Current Topics in Medicinal Chemistry - Volume 4, Issue 9, 2004

Human immunodeficiency virus type 1 (HIV-1) gene expression and transcription is an essential step in the viral life cycle, which is considered to be a possible target for inhibition of HIV-1 replication. Among the factors involved in this step, the cellular transcription factor nuclear factor (NF)-kB is the most potent inducer of HIV-1 gene expression, while the viral transactivator protein Tat seems to play a central role in sustaining a high level of HIV-1 replication. Another important mechanism of HIV-1 gene expression is the nuclear export control of viral mRNA conducted by the viral regulatory protein Rev. Various attempts have been undertaken to discover selective inhibitors of HIV-1 gene expression and transcription. Several small-molecule compounds were reported to inhibit Tat functions though blocking either the Tat / TAR RNA interaction or the kinase activity of cellular cofactors, such as cyclin T1 / CDK9. In the case of Rev inhibitors, it appears to be more difficult to find them than Tat inhibitors, and only a few compounds have been identified as Rev inhibitors. However, the selectivity of these Tat and Rev inhibitors was not high enough to eliminate the cytotoxicity to the host cells. Since the signal transduction pathways leading to NF-kB activation are redox regulated, several antioxidants have been shown to block HIV-1 transcription. Although some of them have progressed into clinical trials in HIV-1-infected patients, the results were not conclusive. In addition, various compounds have been identified as inhibitors of HIV-1 gene expression and transcription, yet their precise mechanisms are still unknown.

The chemokine receptors CXCR4 and CCR5 are used as the main co-receptors by the T-cell-tropic (CXCR4-dependent, X4) and macrophage-tropic (CCR5-dependent, R5) HIV-1 strains, respectively, for entering their target cells. The natural ligands for CXCR4, the CXCchemokine SDF-1 and CCR5, the CC-chemokines RANTES, MIP-1a and MIP-1β are described to inhibit viral entry. In this review we focus on chemokine receptor / HIV co-receptor inhibitors. Modified chemokines such as Met-RANTES and AOP-RANTES showed antiviral activity against R5 viruses. Several low-molecular weight CCR5 antagonists have been described (such as TAK-779 and SCH-C) with potent antiviral activity. The latter compound is also orally available and is able to decrease R5 viral load levels in HIV-infected subjects. Several peptidic compounds, such as T22 (an 18-mer), T134 (a 14-mer), ALX40-4C (a 9-mer) and CGP 64222 (also a 9- mer) have anti-HIV activity and have been identified as CXCR4 antagonists. Also, the HIV-1 Tat protein has been described as a “natural” CXCR4 antagonist with anti-HIV-1 activity. The most potent and specific CXCR4 antagonists are the bicyclam derivatives, which also potently inhibit X4 HIV replication. AMD3100 has proved to be a highly specific CXCR4 antagonist, which consistently blocks X4 viral replication in any target cell-type evaluated so far. AMD3100 was selected as the clinical drug candidate, which, after initial phase I (safety) studies, had proceeded to phase II (efficacy) trials. The compound dose-dependently inhibited X4 viruses after 10 days of continuous infusion of the drug. Recently, the orally bioavailable CXCR4 antagonist, AMD070, is presented as a candidate HIV drug. We believe that chemokine receptor antagonists will become important new antiviral drugs to combat AIDS. Both (CXCR4 and CCR5) chemokine receptor inhibitors will be needed in combination to inhibit viral replication and even in combinations of antiviral drugs that also target other aspects of the HIV replication cycle, such as reverse transcriptase and protease, to obtain optimum therapeutic effects.

