Mechanism of action of drugs

This capsule helps learners explain how drugs produce effects through receptor-mediated and non-receptor mechanisms.

By the end of the module, learners should be able to classify common mechanisms of action, interpret agonist-antagonist relationships, and use mechanism-based reasoning to predict therapeutic effects and adverse effects.

Self-directed learning

Receptor-mediated drug action

Most clinically useful drugs act by binding to molecular targets. These targets include receptors, enzymes, ion channels, transporters, and structural proteins.

For receptor-mediated action, the key terms are affinity, intrinsic activity, efficacy, potency, selectivity, and reversibility. An agonist binds and activates a receptor response. An antagonist binds without activating the receptor and reduces the effect of an agonist.

Clinical reasoning starts by asking: What is the target? What normal signal does the target control? What happens when the drug increases, decreases, or blocks that signal?

Which pair of properties best distinguishes an agonist from a pure antagonist?

  1. Affinity and intrinsic activity Correct. Both bind receptors, but an agonist has intrinsic activity while a pure antagonist does not.
  2. Absorption and distribution These are pharmacokinetic properties, not receptor activation properties.
  3. Half-life and clearance These affect duration and exposure, not whether a receptor is activated.

Non-receptor and physical mechanisms

Not every drug effect depends on a classical receptor. Some drugs act by enzyme inhibition, ion channel blockade, transporter modulation, osmotic action, chemical neutralisation, or physical adsorption.

Mechanism-based grouping is more useful than memorising isolated examples. Enzyme inhibitors alter product formation. Ion channel blockers alter excitability. Osmotic agents move water. Antacids neutralise acid chemically.

A strong explanation connects mechanism to clinical effect: if a drug inhibits cyclooxygenase, prostaglandin synthesis falls, so inflammation and pain can fall but gastric protection may also fall.

Which option is the best example of a non-receptor physical or chemical mechanism?

  1. Activated charcoal adsorbing a toxin in the gut Correct. Adsorption is a physical mechanism rather than receptor activation.
  2. Salbutamol activating beta-2 receptors That is receptor-mediated agonism.
  3. Naloxone blocking opioid receptors That is receptor antagonism.

Mechanism-based clinical reasoning

Mechanism of action is clinically useful because it supports prediction. A drug that increases a physiological pathway can produce both desired and unwanted effects through the same pathway.

Dose-response reasoning matters. A competitive antagonist often shifts an agonist concentration-response curve to the right. A non-competitive antagonist or irreversible blocker can reduce the maximal effect.

Mechanism also helps interpret combinations. Two drugs acting at different steps of the same pathway may produce additive benefit or toxicity. Learners should explicitly state the target, direction of effect, clinical outcome, and predictable harm.

What is expected with a reversible competitive antagonist in a simple agonist dose-response experiment?

  1. A rightward shift that can be overcome by more agonist Correct. The antagonist competes at the receptor, so higher agonist concentration can restore response.
  2. A permanent fall in maximum response in every case That is more typical of non-competitive or irreversible antagonism.
  3. No change in the response curve Competitive antagonism changes apparent potency.

Interactive lecture

Quizzes

Practice quiz: Mechanism vocabulary

A drug binds a receptor and produces a submaximal response even when all receptors are occupied. What is it called?

  1. Partial agonist Correct. A partial agonist has lower efficacy than a full agonist.
  2. Irreversible antagonist An irreversible antagonist reduces response by persistent receptor blockade.
  3. Chemical antagonist A chemical antagonist neutralises or binds another substance directly.

Which target class is directly affected by a proton pump inhibitor?

  1. Enzyme or pump involved in acid secretion Correct. Proton pump inhibitors suppress gastric acid secretion by inhibiting the H+/K+ ATPase pump.
  2. Opioid receptor That target is relevant to opioid agonists and antagonists.
  3. Voltage-gated sodium channel only Sodium channels are important targets, but not the proton pump inhibitor target.

Graded quiz: Applying mechanism reasoning

A patient develops bronchospasm after a non-selective beta blocker. Which mechanism-based explanation is best?

  1. Blockade of beta-2 receptors can reduce bronchodilation Correct. Loss of beta-2 mediated bronchodilation can worsen bronchospasm.
  2. Activation of beta-2 receptors always causes bronchospasm Beta-2 activation usually promotes bronchodilation.
  3. The effect is unrelated to receptor selectivity Selectivity is central to this adverse effect.

Which explanation best links enzyme inhibition to adverse effects?

  1. Blocking an enzyme may reduce both harmful and protective products of the pathway Correct. Mechanism-based adverse effects often arise from the same pathway as benefit.
  2. Enzyme inhibition never affects normal physiology Enzymes are part of normal physiology, so inhibition can affect useful processes.
  3. Adverse effects can only be allergic Allergy is one mechanism of harm, but predictable pharmacological effects are common.

Gate quiz: PH1.7 readiness

Threshold: 80%

A competitive antagonist most often changes an agonist response curve by causing:

  1. A surmountable rightward shift Correct. More agonist can overcome reversible competitive antagonism.
  2. A leftward shift with increased potency Competitive antagonists reduce apparent potency.
  3. Immediate receptor activation Antagonists do not activate receptors.

A concise mechanism-of-action explanation should include:

  1. Target, direction of effect, clinical effect, and predictable harm Correct. This structure supports clinical reasoning.
  2. Only the brand name Brand names do not explain mechanism.
  3. Only route of administration Route may matter clinically, but it is not the mechanism.

Assignments

Shows-how assignment: Explain one mechanism

Choose one familiar drug. Explain its molecular target, direction of action, therapeutic effect, and one predictable adverse effect. Use no more than 250 words and include one simple mechanism diagram or flowchart.

Discussions and PBL

Small group discussion: Selectivity is not absolute

Discuss why receptor selectivity reduces adverse effects but does not eliminate them. Use beta blockers, antihistamines, or any familiar class as the example.

PBL discussion: Antagonist reasoning in a symptomatic patient

A patient receives a drug that blocks a receptor-mediated response. Symptoms improve, but a predictable adverse effect appears in another organ system. Identify the likely target, explain the beneficial effect, and reason through the adverse effect using the same mechanism.