Objectives: To estimate the in vivo fitness cost of enfuvirtide (ENF) resistance, we analyzed dynamic shifts in the HIV-1 quasispecies under changing selective pressure in 3 subjects on failing ENF-based regimens who interrupted ENF while maintaining stable background regimens. Subsequently, ENF was readministered for 4 weeks as "pulse intensification."
Methods: The proportion of plasma virus carrying the V38A mutation in gp41 was quantified by allele-specific real-time polymerase chain reaction in serial samples collected from 3 subjects at 1- to 4-week intervals. Fitness differences were calculated using a method that corrected for time dependence of the viral replication rate.
Results: The V38A mutant made up >or=85% of the quasispecies at baseline and decayed to <5% over 12-24 weeks; plasma HIV-1 RNA levels remained stable during this time. Fitness differences for mutant versus wild type ranged from -25% to -65%, providing in vivo evidence for the reduced fitness of ENF-resistant HIV-1. The V38A mutant virus reemerged rapidly during the ENF pulse.
Conclusions: These results demonstrate that the HIV-1 quasispecies undergoes dynamic changes in response to withdrawal and reinitiation of fusion inhibitor therapy. The relative stability of plasma HIV-1 titers during decay of V38A suggests that factors other than viral fitness likely define viral load set-point in patients with advanced disease.