Two questions about every drug
When you study how a drug binds its target and changes biology, you are studying pharmacodynamics — what the drug does to the body. But there is a second, equally important question: what does the body do to the drug? That is pharmacokinetics (PK). A molecule with beautiful affinity and selectivity is worthless if it never reaches the tissue where the target lives, or if the liver destroys it in minutes.
PK is summarized by four processes, captured in the acronym ADME. Together with toxicity they form ADMET, the profile a drug candidate must satisfy. Think of ADME as the supply chain that delivers your molecule to the front line.
The four letters
- A — [[absorption|Absorption]]: getting the drug from the dosing site into the bloodstream. For a pill, the molecule must dissolve in the gut and cross the intestinal wall. How much survives this journey is its oral bioavailability.
- D — [[distribution|Distribution]]: once in the blood, the drug spreads into tissues. Whether it stays in plasma or partitions into fat, muscle, or brain shapes its volume of distribution.
- M — Metabolism: enzymes, mostly in the liver, chemically modify the drug — often to make it easier to excrete. This is where metabolic stability and first-pass metabolism enter the story.
- E — [[excretion|Excretion]]: the drug and its metabolites leave the body, mainly through the kidneys (into urine) or the liver (into bile). Together, metabolism and excretion set how fast the drug is removed.
Why chemists care
Historically, more drug candidates failed for poor PK than for lack of potency. That is why modern lead optimization balances potency against ADME from the very start. Crucially, ADME is designable: small structural changes alter how a molecule dissolves, crosses membranes, and is recognized by metabolizing enzymes. A medicinal chemist who understands ADME can steer a series toward compounds that are not just potent but *behave* well in the body.