3.16
Drug Biotransformation: Overview
Pharmaceutical substances known as xenobiotics are predominantly lipophilic and nonionized. This enables them to permeate lipid bilayers, such as cell…
Drugs are special xenobiotics that are mostly lipophilic and nonionized. This allows the drug to pass through the lipid bilayers to reach target receptors in the cell.
Lipophilic drugs, however, often exhibit limited renal excretion capacity as they are reabsorbed rapidly from the glomerular filtrate into the blood. Their accumulation in the body prolongs drug action and precipitates toxic reactions.
To prevent this, drugs can be biotransformed into hydrophilic derivatives, which allows their enhanced elimination through urine.
Although several body tissues can metabolize drugs, the liver is the primary site for biotransformation.
Drug biotransformation involves two primary reactions that occur sequentially and are catalyzed by specific cellular enzymes.
Phase I reactions are catabolic and involve functionalization, whereas phase II reactions are anabolic and involve conjugation.
The process of metabolism often terminates the drug's biological activity, but in prodrugs, enhanced activity is attained.
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Q1: Why do lipophilic drugs need to be biotransformed?
Lipophilic drugs are poorly eliminated by the kidneys because they are rapidly reabsorbed from the glomerular filtrate back into the bloodstream through drug elimination by renal route tubular reabsorption. This reabsorption causes drug accumulation in the body, prolonging effects and triggering toxic reactions. Biotransformation converts lipophilic drugs into hydrophilic derivatives, enabling efficient elimination through urine and preventing harmful accumulation.
Q2: What is the role of the liver in drug biotransformation?
The liver is the primary site for drug biotransformation, though other body tissues can also metabolize drugs. Specialized enzymes in the liver catalyze the sequential phase I and phase II reactions that convert lipophilic drugs into water-soluble metabolites. This hepatic metabolism is essential for preparing drugs for efficient elimination from the body.
Q3: How do phase I and phase II reactions differ in drug metabolism?
Phase I reactions are catabolic and involve functionalization, introducing or exposing functional groups like hydroxyl, amino, or carboxyl groups to the drug molecule. Phase II reactions are anabolic and involve conjugation, adding water-soluble molecules such as glucuronic acid, sulfate, or glutathione. Together, these sequential reactions progressively increase drug water solubility for elimination.
Q4: What chemical properties allow drugs to cross cell membranes?
Drugs are predominantly lipophilic and nonionized, allowing them to pass through lipid bilayers and reach intracellular target receptors. This lipophilic nature is advantageous for crossing biological barriers and reaching sites of action. However, this same property limits renal excretion, necessitating biotransformation into hydrophilic derivatives for elimination.
Q5: How does biotransformation affect drug activity?
Biotransformation generally terminates a drug's biological activity by converting it into inactive metabolites. However, prodrugs represent an exception where biotransformation enhances activity rather than eliminating it. In prodrugs, metabolism converts an inactive compound into an active form, leading to improved therapeutic outcomes and more effective treatment.
Q6: What functional groups are introduced during phase I drug metabolism?
Phase I reactions introduce or expose functional groups such as hydroxyl (-OH), amino (-NH2), or carboxyl (-COOH) groups to the drug molecule. These functionalization reactions create more polar metabolites than the parent drug, preparing the molecule for phase II conjugation reactions that further increase water solubility.
Q7: What molecules are typically added during phase II conjugation reactions?
Phase II conjugation reactions add larger, water-soluble molecules to drugs or their phase I metabolites, including glucuronic acid, sulfate, or glutathione. These conjugation reactions significantly increase the drug's water solubility and prepare it for efficient elimination from the body through urine.