Animal models for anti-AIDS therapy

Primate and non-primate species have been used to study the pathobiology of the simian immunodeficiency virus (SIV) and of the human immunodeficiency virus type 1 (HIV-1), respectively, and to develop new therapeutic regimes. Transgenic mice which express either the entire HIV-1 provirus or subgenomic fragments have been used to analyze viral gene products in vivo and may serve as models for the development of agents targeted to select viral functions. Chimeric mice which were created by transplanting human hematolymphoid cells into mice suffering from congenital severe combined immunodeficiency (scid/scid or so called SCID mice), can be infected with HIV-1 and allow one to study the entire HIV-1 replicative cycle. Type C murine leukemia virus models have been used to develop new prophylactic and therapeutic strategies but their use is restricted to the evaluation of select antiviral drug inhibition, targeted to retroviral genes common to both Lentivirinae and Oncovirinae. The role of various animal model systems in the development of anti-HIV-1 and anti-AIDS therapies is summarized.

Development of antiviral treatment strategies in murine models

Murine models with type C murine leukemia viruses have been used to develop major new prophylactic and therapeutic strategies in vaccination, drug therapy of acute virus exposure and chronic viremia, combination therapy, prevention of maternal transmission, and therapy targeted to the central nervous system. Transgenic mice expressing either the whole human immunodeficiency virus type 1 (HIV-1) provirus or subgenomic sequences allow the in vivo analysis of selected HIV-1 functions. The full replicative cycle of HIV-1 can be studied in human/mouse chimerae which were created by transplanting human hematolymphoid cells into SCID mice. The chimeric SCID mouse models have been used successfully to evaluate anti-HIV-1 drugs. The role of the various murine retrovirus systems in the development of anti-HIV-1 and anti-AIDS therapies is summarized.

Synergistic inhibition of human immunodeficiency virus type 1 replication in vitro by two-drug and three-drug combinations of 3'-azido-3'-deoxythymidine, phosphonoformate, and 2',3'-dideoxythymidine

The effects of 3'-azido-3'-deoxythymidine (AZT), phosphonoformate (PFA), and 2',3'-dideoxythymidine (ddT) and their combination on human immunodeficiency type 1 (HIV-1) replication were studied by measuring the HIV-1 p24 antigen expression and reverse transcriptase (RT) release in HIV-1-infected MT4 cells in vitro. RT activity was also measured in a cell-free system by using poly(rA)-oligo(dT) as the primer-template, and cell growth inhibition was measured in noninfected MT4 cells. The interactions of these two- and three-drug combinations were evaluated by the combination index (CI) method and isobologram techniques. The 50% effective concentrations (EC50s) of AZT, PFA, and ddT were 0.014 to 0.005, 9.4 to 8.8, and 8.4 to 2.5 microM, respectively, for p24 enzyme-linked immunosorbent assays (ELISAs) and 0.005 to 0.0034, 1.43 to 1.37, and 2.87 to 2.83 microM, respectively, for RT activity in vitro; for RT activity in the cell-free system, the EC50s were 0.00019 to 0.00024, 0.012 to 0.02, and 0.00074 to 0.0005 microM, for AZT-5'-triphosphate, PFA, and ddT-5'-triphosphate, respectively. AZT in combination with PFA (1:200) or ddT (1:5) as well as the combination of these three drugs (1:200:5) synergistically inhibited HIV-1 replication and RT activity in the cell-free system over a wide range of drug concentrations, with the CIs ranging from 0.5 to 0.09, in which CIs of less than 1, 1, and greater than 1 indicate synergism, additive effect, and antagonism, respectively. Three- and two-drug combinations of AZT, PFA, and ddT showed similar degrees of synergism against HIV-1 replication in p24 assays and RT release assays, whereas the combination of AZT and ddT was found to be the most selective in terms of its anti-HIV-1 effect versus cytotoxicity. Dose reduction indices calculated from both HIV-1 replication inhibition, as measured by p24 ELISA and by RT activity in the cell-free system, indicated that two- and three-drug combinations at high effect levels and the selected combination ratios allow 2- to 240-fold dose reduction over the single drug alone in terms of their anti-HIV-1 effects. The three-drug combination showed the highest dose reduction index. These finding suggest that increased efficacy and reduced toxicity may be achieved in AIDS therapy by using AZT, PFA, and ddT in two- or three-drug combinations.

