enfuvirtide
T-20, DP-178, pentafuside
Enfuvirtide is a 36-amino-acid synthetic peptide that was the first HIV entry inhibitor, and it remains the only fusion inhibitor in clinical use. It is not an oncology drug. It is an antiretroviral, filed in this collection because it is one of the most clinically important therapeutic peptides ever made. It works entirely outside the cell, jamming the conformational change that gp41 must make to pull the viral and host membranes together. Twice-daily subcutaneous injection and near-universal injection-site reactions have limited it to salvage therapy.
Mechanism
HIV-1 entry proceeds through a choreographed series of conformational changes. After gp120 engages CD4 and a chemokine coreceptor (CCR5 or CXCR4), the transmembrane subunit gp41 springs into an extended 'pre-hairpin intermediate' in which its N-terminal fusion peptide is buried in the host membrane and its two heptad repeat regions, HR1 and HR2, are transiently exposed and separated. The fusion step itself requires HR2 to fold back and pack into the grooves of a central trimeric HR1 coiled coil, creating a stable six-helix bundle. That folding is what physically drags the viral and cellular membranes into apposition so the lipid bilayers can merge.
Enfuvirtide is a linear peptide corresponding to residues 643-678 of gp41, the HR2 region itself. It binds into the hydrophobic HR1 groove during the window when the pre-hairpin intermediate is exposed, acting as a competitive decoy: with enfuvirtide occupying the groove, the virus's own HR2 cannot dock, the six-helix bundle never forms, and fusion arrests. Because the drug never needs to enter a cell, it has no interaction with host nucleic acid or enzyme machinery, and it is active against virus resistant to every intracellular antiretroviral class. It is specific to HIV-1; HIV-2 gp41 heptad repeats diverge sufficiently that enfuvirtide is inactive. Resistance arises rapidly and with a low genetic barrier through substitutions in the gp41 HR1 residues 36-45, notably in the GIV motif (G36D/S/V, V38A/M/E, Q40H, N42T, N43D), which alter the groove and reduce binding.
What the research shows
The evidence base rests on two parallel, identically designed randomised trials in heavily treatment-experienced patients. TORO 1, conducted in North and South America, randomised 501 patients, and TORO 2, conducted in Europe and Australia, randomised 512, each 2:1 to enfuvirtide plus an optimised background regimen or the optimised regimen alone. At 24 weeks TORO 1 found a mean decrease in plasma HIV-1 RNA of 1.696 log₁₀ copies/mL with enfuvirtide versus 0.764 log₁₀ in controls, and TORO 2 found 1.429 versus 0.648 log₁₀, roughly a one-log additional reduction in both, with correspondingly larger CD4 cell gains. The pre-specified 48-week pooled analysis showed the effect was durable rather than transient: 30.4% of enfuvirtide recipients versus 12.0% of controls had HIV-1 RNA below 400 copies/mL, and CD4 count increases were about twice as large in the enfuvirtide group.
What those numbers meant in 2003 was substantial. Patients with multiclass-resistant virus and few remaining options gained a genuinely new mechanism. What has changed since is the comparator landscape. Integrase inhibitors, second-generation protease inhibitors and newer entry agents now deliver equivalent or better salvage activity by mouth, and enfuvirtide's requirement for twice-daily subcutaneous injection with injection-site reactions in nearly every patient makes it unattractive when alternatives exist. It remains relevant in very heavily pre-treated individuals and has found niche use in some transplant and paediatric settings. The TORO trials also produced an uncomfortable secondary observation that has never been fully explained: bacterial pneumonia occurred more frequently in enfuvirtide recipients (roughly 4.7 versus 0.6 events per 100 patient-years), a signal that persists in the labelling.
Evidence assessment
High-quality evidence
Two independently conducted, geographically separate randomised controlled trials (TORO 1 and TORO 2) both demonstrated significant additional virological suppression over an optimised background regimen, with durability confirmed in a pre-specified 48-week pooled analysis, and full regulatory approval in the US and EU. All three citations verified against PubMed. The tier describes the strength of the efficacy evidence, not the drug's current place in therapy, which is now marginal.
Tiers are applied consistently across the library and re-checked when new trials read out. Read the grading method.
