Antiviral Properties of the HIV-1 Proteinase Inhibitor RO 31-8959

The peptide derivative Ro 31-8959 has been shown to be a potent inhibitor of HIV proteinase with an IC50 of 2 × 10−9M, against HIV-1RF in acutely infected lymphoblastoid cells. This inhibition was not overcome by increasing the infectious dose or by extending the culture time. Similar antiviral activity was also obtained against HIV-2, SIV and several AZT-resistant strains of HIV-1. The time of addition of the inhibitor could be delayed for 22 h without significant loss of activity, supporting its mode of action as taking place late in the replication cycle of HIV-1. Ro 31-8959 also showed activity against chronically infected cells.

Long-Term Treatment of HIV-Infected MT-4 Cells in Culture with HIV Proteinase Inhibitor RO 31-8959 Leads to Complete Cure of Infection

An HIV-infected cell culture was treated with the specific HIV proteinase inhibitor Ro 31-8959 for three months to analyse the antiviral effect and possible cytotoxicity of the drug in long-term treatment. The drug was added 1 h after HIV infection with 0.002 m.o.i. and maintained for 87 days in the cell culture. There was no detectable cell death nor any evidence of HIV production in this time. Cells were proven to be initially infected, since premature drug removal led to a re-emergence of infectious HIV and cell death. However, after 87 days of treatment the drug could be removed safely and HIV was cleared demonstrably from the culture. These data suggest that long-term dosage may be advantageous to the clinical treatment of HIV infection by HIV proteinase inhibitors.

Antiviral Synergy between Inhibitors of HIV Proteinase and Reverse Transcriptase

Ro 31–8959, a potent and specific inhibitor of HIV proteinases, is shown to interact synergistically in combination with nucleoside analogue inhibitors of HIV reverse transcriptase (AZT, ddC, 2′-FddC). Ninety per cent inhibition endpoints (IC90), obtained from checkerboard titrations of compound mixtures on CEM-T4 cells infected with HIV-i strain GB8, have been further analysed. Compared with concentrations needed when inhibitors are used individually, reductions of between 2- and 30-fold were observed in combination, depending on the nucleoside analogue and the ratio of concentrations employed. The results suggest that combinations of AZT or ddC with Ro 31–8959 should be considered for development as candidate antiviral treatments of patients with HIV infection.

Natural product-based anti-HIV drug discovery and development facilitated by the NCI developmental therapeutics program

During the decade 1987-1996, the Developmental Therapeutics Program (DTP) of the National Cancer Institute (NCI) provided infrastructure support for both intramural and extramural anti-HIV (human immunodeficiency virus) drug discovery research and development. This retrospective review describes some of the anti-HIV lead discovery and development that took place under DTP auspices or which was substantially facilitated by resources made available through the DTP. Examples highlighted include leads identified through the initial screening of pure natural product derived compounds and those derived from bioassay-guided fractionation of crude natural product extracts, and these are classified according to the mechanism of action targeting the critical steps within the replication cycle of HIV.

Inhibition of the early phase of HIV replication by an isothiazolone, PD 161374

A new class of substituted 2'-benzisothiazolone represented by PD 161374 was discovered with antiviral activity against retroviruses similar to previously described nucleocapsid inhibitor PD 159206 (DIBA-4). In T cell culture, the 50% inhibitory concentrations (EC(50)) of PD 161374 and PD 159206 were on average 2.5 microM (ranges of 1.2-13.5 microM) without any cytotoxic effect up to 100 microM. PD 161374 inhibited acute HIV infection and it was effective when added during the early phase of HIV infection. However, very modest effects were observed in chronically infected H9 cells and the HIV latency model line OM-10.1. Direct PCR analysis of infected cells demonstrated that PD 161374 delayed the appearance of completed HIV-cDNA products including 2LTR circles. Together all these results suggest that PD 161374 exerts its antiviral effect at pre-integration steps in the early phase of the virus life cycle. When combined with a protease inhibitor, PD 161374 did not show any antagonism and combination with a reverse transcriptase inhibitor (AZT) resulted in a synergistic effect.

Human immunodeficiency virus protease inhibitors

Human immunodeficiency virus (HIV) protease inhibitors have revolutionized the treatment of individuals infected with this pathogen. The protease is an enzyme that is essential for viral replication because it cleaves both structural and functional proteins from precursor viral polyprotein strands. Inhibition of this process suppresses viral replication, which produces immature noninfectious virions. When combined with reverse transcriptase inhibitors, these agents are very potent in suppressing viral replication. Pharmacologic properties, toxic profile, drug interactions, and resistance patterns differ among protease inhibitors, and all must be considered when selecting the drugs for therapeutic use in humans. The best combination, sequence of use, durability of response, and magnitude of immune reconstitution and function are issues that have yet to be fully elucidated.

