Background:  Antiretroviral drugs often exert a persistent benefit despite the presence of drug resistance-associated mutations. Several mechanisms may account for this benefit, including partial anti-HIV activity of the drug against the resistant variant and/or the selective maintenance of mutations that reduce replicative capacity. The objective of this study is to define the degree to which enfuvirtide (ENF) has residual benefit during virologic failure, and to define the mechanism for this benefit.

Methods:  This is a prospective study of ENF-treated subjects with detectable viremia (> 400). ENF was interrupted while background drugs were continued. ENF phenotypic susceptibility and tropism were measured using single cycle replication entry assay in which patient-derived env genes was co-transfected with a luciferase-containing env-defective HIV genomic vector; infectivity is assessed by measuring the production of relative light units in cell lines expressing CCR5 or CXCR4. A segment of the gp41-coding region was also amplified, and multiple independent clones sequenced to assess resistance mutations.

Results:  We studied 20 subjects. The median baseline viral load was 5.12 log copies/mL (IQR 4.65 to 5.47) and the median CD4 cell count was 89 cells/mm³ (46 to 158). At week 2 off ENF, the median change in viral load was +0.19 (–0.07 to 0.36) (p = 0.05) and the median change in CD4 cells was +2 (–10 to +15) (p = 0.30). At week 16, the median change in viral load at week 16 was +0.14 (+0.04 to +0.28) (p = 0.04), while the change in CD4 cell count was –4 (–25 to + 1) (p = 0.14). ENF susceptible virus emerged by week 16 in the absence of therapy in most individuals (p = 0.003); this shift was associated with an increase in relative light units the dominant virus (R5 or X4) (median 0.26 log₁₀ relative light units increase, p = 0.01). Clonal analysis of HR-1 sequences revealed that ENF-resistance mutations could no longer be detected within 16 weeks of ENF cessation in most subjects.

Conclusions:  Interrupting ENF therapy in patients with ENF-resistant HIV was associated with a modest but measurable increase in viremia, indicating persistent low-level activity of the drug. ENF resistance waned rapidly in the absence of drug pressure. This observation is consistent with a fitness cost (as measured in the absence of drug) associated with ENF-resistance-associated mutations.  

Keywords: enfuvirtide; viral fitness; treatment interruptions