ReviewFifteen years of HIV Protease Inhibitors: raising the barrier to resistance
Section snippets
HIV protease: function and structure
HIV protease plays an essential role in the viral life cycle. It generates mature infectious virus particles through cleavage of the viral Gag and GagPol precursor proteins (Kohl et al., 1988). The Gag precursor protein codes for all the structural viral proteins, matrix (p17, MA), capsid (p24, CA) and nucleocapsid (p7, NC), the p6 protein and the two spacer proteins p2 (SP1) and p1 (SP2) (Fig. 1A). The GagPol polyprotein is generated through a −1 ribosomal frameshift event, occurring at a
Inhibitors of the HIV protease
Detailed knowledge of the structure of HIV protease and its substrate has led to the development of specific protease inhibitors (PIs). They have been designed to bind the viral protease with high affinity but tend to occupy more space than the natural substrates. Currently, there are nine PIs approved for clinical use: saquinavir, ritonavir, indinavir, nelfinavir, amprenavir, lopinavir, atazanavir, tipranavir and darunavir (Fig. 3, Table 1). Most of them are prescribed with a concomitant low
Saquinavir
The first protease inhibitor to be approved in 1995 by the FDA in the US under accelerated approval regulations for use in antiretroviral combination therapy was saquinavir (Fig. 3, Table 1). Saquinavir showed in vitro activity against HIV in human lymphoblastoid and monocytic cell lines and in peripheral blood mononuclear cells (PBMCs) (Plosker and Scott, 2003). The concentration of saquinavir required to inhibit 50% replication (IC50) ranged from 20 to 500 nM in studies using cells that are
Second-generation protease inhibitor therapy; boosting of protease inhibitors
As indicated, the next major advance in the use of protease inhibitors came when it was recognized that ritonavir reduces the metabolism of concomitantly administered PIs through hepatic and intestinal cytochrome P450 3A4 inhibition, leading to dramatically improved bioavailability and half-life of these PIs. The first combination used in clinical trials was saquinavir and ritonavir (400 mg each bid).
This combination with both PIs in therapeutic doses, delivered a powerful high antiviral
Double-boosting protease inhibitor based therapy
Preceding the recent approval of several new antiretroviral compounds physicians experienced difficulties building an effective regimen for a group of heavily therapy-experienced patients with extensive drug resistance. In this setting of limited therapeutic options, the use of double-boosted PI was explored to gain possible synergistic or added antiviral activity of both drugs and to increase the genetic barrier to PI resistance. Although no large randomised trials assessing the clinical
Ritonavir-boosted protease inhibitor-monotherapy
Although NRTIs had been the cornerstone of highly active antiretroviral therapy, the finding that the originally to PIs attributed lipo-atrophy was mainly induced by these NRTIs, fuelled a search for alternative regimens. Moreover, the profound efficacy of boosted PI-based HAART and the high genetic barrier to resistance questionned the paradigm of a three-drug regimen. Combined with the challence of life long adherence and the risk for selection of multidrug resistance these considerations led
Evolution of resistance
Resistance to all the protease inhibitors has been observed and the genetic basis of resistance has been well documented. Within a chronically infected patient in the absence of effective treatment, continuous high level replication, lack of proofreading by the viral reverse transcriptase and recombination lead to the generation of massive numbers of genetically distinct viral variants, referred to as a viral quasispecies (Domingo et al., 1996). Within the viral quasispecies, wild-type is
Mechanisms of HIV protease resistance
The development of protease inhibitor resistance is a stepwise process in which a substitution in the substrate-binding cleft of the viral protease is usually observed first. These resistance mutations in the viral protease result in an overall enlargement of the catalytic site of the enzyme. This leads to decreased binding to the inhibitor (causing a decrease in drug sensitivity) and, in parallel, to some decrease in binding to the natural substrate and thus to decreased viral replication (
The influence of genetic diversity on protease inhibitor efficacy and selection of resistance
The vast majority of data regarding efficacy and selection of drug resistance to boosted PI have been generated for subtype B infections. Among different subtypes the variation in nucleotide sequence in the protease coding pol gene is approximately 10–15%, leading to distinct polymorphisms at amino acid level. These genetic differences have been reported to influence baseline susceptibility of PIs, the genetic barrier for selection of PI drug resistance and mutational pathways, but overall high
The use of boosted PIs in resource-limited settings
HAART became available in resource-limited settings after adoption of the Doha-declaration, enabling developing countries to circumvent patent rights for better access to essential medicines. Two years later the World Health Organization (WHO) launched their “3by5” initiative aiming at provision of antiretroviral therapy to three million people by the end of 2005. Even though this goal was not met, the initiative led to an unprecedented increase in therapy roll out and 2 years later, by the end
Novel boosting and PI strategies
Boosted PI containing regimens have proven to be a solid cornerstone of HIV-therapy. Still there are several concerns regarding use of PIs that require compelling attention, with respect to toxicity, and cross-resistance.
Recent in vitro experiments have demonstrated that several PIs show decreased insulin-mediated glucose disposal through an acute blockade of glucose transporter-4 (GLUT4) (Hruz, 2008). In addition, new onset diabetes mellitus, exacerbation of pre-existing diabetes mellitus and
Acknowledgements
We thank Dr. Pavlina Rezacova from the Institute of Organic Chemistry and Biochemistry Insitute of Molecular Genetics in Prague, Czech Republic for preparing Fig. 2. Funding resource: EU grant (LSHP-CT-2007-037693), a Dutch AIDSfonds grant (2006028) and the Netherlands Organization for Scientific Research (NWO) VIDI Grant (91796349).
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