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Enfuvirtide (Fuzeon)36 residuesYTSLIHSLIEESQNQQEKNEQELLELDKWASLWNWFEach bubble = one amino acid. Size = residue mass. Color = chemical class.Also known as: T-20, Fuzeon, DP-178
A 36-amino-acid peptide that blocks HIV at the front door. Enfuvirtide (Fuzeon) was the first FDA-approved fusion inhibitor, stopping the virus from physically merging with CD4+ T-cells. In the TORO trials, it cut viral loads by 1.48 log10 copies/mL more than background therapy alone across 995 patients. The catch: injection site reactions hit 98% of users, and twice-daily subcutaneous dosing made adherence brutal. Genentech pulled it from the US market in February 2025 after integrase inhibitors and long-acting injectables made it obsolete. The science still holds. The product does not.
Enfuvirtide (brand name Fuzeon, T-20, CAS 159519-65-0) is a 36-amino-acid synthetic peptide derived from the HR2 domain of HIV-1 gp41. It works exactly as designed, but 98% of patients got injection site reactions, and that single number explains why it's commercially dead. It was approved by the FDA on March 13, 2003, as the first member of the fusion inhibitor class of antiretrovirals. The mechanism is different from everything else in HIV medicine. Instead of targeting enzymes inside infected cells, enfuvirtide parks itself on the viral surface. It binds the HR1 trimer groove of gp41, blocking the six-helix bundle formation that normally pulls viral and cellular membranes together. No bundle, no fusion, no entry. Because it works extracellularly, it retains activity against strains resistant to NRTIs, NNRTIs, and protease inhibitors. The TORO-1 and TORO-2 trials (Lazzarin et al., NEJM 2003)[1] enrolled 995 treatment-experienced adults. At 48 weeks, the enfuvirtide arm showed a mean viral load reduction of 1.48 log10 copies/mL over optimized background regimen alone (which managed only 0.63 log10). CD4+ recovery was +91 cells/mm3 versus +45 cells/mm3. The 96-week extension [2] confirmed durable results. Today, enfuvirtide is a historical artifact. Genentech discontinued Fuzeon in the US on February 28, 2025. Oral integrase inhibitors like dolutegravir and long-acting cabotegravir/rilpivirine injections replaced it entirely. The pharmacology is solid; the clinical niche vanished.
Enfuvirtide works outside the cell. That distinction matters more than it sounds. During HIV infection, the viral envelope protein gp120 first binds the CD4 receptor and a coreceptor (CCR5 or CXCR4) on the T-cell surface. This triggers a conformational shift in gp41, exposing its HR1 coiled-coil trimer. Normally, the HR2 regions fold back onto HR1 to form a thermostable six-helix bundle. That bundle pulls the viral membrane and the cell membrane together, completing fusion. Enfuvirtide is a synthetic copy of the HR2 domain. It competes for the HR1 trimer groove, blocking HR2 from folding back. The result: gp41 gets stuck in a pre-hairpin intermediate. Fusion stalls. The virus cannot enter. Every other antiretroviral class (NRTIs, NNRTIs, protease inhibitors, integrase inhibitors) works inside the cell after infection has already occurred. Enfuvirtide acts before the virus ever crosses the membrane. This extracellular mechanism explains why it stays active against strains that have accumulated resistance to intracellular drug targets. Resistance does develop, though. Single mutations at positions 36 through 45 in the gp41 HR1 domain can confer 5 to 100-fold decreases in susceptibility. The genetic barrier is low, which is why enfuvirtide should never be used without combination partners.
Fixed 90 mg SC twice-daily dosing is robustly validated by Phase III TORO-1 and TORO-2 trials (NEJM 2003) and confirmed through 96-week extension data. Mean -1.48 log10 copies/mL viral load reduction over optimized background regimen alone (-0.63 log10) at 48 weeks. CD4+ recovery +91 vs +45 cells/mm³. No dose-finding uncertainty: fixed-dose antiretroviral. Now discontinued commercially (Feb 28, 2025); pharmacological data remains valid.
TORO-1 and TORO-2 pivotal Phase III trials: Lalezari et al. NEJM 2003 (PMID 12773645)
Drug commercially discontinued in the US as of February 28, 2025. No new clinical studies anticipated. Evidence base is from 2003–2007 trials. ISRs occur in ~98% of patients (near-universal). Resistance develops rapidly without combination ART partners. Superseded clinically by integrase inhibitors and long-acting injectables.
