6.14
Modelos de depuración: modelos compartimentales
La depuración mide la eliminación de fármacos del compartimento central, que incluye el plasma y órganos altamente perfundidos como los riñones y el h…
El aclaramiento mide directamente la eliminación del fármaco desde el compartimento central, que comprende plasma y órganos altamente perfundidos como los riñones y el hígado.
Los cálculos del aclaramiento varían en función de los modelos farmacocinéticos y de la vía de administración.
El modelo de un compartimento describe la farmacocinética de los fármacos polares, como los antibióticos aminoglucósidos, que se administran por vía intravenosa y se excretan fácilmente en la orina.
Aquí, el producto de la constante de velocidad terminal, λz y el volumen total de distribución, Vss da el espacio libre.
Si la absorción de un fármaco oral es más rápida que su eliminación, la constante de velocidad terminal representa la eliminación. Sin embargo, en el fenómeno flip-flop, cuando la absorción oral es más lenta que la eliminación, la constante de velocidad refleja la absorción.
El modelo de dos compartimentos representa con precisión la farmacocinética de los fármacos menos polares distribuidos en un compartimento con poca perfusión de sangre. Clínicamente, la farmacocinética del antibiótico vancomicina se predice eficazmente mediante este modelo.
En particular, el espacio libre distributivo refleja el espacio libre entre dos compartimentos: el central, que incluye plasma y los órganos bien perfundidos, y el periférico, que alberga los órganos menos perfundidos.
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Q1: What does clearance measure in pharmacokinetics?
Clearance directly measures drug elimination from the central compartment, which comprises plasma and highly perfused organs like kidneys and liver. It quantifies how efficiently the body removes a drug and is essential for determining appropriate dosing regimens to maintain therapeutic drug levels while minimizing potential side effects.
Q2: How does the one-compartment model calculate drug clearance?
In the one-compartment model, clearance is calculated as the product of the terminal rate constant (λz) and total volume of distribution (Vss). This model effectively describes polar drugs like aminoglycoside antibiotics administered intravenously and readily excreted in urine, making it ideal for drugs with rapid, uniform distribution.
Q3: What is the flip-flop phenomenon in oral drug pharmacokinetics?
The flip-flop phenomenon occurs when oral drug absorption is slower than elimination. In this case, the terminal rate constant reflects absorption kinetics rather than elimination kinetics, potentially altering clearance calculations and requiring careful interpretation of pharmacokinetic parameters for accurate dosing.
Q4: Why is the two-compartment model used for less polar drugs?
The two-compartment model accurately represents pharmacokinetics of less polar drugs that distribute into poorly blood-perfused compartments. Vancomycin, a clinically important antibiotic, is effectively predicted by this model because it accounts for both central and peripheral compartment distribution, providing more accurate clearance predictions.
Q5: What is distributional clearance and how does it differ from total clearance?
Distributional clearance reflects drug movement between two compartments: the central compartment including plasma and well-perfused organs, and the peripheral compartment housing less-perfused organs. Unlike total clearance, which measures elimination from the body, distributional clearance describes the rate of drug redistribution between tissue compartments.
Q6: How does administration route affect clearance calculations in compartment models?
Clearance calculations vary depending on the administration route and pharmacokinetic model used. Intravenous administration with the one-compartment model provides straightforward clearance from the product of terminal rate constant and volume of distribution, while oral administration requires consideration of absorption kinetics and potential flip-flop phenomena.
Q7: What organs comprise the central compartment in clearance models?
The central compartment includes plasma and highly perfused organs such as kidneys and liver. These organs receive rapid blood flow and are where most drug elimination occurs, making them critical for understanding how clearance directly measures drug removal from this compartment.