetics

Table of Contents

What is Pharmacokinetics?

When you are researching substance uses, you may come across some sophisticated terminology. Most scientific terms are used on websites or in articles that are not common knowledge. One of the terms that you should be familiar with is pharmacokinetics. It is one of the more commonly used, and one of the more important, terms that you will see in journals and online.

Pharmacokinetics is a specific branch of pharmacology. It is focused on the study of the movement of drugs throughout the body. The term “body” in this definition refers specifically to the living organism that the drug is being administered to. It can mean human or animal bodies, but ultimately the study is concerned with the way drugs travel in and affect the human body.1   

Factors Affecting Pharmacokinetics

Several factors can affect the way that drugs are distributed throughout the body. They’re largely related to variables that are specific to patients. Some of the most commonly referred to factors that affect pharmacokinetics are:

  • The action of enzymes in metabolizing the substance: This factor refers to whether or not the drug is water-soluble or fat-soluble. Both of these play a large part in the body.
  • Genetic makeup: Certain genetics can cause issues in the interactions of drugs in the body.
  • Sex: Different hormones and bodily processes can affect the way drugs are absorbed.
  • Age: As we age, our bodies absorb and hold onto drugs for longer periods, affecting the drug’s half-life. Age plays a significant factor in the study of pharmacokinetics.  

Metrics of Pharmacokinetics

There are many different metrics used when discussing pharmacokinetics. Some of them are familiar, while others are a bit more scientific in nature. A few examples of common metrics are listed below:

  • Dose: The amount of the drug being administered.
  • Dosing interval: How often the drug is to be administered.
  • Bioavailability: The proportion of a drug or substance that enters the circulation when introduced to the body.
  • Concentration: The amount of a drug in a given volume.
  • Absorption half-life: The amount of time that it takes for the drug to be at half the amount it was at introduction.
  • Elimination rate constant: The amount of drug eliminated per a specified period. 

Why is Pharmacokinetics Important in Clinical Trials?

The knowledge of how a drug behaves informs the suitability of a drug for its target. If this information is not gathered, the drug being used may cause death or adverse effects. Pharmacokinetics provides accurate data for preclinical trials, which then inform clinical trials. It allows the professionals studying the drug to create suitable initial doses, and any possible side-effects can be managed accordingly.

When preparing for clinical trials, the most important pharmacokinetic principles are distribution, absorption, metabolism, and excretion. All of these areas of study inform the way that the drug is given and distributed, and how it can be expected to process and exit the body. 

Pharmacokinetics Change the Effects Drugs Have on the Brain

Pharmacokinetics studies the way that drugs move throughout the body. As such, pharmacokinetics can be applied to factors that change the effects that drugs have on the brain. Understanding how the drug is going to be absorbed in the body is important.1

If the body absorbs the drug before it reaches the brain, it will be less effective. Pharmacokinetics can inform how the drug should be absorbed (water-solubility vs. fat-solubility), thus affecting the brain. 

LADME

When looking into pharmacokinetics, you are likely to come across the acronym LADME. The acronym breaks down to the following terms:

  • Liberation: The way that the drug is released from its dosage form
  • Absorption: The movement of the drug from the site of administration to the blood
  • Distribution: The way that the drug is diffused in the body tissues
  • Metabolism: The chemical conversion of drugs into compounds that are easier to eliminate
  • Excretion: The way that the drug or converted product is removed from the body 

How Are Drugs Distributed Around the Body?

Drugs are absorbed into the body’s bloodstream. When they are in the bloodstream, they circulate throughout the body and are absorbed by the body’s tissues. Once they are absorbed, they do not spread evenly throughout the rest of the body. They target specific parts of the body. 

How Are Drugs Absorbed in the Body?

There are a number of ways that drugs are absorbed in the body. Most are absorbed through passive diffusion, however, meaning that they are taken orally, and make their way to the gastrointestinal system. At that point, they are dissolved and passed into the bloodstream. 

How Are Drugs Metabolized?