Pentoxifylline (Trental) decreases the replication of the human immunodeficiency virus type 1 in human peripheral blood mononuclear cells and in cultured T cells

Pentoxifylline (Trental), used routinely for the treatment of intermittent claudication, has been shown previously to decrease the levels of tumor necrosis factors-alpha (TNF-alpha) RNA in cancer patients and to lead to a general improvement of well being. Increased TNF-alpha levels have been observed not only in cancer patients but also in cachectic patients with the acquired immunodeficiency syndrome (AIDS), and TNF-alpha is known to increase the expression of the human immunodeficiency virus type 1 (HIV-1) via activating its long terminal repeat (LTR). Moreover, TNF-alpha decreases the therapeutic efficacy of zidovudine (AZT). Here we show a significant decrease in HIV-1 replication by pentoxifylline in infected human peripheral blood mononuclear cells. The reduction was proportional to the downregulation of expression of a reporter gene, the bacterial gene for chloramphenicol acetyl transferase, linked to the HIV-1 LTR in human monocytoid cells. We conclude that patients with AIDS may benefit from pentoxifylline treatment because of its blockage of TNF-alpha-mediated HIV-1 upregulation, from increased efficacy of AZT, and also from improvement in TNF-alpha-induced cachexia.

N-carboxymethylchitosan-N,O-sulfate as an anti-HIV-1 agent

N-carboxymethylchitosan-N-O-sulfate (NCMCS), a sulfated polysaccharide derivative of chitin, inhibited the propagation of the human immunodeficiency virus type 1 (HIV-1) in human CD4+ cells and that of Rauscher murine leukemia virus (RLV) in murine fibroblasts. A dose-dependent inhibition of both viruses was observed without significant cytotoxicity. NCMCS blocked the binding of HIV-1 to human CD4+ target cells and competitively inhibited HIV-1 reverse transcriptase. Thus, NCMCS may prevent HIV-1 infection by inhibiting viral adsorption to the CD4 receptor and reverse transcription of the viral genome.

Protective cellular retroviral immunity requires both CD4+ and CD8+ immune T cells

We have found previously that postexposure chemoprophylaxis with 3'-azido-3'-deoxythymidine (also known as zidovudine or AZT) in combination with recombinant human alpha A/D interferon fully protected mice exposed to a lethal dose of Rauscher murine leukemia virus (RLV) against viremia and disease. After cessation of therapy, over 90% of these mice were able to resist rechallenge with live RLV, thus demonstrating an acquired immunity. Adoptive cell transfer of 4 x 10(7) cells from immunized mice fully protected naive recipients from viremia and splenomegaly after RLV challenge. However, when these immune T cells were fractionated into CD4+ and CD8+ subpopulations, only partial protection was found when 4 x 10(7) T cells of either subset were given. Full protection against RLV challenge was seen again when the T-cell subsets from immunized mice were recombined and transferred at the same number into naive mice. We conclude that cellular immunity alone is protective and that both CD4+ and CD8+ cell types are required for conferring full protection against live virus challenge.

