Viral Entry and Fusion Inhibition Module

Module viral_entry_fusion_inhibition 0 disorders Pathograph 5

The viral cell-entry pathway and the successive steps at which entry-targeting antivirals interrupt it. Every enveloped virus must cross the host plasma membrane to deliver its genome into the cytoplasm, and it does so through an ordered, virus-specific program built from surface glycoproteins and the host receptor/co-receptor/uptake factors they hijack. This module traces that program as a causal cascade: a virion first attaches to its primary receptor; for viruses that require one, a co-receptor is then engaged; the apposed viral and host membranes fuse to deliver the genome into the cytoplasm; productive entry is the biological output that entry inhibitors suppress. Three pharmacological strategies act on distinct steps of this cascade. Attachment inhibitors block receptor binding — fostemsavir clamps the HIV gp120 envelope so it cannot engage CD4, and the HBV/HDV peptide bulevirtide blocks docking of the large surface protein at the NTCP bile-acid transporter on hepatocytes. Co-receptor antagonists block the second step — the CCR5 antagonist maraviroc occupies CCR5 and works only against CCR5-tropic HIV. Fusion inhibitors block the committed third step — the gp41 fusion inhibitor enfuvirtide disrupts the six-helix-bundle conformational change that drives membrane merger. The membrane-fusion step is the committed, most-druggable point of no return for entry and is the canonical conformance/treatment target of the module, while attachment and co-receptor engagement are additional, upstream drug-target steps. The net biological effect of engaging any of these steps is suppression of productive viral entry before any cytoplasmic replication occurs. Because the targets are virus- and tropism-specific surface proteins rather than conserved enzymes, the same pathway generates the escape route: a tropism switch (CCR5-tropic to CXCR4-tropic HIV) or envelope-glycoprotein mutation abolishes inhibitor activity. Exemplar agents: fostemsavir and bulevirtide (attachment), maraviroc (co-receptor), enfuvirtide (fusion).

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Pathophysiology Nodes

5 shared nodes are defined in this module.

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Cell Types

No cell types are annotated for this module.

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Biological Processes

Notes

This is an antiviral drug-mechanism module structured as a biological pathway, not a specific disease. Its nodes are successive biological steps of the viral entry program (receptor attachment -> co-receptor engagement -> membrane fusion and cytoplasmic genome delivery -> suppression of productive entry), with an adaptive tropism/resistance branch; the entry-inhibitor classes that act on each step are described in the node text rather than modelled as separate nodes. Disorder entries reference individual nodes via conforms_to (e.g., "viral_entry_fusion_inhibition#Membrane Fusion and Delivery into the Cytoplasm"), and an entry-targeting treatment points its target_mechanisms edge at the step it inhibits (the attachment node for attachment/NTCP inhibitors, the co-receptor node for CCR5 antagonists, the membrane-fusion node for fusion inhibitors). Key conformance / treatment target: "Membrane Fusion and Delivery into the Cytoplasm" — the committed, most-druggable entry step (the point of no return), with attachment and co-receptor engagement as additional upstream drug-target steps. The resistance node captures the gating knowledge that distinguishes "an entry inhibitor exists" from real drug selection: viral tropism and receptor usage determine whether the target exists at all, and a tropism switch or envelope mutation escapes the inhibitor without any change to the cytoplasmic replication machinery. See projects/ANTIVIRAL.md.

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Used By Disorder Entries

No disorder entries currently reference this module via conforms_to.

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Pathograph

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Pathophysiology

Viral Attachment to the Host-Cell Receptor

trigger

Entry begins when surface glycoproteins on the virion bind the primary host-cell receptor, the committed first step that anchors the virus to the target cell. For HIV this is engagement of CD4 by the gp120 envelope; for HBV/HDV it is docking of the large surface protein at the NTCP bile-acid transporter on hepatocytes. This step is itself a drug target: attachment inhibitors block receptor binding — fostemsavir clamps gp120 so it cannot engage CD4, and the first-in-class peptide bulevirtide blocks HBV/HDV docking at NTCP. Because attachment depends on a virus- and host-specific receptor-binding interaction rather than a conserved enzyme, attachment inhibitors are exquisitely specific to a single virus.

virion attachment to host cell GO:0019062

Downstream

Co-receptor Engagement After primary-receptor attachment, viruses that require one engage a co-receptor to license fusion.

