Research information only. PrecisePep is an educational library. It is not medical advice and is not intended to assist, encourage or enable self-administration. The compounds documented here are research chemicals — most are not approved for human use. Read the full disclaimer.
PrecisePep/Peptide Library/Enfuvirtide
Immune & Anti-Inflammatory
Fuzeon® · T-20 · pentafuside
A 36-residue peptide that blocks HIV from fusing with the cell it is trying to enter — the largest peptide drug in this library, the first entry inhibitor ever approved, and a case study in why peptides make difficult medicines.
Almost every HIV drug works after the virus is already inside the cell. Reverse transcriptase inhibitors, integrase inhibitors and protease inhibitors all act on steps of the viral life cycle that happen in the cytoplasm or nucleus. Enfuvirtide acts before any of that — it stops the virus from getting in.
HIV entry requires the viral envelope protein gp41 to fold in on itself. Two helical regions, HR1 and HR2, zip together into a six-helix bundle, and that folding is what physically pulls the viral and cell membranes into contact so they can fuse. Enfuvirtide is a synthetic 36-residue peptide copied from HR2. It binds HR1 first, occupying the groove that the real HR2 needs, and the bundle cannot form. The virus is left attached to the cell surface and unable to enter.[1]
It worked. The TORO trials in heavily treatment-experienced patients with multidrug-resistant virus reported significantly greater viral load reduction when enfuvirtide was added to an optimised background regimen, at a time when options for that population were close to exhausted. For a period in the mid-2000s it was the difference between a viable regimen and none.[1]
It has since been largely displaced — not because it stopped working, but because integrase inhibitors and second-generation agents arrived in oral form. Enfuvirtide requires reconstitution and two subcutaneous injections every day, and produces injection site reactions in almost everyone who takes it.[2] When a once-daily tablet with a comparable resistance profile appeared, that comparison was not close.
Enfuvirtide is a prescription medicine used within specialist HIV care, and this page publishes no unsupervised dosing for it. It appears because the conditions index names HIV among conditions with approved peptide treatments — and because it is the single best illustration on this site of the practical limitations of peptide therapeutics. It is not a coincidence that the peptide drug in this class became the one nobody wanted to take.
Blocking six-helix bundle formation. After gp120 engages CD4 and a chemokine co-receptor, gp41 inserts its fusion peptide into the host membrane and then collapses: HR1 and HR2 regions from three gp41 subunits assemble into a six-helix bundle, drawing the two membranes together. Enfuvirtide is an HR2 mimic that binds the HR1 trimeric coiled coil, competitively preventing the real HR2 from docking. No bundle, no fusion, no entry.
Extracellular action. Because it works outside the cell, it needs no intracellular penetration and no metabolic activation. That also means it has no cytochrome P450 interactions — an unusual advantage in a therapeutic area defined by them.
Resistance. Mutations in the HR1 region — most characteristically in the gp41 36–45 codon range — reduce binding. Resistance emerges relatively quickly when the background regimen is not fully suppressive, which is why it was only ever used as part of an optimised combination.
Why it must be injected. A 36-residue peptide is destroyed in the gut and is far too large to cross mucosal barriers usefully. There is no oral formulation and no realistic prospect of one. Its half-life also requires twice-daily administration — the size and fragility that make it a good fusion inhibitor make it a poor medicine.
Effects that have been reported. A study-tier entry means a result was published — not that the effect is established, reproducible, or transferable from a rodent to a person.
Highest confidenceReported in peer-reviewed literature, registered clinical trials, or regulatory filings. Note the model: much peptide literature is in vitro or rodent work, not human trial data.
The TORO 1 and TORO 2 randomised trials in treatment-experienced patients reported significantly greater reductions in HIV-1 RNA and greater CD4 recovery when enfuvirtide was added to an optimised background regimen, compared with the background regimen alone.
As the first entry inhibitor, it retained activity against virus resistant to every other class available at the time — the property that made it valuable in salvage therapy.
It is catabolised to its constituent amino acids rather than metabolised hepatically, which removes an entire category of interaction problems in patients typically taking several other antiretrovirals.
Studied and used in children, with weight-based dosing, in the same salvage context.
Its approval validated viral entry as a druggable target and informed subsequent work on entry inhibitors including maraviroc, ibalizumab and fostemsavir.
Unproven — inference onlyExtrapolated from a known mechanism, receptor profile, or adjacent compound class. Biologically plausible but not directly demonstrated for this compound in this context.
