Pharmacology · Antiviral Pharmacology

Influenza and Respiratory Virus Pharmacology

Mechanisms, resistance, and clinical use across respiratory antiviral agents

The adamantanes (amantadine, rimantadine) are clinically obsolete for influenza — the S31N M2 channel mutation is now present in essentially all circulating H3N2 and pandemic H1N1 strains, conferring universal resistance; amantadine retains value for Parkinson's disease and drug-induced extrapyramidal symptoms through a completely separate dopaminergic mechanism, but should never be used for influenza treatment or prophylaxis.
Oseltamivir and zanamivir inhibit neuraminidase by mimicking the transition state of sialic acid cleavage — without functional neuraminidase, newly assembled virions remain tethered to the infected cell surface and to adjacent cells, preventing efficient spread; both are active against influenza A and B, and oseltamivir's H275Y cross-resistance (which spares zanamivir) is explained by the structural difference in how each drug interacts with the N1 binding pocket.
Baloxavir marboxil introduces a mechanistically distinct target — the cap-dependent endonuclease (CEN) of the viral PA subunit — blocking cap-snatching, which is how influenza steals 5′ cap structures from host mRNA to prime viral mRNA synthesis; this mechanism is entirely independent of neuraminidase, so baloxavir retains full activity against H275Y oseltamivir-resistant strains, though its own resistance mutation (PA-I38T) emerges in approximately 2–23% of treated patients.
Nirsevimab has displaced palivizumab as the standard RSV prophylaxis for healthy infants — its Fc-engineered extended half-life allows a single injection per RSV season (versus monthly palivizumab injections), and it targets a highly conserved prefusion F protein epitope with superior potency; palivizumab is now reserved for high-risk groups including infants born at 28 weeks gestation or less and those with hemodynamically significant congenital heart disease.
Nirmatrelvir-ritonavir's 89% efficacy against COVID-19 hospitalization rests on protease inhibition — nirmatrelvir blocks the SARS-CoV-2 main protease that cleaves the viral polyprotein, and ritonavir pharmacokinetically boosts nirmatrelvir by CYP3A4 inhibition; this boosting mechanism makes nirmatrelvir-ritonavir subject to the same drug interaction risks as any boosted PI regimen, with tacrolimus, cyclosporine, and lipid-lowering statins among the most clinically consequential co-administered drugs.

Abbreviations: NA = neuraminidase · HA = hemagglutinin · CEN = cap-dependent endonuclease · RdRp = RNA-dependent RNA polymerase · RSV = respiratory syncytial virus · F protein = fusion protein · SARS-CoV-2 = severe acute respiratory syndrome coronavirus 2 · Mpro = main protease · IMPDH = inosine monophosphate dehydrogenase · HSCT = hematopoietic stem cell transplant

Influenza — Three Drug Target Classes

M2 Ion Channel — Obsolete

Adamantanes

Amantadine and rimantadine block M2 proton channel → prevent viral uncoating; influenza A only (B lacks M2)
OBSOLETE for influenza: S31N resistance is universal in current H3N2 and pandemic H1N1 strains — do not use for influenza treatment or prophylaxis
Amantadine retains non-antiviral clinical uses: Parkinson's disease treatment and drug-induced extrapyramidal symptoms — separate dopaminergic mechanism

Neuraminidase Inhibitors

Oseltamivir, Zanamivir, Peramivir

Transition-state analogues — competitively inhibit NA active site → virions remain tethered to cell surface, preventing spread
Active against influenza A and B; start within 48 hours of symptom onset for maximum benefit
Oseltamivir: oral prodrug; renal dose adjustment required if CrCl <30 mL/min
Zanamivir: inhaled only — avoid in asthma or COPD due to bronchospasm risk
Peramivir: IV single dose; for hospitalized patients unable to take oral or inhaled agents

Cap-Dependent Endonuclease

Baloxavir Marboxil

Inhibits CEN of PA subunit → blocks cap-snatching → halts all viral mRNA transcription at its origin
Single oral dose (40 mg if <80 kg; 80 mg if ≥80 kg) — superior convenience vs. 5-day oseltamivir course
Active against influenza A and B; retains full activity against H275Y oseltamivir-resistant strains
Resistance: PA-I38T/F/M → reduces baloxavir CEN binding; emerges in ~2–9% adults, up to 23% children
Approved for treatment and post-exposure prophylaxis

Neuraminidase Inhibitor Resistance

Oseltamivir Resistance

H275Y Mutation — N1 Neuraminidase

H275Y in N1 neuraminidase: high-level oseltamivir resistance with largely preserved zanamivir susceptibility
Structural basis: oseltamivir requires rotation of Glu276 in the N1 binding pocket; H275Y sterically prevents this rotation; zanamivir does not depend on Glu276 movement
Global spread of H275Y in 2008–2009 seasonal H1N1 occurred without drug selection pressure — demonstrates resistant strains can transmit efficiently even at fitness cost
Management of confirmed H275Y resistance: switch to zanamivir inhaled or IV peramivir; baloxavir retains full activity

