Page 50 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

48 CHAPTER 3 Immune assay assisted cancer diagnostic

vivo include a cellular versus a subcellular location, antigenic stability, binding site
availability, degree of homology with other molecules, and degree of tumor antigen
expression throughout the tumor. Additional variables include type of tumor, size,
viability, position and vascularity of the tumor, route of management, and the proce-
dure and scanning instrument have used.
Furthermore, antibodies which have been injected into the bloodstream pass
through a number of compartment, including vascular and extravascular spaces
(organs, tissues, and body fluids), and there are many obstacles to the delivery of
an injected MoAb. It has demonstrated that only a small amount of the injected Ig
(approximately between 0.0007% and 0.01% of the injected dose per gram of tis-
sue) targets the tumor. For instance, the binding of MoAbs to circulating antigen
(secreted from the tumor) or to antigen present on normal tissue can deflect MoAbs
from the tumor. At the tumor site, poor vascular penetration and vascularization
can also decrease MoAb availability and prevent binding to tumors. At the cellular
level, antigen modulation, unavailability, and low density can all affect the amount of
MoAb bound to tumor. The site of tumor(s) in the body may affect the technique of
MoAb management. Intravenous injection is the most popular technique, but when
the malignancy is limited to a region or body cavity, intraregional or intracavitary
usage, such as intraperitoneal or intrathecal injection, has demonstrated effective for
imaging.
Effectiveness of both immune scintigraphy and immunotherapy depends on the
portion of antibody taken up in the tumor. This depends on penetrability of MoAbs in
the tumor cells. Tumors are characterized by heterogeneous and high interstitial fluid
pressure with high viscosity of tumor blood supply. MoAbs have to penetrate through
this pressure gradient which finally depends on the molecular size of MoAbs. Thus,
the large size of MoAbs, because of the presence of Fc region, is an inconvenience
in cases where tumor penetration is difficult due to heterogeneity. Whole antibod-
ies have a multinodular nature with each domain having a particular function, that
is, Fab region for antigen binding and Fc domain for effector functions. Interaction
of the neonatal Fc receptor (FcRn) and Fc domain of antibody aids to increase its
biological half-life. However, recently the concept of engineered antibody fragments
and single-domain antibody (sdAb) known as mini bodies has been developed by
through selection of suitable molecular domains of MoAbs in efforts to control in
vivo valence, affinity, avidity, and tissue [8]. Antibody fragments are often used in
cases such as radiolabeling [9] and the combination of imaging and immune diagnos-
tic and will be discussed later in this chapter.
The most important function of anticancer MoAbs has been for the measurement
of circulating TAAs, such as CEA, AFP, hCG, PSA, CA125, in body fluid immuno-
assays, and so forth.
For example, RP215 is a MoAb produced in mice immunized against OC-3-VGH
ovarian cancer extract and it was shown that it mainly reacts with protein bands of
55 kDa, when Western Blot assays were carried out either with cancer cell extract
or with affinity-purified CA215 [10]. RP215 is specific to carbohydrate-associated
epitope of CA215, consists mainly of cancerous antigen receptors. So RP215 can be