Biomedical Engineering Reference
In-Depth Information
have employed to specifically eliminate malignant cells
while leaving normal cells unaffected [8].
17.2 TOXINS
A general mechanism of microbial or plant toxins is their
ability to escape lysosomal degradation. Besides the many
variables such as antigen expression, trafficking kinetics,
extracellular immunotoxin concentration, and exposure
time, the real rate-limiting step determining potency seems
to be the endosomal escape [9]. Most of the naturally
occurring toxins contain a binding domain that enables
attachment to cell surface molecules. This binding domain
can easily be replaced by another domain that binds specifi-
cally to markers on malignant cells. Nowadays, a multitude
of these extracellular markers are known.
On the basis of their mechanism of action, toxins can be
classified into three categories: (a) disruption of membrane
integrity, (b) interruption of electrical signaling, and
(c) interference with enzymatic processes. Interestingly, so
far only members of the last group (c) have been used for
therapeutic approaches in the form of fusion proteins. Since
the early 1970s with chemically conjugated complex toxins,
much progress has been achieved in the last 40 years.With the
rise of genetic engineering, toxin variants could be generated,
which solved many problems of first generation targeted
toxins, such as heterogeneity and low production yields,
poor tumor penetration due to size limitations, and unspecific
toxicity. In general, immunotoxins have been primarily used
to treat hematological cancers because of better accessibility
of the malignant cells and the immunocompromised status of
the patients [10]. Table 17.1 summarizes the receptors and
activities of natural toxins. An overview on the current status
of immunotoxins is presented in Chapter 18.
FIGURE 17.1
Toxic activities of fusion proteins. Fusion pro-
teins that do not require internalization frequently involve the
activity of immune cells. They either attract them by cytokines or
engage them through Fc receptors. In the case of bifunctional
antibodies, instead of an Fc-receptor leucocyte specific surface
molecules such as CD3 are addressed (see part IIIb of this topic).
Fc receptors can cause antibody-derived cellular cytotoxicity
(ADCC), antibody-dependent cell-mediated phagocytosis
(ADCP), complement-dependent toxcitiy (CDC), and comple-
ment-dependent cell-mediated cytotoxicity (CDCC). Another
case involving immune cells is tumor-targeted superantigens
(TTSA). Special applications such as ADEPT (antibody-directed
enzyme prodrug therapy) generate extracellularly toxic mole-
cules that can enter the cell. Death receptor (DR) ligands trigger
apoptosis. The two other classes of toxic fusion proteins exert
their activities only within the cell. They contain either toxins
from bacteria or plants, or human enzymes that destroy cellular
functions.
17.2.1 Microbial Toxins
progress has been achieved in engaging cytotoxic T-cells
with bifunctional antibodies. Part IIIb of this topic is dedi-
cated to elucidate the concept of multifunctional antibodies.
Ideally, the stimulation of the immune system is restricted
to the tumor site to avoid any unwanted autoimmune effects
when distributed systemically. However, on the other hand,
immunotherapy is most efficient if a long lasting memory
effect can be obtained [5].
Owing to the immunogenic limitations of natural toxins,
immunotoxins based on human proteins have been estab-
lished. This group consists of enzymes that interfere with
cellular signaling and protein translation [6], cause proteo-
lytic degradation, or trigger apoptosis [7].
The following paragraphs introduce the different concepts
of targeted cell killing therapy based on fusion proteins. They
describe the different designs and improvements researchers
Bacterial toxins typically consist of several domains and are
able to separate the binding domain form the catalytic domain.
This is their key advantage when compared with plant toxins.
Most research has been done with diphtheria toxin (DT) and
Pseudomonas exotoxin A (PE) [11]. Their mode of action is
illustrated in Figure 17.2. Bacterial toxins are usually highly
immunogenic; therefore, mutagenesis or truncation was per-
formed to optimize the protein sequence [12].
(DT) is the prototype for ADP-ribosylating toxins. It is
secreted by the Gram-positive, aerobic Corynebacterium
diphtheria. The full-length toxin comprises three domains
and stretches over 535 amino acid residues. The toxic
activity is contained in the N-terminal fragment (A). The
C-terminal domain is divided into two parts, the trans-
location domain (T) and the receptor-binding domain
(B) at the carboxyl end. Upon binding to its native receptor
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