Historically characterized by near-universal frustration with injection site reactions and twice-daily SC burden. Patients who used it reported it worked virologically but was difficult to tolerate long-term. Minimal community presence post-2010 as oral ARV options improved. No performance/wellness use community exists.
The minimal available patient experience aligns with clinical trial data: virologic efficacy confirmed, ISR burden and injection fatigue are exactly as documented in TORO trials. No community dose experimentation exists. Discontinued drug with no active user community.
| Level | Dose / Injection | Frequency |
|---|---|---|
| Beginner | 90mg | 2x Daily |
| Moderate | 90mg | 2x Daily |
| Aggressive | 90mg | 2x Daily |
Each vial contains 108 mg of lyophilized powder. Add 1.1 mL of Sterile Water for Injection (SWFI only, not bacteriostatic water) to get roughly 90 mg/mL. Draw 1.0 mL to deliver the 90 mg dose. The reconstitution takes patience. Tap the vial gently for 10 seconds, roll between your palms, then wait. Full dissolution can take up to 45 minutes. Don't rush it. Injecting undissolved powder is a real problem, not a theoretical one. The solution should be completely clear, colorless, and free of bubbles before you draw. Refrigerate after reconstitution at 2 to 8 degrees C. Use within 24 hours. There's no preservative, so leftover solution gets discarded. One thing most people miss: never inject cold solution. Pull the vial from the fridge and let it warm to room temperature for 10 to 15 minutes before injection. Cold subcutaneous injections increase local pain considerably.
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.
Enfuvirtide is an antiretroviral and must never be cycled or interrupted. Interruption of ART causes HIV viral rebound (typically within 1–3 weeks) and can select for resistant viral quasispecies. The low genetic barrier to enfuvirtide resistance (single gp41 HR1 mutations sufficient for 5–100× resistance fold-change) makes intermittent use especially dangerous. Continuous administration is mandated by the FDA label and by all HIV treatment guidelines (DHHS, EACS, WHO). The cyclingProtocol.onWeeks: 52, offWeeks: 0 in the database correctly reflects this.
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Expected: Mean -1.48 log10 copies/mL viral load reduction at 48 weeks (over optimized background). Mean +91 CD4+ cells/mm³ over OBR-alone arm. ~37% achieving VL <400 copies/mL at 48 weeks vs. 16% in OBR-alone arm. Durable suppression confirmed through 96 weeks.
Monitor: HIV-1 RNA viral load at baseline, 4–8 weeks, then every 3–6 months. CD4+ count every 3–6 months. Watch for bacterial pneumonia (especially in patients with CD4 <200, smokers, IVDU). Monitor injection sites for severe ISRs requiring medical attention (~3% of patients).
Remove one 108 mg enfuvirtide vial and one 1.1 mL SWFI vial from storage.
Tap gently for 10 seconds, then roll between palms. Do not shake. Wait up to 45 minutes for complete dissolution.
Confirm the solution is clear, colorless, and free of particulate matter or bubbles.
If refrigerated, allow the reconstituted vial to reach room temperature (10 to 15 minutes).
Use a 27-gauge, 0.5-inch needle for subcutaneous injection.
Inject subcutaneously into the upper arm, anterior thigh, or abdomen. Rotate sites with every dose. Never inject into an active ISR site, scar tissue, moles, or bruises.
Administer at the same times daily, approximately 12 hours apart (for example, 8 AM and 8 PM).
Discard any unused reconstituted solution after 24 hours.
Enfuvirtide is always used as add-on therapy to ≥2 other active ARVs. NRTIs form the backbone of most OBRs. No pharmacokinetic interaction with enfuvirtide: no dose adjustment needed.
No pharmacokinetic interaction with enfuvirtide. Preferred OBR backbone partners if integrase inhibitor resistance has not yet emerged.
Post-attachment CD4-directed inhibitor; different mechanism, no direct cross-resistance with enfuvirtide. Ibalizumab has largely replaced enfuvirtide as the injectable entry inhibitor of choice for ultra-salvage therapy.
Oral attachment inhibitor targeting gp120; does not cross-resist with enfuvirtide (different mechanism and binding site). Currently the preferred oral salvage option for MDR-HIV when enfuvirtide is unavailable.
PK interaction: enfuvirtide increases tipranavir/ritonavir (TPV/r) AUC by ~45%. Monitor TPV/r concentrations; the interaction direction in peptides.ts is reversed and should be corrected.
