3.1 : Pharmacokinetics: Overview

Pharmacokinetics is a scientific discipline that focuses on the journey of a drug within the body, encompassing four key stages: absorption, distribution, metabolism, and elimination. The first stage, absorption, involves the drug's transfer into the bloodstream. Several factors dictate the extent and speed of this process. For example, the liver often metabolizes oral drugs before they reach systemic circulation, leading to only partial absorption. In contrast, intravenous (IV) administration bypasses this initial metabolism, allowing the drug to enter the bloodstream directly and be nearly entirely absorbed. Following absorption, the drug is disseminated to different tissues in the body, marking the distribution phase.

The next stage, metabolism, alters the drug's structure, making it more suitable for elimination. This is particularly important for lipophilic substances, which tend to be reabsorbed rather than eliminated. To prevent this, these lipid-soluble drugs are metabolized into water-soluble substances.

Finally, the elimination stage involves expelling the drug from the body, either in its original form or as a metabolite. Renal elimination is primarily limited to hydrophilic substances due to the reabsorption tendency of lipophilic compounds. So, the transformation of lipid-soluble drugs to water-soluble forms is crucial to enable excretion.