Biomedical Engineering Reference
In-Depth Information
FIGURE 25.1
Transport of large molecules through the blood-brain barrier (BBB). The BBB is
formed of endothelial cells that are connected through tight junctions (TJ). (A) Large molecules that
are the ligands (L) of specific receptors (R) at the luminal side of the BBB are transported via
receptor-mediated transcytosis (RMT) to the apical side. Typical ligands are insulin, transferring, or
fusion proteins. (B) Other large cationic molecules such as human serum albumin (HSA) can enter
the brain by adsorptive-mediated transcytosis. This principle relies on the negative charge of
endothelial cells at the luminal side. (C) In the molecular Trojan horse (MTH) approach, antibodies
that bind to receptors of the RMT pathway are utilized. Fusion of drug (D) molecules to the antibody
facilitates a passage into the brain. (D) Similarly, MTH can be performed with bispecific antibodies as
well. One arm docks to the receptor, the other arm binds a brain-specific target.
Only small lipid soluble molecules are able to pass by lipid-
mediated free diffusion. Other molecules depend on trans-
porter proteins or receptor mediated transcytosis (RMT). The
receptors for insulin (IR) and transferrin (TfR) are highly
expressed at the BBB, thus being interesting targets for
transporting molecules to the brain [9]. Current strategies
for BBB transport are summarized in Figure 25.1.
Initially, transferrin (Tf) fusion proteins were designed,
which should cross the BBB via the TfR. The first described
construct consisted of Tf fused to an antibody and success-
fully passed over the BBB. However, the concentration of
this fusion protein in the brain reached only
1
accumulation of Mf in liver and kidney, the level of Mf in
brain begins to rise to a maximum uptake at 6 h post
intravenous (i.v.) injection. Overall, the enrichment of Mf
in brain is better than many other carrier systems [13].
Although no fusion proteins utilizing Mf or Lf have been
described so far, both have a good potential as transporter.
Right now they served mainly as large molecule carrier to
shuttle conjugated small molecules across the BBB. This
approach might be superior to direct fusion of therapeutic
proteins or peptides since more drug molecules can simul-
taneously be transported by a single carrier.
Owing to the excellent uptake of the antibody against
TfR, a wide range of fusion partners was evaluated. Simul-
taneously, antibodies against the IR were generated. Com-
paring the BBB delivery of TfR- and IR-directed antibodies,
the IR antibody was ninefold better, and therefore selected
for all further therapeutic development [14]. Utilizing an
antibody as molecular Trojan horse (MTH) combines the
advantage of long half-life and stabilization of labile neuro-
trophic factors with the potential of dimerization. So far,
molecules from the class of neurotrophic factors, lysosomal
enzymes, and antibodies have been fused to the MTH
=
3
of an anti-TfR
antibody [10]. Later, Tf connected by helical linkers to nerve
growth factor (NGF) as payload was tested in cell culture,
but never in vivo [11].
Other members of the transferrin family are lactoferrin
(Lf) and melanoferrin (Mf). Both can cross the BBB as well
and have been used as carrier molecule. Despite the sixfold
shorter plasma half-life of Lf than Tf, the brain uptake was
much better for Lf when tested in rats [12]. On the other
hand, Mf has 10-15 times higher brain transport rates than
the other iron-binding proteins Tf and Lf. After an initial
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