Introduction: Idoxuridine and Acyclovir are important antiviral agents studied in B. Pharm courses for their distinct chemistry, synthesis, and structure–activity relationship (SAR). Idoxuridine is a 5‑iodinated pyrimidine nucleoside used topically against DNA viruses; its synthesis involves selective 5‑iodination and nucleoside construction, while SAR highlights the crucial 5‑halogen and deoxyribose for activity. Acyclovir is an acyclic guanosine analogue activated by viral thymidine kinase; its synthesis and prodrug design (valacyclovir) improve oral bioavailability. Key keywords: Idoxuridine, Acyclovir, synthesis, nucleoside analog, SAR, antiviral mechanism, phosphorylation, thymidine kinase, prodrug. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What is the chemical identity of idoxuridine?
- 5‑iodo‑2’‑deoxyuridine
- 9‑(2‑hydroxyethoxymethyl)guanine
- 5‑fluoro‑2’‑deoxyuridine
- L‑valyl ester of acyclovir
Correct Answer: 5‑iodo‑2’‑deoxyuridine
Q2. Which feature is essential in acyclovir’s structure for selective activation by viral enzymes?
- Aromatic halogen substitution on the base
- An acyclic side chain lacking a complete sugar ring
- 5’‑Methylation of the sugar
- Ribose 2’‑OH group
Correct Answer: An acyclic side chain lacking a complete sugar ring
Q3. Which enzyme primarily phosphorylates acyclovir to its monophosphate form in infected cells?
- Viral thymidine kinase
- Cellular adenosine kinase
- DNA polymerase
- Guanine deaminase
Correct Answer: Viral thymidine kinase
Q4. In idoxuridine SAR, what is the main role of the 5‑iodo substituent on the uracil ring?
- Enhances incorporation into viral DNA and increases mutagenicity
- Prevents cellular uptake
- Improves oral bioavailability
- Acts as a prodrug moiety
Correct Answer: Enhances incorporation into viral DNA and increases mutagenicity
Q5. A common laboratory route to introduce iodine at the 5‑position of deoxyuridine uses which reagent class?
- Electrophilic iodinating agents (e.g., I2 with oxidant or N‑iodosuccinimide)
- Nucleophilic fluorinating agents
- Organometallic lithium reagents for substitution
- Peracid epoxidation
Correct Answer: Electrophilic iodinating agents (e.g., I2 with oxidant or N‑iodosuccinimide)
Q6. The Koenigs–Knorr or silyl‑Hilbert–Johnson methods are referenced in nucleoside synthesis for what purpose?
- Glycosylation to form the N‑glycosidic bond between base and sugar
- Oxidation of alcohols to ketones
- Selective halogenation of heterocycles
- Hydrogenation of double bonds
Correct Answer: Glycosylation to form the N‑glycosidic bond between base and sugar
Q7. Which SAR principle explains why acyclovir is more selective for virus‑infected cells than for uninfected cells?
- It is a strong inhibitor of human DNA polymerase
- Its initial phosphorylation depends on viral kinase, concentrating active drug in infected cells
- Its bulky sugar prevents cellular uptake
- Its halogen substituent targets viral membranes
Correct Answer: Its initial phosphorylation depends on viral kinase, concentrating active drug in infected cells
Q8. Which of the following best describes the mechanism by which acyclovir inhibits viral replication?
- Competitive inhibition of viral protease
- Chain termination after incorporation by viral DNA polymerase
- Intercalation into viral RNA
- Inhibition of viral entry into host cell
Correct Answer: Chain termination after incorporation by viral DNA polymerase
Q9. Valacyclovir is a prodrug of acyclovir. What is the main pharmacokinetic advantage of valacyclovir?
- Decreased renal clearance compared to acyclovir
- Higher oral bioavailability via peptide transporter uptake
- Longer intracellular half‑life without activation
- Direct antiviral activity without conversion
Correct Answer: Higher oral bioavailability via peptide transporter uptake
Q10. Resistance to acyclovir in herpes viruses is most commonly due to mutations in which gene?
