Pharmacology
Summary
Elimination refers to the irreversible expulsion of drugs from the body, with various routes facilitating this process. The primary mode of elimination for most medications is renal elimination, which occurs via urine through the glomerulus in the kidneys. In determining drug choice and dosing adjustments, particularly in cases of renal impairment, the glomerular filtration rate (GFR) is a key metric. Other elimination pathways include elimination through the gut via feces, the lungs by exhalation, the liver either through bile or biotransformation, and the skin via sweat. The rate of elimination of drugs via urination can be influenced by the pH of the urine. For instance, weakly acidic drugs are expelled more rapidly in urine with a higher pH (basic), whereas weakly basic drugs are cleared faster in a lower pH (acidic) environment.
The concept of clearance (CL) also plays a fundamental role in pharmacokinetics. It defines the volume of plasma from which a substance, such as a drug, is entirely removed within a specific timeframe, typically expressed in L/h or mL/min. The elimination rate, representing the amount of drug removed from the body over a set period, is shown in mass per time units, like mg/h. To deduce the clearance, one can divide the elimination rate by the plasma concentration of the drug. Another method to determine clearance employs the area under the curve (AUC), which denotes the extent of exposure to a drug or the amount of it reaching the circulation. For drugs administered extravascularly, the clearance is determined by accounting for F (bioavailability) and dose, calculated as clearance = (F x dose)/AUC. However, for intravascularly administered drugs, clearance is deduced without considering F (bioavailability) since F equals 1, making the equation clearance = dose/AUC.
Lesson Outline
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FAQs
Clearance refers to the volume of plasma from which a substance, such as a drug, is completely removed per unit of time. This is usually measured in liters per hour (L/h) or milliliters per minute (mL/min). It is an essential concept in pharmacokinetics, and is used in decision-making regarding drug choice and dosage adjustments.
The area under the curve (AUC) is a measure of the extent of drug exposure, or how much of the drug has reached the circulation. Clearance can be calculated using AUC. When considering extravascular administration, the calculation needs to factor in bioavailability (F) and the drug dose. Thus, clearance is determined as (F x dose)/AUC. On the other hand, for intravascular administration, bioavailability (F) is always 1. In this case, clearance is simply calculated as dose/AUC.
The elimination rate of a drug can be affected by its nature. For example, weakly acidic drugs are eliminated faster in higher pH (basic) urine. On the other hand, weakly basic drugs are eliminated faster in lower pH (acidic) urine. This highlights the importance of understanding the chemical properties of drugs and how they interact with the body's different pH levels.
There are several routes of drug elimination from the body: renal elimination via urine through the glomerulus in the kidneys, elimination from the lungs by exhalation, hepatic elimination via bile and/or biotransformation, gut elimination through feces, and elimination through the skin via sweat. The primary route of elimination for most drugs is renal elimination.
The concept of clearance can be especially significant in patients with renal impairment. Considering that renal elimination is the primary route of elimination for most drugs, any impairment in renal function can greatly affect drug clearance. The glomerular filtration rate (GFR) is often used in clinical settings to adjust drug choices and dosages in patients with renal impairment to prevent toxic drug levels and facilitate effective drug clearance.
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