No pharmacokinetic interaction, but elevated post-injection bleeding and hematoma risk at injection sites in anticoagulated patients. Caution with injection technique; monitor INR/coagulation parameters.
Pricing updated 2026-04-09
Injection site reactions affected 98% of patients in clinical trials. That is not a typo, and it is not something that can be managed away with better technique. The ISR profile from TORO trial data: pain and discomfort in 96% of patients, erythema in 91%, induration in 90%, nodules and cysts in 80%, pruritus in 65%, ecchymosis in 52%. Most reactions were graded mild to moderate. About 3% of patients needed medical intervention for severe ISRs, including deep nodules and abscess formation. Systemic side effects were also common. Diarrhea occurred in 31.7% of patients, nausea in 22.8%, and fatigue in 20.2%. These overlap with side effects from combination antiretroviral partners, so attributing them solely to enfuvirtide is difficult. The bacterial pneumonia signal deserves attention. TORO data showed higher bacterial pneumonia rates in enfuvirtide-treated patients compared to the control group. Patients at highest risk had CD4+ counts below 200 cells/mm3, active IV drug use, smoking history, or pre-existing lung disease. Any fever with productive cough warrants same-day medical evaluation. Hypersensitivity reactions are rare but serious. Reports include rash, fever, nausea, vomiting, chills, rigors, and hypotension. If systemic hypersensitivity develops, stop enfuvirtide immediately and do not rechallenge. This is not a "push through it" situation. Immune reconstitution inflammatory syndrome (IRIS) can occur when starting combination ART, as the recovering immune system mounts inflammatory responses against previously subclinical infections. Eosinophilia has also been observed in some patients. Anticoagulated patients face additional bleeding risk at injection sites. If you're on warfarin or heparin, monitor coagulation parameters closely and discuss injection technique with your provider. Pregnancy is FDA Category B. Use only if clearly needed. Breastfeeding is contraindicated in HIV-positive mothers regardless of treatment regimen.
Verify Enfuvirtide (Fuzeon) dosing and safety with a second opinion
Enfuvirtide (Fuzeon) is commercially discontinued in the US as of February 28, 2025. No authorized US manufacturer or distributor exists. Any currently available product is from pre-discontinuation supply, foreign-market diversion, or compounding pharmacies (no FDA oversight for this indication/formulation). Compounded enfuvirtide lacks the lyophilization stability and sterility validation of the original branded product. This is a 36-amino-acid SC peptide requiring specific sterile manufacturing standards.
| Test | When | Target |
|---|---|---|
| HIV-1 RNA viral load (PCR) | Baseline, 4–8 weeks after initiation, then every 3–6 months | Undetectable (<20–50 copies/mL depending on assay); minimum <400 copies/mL |
| CD4+ T-cell count | Baseline, 3 months, then every 3–6 months | >200 cells/mm³ (reduces major OI risk); +91 cells/mm³ over background expected at 48 weeks |
| Gp41 HR1 resistance genotype | At virologic failure (confirmed VL rebound after suppression) | Wild-type at positions 36–45; any HR1 mutation warrants therapy reassessment |
| Injection site assessment | Every clinic visit; patient self-assessment every injection | — |
| Bacterial pneumonia surveillance (clinical exam ± chest X-ray) | Baseline; any fever/cough/dyspnea episode; quarterly screen in high-risk patients | — |
| Liver function tests (ALT, AST, bilirubin) | Baseline, then per standard ART monitoring schedule | — |
Primary efficacy endpoint: confirms virologic suppression. Detectable rebound warrants gp41 HR1 resistance genotyping.
Immunologic recovery marker. CD4 count guides opportunistic infection prophylaxis decisions.
Mutations at positions 36–45 of gp41 HR1 can confer 5–100× enfuvirtide resistance. Determines whether to switch to ibalizumab, fostemsavir, or restructure OBR.
ISRs in 98% of patients. Grade 3+ ISRs (deep nodules, abscesses, severe pain) require site rest and technique review; ~3% of patients need medical intervention.
TORO trials showed increased bacterial pneumonia incidence vs OBR alone. High-risk: CD4 <200, active IVDU, smoking, prior pulmonary disease.
OBR partners (PIs, NNRTIs) carry hepatotoxicity risk. Enfuvirtide itself is metabolized to amino acids with minimal hepatic burden, but baseline and interval monitoring is standard ART care.
