2.4 Anti-HSV mechanisms and targets of natural products
The life cycle of HSV includes entry of the virus to the cell,
dissociation of its tegument, transport of incoming capsids to the
nuclear pore, and release of viral DNA into the nucleus, where
transcription occurs in a cascade-like fashion and DNA
replication ensues; this is followed by capsid assembly and
DNA packaging.106 The virus can also establish latency in the
nervous system, and, once stimulated, the virus repeats the
replication cycle and induces another infection.107
As shown in Table 10, natural products generally target the
viral attachment and entry stage. These types of compounds
account for 73.4% of the approximately 400 total reports
collected. The next most highly targeted stage is DNA
replication. There are many fewer reports of drugs or
compounds that inhibit virion assembly and egress. Targeting
cellular proteins or promoting the immune response is also a
potential mechanism of action for natural products, although
this is the least commonly reported. While many studies have
been conducted to elucidate the mechanism of action of antiHSV compounds, very few have confirmed the specific antiviral
target. The underlying mechanisms of many active natural
products therefore still need to be investigated.
Nevertheless, some studies have discovered some novel or
alternative potential mechanisms of active natural products. For
example, our previous work found that PGG, a hydrolysable
polyphenol isolated from Phyllanthus emblica, could downregulate cofilin1, a key regulator of actin cytoskeleton dynamics,
and inhibit HSV-1 induced rearrangements of actin cytoskeleton,
which is important for infectivity.80 Most of the identified
products target viral gene expression and viral glycoproteins
(Table 11).
In addition, the suitable animal model to evaluate the
mechanisms of action of these potent anti-HSV molecules are
considered as a very important and very actual factor, since the
biological activity in vitro does not always correlate with that in
vivo. Murine herpetic stromal keratitis or cutaneous infection
models are the most commonly used in in vivo studies. FK-3000
(27), at doses of 10 and 25 mg kg21, significantly delayed skin
lesion, and prolonged the mean survival time of BALB/c mice
cutaneously infected with HSV-1 strain 7401H.116 Shimizu et al.
evaluated the efficacies of ethanol extracts from Brazilian
propolis against HSV-1 infection by administrating orally to
cutaneously HSV-1 infected mice.117 Propolis significantly
enhanced delayed-type hypersensitivity (DTH) to inactivated
HSV-1 antigen or augmented interferon (IFN)-c production by
HSV-1 antigen from splenocytes. Eugenol (12) treatment was
found to alter the development of stromal keratitis at 1 mg
mL21, suggesting its potential application in ocular herpetic
infections.71 Petrera and Coto evaluated the therapeutic effect of
meliacine in a female BALB/c model of genital herpetic infection
by intravaginal administration, and found that virus shedding in
vagina fluids was diminished, and the amount of virus that
migrated to the brain and vaginal fluids were reduced.89 These