Baloxavir Resistance

PA-I38 Variants

PA-I38T/F/M reduce baloxavir acid affinity for CEN active site → prolonged viral shedding, slower symptom resolution
Emergence rate: ~2–9% in adults, up to 23% in children in some studies — higher in pediatric patients
PA-I38 variants retain full neuraminidase inhibitor susceptibility — no cross-resistance
Fitness cost of PA-I38T is moderate — limits but does not prevent community spread
Management: neuraminidase inhibitors if baloxavir resistance confirmed or clinically suspected

RSV — Prophylaxis and Treatment

RSV Prophylaxis

Monoclonal Antibodies

Nirsevimab (Beyfortus)Long-acting mAb targeting highly conserved prefusion F protein epitope; single IM injection per RSV season; ~74–83% efficacy; ACIP-recommended for all infants under 8 months entering first RSV season
Fc-engineered extended half-life provides season-long protection from a single dose — replaces the monthly palivizumab dosing burden for most infants
Palivizumab (reserved)Now reserved for high-risk groups: gestational age ≤28 weeks (first year), chronic lung disease of prematurity, hemodynamically significant congenital heart disease
Palivizumab: monthly IM 15 mg/kg during RSV season; ~55% reduction in RSV hospitalization

RSV Treatment

Ribavirin

Broad-spectrum nucleoside analogue: inhibits IMPDH (depletes GTP pool), inhibits viral RdRp, causes lethal mutagenesis
Inhaled via SPAG-2 device for respiratory virus infections
Weak evidence of benefit in otherwise healthy infants — largely abandoned in routine pediatric RSV
Reserved for immunocompromised patients with severe RSV lower respiratory tract disease — especially HSCT recipients (observational benefit)
Teratogenic: healthcare workers of childbearing potential require respiratory protection during administration

COVID-19 Antivirals — Key Mechanisms

Oral Protease Inhibitor

Nirmatrelvir-Ritonavir (Paxlovid)

Nirmatrelvir: peptidomimetic inhibitor of SARS-CoV-2 main protease (Mpro/3CLpro) → blocks viral polyprotein cleavage → halts replication
Ritonavir: CYP3A4/P-gp inhibitor added as pharmacokinetic booster only — no antiviral activity at this dose
EPIC-HR trial: 89% reduction in hospitalization or death in high-risk unvaccinated adults treated within 3 days of symptom onset
Critical drug interactions: raises concentrations of all CYP3A4 substrates — tacrolimus, cyclosporine, simvastatin, lovastatin, many anticoagulants require management
COVID-19 rebound of symptoms and viral load reported after completing 5-day course — mechanism under investigation

IV RdRp Inhibitor

Remdesivir

Phosphoramidate prodrug of adenosine analogue → activated to triphosphate → delayed chain termination of viral RdRp
Broad-spectrum RNA virus activity; originally developed for Ebola and hepatitis C
IV administration for most indications; 3-day outpatient course available for high-risk COVID-19
Greatest clinical benefit in COVID-19: hospitalized patients on low-flow oxygen who have not yet required mechanical ventilation
Also investigated for RSV and other RNA respiratory virus infections

RNA Virus Resistance — General Principle

RNA respiratory viruses (influenza, SARS-CoV-2) use error-prone RdRp — every possible single-nucleotide mutation is generated many times daily within a single infected host. Resistant variants pre-exist before drug exposure; treatment selects and amplifies them. The fitness cost of the resistance mutation determines whether resistant strains spread in the community: low fitness cost (influenza S31N) allowed global spread without drug pressure; moderate fitness cost (baloxavir PA-I38T) limits but does not prevent spread.

Clinical implication: stockpile mechanistically diverse agents — neuraminidase inhibitors, CEN inhibitors, M2 blockers (when effective), RdRp inhibitors, protease inhibitors — so that resistance to one class does not eliminate all treatment options. Establish resistance surveillance before widespread deployment of any new agent.

Suggested References

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Brunton L, Knollmann B, Hilal-Dandan R, eds.	Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. — Chapter 56: Antiviral Agents (Non-Retroviral)	McGraw-Hill; 2023
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Hammitt LL et al.	Nirsevimab for prevention of RSV in healthy late-preterm and term infants	N Engl J Med. 2022
Waghmare A et al.	Respiratory syncytial virus lower respiratory disease in hematopoietic cell transplant recipients	Clin Infect Dis. 2013
Hammond J et al.	Oral nirmatrelvir for high-risk, nonhospitalized adults with Covid-19	N Engl J Med. 2022