Lecture-7 - Thus far we have been assuming that the rates...

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Thus far we have been assuming that the rates of various processes (distribution & elimination) are proportional to the drug concentration in the compartment from which drug molecules exit. This is a valid assumption for processes controlled by diffusion (eg. glomerular filtration) and for carrier systems that are not close to being saturated . However, once a carrier system (eg. kidney tubular secretion) becomes saturated, the rate of drug movement becomes independent of drug concentration (i.e., it becomes a zero-order process).
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A comparison of first-order and zero-order kinetics. Processes such as drug binding to plasma proteins or catalyzing enzymes may become saturated at high drug concentrations. Example: ethanol undergoes zero-order metabolism until the concentration in the body is very low.
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The duration of a drug’s effects is proportional to the half-life of elimination and to the logarithm of the dose, providing the absorption and distribution of the drug are rapid in comparison with elimination. One must increase dosage exponentially to see linear increases in duration of action. The duration of drug action is extended by one half-life when the dose is doubled.
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Example: Assume that a drug has a t 1/2 of 4 hours, the V D is 5 L, and the therapeutically-effective concentration is 5 mg/L. Because 100 mg has been administered (i.v. so it’s fast acting), the C 0 is 20 mg/L. In two half-lives (8 hours), the plasma level of drug will fall below the therapeutically-effective level. If 200 mg of drug is administered instead of 100 mg,
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Lecture-7 - Thus far we have been assuming that the rates...

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