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Antivirals acting on viral envelopes via biophysical mechanisms of action
Most antivirals target viral proteins and are specific for only one virus, or viral type. Whereas viral proteins are encoded in the plastic viral genome, virion lipids are not and their rearrangements during fusion are conserved
among otherwise unrelated enveloped viruses. Antivirals that inhibit these lipid rearrangements could thus pose
a high barrier to resistance and have broad-spectrum activity.
Fusion occurs through a hemifusion stalk in which only the outer leaflets are fused and thus curved with a
smaller radius for the polar heads than for the hydrophobic tails (negative curvature). Outer leaflets enriched in
phospholipids with head groups of larger cross sections than their lipid tails (“inverted cone”) disfavor negative
curvature, inhibiting fusion. The rigid amphipathic fusion inhibitors (RAFIs) are synthetic compounds of inverted
cone molecular geometry. They inhibit infectivity of otherwise unrelated enveloped viruses. The leading
RAFI, aUY11, has an ethynyl-perylene hydrophobic and an uracil-arabinose polar moiety. aUY11 intercalates in
viral envelopes and inhibits virion-to-cell fusion of a broad spectrum of otherwise unrelated enveloped viruses.
Previous studies showed that amphipathicity, rigidity, and inverted cone molecular geometry were required. We
propose that the inverted cone molecular geometry of the RAFIs increases the energy barrier for the hemifusion
stalk, inhibiting fusion. Then, chemically distinct compounds with similar amphipathicity, rigidity, and inverted
cone shape would have similar antiviral potencies, regardless of specific chemical groups. Alternatively, the
perylene group exposed to visible light may induce viral lipid peroxidation. Then, the perylene group and absorbance
at visible spectrum would be required. We now evaluated twenty-five chemically distinct RAFIs. The
perylene moiety and absorption at visible spectrum were not required, but a minimum length of the hydrophobic
moiety was, 10.3 Å. The arabino moiety could be modified or replaced by other groups. Cytidine was not
tolerated. Bilayer intercalation was required but not sufficient. The vast majority of RAFIs had no overt cytotoxicity
(CC50 > 20 μM; TI > 250–1200). Carbonyl or butylamide substitutions for arabino, or cytidine replacement
for uracil, increased cytotoxicity. Cytotoxicity was mainly determined by the polar moiety and there
was no correlation between antiviral and cytostatic activities.
The definition of the effects of shape and chemical groups of the RAFIs opens the possibility to the rational
design of lipid-acting antivirals active against a broad spectrum of enveloped viruses.