5.1
Biotransformación de fármacos: descripción general
La biotransformación, también conocida como metabolismo de fármacos, es un proceso fisiológico vital que altera químicamente los fármacos, facilitando…
La biotransformación o metabolismo de fármacos conlleva una alteración química de los xenobióticos, ya sea activando o desactivando sus propiedades farmacológicas.
El metabolismo convierte los fármacos lipofílicos en productos polares solubles en agua, lo que ayuda a la excreción. Por ejemplo, la conversión de codeína en morfina.
El hígado es el sitio principal para el metabolismo de los medicamentos, pero los pulmones, los riñones, los intestinos, la placenta y la piel también contribuyen.
La mayoría de los fármacos se metabolizan en dos fases secuenciales. Las reacciones de fase I introducen o desenmascaran grupos funcionales polares a través de reacciones oxidativas, reductoras e hidrolíticas, preparando el fármaco para las reacciones de fase II.
En la fase II, los productos de las reacciones de la fase I se conjugan con pequeñas moléculas polares como el ácido glucurónico, el sulfato, la glicina y el glutatión, formando metabolitos altamente solubles en agua.
Las enzimas microsomales que se encuentran en el retículo endoplásmico del hígado, junto con las enzimas no microsomales en el citoplasma y las mitocondrias, catalizan la mayoría de las reacciones de biotransformación de fármacos.
Las propiedades fisicoquímicas del fármaco, las variaciones biológicas y los factores químicos influyen en el metabolismo del fármaco, donde los fármacos forman metabolitos reactivos, lo que provoca toxicidad.
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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.