Genome-Wide RNAi Screening to Identify Human Host Factors Crucial for In fl uenza Virus Replication - RNA Interference from Biology to Therapeutics - page 249

12.3 Applications

The human genes ostensibly involved in the influenza A infection and replication
process as identified in the screens discussed constitute potential future targets for
antiviral strategies. Therapeutically targeting a host factor pivotal for IAV infection
is anticipated to minimize the likelihood of developing drug-resistant variants and,
thus, also to protect from new emerging influenza viruses. There are two main strat-
egies of how to apply this information with respect to drug development which will
be discussed in more detail below: The first strategy focuses on RNAi-based post-
transcriptional prevention of expression of the respective human genes. The second
approach is based upon posttranslational binding and inactivation of the correspond-
ing human gene products (proteins) by small molecule inhibitors or new chemical
entities. By targeting cellular proteins required by the virus, both approaches reduce
the chances of drug-resistances arising and also enhance the possibility of identify-
ing a broad-spectrum antiviral blocking certain cellular pathways shared by differ-
ent viruses.

12.3.1 Therapeutic Applications of siRNA Directed

Against In fl uenza

Since siRNAs are regulators of gene expression in cells, and are specific, efficient,
and considered nontoxic, silencing of host genes essential for viral replication not
only holds great therapeutic promise but also avoids the problem of resistance
development often encountered when employing the traditional approach of inhibit-
ing expression of viral genes.
Although great progress in the drug-like use of siRNAs has been made in recent
years, the success of siRNAs as therapeutics to block and subsequently degrade
mRNA of a specific host gene still depends to a great extent on the development of
ef fi cient in vivo delivery methods. siRNAs need to cross the cell membranes and to
enter cells at the site of influenza infection to then be incorporated into the silencing
complex. In addition to delivery, specificity and stability are two major obstacles in
the development of therapeutic siRNA. The two most important caveats encountered
in the siRNA selection process, with respect to specificity, are off-target effects caused
by unsolicited gene homologies and immune stimulation due to recognition of
specific, unmodified siRNA by the innate immune system [ 35 ] . Off-target effects
induced by mismatch tolerance are influenced mainly by the thermodynamic stability
of the first few base pairs of both ends of an siRNA duplex, crucial for the determina-
tion of which siRNA strand will be incorporated into the RISC. A common method
utilized to improve stability and to avoid off-target effects is the inclusion of various
chemical modifications of the ribose at the 2¢ position of the sense strand [ 36- 38 ] .
It was demonstrated in two preclinical investigations targeting viral components
inhibiting IAV replication that antiviral siRNAs provided both prophylactic and
therapeutic benefits during influenza infection in mice [ 39 ] . Also, siRNAs targeting