Medicinal Chemistry SAR: COX1/COX2 inhibitors and adrenergic/cholinergic agents MCQs With Answer

Introduction:

This quiz collection focuses on structure–activity relationships (SAR) of COX-1/COX-2 inhibitors and adrenergic/cholinergic agents — high-yield topics in MPC 103T Advanced Medicinal Chemistry for M.Pharm students. Questions probe how small structural changes (e.g., sulfonamide vs carboxylate, aryl heterocycles, N‑substitution, ester vs carbamate) influence receptor/enzyme selectivity, potency, metabolism and adverse effects. The set combines mechanistic SAR principles with examples of marketed drugs, metabolism pathways and pharmacodynamic consequences to develop critical thinking for drug design and analysis. Use these MCQs to test and deepen understanding of molecular features that govern activity, selectivity and safety in these therapeutic classes.

Q1. Which structural feature is most commonly associated with selective COX-2 inhibition in classical diaryl heterocycle inhibitors?

• An aryl para-substituted sulfonamide or sulfone group providing access to the COX-2 side pocket
• A free primary alcohol on the aryl ring
• A small unsubstituted phenyl ring to increase planarity
• A terminal aliphatic carboxylic acid only

Correct Answer: An aryl para-substituted sulfonamide or sulfone group providing access to the COX-2 side pocket

Q2. The core heterocyclic scaffold present in celecoxib that helps orient two aryl rings for COX-2 binding is:

• A benzimidazole ring
• A pyrazole ring
• A thiazole ring
• A benzofuran ring

Correct Answer: A pyrazole ring

Q3. Why do selective COX-2 inhibitors generally produce less gastrointestinal (GI) toxicity compared with nonselective NSAIDs?

• COX-2 inhibitors continuously stimulate mucus secretion in the stomach
• They spare COX-1 mediated production of protective gastric prostaglandins
• They do not inhibit prostaglandin synthesis systemically
• They are rapidly excreted unchanged, avoiding gastric exposure

Correct Answer: They spare COX-1 mediated production of protective gastric prostaglandins

Q4. Which NSAID class is correctly paired with representative drugs that exemplify arylpropionic acid SAR?

• Arylpropionic acids (e.g., ibuprofen, naproxen)
• Arylacetic acids (e.g., celecoxib, rofecoxib)
• Diarylpyrazoles (e.g., aspirin, diclofenac)
• Oxicams (e.g., ibuprofen, mefenamic acid)

Correct Answer: Arylpropionic acids (e.g., ibuprofen, naproxen)

Q5. In adrenergic agents, which aromatic substitution is rapidly metabolized by catechol-O-methyltransferase (COMT) and thus decreases oral bioavailability?

• A para-methoxy substituent
• A 3,4-dihydroxy (catechol) substitution on the phenyl ring
• A single meta-hydroxyl group only
• An ortho-chloro substituent

Correct Answer: A 3,4-dihydroxy (catechol) substitution on the phenyl ring

Q6. Which modification on phenethylamine adrenergic agonists generally increases beta-receptor selectivity (especially β2) relative to alpha activity?

• Introduction of a bulky N‑alkyl group (e.g., tert‑butyl)
• Removal of the beta‑hydroxyl group
• Conversion of the amine to a primary ammonium salt
• Adding an ortho‑nitro substituent on the ring

Correct Answer: Introduction of a bulky N‑alkyl group (e.g., tert‑butyl)

Q7. To increase duration of action and reduce hydrolysis by cholinesterases, muscarinic agonists are often converted from esters to which functional class?

• Thioesters
• Carbamates
• Phosphonates
• Simple ethers

Correct Answer: Carbamates

Q8. Quaternary ammonium cholinomimetics (e.g., bethanechol) are characteristically:

• Highly lipophilic and penetrate the CNS easily
• Permanently positively charged and largely restricted to peripheral tissues
• Readily dealkylated to tertiary amines in vivo
• Preferentially selective for nicotinic receptors in the brain

Correct Answer: Permanently positively charged and largely restricted to peripheral tissues

Q9. Which class of acetylcholinesterase inhibitors produces essentially irreversible enzyme inhibition by phosphorylating the active site serine?

• Carbamate inhibitors (e.g., neostigmine)
• Organophosphorus agents (e.g., sarin, malathion)
• Reversible aromatic esters (e.g., physostigmine)
• Synthetic peptidomimetics

Correct Answer: Organophosphorus agents (e.g., sarin, malathion)

Q10. Regarding the chiral center at the α-carbon of arylpropionic acids (e.g., ibuprofen), which statement reflects SAR and metabolic reality?

