Pharmacokinetics: Drug absorption and distribution

Pharmacokinetics refers to the movement and modification of a medication inside the body. In other words, it’s what the body does to a medication and how it does it.

Okay, first things first. A medication needs a way to be administered, or a route of administration. Depending on the form of the chemical preparation, like a pill, solution, spray, or ointment; and the part of the body being treated, the medication can be administered through various means or routes: such as swallowed by the mouth or orally, injected into a vein or intravenously, injected into a muscle or intramuscularly, inhaled into the lungs, sprayed into the nose or nasally, and applied onto the skin or cutaneously.

Once a medication is administered, it first has to be absorbed into the circulation; then distributed throughout the body; metabolized or broken down; and finally, excreted in the urine or feces. This process can be broken down into four components with the acronym ADME; which stands for Absorption, Distribution, Metabolism, and Excretion. Now, the loss of drug through chemical metabolism, which makes it inactive, and through physical excretion out of the body, can together be referred to as elimination.

Okay, so let’s start with absorption. Absorption is the process of moving the medication from the site of administration into the circulation. With the exception of intravenous administration, a medication will need to cross one or more cell membranes before it reaches the circulation. Movement across the cell membrane can occur via passive transport, which requires no energy, and active transport, which requires energy in the form of adenosine triphosphate, or ATP.

Two types of passive transport are used; facilitated diffusion and passive diffusion. Facilitated diffusion helps larger, water-soluble, and polar medications move across the membrane through transport proteins like channels and carrier proteins. Passive diffusion helps small, lipid-soluble, and nonpolar medications move across the membrane, from an area of high concentration to low concentration.