Background:  Resistance development to the first HIV-1 fusion inhibitor enfuvirtide (ENF) in gp41 is being clarified. A pivotal ENF binding site in the N-terminal part of the heptad repeat (HR1) comprises important resistance mutations previously defined (ie, ³⁶GIVQQQNN⁴³). The HR1 domain interacts in a complex secondary and tertiary manner with HR1 and HR2 domains in the envelope trimeric structure. We wanted to identify compensatory mutations that had an impact on the fitness of primary ENF resistance mutations.

Methods:  We followed a group of HAART-experienced patients treated with the fusion inhibitor ENF as part of their HAART regimen. By sequencing of virus obtained from plasma we followed genotypic changes in HR1 and HR2 domains of gp41. Fusion competence of selected variants was investigated in a single round viral infectivity assay.

Results:  The study confirmed the development of mutations in HR1 of primary importance in resistance development (N43D), but revealed an interesting compensatory mutation in HR2 (E137K) that either was present in the pre-treatment virus or developed during therapy. In a functional assay of viral envelope fusogenicity we found that the 43D genotype had a 1.4 log₁₀ decrease in infectivity which could be restored by introduction of 137K. Further studies of the 43D genotype strongly implied that the decreased fitness is a restriction at the membrane fusion step induced by the envelope. Of importance is that the 137K mutant in itself had a fusion capacity comparable to the wildtype. We investigated the prevalence of the 43D resistance mutation from 7 previously published ENF studies and found a significant increase of 43D when position 137 was polymorphic (ie, 137K/Q) compared with 137E (p = 0.014).

Conclusions:  Polymorphism at position 137 in HIV-1 gp41 is commonly observed in ENF-naďve patients. The 137K mutation increases the fitness of the 43D ENF-resistant genotype and the presence of 137K may therefore have a value for predicting the effect of ENF treatment.