• The R-enantiomer is the active form and the S-enantiomer is inactive and cannot interconvert
• The S-enantiomer has greater COX inhibitory activity; R can be enzymatically inverted to S in vivo
• Both enantiomers are completely inactive; racemization produces the active form
• Only the racemate is active and individual enantiomers lack affinity

Correct Answer: The S-enantiomer has greater COX inhibitory activity; R can be enzymatically inverted to S in vivo

Q11. A key amino-acid difference between COX-1 and COX-2 that creates the additional COX-2 side pocket is:

• The substitution of Val for Ile at a residue lining the active site (Val523 vs Ile523)
• A serine to threonine substitution at position 120
• The presence of a cysteine residue only in COX-1
• Loss of the Arg120 anchor in COX-2

Correct Answer: The substitution of Val for Ile at a residue lining the active site (Val523 vs Ile523)

Q12. Rofecoxib (withdrawn COX-2 inhibitor) is characterized by which para‑aryl substituent important for COX-2 binding?

• A para-methylsulfone (–SO2CH3) group on one aryl ring
• A para-carboxylate group (–COOH)
• A para-hydroxyl group (–OH)
• A para-amino group (–NH2)

Correct Answer: A para-methylsulfone (–SO2CH3) group on one aryl ring

Q13. Which aromatic pharmacophore is essential for high potency of endogenous catecholamine agonists (e.g., norepinephrine) at adrenergic receptors?

• A single para-hydroxyl substituent
• A 3,4-dihydroxy (catechol) motif on the benzene ring
• An unsubstituted phenyl ring only
• A para-nitro group to increase acidity

Correct Answer: A 3,4-dihydroxy (catechol) motif on the benzene ring

Q14. Introducing an α‑methyl group into phenethylamine-derived sympathomimetics (e.g., amphetamine vs phenethylamine) primarily:

• Increases susceptibility to COMT metabolism
• Protects the molecule from MAO metabolism, increasing oral activity and duration
• Converts the agent into a muscarinic ligand
• Removes activity at adrenergic receptors altogether

Correct Answer: Protects the molecule from MAO metabolism, increasing oral activity and duration

Q15. The acidic carboxylate of classical NSAIDs (e.g., diclofenac, indomethacin) interacts in the COX active site primarily with which residue?

• Ser530
• Arg120
• Cys521
• Gly526

Correct Answer: Arg120

Q16. According to muscarinic agonist SAR, which arrangement is typically required for high affinity at muscarinic receptors?

• A positively charged nitrogen separated from an ester-like oxygen by two carbon atoms
• A quaternary ammonium directly bonded to an aromatic ring without spacer
• An aldehyde functional group adjacent to the ammonium
• A long flexible polyethylene glycol spacer (>6 carbons) between the N and oxygen

Correct Answer: A positively charged nitrogen separated from an ester-like oxygen by two carbon atoms

Q17. Which of the following drugs is a prototypical selective COX-2 inhibitor used to illustrate COX-2 directed SAR?

• Celecoxib
• Aspirin
• Indomethacin
• Ketoprofen

Correct Answer: Celecoxib

Q18. A widely accepted mechanistic rationale for the increased cardiovascular risk observed with some COX-2 selective inhibitors is:

• Enhanced platelet inhibition via COX-1 overactivation
• Selective suppression of endothelial prostacyclin (PGI2) with preserved platelet thromboxane A2, shifting balance toward thrombosis
• Direct toxic effects on myocardial calcium channels
• Induction of severe systemic hypotension causing ischemia

Correct Answer: Selective suppression of endothelial prostacyclin (PGI2) with preserved platelet thromboxane A2, shifting balance toward thrombosis

Q19. Which beta‑blocking agent is known to possess intrinsic sympathomimetic activity (partial agonist), an SAR property associated with reduced resting bradycardia?

• Pindolol
• Propranolol
• Metoprolol
• Atenolol

Correct Answer: Pindolol

Q20. Carbamate inhibitors of acetylcholinesterase (e.g., neostigmine) differ from organophosphates in that:

• Carbamates produce permanent phosphorylation of the enzyme
• Carbamate-mediated carbamylation is reversible and the enzyme can be regenerated over time
• Carbamates have no effect on cholinesterase activity
• Carbamates irreversibly alkylate DNA as their primary mechanism

Correct Answer: Carbamate-mediated carbamylation is reversible and the enzyme can be regenerated over time

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