Targeting the DEAD-Box RNA Helicase eIF4A with Rocaglates-A Pan-Antiviral Strategy for Minimizing the Impact of Future RNA Virus Pandemics

The increase in pandemics caused by RNA viruses of zoonotic origin highlights the urgent need for broad-spectrum antivirals against novel and re-emerging RNA viruses. Broad-spectrum antivirals could be deployed as first-line interventions during an outbreak while virus-specific drugs and vaccines are developed and rolled out. Viruses depend on the host’s protein synthesis machinery for replication. Several natural compounds that target the cellular DEAD-box RNA helicase eIF4A, a key component of the eukaryotic translation initiation complex eIF4F, have emerged as potential broad-spectrum antivirals. Rocaglates, a group of flavaglines of plant origin that clamp mRNAs with highly structured 5′ untranslated regions (5′UTRs) onto the surface of eIF4A through specific stacking interactions, exhibit the largest selectivity and potential therapeutic indices among all known eIF4A inhibitors. Their unique mechanism of action limits the inhibitory effect of rocaglates to the translation of eIF4A-dependent viral mRNAs and a minor fraction of host mRNAs exhibiting stable RNA secondary structures and/or polypurine sequence stretches in their 5′UTRs, resulting in minimal potential toxic side effects. Maintaining a favorable safety profile while inducing efficient inhibition of a broad spectrum of RNA viruses makes rocaglates into primary candidates for further development as pan-antiviral therapeutics.

Comparison of broad-spectrum antiviral activities of the synthetic rocaglate CR-31-B (-) and the eIF4A-inhibitor Silvestrol

Rocaglates, a class of natural compounds isolated from plants of the genus Aglaia, are potent inhibitors of translation initiation. They are proposed to form stacking interactions with polypurine sequences in the 5′-untranslated region (UTR) of selected mRNAs, thereby clamping the RNA substrate onto eIF4A and causing inhibition of the translation initiation complex. Since virus replication relies on the host translation machinery, it is not surprising that the rocaglate Silvestrol has broad-spectrum antiviral activity. Unfortunately, synthesis of Silvestrol is sophisticated and time-consuming, thus hampering the prospects for further antiviral drug development. Here, we present the less complex structured synthetic rocaglate CR-31-B (−) as a novel compound with potent broad-spectrum antiviral activity in primary cells and in an ex vivo bronchial epithelial cell system. CR-31-B (−) inhibited the replication of corona-, Zika-, Lassa-, Crimean Congo hemorrhagic fever viruses and, to a lesser extent, hepatitis E virus (HEV) at non-cytotoxic low nanomolar concentrations. Since HEV has a polypurine-free 5′-UTR that folds into a stable hairpin structure, we hypothesized that RNA clamping by Silvestrol and its derivatives may also occur in a polypurine-independent but structure-dependent manner. Interestingly, the HEV 5′-UTR conferred sensitivity towards Silvestrol but not to CR-31-B (−). However, if an exposed polypurine stretch was introduced into the HEV 5′-UTR, CR-31-B (−) became an active inhibitor comparable to Silvestrol. Moreover, thermodynamic destabilization of the HEV 5′-UTR led to reduced translational inhibition by Silvestrol, suggesting differences between rocaglates in their mode of action, most probably by engaging Silvestrol's additional dioxane moiety.

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The synthetic rocaglate CR-31-B (−) has broad-spectrum antiviral activity comparable to that of Silvestrol.

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Both compounds show remarkably low cytotoxicity in primary cells.

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Silvestrol and CR-31-B (−) are highly efficient against HCoV-229E in a primary human bronchial epithelial cell system.

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Both compounds reduce LASV and CCHFV titers by about 3–4 logs in primary murine hepatocytes.

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Only Silvestrol with its characteristic dioxane moiety can clamp polypurine-free structured RNAs onto the eIF4A helicase.