ReviewNon-nucleoside reverse transcriptase inhibitors (NNRTIs), their discovery, development, and use in the treatment of HIV-1 infection: A review of the last 20 years (1989–2009)
Introduction
We are nearing the end of the third decade of the Acquired Immune Deficiency Syndrome (AIDS) pandemic, and in the developed countries at least, the times when diagnosis of human immunodeficiency virus (HIV) infection represented a death sentence (Rothenberg et al., 1987) are over. Researchers around the world have worked hard, and nowadays 25 antiretroviral drugs have been approved by regulatory authorities for the treatment of HIV infection. Treatment guidelines recommend that patients be administered a combination of at least two to three active drugs from at least two different classes, and that the goal of therapy for all patients, regardless of the number of therapies they have been subjected to, should be to control virus replication, as measured by a plasma viral load below the quantification limit of current available assays, i.e. 50 or 40 HIV RNA copies/mL (DHHS, 2007, Gazzard, 2008). Available drugs belong to 6 different classes: eight nucleoside (nucleotide) reverse transcriptase (RT) inhibitors (N[t]RTIs), four non-nucleoside RT inhibitors (NNRTIs), ten protease inhibitors (PIs) and one integrase inhibitor, which are targeted at viral enzymes; a fusion inhibitor, which prevents the fusion of the virus envelope with the host-cell membrane; and a CCR5 inhibitor, which blocks the interaction of the virus with one of its receptors on the host cell (De Clercq, 2009). Although some of these drugs are minimally used, because of their side effect profiles, high pill burden and/or inconvenient administration schedules, or difficult to manage drug-drug interactions, patients and their treating physicians still have at their disposition many options to construct the most appropriate combination regimen, the one that will best fit the requirements of the individual patient.
NRTIs were the first class of antiretroviral drugs to gain regulatory approval: in 1987 zidovudine (AZT) was the first drug to be licensed for the treatment of HIV infection (Ezzell, 1987), a mere four years after the identification of HIV as the etiology agent for AIDS (Barre-Sinoussi et al., 1983, Gallo et al., 1983). In those days, AZT was administered as monotherapy and as such would prolong the lives of patients for 6 to 18 months. The introduction of a second class of antiretroviral drugs, i.e. the PIs, with the approval of saquinavir in 1995, changed the picture of treatment of HIV infection. This opened the era of Highly Active Antiretroviral Therapy, or HAART. The recommended treatment regimens should combine three drugs, from at least two different classes, in order to keep virus replication under control, and to observe a concomitant increase in CD4 cells, and thus an immunologic improvement. HAART drastically reduced the incidence of opportunistic infections and death in AIDS patients. Shortly after the approval of the first protease inhibitors, a new class of antiretroviral drugs was introduced: the NNRTIs. Nevirapine was approved by the FDA in 1996, followed by delavirdine in 1997, and efavirenz in 1998. However, a significant proportion of patients did not fully benefit from HAART, as their virus had accumulated resistance to the available drugs, as a consequence of monotherapy, and/or suboptimal combination therapy regimens. In recent years, several molecules with better resistance profiles have been developed to address this issue. Among those, etravirine was the first NNRTI to demonstrate clinical efficacy in patients with NNRTI-resistant HIV-1 infection, and was approved by the FDA in 2008. Other NNRTIs with an improved resistance profile are currently being developed.
This review covers the NNRTI class of anti-HIV-1 drugs, from the initial discovery of the class in 1990 to the current compounds in clinical development, i.e. around 20 years of research and development efforts. It describes the characteristics of the NNRTIs, their mechanisms of action, HIV-1 resistance to the inhibitors, and the drugs that have been approved for the treatment of HIV-1 infection, or are currently in clinical development. The role of NNRTIs in prevention of HIV transmission is also addressed.
