Discovery of Pentacyclic Triterpenoids as Potential Entry Inhibitors of Influenza Viruses

Abstract

Entry inhibitors are of particular importance in current efforts to develop a new generation of anti-influenza virus drugs. Here we report certain pentacyclic triterpenes exhibiting conserved structure features and with in vitro anti-influenza virus activity comparable to and even higher than that of oseltamivir. Mechanistic studies indicated that these lead triterpenoids bind tightly to the viral envelope hemagglutinin (HA), disrupting the interaction of HA with the sialic acid receptor and thus the attachment of viruses to host cells. Docking studies suggest that the binding pocket within HA for sialic acid receptor potentially acts as a targeting domain, and this is supported by structure-activity data, sialic acid competition studies, and broad anti-influenza spectrum as well as less induction of drug resistance. Our study might establish the importance of triterpenoids for development of entry inhibitors of influenza viruses.

Triterpenoids Manipulate a Broad Range of Virus-Host Fusion via Wrapping the HR2 Domain Prevalent in Viral Envelopes

“…The interactions of residues of the HR2 core with Y11, elucidated by the NMR data, were clearly reflected by the docking calculations 11,22,28,29 showing that most hydrophobic residues were shielded by the triterpenoid hydrophobic scaffold ( Fig. 3d and Supplementary Fig.…”

“…This result is markedly different from that of E-64d, which inhibits EBOV entry into cells by targeting the host cysteine protease. Furthermore, the subsequent addition of Y11 to the cultured virus/host complexes at a low temperature, allowing virion attachment but not membrane fusion due to the energy requirements 11 , had a greater inhibitory effect than the other treatments ( Supplementary Fig. 2b).…”

“…To alleviate safety concerns, Ebola pseudoparticles (EBOVpp), i.e., Ebola glycoprotein (GP) assembled onto retroviral core particles, were used to screen a triterpenoid library consisting of hundreds of chemicals. Vesicular stomatitis virus (VSV) GP and WSN influenza envelope hemagglutinin (HA)/neuraminidase (NA) protein pseudoparticles were tested in parallel to exclude toxic and non-specific effects 11,13 . We identified a panel of triterpenoid compounds that inhibited the EBOVpp to different extents, which were strongly related to their structures, but did not inhibit VSV and WSN (Fig.…”

“…As the viral envelope was the only difference between EBOVpp and the VSV and WSN pseudoviruses 19 , we determined whether EBOV GP was the potential target of the triterpenoid lead Y11 by a series of time-of-addition experiments 11,13,19 , including pretreatment of the viruses, pretreatment of the cells, and pre-attachment, postattachment and post-entry assays conducted in parallel, as previously reported (Supplementary Fig. 2a).…”

“…Very recently, analogues/derivatives of OA and echinocystic acid (EA), an OA-type triterpene, were found to exert a variety of inhibitory activities, strongly related to their structures, against influenza virus and HCV infection [11][12][13] . Further investigations suggested that these triterpenoid leads might bind to viral fusion proteins, including hemagglutinin (HA2) of influenza, E2 of HCV and GP41 of HIV, thereby disrupting viral entry into the host cells 11,13,14 . Whether the observed antiviral aspects of triterpenoids extend to other viruses and translate into a shared mechanism that can chemically antagonize the fusion of viruses with cellular membranes remains a topic of ongoing investigation.…”