The same structural strategy has been explored against other class I fusion proteins, including in coronaviruses and respiratory syncytial virus. Interesting as a design template; no approved product has followed by this route.
Mechanistically coherent — blocking entry is the ideal point for prophylaxis — but twice-daily injections make it entirely impractical against long-acting alternatives.
Extended-half-life fusion inhibitors have been pursued to remove the twice-daily burden. The practical lesson of enfuvirtide is that mechanism is necessary and delivery is decisive.
Uncontrolled reportsAggregated from research-community logs, forums, vendor literature and self-reported experience. Uncontrolled, unverified, subject to placebo, polypharmacy and product-purity confounds. Lowest evidentiary weight.
Patient reporting from the mid-2000s is consistent on both points. In salvage therapy it was the difference between suppression and progression, and the injection burden was severe enough that adherence was the limiting factor rather than efficacy.
Firm subcutaneous nodules at injection sites are near-universal and can persist for months, progressively reducing usable sites.
The powder dissolves slowly — historically up to 45 minutes — which for a twice-daily drug is a substantial daily burden. Patients commonly prepared doses in advance and refrigerated them, within the labelled window.
A 36-residue peptide requiring twice-daily injection, for a condition with excellent oral treatment, has no research-chemical presence. There is nothing to gain from unlicensed supply.
Adverse effects, contraindications and unknowns. The grey-market tier is where under-reporting is worst: uncontrolled use produces no systematic safety surveillance at all, so absence of a reported harm is not evidence of safety.
Highest confidenceReported in peer-reviewed literature, registered clinical trials, or regulatory filings. Note the model: much peptide literature is in vitro or rodent work, not human trial data.
98% of patients experienced at least one injection site reaction in the pivotal T20-301 and T20-302 trials — erythema and induration in roughly 80%, nodules in about 56%, pain or discomfort in about 52%, and pruritus in about 44%. This is the defining characteristic of the drug, not a footnote to it.
A higher incidence of bacterial pneumonia was observed in enfuvirtide recipients in the trial programme. The mechanism is not established, and the label carries a warning to monitor for it.
Systemic hypersensitivity including rash, fever, nausea, hypotension and elevated transaminases has been reported, and recurred on rechallenge in some cases. Rechallenge after a hypersensitivity reaction is not advised.
The most common systemic adverse events, generally mild.
A class-wide consequence of effective antiretroviral therapy rather than of this drug specifically — a recovering immune system mounts an inflammatory response to previously indolent infections.
gp41 HR1 mutations reduce susceptibility, and emerge readily where the background regimen does not fully suppress replication.
Unproven — inference onlyExtrapolated from a known mechanism, receptor profile, or adjacent compound class. Biologically plausible but not directly demonstrated for this compound in this context.
With two injections a day and persistent nodules, the practical constraint eventually becomes finding usable tissue. This is a real reason regimens were changed.
The consistency and character of the reactions suggest a local immune response to the peptide rather than simple irritation, though the mechanism has not been definitively established.
Uncontrolled reportsAggregated from research-community logs, forums, vendor literature and self-reported experience. Uncontrolled, unverified, subject to placebo, polypharmacy and product-purity confounds. Lowest evidentiary weight.
Widely described as persisting well after switching regimens.
Patients describe elaborate rotation schemes as essential rather than advisable.
The consistent theme once integrase inhibitors became available.
Figures below are documented, not recommended. Study ranges come from published protocols in the stated model; grey-market ranges are what the research community reports using and carry no safety validation of any kind.
Highest confidenceReported in peer-reviewed literature, registered clinical trials, or regulatory filings. Note the model: much peptide literature is in vitro or rodent work, not human trial data.
Injected into the upper arm, anterior thigh or abdomen, into a site with no current injection site reaction and away from the navel, scar tissue, bruises and moles. Reconstituted only with the 1 mL of sterile water for injection supplied in the convenience kit.
Indicated in combination with other antiretroviral agents in treatment-experienced patients with ongoing replication. Monotherapy would select resistance rapidly and is not a use of this drug.
Children are dosed by body weight, defined in the label.
Systematic rotation is specified in the instructions for use, and is the only meaningful mitigation for the injection site reactions.
| Parameter | Value | Note |
|---|---|---|
| Adult dose | 90 mg (1 mL) twice daily | Subcutaneous |
| Sites | Upper arm, anterior thigh, abdomen | Rotate systematically |
| Reconstitution | 1 mL sterile water, supplied | Kit diluent only |
| Paediatric | Weight-based | See label |
| Use | Combination only | Never as monotherapy |
| Injection site reactions | 98% of patients | The defining adverse effect |
| Monitor for | Bacterial pneumonia, hypersensitivity | Labelled warnings |
Unproven — inference onlyExtrapolated from a known mechanism, receptor profile, or adjacent compound class. Biologically plausible but not directly demonstrated for this compound in this context.
