A plant-derived compound and its analogues may help prevent and treat hepatitis B virus (HBV) infection and could contribute to the development of well-tolerated and broadly active inhibitors of HBV, including activity against treatment-resistant strains, a new Japanese study has found.

Unlike the hepatitis C virus (HCV) infection, against which direct-acting antivirals have radically improved treatment outcomes, the development of new antivirals against HBV, especially viral-targeting compounds, is limited by HBV requiring just four viral genes for its efficient replication and infection.

"The standard strategy for developing antivirals is to inhibit viral-enzymes, such as HIV reverse transcriptase, integrase, and protease, and HCV protease and polymerase, but HBV has only one viral protein conferring enzymatic activity, polymerase," study corresponding author Koichi Watashi, a senior researcher in the Department of Virology II at the National Institute of Infectious Diseases in Tokyo, told BioWorld Today.

As a result, he said, "currently, anti-HBV therapy is based mainly on nucleoside/tide analogues and interferons, which can effectively reduce the viral load in HBV-infected patients and lead to remission of hepatitis.

"However, these agents rarely achieve complete elimination of HBV. In addition, interferons can cause serious adverse effects, while nucleoside/tide analogues cannot completely prevent the emergence of drug-resistant viruses," said Watashi. "Therefore, we need new anti-HBV agents, especially those with a different mode of action for better treatment."

In their new study, the researchers showed that the plant-derived flavonoid proanthocyanidin (PAC) and its analogues, the oolonghomobisflavanes, acted directly on large HBV viral surface proteins (LHBs), inhibiting entry of HBV into host cells, they reported in the Jan. 17, 2017, edition of Hepatology.

Through cell-based chemical screening, they showed that PAC inhibited HBV infection with little cytotoxic effect and prevented attachment of the preS1 region in LHBs to its receptor, sodium taurocholate cotransporting polypeptide (NTCP).

"PAC represents a specific inhibitor against HBV that is a less toxic grape seed-derived agent [that is] available as a dietary supplement in some countries," Watashi said.

"Previous work in mice has shown that after the oral intake of PAC or its analogues, serum levels of 10 mcg/mL or more cause no apparent adverse effects, and this concentration reduced HBV infection in the present study," he said.

"We showed the inhibitory effect of PAC on HBV preS1-NTCP attachment in both cell culture and in vitro," said Watashi. "In cell culture, fluorescent-labeled HBV preS1 attaches to host cells expressing NTCP, but this was drastically inhibited by PAC. In in vitro assay, biotin-labeled HBV preS1 capture recombinant NTCP protein was also interrupted by PAC treatment." PAC was further shown to target HBV particles directly and impair their infectivity, whereas it did not affect NTCP function.

Importantly, PAC was shown to have pan-genotypic anti-HBV activity and was also effective against a clinically relevant nucleoside analogue-resistant HBV isolate. "There are 10 known HBV genotypes and we tested for the four most prevalent of these [A, B, C, D], infection by all of which was inhibited by PAC," said Watashi.

"We used an HBV isolate carrying mutations at amino acids 180, 202 and 204 of polymerase, which is a typical resistant clone to entecavir and lamivudine. PAC and its analogues also inhibited the infection of this mutant HBV."

The researchers further demonstrated that PAC augmented the ability of the nucleoside analogue tenofovir to interrupt HBV spread in primary human hepatocytes.

"Supplementation of tenofovir with PAC enhanced anti-HBV activity, almost completely blocking viral spread, suggesting that multidrug treatment with PAC or its analogues and nucleoside/tide analogues has a better antiviral outcome."

Taken together, those findings strongly suggest that PAC and its analogues represent a novel class of safe and broadly active inhibitors of HBV infection. "Given the success of antiviral treatments against HIV and HCV, multidrug treatment is clearly a powerful strategy to improve treatment outcomes," noted Watashi. However, "there is no consensus regarding whether co-treatment with a nucleoside/tide analogue and interferon provides an improved outcome."

Nevertheless, "we do need drugs with a different mode of action. PAC and its analogues, which target the HBV viral envelope protein, will potentially provide additional choices for multidrug HBV therapy in the future, improving the treatment of this infection."