Inhibitory activity of S-adenosylhomocysteine hydrolase inhibitors against human cytomegalovirus replication

Functional and Pathological Roles of AHCY

Adenosylhomocysteinase (AHCY) is a unique enzyme and one of the most conserved proteins in living organisms. AHCY catalyzes the reversible break of S-adenosylhomocysteine (SAH), the by-product and a potent inhibitor of methyltransferases activity. In mammals, AHCY is the only enzyme capable of performing this reaction. Controlled subcellular localization of AHCY is believed to facilitate local transmethylation reactions, by removing excess of SAH. Accordingly, AHCY is recruited to chromatin during replication and active transcription, correlating with increasing demands for DNA, RNA, and histone methylation. AHCY deletion is embryonic lethal in many organisms (from plants to mammals). In humans, AHCY deficiency is associated with an incurable rare recessive disorder in methionine metabolism. In this review, we focus on the AHCY protein from an evolutionary, biochemical, and functional point of view, and we discuss the most recent, relevant, and controversial contributions to the study of this enzyme.

Identification of 6'-β-fluoro-homoaristeromycin as a potent inhibitor of chikungunya virus replication

We have reported on aristeromycin (1) and 6′-fluorinated-aristeromycin analogues (2), which are active against RNA viruses such as Middle East respiratory syndrome coronavirus (MERS-CoV), severe acute respiratory syndrome coronavirus (SARS-CoV), Zika virus (ZIKV), and Chikungunya virus (CHIKV). However, these exhibit substantial cytotoxicity. As this cytotoxicity may be attributed to 5′-phosphorylation, we designed and synthesized one-carbon homologated 6′-fluorinated-aristeromycin analogues. This modification prevents 5′-phosphorlyation by cellular kinases, whereas the inhibitory activity towards S-adenosyl-l-homocysteine (SAH) hydrolase will be retained. The enantiomerically pure 6′-fluorinated-5′-homoaristeromycin analogues 3a-e were synthesized via the electrophilic fluorination of the silyl enol ether with Selectfluor, using a base-build up approach as the key steps. All synthesized compounds exhibited potent inhibitory activity towards SAH hydrolase, among which 6′-β-fluoroadenosine analogue 3a was the most potent (IC₅₀ = 0.36 μM). Among the compounds tested, 6′-β-fluoro-homoaristeromycin 3a showed potent antiviral activity (EC₅₀ = 0.12 μM) against the CHIKV, without noticeable cytotoxicity up to 250 μM. Only 3a displayed anti-CHIKV activity, whereas both3a and 3b inhibited SAH hydrolase with similar IC₅₀ values (0.36 and 0.37 μM, respectively), which suggested that 3a’s antiviral activity did not merely depend on the inhibition of SAH hydrolase. This is further supported by the fact that the antiviral effect was specific for CHIKV and some other alphaviruses and none of the homologated analogues inhibited other RNA viruses, such as SARS-CoV, MERS-CoV, and ZIKV. The potent inhibition and high selectivity index make 6′-β-fluoro-homoaristeromycin (3a) a promising new template for the development of antivirals against CHIKV, a serious re-emerging pathogen that has infected millions of people over the past 15 years.

Saikosaponins: a review of pharmacological effects

Over the past decades, a number of phytochemicals have been reported to possess potent pharmacological effects. Saikosaponins represent a group of oleanane derivatives, usually as glucosides, which are commonly found in medicinal plants Bupleurum spp., which have been used as traditional Chinese medicine for more than 1,000 years in China. Emerging evidence suggests that saikosaponins have many pharmacological effects, including sedation, anticonvulsant, antipyretic, antiviral, immunity, anti-inflammation, antitumor properties, protecting liver and kidney and so on. The present review provides a comprehensive summary and analysis of the pharmacological properties of saikosaponins, supporting the potential uses of saikosaponins as a medicinal agent.

Sirtinol regulates the balance of Th17/Treg to prevent allograft rejection

BACKGROUND

Current immunosuppressive medications used after transplantation induce significant toxicity , and a new medication regimen is needed. Based on recent research, Sirt1 exerts a proinflammatory effect on the immune response. Sirtinol is a Sirt1 inhibitor, but its impact on allograft rejection and its molecular mechanisms of action have not yet been reported.

