Lead discovery based on drug metabolism MCQs With Answer

Drug metabolism is central to lead discovery because metabolic stability, bioactivation and ADME determine a candidate’s safety and efficacy. This concise, keyword-rich set of MCQs for B. Pharm students explores phase I and II reactions, major CYP450 isoenzymes, in vitro models (microsomes, hepatocytes), metabolite identification by LC-MS/MS, IVIVE and PBPK concepts, drug–drug interactions, genetic polymorphisms, transporter interplay, and strategies to improve metabolic profiles such as deuteration, bioisosteres and soft-spot modification. Questions focus on interpreting metabolic data, predicting clearance, recognizing reactive metabolites and applying design approaches to optimize leads and practical applications in drug development. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which reaction is a typical phase I metabolic transformation?

• Oxidation by cytochrome P450 enzymes
• Glucuronidation by UGT enzymes
• Sulfation by SULT enzymes
• Conjugation with glutathione

Correct Answer: Oxidation by cytochrome P450 enzymes

Q2. Which in vitro system is most suitable for studying both phase I and phase II metabolism?

• Human liver microsomes
• Recombinant single CYP450 enzyme
• Primary human hepatocytes
• Plasma protein binding assay

Correct Answer: Primary human hepatocytes

Q3. A metabolic “soft spot” refers to:

• A region in a drug molecule prone to rapid metabolic conversion
• A high-affinity binding site on a metabolic enzyme
• An inactive metabolite
• A site on a transporter that binds drug

Correct Answer: A region in a drug molecule prone to rapid metabolic conversion

Q4. Which CYP450 isoform is most commonly associated with polymorphic metabolism affecting many CNS drugs?

• CYP3A4
• CYP1A2
• CYP2D6
• CYP2E1

Correct Answer: CYP2D6

Q5. In vitro intrinsic clearance (CLint) measured in microsomes is primarily used to predict:

• Oral bioavailability directly without scaling
• Potential hepatic metabolic clearance after scaling to in vivo
• Renal clearance
• Plasma protein binding percentage

Correct Answer: Potential hepatic metabolic clearance after scaling to in vivo

Q6. Which analytical technique is standard for metabolite identification and structural elucidation?

• UV-visible spectroscopy
• LC-MS/MS coupled with high-resolution mass spectrometry
• Gel electrophoresis
• Northern blotting

Correct Answer: LC-MS/MS coupled with high-resolution mass spectrometry

Q7. Glucuronidation is catalyzed by which enzyme family?

• SULT (sulfotransferases)
• UGT (UDP-glucuronosyltransferases)
• GST (glutathione S-transferases)
• CYP450 (cytochrome P450s)

Correct Answer: UGT (UDP-glucuronosyltransferases)

Q8. Which strategy is commonly used to reduce metabolic liability at a benzylic oxidation site?

• Introduce a hydrogen bond donor
• Replace the benzylic hydrogen with deuterium or a fluorine-containing bioisostere
• Add a bulky polar group to increase clearance
• Increase lipophilicity by adding alkyl chains

Correct Answer: Replace the benzylic hydrogen with deuterium or a fluorine-containing bioisostere

Q9. A reactive metabolite that forms covalent adducts often requires which additional in vitro test for identification?

• Glucuronidation assay
• GSH trapping assay with LC-MS detection
• Protein precipitation assay
• pKa determination

Correct Answer: GSH trapping assay with LC-MS detection

Q10. Which factor most strongly influences species differences in drug metabolism?

• Identical transporter expression across species
• Differences in enzyme expression and substrate specificity across species
• Same hepatic blood flow values
• Uniform plasma protein binding across species

Correct Answer: Differences in enzyme expression and substrate specificity across species

Q11. Which parameter is essential for IVIVE (in vitro–in vivo extrapolation) to predict human clearance?

• Microsomal protein per gram liver and liver weight scaling factors
• pH of the assay buffer only
• UV absorbance of parent compound
• Manufacturer of microsomes

Correct Answer: Microsomal protein per gram liver and liver weight scaling factors

Q12. Which transporter often contributes to hepatic uptake and affects metabolic clearance?

• P-glycoprotein (P-gp)
• OATP (organic anion transporting polypeptides)
• Na+/K+ ATPase
• GLUT transporters

Correct Answer: OATP (organic anion transporting polypeptides)

Q13. Which outcome suggests time-dependent inhibition of a CYP enzyme in vitro?

• Immediate increase in enzyme activity upon incubation
• Progressive loss of activity with pre-incubation and NADPH dependence
• No change after prolonged incubation
• Reversible inhibition that disappears after dilution

Correct Answer: Progressive loss of activity with pre-incubation and NADPH dependence

Q14. Why is metabolic stability important during lead optimization?

• Low metabolic stability guarantees better efficacy
• Appropriate metabolic stability can improve half-life, exposure and reduce dosing frequency
• Metabolic stability only affects formulation, not pharmacokinetics
• High clearance always improves safety

Correct Answer: Appropriate metabolic stability can improve half-life, exposure and reduce dosing frequency

Q15. Prodrugs are designed to:

• Directly inhibit CYP450 enzymes
• Be inactive precursors that are metabolically converted to active drugs to improve ADME
• Increase renal elimination only
• Prevent any metabolic transformation

Correct Answer: Be inactive precursors that are metabolically converted to active drugs to improve ADME

Q16. Which CYP isoform is primarily responsible for metabolizing the largest share of marketed drugs?

