RNAi as New Class of Nanomedicines - Harnessing Biomaterials in Nanomedicine: Preparation, Toxicity, and Applications - page 170

6.2.1 Antibody-Mediated siRNA Delivery

Antibodies are proteins expressed by B cells that recognize and bind
specific antigens with high affinity and selectivity. Under normal
circumstances, the immune system produces antibodies to defend
against foreign pathogens, but antibodies can also be produced
against non-pathogen-associated molecules when a specific protein
or peptide is introduced into a foreign species. The use of antibodies
for therapeutic purposes is limited by their large and complex
molecular structure, the cost of production and purification, and the
potential for binding to complement or Fc receptors inducing cell
lysis, opsonization and degranulation of mast cells, eosinophils and
basophils [1, 149, 154]. To circumvent some of these limitations,
antibody fragments have been used that retain the antigen recognition
domain, but have lost the heavy chain Fc domain. This reduces the
overall size of the molecule and reduces many of the negative side
eff ects of antibody treatments. To facilitate siRNA delivery, the
antigen recognition domain of an antibody can be fused to a cationic
polymer to which siRNAs bind. Using a fragment antibody (FAb) that
specifically recognizes the HIV-1 envelope glycoprotein (gp160)
fused to protamine, Song and colleagues were able to deliver anti-
HIV siRNAs exclusively to hard-to-transfect, HIV-1 infected CD4+ T
lymphocytes resulting in the suppression of the viral infection [137].
In addition, the growth of subcutaneously implanted tumors that
transgenically expressed the HIV-1 envelope, but not of envelope
negative control tumors, was inhibited following the intratumoral
or intravenous injection of a cocktail of siRNAs (c-MYC, VEGF, and
MDM2 siRNAs), bound to the antibody-protamine fusion protein.
In a similar manner, a single-chain antibody specific for the pan T
cell marker CD7 fused to a polyarginine tract (scFvCD7-9R) was able
to deliver anti-HIV siRNAs to T lymphocytes in a humanized mouse
model of HIV-1 [91]. This treatment resulted in the suppression of
viral replication and prevention of the disease-associated loss of CD4+
T cells. Since antibodies can be generated to very specific epitopes, it
is possible to target distinct isoforms or conformations of a protein.
Peer and colleagues demonstrated this exquisite specificity using an
antibody that recognized a conformationally distinct, activated form
of the lymphocyte function-associated antigen-1 (LFA-1) [123]. This
reagent facilitated siRNA delivery to cells that bore the activated,
but not the latent form of the receptor. Although these studies