RESLUTS

In this study, we examined the effect of sirtinol on prolonging allograft survival in a mouse cervical heterotopic heart transplantation model. Based on an examination of the allograft, allografts from sirtinol-treated recipients show significantly lower levels of IL-17A expression and higher levels of Foxp3 expression. In vivo, sirtinol reduces the proportion of Th17 cells and increases the proportion of Treg cells in splenocytes from recipients. In vitro, sirtinol reduces the proportion of Th17 cells and decreases the expression of IL-17A and RORγt in an isolated CD4⁺ T cell population. Moreover, we identified synergistic effects of sirtinol and FK506 on prolonging allograft survival, and sirtinol synergizes with FK506 to promote Foxp3 expression.

CONCLUSION

Sirtinol, a Sirt1 inhibitor, may be a promising immunosuppressive drug to prevent the rejection reaction in combination with FK506.

Characterization of human S-adenosyl-homocysteine hydrolase in vitro and identification of its potential inhibitors

Human S-adenosyl-homocysteine hydrolase (SAHH, E.C.3.3.1.1) has been considered to be an attractive target for the design of medicines to treat human disease, because of its important role in regulating biological methylation reactions to catalyse the reversible hydrolysis of S-adenosylhomocysteine (SAH) to adenosine (Ado) and l-homocysteine (Hcy). In this study, SAHH protein was successfully cloned and purified with optimized, Pichia pastoris (P. pastoris) expression system. The biological activity results revealed that, among the tested compounds screened by ChemMapper and SciFinder Scholar, 4-(3-hydroxyprop-1-en-1-yl)-2-methoxyphenol (coniferyl alcohol, CAS: 458-35-5, ZINC: 12359045) exhibited the highest inhibition against rSAHH (IC₅₀= 34 nM). Molecular docking studies showed that coniferyl alcohol was well docked into the active cavity of SAHH. And several H-bonds formed between them, which stabilized coniferyl alcohol in the active site of rSAHH with a proper conformation.

Inhibition of histone methyltransferase EZH2 ameliorates early acute renal allograft rejection in rats

BACKGROUND

Although histone methyltransferases EZH2 has been proved to have significant regulatory effect on the immune rejection after hematopoietic stem cell transplantation, its role in solid-organ transplantation remains uncovered. In this study, we investigate whether histone methylation regulation can impact renal allograft rejection in rat models.

RESULTS

Allogeneic rat renal transplantation model (Wistar to Lewis) was established, and the recipients were administrated with EZH2 inhibitor DZNep after transplantation. Renal allografts and peripheral blood were collected on day 5 after transplantation for histological examination and mechanism investigation. We found that inhibition of EZH2 by DZNep after transplantation significantly ameliorated acute rejection (AR), with decreased histological injury and reduced inflammatory infiltration in renal allografts. Attenuation of AR was due to the prohibited activation of alloreactive T cells, the subsequent impaired production of inflammatory cytokines, and also the elevated apoptosis of alloreactive T cells in both renal allografts and periphery. However, inhibition of EZH2 did not increase the regulatory T cells during the AR.

CONCLUSIONS

Disruption of EZH2 by DZNep suppressed the immune responses of alloreactive T cells and ameliorated AR of renal allografts. This suggests a therapeutic potential of targeting histone methyltransferases EZH2 in treating allograft rejection after solid organ transplantation.

Structure-Activity Relationships of Neplanocin A Analogues as S-Adenosylhomocysteine Hydrolase Inhibitors and Their Antiviral and Antitumor Activities

On the basis of the potent inhibitory activity of neplanocin A (1) against S-adenosylhomocysteine (AdoHcy) hydrolase, we analyzed the comprehensive structure-activity relationships by modifying the adenine and carbasugar moiety of 1 to find the pharmacophore in the active site of the enzyme. The introduction of 7-deazaadenine instead of adenine eliminated the inhibitory activity against the AdoHcy hydrolase, while 3-deazaadenine maintained the inhibitory activity of the enzyme, indicating that N-7 is essential for its role as a hydrogen bonding acceptor. The substitution of hydrogen at the 6'-position with fluorine increased the inhibitory activity of the enzyme. The one-carbon homologation at the 5'-position generally decreased the inhibitory activity of the enzyme, indicating that steric repulsion exists. A molecular docking study also supported these experimental data. In this study, 6'-fluoroneplanocin A (2) was the most potent inhibitor of AdoHcy hydrolase (IC50 = 0.24 μM). It showed a potent anti-VSV activity (EC50 = 0.43 μM) and potent anticancer activity in all the human tumor cell lines tested.

