1.1. Explain the process of drug pharmacokinetics

This guide will help you answer 1.1. Explain the process of drug pharmacokinetics.

Pharmacokinetics refers to how drugs are absorbed, distributed, metabolised, and excreted by the body. These processes determine the drug’s movement within the body and its effectiveness. It focuses on what the body does to the drug, rather than the effects the drug has on the body.

Pharmacokinetics is divided into four key stages:

1. Absorption
2. Distribution
3. Metabolism
4. Excretion

Each stage plays an important role in the overall process of how a drug reaches the target area, acts, and leaves the body.

Absorption

Absorption is the process where the drug enters the bloodstream after being administered. This stage depends on how the drug is taken (known as the route of administration). Common routes include:

• Oral (by mouth)
• Intravenous (injected into a vein)
• Subcutaneous (injected under the skin)
• Transdermal (through the skin)
• Inhalation (breathed in)

The chosen route affects how quickly the drug is absorbed. For example, drugs given intravenously (IV) go directly into the bloodstream, so they work almost immediately. Drugs taken orally, such as tablets, have to pass through the digestive system before they can enter the bloodstream, which slows down absorption.

Key Factors Affecting Absorption:

• Drug solubility: Drugs that dissolve easily in water or fat are absorbed faster.
• Surface area: The larger the surface area the drug passes through, the faster the absorption. For example, the intestines have a huge surface area for absorption.
• Blood flow: Good blood flow to the absorption site increases the speed of absorption.
• pH levels: The acidity or alkalinity of the site can either help or hinder drug absorption. For instance, the stomach’s acidic environment can break down some drugs.

After being absorbed, the drug enters the bloodstream and moves to the next stage: distribution.

Distribution

Distribution refers to how the drug travels throughout the body’s blood and tissues. Once a drug enters the bloodstream, it needs to reach the specific part of the body where it will have its effect. For example, a painkiller may need to target the brain, while an antibiotic might focus on a localised infection.

Factors Influencing Distribution:

• Blood flow: Drugs are delivered to tissues that have a good blood supply more quickly, such as the liver, heart, and kidneys. Areas with lower blood flow, like fat and bone, take longer.
• Binding to proteins: Some drugs attach to proteins in the blood, such as albumin. This prevents them from leaving the bloodstream and spreading to tissues, reducing their effect. Drugs that are not bound to proteins (referred to as “free drugs”) are active and can enter tissues to produce their effect.
• Barriers: Some body tissues, like the brain, have protective barriers. The blood-brain barrier is highly selective, allowing only certain substances to pass through. This can limit the distribution of drugs to these areas.

After the drug is distributed to the target site, the body starts metabolising it.

Metabolism

Metabolism (also called biotransformation) is the process where the body chemically changes the drug. This happens so the drug can be easier to excrete. The liver is the primary organ for drug metabolism, although the kidneys, lungs, and even the digestive system may also be involved.

In the liver, enzymes break down the drug into smaller compounds called metabolites. These can either be active (still affect the body) or inactive (no longer have an effect). Some drugs need to be metabolised before they can work—these are called “prodrugs.”

Factors That Influence Metabolism:

1. Age: Metabolism may be slower in infants and the elderly. This affects drug clearance from the body, potentially leading to higher levels in the blood.
2. Genetics: People can inherit different enzyme activity levels, affecting how fast or slow drugs are broken down.
3. Liver health: If the liver is damaged (e.g., by disease or alcohol use), it may not metabolise drugs properly, leading to drug accumulation.
4. Other medications: Some drugs speed up or slow down the activity of liver enzymes. This can change how quickly another drug is broken down, potentially resulting in interactions.

Once a drug is metabolised, the body needs to remove it. This brings us to the final stage.

Excretion

Excretion is the process of removing the drug and its metabolites from the body. The kidneys are the primary organ involved in this process, where waste is filtered out from the blood and passed out through urine. Other routes of excretion include:

• Bile (leaving the body in faeces)
• Lungs (e.g., exhalation of anaesthetic gases)
• Sweat
• Saliva
• Breast milk

Kidney function is a major factor in excretion. Poor kidney function can lead to drug accumulation in the body, increasing the risk of side effects or toxicity.

Key Points About Excretion:

• Drugs that are water-soluble are more easily excreted through urine.
• Fat-soluble drugs may need to be metabolised further in the liver into water-soluble substances before excretion.

Pharmacokinetics and Half-Life

A key concept in pharmacokinetics is the “half-life” of a drug. This refers to the time it takes for the concentration of the drug in the blood to reduce by half. The half-life determines:

• How often a drug needs to be taken
• How long it stays in the body

For example, if a drug has a half-life of 4 hours, its concentration will be halved every 4 hours after taken. A longer half-life might mean the drug only needs to be taken once a day, while a shorter half-life may require taking it more often.

Factors Affecting the Pharmacokinetics Process

Several elements influence how pharmacokinetics works:

1. Individual differences: Age, body weight, genetics, and organ function can all affect how a person absorbs, metabolises, or excretes drugs.
2. Drug formulation: Liquid medications absorb faster than tablets, and extended-release tablets are designed to release the drug slowly over time.
3. Interaction with food: Taking some medications with food can interfere with absorption, either reducing or delaying their effect.

The Role of Pharmacokinetics in Healthcare

Understanding pharmacokinetics helps healthcare professionals make informed decisions about drug prescribing. It ensures that patients receive the right dose, at the right time, with minimal risk of harm. For example:

• Dosing adjustments: If a person has impaired kidney or liver function, they may need a lower dose to prevent drug accumulation.
• Drug choice: Some drugs might not work well for a patient with genetic differences in metabolism enzymes, leading to the selection of an alternative.
• Timing: Dosage timing can be adjusted to maximise absorption or minimise side effects (e.g., taking certain antibiotics with food to avoid stomach irritation).

Pharmacokinetics also assists in predicting how different medications interact. This allows healthcare providers to avoid harmful drug combinations and improve patient safety.

Conclusion

Pharmacokinetics is the study of how drugs move through the body, from absorption to excretion. It provides the foundation for administering medicines safely and effectively. By understanding the four stages—absorption, distribution, metabolism, and excretion—healthcare workers can better predict how a drug will act on a patient. This knowledge helps in delivering personalised care and improving patient outcomes.