Inhibition of replication of hepatitis B virus by cytallene in vitro

Palladium-Catalyzed Stereoselective Construction of 1,3-Stereocenters Displaying Axial and Central Chirality via Asymmetric Alkylations

The concurrent construction of 1,3-stereocenters remains a challenge. Herein, we report the development of stereoselective union of a point chiral center with allenyl axial chirality in 1,3-position by Pd-catalyzed asymmetric allenylic alkylation between racemic allenyl carbonates and indanone-derived β-ketoesters. Various target products bearing a broad range of functional groups were afforded in high yield (up to 99%) with excellent enantioselectivities (up to 98% ee) and good diastereoselectivities (up to 13:1 dr).

(Z)- and (E)-2-(Hydroxymethylcyclopropylidene)-Methylpurines and Pyrimidines as Antiviral Agents

Several Z- and E-methylenecyclopropane nucleoside analogues were synthesized and evaluated for antiviral activity. Reaction of the Z- and E-2-amino-6-chloropurine methylenecyclopropanes with ammonia or cyclopropylamine gave 2,6-diamino or 2-amino-6-cyclopropylamino analogues. Alkylation elimination of N4-acetylcytosine with ethyl Z- and E-2-bromo-2-bromomethylcyclopropane-1-carboxylates gave a mixture of the Z-and E-methylenecyclopropane derivatives of cytosine. Reduction furnished a mixture of syncytol and the E isomer. Benzoylation led to the respective N4-benzoyl derivatives which were separated by chromatography. Debenzoylation afforded pure syncytol and the E isomer. Alkylation of 2,4-bis-O-trimethylsilylthymine with ethyl Z- and E-2-bromo-2-bromomethylcyclopropane-1-carboxylates gave the corresponding Z- and E-1-bromo-cyclopropylmethylderivatives of thymine. Base-catalysed elimination of HBr gave Z- and E-methylenecyclopropane carboxylic esters. Reduction furnished, after chromatographic separation, synthymol and the E isomer. The Z/E isomeric assignment of the obtained products followed from 1H NMR spectroscopy. The methylenecyclopropane analogues were tested for antiviral activity in vitro against human and murine cytomegalovirus (HCMV, MCMV), Epstein–Barr virus (EBV), varicella zoster virus (VZV), hepatitis B virus (HBV), herpes simplex virus types 1 and 2 (HSV-1, HSV-2), human herpesvirus 6 (HHV-6) and human immunodeficiency virus type 1 (HIV-1). The Z-2-amino-6-cyclopropylaminopurine analogue was the most effective agent against HCMV (EC50 or EC90 0.4–2 μM) followed by syncytol and the Z-2,6-diaminopurine analogues (EC50 or EC90 3.4–29 and 11–24 μM, respectively). The latter compound was also a strong inhibitor of MCMV (EC50 0.6 μM). Syncytol was the most potent against EBV (EC50 <0.41 and 2.5 μM) followed by the Z-2,6-diaminopurine (EC50 1.5 and 6.9 μM) and the Z-2-amino-6-cyclopropylaminopurine derivative (EC50 11.8 μM). Syncytol was also most effective against VZV (EC50 3.6 μM). Activity against HSV-1, HSV-2 and HHV-6 was generally lower; synthymol had an EC50 of 2 μM against HSV-1 (ELISA) and 1.3 μM against EBV in Daudi cells but was inactive in other assays. The 2-amino-6-cyclopropylamino analogue displayed EC50 values between 215 and >74 μM in HSV-1 and HSV-2 assays. 2-Amino-6-cyclopropylaminopurine and 2,6-diaminopurine derivatives were effective against HBV (EC50 2 and 10 μM, respectively), whereas none of the analogues inhibited HIV-1 at a higher virus load. Syncytol and the E isomer were equipotent against EBV in Daudi cells but the E isomer was much less effective in DNA hybridization assays. The E-2,6-diaminopurine analogue and E isomer of synthymol were devoid of antiviral activity.

Discovery and development of anti-HBV agents and their resistance

Hepatitis B virus (HBV) infection is a prime cause of liver diseases such as hepatitis, cirrhosis and hepatocellular carcinoma. The current drugs clinically available are nucleot(s)ide analogues that inhibit viral reverse transcriptase activity. Most drugs of this class are reported to have viral resistance with breakthrough. Recent advances in methods for in silico virtual screening of chemical libraries, together with a better understanding of the resistance mechanisms of existing drugs have expedited the discovery and development of novel anti-viral drugs. This review summarizes the current status of knowledge about and viral resistance of HBV drugs, approaches for the development of novel drugs as well as new viral and host targets for future drugs.

