BMS 123 · Common Core (Tronc commun)Section 5.1 · Pharmacodynamics: mechanisms and receptors
Section 18 of 27
What pharmacodynamics studies
Define pharmacodynamics, distinguish effects from mechanisms, and list the five questions pharmacodynamics answers.
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Why this matters
Pharmacodynamics is the half of pharmacology that explains the effect itself. Without it, a plasma concentration is only a number: you cannot say whether the drug will work, how strongly, or which unwanted effects to expect. It also gives you the questions to ask about any new medicine — what does it hit, what does it do, and at what dose does the response appear.
What you need to know first
You should already be able to say what pharmacokinetics covers — absorption, distribution, metabolism and excretion — and that a drug must reach its target before anything happens. No receptor biology is assumed here; the four main targets are introduced in section 5.3.
Words to know (5)
Pharmacodynamics
The branch of pharmacology that studies what a drug does to the body: its mechanism of action and its effects. Explaining how aspirin eases pain by blocking an enzyme is pharmacodynamics.
Pharmacokinetics
The branch that studies what the body does to the drug: absorption, distribution, metabolism and excretion. A tablet crossing the gut wall is a pharmacokinetic event.
Response (drug action)
The observable change a drug produces in the body, whether it is wanted or unwanted. A fall in fever after paracetamol is a response.
Mechanism of action (MOA)
The molecular steps by which a drug produces its effect, normally beginning at the target it binds. Blocking a sodium channel in a nerve is a mechanism of action.
Macromolecule
A very large molecule, usually a protein, that a drug can bind; drug targets are mostly macromolecules. An enzyme such as xanthine oxidase is a macromolecule.
One idea at a timeIdea 1 / 5
In one sentence Pharmacodynamics is the study of how a drug interacts with macromolecules in the body to produce a physiological or biochemical change.
Pharmacodynamics is what the drug does to the body
Drugs that have been administered undergo absorption, distribution, metabolism and excretion — the work of pharmacokinetics. At the same time they are carried to their sites of action, where they interact with components of the cells and bring about precise changes. Pharmacodynamics is the study of how drugs interact with macromolecules in the living system to bring about physiological or biochemical changes. It is the branch of pharmacology that focuses on the biochemical and physiological effects of drugs in the body and on the mechanisms that produce those effects. Put briefly: pharmacokinetics follows the journey, pharmacodynamics explains the destination.
For example
Aspirin relieves pain and lowers fever by blocking an enzyme that makes prostaglandins. Naming that enzyme, describing how aspirin blocks it and explaining the change that follows in the body is pharmacodynamics.
Think of it like
Pharmacokinetics is the delivery service and pharmacodynamics is what the parcel does once it is opened. The limit of the image is that a drug is not a passive parcel: it alters the machine it enters, and that alteration can change how the body handles the drug in return.
Pause and check yourself
In one sentence, what does pharmacodynamics study?
Show the answer
It studies how a drug interacts with macromolecules in the body to produce physiological or biochemical changes — in short, what the drug does to the body.
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From one occupied receptor to a visible effect
Binding is only the first link in a chain. When a drug occupies its receptor, the receptor changes shape; that change activates an amplifying protein such as a G protein, which switches on an enzyme, which manufactures a second messenger such as cyclic AMP, which opens an ion channel or alters the activity of another enzyme inside the cell. Each step multiplies the signal, so a few thousand occupied receptors can change the behaviour of an entire tissue. This amplification explains two things beginners find surprising. First, a drug can produce a large effect while occupying only a small fraction of the available receptors, because the tissue holds receptors in reserve; the fraction actually needed for a half-maximal response is often far below half. Second, two drugs that bind the same receptor with the same affinity can produce very different effects, because what matters is not only how many receptors are occupied but what each occupied receptor does next. The chain also explains why the same drug produces one effect in the bronchi and another in the heart: the receptors are similar, but the machinery behind them is not.
