The development of antiviral drugs has been one of the great successes of modern medicine, transforming human immunodeficiency virus (HIV) infection from a fatal disease to a manageable chronic condition and providing effective treatments for hepatitis B and C, influenza, herpesviruses, and most recently SARS-CoV-2. Antiviral therapy targets essential viral proteins or processes, but the error-prone nature of viral replication, particularly for RNA viruses, drives rapid emergence of drug-resistant variants.
Targets in the Viral Life Cycle
Antiviral drugs can inhibit any step of the viral replication cycle. Entry inhibitors block viral attachment (maraviroc, a CCR5 antagonist for HIV), receptor binding, or membrane fusion (enfuvirtide, a fusion inhibitor peptide for HIV). Uncoating inhibitors such as amantadine and rimantadine block the M2 ion channel of influenza A. Polymerase inhibitors are the largest class of antivirals and include nucleoside and nucleotide analogs that are incorporated into viral DNA or RNA by the viral polymerase, causing chain termination. Acyclovir, a guanosine analog, is phosphorylated by herpes simplex virus thymidine kinase and then by cellular kinases to acyclovir triphosphate, which competitively inhibits the viral DNA polymerase. Tenofovir and entecavir are nucleotide/nucleoside analogs used for HIV and hepatitis B. Sofosbuvir, a uridine nucleotide prodrug, targets the hepatitis C virus NS5B RNA polymerase. Remdesivir, an adenosine analog with a unique mechanism of delayed chain termination, is active against multiple RNA viruses including SARS-CoV-2. Protease inhibitors block viral polyprotein processing, which is essential for producing functional viral proteins in HIV (ritonavir, darunavir), hepatitis C (grazoprevir), and SARS-CoV-2 (nirmatrelvir, the antiviral component of Paxlovid). Integrase strand transfer inhibitors (raltegravir, dolutegravir) block HIV integration into the host genome. Neuraminidase inhibitors (oseltamivir, zanamivir) prevent influenza virus release from infected cells.
HIV Antiretroviral Therapy
The advent of combination antiretroviral therapy (ART) in the mid-1990s revolutionized HIV treatment. Standard ART combines three active drugs from at least two classes, typically two nucleoside reverse transcriptase inhibitors (NRTIs, such as tenofovir disoproxil fumarate and emtricitabine) plus a third agent from a different class, such as an integrase strand transfer inhibitor (dolutegravir, bictegravir), a non-nucleoside reverse transcriptase inhibitor (NNRTI, such as efavirenz), or a protease inhibitor. ART suppresses HIV replication to below detectable levels, allows CD4+ T cell counts to recover, prevents progression to AIDS, and reduces the risk of sexual transmission to zero when viral load is undetectable. Pre-exposure prophylaxis (PrEP) using tenofovir-emtricitabine is highly effective at preventing HIV acquisition in uninfected individuals. Long-acting injectable regimens (cabotegravir and rilpivirine) administered monthly or every two months are alternatives to daily oral therapy. Despite ART, HIV persists in latent reservoirs of resting CD4+ T cells that harbor integrated provirus, which is not eliminated by current drugs and is the major barrier to HIV cure.
Direct-Acting Antivirals for Hepatitis C
The development of direct-acting antivirals (DAAs) for hepatitis C virus (HCV) represents a landmark achievement in antiviral therapy, achieving cure rates exceeding 95% with all-oral, interferon-free regimens of 8–12 weeks. DAAs target three HCV proteins: NS3/4A protease (grazoprevir, glecaprevir), NS5A (ledipasvir, velpatasvir), and NS5B RNA polymerase (sofosbuvir). Combination regimens such as sofosbuvir-velpatasvir are pangenotypic, effective against all six major HCV genotypes. The high cure rates and tolerability of DAA therapy have led the World Health Organization to set a goal of eliminating viral hepatitis as a public health threat by 2030. DAA therapy reduces the risk of liver cirrhosis, hepatocellular carcinoma, and liver-related mortality, though patients with advanced fibrosis remain at risk of hepatocellular carcinoma and require continued surveillance.
Mechanisms of Antiviral Resistance
Drug resistance arises when mutations in the viral target protein reduce drug binding while maintaining essential viral functions. RNA viruses have high mutation rates of 10⁻³–10⁻⁵ per nucleotide per replication due to error-prone RNA-dependent RNA polymerases lacking proofreading activity, producing a swarm of closely related variants termed a quasispecies. Pre-existing resistant variants present at low frequencies in the viral population can be selected under drug pressure and become dominant within days to weeks. Resistance mechanisms include reduced drug binding (mutations in the drug binding site), increased target expression, drug exclusion from cells, and enhanced drug metabolism. In HIV, resistance to NRTIs occurs through mutations that increase discrimination against the nucleoside analog (K65R, M184V) or that promote excision of the incorporated analog (thymidine analog mutations). NNRTI resistance mutations, such as K103N and Y181C, reduce binding to a hydrophobic pocket near the reverse transcriptase active site. Integrase resistance mutations (R263K, G140S/Q148H) reduce dolutegravir binding. In influenza, the H275Y mutation in N1 neuraminidase confers oseltamivir resistance, and the S31N mutation in M2 confers adamantane resistance.
Combination Therapy and Resistance Management
Combination therapy with multiple drugs targeting different viral proteins is the standard approach to prevent resistance, as the probability of a single virus simultaneously acquiring multiple resistance mutations is the product of individual mutation probabilities and therefore extremely low. For HIV, combination ART with three drugs from at least two classes is the standard of care. For HCV, combinations of two or three DAAs with distinct resistance profiles achieve cure rates above 95% and prevent emergence of resistance during treatment. Treatment adherence is critical for resistance prevention, as suboptimal drug levels from missed doses allow low-level viral replication and selection of resistant variants. Therapeutic drug monitoring and resistance testing (genotypic and phenotypic) guide regimen selection and switching in patients with detectable viremia. For influenza, neuraminidase inhibitors remain effective against most circulating strains despite seasonal variation in resistance prevalence, and baloxavir provides an alternative with a novel mechanism.
Novel Antiviral Strategies
Emerging antiviral approaches aim to improve efficacy, broaden activity, and reduce resistance risk. Host-directed therapies target host factors required for viral replication rather than viral proteins, presenting a higher barrier to resistance because host genes do not mutate under drug pressure. Examples include inhibitors of the host protease TMPRSS2, which processes the SARS-CoV-2 spike protein for entry, and inhibitors of cyclophilins, which are required for HCV and HIV replication. Broad-spectrum antivirals targeting conserved viral functions or common host dependencies could provide preparedness against future pandemic threats, including favipiravir (a purine analog active against many RNA viruses) and GS-5734 (remdesivir). Monoclonal antibody therapies, such as bamlanivimab for SARS-CoV-2 and palivizumab for respiratory syncytial virus, provide passive immunity for high-risk patients. RNA interference-based therapies using siRNAs or antisense oligonucleotides represent a programmable approach, while CRISPR-Cas systems are being explored for direct cleavage of viral genomes or proviral DNA.
