Chemistry in Everyday Life • Topic 1 of 3

Drugs & Their Classification

A drug is a chemical of low molecular mass (about 100–500 u) that interacts with a biological target (a macromolecule) and produces a response. When a drug is used to diagnose, prevent or treat disease, it is called a medicine. The branch of science that studies how drugs interact with the body is pharmacology; the use of chemicals for therapeutic effect is chemotherapy. Most useful drugs work in very small doses, but in higher doses many become poisons — the boundary between cure and harm is narrow.

Drug–target interaction

Drugs act by binding to biological targets. The most important targets are enzymes (catalytic proteins) and receptors (proteins on cell membranes that pass chemical messages into the cell).

Enzymes as targets. An enzyme first holds the substrate at its active site by weak attractions, then catalyses the reaction. Drugs can interfere in two ways. A drug may compete with the natural substrate for the active site — this is competitive inhibition. Alternatively, a drug may bind at a different site (an allosteric site), changing the shape of the active site so the substrate no longer fits — this is non-competitive (allosteric) inhibition. If the drug binds the enzyme by strong covalent bonds the inhibition is essentially permanent.

Receptors as targets. Receptors sit in the cell membrane. The body’s own messenger molecules (such as hormones and neurotransmitters) bind the receptor’s active site, change its shape and transfer the message inside the cell. Drugs that act on receptors fall into two classes:

  • Agonists mimic the natural messenger, switch the receptor on and trigger the same response. They are useful when the natural chemical is deficient.
  • Antagonists bind the receptor but produce no response; by occupying the binding site they block the natural messenger. They are useful when a chemical message must be stopped.

Classification of drugs

Drugs are classified on four bases:

  • By pharmacological effect — the kind of biological action, e.g. analgesics relieve pain, antiseptics kill micro-organisms. Useful to a doctor choosing a treatment.
  • By drug action (chemical action) — the biochemical process the drug acts on, e.g. antihistamines act against the chemical histamine.
  • By chemical structure — drugs sharing a common structural feature are grouped together; they often show similar effects, e.g. sulphonamides.
  • By molecular target — drugs binding the same biomolecule (enzyme or receptor). Most useful to medicinal chemists designing new drugs.

Therapeutic index

Safety is measured by the therapeutic index, the ratio of the dose that is toxic to the dose that is effective. A large therapeutic index means the drug is safe (a big gap between the useful and the harmful dose); a small index means the drug must be dosed very carefully.

Antacids

Excess hydrochloric acid in the stomach causes irritation, even ulcers. Antacids remove this excess acid. Mild antacids such as sodium hydrogencarbonate or a mixture of aluminium and magnesium hydroxides merely neutralise acid that is already present, but cannot stop its over-production. The breakthrough came with drugs that prevent acid formation. Histamine stimulates the stomach’s acid-secreting cells through H2 receptors; drugs that block these receptors stop the message and cut acid output. Cimetidine and ranitidine are such H2-receptor blockers and revolutionised ulcer treatment.

Antihistamines

Histamine is a powerful body chemical: it dilates blood vessels and causes the inflammation, redness and runny nose of allergies, and it triggers stomach-acid secretion. Antihistamines (antiallergic drugs) such as brompheniramine and terfenadine interfere with histamine’s action at H1 receptors and relieve allergy symptoms. Note that these ordinary antihistamines do not affect acid secretion, because that is controlled by a different receptor (H2); that is why a separate class of H2 blockers (cimetidine, ranitidine) was needed for acidity.

Agonist and antagonist binding a receptor (lock and key)Drug-Receptor Interaction (Lock and Key)Receptor (active site)Agonistfits and switches ON✓Receptor (blocked)Antagonistbinds but no response✗Agonist mimics the natural messenger; antagonist occupies the site and blocks it.
Solved Examples
1
Worked Example
Define a drug and a medicine. Are all drugs medicines?
Solution
  1. A drug is a chemical of low molecular mass (about 100–500 u) that interacts with a biological macromolecule (target) and produces a biological response.
  2. When that response is helpful and the drug is used to diagnose, prevent or treat disease, it is called a medicine.
  3. Not every drug is a medicine: at higher doses many drugs become poisons, and substances of abuse are drugs but not medicines.

Answer: A drug interacts with a body target to give a response; a medicine is a drug used therapeutically. All medicines are drugs, but not all drugs are medicines.

