Antiviral effects of 3'-azido-3'-deoxythymidine, 2',3'-dideoxycytidine, and 2',3'-dideoxyadenosine against simian acquired immunodeficiency syndrome-associated type D retrovirus in vitro

Advances in HIV therapeutics and cure strategies: findings obtained through non-human primate studies

Human immunodeficiency virus (HIV), the main contributor of the ongoing AIDS epidemic, remains one of the most challenging and complex viruses to target and eradicate due to frequent genome mutation and immune evasion. Despite the development of potent antiretroviral therapies, HIV remains an incurable infection as the virus persists in latent reservoirs throughout the body. To innovate a safe and effective cure strategy for HIV in humans, animal models are needed to better understand viral proliferation, disease progression, and therapeutic response. Nonhuman primates infected with simian immunodeficiency virus (SIV) provide an ideal model to study HIV infection and pathogenesis as they are closely related to humans genetically and express phenotypically similar immune systems. Examining the clinical outcomes of novel treatment strategies within nonhuman primates facilitates our understanding of HIV latency and advances the development of a true cure to HIV.

Comprehensive in vitro analysis of simian retrovirus type 4 susceptibility to antiretroviral agents

Simian retrovirus type 4 (SRV-4), a simian type D retrovirus, naturally infects cynomolgus monkeys, usually without apparent symptoms. However, some infected monkeys presented with an immunosuppressive syndrome resembling that induced by simian immunodeficiency virus infection. Antiretrovirals with inhibitory activity against SRV-4 are considered to be promising agents to combat SRV-4 infection. However, although some antiretrovirals have been reported to have inhibitory activity against SRV-1 and SRV-2, inhibitors with anti-SRV-4 activity have not yet been studied. In this study, we identified antiretroviral agents with anti-SRV-4 activity from a panel of anti-human immunodeficiency virus (HIV) drugs using a robust in vitro luciferase reporter assay. Among these, two HIV reverse transcriptase inhibitors, zidovudine (AZT) and tenofovir disoproxil fumarate (TDF), potently inhibited SRV-4 infection within a submicromolar to nanomolar range, which was similar to or higher than the activities against HIV-1, Moloney murine leukemia virus, and feline immunodeficiency virus. In contrast, nonnucleoside reverse transcriptase inhibitors and protease inhibitors did not exhibit any activities against SRV-4. Although both AZT and TDF effectively inhibited cell-free SRV-4 transmission, they exhibited only partial inhibitory activities against cell-to-cell transmission. Importantly, one HIV integrase strand transfer inhibitor, raltegravir (RAL), potently inhibited single-round infection as well as cell-free and cell-to-cell SRV-4 transmission. These findings indicate that viral expansion routes impact the inhibitory activity of antiretrovirals against SRV-4, while only RAL is effective in suppressing both the initial SRV-4 infection and subsequent SRV-4 replication.

Susceptibilities of simian immunodeficiency virus to protease inhibitors

We used a focal infectivity assay with HeLa H1-JC.37 cells to directly compare susceptibilities of simian immunodeficiency virus (SIV) and human immunodeficiency virus type 1 (HIV-1) to protease inhibitors. SIVmac239 was inhibited by indinavir, saquinavir, and ritonavir, with 50% effective concentrations (means +/- standard deviations) of 39 +/- 8, 55 +/- 3, and 13 +/- 5 nM, respectively. The corresponding values for inhibition of HIV-1 were 66 +/- 4, 47 +/- 10, and 25 +/- 14 nM, respectively.