Different effect of benzylacyclouridine on the toxic and therapeutic effects of azidothymidine in mice

It has been reported that in vitro uridine (Urd) can reverse azidothymidine (AZT) cytotoxicity without decreasing anti-human immunodeficiency virus (HIV) activity. Our studies in mice have shown that daily oral doses of benzylacyclouridine (BAU), an inhibitor of Urd breakdown, also reduces AZT hematologic toxicity, presumably by elevating the plasma concentration of Urd. We now extend these murine studies and report the effect of various doses of exogenous Urd, various doses of BAU, or the combination of BAU and Urd, administered daily, on AZT-induced toxicity. In mice receiving concomitant AZT, daily doses of Urd of 1,000 to 2,000 mg/kg increase peripheral reticulocytes and slightly reduce AZT-induced hematologic toxicity. However, the range of effective doses is narrow, and higher doses of Urd (greater than 3,000 mg/kg/d) significantly enhance hematologic toxicity. At its most effective dose, (2,000 mg/kg/d), Urd produces 28% mortality. In contrast, BAU doses up to 300 mg/kg/d reduced AZT-related hematologic toxicity in a dose-dependent manner without mortality. Higher daily doses of BAU and the combination of BAU with low doses of Urd were not more effective. Studies conducted in mice infected with the Rauscher murine leukemia virus (RLV) indicate that BAU does not impair the antiretroviral effect of AZT when administered at doses that reduce AZT-induced anemia and leukopenia. These findings may be significant for the treatment of patients with acquired immunodeficiency syndrome (AIDS) and AIDS-related complex.

Inhibition of human immunodeficiency virus type 1 replication by phosphonoformate esters of 3'-azido-3'-deoxythymidine

The water-soluble ammonium salt of 3'-azido-5'-(O-ethoxycarbonylphosphinyl)-3'-deoxythymidine (ECP-AZT), the prototype of a novel class of compounds incorporating two active antiretroviral agents, in this case 3'-azido-3'-deoxythymidine (AZT) and phosphonoformic acid (PFA), within the same structure, was synthesized and tested as an inhibitor of the replication of human immunodeficiency virus type 1 (HIV-1) in Jurkat cells, a CD4+ human T-lymphocyte cell line. The corresponding 5'-(O-methoxycarbonylphosphinyl) derivative (MCP-AZT) was also prepared. The rationale for the synthesis of ECP-AZT and MCP-AZT was that they may be cleaved intracellularly to AZT and PFA via hydrolysis of the phosphate ester bond or to AZT 5'-monophosphate by oxidative cleavage of the carbon-phosphorus bond. ECP-AZT was found to block viral replication at a 50% inhibitory concentration (IC50) of ca. 10(-6) M as measured by reverse transcriptase (RT) activity in supernatants from cultures of infected cells. Little or no inhibition of cell growth was observed at this concentration, and there was less than 20% inhibition of cell growth at 10(-4) M. AZT itself was a more potent inhibitor of HIV-1 replication than ECP-AZT, but was also more cytotoxic. The antiviral selectivity of ECP-AZT, defined as the ratio IC50 (virus inhibition)/IC50(cell growth inhibition), was in the range considered to be therapeutic for anti-AIDS nucleosides.

Murine models for evaluating antiretroviral therapy

The pandemic of the acquired immunodeficiency syndrome (AIDS), caused by the human immunodeficiency virus type 1 (HIV-1), requires rapid development of effective therapy and prevention. Analysis of candidate anti-HIV-1 drugs in animals is problematic since no ideal animal model for HIV-1 infection and disease exists. For many reasons, including small size, availability of inbred strains, immunological reagents, and lymphokines, murine systems have been used for in vivo analysis of antiretroviral agents. Here we review currently available murine models involving HIV-1 in transgenic mice and in chimeric mice reconstituted with human cells, as well as murine systems using retroviruses of the subfamily Oncovirinae rather than Lentivirinae. We report our results on various antiretroviral treatment strategies, including chemoprophylaxis after acute retroviral exposure, therapy of chronic viremia, quantitative analysis of combination therapy, and therapy during pregnancy and in the neonatal period aimed at preventing viremia in the offspring. Due to our highly effective postexposure treatment protocols with 3'-azido-3'-deoxythymidine (zidovudine) combined with recombinant human interferon-alpha A/D, retrovirus-inoculated mice developed immunity to the virus to which they were exposed, which will allow us to determine the nature of protective antiretroviral immunity in inbred mice.