Given the burden and the availability of well-tolerated oral options, current use is confined to patients whose resistance profile leaves few alternatives.
Uncontrolled reportsAggregated from research-community logs, forums, vendor literature and self-reported experience. Uncontrolled, unverified, subject to placebo, polypharmacy and product-purity confounds. Lowest evidentiary weight.
This is an antiretroviral for multidrug-resistant HIV. It is prescribed within specialist care and there is no unsupervised use to document.
Combination effects. Almost no peptide combination has been studied as a combination in humans; stack logic is overwhelmingly mechanistic inference and community practice. Each linked compound has its own page.
Highest confidenceReported in peer-reviewed literature, registered clinical trials, or regulatory filings. Note the model: much peptide literature is in vitro or rodent work, not human trial data.
The TORO trials tested exactly this: enfuvirtide added to a regimen selected on resistance testing. The added agent and the optimised background are one intervention, and adding it to a failing regimen wastes it.
Catabolism to amino acids rather than hepatic metabolism means it does not interact with the protease inhibitors and non-nucleoside agents it is given alongside.
Unproven — inference onlyExtrapolated from a known mechanism, receptor profile, or adjacent compound class. Biologically plausible but not directly demonstrated for this compound in this context.
Maraviroc blocks the CCR5 co-receptor, ibalizumab blocks CD4 binding, fostemsavir blocks gp120 attachment. Different steps of the same entry process, and combinations have been explored in deep salvage.
No research peptide has any established role alongside antiretroviral therapy, and immunomodulation of unknown direction in someone managing HIV is a poor idea. Thymosin alpha-1 has genuine approved use in chronic hepatitis, which is a different virus and a different context.
Uncontrolled reportsAggregated from research-community logs, forums, vendor literature and self-reported experience. Uncontrolled, unverified, subject to placebo, polypharmacy and product-purity confounds. Lowest evidentiary weight.
There is no community stacking practice around this compound.
Whole schedules — dosing, titration, cycle length, and off-time — as documented in trials, as proposed from mechanism, and as circulated in the research community. Reproduced for study and comparison only.
Highest confidenceReported in peer-reviewed literature, registered clinical trials, or regulatory filings. Note the model: much peptide literature is in vitro or rodent work, not human trial data.
Randomised open-label trials in treatment-experienced patients, comparing an optimised background regimen with and without enfuvirtide, using change in plasma HIV-1 RNA as the primary endpoint.
Genotypic and phenotypic resistance testing to build the background regimen before adding enfuvirtide — the standard of care in salvage therapy.
The instructions for use are unusually detailed, and adherence to them is the main determinant of how long a patient can continue.
Unproven — inference onlyExtrapolated from a known mechanism, receptor profile, or adjacent compound class. Biologically plausible but not directly demonstrated for this compound in this context.
The clinical trajectory for most patients who used it was toward an oral regimen as soon as resistance allowed. That is a protocol decision as much as a preference.
Uncontrolled reportsAggregated from research-community logs, forums, vendor literature and self-reported experience. Uncontrolled, unverified, subject to placebo, polypharmacy and product-purity confounds. Lowest evidentiary weight.
Reconstituting in advance and refrigerating within the labelled window was a widely used strategy to manage the twice-daily burden.
The approved indication is covered above. This section exists for one reason: enfuvirtide is the best illustration in this library of a peptide drug that worked and was abandoned anyway, and the reason it was abandoned is the reason most peptide drugs never reach a market at all.
Still licensed, rarely used. Integrase inhibitors and newer agents provide comparable or better activity in oral form, and enfuvirtide is now largely reserved for deep salvage where resistance leaves few options.
Pathophysiology
A 36-residue peptide cannot be swallowed, requires reconstitution, needs twice-daily subcutaneous injection, and produces injection site reactions in almost everyone who takes it.
Mechanistic rationale
Injection site reactions affected 98% of patients — erythema and induration in roughly 80%, nodules in about 56%, pain in about 52% — and the nodules persist for months, progressively using up available sites. Reconstitution historically took up to 45 minutes, twice a day. When a once-daily tablet with a comparable resistance profile appeared, the comparison was not close. Efficacy was never the problem. That is the constraint the entire peptide therapeutics field works under, and it is why linaclotide — which is swallowed because it never needs absorbing — is the interesting counterexample.