• CYP2C9
• CYP3A4
• CYP2E1
• CYP2B6

Correct Answer: CYP3A4

Q17. Which outcome from a hepatocyte clearance assay indicates low hepatic clearance potential?

• Very rapid disappearance of parent compound
• Apparent half-life much longer than 120 minutes in human hepatocytes
• Formation of many polar metabolites quickly
• Extensive protein binding in hepatocyte medium

Correct Answer: Apparent half-life much longer than 120 minutes in human hepatocytes

Q18. Which conjugation pathway often detoxifies electrophilic metabolites by direct conjugation?

• Glucuronidation by UGT
• Glutathione conjugation via GST
• Sulfation by SULT
• Oxidation by CYP

Correct Answer: Glutathione conjugation via GST

Q19. Deuteration of metabolically labile C–H bonds primarily results in:

• Increased polarity of the molecule
• Reduced rate of metabolic oxidation at that position due to kinetic isotope effect
• Complete elimination of CYP-mediated metabolism
• Higher renal clearance

Correct Answer: Reduced rate of metabolic oxidation at that position due to kinetic isotope effect

Q20. In PBPK modeling for clearance prediction, which input is critical?

• Only the chemical synthesis route
• Physicochemical properties, in vitro metabolic parameters and physiological system data
• High-resolution NMR data exclusively
• Marketing authorization status

Correct Answer: Physicochemical properties, in vitro metabolic parameters and physiological system data

Q21. Covalent binding of metabolites to proteins is a red flag because it may indicate:

• Improved oral bioavailability
• Potential for idiosyncratic toxicity or immune response
• Enhanced metabolic clearance
• Reduced protein binding

Correct Answer: Potential for idiosyncratic toxicity or immune response

Q22. Which approach can be used to identify the site of metabolism on a small molecule?

• Incubation with UGT inhibitors only
• Metabolite mapping using LC-MS/MS fragment analysis and comparison with synthesized standards
• Measuring aqueous solubility at pH 7.4 only
• Determining melting point

Correct Answer: Metabolite mapping using LC-MS/MS fragment analysis and comparison with synthesized standards

Q23. Which in vitro system lacks phase II conjugation capacity but is useful for studying CYP-mediated metabolism?

• Primary human hepatocytes
• Human liver microsomes (with UDPGA or PAPS added as needed)
• Recombinant single CYP enzymes or microsomes without cofactors
• Whole blood

Correct Answer: Recombinant single CYP enzymes or microsomes without cofactors

Q24. Which metabolic change would likely increase a compound’s renal excretion?

• Conversion to a more lipophilic metabolite
• Formation of a polar conjugated metabolite (e.g., glucuronide)
• Removal of a polar group
• Increasing molecular weight above 1000 Da

Correct Answer: Formation of a polar conjugated metabolite (e.g., glucuronide)

Q25. A strong CYP3A4 inhibitor co-administered with a CYP3A4-metabolized drug will typically cause:

• Decreased exposure of the drug
• Increased exposure and potential toxicity of the drug
• No change in pharmacokinetics
• Immediate elimination of the drug via lungs

Correct Answer: Increased exposure and potential toxicity of the drug

Q26. Which probe substrate is commonly used to assess CYP2D6 activity in vitro?

• Midazolam
• Diclofenac
• Dextromethorphan
• Acetaminophen

Correct Answer: Dextromethorphan

Q27. Which metabolic liability is best addressed by introducing steric hindrance near the metabolized site?

• High aqueous solubility
• Rapid oxidative metabolism at an exposed site
• Excessive basicity
• Low logP only

Correct Answer: Rapid oxidative metabolism at an exposed site

Q28. In a metabolite safety assessment, which metabolites require further toxicological evaluation?

• Metabolites present at negligible levels compared to parent
• Human-specific metabolites or those exceeding a defined exposure threshold relative to parent
• All metabolites regardless of exposure or relevance
• Only metabolites detected in rodent plasma

Correct Answer: Human-specific metabolites or those exceeding a defined exposure threshold relative to parent

Q29. Which experimental observation suggests a drug is a P450 inducer in vivo?

• Decreased clearance of co-administered P450 substrates
• Increased clearance of co-administered P450 substrates after repeated dosing
• No change in pharmacokinetics after repeated dosing
• Immediate hypersensitivity reaction

Correct Answer: Increased clearance of co-administered P450 substrates after repeated dosing

Q30. Which design tactic helps avoid formation of reactive quinone-imine metabolites from phenolic precursors?

• Increase electron density on the aromatic ring
• Mask the phenol as a conjugatable prodrug or replace with a less oxidizable bioisostere
• Add an ortho-alkyl group to promote oxidation
• Remove polar functionality to increase lipophilicity

Correct Answer: Mask the phenol as a conjugatable prodrug or replace with a less oxidizable bioisostere

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