What is the basis of most useful classification of medications in medical chemistry?

Question

UnlockIAS · Accepted Answer

Correct — B, Molecular targets. The stem is lifted almost word for word from the NCERT Class 12 Chemistry chapter 'Chemistry in Everyday Life', which sets out four different bases on which drugs may be classified and then names one of them as the most useful for medicinal chemists. The four are these. Classification on the basis of pharmacological effect gathers every drug that produces the same biological result, so all analgesics sit together whatever their chemistry — the arrangement a prescribing doctor wants, because it shows the full range of drugs available for one kind of problem. Classification on the basis of drug action groups drugs by the biochemical process they interfere with, which is how all antihistamines come to be one class. Classification on the basis of chemical structure groups molecules that share a common skeleton, as the sulpha drugs all share the sulphanilamide nucleus. And classification on the basis of molecular targets groups drugs by the biomolecule they actually bind to — proteins in the form of enzymes and receptors, and beyond them nucleic acids, carbohydrates and lipids. NCERT's own conclusion is that drugs possessing some common structural features may share the same mechanism of action on a particular target, and that this target-based classification is the most useful one for medicinal chemists. The reason is occupational. A medicinal chemist is not choosing a therapy; he or she is designing a molecule, and only the target tells you the three-dimensional pocket the molecule has to fit, the charges it has to complement and the bond it has to make or block. Aspirin is understood as a drug that acetylates a serine residue in the active site of cyclo-oxygenase; cimetidine and ranitidine as antagonists that occupy the histamine H2 receptor on the stomach's acid-secreting cells; the sulpha drugs as inhibitors of the bacterial enzyme dihydropteroate synthase. Each of those descriptions is a design brief. 'Analgesic' and 'antacid' are not.

Why the other options are wrong — (a) A perfectly real classification and the first one NCERT lists, which is what makes it the strongest wrong answer here — nothing in it is false. It groups drugs by the effect they produce on the body, so all analgesics form one class and all antipyretics another, and it is the most useful arrangement for a physician because it displays every drug available against a given complaint. Its limitation is precisely what the stem is probing: the class 'analgesic' contains aspirin, morphine and paracetamol, three molecules with different skeletons acting on different targets, so knowing the effect tells a chemist nothing whatever about what to synthesise.  (c) Also a genuine basis, and the one a candidate is likeliest to pick after ruling out the pharmacological option, on the reasoning that a chemistry question must have a chemistry answer. It groups drugs sharing a common structural skeleton, which is how the sulphonamides became a named family. But NCERT flags its weakness: drugs built on the same nucleus often, yet not always, share a pharmacological activity, so structure alone does not predict what a molecule will do in the body. Structure is the answer to the question, and the target is the question — which is why the target and not the skeleton organises the discipline.  (d) Wrong because one of the listed bases is exactly the one the textbook singles out. It is worth noticing that a fourth genuine basis, classification by drug action, is missing from the options altogether, which can make a well-prepared candidate suspect the list is incomplete and reach for the escape route. The list being incomplete does not make it wrong: the stem asks which basis is the most useful, and the most useful one is present.

Concept: Medicinal chemistry is the branch of chemistry concerned with the design, synthesis and mechanism of substances used as medicines. NCERT defines drugs as chemicals of low molecular mass, roughly 100 to 500 u, which interact with macromolecular targets in the body and produce a biological response; when that response is therapeutic and the substance is safe at the dose used, the drug is called a medicine. The macromolecules they act on are the drug targets, and two kinds dominate. Enzymes are biological catalysts, and a drug that blocks one is an inhibitor: it may compete with the natural substrate for the active site, or bind at an allosteric site and change the shape of the active site so the substrate no longer fits. Receptors are proteins embedded in cell membranes that receive chemical messengers; a drug that mimics the messenger and switches the receptor on is an agonist, and one that occupies the site and blocks the natural messenger is an antagonist. The reason this framework matters beyond the exam is selective toxicity — an antibacterial works only because the target it attacks exists in the bacterium and not in us, which is why the sulpha drugs, aimed at an enzyme of bacterial folate synthesis that human cells do not possess, can be swallowed safely.

State PCS 2023 Q28 — Science & Technology PYQ with Answer

State PCS 2023 Q28 — Science & Technology PYQ with Answer