Drugs are metabolized in a number of ways in the body. They can be metabolized through oxidation, reduction, hydrolysis, hydration, conjugation, condensation, or isomerization. These processes break the drug down into a more manageable compound, making it easier to excrete from the body. Most of the enzymes that metabolize the drugs are located in the tissues of the body and are heavily concentrated in the liver. 

How Are Drugs Excreted From the Body?

Drugs are excreted from the body in several different ways. The method of excretion is dependent upon how the drug is metabolized. Water-soluble drugs are eliminated by the kidneys and are excreted via urine. Others are fat-soluble and are excreted via bile created by the liver. Other forms of excretion occur through sweat, saliva, and milk (in women who are lactating). 

Pharmacokinetics Models

Two models are used to determine a drug’s pharmacokinetics. They differ from one another and are used in different trials to inform different aspects of each drug. 

Noncompartmental Analysis

This form of analysis is model-independent, meaning that it does not rely on assumptions about body compartments. Noncompartmental methods are known for providing more consistency throughout testing. They are also less complex than compartmental methods and require less mathematical rigor. Because of this aspect, they are often faster and more cost-effective than compartmental models.

Compartmental Analysis

Compartmental methods take into consideration all different systems in the body. They are referred to as compartments. The compartmental method lends itself to the belief that all of the systems are interconnected. As such, the regression from system to system is taken into consideration.

This model requires more math, as well as more testing. These methods are highly complex and take a large amount of money to accomplish. Each test or method may take into account different variables, as each pharmacokinetic professional will have different assumptions.3  

What is the Relationship Between Single Dose Concentration and Time?

The relationship between the single-dose concentration of a drug and time is called the drug concentration-effect relationship. It describes the way that drug concentration in the body falls exponentially with time. The highest concentration is in the body just after the absorption of the drug. Then, the concentration falls exponentially for a specific period afterward. However, after the half-life has been reached, the excretion of the drug slows exponentially as well. This fact is, again, due to the half-life of the drug itself. See the example below.4

  • If a drug has a half-life of three hours at the dose it is administered for, it reduces by 50% in three hours.
  • The remaining 50% of the drug then reduces by half again, down to 25%, in another three hours.
  • This process continues until the drug is removed from the body entirely (>99%). 

What is the Relationship Between Repeated Dose Concentration and Time?

There is a superstition that taking repeated doses creates a larger buildup of the drug in the body, therefore taking longer to excrete. However, this belief is untrue. The drug still has the same half-life, meaning that it leaves the body at the same rate.

As such, adding a repeated dose does not increase concentration in the way that you would think. It can increase the time that it takes the drug to leave the body from the first dose, but the amount of time to be excreted from the repeated dose to no substance being left in the body is the same. 

Pharmacokinetics vs Pharmacodynamics

When you are researching pharmacokinetics, you’ll likely come across the term pharmacodynamics. The two terms are very different, but they are related.

Pharmacokinetics, as described before, is the study of the movement of drugs in the body. It focuses on the drug when it comes to the area being studied. It characterizes absorption, distribution, bioavailability, metabolism, and excretion in terms of time.5

Pharmacodynamics, on the other hand, is the study of the body’s biological response to drugs. It focuses on the biochemical and molecular responses triggered by the substance being introduced to the body.5

The two can be compared through the exposure-response relationship. It characterizes drug exposure, predicts dosage requirements, estimates rates of absorption and elimination. By analyzing pharmacokinetics and pharmacodynamics side by side, most information regarding the way a drug will affect a person can be predicted, which results in the information provided on most approved drug labels, as well. 

Key Takeaways

Overall, you should care about pharmacokinetics because they help to determine any key factors associated with drugs. Through testing and analysis, pharmacokinetics inform standard doses, dosing intervals, bioavailability, concentration, and excretion.

When used in conjunction with pharmacodynamics, all information about how the drug will affect the body, as well as how the body will react to the drug, can be determined. It is the standard for approving any medications in the pharmaceuticals industry. 

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