Enfuvirtide

In plain language

Enfuvirtide was the first HIV drug that worked by physically blocking the virus from entering cells, rather than targeting enzymes inside them. It was sold as Fuzeon, a twice-daily injection under the skin.

It mimics one of the coils HIV throws out like a harpoon to fuse with the immune cell, and it jams the mechanism mid-throw. It was a breakthrough in 2003 but required two injections a day and caused skin nodules; better oral drugs eventually made it obsolete, and U.S. sales ended in February 2025.

For students

Enfuvirtide is a 36-residue synthetic peptide (about 4,492 Da) derived from the HIV-1 gp41 C-terminal heptad repeat (HR2) region. It binds the N-terminal heptad repeat (HR1) of gp41 in the pre-hairpin intermediate state, preventing formation of the six-helix bundle that pulls viral and cellular membranes together, a mechanism termed fusion inhibition.

The TORO 1 and TORO 2 trials (n=1,013 combined) showed significant viral load reductions and CD4 gains when added to failing regimens in 2003-era patients, supporting approval. Its history teaches both the power and limits of entry inhibition: potency was real, but twice-daily 90 mg injections, frequent injection-site reactions (nearly all patients), and its use as salvage therapy for multi-drug-resistant virus, followed by the arrival of integrase inhibitors, limited its role. Roche ended U.S. distribution at the end of February 2025; it remains marketed in some other regions.

For researchers and instructors

Biochemistry. The first approved drug targeting a protein-protein interaction in a dynamic intermediate state (pre-hairpin) is the structural lesson; the six-helix bundle formation is a classic membrane fusion teaching model.

Pharmacology. 90 mg twice-daily subcutaneous injection; nearly universal injection-site reactions; hypersensitivity (including rare cases with hypotension and renal failure) in about 4-5% of patients; no food or renal-dosing considerations.

Human evidence. TORO 1 and TORO 2 (n=1,013 combined) are the pivotal trials; the accompanying long-term cohort data spanned the salvage-therapy era of the mid-2000s.

Teaching point. Teaches that mechanistic elegance does not guarantee commercial survival, and how a drug's fate can be sealed by the arrival of a more convenient drug class rather than by its own failures.