Differential susceptibility of retroviruses to nucleoside analogues

Retroviruses may cause diseases in their vertebrate hosts. They are distinguished by their common means of replication involving reverse transcription, a process inhibited by nucleoside reverse transcriptase inhibitors (NRTIs) and other compounds used in antiretroviral chemotherapy. Previous work on NRTIs has been limited to their effect on human immunodeficiency virus (HIV) (for review see Ho & Hitchcock, 1989; Weller, 1999) and little information exists regarding the efficacy and therapeutic potential of these drugs against other retroviruses. We have tested all six NRTIs licensed for HIV treatment [didanosine (ddI), zalcitabine (ddC), lamivudine (3TC), stavudine (d4T), zidovudine (AZT) and abacavir (ABC)] against seven retroviruses representative of the traditional subfamilies: Spumavirinae, Lentivirinae and the Oncovirinae. As expected, each drug showed a range of activities against the panel of retroviruses, some drugs inhibiting other viruses at concentrations well below those required for HIV. Overall, AZT was the most active inhibitor (IC50 range, 0.032-1.0 microM), being most active against the Spuma (foamy) viruses. Abacavir was inhibitory for HIV-1, MN strain (HIV-1 MN), amphotrophic murine leukemia virus (MLV-A) and simian foamy virus type 6 (SFV-6). The least effective inhibitor, 3TC (IC50 range, 0.32->100 microM), was most potent against simian retrovirus types 1 and 2 (SRV-1, SRV-2) and HIV-1, but did not inhibit foamy viruses and MLV-A. Additionally, there were differences in the concentration of drug required to inhibit closely related viruses. Taken together, these data suggest that NRTIs have a wide spectrum of antiretroviral activity and the activity of compounds, even against closely related retroviruses, cannot be predicted.

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.

Antiviral agents: characteristic activity spectrum depending on the molecular target with which they interact

The target protein (enzyme) with which antiviral agents interact determines their antiviral activity spectrum. Based on their activity spectrum, antiviral compounds could be divided into the following classes: (1) sulfated polysaccharides (i.e., dextran sulfate), which interact with the viral envelope glycoproteins and are inhibitory to a broad variety of enveloped viruses (i.e., retro-, herpes-, rhabdo-, and arenaviruses): (2) SAH hydrolase inhibitors (i.e., neplanocin A derivatives), which are particularly effective against poxvirus, (-)RNA viruses (paramyxovirus, rhabdovirus), and (+/-)RNA virus (reovirus); (3) OMP decarboxylase inhibitors (i.e., pyrazofurin) and CTP synthetase inhibitors (i.e., cyclopentenylcytosine), which are active against a broad range of DNA, (+)RNA, (-)RNA, and (+/-)RNA viruses; (4) IMP dehydrogenase inhibitors (i.e., ribavirin), which are also active against various (+)RNA and (-)RNA viruses and, in particular, ortho- and paramyxoviruses; (5) acyclic guanosine analogs (i.e., ganciclovir) and carbocyclic guanosine analogs (i.e., cyclobut-G), which are particularly active against herpesviruses (i.e., HSV-1, HSV-2, VZV, CMV); (6) thymidine analogs (i.e., BVDU, BVaraU), which are specifically active against HSV-1 and VZV because of their preferential phosphorylation by the virus-encoded thymidine kinase; (7) acyclic nucleoside phosphonates (i.e., HPMPA, HPMPC, PMEA, FPMPA), which, depending on the structure of the acyclic side chain, span an activity spectrum from DNA viruses (papova-, adeno-, herpes-, hepadna-, and poxvirus) to retroviruses (HIV); (8) dideoxynucleoside analogs (i.e., AZT, DDC), which act as chain terminators in the reverse transcriptase reaction and thus block the replication of retroviruses as well as hepadnaviruses; and (9) the TIBO, HEPT, and other TIBO-like compounds, which interact specifically with the reverse transcriptase of HIV-1 and thus block the replication of HIV-1, but not of HIV-2 or any other retrovirus.

Antileukemic activity of Viva-Natural, a dietary seaweed extract, on Rauscher murine leukemia in comparison with anti-HIV agents, azidothymidine, dextran sulfate and pentosan polysulfate

An antileukemic activity of partially purified polysaccharide of an edible seaweed. Viva-Natural, against Rauscher murine retrovirus-induced erythroleukemia has been demonstrated. This antileukemic effect is compared with standard anti-human immunodeficiency virus (HIV) agents, azidothymidine (AZT), dextran sulfate and pentosan polysulfate. Pretreatment with Viva-Natural, as an immunomodulator, on day 3 prior to the virus inoculation demonstrated definite prophylactic activity, while pretreatment with the other three anti-HIV agents showed no prophylactic activity. The replication of Rauscher virus in BALB/3T3 cell cultures accompanied by direct cytopathic effect (syncytia formation) was suppressed in the presence of Viva-Natural or the other anti-HIV agents in the culture medium. In spite of the antiviral potentials of the four agents in vitro, only Viva-Natural and AZT demonstrated therapeutic efficacy against Rauscher leukemia in mice.

