The HIV-1 reverse transcriptase E138A natural polymorphism decreases the genetic barrier to resistance to etravirine in vitro

Objectives: The HIV-1 reverse transcriptase (RT) natural polymorphism E138A is included among the mutations with a minor impact on response to etravirine. However, the interpretation of E138A on etravirine susceptibility is not consistent across different genotypic resistance algorithms. The aim of the study was to investigate the effect of E138A on the genetic barrier to resistance to etravirine in vitro. Methods: A panel of 20 clinically derived recombinant viruses (10 with WT 138E and 10 with 138A, all without any other resistance mutation) were cultured in the presence of increasing etravirine concentrations and analysed for genotypic changes at virus breakthrough. Parallel experiments were conducted with 138E/A/G/K/Q NL4-3-based clones. Results: In the NL4-3 background, codon 138 changes increased etravirine resistance in the following order: Q>K>A>G>E. The 138A viruses were less susceptible to etravirine compared with the 138E viruses [median (IQR) fold change, 1.8 (1.5-2.8) versus 1.3 (0.8-1.8); P=0.026], overcame etravirine pressure earlier [HR (95%CI) for viral outgrowth with 138A, 5.48 (2.95-28.24); P<0.001] and grew at higher drug concentrations [median (IQR), 1350 (1350-1350) versus 0 (0-1350) nM; P=0.005]. A variety of etravirine resistance-related mutations and changes in the RT connection and RNase H domains accumulated without any consistent pattern depending on baseline codon 138. Conclusions: E138A can contribute to reduced response to etravirine through a decreased genetic barrier to resistance. In vitro drug resistance selection is a valuable complement to define the full potential of low-level resistance mutations.