Peptide Schedule
Enfuvirtide (Fuzeon)36 residuesYTSLIHSLIEESQNQQEKNEQELLELDKWASLWNWFEach bubble = one amino acid. Size = residue mass. Color = chemical class.Enfuvirtide (Fuzeon)
Benefits
About Enfuvirtide (Fuzeon)
Enfuvirtide (brand name Fuzeon) is a first-in-class 36-amino-acid synthetic peptide that represents the first member of the HIV fusion inhibitor class of antiretroviral drugs. Approved by the FDA on March 13, 2003, enfuvirtide works by a completely novel mechanism compared to other antiretrovirals — it blocks HIV-1 from physically fusing with and entering host CD4+ T-cells. The peptide sequence is derived from the heptad repeat 2 (HR2) domain of the HIV-1 transmembrane glycoprotein gp41, enabling it to competitively bind to the HR1 domain and prevent the conformational rearrangement essential for viral-cell membrane fusion. In pivotal Phase III clinical trials (TORO-1 and TORO-2), enfuvirtide plus an optimized background regimen produced significantly greater viral load reductions (mean -1.48 log10 copies/mL at 48 weeks) compared to the optimized background regimen alone (-0.63 log10), along with superior CD4+ T-cell count recovery (+91 vs. +45 cells/mm3). Enfuvirtide is exclusively indicated for treatment-experienced adults and pediatric patients (weighing at least 11 kg) who have evidence of HIV-1 replication despite ongoing antiretroviral therapy. Because it targets viral entry rather than intracellular replication machinery, enfuvirtide retains activity against strains resistant to NRTIs, NNRTIs, and protease inhibitors. The drug has a favorable pharmacokinetic profile with 84.3% subcutaneous bioavailability and a short elimination half-life of approximately 3.8 hours, necessitating twice-daily dosing. Although injection site reactions are nearly universal, enfuvirtide remains a critical salvage therapy option for patients with multi-drug-resistant HIV-1 infection.
Who Should Consider Enfuvirtide (Fuzeon)
- Treatment-experienced adults with multi-drug-resistant HIV-1 infection
- Pediatric patients weighing at least 11 kg with treatment-resistant HIV-1
- Patients with documented resistance to NRTIs, NNRTIs, and protease inhibitors
- HIV-1-positive individuals requiring salvage therapy regimens
- Patients intolerant to integrase inhibitor-based regimens who need alternative ART options
How Enfuvirtide (Fuzeon) Works
Enfuvirtide is a synthetic 36-amino-acid peptide that mimics the heptad repeat 2 (HR2) domain of the HIV-1 transmembrane glycoprotein gp41. During normal HIV entry, the viral envelope glycoprotein gp120 binds to the CD4 receptor and a coreceptor (CCR5 or CXCR4) on the host T-cell surface, triggering a conformational change in gp41 that exposes its HR1 coiled-coil trimer. Normally, the HR2 regions fold back onto HR1 to form a thermostable six-helix bundle structure that pulls the viral and cellular membranes together, enabling fusion and viral entry. Enfuvirtide competitively binds to the HR1 trimer groove, blocking the HR1-HR2 interaction and preventing six-helix bundle formation. This arrests the fusion process at an intermediate stage, effectively locking gp41 in a pre-hairpin conformation. Because enfuvirtide acts extracellularly at the viral surface before membrane fusion occurs, it represents a fundamentally different mechanism from all intracellular antiretroviral classes. Resistance to enfuvirtide maps primarily to a 10-amino-acid motif (positions 36-45) in the HR1 domain of gp41.
What to Expect
Enfuvirtide reaches steady-state plasma concentrations within the first week of twice-daily dosing. Injection site reactions typically appear within the first few days. Early viral load decline begins as HIV fusion is inhibited.
Significant viral load reduction expected, typically 1.0-1.5 log10 copies/mL decline when combined with optimized background therapy. CD4+ T-cell count begins to increase. Injection site management routine becomes established.
Continued virologic suppression with sustained CD4+ recovery. Many patients achieve viral loads below 400 copies/mL. Injection site reactions may improve with consistent site rotation and proper technique.
Durable viral suppression maintained. TORO trial data showed mean CD4+ increase of +91 cells/mm3 in the enfuvirtide group. Immunologic recovery supports reduced opportunistic infection risk.
Long-term suppression sustained in patients with adequate adherence. 48-week TORO data confirmed durable efficacy with mean viral load reduction of -1.48 log10. Ongoing monitoring for resistance mutations in gp41 HR1 region recommended.