The development of novel compounds that can effectively inhibit both wild type and the most consensus resistant strains of human immunodeficiency virus type 1 (HIV-1) is the primary focus in HIV disease management. Combination therapy, comprising at least three classes of drugs, has become the standard of care for acquired immunodeficiency syndrome (AIDS) or HIV-infected individuals. The drug cocktail can comprise all three classes of HIV inhibitors, including nucleoside reverse transcriptase inhibitors (NRTI), non-nucleoside reverse transcriptase inhibitors (NNRTI) and protease inhibitors (PI). Due to their competitive mode of inhibition and requirement for metabolic activation, almost all NRTI drugs lack the virological potency of NNRTI or PI drugs. However, data from clinical trials indicate that sustained viral suppression could not be achieved with NRTI, NNRTI or PIs alone. Therefore, the NRTIs will remain essential components of highly active antiretroviral therapy (HAART) for the foreseeable future, because they enhance the virological potency of the regimen, they do not bind excessively to protein and most regimens are small pills / tablets given once a day. It has become apparent in recent years that the prolonged use of certain NRTIs exhibits adverse events as a class, limiting the length of time for which they can be safely used. Of major clinical concern is their association with the potentially fatal lactic acidaemia and hepatic steatosis. These class events, as well as individual drug effects, such as peripheral neuropathy, are linked to delayed mitochondrial destruction. In addition to toxicity, the development of resistance-conferring mutations against exposure to nucleoside analogs currently in use influences longterm therapeutic benefits. Of critical importance for the evaluation of new NRTIs are recent studies showing that the efficiency of discrimination or excision by pyrophosphorolysis in the presence of nucleotides of a given NRTI is a key determinant in the emergence of one or the other resistance pathway.

Almost fifteen years ago, the first non-nucleoside reverse transcriptase (RT) inhibitor (NNRTI) lead compounds have been discovered. Nowadays, three NNRTIs are approved for treatment of HIV-1-infected individuals and several others are subject of (advanced) clinical trials. Although the NNRTIs target HIV-1 RT, they are clearly different from the nucleoside RT inhibitors (NRTIs). They are highly selective for HIV-1 and do not inhibit HIV-2 or any other (retro)virus. They target HIV-1 RT by a direct interaction without the need to be metabolised by cellular enzymes, and they interact at a site on the HIV-1 RT that is near to, but distant from, the substrate-binding site. The majority of NNRTIs share common conformational properties and structural features that let them fit in a hydrophobic pocket at the HIV-1 RT, which is nowadays well-characterized. A wide variety of crystal structures of RT complexed with NNRTIs have been obtained. They provide detailed insights in the molecular interaction of the NNRTIs with the amino acids lining the pocket in HIV-1 RT. Due to their unprecedented specificity, the NNRTIs are relatively non-toxic in cell culture, and the most potent compounds reach selectivity indices that exceed 100,000 or more. However, inherent to their high specificity, the NNRTIs easily select for mutant virus strains with several degrees of drug resistance. The first-generation NNRTIs such as nevirapine and delavirdine easily loose their inhibitory potential against mutant virus strains that contain single amino acid mutations in their RT. The second-generation NNRTIs such as efavirenz, capravirine and dapivirine usually require two or more mutations in the HIV-1 RT before significantly decreasing their antiviral potency. Evidently, it requires a markedly longer time period to obtain significant resistance against second-generation NNRTIs. The resistance spectrum of NNRTIs is entirely different from the NRTI resistance spectrum, and, as a rule, NRTI-resistant mutant virus strains keep full sensitivity to the inhibitory effects of NNRTIs, and vice versa NNRTI-resistant and mutant virus strains keep full sensitivity to the inhibitory effects of NRTIs. NNRTIs have proven beneficial when included in drug combination (triple or quadruple) therapy, preferably in the presence of protease inhibitors and NRTIs.

Emergence of drug-resistant viral strains is one of the major milestones and the main cause for the failure of antiretroviral therapy. Combination of different anti-HIV agents has become the standard clinical practice to keep the viral load at low or even undetectable levels and to prevent emergence of virus-drug resistance. Among the human immunodeficiency virus (HIV) reverse transcriptase (RT) inhibitors, the so called nonnucleoside RT inhibitors (NNRTIs) have gained a definitive place in the treatment of HIV infections in combination with nucleoside analogue RT inhibitors (NRTIs) and HIV protease inhibitors (PIs). The virus can be markedly suppressed for a relatively long period of time when exposed to multiple drug combination therapy (highly active antiretroviral therapy, HAART). TSAO derivatives are a peculiar group of highly functionalized nucleosides that belong to the so-called nonnucleoside RT inhibitors (NNRTIs). They exert their unique selectivity for HIV-1 through a specific interaction with the p51 subunit of HIV-1 RT. They are the first small molecules that seem to interfere with the dimerization process of the enzyme. This review covers the work carried out with this unique class of specific inhibitors of HIV-1 reverse transcriptase, including structure activity relationship studies (SAR), its mechanism of action, resistance studies, model of interaction with the enzyme, etc.