Vaccination with a live retrovirus: the nature of the protective immune response

We tested 3'-azido-3'-deoxythymidine (zidovudine) combined with interferon alpha as chemoprophylaxis after exposing mice to Rauscher murine leukemia virus. Therapy started 4 hr after inoculation and administered for 20 days prevented viremia and disease in all 234 mice tested. When the animals were rechallenged with live virus after cessation of therapy, 96% were resistant. The nature of this protective immune response was analyzed: Passive serotherapy of naive mice challenged subsequently with Rauscher murine leukemia virus was only protective at a high dose of immune serum. Immune, but not naive, T cells alone were fully protective against virus challenge. We conclude that vaccination with a live retrovirus that cannot replicate because of pharmacological blockade induces a T-cell response capable of protecting against a lethal retrovirus-induced disease.

Analysis of 3'-azido-3'-deoxythymidine levels in tissues and milk by isocratic high-performance liquid chromatography

A sensitive high-performance liquid chromatographic (HPLC) assay was established to analyze levels of the antiretroviral agent 3'-azido-3'-deoxythymidine (AZT, zidovudine) in serum, milk and tissue extracts. After methanol precipitation, serum samples could be injected directly into the HPLC apparatus, whereas tissue extracts required further clarification. Recovery of AZT was virtually complete. Isocratic elution with a mobile phase consisting of 6% acetonitrile and 0.1 M ammonium acetate, pH adjusted to 4.5 with glacial acetic acid, resulted in good resolution of AZT and its metabolites; retention times for AZT and the internal standard, p-nitrophenol, were 20 and 37 min, respectively. Using this method, we have demonstrated that AZT crosses both the blood-brain and placental barriers and is excreted into milk at high levels.

Human cell lines stably expressing HIV env and tat gene products

A DNA fragment containing the tat, rev and env genes of the human immunodeficiency virus type 1 was inserted into the retroviral vector pZIPneoAU3. The resulting plasmid penvAU3 was transfected into HeLa and psi CRIP cells. Resulting recombinant retroviruses were used to infect HeLa and Jurkat cells. Immunoprecipitation analysis of stable transformants showed the expression of HIV env glycoproteins gp160, gp120 and gp41. Transactivation assays with a plasmid containing the gene for chloramphenicol acetyltransferase linked to HIV promoter-enhancer sequences demonstrated the expression of functional tat. These cells constitute virus-free tools for functional and structural studies of native env and tat.

Unusual DNA structures at the integration site of an HIV provirus

Supercoiled pHXBc2 DNA (containing the genome of the human immunodeficiency virus type 1 and human sequences) migrated more slowly than linear DNA in native and ethidium bromide agarose gel electrophoresis at 4.5 volts/cm, suggesting the presence of unusual DNA structures. S1 nuclease analysis of pHXBc2 revealed two S1 hypersensitive sites. Site I was located within a 25 bp direct repeat in host DNA 0.6 kB upstream from the 5' LTR. Site II was mapped 0.2 kB upstream from the vif gene start site. Sequence analysis showed that Site I sequences could assume different unusual DNA structures, whereas sequences at Site II could assume either slipped or H-DNA forms. Unusual DNA structures in host DNA may be associated with active chromatin regions and may favor proviral integration.

Synthesis of the 2-chloro analogues of 3'-deoxyadenosine, 2',3'-dideoxyadenosine, and 2',3'-didehydro-2',3'-dideoxyadenosine as potential antiviral agents

