A Novel Substrate-Based HIV-1 Protease Inhibitor Drug Resistance Mechanism

BACKGROUND: HIV protease inhibitor (PI) therapy results in the rapid selection of drug resistant viral variants harbouring one or two substitutions in the viral protease. To combat PI resistance development, two approaches have been developed. The first is to increase the level of PI in the plasma of the patient, and the second is to develop novel PI with high potency against the known PI-resistant HIV protease variants. Both approaches share the requirement for a considerable increase in the number of protease mutations to lead to clinical resistance, thereby increasing the genetic barrier. We investigated whether HIV could yet again find a way to become less susceptible to these novel inhibitors. METHODS AND FINDINGS: We have performed in vitro selection experiments using a novel PI with an increased genetic barrier (RO033-4649) and demonstrated selection of three viruses 4- to 8-fold resistant to all PI compared to wild type. These PI-resistant viruses did not have a single substitution in the viral protease. Full genomic sequencing revealed the presence of NC/p1 cleavage site substitutions in the viral Gag polyprotein (K436E and/or I437T/V) in all three resistant viruses. These changes, when introduced in a reference strain, conferred PI resistance. The mechanism leading to PI resistance is enhancement of the processing efficiency of the altered substrate by wild-type protease. Analysis of genotypic and phenotypic resistance profiles of 28,000 clinical isolates demonstrated the presence of these NC/p1 cleavage site mutations in some clinical samples (codon 431 substitutions in 13%, codon 436 substitutions in 8%, and codon 437 substitutions in 10%). Moreover, these cleavage site substitutions were highly significantly associated with reduced susceptibility to PI in clinical isolates lacking primary protease mutations. Furthermore, we used data from a clinical trial (NARVAL, ANRS 088) to demonstrate that these NC/p1 cleavage site changes are associated with virological failure during PI therapy. CONCLUSIONS: HIV can use an alternative mechanism to become resistant to PI by changing the substrate instead of the protease. Further studies are required to determine to what extent cleavage site mutations may explain virological failure during PI therapy

Determination of Phenotypic Drug Susceptibility of C-Terminal Gag Clones

<p>Representation of the fold increases in phenotypic drug susceptibility of the C-terminal Gag clone derived from IVS-1, containing the 436E+437T amino acid change as compared to wild-type HIV (HXB2). The black bars indicate the fold increase as determined in the multiple-cycle MTT drug susceptibility assay, and the gray bars indicate the fold increase as determined in the single-cycle PhenoSense drug susceptibility assay. Drug susceptibility to the PI RO033-4649, LPV, TPV, ATV, AMP, ritonavir (RTV), saquinavir (SQV), indinavir (IDV), and nelfinavir (NFV), and as a control to the RT inhibitor NVP was determined. RO033-4649 and TPV were not available at the time of PhenoSense assay testing.</p

Schematic Representation of the Viral Frameshift Region and Investigation of Frameshift Efficiency

<div><p>The RNA structure of the frameshift stimulatory signal is represented here [<a href="http://www.plosmedicine.org/article/info:doi/10.1371/journal.pmed.0040036#pmed-0040036-b030" target="_blank">30</a>,<a href="http://www.plosmedicine.org/article/info:doi/10.1371/journal.pmed.0040036#pmed-0040036-b051" target="_blank">51</a>]. The nucleotide and amino acid changes (between brackets) as observed in our in vitro selection experiments are indicated.</p> <p>(A) The relative frameshift efficiency of wild-type HIV (HXB2) and the p1 constructs of IVS-1 (436E+437T), IVS-32 (437V), and IVS-34 (437T). The values of the frameshift efficiency are the means of four independent experiments, with bars representing the standard errors.</p> <p>(B) Analysis of differences in levels of HIV GagPol products (RT, p66, and p51) as compared to HIV Gag products (CA, p24) by Western blot analysis.</p></div

Analysis of Gag Polyprotein Processing

<p>Western blot analysis of cell lysates (left panel) or virus particles (right panel) obtained by ultracentrifugation of cell culture supernatants from 293T cells transfected with the respective recombinant virus plasmids in the absence of inhibitor or in the presence of 50 or 500 nM of RO033-4649. Molecular mass standards are depicted on the left, Gag-derived proteins are identified on the right. Protein detection was performed following incubation with a specific antibody against NC.</p

Schematic Representation of the Distribution of all Amino Acid Changes Appearing during in vitro Selection Experiments Using RO033-4649

<p>Horizontal bars below the gene organization scheme represent the genomes of viruses from the three in vitro selection experiments: IVS-1, IVS-32, and IVS-34. Vertical lines illustrate the observed amino acid changes. The C-terminal portion of Gag was expanded to precisely map the nucleotide changes leading to amino acid changes (bold) in both translational reading frames (Gag and GagPol). In addition, the three different cleavage sites (NC/p1, NC/TFP, and TFP/p6^pol) are indicated. The control experiments, in which no drug was added during the in vitro selections, demonstrated no amino acid changes in the viral Gag and protease region.</p