• Molecular Mechanism of I50V, I54L and I54M Resistance to Amprenavir and Other HIV-1 Protease Inhibitors
  Abstract 563-T
  Authored by R. Xu, W. Andrews, A. Spaltenstein, D. Danger, W. Dallas, L. Carter, M. Hanlon, L. Wright, and E. Furfine (GlaxoSmithKline, Res. Triangle Park, NC)
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Emergence of drug resistance remains one of the most challenging issues in the treatment of HIV infection. Understanding the genetic pathways for resistance and the biochemical mechanisms of resistance have facilitated treatment planning and even aided the discovery and design of new drugs. For the nucleoside reverse transcriptase inhibitors and protease inhibitors, this understanding has led to the concept of rational sequencing of treatments, with the goal of having several rounds or successful therapy.

Resistance to the protease inhibitor amprenavir (APV, Agenerase) is mediated by one of several genetic pathways. The most common is one that is mediated by a unique resistance mutation at codon 50 of the protease gene, resulting in the substitution of the amino acid valine (V) for the wild-type isoleucine residue (I; called "I50V"). Viruses with this mutation have reduced susceptibility to amprenavir, but have retained susceptibility to the other protease inhibitors.

The second most prevalent mutational pathway involves mutations at protease gene codon 54. Amprenavir-selected codon 54 mutations are leucine (L) or methionine (M; "I54L" or "I54M"); these mutations are relatively unique to amprenavir -- other protease inhibitors select for different amino acid substitutions at the 54 position.

The susceptibility pattern of virus with the I50V mutation to other protease inhibitors has been previously described. Previously, the Xu group found that purified I50V-mutated protease enzyme has decreased binding affinity to amprenavir (140-fold) and ritonavir (50-fold) but only modest reduction in binding to nelfinavir (NFV, 10-fold) when compared to wild-type protease (Xu et al., 8th CROI). In the current study, I50V enzyme is noted to have 15-fold decrease in indinavir (IDV) binding. Atomic level (crystallographic) modelling of APV, IDV or NFV bound to the mutant enzymes provides a detailed look into why resistance happens. When the protease has the I50V substitution, a favorable environment for the binding of APV in the active site of protease is lost. APV is less likely to be bound to the inhibitor; hence HIV replication is less likely to be inhibited. The inhibitors IDV and NFV are less affected by these spatial alterations and therefore retain more activity in the enzyme (or virus) that contains the I50V substitution.

For viruses with the 54L or 54M substitutions, a similar pattern emerges, with alteration in the shape of the protease inhibitor binding site. These changes exclude APV from binding into the enzyme. These shifts are very similar in shape to the binding conformation with IDV or NFV, consistent with the retention of activity of IDV and NFV.

These data provide an important extension of our understanding of how resistance to amprenavir occurs and explains the lack of cross resistance with the two most common resistance pathways to amprenavir. Having this information may not have immediate implications for the use of amprenavir -- because the drug has been limited by high pill count and size. A newer chemical formulation of amprenavir (called 908) is in advanced clinical trials; this drug has a much lower pill count (two pills twice daily) and, when boosted with ritonavir, may be given once daily. The importance of this data is that it aids in understanding how amprenavir resistance occurs and suggests, mechanistically at least, how other drugs might retain activity in viruses that have acquired APV resistance. Since lopinavir (LPV, the active component of Kaletra) bears significant structural similarity to IDV and RTV, it is attractive to speculate that the APV-selected resistant viruses should retain activity to LPV/RTV (Kaletra) too.

I always caution physicians and patients alike to look at data with a careful eye to the limitations of a given study. While having details into the structure of the viral enzyme is important, clinical therapeutic decisions should be made after knowing the results of clinical therapeutic studies. Such studies are well underway with 908 in both the treatment-naïve and treatment-experienced patient population. In our clinic, the new protease appears to be very well tolerated and has good viral potency. Results from these studies should be presented by the 4th quarter of this year. A new, low pill count protease inhibitor will certainly be a welcome addition to treatment options.