Where pharmacokinetics meets pharmacodynamics
Pharmacokinetics describes a concentration that rises and falls with time; pharmacodynamics describes the effect that a given concentration produces. Put the two curves together and you have the dose schedule: the concentration must stay in the range where the pharmacodynamic curve is steep enough to give benefit without reaching the flat top where toxicity begins. This is why a drug with a short half-life may need several doses a day or a slow-release form, and why a drug with a long half-life can be given once daily but accumulates if the dose is repeated too quickly. Two practical consequences follow. First, the effect does not always follow the plasma level immediately: after a dose, the concentration peaks before the effect does, and when the concentration falls the effect may lag behind, a phenomenon called hysteresis. Second, repeated exposure can change pharmacodynamics itself. The tissue may reduce the number of receptors or become less responsive, which is tolerance, so the same concentration produces a smaller effect than it did at the start of treatment. Unit 6 develops the dose–response curves that make these relationships measurable.
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Pharmacokinetics and pharmacodynamics in one loop
The journey of a drug through the body ends at a concentration at the site of action, where pharmacodynamics begins and turns binding into a response.
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01 / 6Pharmacokinetics (PK)
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Pharmacokinetics (PK)
The branch that studies what the body does to the drug: absorption from the site of administration, distribution into tissues, metabolism in the liver and excretion by the kidney. It decides how much drug arrives and how quickly it leaves.
Schematic, not to scale. The four ADME steps are drawn in order for clarity; in the body they overlap in time, and a drug may act on its target while it is still being absorbed.
Key points to remember
✓Pharmacodynamics studies what a drug does to the body: its mechanism of action and its effects.
✓The effect is the observable change; the mechanism is the molecular route that produces it.
✓Response size depends on the concentration at the site of action and on the drug–target interaction there.
✓Pharmacodynamics answers five questions: target, mechanism, effects, similar drugs, and dose–effect relationship.
✓Understanding the mechanism is what allows effects and adverse reactions to be predicted rather than merely observed.
Go deeper
Worked example
Reading a new antihypertensive in pharmacodynamic terms. A drug company presents a molecule that lowers blood pressure in laboratory animals. You have no clinical experience of it. Using the five pharmacodynamic questions, you must produce a short profile of what this drug is likely to do in a patient.
Find the target: on which macromolecule does the molecule act? If it is an enzyme, a channel, a carrier or a receptor, that first answer already limits the possible effects.
Describe the mechanism: does it switch the target on or off, and what happens inside the cell as a result? This is the step that lets you predict rather than merely observe.
List the effects: separate the pharmacological effect you want from the side, toxic and lethal effects that the same mechanism could produce elsewhere in the body.
Ask which drugs behave similarly: placing the molecule in a class tells you which interactions, cautions and adverse effects to look for from experience with its neighbours.
Ask about the dose–response relationship: how much is needed for the effect, and how much for toxicity? This introduces potency and efficacy, developed in Unit 6.
Pharmacodynamics cannot tell you whether the drug is safe in every patient, but it gives you an ordered list of predictions you can then check against evidence. A profile built this way is far more useful than a single blood-pressure reading.
Common misconception
A very common beginner model is that pharmacodynamics simply means “how strong a drug is”. Potency and efficacy are outputs of pharmacodynamics, but the subject is the whole chain from target to mechanism to observable effect. A drug can be extremely strong and still useless if it acts on the wrong target.
Check your reasoning
A patient’s blood pressure has not fallen after a week of treatment. Using pharmacodynamics rather than pharmacokinetics, what would you examine first?
Reveal an answer
First, whether the drug is reaching the target that matters for this patient’s illness: the right receptor, in the right tissue, in sufficient quantity. Then whether the body is compensating against the effect, and whether the chosen mechanism actually suits the cause of the hypertension. Only after that would you suspect that the molecule cannot work in this patient.
Where this comes from
BMS 123 General Pharmacology II lecture notes · Part Two, Principles of Pharmacodynamics, §1.1 Definition (What is pharmacodynamics?)
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About this course and its sources
Bilingual lessons organised from the supplied BMS 123 General Pharmacology II lecture notes (Principles of Pharmacokinetics and Principles of Pharmacodynamics). Diagrams are original educational schematics; pharmacology, nursing and French-language review are pending.