Pharmacokinetics: Overview

Pharmacokinetics encompasses four key stages: absorption, distribution, metabolism, and elimination. Absorption involves the drug's transfer into the bloodstream. Distribution spreads the drug to different tissues. Metabolism alters the drug's structure for elimination. Finally, elimination removes the drug from the body in its original or metabolized form.

Orally administered drugs are often metabolized by the liver before entering systemic circulation, a process called first-pass metabolism. This hepatic metabolism reduces the amount of unchanged drug reaching the bloodstream, resulting in only partial absorption compared to intravenous administration. Understanding factors influencing bioavailability first pass elimination helps explain this phenomenon.

Intravenous administration bypasses hepatic metabolism and delivers the drug directly into the bloodstream, allowing nearly complete absorption. In contrast, oral drugs undergo first-pass metabolism in the liver, resulting in only partial absorption of the administered dose. This route difference significantly impacts drug bioavailability and therapeutic effectiveness.

Lipophilic (lipid-soluble) substances tend to be reabsorbed in the kidneys rather than eliminated. Metabolism converts these lipid-soluble drugs into water-soluble substances, preventing reabsorption and enabling renal elimination. This transformation is essential for effective drug clearance from the body and preventing drug accumulation.

Renal elimination is primarily limited to hydrophilic (water-soluble) substances. Lipophilic compounds tend to be reabsorbed in the renal tubules, preventing their elimination. Therefore, drugs must be water-soluble or metabolized into water-soluble forms to be effectively eliminated through the kidneys and removed from the body.

Multiple factors affect drug absorption, including the route of administration and hepatic metabolism. Oral drugs undergo first-pass hepatic metabolism, reducing absorption, while intravenous drugs bypass this process for nearly complete absorption. Drug properties, gastrointestinal conditions, and membrane transport mechanisms also significantly influence absorption rates and extent.

Following absorption, the drug is distributed to various tissues throughout the body during the distribution phase. Metabolism then alters the drug's chemical structure, making it more suitable for elimination. This sequential process ensures drugs reach target tissues and are subsequently transformed into water-soluble forms the body can effectively excrete through renal or non-renal routes.