What is the mechanism of action of oseltamivir and zanamivir?

Explanation

## Correct Answer: C. Neuraminidase inhibition Oseltamivir (Tamiflu) and zanamivir (Relenza) are **neuraminidase inhibitors** — a class of antiviral agents that target influenza virus neuraminidase, a surface glycoprotein essential for viral release from infected cells. Neuraminidase cleaves sialic acid receptors on the host cell membrane, allowing newly synthesized viral particles to detach and spread to adjacent cells. By inhibiting this enzyme, these drugs prevent viral egress, trapping virions on the cell surface and blocking further infection. Both agents are effective against influenza A and B, and are particularly valuable in India during seasonal flu outbreaks and pandemic preparedness. Oseltamivir is administered orally (75 mg twice daily for 5 days), while zanamivir is inhaled. These drugs must be started within 48 hours of symptom onset for maximum efficacy. They reduce symptom duration by approximately 1 day and lower the risk of complications in high-risk patients (elderly, immunocompromised, pregnant women). Unlike antiretrovirals or antibiotics that target host or bacterial machinery, neuraminidase inhibitors are virus-specific, making them a cornerstone of influenza management in Indian clinical practice. ## Why the other options are wrong **A. DNA polymerase inhibition** — This is wrong because influenza is an RNA virus, not a DNA virus, so it does not encode or require DNA polymerase. DNA polymerase inhibitors (e.g., acyclovir for HSV, ganciclovir for CMV) target DNA viruses. This is a classic NBE trap pairing antiviral mechanisms with incorrect viral targets — students may confuse oseltamivir with nucleoside analogues used for herpes or HIV. **B. Protein synthesis inhibition** — This is wrong because oseltamivir and zanamivir do not inhibit viral or host protein synthesis. Protein synthesis inhibitors (e.g., chloramphenicol, tetracyclines) target bacterial ribosomes or are used in some antiparasitic contexts, not antiviral therapy. This option may trap students who confuse broad antiviral categories or conflate different drug classes. **D. Nucleotide analogue** — This is wrong because oseltamivir and zanamivir are not nucleotide analogues. Nucleotide analogues (e.g., acyclovir, AZT, tenofovir) mimic natural nucleotides and inhibit viral polymerases. Oseltamivir and zanamivir are small-molecule sialic acid analogues that directly inhibit neuraminidase enzyme activity — a fundamentally different mechanism. NBE may use this to test whether students confuse antiviral drug classes. ## High-Yield Facts - **Neuraminidase inhibitors** (oseltamivir, zanamivir) prevent viral release by blocking sialic acid cleavage on infected cell membranes. - **Oseltamivir dosing**: 75 mg orally twice daily for 5 days; must start within 48 hours of symptom onset for maximum benefit. - **Zanamivir** is inhaled (5 mg × 2 inhalations twice daily × 5 days); contraindicated in chronic respiratory disease due to bronchospasm risk. - **Efficacy**: Reduces symptom duration by ~1 day and prevents complications in high-risk groups (elderly, pregnant, immunocompromised). - **Influenza is an RNA virus** — requires RNA-dependent RNA polymerase, not DNA polymerase; neuraminidase is a surface glycoprotein, not a polymerase. - **Resistance**: Emerges via mutations in neuraminidase or hemagglutinin; H1N1 pandemic strains showed oseltamivir resistance in some regions. ## Mnemonics **NANI = Neuraminidase Inhibitors** **N**euraminidase **A**ntiviral **N**ew **I**nhibitors — oseltamivir and zanamivir block neuraminidase to prevent viral release. Use this when you see 'oseltamivir' or 'zanamivir' in a question. **48-Hour Window** Start neuraminidase inhibitors **within 48 hours** of symptom onset — this is the critical time window for efficacy in Indian clinical practice. After 48 hours, benefit is minimal. ## NBE Trap NBE pairs neuraminidase inhibitors with DNA polymerase inhibition or nucleotide analogues to exploit confusion between antiviral drug classes. Students may conflate oseltamivir (enzyme inhibitor) with acyclovir or AZT (nucleotide analogues) if they lack clear mechanistic understanding of influenza virology. ## Clinical Pearl In Indian hospitals during flu season, oseltamivir is the first-line choice for symptomatic influenza in high-risk patients (pregnant women, elderly, immunocompromised). Starting it within 48 hours of fever onset can prevent progression to pneumonia and reduce ICU admissions — a critical intervention in resource-limited settings. _Reference: KD Tripathi Pharmacology Ch. 59 (Antivirals); Harrison Principles of Internal Medicine Ch. 195 (Influenza)_