Evaluation of reverse transcriptase and protease inhibitors in two-drug combinations against human immunodeficiency virus replication

Current treatments for human immunodeficiency virus (HIV) include both reverse transcriptase and protease inhibitors. Results from in vitro and clinical studies suggest that combination therapy can be more effective than single drugs in reducing viral burden. To evaluate compounds for combination therapy, stavudine (d4T), didanosine (ddI), or BMS-186,318, an HIV protease inhibitor, were combined with other clinically relevant compounds and tested in a T-cell line (CEM-SS) that was infected with HIV-RF or in peripheral blood mononuclear cells infected with a clinical HIV isolate. The combined drug effects were analyzed by the methods described by Chou and Talalay (Adv. Enzyme Regul. 22:27-55, 1984) as well as by Prichard et al. (Antimicrob. Agents Chemother. 37:540-545, 1993). The results showed that combining two nucleoside analogs (d4T-ddI, d4T-zidovudine [AZT], and d4T-zalcitabine [ddC]), two HIV protease inhibitors (BMS-186,318-saquinavir, BMS-186,318-SC-52151, and BMS-186,318-MK-639) or a reverse transcriptase and a protease inhibitor (BMS-186,318-d4T, BMS-186,318-ddI, BMS-186,318-AZT, d4T-saquinavir, d4T-MK-639, and ddI-MK-639) yielded additive to synergistic antiviral effects. In general, analysis of data by either method gave consistent results. In addition, combined antiviral treatments involving nucleoside analogs gave slightly different outcomes in the two cell types, presumably because of a difference in phosphorylation patterns. Importantly, no strong antagonism was observed with the drug combinations studied. These data should provide useful information for the design of clinical trials of combined chemotherapy.

Resistance of HIV type 1 to proteinase inhibitor Ro 31-8959

During replication of human immunodeficiency virus type 1 (HIV-1), proteolytic cleavage of Gag and Gag-Pol precursor proteins into different functional protein subunits is catalyzed by the viral proteinase, and this enzyme is the target of the antiviral proteinase inhibitor, Ro 31-8959. We investigated in vitro which HIV mutants with reduced sensitivity to Ro 31-8959 emerged during proteinase inhibition treatment; from three different HIV-1 strains, comparable progeny virus resistant to proteinase inhibitor were found, whereas the same experimental protocol detected no resistant HIV-2 mutants. Molecular analysis of the mutations underlying resistance revealed a multistep mechanism in which an amino acid exchange was common to all resistant isolates, and in all experiments preceded further exchanges at position 90 (leucine to methionine) and/or at position 54 (isoleucine to valine). For wild-type strains the 90% inhibitory concentrations of Ro 31-8959 were close to 20 nM, whereas HIV-1 mutants with all 3 amino acid exchanges had more than 50-fold increased 90% inhibitory concentrations (above 1000 nM). The primary event (Gly-48 to valine) occurs at the hinge of the flaps of the proteinase, thus hampering entry of the inhibitor to the active center and suggesting steric hindrance. Detailed knowledge of this stereotypic process could open inhibitor design, thus preventing conceivable escape of resistant virus on proteinase inhibitor action.

Antiviral action of oligodeoxyguanylic acids against human immunodeficiency virus type 1

Deoxyguanylic acids, but not other deoxynucleotides, as short as 3- to 4-mer, were effective in preventing HIV-1-induced cytopathicity. In addition, they prevented giant cell formation of infected Sup-T1 cells, and p24 production in HIV-1 infected H9 cells. Phosphorylation at either the 5'- or 3'-end enhanced these activities. Furthermore, 5'-phosphorylated phosphorothioate tetradeoxyguanylic acid was effective in reducing HIV production in chronically infected cells (H9/IIIB). The search for the target steps of this compound revealed that it inhibits at least 3 steps in the life cycle of HIV: interaction with CD4 (measured by inhibitory effect on the syncytia formation between Sup-T1 and H9/IIIB cells), reverse transcriptase, and step(s) after integration. These results suggest that phosphorylated phosphorothioate tetradeoxyguanylic acid may be a novel candidate for a therapeutic agent of AIDS.

Discovery and in vitro development of AIDS antiviral drugs as biopharmaceuticals

The goal of developing an effective drug against HIV-1 and AIDS has been approached by several routes, with enough encouraging results to stimulate further efforts. Compounds active against HIV-1 have been discovered for many of the functions in the reproductive cycle recognized as virus-specific targets. Discoveries have been made in cell-based assays as well as mechanistic assays, and the value of both types of assays in the drug discovery process has been discussed. Although the final test of a drug's efficacy comes in the clinical experience, submission of an antiviral compound to an in vitro developmental gauntlet can save much time, effort, expense, and human resource in the in vivo developmental regimen required prior to human use. Emergence of viral resistance to drugs in several structural classes has compromised their clinical efficacy, suggesting that development of other potential drugs in those classes may not be good investments. Strains of HIV-1 resistant to specific compound classes are used to categorize new active discoveries for possible developmental exclusion, and defining the mechanism of action of such a new compound may confirm the discouraging judgement. On the other hand, novel compounds which exhibit a broad range of activity in drug-resistant and other HIV-1 strains deserve greater scrutiny. Clinicians most likely will be hesitant to treat patients with compounds shown to act on virus-cell surface interactions, given the failure in the past of several such compounds in clinical studies. But a compound shown to have a unique and novel mechanism of action will be looked upon more favorably, and surviving other tests of potency, solubility, and stability will be unhesitatingly presented for in vivo development. The partial successes of drugs currently in clinical use against AIDS offers great encouragement that other more-effective, less-toxic drugs will be found. Exquisite techniques for identifying new targets on virus gene products, the selection of compounds on activity paradigms, and the enormous variety of compounds becoming available through synthesis libraries, all offer opportunities for anti-HIV drug discovery, which, in our view, cannot fail to present potent antiviral compounds which will survive the rigorous preclinical and clinical tests leading to a drug effective against AIDS.

