Antiviral drug Study Guide

📖 Core Concepts Antiviral drugs – medications that treat viral infections by targeting virus‑specific processes; usually safe for host cells. Broad‑spectrum vs. virus‑specific – broad‑spectrum act against many viruses; most are designed for a single virus (HIV, herpes, HBV, HCV, influenza). Direct‑acting antivirals (DAAs) – bind viral proteins (e.g., polymerase, protease, entry proteins) to block replication. Host‑targeting antivirals (HTAs) – inhibit host factors essential for viral life‑cycle, giving a higher genetic barrier to resistance. Genetic barrier – how many mutations a virus must acquire to become resistant; low for DAAs, high for HTAs. Resistance – viral genetic changes that reduce drug susceptibility; driven by high replication error rates (especially in RNA viruses). --- 📌 Must Remember Entry inhibitors block virus‑receptor binding or fusion (e.g., HIV fusion blockers). Uncoating inhibitors = amantadine, rimantadine (influenza). Nucleoside analogues → chain terminators (e.g., acyclovir for herpes, NS5B inhibitors for HCV). Integrase inhibitors prevent viral DNA integration (HIV). Protease inhibitors stop polyprotein cleavage (HIV, HCV NS3/4A). Neuraminidase inhibitors (zanamivir, oseltamivir) block influenza virus release. Resistance mechanisms – point mutations (e.g., neuraminidase mutations), quasispecies selection, reassortment. Combination therapy = multiple mechanisms → lower chance of a single resistance mutation. HTA advantage – host genome stability → higher barrier to resistance. --- 🔄 Key Processes Viral Entry Inhibition Drug mimics cellular receptor → binds viral protein → prevents attachment. Drug mimics viral protein → binds cellular receptor → blocks binding site. Uncoating Block Amantadine/rimantadine bind M2 ion channel → prevent acid‑mediated uncoating of influenza virions. Reverse Transcription Inhibition (HIV) Nucleoside analogue → incorporated by reverse transcriptase → terminates DNA chain. Integrase Inhibition (HIV) Drug binds integrase active site → blocks integration of proviral DNA into host genome. Protease Inhibition (HIV, HCV) Inhibitor binds viral protease → polyprotein remains uncleaved → non‑functional virions. Neuraminidase Inhibition (Influenza) Zanamivir/Oseltamivir bind neuraminidase → prevent cleavage of sialic acid → virions stay attached to host cell. Antisense / Morpholino Action Short DNA/RNA segment binds complementary viral RNA → blocks translation/replication. Combination Therapy Design Choose drugs with non‑overlapping targets → e.g., reverse‑transcriptase inhibitor + protease inhibitor for HIV. --- 🔍 Key Comparisons DAA vs. HTA Target: viral protein vs. host protein. Resistance: low genetic barrier vs. high genetic barrier. Safety: generally higher specificity (DAA) but risk of off‑target host effects (HTA). Entry Inhibitor vs. Uncoating Inhibitor Step: attachment/fusion vs. post‑entry capsid release. Typical drugs: HIV fusion blockers vs. amantadine (influenza). Nucleoside Analogue vs. Non‑nucleoside Polymerase Inhibitor (HCV) Mechanism: chain termination after incorporation vs. allosteric block of polymerase active site. Neuraminidase Inhibitor vs. HA (Hemagglutinin) Inhibitor Stage: release of progeny virions vs. initial attachment. --- ⚠️ Common Misunderstandings “All antivirals are virucides.” – Virucides chemically destroy viruses; antivirals are therapeutic drugs that act inside the host. “Broad‑spectrum antivirals have no resistance.” – Even broad‑spectrum agents can select for resistant mutants if they target viral proteins. “HTAs are always safer.” – Inhibiting host pathways can cause toxicity; therapeutic window must be carefully evaluated. “Resistance only arises from point mutations.” – Reassortment, recombination, and gene amplification also generate resistance, especially in influenza. --- 🧠 Mental Models / Intuition “Lock‑and‑Key” for entry – Think of the virus as a key that must fit a lock (cell receptor). Entry inhibitors either change the lock or jam the key. “Chain‑Termination” analogy – Nucleoside analogue = a missing link that stops the polymerase train from moving forward. “Assembly line” for proteases – Viral polyprotein is a raw product; protease is the cutter that creates final parts. Inhibit the cutter → unfinished product. “Quasispecies cloud” – Visualize a swarm of slightly different viruses; drug pressure selects the few that survive. --- 🚩 Exceptions & Edge Cases Amantadine resistance – Common in modern influenza A; not effective for most circulating strains. Low genetic barrier DAAs – Some DAAs (e.g., early HCV protease inhibitors) rapidly select resistance; newer DAAs have higher barriers. HTA toxicity – Kinase inhibitors can affect normal cell signaling; dose‑limiting side effects may restrict use. Neuraminidase inhibitor cross‑resistance – Mutations may affect both oseltamivir and zanamivir, but some mutations confer resistance to only one. --- 📍 When to Use Which Choose DAAs when a virus has a well‑characterized, virus‑specific protein (e.g., HIV protease, HCV NS5A). Choose HTAs for emerging viruses lacking virus‑specific drugs or when resistance to DAAs is high. Combine drugs if monotherapy has a low genetic barrier (e.g., HIV, HCV) → pair reverse‑transcriptase inhibitor + integrase inhibitor. Use neuraminidase inhibitors for acute influenza within 48 h of symptom onset; avoid if known resistance mutations are present. Select entry inhibitors for viruses where attachment is the dominant bottleneck (e.g., HIV, some coronaviruses). --- 👀 Patterns to Recognize Resistance ↔ High replication error rate – RNA viruses → frequent mutations → look for resistance in influenza, HIV, HCV. Drug class ↔ Life‑cycle stage – Entry → fusion blockers; Replication → nucleoside analogues, polymerase inhibitors; Maturation → protease inhibitors; Release → neuraminidase inhibitors. Combination therapy → Multiple life‑cycle steps – If a question lists drugs from different stages, it likely describes a regimen to prevent resistance. Host‑targeted → Broad‑spectrum – HTAs often mentioned alongside “emerging viruses” or “high genetic barrier”. --- 🗂️ Exam Traps Confusing virucides with antivirals – Virucides are disinfectants; antivirals are systemic drugs. Assuming all DAAs have high genetic barriers – Many early DAAs are low barrier; remember “single mutation → resistance”. Mix‑up between neuraminidase and hemagglutinin inhibitors – Only neuraminidase inhibitors block release; HA inhibitors are not listed in the outline. Attributing “immune enhancement” to a drug class – Interferon‑alpha is an immune‑modulating antiviral, not a direct‑acting agent. Over‑generalizing HTA safety – HTAs can be toxic; the outline stresses balancing efficacy vs. host toxicity. ---