HIV-1 integrase is a multidomain enzyme which is required for the integration of viral DNA into the host genome. It is one of three enzymes of HIV, the others being the Reverse Transcriptase and the Protease. It is an attractive target for therapeutic drug design. The enzyme consists of three domains. The N-terminal domain has a His2Cys2 motif which chelates zinc, the core domain has the catalytic DDE motif which is required for its enzymatic activity, and the C-terminal domain has an SH3-like fold which binds DNA nonspecifically. We review the structures of various integrase fragments, the core domain with inhibitors bound, and propose a model for DNA binding.

Since the beginning of the HIV epidemic almost 70 million people have been infected with HIV. It is estimated that 42 million people are currently living with HIV/AIDS. The spread of HIV continues throughout the world and current estimates indicate that in 2002, 5 million people were newly infected with HIV and 3 million people died. Current treatments employ a combination of therapeutic agents that target the viral reverse transcriptase and protease enzymes and viral entry. However the clinical benefit of these agents is often limited due to issues of regimen compliance, significant side effects, and the emergence of viral strains that are drug resistant. The introduction of novel agents that interfere with alternate stages in the viral life cycle represent potential solutions to these problems. The integration of the HIV genome into the cellular chromosome, a process catalyzed by the viral enzyme integrase, has been shown to be essential for viral replication. Since HIV integrase has no direct cellular counterpart it presents itself as an attractive target for therapeutic intervention. This review summarizes recent and promising developments both in the HIV integrase field and the global quest for therapeutically useful inhibitors of HIV integrase.

The identification of HIV-1 protease (HIVp) as a target for therapeutic intervention against AIDS was soon followed by major efforts to understand its substrate specificity, reaction kinetics and three-dimensional structure, both in the free state and in complex with a number of ligands including substrate mimics, products, and inhibitors. On the whole these studies have been extremely successful and have had a major impact on our understanding of ligand-receptor interactions and enzyme inhibition mechanisms. HIVp has also become a paradigm for the development and testing of new drug-design methodologies both in vitro and in silico. Even though thousands of potential HIVp inhibitors exhibiting amazing chemical diversity have been synthesized or identified from natural sources, only a few have turned out to be useful for human therapy. Although the alternative goal of preventing enzyme dimerization has been achieved as a proof of concept, this approach has not yet yielded a clinical candidate. The review covers the general strategies that led to some of the most useful inhibitors, the reasons for our limited success in effectively inhibiting this retroviral target in a clinical setting, current progress with second-generation inhibitors, and new avenues for research.

Macrophages (M / M) are identified as the second cellular target of HIV and a crucial virus reservoir. M / M are persistently infected cells and not susceptible to the HIV cytophatic effects typical of infected CD4+ T-lymphocytes. HIV replication in M / M is a crucial pathogenetic event during the whole course of the disease. Moreover, the dynamics of HIV-1 replication and cumulative virus production is quite different in M / M and CD4+ T-lymphocytes in the presence or in the absence of antiviral drugs. Thus, for their unique cellular characteristics, the activity of anti-HIV compounds could be different in M / M than in CD4+ T-lymphocytes. Indeed, nucleoside analogues inhibitors of HIV-reverse transcriptase (NRTIs) show potent antiviral activity in macrophages, although the limited penetration of these compounds in sequestered body compartments and the scarce phosphorylation ability of macrophages, suggest that a phosphate group linked to NRTIs may confer a greater anti-HIV activity in such cells. The antiviral activity of non-nucleoside reverse transcriptase inhibitors (NNRTIs) in macrophages is similar to that found in CD4-lymphocytes. Interestingly, protease inhibitors (PIs), acting at post-integrational stages of virus replication, are the only drugs able to interfere with virus production and release from macrophages with established and persistent HIV infection. For these reasons, a careful analysis of the distribution of antiviral drugs, and the assessment of their activity in cells of macrophage lineage, represent key factors in the development of therapeutic strategies aimed to the treatment of the HIV-infected patients.