- Viral thymidine kinase gene
- Viral RNA polymerase gene
- Host cytochrome P450 gene
- Viral integrase gene
Correct Answer: Viral thymidine kinase gene
Q11. During idoxuridine synthesis, protecting groups on the sugar hydroxyls are used primarily to:
- Enhance water solubility of the final product
- Direct selective glycosylation and prevent side reactions
- Introduce the iodine atom at 5‑position
- Facilitate ring opening of uracil
Correct Answer: Direct selective glycosylation and prevent side reactions
Q12. Which structural modification on guanine analogs typically reduces antiviral potency against HSV?
- Maintaining the 2‑amino group on the purine ring
- Replacing the acyclic side chain with a bulky aromatic group
- Keeping the N9 linkage intact
- Retaining the heterocyclic base hydrogen bond pattern
Correct Answer: Replacing the acyclic side chain with a bulky aromatic group
Q13. Idoxuridine’s clinical use is limited compared to acyclovir mainly because:
- Idoxuridine has higher oral bioavailability
- Idoxuridine is more mutagenic and less selective than acyclovir
- Idoxuridine is a prodrug requiring viral enzymes
- Idoxuridine targets RNA viruses preferentially
Correct Answer: Idoxuridine is more mutagenic and less selective than acyclovir
Q14. In acyclovir activation, after viral thymidine kinase forms acyclovir monophosphate, which cellular enzymes complete activation to the triphosphate?
- Cellular kinases (guanylate kinase and other kinases)
- Viral proteases
- CYP450 enzymes
- Topoisomerases
Correct Answer: Cellular kinases (guanylate kinase and other kinases)
Q15. Which reagent choice is most appropriate for selective iodination of an activated pyrimidine at the 5‑position under mild conditions?
- N‑iodosuccinimide (NIS)
- Sodium borohydride
- m‑CPBA (meta‑chloroperoxybenzoic acid)
- Grignard reagent (RMgBr)
Correct Answer: N‑iodosuccinimide (NIS)
Q16. Which property of acyclovir contributes most to its low oral bioavailability in the parent form?
- High lipophilicity and membrane retention
- Poor intestinal absorption due to polar character and limited transport
- Extensive first‑pass metabolism to active metabolites
- High plasma protein binding
Correct Answer: Poor intestinal absorption due to polar character and limited transport
Q17. For synthetic modification of idoxuridine to study SAR, altering which position on the pyrimidine ring is most informative?
- 5‑position halogen or substituent
- N1 position alkylation
- 2’‑OH group modification on ribose
- 5’‑phosphate esterification only
Correct Answer: 5‑position halogen or substituent
Q18. Which analytical technique is most useful to confirm the incorporation of iodine at the 5‑position in idoxuridine synthesis?
- Mass spectrometry and NMR spectroscopy
- Infrared spectroscopy only
- Polarimetry exclusively
- Thin layer chromatography without standards
Correct Answer: Mass spectrometry and NMR spectroscopy
Q19. Acyclovir triphosphate primarily inhibits viral DNA polymerase by:
- Covalently modifying the polymerase active site
- Competing with dGTP and causing chain termination after incorporation
- Binding to viral envelope proteins
- Blocking nucleotide synthesis de novo
Correct Answer: Competing with dGTP and causing chain termination after incorporation
Q20. In SAR studies, which change to the sugar moiety of a nucleoside analog typically abolishes antiviral activity?
- Complete removal of the sugar (no substitute chain)
- Replacement with a small acyclic chain that supports phosphorylation
- Minor stereochemical inversion at non‑critical centers
- Introduction of a bioisosteric hydroxymethyl group
Correct Answer: Complete removal of the sugar (no substitute chain)
Q21. The rationale for designing valacyclovir involved conjugating acyclovir to which type of promoiety?
