5.1
Drug Biotransformation: Overview
Biotransformation, also known as drug metabolism, is a vital physiological process that chemically alters drugs, facilitating their elimination from t…
Drug biotransformation or metabolism entails chemical alteration of xenobiotics, either activating or deactivating their pharmacological properties.
Metabolism converts lipophilic drugs into polar, water-soluble products, aiding excretion. For instance, the conversion of codeine to morphine.
The liver is the primary site for drug metabolism, but the lungs, kidneys, intestines, placenta, and skin also contribute.
Most drugs are metabolized in two sequential phases. Phase I reactions introduce or unmask polar functional groups through oxidative, reductive, and hydrolytic reactions, preparing the drug for phase II reactions.
In phase II, products from phase I reactions conjugate with small polar molecules like glucuronic acid, sulfate, glycine, and glutathione, forming highly water-soluble metabolites.
Microsomal enzymes found in the liver's endoplasmic reticulum, along with non-microsomal enzymes in the cytoplasm and mitochondria, catalyze most drug biotransformation reactions.
Drug's physicochemical properties, biological variations, and chemical factors influence drug metabolism, where drugs form reactive metabolites, leading to toxicity.
View the full transcript and gain access to JoVE Core videos
Q1: What is drug biotransformation and why is it important?
Drug biotransformation, also called drug metabolism, chemically alters drugs to facilitate their elimination from the body and terminate their action. This process converts lipophilic drugs into polar, water-soluble products that are readily excreted via urine or bile. Biotransformation detoxifies xenobiotics, activates prodrugs, and prevents drug accumulation, reducing toxicity risk and ensuring patient safety.
Q2: How do phase I and phase II reactions differ in drug metabolism?
Phase I reactions introduce or unmask polar functional groups through oxidation, reduction, and hydrolysis, increasing water solubility. Phase II reactions further enhance solubility by conjugating phase I products with small polar molecules like glucuronic acid, sulfate, glycine, and glutathione, forming highly water-soluble metabolites ready for excretion.
Q3: Which organs are involved in drug biotransformation?
The liver is the primary site for drug metabolism, containing abundant drug-metabolizing enzymes. However, the lungs, kidneys, intestines, placenta, and skin also contribute to biotransformation. Microsomal enzymes in the liver's endoplasmic reticulum catalyze most reactions, while non-microsomal enzymes in the cytoplasm and mitochondria play subsidiary roles.
Q4: What types of phase II conjugation reactions occur during drug metabolism?
Phase II conjugation reactions include glucuronidation, sulfation, acetylation, methylation, and glutathione conjugation. These reactions attach small polar molecules to drugs or their phase I metabolites, dramatically increasing water solubility. Each conjugation type targets specific functional groups, ensuring efficient conversion of diverse drug structures into excretable metabolites.
Q5: How does biotransformation affect drug solubility and excretion?
Biotransformation converts lipophilic drugs into hydrophilic, water-soluble metabolites through phase I and phase II reactions. This increased water solubility prevents reabsorption in the lipophilic environment of renal tubules, ensuring efficient excretion via urine or bile. The process is essential for terminating drug action and preventing toxic accumulation.
Q6: What are the consequences of forming reactive metabolites during drug biotransformation?
While biotransformation generally detoxifies drugs, it can also form reactive metabolites that cause adverse effects and toxicity. These reactive intermediates may bind to cellular proteins or DNA, potentially causing tissue damage or triggering immune responses. Understanding how physicochemical and chemical properties influence reactive metabolite formation is critical for drug safety assessment.
Q7: Can you provide an example of how a drug is biotransformed?
Codeine is converted to morphine through biotransformation, demonstrating how metabolism can activate a prodrug into a more potent form. This conversion involves phase I reactions that modify the drug's chemical structure, followed by phase II conjugation reactions that enhance water solubility, enabling efficient excretion while the active metabolite exerts therapeutic effects.