3-Deazaneplanocin A and neplanocin A analogues and their effects on apoptotic cell death

3-Deazaneplanocin A (DzNep) is a potential epigenetic drug for the treatment of various cancers. DzNep has been reported to deplete histone methylations, including oncogenic EZH2 complex, giving rise to epigenetic modifications that reactivate many silenced tumor suppressors in cancer cells. Despite its promise as an anticancer drug, little is known about the structure-activity relationships of DzNep in the context of epigenetic modifications and apoptosis induction. In this study, a number of analogues of DzNep were examined for DzNep-like ability to induce synergistic apoptosis in cancer cells in combination with trichostatin A, a known histone deacetylase (HDAC) inhibitor. The structure-activity relationship data thus obtained provide valuable information on the structural requirements for biological activity. The studies identified three compounds that show similar activities to DzNep. Two of these compounds show good pharmacokinetics and safety profiles. Attempts to correlate the observed synergistic apoptotic activities with measured S-adenosylhomocysteine hydrolase (SAHH) inhibitory activities suggest that the apoptotic activity of DzNep might not be directly due to its inhibition of SAHH.

The downregulation of GFI1 by the EZH2-NDY1/KDM2B-JARID2 axis and by human cytomegalovirus (HCMV) associated factors allows the activation of the HCMV major IE promoter and the transition to productive infection

Earlier studies had suggested that epigenetic mechanisms play an important role in the control of human cytomegalovirus (HCMV) infection. Here we show that productive HCMV infection is indeed under the control of histone H3K27 trimethylation. The histone H3K27 methyltransferase EZH2, and its regulators JARID2 and NDY1/KDM2B repress GFI1, a transcriptional repressor of the major immediate-early promoter (MIEP) of HCMV. Knocking down EZH2, NDY1/KDM2B or JARID2 relieves the repression and results in the upregulation of GFI1. During infection, the incoming HCMV rapidly downregulates the GFI1 mRNA and protein in both wild-type cells and in cells in which EZH2, NDY1/KDM2B or JARID2 were knocked down. However, since the pre-infection levels of GFI1 in the latter cells are significantly higher, the virus fails to downregulate it to levels permissive for MIEP activation and viral infection. Following the EZH2-NDY1/KDM2B-JARID2-independent downregulation of GFI1 in the early stages of infection, the virus also initiates an EZH2-NDY1/ΚDM2Β-JARID2-dependent program that represses GFI1 throughout the infection cycle. The EZH2 knockdown also delays histone H3K27 trimethylation in the immediate early region of HCMV, which is accompanied by a drop in H3K4 trimethylation that may contribute to the shEZH2-mediated repression of the major immediate early HCMV promoter. These data show that HCMV uses multiple mechanisms to allow the activation of the HCMV MIEP and to prevent cellular mechanisms from blocking the HCMV replication program.

Modulation of allogeneic CD8+ T-cell response by DZNep controls GVHD while preserving hematopoietic chimerism

BACKGROUND

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) combined with solid-organ transplantation is a feasible method to achieve long-lasting organ allograft tolerance through the induction of hematopoietic chimerism in recipients. However, the allo-HSCT engraftment puts recipients at risk of life-threatening graft-versus-host disease (GVHD). Novel immunomodulatory approaches are required to effectively control GVHD while preserving the status of hematopoietic chimerism. We have reported that histone methylation inhibitor 3-deazaneplanocin A (DZNep) can control ongoing GVHD in mice by selectively inducing apoptosis of alloreactive effector T cells.

METHODS

Using donor-derived CD8 T cell-mediated mouse GVHD model, we further investigated the effect of in vivo administration of DZNep on allogeneic CD8 T cell response and the hematopoietic chimerism in recipients.

