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Enfuvirtide
A 36-amino acid synthetic peptide that prevents HIV-1 entry into host cells by blocking the viral fusion machinery, representing the first approved HIV fusion inhibitor.
Overview
Enfuvirtide is a synthetic 36-amino acid peptide that was the first antiretroviral drug to target the HIV entry/fusion step and the first peptide-based antiretroviral approved by the FDA (2003). Developed by Trimeris and Roche and marketed as Fuzeon, enfuvirtide works by binding to the heptad repeat 1 (HR1) region of the HIV-1 gp41 transmembrane glycoprotein, preventing the conformational change required for viral-host cell membrane fusion. By blocking this fusion step, enfuvirtide prevents HIV-1 from entering and infecting CD4+ T cells.
The mechanism of enfuvirtide is fundamentally different from all other classes of antiretroviral drugs that target intracellular viral processes (reverse transcription, integration, protease cleavage). Because enfuvirtide acts extracellularly at the viral entry step, it retains full activity against HIV strains resistant to nucleoside/non-nucleoside reverse transcriptase inhibitors, protease inhibitors, and integrase inhibitors. This made enfuvirtide particularly valuable for treatment-experienced patients with multidrug-resistant HIV.
The TORO (T-20 vs. Optimized Regimen Only) trials demonstrated that adding enfuvirtide to an optimized background regimen significantly improved virologic and immunologic outcomes in heavily treatment-experienced patients with multidrug-resistant HIV. At 24 weeks, enfuvirtide-treated patients achieved approximately 1 log greater HIV RNA reduction and 30-35 more CD4+ cells compared to the control group.
Despite its efficacy, enfuvirtide's clinical use has been limited by the requirement for twice-daily subcutaneous injections, frequent and often painful injection site reactions, the complexity of reconstitution, and high cost. These practical barriers, combined with the development of newer oral antiretroviral classes (integrase inhibitors, CCR5 antagonists) that are effective against resistant virus, have made enfuvirtide a drug of last resort. However, it remains an important option for patients with extremely limited treatment alternatives.