Biomedical Engineering Reference
In-Depth Information
subsequent to spinal cord injury [36] Recently, a CD4-
VH3-IgG fusion protein has been described that partially
neutralizes HIV by inhibiting entry of the virus into cells
[37], while a TWEAK cytokine scavenging IgG fusion
protein has shown promise in the treatment of athero-
sclerosis in mice [38].
Unlike the IgG fusions outlined above, SHG2210 was
engineered to use the TF-TFR. As such, SHG2210 joins a
long list of therapeutic candidates and drugs that utilize this
ubiquitous ligand/receptor pathway (reviewed in Reference
[39]). However, unlike SHG2210, which seeks to use the
TFR as a clearance mechanism, most drugs that utilize TF or
target the TFR are doing so in an effort to target the
therapeutic to a particular cell type or to gain access to
difficult drug niches within the body. The most common
TFR targeting fusion protein drugs are those used to treat
cancer. Examples include TF-doxorubicin conjugates for the
treatment of drug-resistant cells [40-42] and transferrin
diphtheria toxin subunit fusions for the treatment of solid
human gliomas [43]. The TF-TFR axis is also an attractive
candidate for the delivery of therapeutics across the blood
brain barrier; a host of conjugates based on the anti-TFR
monoclonal antibody OX26 have shown promise in vitro and
in animal models as well [44,45]. Finally, the development
of modular drug delivery scaffolds built upon lipoplex and
liposome technology have been adapted for TF-TFR specific
targeting in vitro and in vivo [46-48].
FIGURE 12.6
Prevention of AChR antigenic modulation by
SHG2210 fusion protein. Data depict the mean fluorescence inten-
sity (MFI) of AChR staining on DB40 cells by flow cytometry
following overnight treatment of cells with myasthenic serum from
an MG patient. Data are the mean of triplicate samples,
SD.
those with predominantly anti-
a
-subunit responses. The
general concept of a neutralizing fusion protein for the
binding and sequestration of anti-AChR autoantibodies
has been approached with various fusion proteins. Barchan
et al. demonstrated that GST fusions of full length or partial
a
-subunit had a protective effect on antigenic modulation of
the AChR both in vitro and development of EAMG in vivo
[14,30]. Recombinant AChR
a
-subunit fragments have been
fused to toxins to produce “smart bombs,” which specifically
target and eliminate autoreactive B cells expressing anti-
AChR antibodies on their cell surface [31]. Finally, a fusion
composed of the extracellular domain of the human
a
1-
subunit and the Fc domain of the human IgG1 heavy chain
was shown to immunomodulate AChR-specific autoreactive
B cells [32].
Apart from myasthenia gravis, fusion proteins that both
neutralize an unwanted entity and have an intrinsic uptake or
clearance pathway are attractive drug candidates for many
indications. For example, an LDL receptor-transferrin
fusion protein was shown to reduce levels of plasma LDL
cholesterol and has been proposed as a potential therapeutic
for the treatment of familial hypercholesterolemia [33].
There is also a wealth of literature describing the use of
IgG fusion proteins to scavenge or modulate soluble media-
tors of inflammation and prevent unwanted interactions
between cells and potential pathogens. Several examples
are described below and are beyond the scope of this chapter.
Tumor necrosis factor receptor/IgG fusion proteins, which
neutralize and clear the proinflammatory cytokine TNF-
a
are well described in the literature and have been commer-
cialized for the treatment of rheumatoid arthritis [34,35],
while TrkA-IgG fusions that bind and neutralize intraspinal
NGF have been used to help attenuate dysreflexia
12.5 FUTURE PERSPECTIVES
In vitro studies show SHG2210 is able to bind both TF and
a
-AChR antibodies. However, it is likely that SHG2210
complexed with
a
-AChR is sterically hindered relative to TF
and thus, cellular trafficking is distinct from TF. Although
SHG2210 is able to prevent antigenic modulation, its effi-
cacy is limited as not all patient sera responded favorably.
The engineering of a novel AChR-TF fusion protein may be
needed to improve the efficacy and is faced with several
challenges. For example, efficacy may be improved by
generating a molecule with multiple AChR subunits, fused
to transferrin to enable binding to a broader class of anti-
bodies. Psaridi-Linardaki et al. demonstrated that the extrac-
ellular domain of the
a
-subunit, immobilized on sepharose
columns, could remove myasthenic antibodies from the sera
of MG patients [49]. In a follow-up to this study, this same
group demonstrated that a combinatorial approach, in which
a
-subunit was combined with
b
,
g
, and
subunits on the
same column, greatly enhanced the efficiency of clearance
[11] However, by including the entire extracellular domain
of each AChR subunit, it is likely that the resultant fusion
protein is simply too large to engage the transferrin receptor.
In turn, this would increase the risk of aberrant TFR-
mediated uptake and trafficking.
e
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