Novel ring-expanded nucleoside analogs exhibit potent and selective inhibition of hepatitis B virus replication in cultured human hepatoblastoma cells

Novel ring-expanded nucleoside (REN) analogs (1-3) containing 5:7 fused ring systems as the heterocyclic base were found to be potent and selective inhibitors of hepatitis B virus (HBV) replication in cultured human hepatoblastoma 2.2.15 cells. The most active compound, 6-amino-4,5-dihydro-8H-1-(beta-D-ribofuranosyl)imidazo[4,5-e][1,3]diazepine-4,8-dione (1), inhibited the synthesis of intracellular HBV replication intermediates and extracellular virion release in 2.2.15 cells with 50% effective concentration (EC50) of 0.604 and 0.131 microM, respectively. All three compounds had no effect on the synthesis of viral ribonucleic acids (RNA) in 2.2.15 cells. These compounds also exhibited low cellular toxicity in stationary and rapidly growing cell systems.

Drug discovery and development of antiviral agents for the treatment of chronic hepatitis B virus infection

A safe and effective vaccine for hepatitis B virus (HBV) has been available for nearly twenty years and currently campaigns to provide universal vaccination in developing countries are underway. Nevertheless, chronic HBV infection remains a leading cause of chronic hepatitis worldwide and there is a strong need for safe and effective antiviral therapies. Attempts to identify and develop antiviral agents to treat chronic HBV infection remains focused on nucleoside analogs such as 3TC (lamivudine), adefovir dipivoxil, (bis-POMPMEA), and others. However, advances in our understanding of the molecular biology of HBV and the development of new assays for HBV polymerase activity, such as the reconstitution of active HBV polymerase in vitro, should facilitate large screening efforts for non-nucleoside reverse transcriptase inhibitors. Recent advances have furthered our understanding of clinical resistance to lamivudine, have provided new approaches to treatment, and have offered new perspectives on the major challenges to the identification and development of antiviral agents for chronic HBV infection. Here, in an update to our previous review article that appeared in this series [59a], we focus on recent advances that have occurred in the areas of virus structure and replication, in vitro viral polymerase assays, cell culture systems, and animal models.

Spiropentane mimics of nucleosides: analogues of 2'-deoxyadenosine and 2'-deoxyguanosine. Synthesis of all stereoisomers, isomeric assignment, and biological activity

Synthesis of spirocyclic analogues of 2'-deoxyadenosine and 2'-deoxyguanosine (12a-15a and 12b-15b) is described. Rhodium-catalyzed reaction of ethyl diazoacetate with methylenecyclopropane 19, obtained from 2-bromo-2-bromomethylcyclopropane 17 via debromination (16), reduction (18), and acetylation (19), gave a mixture of all four isomeric spiropentanes 20a-20d. Hydrolysis afforded hydroxy carboxylic acids 21a-21d. Acetylation of separated proximal + medial-syn isomers 21a + 21b and medial anti + distal isomers 21c + 21d furnished acetates 22a + 22b and 22c + 22d. Curtius rearrangement effected by diphenylphosphoryl azide in tert-butyl alcohol performed separately with mixtures 22a + 22b and 22c + 22d led to BOC-amino spiropentanes 23a + 23b and 23c + 23d. After deacetylation all isomers 24a-24d were separated and deprotected to give aminospiropentane hydrochlorides 25a-25d. Free bases were of limited stability. The heterocyclic moieties were introduced into individual isomers 25a-25d via 6-chloropurine derivatives 26a-26d or 30a-30d. Ammonolysis of 26a-26d furnished the adenine isomeric series 12a-15a, whereas guanine derivatives 12b-15b were obtained by hydrolysis of 30a-30d with formic acid. The isomeric assignments followed from IR spectra of BOC-aminospiropentanes 24a-24d and NMR spectra of 12a-15a including NOE and (H,H) COSY. The proximal and medial-syn isomers 12a and 12b were modest inhibitors of human cytomegalovirus (HCMV) and Epstein-Barr virus (EBV) in culture, whereas the medial-anti isomer 12c was a substrate for adenosine deaminase. The distal isomer 15b was an anti-EBV agent. The medial-syn phosphoralaninate 34 was an effective inhibitor of HCMV replication in vitro. It was also active against herpes simplex virus type 1 (HSV-1), varicella zoster virus (VZV), human immunodeficiency virus (HIV-1), hepatitis B virus (HBV), and EBV with a varying degree of cytotoxicity.

