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PharmD clinical pharmacy program Level 3, Semester 2 Biopharmaceutics & Pharmacokinetics (PT608(

Pharmacokinetics of Drug Absorption

 The systemic drug absorption from the GI tract or any other extravascular site is

dependent on: (1) the physicochemical properties of the drug, (2) the dosage form,

and (3) the anatomy and physiology of the absorption site.

 For oral dosing, such factors as the surface area of the GI tract, stomach-emptying

rate, GI mobility, and blood flow to the absorption site all affect the rate and the

extent of drug absorption.

 The overall rate of drug absorption may be described as either a first-order or zero-

order input process. Most pharmacokinetic models assume first-order absorption

unless an assumption of zero-order absorption improves the model significantly or

has been verified experimentally.

One-compartment pharmacokinetic model for first-order drug absorption and

first-order drug elimination

➔Xa:Mass or amount of absorbable drug remaining in the gut, or at the site of

administration, at time t

➔X:Mass or amount of drug in the blood at a time, t

➔Xu:Mass or amount of drug excreted unchanged in the urine at a time, t
➔Ka: First-order absorption rate constant

➔K: First-order elimination rate constant.

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