Community reports
Patient reporting from the mid-2000s is consistent on both points: in salvage therapy it was the difference between suppression and progression, and adherence rather than efficacy was the limiting factor.
Components carrying the argument: Size, route and burden — not pharmacology
Pathophysiology
Blocking viral entry is the ideal point at which to prevent infection, since nothing enters the cell to establish a reservoir.
Mechanistic rationale
Mechanistically this is the best possible place for a prophylactic agent to act. Twice-daily injections make it entirely impractical against oral and long-acting injectable options that now dominate PrEP. Another instance of the same lesson.
Community reports
Not used for this.
Components carrying the argument: gp41 fusion inhibition
Pathophysiology
Many enveloped viruses use class I fusion proteins with a comparable six-helix bundle mechanism.
Mechanistic rationale
The same structural strategy — a peptide mimicking one helical region to prevent the bundle forming — has been explored against coronaviruses and respiratory syncytial virus. It remains a design template rather than an approved product by that route, and the delivery constraint above is a large part of why.
Community reports
Occasionally cited in antiviral peptide discussion.
Components carrying the argument: Class I fusion protein mechanism
| Question | Position |
|---|---|
| Did it work? | Yes — TORO trials, in multidrug-resistant HIV |
| Why is it rarely used? | Twice-daily injection; 98% injection site reactions |
| The lesson | Delivery, not mechanism, decides whether a peptide drug survives |
| Counterexample | Linaclotide — oral, because it needs no absorption |
| Grey market | None — HIV treatment is effective and widely subsidised |
For HIV: modern antiretroviral therapy is highly effective, generally a single daily tablet, and free or subsidised in most health systems. Undetectable viral load means untransmittable. Long-acting injectable regimens now exist for people who prefer them. Resistance testing guides regimen construction in treatment-experienced patients, and specialist HIV care is the setting for all of it — a person managing HIV outside that care is not solving a problem, they are creating several.
Physical-chemistry reference for laboratory handling of the lyophilised material. Reproduced so that stability and concentration mathematics can be studied — see the calculator for the arithmetic and the reconstitution guide for sterile technique.
Lyophilised powder reconstituted with exactly 1 mL of the sterile water for injection supplied in the convenience kit — not bacteriostatic water, and not a substituted diluent. The powder dissolves slowly and the vial should be rolled gently rather than shaken; foaming indicates rough handling. This site publishes no alternative reconstitution arithmetic for it.
Powder at controlled room temperature. Reconstituted solution refrigerated at 2–8 °C and used within the labelled window, brought to room temperature before injection to reduce injection site discomfort.
Licensed: 108 mg single-use vial delivering 90 mg/mL after reconstitution, supplied in a convenience kit with diluent, syringes and alcohol wipes.
A 36-residue peptide in aqueous solution is not a stable object. The labelled in-use window is short by the standards of the rest of this library, and the supplied diluent is sterile rather than bacteriostatic water for that reason.
Regulatory status changes and differs by country. This is a summary for orientation, not legal advice — check your own jurisdiction.
Approved prescription medicine. Approved in 2003 as Fuzeon, indicated in combination with other antiretroviral agents for the treatment of HIV-1 infection in treatment-experienced patients with evidence of viral replication despite ongoing antiretroviral therapy.
Still licensed, but rarely used. Integrase inhibitors and newer agents provide comparable or better activity in oral form, and enfuvirtide is now largely reserved for deep salvage where resistance leaves few options.
There is no unlicensed supply, and no reason there would be. HIV treatment is highly effective, widely available and free or subsidised in most health systems — a person managing HIV outside specialist care is not solving a problem, they are creating several.
Regulatory and trial claims re-checked against primary sources on 16 August 2026. Checked: injection site reaction rates in the current label. Approvals, trial readouts and compounding decisions move faster than anything else on this page — a date here means someone looked, not that nothing has changed since.
Not a performance compound and not addressed as such under the WADA Prohibited List. Verify against the current list if competing.
Links resolve to PubMed records, trial registries, publisher pages or primary documents. Where a body of work rather than a single paper is cited, the link opens a PubMed query so the full result set — including newer papers than this page — can be reviewed. Verify every claim against the primary source before relying on it.
For educational and research reference only. Nothing on this site is medical advice, a recommendation, or an instruction to administer any substance to a human being. Every figure on this page is a record of what has been reported, not a recommendation. Full disclaimer.