- An amino acid (L‑valine) to exploit peptide transporters
- A phosphate group to increase polarity
- Polyethylene glycol to reduce renal clearance
- A lipid chain to increase fat solubility
Correct Answer: An amino acid (L‑valine) to exploit peptide transporters
Q22. During laboratory synthesis, which protecting group is commonly used for hydroxyl functions on sugars to allow selective reactions?
- Silyl ethers (e.g., TBDMS) or acyl groups (e.g., acetyl)
- Nitro groups
- Nitrile protection
- Methylcarbamate groups for bases only
Correct Answer: Silyl ethers (e.g., TBDMS) or acyl groups (e.g., acetyl)
Q23. Which adverse effect is more associated with systemic nucleoside analog therapy compared to topical idoxuridine?
- Systemic nephrotoxicity or crystal nephropathy with high IV doses
- Localized ocular irritation only
- Complete hair loss as a common effect
- Immediate anaphylaxis in most patients
Correct Answer: Systemic nephrotoxicity or crystal nephropathy with high IV doses
Q24. Which structural requirement of guanine analogs is critical for hydrogen bonding with viral polymerase/nascent strand?
- Presence of the 2‑amino and 6‑oxo (or equivalent) functionalities on the purine ring
- Replacement of nitrogen atoms with carbon
- Large lipophilic substituents at C8
- Absence of hydrogen bond donors on the base
Correct Answer: Presence of the 2‑amino and 6‑oxo (or equivalent) functionalities on the purine ring
Q25. For teaching purposes, which step is emphasized as key in converting a guanine derivative into acyclovir in synthetic schemes?
- Installing the acyclic hydroxyalkyl side chain at N9 via alkylation
- Oxidation of guanine to xanthine
- Halogenation at C8 of the purine
- Glycosylation with ribose via Koenigs–Knorr
Correct Answer: Installing the acyclic hydroxyalkyl side chain at N9 via alkylation
Q26. Which factor is least relevant when designing nucleoside analogs for improved selectivity?
- Specificity for viral kinases over host kinases
- Ability to be incorporated by viral polymerase but not host polymerases
- Enhancing general cytotoxicity to rapidly kill host cells
- Optimizing prodrug strategies for targeted uptake
Correct Answer: Enhancing general cytotoxicity to rapidly kill host cells
Q27. In structure–activity relationship studies of idoxuridine analogs, replacing iodine with a smaller halogen like fluorine typically results in:
- Different electronic effects and usually reduced antiviral incorporation potency
- Improved mutagenic potential and selectivity
- Complete conversion to a guanine analog
- No change in biological activity
Correct Answer: Different electronic effects and usually reduced antiviral incorporation potency
Q28. Which laboratory observation would indicate successful conversion of acyclovir to valacyclovir during synthesis?
- Mass increase corresponding to an L‑valyl ester and changed chromatographic retention time
- Disappearance of all hydroxyl signals in NMR without mass change
- Complete loss of UV absorbance of the purine ring
- Spontaneous crystallization into a gas
Correct Answer: Mass increase corresponding to an L‑valyl ester and changed chromatographic retention time
Q29. Which therapeutic advantage does acyclovir have over idoxuridine for systemic herpes infections?
- Oral and systemic activity with favorable selectivity and lower host toxicity
- Higher topical ocular potency only
- Ability to target RNA polymerase directly
- Longer environmental stability as a topical agent
Correct Answer: Oral and systemic activity with favorable selectivity and lower host toxicity
Q30. When discussing SAR for nucleoside antivirals in exams, which key concept should students always emphasize?
- Balance between structural mimicry of natural nucleosides and modifications that confer selectivity and proper activation
- Maximizing lipophilicity for all antiviral drugs
- Avoiding any prodrug strategies as they are ineffective
- Favoring bulky aromatic substitutions to enhance incorporation
Correct Answer: Balance between structural mimicry of natural nucleosides and modifications that confer selectivity and proper activation
