Pharmacokinetics The In Vivo Movement of Drugs

PK is the time course studies of drug absorption, distribution, metabolism, and elimination in vivo. Drug absorption is a highly variable process dependent on inherent drug physiochemical properties. Passive diffusion is by far the most important process by which drugs move across cell membranes. Both the thickness of the cell membrane and presence of drug efflux pumps (e.g., ATP-binding cassette (ABC) transporters such as P-glycoprotein) will determine the rate and extent of drug absorption. While oral (enteral) drug administration is convenient, safe, and economical, the majority of cancer drugs often have a high first-pass effect and this results in low bioavailability (F), describing the fractional extent of the dose reaching the systemic circulation. This first-pass effect results from poor absorption in the gastrointestinal tract, or significant metabolism or excretion by gastrointestinal mucosa and/or liver prior to entering the systemic circulation.

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