Manipulation of Leukocytes Using Therapeutic RNAi Delivered by Targeted and Stabilized Nanoparticles - Nanotechnology for the Delivery of Therapeutic Nucleic Acids - page 186

interfering with the adhesive interactions of leukocytes. Efalizumab
(Raptiva) and natalizumab (Tysabri) are examples of humanized,
integrin-blocking antibodies that have been approved by the FDA
for the treatment of autoimmune diseases. However, in April 2009,
efalizumab was withdrawn from markets worldwide because of an
increased risk of progressive multifocal leukoencephalopathy.

8.7 Leukocyte Integrins as Targets for

siRNA Delivery

The potential use of integrins as receptor targets for RNAi delivery to
leukocytes is supported by the following observations: (i) Two of the
family members,

β β

integrins, are expressed exclusively on
leucocytes, enabling the selective targeting of hematopoietic cells;
(ii) integrins are constitutively internalized and recycled to enable
the migration of leukocytes. Thus, integrin recycling supports the
internalization of bound antibodies and peptide, a prerequisite for
siRNA-mediated activation of the RNAi pathway; (iii) the most unique
feature of integrins is their ability to bind ligands in a dynamically
upregulated manner, via a conformational change from a low-affinity
conformation in resting cells to a high-affinity conformation upon
activation. Various leukocyte-associated diseases are characterized
by the high-affinity conformation form of integrins [20, 21]; this
observation is expected to increase delivery selectivity by leaving
naïve cells untouched. As proof of principle that leukocyte integrins
are excellent receptor targets for siRNA delivery to leukocytes, an
scFV against the high-affinity form of integrin lymphocyte function-
associated antigen 1 (LFA-1) was fused to protamine (Fig. 8.2H).
This fusion protein selectively delivered siRNAs into activated
lymphocytes, both

and

2 7

in vitro and in vivo .

8.8 The Construction and Characterization

of I-tsNP

I-tsNPs are made from neutral unilamellar nanoscale liposomes
with a defined size of approximately 80 nm in diameter (Fig. 8.3).
The neutral charge of the I-tsNPs eliminates toxic effects, such as
the induction of cytokines and liver damage, as caused by cationic
lipids and polymers [22]. The unilamellar liposomes are coated