What is enfuvirtide (Fuzeon)? The first peptide HIV fusion inhibitor
Peptide drugs
Enfuvirtide (brand name Fuzeon, also called T-20) is a 36-amino-acid synthetic peptide that was approved by the US FDA in 2003 as the first peptide-based HIV entry inhibitor. Instead of attacking the virus after it gets inside a cell, it acts on the viral envelope protein gp41 to interfere with the step in which the virus and the host cell membrane merge.
What is enfuvirtide?
According to the FDA prescribing information, enfuvirtide is a linear 36-amino-acid synthetic peptide whose N-terminus is acetylated and whose C-terminus is a carboxamide, so both ends are chemically capped. The label gives the molecular formula as C204H301N51O64 and the molecular weight as 4492. For comparison with other peptide drugs, that puts it well above the size of small-molecule antiretrovirals; our explainer on peptides versus small molecules covers why that size matters for delivery.
The label describes it as an inhibitor of the fusion of HIV-1 with CD4 cells, indicated together with other antiretroviral agents for treatment-experienced patients who still have ongoing viral replication. Reviews of the field describe it as the first peptide-based HIV entry inhibitor, approved in 2003, which stimulated later work on other protein- and peptide-based entry inhibitors. If you are curious about the naming pattern, see why peptide drug names end in “tide”.
How enfuvirtide is studied to work
HIV entry is a multi-stage process: the virus attaches and binds CD4, then a co-receptor, and finally the viral and cell membranes fuse. A 2013 review of entry inhibitors notes that different drugs act at different stages. Maraviroc blocks the interaction with the CCR5 co-receptor, whereas enfuvirtide is a fusion inhibitor that disrupts conformational changes in gp41 that drive membrane fusion. The FDA label puts the same idea in more specific terms: enfuvirtide binds the first heptad-repeat (HR1) region of the gp41 subunit and blocks the shape changes needed for fusion.
Because gp41 is a viral protein displayed on the outside of the virus, the drug works in the extracellular space. That is also why peptides can be a good fit here: the target is a protein-protein interaction surface, something peptides can mimic or block more naturally than a typical small molecule can. (For a related receptor-side perspective, see what is a GPCR.)
What the enfuvirtide trials reported
Two phase 3 trials, TORO 1 and TORO 2, tested enfuvirtide plus an optimized background regimen against the optimized background regimen alone in people with extensive prior antiretroviral exposure and resistant virus. Participants were randomized 2:1. In TORO 1 (North and South America; 501 randomized), the mean fall in HIV-1 RNA at week 24 was 1.696 log10 copies/mL with enfuvirtide versus 0.764 in the control group, and mean CD4+ counts rose by 76 versus 32 cells per cubic millimeter. In TORO 2 (Europe and Australia; 512 randomized), plasma HIV-1 RNA fell 0.781 log10 copies/mL more than in controls at week 24, with CD4+ gains of 65.5 versus 38.0.
The prescribing information also reports pooled week-48 results from the two trials: 23% of enfuvirtide recipients had HIV-1 RNA below 50 copies/mL versus 8% on the background regimen alone, and 46% versus 18% had a drop of at least 1 log10.
Safety findings and practical limits
The same trials documented the drug’s main trade-offs. Injection-site reactions were reported by 98% of enfuvirtide recipients in TORO 1, and the label gives the same 98% figure for subjects with at least one local reaction. Pneumonia was more frequent in the enfuvirtide group in TORO 1; the label reports an incidence of 2.7% (3.2 events per 100 patient-years), and hypersensitivity reactions in fewer than 1%. On the label, the drug is given as a twice-daily subcutaneous injection, which is part of why it is reserved for people with limited remaining options.
Resistance has also been documented. The label notes resistance-associated changes at positions 36 to 38 of the gp41 HR1 region, and combinations of HR1 substitutions can lower susceptibility further. A 2019 study of the HR1 region reports additional substitutions that contribute to resistance to peptide fusion inhibitors.
Handling and reconstitution (research context)
Enfuvirtide is a good example of why vial labels need careful reading. Per the prescribing information, it is supplied as a lyophilized powder that “must only be reconstituted” with the Sterile Water for Injection provided in the kit (1 mL), and each single-dose vial contains 108 mg of enfuvirtide to deliver 90 mg. Unreconstituted vials are stored at room temperature (25 °C, excursions 15 to 30 °C), while the reconstituted solution is refrigerated at 2 to 8 °C and used within 24 hours.
The lesson for lab math is that vial content, diluent volume, and deliverable amount are three different numbers. To work through concentration arithmetic for any lyophilized peptide, use the reconstitution calculator and the concentration converter, and read why units and milliliters are not a dose. Also see our guides on how to reconstitute peptides and sterile technique.
Frequently asked questions
What is a fusion inhibitor?
It is a drug that stops a virus from merging its membrane with the host cell membrane. Enfuvirtide acts on the viral gp41 protein, so it works before the viral contents enter the cell.
Is enfuvirtide taken by mouth?
No. The label describes subcutaneous injection. Like most peptide drugs, it is not designed to survive digestion; see peptide half-life extension strategies for the engineering side of that problem.
How is it different from maraviroc?
Both are HIV entry inhibitors, but maraviroc is a small-molecule CCR5 antagonist that blocks the co-receptor interaction, while enfuvirtide is a peptide that blocks gp41 conformational changes.
How is a peptide like this made?
Peptides of this length are typically produced by chemical synthesis; the trade-offs are covered in recombinant vs synthetic peptide production.
References
- Lalezari JP et al. Enfuvirtide, an HIV-1 fusion inhibitor, for drug-resistant HIV infection in North and South America (TORO 1). N Engl J Med 2003;348:2175-85.
- Lazzarin A et al. Efficacy of enfuvirtide in patients infected with drug-resistant HIV-1 in Europe and Australia (TORO 2). N Engl J Med 2003;348:2186-95.
- Fuzeon (enfuvirtide) prescribing information, as reproduced by Drugs.com (rev. 12/2019): structure, mechanism, handling, adverse events, pooled week-48 results.
- Entry inhibitors and their use in the treatment of HIV-1 infection. Antiviral Res 2013.
- Development of protein- and peptide-based HIV entry inhibitors targeting gp120 or gp41. Viruses 2019.
- Mutations of Glu560 within the HIV-1 envelope N-terminal heptad repeat region and resistance to peptide fusion inhibitors. Retrovirology 2019.
Informational only — not medical advice. This article is for educational purposes. It does not recommend, sell or instruct personal human use of any compound. Consult a qualified healthcare professional for any health decision. Content is intended for adults 21+. Verify scientific details against the primary sources cited.