RESULTS

We found that DZNep delayed the in vivo proliferation of donor-derived alloreactive CD8 T cells and also reduced the interleukin-2 production by these T cells. Moreover, DZNep treatment resulted in a significant decrease of interferon-γ, tumor necrosis factor-α, granzyme B, TRAIL, and Fas ligand expressing donor-derived CD8 T cells, suggesting a multilevel modulation role on T-cell survival and effect in vivo. Notably, DZNep treatment did not hamper the generation of hematopoietic chimerism in recipients.

CONCLUSIONS

These findings suggest that modulation of histone methylation through DZNep may be a potential strategy for the induction of hematopoietic chimerism to achieve donor-specific organ allograft tolerance through donor allo-HSCT combined with solid-organ transplantation.

Inhibition of histone methylation arrests ongoing graft-versus-host disease in mice by selectively inducing apoptosis of alloreactive effector T cells

Histone methylation is thought to be important for regulating Ag-driven T-cell responses. However, little is known about the effect of modulating histone methylation on inflammatory T-cell responses. We demonstrate that in vivo administration of the histone methylation inhibitor 3-deazaneplanocin A (DZNep) arrests ongoing GVHD in mice after allogeneic BM transplantation. DZNep caused selective apoptosis in alloantigen-activated T cells mediating host tissue injury. This effect was associated with the ability of DZNep to selectively reduce trimethylation of histone H3 lysine 27, deplete the histone methyltransferase Ezh2 specific to trimethylation of histone H3 lysine 27, and activate proapoptotic gene Bim repressed by Ezh2 in antigenic-activated T cells. In contrast, DZNep did not affect the survival of alloantigen-unresponsive T cells in vivo and naive T cells stimulated by IL-2 or IL-7 in vitro. Importantly, inhibition of histone methylation by DZNep treatment in vivo preserved the antileukemia activity of donor T cells and did not impair the recovery of hematopoiesis and lymphocytes, leading to significantly improved survival of recipients after allogeneic BM transplantation. Our findings indicate that modulation of histone methylation may have significant implications in the development of novel approaches to treat ongoing GVHD and other T cell-mediated inflammatory disorders in a broad context.

X-ray crystal structure and binding mode analysis of human S-adenosylhomocysteine hydrolase complexed with novel mechanism-based inhibitors, haloneplanocin A analogues

The X-ray crystal structure of human S-adenosylhomocysteine (AdoHcy) hydrolase was first determined as a tetrameric form bound with the novel mechanism-based inhibitor fluoroneplanocin A (4b). The crystallized enzyme complex showed the closed conformation and turned out to be the intermediate of mechanism-based inhibition. It confirmed that the cofactor depletion by 3'-oxidation of fluoroneplanocin A contributes to the enzyme inhibition along with the irreversible covalent modification of AdoHcy hydrolase. In addition, a series of haloneplanocin A analogues (4b-e and 5b-e) were designed and synthesized to characterize the binding role and reactivity of the halogen substituents and the 4'-CH(2)OH group. The biological evaluation and molecular modeling studies identified the key pharmacophores and structural requirements for the inhibitor binding of AdoHcy hydrolase. The inhibitory activity was decreased as the size of the halogen atom increased and/or if the 4'-CH(2)OH group was absent. These results could be utilized to design new therapeutic agents operating via AdoHcy hydrolase inhibition.

Truncated fluorocyclopentenyl pyrimidines as S-adenosylhomocysteine hydrolase inhibitors

On the basis of inhibitory activity of truncated cyclopentenyl cytosine against S-adenosylhomocysteine hydrolase (SAH), its fluorocyclopentenyl pyrimidine derivatives were efficiently synthesized from D-ribose via electrophilic fluorination as a key step. The final nucleosides were evaluated for SAH inhibitory activity, among which the uracil derivative 9 showed significant inhibitory activity (IC(50) = 8.53 microM). They were also evaluated for cytotoxic effects in several human cancer cell lines such as fibro sarcoma, stomach cancer, leukemia, and colon cancer, but they did not show any cytotoxic effects up to 100 microM, indicating that 4'-hydroxymethyl groups are essential for the anticancer activity.

