Biomedical Engineering Reference
In-Depth Information
Protective
moiety
Antibody-enzyme construct
Prodrug
Antigen
Drug
DNA-cross-links
cell death
DNA-cross-links
cell death
Cancer cell
FIGURE 23.1
Antibody-directed enzyme prodrug therapy (ADEPT); the basic principle. First the
antibody-enzyme construct binds to the antigen. Then the prodrug is added. The enzyme converts the
prodrug into a cytotoxic drug by removing the protective moiety. The active drug enters the cell and
causes cell death.
to be the case as it was shown that antibody-enzyme
conjugates localized in human chorionic gonadotrophin
(hCG) producing tumors even in the presence of high
concentrations of hCG in blood [9].
Our studies have used carcino embryonic antigen (CEA)
as the target molecule. CEA is virtually universally
expressed by colorectal carcinomas and perhaps less regu-
larly by several other epithelial cancers. It also has the
advantage that its site of expression in normal intestinal
tissue is not accessible to intravenously administered anti-
CEA antibodies.
conjugated to enzyme in early ADEPT studies in prefer-
ence to intact antibodies that might have proved to be
better delivery agents. Antibody fragments tend to be
cleared from blood more rapidly than intact antibodies
but binding affinity and, maybe, other factors come into the
equation. However, prolonged retention of enzyme in
tumor remains an important objective whatever antibody
format is employed.
The argument that antibody fragments will penetrate
tumors more effectively does not carry much weight in
ADEPT partly because of the additional size of the enzyme
and also because tumor penetration is dependent on the
small drug molecules that are generated.
Most of the antibodies used in clinical studies reported
thus far have been murine in origin although this should not
remain a problem in the future.
23.2.2 The Antibody
The most important characteristics of the antibody compo-
nent are specificity and the ability to retain the enzyme at
tumor sites for prolonged periods of up to several days. This
would allow prodrug to be administered over longer periods.
Antibody or the antibody-enzyme conjugate will accumu-
late in tumors when the concentration in blood exceeds that
in the tumor. When the blood concentration falls below that
of tumor, there may be loss of conjugate from the tumor.
Thus, the binding affinity of the antibody is important for
the retention of antibody-enzyme conjugate in the tumor
such that a situation is attained where there is an effective
concentration of enzyme at tumor sites and an absence of
enzyme in blood.
Our early studies were influenced by experience with
radioimmunolocalization of cancers using radiolabeled anti-
bodies, in which it was found that F(ab
0
)
2
fragments were
preferred because these cleared from blood more rapidly
resulting in a better ratio of tumor to background than intact
IgG [10]. This resulted in our using F(ab
0
)
2
fragments
23.2.3 The Enzyme
The use of native human enzymes in ADEPT is excluded by
their normal tissue distribution. Nonhuman mammalian
enzymes are excluded by their analogous human forms
and by their inherent immunogenicity. This restricts the
choice to a limited repertoire of bacterial enzymes that
have the advantage of high efficiency and the disadvantage
of potent immunogenicity that could limit their repeated use.
The issue of immunogenicity is discussed later.
The possibility of using mutated human enzymes such
that the prodrug is activated only by the mutated enzyme and
not by the native form is a challenge that awaits completion.
The enzymes used in ADEPT systems have included:
Mammalian/human enzymes which included placental
alkaline phosphatise [4], human
b
-glucuronidase [11],
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