Key studies
Enfuvirtide, an HIV-1 fusion inhibitor, for drug-resistant HIV infection in North and South America Preclinical only
Mean decrease in plasma HIV-1 RNA at 24 weeks of 1.696 log₁₀ copies/mL with enfuvirtide versus 0.764 log₁₀ with the background regimen alone, with greater CD4 gains. Injection-site reactions in 98% of enfuvirtide recipients.
Efficacy of enfuvirtide in patients infected with drug-resistant HIV-1 in Europe and Australia Preclinical only
Mean decrease in plasma HIV-1 RNA at 24 weeks of 1.429 log₁₀ copies/mL with enfuvirtide versus 0.648 log₁₀ in controls, an independent replication of the TORO 1 result in a different population.
Durable efficacy of enfuvirtide over 48 weeks in heavily treatment-experienced HIV-1-infected patients in the T-20 versus optimized background regimen only 1 and 2 clinical trials Preclinical only
30.4% of enfuvirtide recipients versus 12.0% of controls achieved HIV-1 RNA below 400 copies/mL at 48 weeks, with CD4 increases roughly twice as large, demonstrating that the effect was durable rather than a transient early response.
Safety
Local injection-site reactions are near-universal, reported by around 98% of patients in the TORO trials: erythema, induration, nodules and cysts, pruritus and ecchymosis, occurring with the great majority of injections and requiring site rotation. Most are mild to moderate but they are the commonest reason for discontinuation. Localised cutaneous amyloidosis at injection sites has been described with long-term use. Systemic hypersensitivity reactions occur in under 1% and may include rash, fever, nausea, vomiting, chills, rigors, hypotension, raised transaminases, respiratory distress, glomerulonephritis and primary immune complex reaction; these can recur on rechallenge, so the drug should not be restarted after a hypersensitivity event. An increased rate of bacterial pneumonia was observed in the pivotal trials, and patients with low CD4 counts, high viral load, intravenous drug use, smoking or prior lung disease warrant particular vigilance. Eosinophilia, peripheral neuropathy, insomnia and depression have been reported. Because enfuvirtide is catabolised to amino acids rather than metabolised by cytochrome enzymes, it is unusually free of pharmacokinetic drug interactions.
Regulatory status
| Jurisdiction | Status |
|---|---|
| United Kingdom | EU marketing authorisation granted May 2003 for combination use in treatment-experienced adults and children aged 6 years and over with HIV-1 infection failing on current regimens. Licensed in Great Britain by the MHRA. In practice it is now a rarely used salvage agent, superseded for most patients by integrase strand transfer inhibitors, second-generation non-nucleoside reverse transcriptase inhibitors and long-acting injectables. |
| United States | FDA approved March 2003 in combination with other antiretroviral agents for treatment of HIV-1 infection in treatment-experienced patients with evidence of viral replication despite ongoing antiretroviral therapy, the first HIV entry inhibitor licensed anywhere. Subcutaneous injection, prescription-only. |
| WADA (sport) | Not on the WADA Prohibited List. Enfuvirtide is an antiretroviral peptide with no anabolic, hormonal, stimulant or masking activity. |
Questions
No. It is an antiretroviral used to treat HIV-1 infection. It appears in this collection because it is one of the most clinically important therapeutic peptides ever made, and it is frequently searched alongside oncology peptides. Its mechanism has nothing to do with cancer.
It is a 36-amino-acid peptide with no protection against gut proteases and no realistic prospect of oral absorption at that size. Subcutaneous injection twice daily is the only viable route, which is a major practical limitation given that around 98% of patients develop injection-site reactions.
No. The heptad repeat sequences of HIV-2 gp41 differ enough from HIV-1 that enfuvirtide does not bind effectively. It is specific to HIV-1.
Because a small number of single amino acid substitutions in the gp41 HR1 groove (around residues 36 to 45, including G36D, V38A, N42T and N43D) are sufficient to reduce enfuvirtide binding while leaving the virus able to fuse. That is a low genetic barrier, which is why enfuvirtide is never used without an active background regimen.
Rarely. It remains licensed and can be valuable in patients with multiclass-resistant virus, but integrase inhibitors, newer protease inhibitors and long-acting injectables now cover most salvage scenarios by less burdensome routes.