Inhibition of transmethylation down-regulates CD4 T cell activation and curtails development of autoimmunity in a model system

Transmethylation affects several cellular events, including T cell activation, and blockade of this pathway may curtail inflammatory/autoimmune responses. Here, we demonstrate that transmethylation inhibition by a novel reversible S-adenosyl-l-homocysteine hydrolase inhibitor leads to immunosuppression by reducing phosphorylation of several key proteins involved in TCR signaling, including Akt, Erk1/2, and NF-kappaB. Remarkably, this effect was largely restricted to CD4 T cells and correlated with reduced arginine methylation of Vav1, an essential guanine nucleotide exchange factor in T cell stimulation. Treatment with the transmethylation inhibitor averted, and even ameliorated, the CD4-mediated autoimmune disease, experimental autoimmune encephalomyelitis. The data suggest that transmethylation is required for CD4 T cell activation, and its inhibition may be a novel approach in the treatment of multiple sclerosis, and other CD4-mediated autoimmune diseases.

John Montgomery's legacy: carbocyclic adenosine analogues as SAH hydrolase inhibitors with broad-spectrum antiviral activity

Ever since the S-adenosylhomocysteine (AdoHcy, SAH) hydrolase was recognized as a pharmacological target for antiviral agents (J. A. Montgomery et al., J. Med. Chem. 25:626-629, 1982), an increasing number of adenosine, acyclic adenosine, and carbocyclic adenosine analogues have been described as potent SAH hydrolase inhibitors endowed with broad-spectrum antiviral activity. The antiviral activity spectrum of the SAH hydrolase inhibitors include pox-, rhabdo-, filo-, arena-, paramyxo-, reo-, and retroviruses. Among the most potent SAH hydrolase inhibitors and antiviral agents rank carbocyclic 3-deazaadenosine (C-c3 Ado), neplanocin A, 3-deazaneplanocin A, the 5'-nor derivatives of carbocyclic adenosine (C-Ado, aristeromycin), and the 2-halo (i.e., 2-fluoro) and 6'-R-alkyl (i.e., 6'-R-methyl) derivatives of neplanocin A. These compounds are particularly active against poxviruses (i.e., vaccinia virus), and rhabdoviruses (i.e., vesicular stomatitis virus). The in vivo efficacy of C-c3 Ado and 3-deazaneplanocin A has been established in mouse models for vaccinia virus, vesicular stomatitis virus, and Ebola virus. SAH hydrolase inhibitors such as C-c3Ado and 3-deazaneplanocin A should in thefirst place be considered for therapeutic (or prophylactic) use against poxvirus infections, including smallpox, and hemorrhagic fever virus infections such as Ebola.

Anti-HIV-1 activity of 3-deaza-adenosine analogs. Inhibition of S-adenosylhomocysteine hydrolase and nucleotide congeners

Eight adenosine analogs, 3-deaza-adenosine (DZA), 3-deaza-(+/-)aristeromycin (DZAri), 2',3'-dideoxy-adenosine (ddAdo), 2',3'-dideoxy-3-deaza-adenosine (ddDZA), 2',3'-dideoxy-3-deaza-(+/-)aristeromycin (ddDZAri), 3-deaza-5'-(+/-)noraristeromycin (DZNAri), 3-deaza-neplanocin A (DZNep), and neplanocin A (NepA), were tested as inhibitors of human placenta S-adenosylhomocysteine (AdoHcy) hydrolase. The order of potency for the inhibition of human placental AdoHcy hydrolase was: DZNep approximately NepA >> DZAri approximately DZNAri > DZA >> ddAdo approximately ddDZA approximately ddDZAri. These same analogs were examined for their anti-HIV-1 activities measured by the reduction in p24 antigen produced by 3'-azido-3'-deoxythymidine (AZT)-sensitive HIV-1 isolates, A012 and A018, in phytohemagglutinin-stimulated peripheral blood mononuclear (PBMCs) cells. Interestingly, DZNAri and the 2',3'-dideoxy 3-deaza-nucleosides (ddAdo, ddDZAri, and ddDZA) were only marginal inhibitors of p24 antigen production in HIV-1 infected PBMC. DZNAri is unique because it is the only DZA analog with a deleted methylene group that precludes anabolic phosphorylation. In contrast, the other analogs were potent inhibitors of p24 antigen production by both HIV-1 isolates. Thus it was postulated that these nucleoside analogs could exert their antiviral effect via a combination of anabolically generated nucleotides (with the exception of DZNAri), which could inhibit reverse transcriptase or other viral enzymes, and the inhibition of viral or cellular methylation reactions. Additionally, QSAR-like models based on the molecular mechanics (MM) were developed to predict the order of potency of eight adenosine analogs for the inhibition of human AdoHcy hydrolase. In view of the potent antiviral activities of the DZA analogs, this approach provides a promising tool for designing and screening of more potent AdoHcy hydrolase inhibitors and antiviral agents.

