HIV Drug Resistance Workshop: new concepts for the clinician

Researchers from Cleveland in a study of nelfinavir (NFV) and its primary mutation D30N which has been shown to reduce replicative competence posed a question that should be of interest to clinicians: can we force HIV-1 to select for and retain the D30N mutation which may lead to a virus with impaired replication capacity? And is this related to the amount or dose of NFV?

Viral genetic evolution was evaluated through serial passages using (i) three different concentrations of NFV, (ii) combinations of NFV and a second PI and (iii) these PIs alone (not NFV). Although all three concentrations of NFV (0.1, 0.5, and 1 uM) lead to similar resistance pathways (D30N followed by M36I and M46I/L), the concentrations did indeed have an impact on resistance evolution. The 1uM NFV dose selected the L23I as an additional mutation. Combining NFV with other PIs resulted in the extinction of this virus following several passages, but the NFV and indinavir (IDV) combination led to an L24I and N88D virus profile. Interestingly, when amprenavir (APV) was selected, the D30N did not survive. On this 1 uM dose, the study found that ritonavir (RTV) or NFV and IDV impaired replicative capacity most profoundly.

The authors confirm that as NFV levels rise, replicative capacity in viruses with the D30N mutation declines. Furthermore, in patients with N88S, an amprenavir (APV) associated mutation, the D30N virus showed hypersusceptibility (HS) to APV – that is, increased susceptibility to APV at a specific drug concentration when compared with a virus lacking that resistance mutation. The study helps to confirm the value of combining therapeutic drug monitoring (TDM) with resistance testing. What we now need is to establish not only therapeutic cut-offs for drugs, but drug exposure levels that enable physicians to sustain their choice of mutations that may confer selective advantage through cultivating a poorly-replicating virus (Quinones-Mateu).

Further evidence that RC and HS are distinctly correlated was demonstrated by a study from Virologic manipulating their sizeable database of clinical samples. HS was defined as PI fold-change in IC₅₀ below 0.4 and RC below 44%. The results were stratified as low RC/PI-HS, high RC/PI-HS, low RC/PI not HS and high RC/PI not HS. They found significant correlation between RC and PI susceptibility depending on the type of PI. Interestingly, confounding the popular conception that PI HS is a marker for impaired RC, they found that the quantity of infectious virus did not in fact impact on PI susceptibility (Bates).

For patients starting therapy and those with an established treatment history, the selection of the appropriate combination of compounds is crucial not only for efficacy, for the management of toxicities and to minimise pharmacokinetic interactions but also to exploit favourable resistance interactions. Data are accumulating that suggests that whilst clinicians should remain alert for mutations that are proven to be deleterious, they should also be mindful of mutations that may confer some synergistic and potentially beneficial interaction. As Dr Francois Clavel of Bichat, Paris, noted “not all mutations are created equal”.

In a study from Erlangen, Germany, genotypic and phenotypic analyses of samples taken from seven patients resistant to all PIs except saquinavir (SQV) and atazanavir (ATV) despite presence of the 82 mutation associated with intermediate-level resistance to ATV. All samples had resistance with the 82 mutation and the lopinavir (LPV) associated mutation at L76V, reported in vitro (conferring >4-fold resistance to LPV).

The presence of the L76V mutation appears to revert resistance, showing a decrease in viral load despite previous PI experience and resistance accumulation (VL reductions between >2.2 log and 3.1 log observed within 12 weeks). The explanation for this response and the best strategy for maintaining it remain unclear. An effect of this drug combination and mutational pattern on replicative capacity cannot be ruled out. What is also unclear at this point is whether the L76V mutation persists after removal of LPV pressure.

Nevertheless, the research findings do suggest that in the salvage context, interactions between mutations should be carefully considered for their potential to improve treatment outcomes. Other data including those from Dr Clavel’s laboratory seem to confirm a similar sequence of resistance evolution(Walter).

What were the key messages from this year's Resistance Workshop?

As researchers learn more about the ways in which HIV develops resistance to entry and attachment inhibitors, the future shape of therapy with these new classes of drugs becomes clearer.

What sort of resistance is occurring in clinic populations, and what does this tell us about clinical practice today?

The threat of nevirapine resistance is forcing a rethink of strategies to prevent mother to child transmission. The workshop learnt more about resistance patterns, and why differences in HIV-1 subtype may need to be taken into account when thinking about preventing mother to child transmission.

As treatment access expands and the proportion of patients in Europe with non-B subtypes grows, understanding differences in resistance patterns between B and non-B HIV subtypes becomes more and more important.