Genome-Wide RNAi Screening to Identify Human Host Factors Crucial for In fl uenza Virus Replication - RNA Interference from Biology to Therapeutics - page 250

IAV viral nucleoprotein or polymerase protected against an otherwise lethal infec-
tion [ 40 ]. Robbins et al., however, were able to demonstrate that the effects observed
in both of these studies were most likely misinterpreted, owing to the use of a green
fluorescent protein (GFP) control siRNA identified as having unusually low immu-
nostimulatory activity compared with the active anti-influenza-formulated siRNAs
[ 41 ]. Remarkably, this GFP siRNA has served as a negative control for a number of
groups reporting therapeutic effects of siRNAs [ 42 ]. Thus, the existence of puta-
tively immunostimulatory motifs within functional siRNAs must be considered
when attempting to minimize immunotoxicity and to reduce the potential for off-
target gene effects.
An important consideration especially when intending to use siRNA targeting a
cellular host factor in a therapeutic application for influenza virus infection is that
knockdown kinetics of siRNA will always lag behind IAV replication kinetics and
spread in the body. This could interfere with treatment implementation since a
potential antiviral siRNA therapeutic would have to be administered within hours
after virus infection. Nonetheless, therapeutic siRNAs can be very well envisioned
for prophylactic treatment when attempting, e.g., to protect family members of an
infected person. Further studies will show if the benefits of such treatment outweigh
the conceivable risks.

12.3.2 Small Molecule Inhibitors

An alternative to siRNA to block the human target genes identified and pivotal for
IAV propagation is the use of small molecule inhibitors. Protein-protein interac-
tions are central to many biological processes in the human body and in the course
of viral infection. Therefore, inhibiting these interactions by small organic molecule
antagonists offers an attractive and feasible opportunity for therapeutic intervention.
However, development of small molecule inhibitors is a complex task due to factors
such as a) the general lack of small molecule starting points for drug design; b)
difficulty of small molecule compounds in competing against the large surface area
typically involved in a protein-protein interaction, which furthermore tends to be
fairly flat and devoid of small molecule binding pockets; c) difficulty of distinguish-
ing real binding from that present only as an artifact; and d) the size and quality and
makeup of small molecule libraries [ 43 ] .
The development of a suitable assay system is a prerequisite for screening com-
pound libraries for the identification of small molecule ligands able to bind and to
interfere with IAV essential targets and to thereby reduce virus pathology.
Despite these hurdles, administering small molecule antagonist targeting a
cellular protein required by the virus appears to hold great therapeutic promise,
ensuring of course that inhibition of the cellular protein is not detrimental to the
host. A striking example in support of the idea is the protection of humans from HIV
infection conferred from the homozygous deletion in the HIV co-receptor gene