Many doors into the body

A medicine only helps if enough of it reaches the right place at the right time. Pharmacology is the science of how that happens, and it has two halves:

• Pharmacokinetics (PK): what the body does to the drug. How it gets in, where it goes, how it is changed and how it leaves.
• Pharmacodynamics (PD): what the drug does to the body. Which target it hits and what effect that produces.

This world follows the PK half. Throughout this lab the medicines are made up: "Drug A", "Drug B" and "Drug C", with invented numbers. They teach the ideas without being anything you could take.

The first decision about any medicine is the route of administration, the door it uses to enter the body. Common routes:

• Oral (swallowed): tablets, capsules, liquids. Convenient, but the drug must survive the gut and the liver.
• Intravenous (IV): straight into a vein, usually by a nurse or doctor. No absorption step at all.
• Intramuscular and subcutaneous: injected into muscle or just under the skin, then absorbed into the blood.
• Sublingual or buccal: dissolved under the tongue or in the cheek. The thin lining there lets some drugs pass directly into small veins.
• Transdermal (skin patch) and topical (cream): through or onto the skin.
• Inhaled: into the lungs, which have a huge, thin surface for quick absorption and also act directly on the airways.

Different routes give different speed of onset (usually fastest for IV and inhaled, slower for oral and patches) and meet different obstacles on the way to the blood.

Scientists organise the journey with four letters, ADME:

• Absorption: getting from the outside into the blood.
• Distribution: spreading from the blood into the tissues.
• Metabolism: being chemically changed, mostly by the liver.
• Excretion: leaving the body, mostly through the kidneys.

The key number for the first step is bioavailability, F: the fraction of the dose that reaches the general blood circulation unchanged. By definition an IV dose has F = 1 (100%), because all of it is put straight into the blood. For other routes, F is found by comparing the area under the blood-level curve (AUC) with the IV one for the same dose: F = AUC(route) / AUC(IV).