704 
Contribution of Non-Thymidine Analog Nucleoside RT Inhibitor Associated Mutations to Phenotypic Hypersusceptibility to Efavirenz
E Coakley and Neil Parkin*
ViroLogic, Inc, South San Francisco, CA, USA
Background: Efavirenz (EFV) hypersusceptibility has
been associated with thymidine analog mutations (TAM)
and superior clinical outcomes in subjects treated with EFV. Recent studies
have highlighted the activity and resistance profiles of a variety of thymidine analog-sparing nucleoside analog combinations
(TA-NAM). Therefore we evaluated the effect of non-TA NAM on EFV
susceptibility within the virologic database.
Methods: EFV hypersusceptibility
was defined as fold-change < 0.4 using the phenosense
assay. Isolates with unmixed nucleoside reverse transcriptase inhibitor (NRTI)
mutations (K65R, T69X, L74IV, V75X, M184IV; X = any non-wild type amino acid)
but no TAM (M41L, D67N, K70R, L210W, T215FY, K219X), Q151M, T69 insertions or
NNRTI mutations (A98G, L100I, K101EP, K103NS, V106AM, Y181X, Y188X, G190X,
P225X, F227X, M230L, P236L) were identified. Isolates without NRTI, NNRTI or PI
mutations served as a wild type reference group.
Results:
*p =
0.03 excluding a single outlier with EFV fold change = 13.
Mean EFV fold change was
lower for isolates with 1 NAM
than wild type (p < 0.0001) and 2 than
1 NAM
(p = 0.0006). When analyzed by
specific mutation, samples with only K65R, T69X OR M184IV demonstrated
significantly reduced EFV fold change than wild tyype.
Further, the effect was greater for K65R than T69X (p = 0.041) but M184IV (p =
0.1). Isolates with K65R+M184IV had lower mean EFV fold change than M184IV
alone (p < 0.0001) but not lower
than K65R alone (p = 0.12). NNRTI
susceptibilities were measured in site-directed mutants bearing K65R in an NL4-3
background; fold change to EFV, nevirapine (NVP), and
delavirdine (DLV) were 0.56 ± 0.02, 0.53 ± 0.05, and
0.54 ± 0.03, respectively (mean ± SD of 18 replicates).
Conclusions:
Groups of viruses containing the
specific non-TA NAM
K65R, T69X, M184IV, and K65R+M184IV demonstrate enhanced EFV susceptibility. An NL4-3/K65R site-directed mutants demonstrated
significantly lower mean fold change to EFV, NPV, and DLV. Thus EFV hypersusceptibility is associated with both non-TA NAM in addition
to TAM. These observations may have relevance to the use of NNRTI in non-TA-NRTI
combination regimens and may help explain the superior virologic
efficacy of these regimens.
Keywords: efavirenz; hypersusceptibility; reverse transcriptase