2-Chloro-3'-deoxyadenosine (2-chlorocordycepin), 2-chloro-2',3'-dideoxyadenosine (2-ClddAdo), and 2-chloro-2',3'-didehydro-2',3'-dideoxyadenosine (2-ClddeAdo) were synthesized from 2-chloroadenosine (2-ClAdo) as candidate antiretroviral agents on the basis that 2-chloro substitution would prevent enzymatic deamination and increase efficacy relative to 2',3'-dideoxyadenosine (ddAdo). Reduction of 2-chloro-5'-(4,4'-dimethoxytrityl)-2',3'-O-thiocarbonyladenosine with n-Bu3SnH, followed by detritylation with AcOH, unexpectedly gave a mixture of 2-chlorocordycepin and 2-chloroadenine. Treatment of the crude n-Bu3SnH reduction product with 1,1'-thiocarbonyldiimidazole, followed by another cycle of n-Bu3SnH reduction and detritylation with silica gel afforded 2-ClddAdo and a byproduct identified as 2-chloro-2',3'-O-methyleneadenosine. Treatment of 2-chloro-5'-O-(4,4'-dimethoxytrityl)-2',3'-thiocarbonyladenosine with 1,3-dimethyl-2-phenyl-1,3,2-diazaphospholidine followed by silica gel detritylation afforded 2-ClddeAdo. 2-ClddAdo and 2-ClddeAdo were tested for activity against human immunodeficiency virus (HIV) in a cultured human T4+ lymphocyte cell line. At a concentration of 100 microM, 2-ClddAdo inhibited reverse transcriptase (RT) production by 97%, while 2',3'-dideoxyadenosine (ddAdo) gave greater than 99% inhibition. In growth assays against uninfected T4+ cells, however, 100 microM 2-ClddAdo gave 23% inhibition while 100 microM ddAdo was nontoxic. At a nontoxic concentration of 20 microM, RT production was 75% inhibited by ddAdo but only 43% inhibited by 2-ClddAdo. Thus, a 2-chloro substituent increased host cell toxicity but decreased antiretroviral activity. The unsaturated analogue 2-ClddeAdo was more cytotoxic than 2-ClddAdo, and antiviral effects could not be measured above 20 microM, where there was only 75% inhibition of RT production. Because of the decreased therapeutic index of 2-ClddeAdo relative to 2-ClddAdo and ddAdo, greater than 90% inhibition of viral protein synthesis at a noncytotoxic concentration could not be achieved. In growth assays with cultured human T and B lymphocytes, 100 microM 2-chlorocordycepin gave 60-70% growth inhibition, while the IC50 against mouse fibroblasts was only 30 microM. The high cytotoxicity of 2-chlorocordycepin precluded consideration of this compound as an antiviral agent.

Maternal transmission of retroviral disease and strategies for preventing infection of the neonate

Moloney murine leukemia virus is efficiently transmitted from viremic mothers to offspring, primarily via virus-containing milk. To determine the level in the infectious process at which an antiviral agent can interfere most effectively with perinatal viral transmission, we examined the effect of the drug 3'-azido-3'-deoxythymidine (AZT) on transmission of Moloney murine leukemia virus from viremic mothers to offspring. Although AZT treatment did not affect the titer of virus in milk, it did suppress the development of viremia in all offspring. AZT, however, did not prevent transmission of virus from viremic mothers to 25% of the offspring, but did lead to a marked reduction in virus load in these infected mice. These results provide evidence for effective antiretroviral therapy during gestation and in the perinatal period and are of potential significance for the management of maternal transmission of human retroviruses.

Murine models for antiretroviral therapy

Rauscher murine leukemia virus (RLV) infection of adult mice results in splenomegaly that is proportional to the virus titer. We have used this model to study the therapy of chronic viremia as well as chemoprophylaxis after viral exposure, both with single agents and with combination therapy. We showed that prompt initiation of effective antiviral therapy after viral exposure can prevent de novo infection. We have also developed a murine neurotropic retrovirus model which allows analysis of candidate antiviral agents for activity across the blood-brain barrier. This model can also be used to study therapeutic approaches to retroviral infections acquired at midgestation or during the neonatal period.