Cell-based and animal models for hepatitis B and C viruses

Reliable cell-based assays and animal models have been developed for evaluating agents against hepatitis B virus. Although much progress has been made, in vitro and in vivo assays for hepatitis C virus are still on the horizon. Advances towards establishing inexpensive and reliable experimental models have accelerated the development of therapeutic modalities for these life-threatening viral infections. The characterization of well-defined viral targets coupled with improved molecular diagnostic technologies have illuminated this field.

The identification and development of antiviral agents for the treatment of chronic hepatitis B virus infection

Hepatitis B virus (HBV) is the leading cause of chronic hepatitis throughout the world. Notwithstanding the availability of a safe and effective vaccine, the world prevalence of HBV has not declined significantly, thus resulting in the need for a selective antiviral agent. HBV is a small, partially double-stranded DNA virus which replicates through an RNA intermediate. Most efforts to develop anti-HBV agents have been targeted to the viral DNA polymerase which possesses reverse transcriptase activity. Currently, the most promising anti-HBV agents are nucleoside analogs which interfere with viral DNA replication. Although earlier nucleoside analogs such as vidarabine (ara-A) and fialuridine (FIAU) have displayed unacceptable toxicities, newer analogs such as lamivudine (3TC), bis-POM PMEA (GS-840), lobucavir, and BMS-200,475 have demonstrated clinical utility. In particular, the use of lamivudine has generated considerable interest in the development of other L-enantiomeric nucleoside analogs for use against HBV. Here, we provide an overview of HBV structure and replication strategy and discuss the use of cell culture systems, in vitro viral polymerase systems, and animal models to identify and evaluate anti-HBV agents. We also discuss the various classes of nucleoside analogs in terms of structure, mechanism of action, status in clinical development, ability to select for resistant HBV variants, and use in combination therapies. Finally, we present a discussion of novel antiviral approaches, including antisense and gene therapy, and address the various challenges to successful anti-HBV chemotherapeutic intervention.

Metabolism of 2',3'-dideoxy-2',3'-didehydro-beta-L(-)-5-fluorocytidine and its activity in combination with clinically approved anti-human immunodeficiency virus beta-D(+) nucleoside analogs in vitro

2',3'-Dideoxy-2',3'-didehydro-beta-L(-)-5-fluorocytidine [L(-)Fd4C] has been reported to be a potent inhibitor of the human immunodeficiency virus (HIV) in cell culture. In the present study the antiviral activity of this compound in two-drug combinations and its intracellular metabolism are addressed. The two-drug combination of L(-)Fd4C plus 2',3'-didehydro-2'-3'-dideoxythymidine (D4T, or stavudine) or 3'-azido-3'-deoxythymidine (AZT, or zidovudine) synergistically inhibited replication of HIV in vitro. Additive antiviral activity was observed with L(-)Fd4C in combination with 2',3'-dideoxycytidine (ddC, or zalcitabine) or 2',3'-dideoxyinosine (ddI, or didanosine). This beta-L(-) nucleoside analog has no activity against mitochondrial DNA synthesis at concentrations up to 10 microM. As we previously reported for other beta-L(-) nucleoside analogs, L(-)Fd4C could protect against mitochondrial toxicity associated with D4T, ddC, and ddI. Metabolism studies showed that this drug is converted intracellularly to its mono-, di-, and triphosphate metabolites. The enzyme responsible for monophosphate formation was identified as cytoplasmic deoxycytidine kinase, and the K(m) is 100 microM. L(-)Fd4C was not recognized in vitro by human mitochondrial deoxypyrimidine nucleoside kinase. Also, L(-)Fd4C was not a substrate for deoxycytidine deaminase. L(-)Fd4C 5'-triphosphate served as an alternative substrate to dCTP for incorporation into DNA by HIV reverse transcriptase. The favorable anti-HIV activity and protection from mitochondrial toxicity by L(-)Fd4C in two-drug combinations favors the further development of L(-)Fd4C as an anti-HIV agent.