Drug Metabolism

Drugs cause physiological, behavioural, or psychological effects when they enter an organism. Medicines are drugs prescribed by doctors as treatments for diseases or conditions such as fever, pain relief in some surgical procedures or anaesthesia before surgery. These drugs have their own set of pharmacological actions within the body. But what is drug metabolism? Drug metabolism is the biotransformation process of turning a substance into a metabolite till they are excreted from the body.

About Drug Metabolism

A drug is a chemical compound or mixture of compounds that produces some effect in the recipient’s body. It affects the functions of that recipient and is known as a drug metabolite. 

It would be helpful to understand how enzymes work to know how drug metabolism works. Enzymes are proteins that speed up chemical reactions in the body. They are responsible for many essential processes within the human body, including biosynthesis, catabolism and respiration. Drugs alter enzyme activity inside the body.

Drugs, especially those with a powerful effect on the body, break down into several active compounds, known as metabolites. The breakdown and removal of these pharmaceutical substances in the body are called drug metabolism.

Although drugs have direct actions on the body, in most instances, drugs are eliminated from the body by being broken down by enzymes into many other substances also used by the body.

These substances, known as metabolites or breakdown products, are also excreted from the body. They serve a different function within it than that of the original drug. Metabolites exert other effects within the body than those of their parent drug.

Most drugs are metabolised in the liver. But depending on their chemical structure and the metabolic reactions they undergo in the liver, they can be excreted totally or partially in urine. The metabolite function within an organism is determined by how much or how little it differs from its parent compound in its chemical structure. 

Various reactions occur during the metabolism of drugs, including:

• Oxidation: Oxidation is the addition and removal of oxygen to a functional group on a drug molecule. 
• Hydrolysis: Hydrolysis is the reaction that results in a water molecule being added to a drug molecule.
• Reduction: Reduction is the removal of hydrogen from a functional group on a drug molecule.
• Condensation: Condensation is the reaction where two or more drug molecules combine to form a molecular compound. 
• Conjugation: Conjugation is the reaction where a drug becomes attached to a lipid molecule and is excreted in urine as an organic compound. 
• Isomerization: Isomerization is the reaction that changes a drug molecule into its optical isomer, which is either the same or different. 

Phases of Drug Metabolism

Phase 1: During Phase 1 of drug metabolism, the drug’s chemical structure goes through modifications. It is usually by reduction, oxidation, cyclization/decyclization, hydrolysis, and the addition of oxygen or the removal of hydrogen. An inactive prodrug can be converted into a metabolically active drug through this phase. Oxidation usually produces metabolites with some pharmacological activity still intact.

Phase 2: A drug molecule is conjugated with another molecule during Phase II modifications. As a result of conjugation, the compound becomes pharmacologically inert and water-soluble, making it more readily excreted. Conjugation mechanisms include acetylation, methylation, glucuronidation, sulphation, and glutathione or glycine conjugation. Organ systems such as the liver, kidney, lungs, and intestines are where these processes take place.

Drug Metabolism Rate

The drug metabolism rate is the speed at which a drug metabolises after administration. This can vary from person to person for a given drug due to genetic factors, lifestyle, and medical history. Phase 1 is usually the slowest phase of drug metabolism, measured in hours or days. In contrast, Phase 2 is faster (measured in minutes or seconds). 

The liver is the main organ responsible for drug metabolism. Still, other organs such as the gut and lungs can also metabolise drugs. The rate at which these organs metabolise drugs is dependent on many factors:

• Some drugs are metabolised faster than others. Thereby, some can be taken more frequently than others without affecting the effectiveness of drug administration. 
• The length of time it takes to clear a pharmaceutical drug from the body is its half-life.
• Drugs with a short half-life have a rapid elimination process. In contrast, those with long half-lives can be taken less frequently because they have longer elimination processes.

Conclusion

Drug metabolism is a very complex process. This is because drugs are not only metabolised in the body but also by the body. Drugs can be metabolised into different metabolites in various organs of the body. These metabolic processes mainly occur in the liver, where enzymes are found. The main aim of drug metabolism is to change the structure of the drug to promote easy excretion.