6.1
Drug Elimination: Overview
Drug elimination involves many complex processes and does not necessarily differentiate between distribution and elimination. It is divided into two p…
When drugs enter the body, they eventually undergo irreversible elimination. Drug elimination primarily involves excretion and metabolism or biotransformation.
Drug excretion is the removal of the unchanged drug. Polar, nonvolatile drugs are primarily excreted through the kidneys via the urine.
Lipophilic drugs are not directly excreted by the kidneys. Instead, first, they undergo biotransformation by the hepatic cytochrome P450 enzymes. The polar metabolites formed are then excreted via the urine.
Additional excretory routes may include breast milk, feces, hepatic bile, and through the lungs.
Drugs excreted in breast milk may affect the infant's well-being.
Fecal excretion is primarily associated with unabsorbed oral drugs. The unabsorbed oral drugs are excreted through the hepatic bile or directly into the intestines without reabsorption. In case of renal impairment, medications like digoxin are significantly excreted in feces instead of urine.
Volatile drugs exit through the lungs via exhaled air.
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Q1: What are the two main processes involved in drug elimination?
Drug elimination occurs through two primary processes: excretion and biotransformation. Excretion removes drugs from the body either unchanged or as metabolites through various routes including the kidneys, bile, lungs, and other pathways. Biotransformation is the chemical conversion of drugs into metabolites, primarily occurring in the liver through hepatic cytochrome P450 enzymes, making lipophilic drugs more polar for easier removal.
Q2: Why do lipophilic drugs require biotransformation before excretion?
Lipophilic drugs are not readily removed by the kidneys because they are lipid-soluble and cannot be easily filtered into urine. These drugs must first undergo biotransformation into more polar metabolites, which occurs primarily in the liver but also in the kidney, lung, small intestine, and skin. Once converted to polar compounds, the metabolites can be efficiently excreted through the kidneys.
Q3: How do polar and nonvolatile drugs differ from volatile drugs in elimination?
Polar and nonvolatile drugs are primarily excreted unchanged through the kidneys via urine, accounting for 25-30% of total drug elimination in humans. Volatile drugs, such as anesthetic gases, follow a different elimination route and exit the body through the lungs via exhaled air. This difference in elimination pathways depends on the drug's chemical properties and solubility characteristics.
Q4: What role does the liver play in drug elimination?
The liver is the primary site of drug biotransformation, where hepatic cytochrome P450 enzymes convert lipophilic drugs into polar metabolites. This metabolic process is essential for making lipid-soluble drugs suitable for excretion. The liver's biotransformation capacity is crucial for eliminating drugs that cannot be directly excreted by the kidneys, making hepatic metabolism a central component of overall drug clearance.
Q5: What happens to drugs that are not absorbed after oral administration?
Unabsorbed oral drugs are primarily excreted through fecal elimination. These drugs may be excreted through hepatic bile or directly into the intestines without reabsorption. In cases of renal impairment, medications like digoxin are significantly excreted in feces instead of urine, demonstrating how fecal excretion becomes more important when renal function is compromised.
Q6: Why is breast milk excretion of drugs clinically significant despite small amounts?
Although breast milk excretion involves only small amounts of drug, it is clinically significant because drugs excreted in breast milk can directly affect the nursing infant's well-being. Even trace quantities of certain medications can accumulate in infants or cause adverse effects due to their immature metabolism and elimination systems, making this excretion route an important consideration in maternal pharmacotherapy.
Q7: What additional excretory routes exist beyond renal and hepatic elimination?
Beyond the kidneys and liver, drugs can be eliminated through several miscellaneous routes including bile, sweat, saliva, and milk. Volatile drugs are excreted via the lungs through exhaled air. Fecal excretion primarily consists of unabsorbed orally ingested drugs or their metabolites. These alternative pathways collectively contribute to total drug elimination and become particularly important when primary elimination routes are impaired.