Synthesis and biological activity of novel S-adenosyl-L-homocysteine hydrolase inhibitors

Four potential S-adenosyl-L-homocysteine hydrolase inhibitors were prepared and tested against purified recombinant rat liver enzyme. Preliminary studies indicate that three of these compounds, 1, 2, and 4, caused time-dependent inactivation of S-adenosyl-L-homocysteine hydrolase but showed a biphasic nature. Compound 3 was found to be a rapid equilibrium inhibitor of this enzyme.

Design, synthesis, and biological evaluation of fluoroneplanocin A as the novel mechanism-based inhibitor of S-adenosylhomocysteine hydrolase

Fluoroneplanocin A (12) was designed as a novel mechanism-based inhibitor of S-adenosylhomocysteine hydrolase (SAH) and efficiently synthesized via an electrophilic vinyl fluorination reaction (n-BuLi, N-fluorobenzenesulfonimide at -78 degrees C). Fluoroneplanocin A exhibited 2-fold more potent SAH inhibitory activity than the parent neplanocin A. A new mechanism of irreversible inhibition discovered in this work might provide new alternatives in the design of a different class of antiviral agents operating via SAH inhibition.

Intracellular phosphorylation of carbocyclic 3-deazaadenosine, an anti-Ebola virus agent

Carbocyclic 3-deazaadenosine (C-c3Ado) is a potent inhibitor of Ebola virus in mice by infrequent dosing, even though its half life in plasma is only 23-28 min. This prompted studies to determine whether C-c3Ado undergoes intracellular metabolism to derivatives that may promote in vivo activity. In cells, radiolabelled compound readily underwent metabolism to monophosphate, diphosphate and triphosphate (C-c3ATP) forms, with C-c3ATP being the major metabolite detected. A non-polar metabolite was also detected both inside and outside treated cells. The retention time of C-c3ATP was similar but not identical to ATP on a strong anion exchange high performance liquid chromatography (HPLC) column or on a DEAE-Sephadex open column. C-c3ATP and ATP were susceptible to degradation to their respective nucleosides by bovine alkaline phosphatase. Intracellular formation of C-c3ATP reached a plateau by about 4 h after treatment of monkey (Vero 76) and mouse (Balb/3T3 clone A31) cells with 10 or 100 microM extracellular compound. Phosphorylation was linearly dose responsive at 1, 3 and 10 microM. However, the extent of phosphorylation decreased with increasingly higher concentrations (30, 100 and 300 microM). When compound was removed from the medium, the nucleoside cleared the cells within 1 min, whereas C-c3ATP had a half life of decay of 2-3 h in five cell lines. Phosphorylation of C-c3Ado to C-c3ATP was not inhibited by cotreatment of cells (at a 20:1 ratio) with adenosine, guanosine, inosine, xanthosine, cytidine or uridine. There was no evidence of incorporation of C-c3Ado (10 microM) into macromolecules of cells over 72 h, whereas adenosine was readily incorporated. C-c3ATP may represent a form of C-c3Ado that might contribute to extending its intracellular half life or otherwise exhibit antiviral activity and/or toxicity.

Doubly homologated dihalovinyl and acetylene analogues of adenosine: synthesis, interaction with S-adenosyl-L-homocysteine hydrolase, and antiviral and cytostatic effects