2
Worked Example
What are the two main types of biological targets for drugs? Give the role of each.
Solution
  1. Enzymes — catalytic proteins; a drug can inhibit them by competing for the active site or by binding an allosteric site.
  2. Receptors — proteins in the cell membrane that receive chemical messages (from hormones or neurotransmitters) and pass them into the cell.
  3. By acting on these targets a drug can either enhance or block a normal biological process.

Answer: The main targets are enzymes and receptors; enzymes catalyse reactions, receptors transmit chemical messages across the cell membrane.

3
Worked Example
Distinguish between an agonist and an antagonist with one use of each.
Solution
  1. An agonist binds a receptor and mimics the natural messenger, switching the receptor on and producing the natural response.
  2. An antagonist binds the receptor but produces no response; it blocks the natural messenger by occupying the site.
  3. Agonists are used when the natural chemical is deficient; antagonists are used when a chemical message must be stopped.

Answer: Agonist = mimics and activates the receptor (used to replace a deficient messenger); antagonist = binds but blocks the receptor (used to stop an unwanted message).

4
Worked Example
On what four bases are drugs classified? Which is most useful to a medicinal chemist?
Solution
  1. By pharmacological effect (kind of biological action).
  2. By drug action / chemical action (the biochemical process affected).
  3. By chemical structure (shared structural feature).
  4. By molecular target (the biomolecule the drug binds).

Answer: The four bases are pharmacological effect, drug action, chemical structure and molecular target; classification by molecular target is most useful to a medicinal chemist designing new drugs.

5
Worked Example
Define the therapeutic index. Is a large value good or bad?
Solution
  1. The therapeutic index is the ratio of the toxic dose of a drug to its effective (therapeutic) dose.
  2. A large value means there is a wide gap between the dose that helps and the dose that harms.
  3. Therefore a large therapeutic index indicates a safer drug.

Answer: Therapeutic index = (toxic dose)/(effective dose). A large value is good — it means the drug is relatively safe.

6
Worked Example
Why are cimetidine and ranitidine better antacids than sodium hydrogencarbonate? Name the receptor they block.
Solution
  1. Sodium hydrogencarbonate only neutralises acid already present; it cannot stop the stomach from making more, so relief is temporary.
  2. Histamine stimulates the acid-secreting cells of the stomach through H2 receptors.
  3. Cimetidine and ranitidine block these H2 receptors, preventing histamine from triggering acid release, so they cut acid production at its source.

Answer: Cimetidine and ranitidine prevent acid formation by blocking the stomach’s H2 receptors, whereas sodium hydrogencarbonate only neutralises acid already formed.

Key Points

  • A drug is a low molecular mass chemical that interacts with a biological target and gives a response; a medicine is a drug used to diagnose, prevent or treat disease (all medicines are drugs, not all drugs are medicines).
  • The main drug targets are enzymes (catalytic proteins, inhibited competitively or allosterically) and receptors (membrane proteins that pass chemical messages into the cell).
  • Agonists mimic the natural messenger and switch the receptor on; antagonists bind the receptor and block it without producing a response.
  • Drugs are classified by pharmacological effect, by drug (chemical) action, by chemical structure and by molecular target; the molecular-target basis is most useful for drug design.
  • Therapeutic index = (toxic dose)/(effective dose); a large value means a safer drug. Cimetidine and ranitidine are H2-receptor blockers that prevent stomach-acid formation, while ordinary antihistamines act at H1 receptors.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.The two most important molecular targets for drugs in the body are:
Explanation: Drugs act mainly on enzymes (catalytic proteins) and receptors (membrane proteins that transmit chemical messages).
Q2.A drug that binds a receptor but produces no response, thereby blocking the natural messenger, is called:
Explanation: An antagonist occupies the receptor site without activating it, blocking the natural chemical message; an agonist would mimic and activate the receptor.
Q3.Cimetidine and ranitidine relieve acidity by acting as:
Explanation: They block stomach H2 receptors, preventing histamine from stimulating acid secretion. Allergy antihistamines act at H1 receptors.
Q4.A drug with a large therapeutic index is:
Explanation: Therapeutic index = (toxic dose)/(effective dose); a large value means a wide safety gap between the helpful and harmful doses.
Practice more MCQs → 📝 Assignment · 30 marks