Pharmacokinetics: Drug elimination and clearance

Pharmacokinetics refers to the movement and modification of medication inside the body.

Or more simply, it’s what the body does to this medication and how it does it.

Alright, so once the medication is administered, it first has to be absorbed into the circulation, then distributed to various tissues throughout the body, metabolized or broken down, and finally, eliminated or excreted in the urine or feces.

You can remember this as ADME- Absorption, Distribution, Metabolism, and Excretion.

Okay, now let’s focus on a process called elimination, which is often confused with the process of excretion.

Elimination is the removal of a medication from the body.

Now this can be accomplished through metabolism, where the medication is broken down into inactive metabolites, or through excretion which is when the intact medication is transported out of the body.

This can happen through a number of ways, but the most common route is through urination.

So, the major function of the kidneys is to clear metabolic waste material and foreign substances, like medications, from the body by filtering the blood.

Now, zooming in on a nephron’s tubule, each one is lined by cells that have two surfaces.

One is the apical surface, which faces the tubular lumen, and the other is the basolateral surface, which faces the peritubular capillaries, which run alongside the nephron.

Alright, so first, certain medications in the circulation can be filtered out with the other metabolic wastes when the blood goes through the glomerulus.

Second, as the filtrate makes its way through the proximal convoluted tubule, certain medications from the peritubular capillaries get secreted into the tubular lumen.

For polar, water-soluble medications, this is mainly done actively through active secretion, meaning that it requires specific carrier proteins on the basolateral membrane of the tubular cells, which uses ATP for energy.

Non- polar, lipid- soluble drugs, on the other hand, are secreted into the proximal tubule via passive diffusion, meaning without requiring any energy.