Maternal transmission of retroviral disease: transgenic mice as a rapid test system for evaluating perinatal and transplacental antiretroviral therapy

Transgenic mouse strains carrying Moloney murine leukemia virus (Mo-MuLV) in the germ line were found to serve as rapid and quantitative models of in utero and perinatal retroviral infection for evaluating strategies of antiretroviral therapy. In these strains virus activation leads to virus expression, viral spread associated with the development of viremia, and subsequent T-cell leukemia/lymphoma. To test these transgenic strains for their usefulness in evaluating antiretroviral agents, the effect of the drug 3'-azido-3'-deoxythymidine (AZT) on the development of viremia and subsequent disease was examined. The assessment of mice for viremia at 1 month of age appeared to be the most useful assay because it was rapid and quantitative. AZT was most effective in preventing viremia in transgenic strains that activate Mo-MuLV after birth and had more variable effects in strains that activate prior to birth. However, in six of seven of the strains examined, AZT led to a marked improvement in survival and reduced the onset of T-cell leukemia/lymphoma. These results provide evidence for effective antiretroviral therapy during gestation and in the perinatal period and are of potential significance for the management of the maternal transmission of human retroviruses.

Slipped DNA structures within the enhancer region of the Moloney murine leukemia virus

We have examined the S1 nuclease sensitivity of supercoiled plasmids harboring the Moloney Murine Leukemia Virus (MoMuLV) long terminal repeat (LTR). S1 sensitivity was found within the LTR enhancer direct repeats. Transformation of E. coli DH5 cells with a construct containing most of the MoMuLV LTR yielded the precise deletion of one direct repeat and loss of S1 sensitivity. The dependence of S1 sensitivity on the presence of both direct repeats, together with the exact excision of one direct repeat by E. coli, suggests the presence of slipped DNA within the enhancer. Such structures may represent targets for effector proteins which mediate vital functions during viral propagation.

Retroviruses and mouse embryos: a rapid model for neurovirulence and transplacental antiviral therapy

A murine model in which neurotropic retroviral infection can be studied over short periods of time was developed. Microinjection of Cas-Br-E virus into midgestation mouse embryos caused paralysis and death within 25 days after birth, in contrast to virus-infected neonates which develop disease only after 4 months. To evaluate whether antiviral drugs could cross the placental barrier and influence the course of the disease, the drug 3'-azido-3'-deoxythymidine (AZT) was administered to infected embryos through the drinking water of pregnant females. AZT treatment markedly retarded the onset and course of virus-induced central nervous system disease, permitting animals to survive beyond 4 months of age. These results are evidence for effective antiviral treatment during gestation and in the perinatal period and are of potential significance for the management of maternal transmission of the acquired immune deficiency syndrome (AIDS) virus.

Suppression of mouse viraemia and retroviral disease by 3'-azido-3'-deoxythymidine

The retroviruses human T-cell lymphotrophic virus-I (HTLV-I) and HTLV-III/LAV (lymphadenopathy-associated virus) are clearly linked to human diseases. Patients with HTLV-I-positive neoplasms may respond transiently to traditional chemotherapy, but are not cured. For patients with acquired immune deficiency syndrome (AIDS) there is no curative therapy. In retroviruses of different species, viral propagation crucially depends on reverse transcriptase, an enzyme not present in normal mammalian cells and different from mammalian DNA polymerases, making it a target for specific inhibition. Reverse transcriptase has been well conserved through evolution: an LAV isolate contained a 250-amino-acid-long domain, presumably the reverse transcriptase core sequence, which has 21% homology to Moloney murine leukaemia virus (MoMLV). Because HTLV-III infects only humans and chimpanzees, we substituted murine retroviruses for in vivo evaluation of candidate anti-AIDS drugs after ascertaining similar inhibition in vitro of HTLV-III and MLVs, which were chosen for their short incubation time. The triphosphate of 3'-azido-3'-deoxythymidine (AZT) is incorporated into complementary DNA by retroviral reverse transcriptase, causing premature chain termination. Here we show that chronic AZT treatment of mice infected with Rauscher murine leukaemia virus complex (RLV) prevents infection of splenocytes and development of splenomegaly, and suppresses viraemia if started soon after inoculation. Starting AZT late in the course of disease still leads to significant prolongation of life; anaemia, however is a significant side-effect. By analogy, AZT may have a role in preventing retroviral disease in humans if started early after infection, and it may lead to significant survival gains even if started later in the course of disease.