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.
Days 1 to 7: Enfuvirtide hits steady-state plasma levels fast, within the first week of twice-daily dosing (half-life is only 3.8 hours). Injection site reactions show up almost immediately. Pain, redness, and hard nodules at the injection site are essentially universal from day one. The 45-minute reconstitution wait per dose is the first adherence test. Weeks 2 to 4: This is where viral load starts dropping meaningfully. Combined with an optimized background regimen, expect a 1.0 to 1.5 log10 copies/mL decline. CD4+ counts begin climbing. ISRs are still constant, but site rotation technique usually improves. Fatigue from twice-daily injections starts building. Weeks 4 to 12: Viral suppression continues. Many patients reach VL below 400 copies/mL by week 8 to 12. Watch for IRIS as the immune system recovers. A small percentage develop severe ISRs (deep nodules, abscesses) that need medical attention. Patients with CD4 under 200, smokers, or IV drug users should be screened for bacterial pneumonia. Weeks 12 to 48: TORO primary endpoint window. The data at 48 weeks: 1.48 log10 viral load reduction over background alone, +91 CD4+ cells/mm3, and 37.1% reaching VL below 400 versus 16.1% in the control arm. Patients who made it past week 12 tended to stay on. Missed doses, not drug resistance, were the main reason for viral rebound. Week 48 to 96 and beyond: The 96-week extension confirmed durable suppression in responders. Resistance mutations at gp41 HR1 positions 36 to 45 can emerge if viremia is not fully controlled; single mutations can cause 5 to 100-fold resistance. Very few patients stayed on enfuvirtide this long because oral alternatives took over before this window arrived for most.
Enfuvirtide reaches steady-state within the first week of BID dosing (half-life 3.8 hours; steady-state in ~19 hours). Early viral entry inhibition begins from first dose. SC bioavailability 84.3%.
Injection site reactions (nodules, erythema, induration) appear within days 1–3. Pain on injection is near-universal. The 45-min reconstitution wait is the most common adherence complaint in the first week.
Significant viral load decline: mean 1.0–1.5 log10 copies/mL reduction when combined with an active optimized background regimen. CD4+ count begins rising. Peak early response window.
Patients report improvement in energy and fewer OI symptoms as CD4 recovers. ISRs ongoing but site-rotation technique improving. Fatigue from BID injection burden increases.
Continued virologic suppression. Many patients achieve VL <400 copies/mL by weeks 8–12. CD4+ recovery ongoing. IRIS can occur as immune system reconstitutes.
ISR habituation reported by patients who persist past week 6. A minority develop severe ISRs (deep nodules, abscesses) requiring site rest. Watch for bacterial pneumonia in high-risk patients.
TORO primary endpoint at 48 weeks: mean -1.48 log10 VL reduction over OBR alone. +91 CD4+ cells/mm³ over OBR-alone group. 37.1% vs 16.1% achieving VL <400 copies/mL.
Patients who tolerate the injection burden through week 12 tend to continue long-term. Viral rebound is typically due to missed doses, not drug resistance. Key attrition driver is adherence fatigue.
TORO 96-week extension confirmed durable efficacy in responders. Resistance mutations in gp41 HR1 (positions 36–45) can emerge if viremia is not fully suppressed: single mutations can confer 5–100× resistance fold-change.
Very few patients remained on enfuvirtide long-term: oral alternatives displaced it before this window. Those who continued confirmed durable efficacy; primary discontinuation reason was adherence, not virologic failure.
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.
Enfuvirtide received FDA approval on March 13, 2003, under NDA 021481 for the treatment of HIV-1 infection in treatment-experienced adults and pediatric patients weighing at least 11 kg. It was the first drug approved in the fusion inhibitor class. Genentech (Roche) discontinued US production and distribution of Fuzeon effective February 28, 2025. The drug is no longer commercially available in the United States. Any product currently circulating is from pre-discontinuation inventory, foreign-market supply, or compounding pharmacies operating without specific FDA oversight for this formulation. Enfuvirtide is not a WADA-banned substance, though its clinical indication is irrelevant to athletic performance. This content is for informational purposes only and does not constitute medical advice. HIV treatment decisions should be made in consultation with a qualified infectious disease specialist or HIV-treating clinician. Never initiate, modify, or discontinue antiretroviral therapy without medical supervision.
Peptide Schedule Research TeamReviewed Apr 20266 Citations