Dosing Protocol
| Level | Dose / Injection | Frequency |
|---|---|---|
| Beginner | 90mg | 2x Daily |
| Moderate | 90mg | 2x Daily |
| Aggressive | 90mg | 2x Daily |
Note: Enfuvirtide is the first FDA-approved HIV fusion inhibitor, a 36-amino-acid synthetic peptide derived from the HR2 region of HIV-1 gp41. It is administered as 90 mg twice daily via subcutaneous injection as part of combination antiretroviral therapy (cART). Each vial contains 108 mg of lyophilized powder and is reconstituted with 1.1 mL of Sterile Water for Injection (SWFI) to deliver approximately 90 mg/mL. Enfuvirtide is reserved for treatment-experienced patients with ongoing HIV-1 replication despite existing antiretroviral regimens. Rotate injection sites between upper arm, anterior thigh, and abdomen to minimize injection site reactions, which occur in nearly all patients.
How to Inject Enfuvirtide (Fuzeon)
Reconstitute the 108 mg vial with 1.1 mL of Sterile Water for Injection (SWFI). Gently tap the vial for 10 seconds, then roll gently between hands to avoid foaming — do not shake. Allow the vial to stand until the powder is completely dissolved, which may take up to 45 minutes. The reconstituted solution should be clear, colorless, and free of bubbles or particulate matter. Inject 1 mL (90 mg) subcutaneously into the upper arm, anterior thigh, or abdomen. Rotate injection sites with each dose — never inject into the same site as the previous injection, into moles, scar tissue, bruises, or areas affected by current injection site reactions. Administer at approximately the same times each day, 12 hours apart. Allow refrigerated solution to reach room temperature before injection.
Cycling Protocol
Continuous therapy as part of combination antiretroviral treatment (cART). Enfuvirtide is not cycled — it is administered continuously for as long as virologic suppression is maintained and the drug is tolerated. Interruption of ART risks viral rebound and resistance development.
Pharmacokinetics
Source: FDA-approved label: mean elimination half-life 3.8 ± 0.6 hours following 90 mg SC dose (NDA 021481)
Loading the interactive decay curve.
Side Effects
Injection site reactions (ISRs) are the most common adverse effect, occurring in approximately 98% of patients. ISRs include pain and discomfort (96%), erythema (91%), induration (90%), nodules and cysts (80%), pruritus (65%), and ecchymosis (52%). Most ISRs are mild to moderate in severity. Systemic side effects include diarrhea (31.7%), nausea (22.8%), and fatigue (20.2%). An increased rate of bacterial pneumonia has been observed, particularly in patients with low baseline CD4+ counts, high viral load, IV drug use, smoking history, or pre-existing lung disease. Rare but serious hypersensitivity reactions have been reported, including rash, fever, nausea, vomiting, chills, rigors, and hypotension — therapy should be immediately discontinued if these occur. Immune reconstitution inflammatory syndrome (IRIS) may occur when initiating combination ART. Eosinophilia has been observed in some patients.
Contraindications
- Known hypersensitivity to enfuvirtide or any component of the formulation — do not rechallenge after systemic hypersensitivity reaction
- Not indicated for treatment-naive patients — approved only for treatment-experienced individuals with evidence of ongoing viral replication
- Patients with active or history of severe coagulation disorders — increased risk of post-injection bleeding and hematoma
- Severe hepatic impairment — limited safety data; use with caution and monitor closely
- Pregnancy — FDA Category B; use only if clearly needed. Breastfeeding contraindicated in HIV-positive mothers regardless of treatment
Drug Interactions
- No clinically significant CYP450-mediated interactions — enfuvirtide is catabolized to constituent amino acids and does not inhibit or induce hepatic enzymes
- Anticoagulants (warfarin, heparin) — potential increased risk of post-injection bleeding; monitor coagulation parameters
- Concurrent antiretrovirals — enfuvirtide is always used in combination with other ARVs; no dose adjustments needed for NRTIs, NNRTIs, PIs, or integrase inhibitors
- Protease inhibitors (tipranavir/ritonavir) — may slightly reduce enfuvirtide concentrations, but no dose adjustment is recommended
Storage & Stability
Molecular Profile
Related Peptides
References
- Efficacy of enfuvirtide in patients infected with drug-resistant HIV-1 in Europe and Australia (TORO 2) — N Engl J Med 2003PubMed 12773645
- Durable efficacy of enfuvirtide over 48 weeks in heavily treatment-experienced HIV-1-infected patients in TORO 1 and 2 — J Acquir Immune Defic Syndr 2005PubMed 16280694
- TORO: ninety-six-week virologic and immunologic response and safety evaluation of enfuvirtide — AIDS 2007PubMed 17711378
- FUZEON (enfuvirtide) FDA Prescribing Information — 2018 LabelFDA Label
- Enfuvirtide: from basic investigations to current clinical use — HIV Ther 2010PubMed 20977403