Interactive laser cytometric analysis of retroviral protein expression in HIV-infected lymphocytic cell lines

We have used interactive laser cytometry to investigate the expression of human immunodeficiency virus (HIV) envelope glycoproteins gp160, gp41, gp120, and the core protein p24 in the HIV-infected human lymphocyte cell lines H-9, CEM-SS, and C8166. This method allowed for the ultrasensitive detection of fluorescence signals at the single cell level and, when combined with specific anti-HIV antibodies, permitted unique quantitative detection of HIV antigens. Indirect immunofluorescence assays with monoclonal antibodies directed against gp120 revealed that a large proportion of lymphocytic cells expressed increased gp120-associated fluorescence consistent with HTLV-IIIRF infection. Certain monoclonal and polyclonal antibodies were also effective in quantifying gp160, gp41, and p24 expression. Expression of these antigens was found to vary significantly within 48 h. Significant loss (greater than or equal to 50%) of gp120 expression was observed when cells were treated with 1.0 microM AZT. The expression of the HIV-associated protein markers gp160, gp41, and p24 was detectable 24 h after infection of C8166, a cord blood lymphocytic cell line. C8166 cells expressed an additional 6- to 10-fold increase in gp120 in 48 h as well as a 3- to 4-fold increase in gp160, gp41, and p24. AZT (0.01 and 0.1 microM) decreased the expression of gp120, gp160, and p24 in a dose-dependent fashion. This new application of interactive laser cytometry permits early, sensitive, and statistically based distinctions in the expression of HIV-associated antigens in infected target cells at the single-cell level, and allows detection of important changes in HIV-associated antigen expression and the kinectics thereof.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory Effects of Heterogeneous Dextran Sulphate Compounds on Simian Type D Retrovirus in vitro

Nine heterogeneous dextran sulphate compounds were tested in vitro for their ability to inhibit a type D simian retrovirus (SRV-2) known to cause simian acquired immunodeficiency syndrome (SAIDS). Dextran sulphate, a sulphated polysaccharide, is available commercially in a variety of molecular weights ranging from 5000 (5K) to 500000 (500K). Of the four 500K compounds tested, three completely blocked replication and infectivity of SRV-2. Of the remaining five compounds tested, only three completely inhibited the virus. Cytotoxic effects also varied even among compounds of the same mol. wt. All nine compounds were further tested in combination with azidothymidine (AZT) or dideoxycytidine (ddC) for possible additive, synergistic or antagonistic effects at reduced concentrations of both drugs. Six dextran sulphate compounds showed slight additive or synergistic effects with AZT, while no effects were seen with ddC. The results suggest that the antiretroviral activity of dextran sulphate depends on the particular chemical preparation of the compound and is not entirely dependent on mol. wt. Thus, it is important to be aware of the inhibitory and cytotoxic variability in heterogeneous dextran sulphate compounds when considering the potential use of this drug, either alone or in combination, as an antiretroviral agent in vivo.

Toxicity and efficacy of 2',3'-dideoxycytidine in clinical trials of pigtailed macaques infected with simian retrovirus type 2

Four dosing regimens of 2',3'-dideoxycytidine (ddC) were administered intravenously for 10 to 28 days to 18 pigtailed macaques with simian acquired immunodeficiency syndrome. Ten macaques naturally infected with simian acquired immunodeficiency syndrome retrovirus serotype 2 (SRV-2), the etiologic agent of simian acquired immunodeficiency syndrome, received ddC by continuous intravenous infusion or by a daily bolus injection for 10 to 12 days. Another eight macaques that were negative for SRV-2 and antibody received ddC prophylaxis prior to challenge with virus and continued to receive ddC therapy for up to 28 days postchallenge. All monkeys treated with a continuous intravenous dose of ddC, which maintained plasma concentrations of ddC at levels known to inhibit SRV-2 in vitro, developed dose-related toxic effects, including leukopenia, anemia, lethargy, and decreased appetite. Monkeys treated with a daily bolus injection of ddC experienced more severe toxic effects than those on the continuous intravenous regimen, including exfoliative dermatitis and peripheral neuropathy. At the concentrations of ddC administered, no significant inhibition of SRV-2 replication was detected in naturally infected macaques. However, a prophylactic regimen of ddC did have an inhibitory effect on SRV-2. Our findings suggest that ddC may be valuable as a short-term prophylactic treatment rather than as a long-term therapy.