Co-receptor Engagement

effector

For viruses that require a second receptor, primary-receptor attachment is followed by engagement of a co-receptor that triggers the conformational changes licensing membrane fusion. The canonical example is HIV: after CD4 binding, gp120 engages a chemokine co-receptor — CCR5 or CXCR4 depending on viral tropism. This step is a distinct drug target: the CCR5 antagonist maraviroc occupies CCR5 and blocks the gp120–co-receptor interaction, but only against CCR5-tropic (R5) virus, making co-receptor antagonism inherently tropism-restricted.

symbiont entry into host cell GO:0046718

Downstream

Membrane Fusion and Delivery into the Cytoplasm Co-receptor engagement drives the fusion glycoprotein conformational change that merges the viral and host membranes.

Membrane Fusion and Delivery into the Cytoplasm

therapeutic vulnerability

Receptor and co-receptor engagement trigger the fusion glycoprotein to drive merger of the viral envelope with a host membrane, delivering the viral genome into the cytoplasm. For HIV the gp41 transmembrane subunit refolds into a six-helix bundle that pulls the two membranes together; this conformational change is the committed, irreversible point of no return for entry. It is therefore the most-druggable entry step and the canonical conformance/treatment target of the module: the gp41 fusion inhibitor enfuvirtide binds the prehairpin intermediate and disrupts the conformational change that drives membrane fusion, arresting the cascade at its committed step. Attachment and co-receptor engagement are additional upstream drug targets, but membrane fusion is the step at which the genome would otherwise be irreversibly committed to the cytoplasm.

fusion of virus membrane with host plasma membrane GO:0019064

Downstream

Suppression of Productive Viral Entry Blocking membrane fusion prevents delivery of the genome into the cytoplasm, aborting productive entry.
Tropism and Entry-Inhibitor Resistance Virus- and tropism-specific surface proteins gate which entry step is druggable; a tropism switch or envelope mutation escapes the inhibitor.

Suppression of Productive Viral Entry

consequence

When an entry inhibitor engages attachment, co-receptor binding, or membrane fusion, the virus is prevented from delivering its genome into the cytoplasm, so productive infection of the target cell is aborted before any intracellular replication step. A co-receptor antagonist that occupies CCR5 prevents the gp120–co-receptor interaction; a fusion inhibitor that binds gp41 prevents the six-helix-bundle change required for membrane merger; an NTCP-blocking peptide prevents hepatocyte docking. This is the antiviral consequence of engaging the upstream entry-target steps — it sits upstream of, and independent from, every cytoplasmic drug target (polymerase, protease, integrase), which is why entry inhibitors combine with replication inhibitors without overlapping targets.

negative regulation of viral process GO:0048525

Tropism and Entry-Inhibitor Resistance

adaptive escape

Entry inhibition is the most narrowly gated antiviral strategy because the target is defined by viral tropism and receptor usage rather than a conserved enzyme. The clearest example is the HIV co-receptor antagonist maraviroc: it is active only against CCR5-tropic (R5) virus, so a viral population that uses or switches to the CXCR4 co-receptor (X4 tropism) is intrinsically unaffected, and a pre-treatment tropism assay is mandatory because a large fraction of treatment-experienced patients harbor CXCR4-using virus. Beyond tropism, resistance can arise from mutations in the V3 loop of the gp120 envelope that let the virus use the drug-bound co-receptor or shift co-receptor preference. This adaptive branch off the committed fusion node encodes why an entry inhibitor's target may not exist at all in a given virus, and why escape can occur without any change in the cytoplasmic replication machinery. Conforming entries can attach a treatment's tropism-restriction or failure mode to this node.

response to xenobiotic stimulus GO:0009410