Rare strain produces breakthrough compound to fight hepatitis B

Scientists at Japan’s Institute of Microbial Chemistry have isolated selective antiviral compounds from a rare strain to target the hepatitis B virus.The study was published in Antibiotic Journal.

Research: Catenupyizomycins, a novel anti-hepatitis B virus compound from the rare actinomycete bacterium Catenuloplanes sp.  MM782L-181F7. Image source: Explode/ShutterstockResearch: Catenupyizomycins, a novel anti-hepatitis B virus compound from the rare actinomycete bacterium Catenuloplanes sp. MM782L-181F7. Image source: Explode/Shutterstock

background

Hepatitis B virus (HBV) causes chronic liver infection in humans, increasing the risk of cirrhosis and hepatocellular carcinoma. Serological clearance of hepatitis B surface antigen and sustained viral suppression are considered the gold standard for the treatment of HBV infection.

Pegylated interferon-alpha and nucleotide/nucleoside analogues are two classes of approved anti-hepatitis B drugs that inhibit disease progression but do not eliminate viral antigens. Therefore, there is a need to identify new drug classes with different modes of action that can be used in combination with already approved drugs to control HBV infection.

In this study, scientists screened microbial fermentation broths to identify novel non-nucleoside inhibitors of HBV replication. They have determined the structure and mode of action of the identified compounds.

To screen, they used a new system, HBV103-AdV, which uses an adenoviral vector to detect replicating viral genomes. The system can evaluate the inhibitory efficacy of a test drug over a 4-day period.

Important observations

The fermentation broth of a rare actinomycete strain, MM782L-181F7, was screened to identify novel inhibitors of HBV replication. The broth was found to reduce intracellular levels of viral DNA with moderate but significant selectivity.Sequencing results of the strain showed that it was 99% similar to the strain chain wing sp.Based on this discovery, the strain was named chain wing sp. MM782L-181F7.

Three active compounds, including chain pyrazomycins A, B, and C, were isolated from the broth for further characterization. Structural analysis shows that the compound has a thiazole-pyridine skeleton related to kanamycin.

Analysis of the anti-HBV activity of these compounds showed that streptavidin could dose-dependently reduce intracellular viral DNA levels in hepatocytes infected with HBV103-AdV. Induction of extracellular viral DNA was observed in cells treated with streptomycin before cell viability was reduced. Furthermore, these compounds were found to reduce wild-type viral DNA in infected cells at micromolar concentrations. However, the selectivity of these compounds against wild-type virus was lower than that against HBV103-AdV.

Mode of action

HBV polymerase exhibits two metal-dependent enzymatic activities, including reverse transcriptase and RNAse. These activities are required for viral genome replication. Many antiviral drugs work by inhibiting the activity of viral DNA polymerase.

Analysis of the mode of action of chain-linked pyrazomycins showed that these compounds have no significant effect on polymerase activity even at high concentrations. Further analysis showed that these compounds can alter the permeability of the plasma membrane and promote the release of intracellular viral DNA during viral replication. This finding explains the induction of extracellular viral DNA following treatment with streptomycin.

Mechanistically, mature nucleocapsids are enveloped by a lipid bilayer and HB surface antigen and subsequently released as extracellular viral particles. The impact of chain-linked pyrazomycin-induced changes in membrane permeability on virion formation was assessed by harvesting and characterizing extracellular virions.

The results show that treatment with chain-linked pyrazomycin significantly increases the number of non-enveloped naked nucleocapsids in the extracellular medium. This observation suggests that chain-linked pyrazomycin promotes the release of immature virions from infected cells by altering membrane permeability.

Significance

This study describes the isolation and characterization of a novel anti-hepatitis B virus compound (catebopyrazomycin) from a rare strain chain wing sp. MM782L-181F7. Strepzopyrazomycin contains a thiazole-pyridine moiety similar to the antibiotic kanamycin.

Mechanistically, these novel compounds exhibit anti-HBV activity by increasing the membrane permeability of infected cells and inducing the release of immature viral particles from the cells.

Endosomal sorting complexes required for transport (ESCRT) machinery are required for the viral budding process. However, immature viral particles are released from infected cells via an ESCRT-independent pathway. In this study, the scientists were unable to assess the effect of nexatepyrazomycin on these pathways due to the compound’s low selectivity for cytotoxicity.

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