Treatment of the 6-aldehyde derived by Moffatt oxidation of 3-O-benzoyl-1,2-O-isopropylidene-alpha-D-ribo-hexofuranose (2c) with the dibromo- or bromofluoromethylene Wittig reagents generated in situ with tetrabromomethane or tribromofluoromethane, triphenylphosphine, and zinc gave the dihalomethyleneheptofuranose analogues 3b and 3d, respectively. Acetolysis, coupling with adenine, and deprotection gave 9-(7,7-dibromo-5,6, 7-trideoxy-beta-D-ribo-hept-6-enofuranosyl)adenine (5a) or its bromofluoro analogue 5b. Treatment of 5a with excess butyllithium provided the acetylenic derivative 9-(5,6, 7-trideoxy-beta-D-ribo-hept-6-ynofuranosyl)adenine (6). The doubly homologated vinyl halides 5a and 5b and acetylenic 6 adenine nucleosides were designed as putative substrates of the "hydrolytic activity" of S-adenosyl-L-homocysteine (AdoHcy) hydrolase. Incubation of AdoHcy hydrolase with 5a, 5b, and 6 resulted in time- and concentration-dependent inactivation of the enzyme (K(i): 8.5 +/- 0.5, 17 +/- 2, and 8.6 +/- 0.5 microM, respectively), as well as partial reduction of enzyme-bound NAD(+) to E-NADH. However, no products of the "hydrolytic activity" were observed indicating these compounds are type I mechanism-based inhibitors. The compounds displayed minimal antiviral and cytostatic activity, except for 6, against vaccinia virus and vesicular stomatitis virus (IC(50): 15 and 7 microM, respectively). These viruses typically fall within the activity spectrum of AdoHcy hydrolase inhibitors.

S-adenosylhomocysteine hydrolase inhibitors interfere with the replication of human immunodeficiency virus type 1 through inhibition of the LTR transactivation

Various analogues of adenosine have been described as inhibitors of S-adenosylhomocysteine (AdoHcy) hydrolase, and some of these AdoHcy hydrolase inhibitors (e.g., 3-deazaadenosine, 3-deazaaristeromycin, and 3-deazaneplanocin A) have also been reported to inhibit the replication of human immunodeficiency virus type 1 (HIV-1). When evaluated against HIV-1 replication in MT-4 cells, macrophages, or phytohemagglutinin-stimulated peripheral blood lymphocytes infected acutely or chronically with HIV-1IIIB or HIVBaL strains, a wide range of adenosine analogues did not inhibit HIV-1IIIB replication for 50% at subtoxic concentrations. However, they inhibited HIV-1 replication in HeLa CD4+ LTR-LacZ cells at concentrations well below cytotoxicity threshold. A close correlation was found among the inhibitory effect of the compounds on AdoHcy hydrolase activity, their inhibition of HIV-1 replication in Hela CD4+ LTR-LacZ cells, and their inhibition of the HIV-1 Tat-dependent and -independent transactivation of the long terminal repeat, whereas no inhibitory effect was seen on HIV-1 reverse transcription or a Tat-independent cytomegalovirus promoter. Our results suggest that AdoHcy hydrolase and the associated S-adenosylmethionine-dependent methylation mechanism play a role in the process of long terminal repeat transactivation and, hence, HIV replication.

In search of a selective antiviral chemotherapy

This article describes several approaches to a selective therapy of virus infections: (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU [brivudin]) for the therapy of herpes simplex virus type 1 and varicella-zoster virus infections: (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine (HPMPC [cidofovir]) for the therapy of various DNA virus (i.e., herpesvirus, adenovirus, papillomavirus, polyomavirus, and poxvirus) infections; 9-(2-phosphonylmethoxyethyl)adenine (PMEA [adefovir]) for the therapy of retrovirus, hepadnavirus, and herpesvirus infections; (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA) for the therapy and prophylaxis of retrovirus and hepadnavirus infections; and nonnucleoside reverse transcriptase inhibitors (NNRTIs), such as tetrahydroimidazo[4,5,1-jk][1,4]-benzodiazepin-2(IH)-one and -thione (TIBO), 1-[(2-hydroxyethoxy)methyl]-6-(phenylthio)thymine (HEPT), alpha-anilinophenylacetamide (alpha-APA), and 2',5'bis-O-(tert-butyldimethylsilyl)-3'-spiro-5"-(4"-amino-1",2"-oxat hiole- 2",2"-dioxide)pyrimidine (TSAO) derivatives, and thiocarboxanilides for the treatment of human immunodeficiency virus type 1 (HIV-1) infections. For the clinical use of NNRTIs, some guidelines have been elaborated, such as starting treatment with combinations of different compounds at sufficiently high concentrations to effect a pronounced and sustained suppression of the virus. Despite the diversity of the compounds described here and the different viruses at which they are targeted, they have a number of characteristics in common. As they interact with specific viral proteins, the compounds achieve a selective inhibition of the replication of the virus, which, in turn, should be able to develop resistance to the compounds. However, as has been established for the NNRTIs, the problem of viral resistance may be overcome if the compounds are used from the start at sufficiently high doses, which could be reduced if different compounds are combined. For HIV infections, drug treatment regimens should be aimed at reducing the viral load to such an extent that the risk for progression to AIDS will be minimized, if not avoided entirely. This may result in a real "cure" of the disease but not necessarily of the virus infection, and in this sense, HIV disease may be reduced to a dormant infection, reminiscent of the latent herpesvirus infections. Should virus replication resume after a certain time, the armamentarium of effective anti-HIV and anti-herpesvirus compounds now available, if applied at the appropriate dosage regimens, should make the virus return to its dormant state before it has any chance to damage the host. It is unlikely that this strategy would eradicate the virus and thus "cure" the viral infection, but it definitely qualifies as a cure of the disease.