Characterization of human immunodeficiency virus type 1 variants with increased resistance to a C2-symmetric protease inhibitor

Inhibitors of the human immunodeficiency virus type 1 protease represent a promising class of antiviral drugs for the treatment of AIDS, and several are now in clinical trials. Here, we report the in vitro selection of viral variants with decreased sensitivity to a C2-symmetric protease inhibitor (A-77003). We show that a single amino acid substitution (Arg to Gln or Lys) at position 8 of the protease results in a substantial decrease in the inhibitory activity of the drug on the enzyme and a comparable increase in viral resistance. These findings, when analyzed by using the three-dimensional structure of the protease-drug complex, provide a strategic guide for the future development of inhibitors of the human immunodeficiency virus type 1 protease.

Viral proteases as targets for chemotherapeutic intervention

Many viruses encode proteinases that are essential for infectivity, and are consequently attractive chemotherapeutic targets. The biochemistry and structure of the human immunodeficiency virus proteinase have been characterized extensively, and potent peptide-mimetic inhibitors have been developed. Techniques and strategies used to improve the efficiency of these compounds are likely to be applicable to other viral proteinases.

Effect of a human immunodeficiency virus protease inhibitor on human monocyte function

Human immunodeficiency virus (HIV) is the cause of acquired immunodeficiency syndrome (AIDS). Encoded by the HIV genome are several precursor proteins that undergo proteolytic cleavage to yield functional proteins. The env precursor protein is cleaved by a cellular protease. The gag precursor protein of HIV (p55), however, is cleaved by a virally encoded aspartate protease (HIV Protease). Cleavage of p55 is required for viral maturation and infectivity. There are also several host cell aspartate proteases that serve important homeostatic functions. Cathepsins D and E are lysosomal aspartate proteases which are believed to play an important role in macrophage function, and it has been suggested that inhibition of these enzymes by an HIV protease inhibitor may exacerbate immunosuppression in AIDS patients. We have studied the effect of SK&F 107461 (a hydroxyethylene dipeptide isostere inhibitor of HIV protease), on various host defense functions of human monocytes. Pepstatin A (an inhibitor of most aspartate proteases) and leupeptin (an inhibitor of serine and cysteine proteases) were included as controls. Although less potent than the prototypic aspartate protease inhibitor pepstatin, SK&F 107461 inhibited partially purified cathepsin D in vitro. However, in cell-based assays, SK&F 107461 had no effect on the degradation of hemoglobin, antigen processing of the protein antigen streptokinase, or secretion of 17-kD IL-1 beta by monocytes at concentrations which inhibit maturation of intracellular virus in HIV infected monocytes. Furthermore, SK&F 107461 had no effect on constitutive candidacidal activity. In contrast, leupeptin and pepstatin A partially inhibited accessory cell function of monocytes in the proliferative response to the recall antigen streptokinase. In addition, leupeptin partially inhibited degradation of hemoglobin.(ABSTRACT TRUNCATED AT 250 WORDS)

Application of latent HIV-1 infected cellular models to therapeutic intervention

The 10-year period of clinical latency following infection with the human immunodeficiency virus type-1 remains as a tremendous opportunity for therapeutic intervention. To decipher the viral and cellular mechanisms involved in controlling active and nonproductive viral expression, chronically infected cell lines have been developed which mimic in vivo latency at a cellular level. This review compares these models of chronic infection, emphasizing the advantages and limitations of this approach to the understanding of AIDS progression. In addition, it accentuates the utility of these models of chronic infection in the development and testing of novel drugs aimed at altering the efferent component of the HIV--1 life cycle. It is this component of the viral life cycle that has remained largely unexplored and open to novel therapeutic strategies for the prevention of lethal immunosuppression.

The HIV-1 protease as a therapeutic target for AIDS

HIV produces a small , dimeric aspartyl protease which specifically cleaves the polyprotein precursors encoding the structural proteins and enzymes of the virus. This proteolytic activity is absolutely required for the production of mature, infectious virions and is therefore an attractive target for therapeutic intervention. This review summarizes the strategies and multidisciplinary efforts that have been applied to date to the identification of specific inhibitors of this critical viral enzyme. These inhibitors include rationally designed peptide substrate analogs, compounds conceived from tertiary structure information on the enzyme and natural products. Future directions in the discovery and development of HIV-1 protease inhibitors are also discussed.