Section snippets
The HIV-1 RT enzyme
The Pol gene of HIV encodes three enzymes: the protease, the RT with embedded ribonuclease H (RNaseH) activity and the integrase. The HIV-1 RT is an asymmetric heterodimer, comprising a p66 subunit (560 amino acids) and a p51 subunit (440 amino acids) (Kohlstaedt et al., 1992). Both subunits are encoded by the same sequence in the viral genome. RNAseH consists of the last (carboxy terminal) 120 amino acids of the p66 subunit, which correspond to the p15 fragment cleaved from the p66 subunit by
The reverse transcription reaction
Reverse transcription is a complex process that requires different catalytic activities. A tRNA3Lys functions as a primer and hybridizes to the primer-binding site located near the 5′ end of the viral genome. This results in the synthesis of a short stretch of single stranded DNA, which then relocates and hybridizes to the repeat sequence at the 3′-end of the RNA genome. After this strand transfer, the synthesis of the first DNA strand can continue. The synthesis of the first DNA strand
Reverse transcriptase inhibitors
During its replication HIV-1 uses four different viral enzymes, all encoded by the Pol gene: the RT, the RNAseH, which is embedded in the RT, the integrase and the protease. Protease, RT and integrase are targets for antiretroviral drugs. Two classes of antiretroviral drugs inhibit the reverse transcriptase reaction: the N(t)RTIs and the NNRTIs. Table 1 summarizes the characteristics of the two classes of HIV-1 RT inhibitors, and highlights the differences between N(t)RTIs and NNRTIs.
N(t)RTIs
The NNRTI binding pocket
Despite the chemical heterogeneity of NNRTIs, they all bind at the same site in the RT. This binding site is located in the palm domain of the p66 subunit of the heterodimeric protein, between the β6–β10–β9 and β12–β13–β14 sheets, and at the basis of the β4–β7–β8 sheet, at a distance of approximately 10 Å from the catalytic site of the enzyme. This pocket is hydrophobic in nature and is lined by the aromatic (Y181, Y188, F227, W229, and Y232), hydrophobic (P59, L100, V106, V179, L234, and P236),
Mechanisms of inhibition of HIV-1 replication by NNRTIs
The discovery of the TIBO compounds and the unexpected finding that they were RT inhibitors (Kukla et al., 1991, Pauwels et al., 1990) triggered research to define the mechanism of action of this family of compounds. This led to the definition of the NNRTI class. Indeed, even if the HEPT compounds had been described earlier (Baba et al., 1989), they had been originally designed as NRTIs (Miyasaka et al., 1989), and it is only later that it was suggested that they would share a common mechanism
Mechanisms of HIV-1 resistance to NNRTIs
Resistance is the cause and/or the consequence of treatment failure. HIV infection is characterized by a very high replication rate, with the production of 1 to 10 billion new virus particles per day in an untreated infected individual (Perelson et al., 1996). Moreover, HIV-1 RT lacks exonucleolytic proof-reading functionality, and this results in an average error rate per detectable nucleotide incorporated of 1/1700 (Roberts et al., 1988). Combining these two factors with the length of the
Clinical use of NNRTIs: treatment of HIV-1 infection
NNRTIs in combination with other antiretroviral drugs have been used for the treatment of HIV-1 infection for over a decade. So far, four drugs in this class have been approved by regulatory authorities, although several others have entered clinical development, but were discontinued for efficacy, pharmacokinetic and/or safety reasons. This review covers the approved NNRTIs, as well as those currently in clinical development. For an overview of other NNRTIs see Balzarini (2004), De Clercq (2004)
Other potential clinical uses of NNRTIs
Although the armamentarium of antiretroviral drugs for the treatment of HIV infection nowadays answers most of the needs of patients and their physicians in terms of long term control of virus replication, HAART remains heavy and cumbersome for patients, even if progresses have been made on the convenience and tolerability of treatment regimens. Moreover, the vast majority of HIV infected individuals live in the developing world, and despite recent large scale initiatives it is estimated that
Conclusions
In 1994, Kilby and Saag asked the question: “Is there a role for non-nucleoside reverse transcriptase inhibitors in the treatment of HIV infection?” (Kilby and Saag, 1994). The answer 15 years later is definitely “yes, there is”.
Although originally studied in suboptimal conditions, efavirenz and nevirapine have demonstrated over the years that, despite their low genetic barrier to the development of resistance, they afford long and sustained virologic efficacy, when combined with two other
Acknowledgements
The skillful editorial help of Karina Uyttersprot is greatly appreciated. The author wishes to thank Julie Mori (Pfizer) for providing the latest presented information on lersivirine.
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