New neplanocin analogues. 7. Synthesis and antiviral activity of 2-halo derivatives of neplanocin A

The syntheses and the antiviral activities of 2-halo derivatives of neplanocin A (1b,c), (6'R)-6'-C-methylneplanocin A (2b), and dehydroxymethylneplanocin A (3b,c) are described. SN2 reaction of the known cyclopentenyl units 12 and 13 with 2-haloadenines under basic conditions gave the protected carbocyclic nucleosides 14b,c and 15b,c, respectively. Starting from the cyclopentenone derivative 5, the optically active tosyloxycyclopentene derivative 11 was prepared, which was similarly condensed with 2-fluoroadenine to give the protected (6'R)-6'-C-methyl derivative 16b. Deprotection of these compounds afforded the target 2-halo derivatives of neplanocin A. Of these new compounds, 2-fluoroneplanocin A (1b) showed an antiviral potency and a spectrum that was comparable to that of neplanocin A (1a). It was particularly active against vaccinia virus, vesicular stomatitis virus, parainfluenza virus, reovirus, arenaviruses (Junin, Tacaribe), and human cytomegalovirus, i.e., those viruses that fall within the purview of the S-adenosyl-L-homocysteine hydrolase inhibitors.

New neplanocin analogues. 6. Synthesis and potent antiviral activity of 6'-homoneplanocin A1

The design, synthesis, and antiviral activities of 6'-homoneplanocin A (HNPA, 3) and its congeners having nucleobases other than adenine, such as 3-deazaadenine (4), guanine (5), thymine (6), and cytosine (7), were described. Starting from the known cyclopentenone derivative 8, the optically active (mesyloxy)cyclopentene derivative 15 was prepared, which was condensed with nucleobases then deprotected to give target compounds 3-7. Of these compounds, HNPA showed an antiviral activity spectrum that was comparable to, and an antiviral specificity that was higher than, that of neplanocin A. HNPA proved particularly active against human cytomegalovirus, vaccinia virus, parainfluenza virus, vesicular stomatitis virus, and arenaviruses, which is compatible with an antiviral action targeted at S-adenosylhomocysteine hydrolase. HNPA appears to be a promising candidate drug for the treatment of these viruses.

Comparison of the inhibition of type A and type B S-adenosylhomocysteine hydrolase: effects of cofactor content on inhibition behavior and nucleoside binding

The enzyme S-adenosylhomocysteine hydrolase (E.C.3.3.1.1) occurs in two forms in bovine liver: Type A, which carries four moles of NAD+ per mole of enzyme tetramer, and Type B, which carries two moles of NAD+ per mole of tetramer. The inhibition of these two forms of the enzyme with 2',2'-difluoro-2'-deoxyadenosine has been investigated. The studies examined the binding stoichiometry and stability of the enzyme-inhibitor complexes formed from each type of the enzyme, the degree of NAD+ reduction and NAD+ release, and the possibility of covalent bond formation between the enzyme and the inhibitor. Significant differences in the behavior of the two forms of the enzyme were encountered which may have important implications for the design of S-adenosylhomocysteine hydrolase inhibitors as therapeutic agents.