IMMUNOKINASES - Fusion Protein Technologies for Biopharmaceuticals: Applications and Challenges - page 319

21

IMMUNOKINASES

S TEFAN B ARTH , 1,2 S TEFAN G ATTENLOHNER , 3 AND M EHMET K EMAL T UR 3
1 Department of Experimental Medicine and Immunotherapy, Institute of Applied Medical Engineering,
Helmholtz-Institute for Biomedical Engineering, University Hospital RWTH Aachen, Aachen, Germany
2 Department of Pharmaceutical Product Development, Fraunhofer Institute for Molecular Biology and
Applied Ecology, Aachen, Germany
3 Institute of Pathology, University Hospital Giessen and Marburg GmbH (UKGM), Giessen, Germany

21.1 Introduction
21.2 Protein kinases, apoptosis, and cancer
21.3 Therapeutic strategies to restore missing kinase expression
21.4 Analysis of immunokinase efficacy
21.5 Outlook
References

reconstitution of down-regulated tumor suppressor functions
(immunokinases) [6,7].
Many forms of cancer are caused by the inactivation of
tumor suppressor genes, whose normal function is to impose
limits on the extent of cell division and proliferation (often
by inducing apoptosis) [8], where the missing function
provided by the tumor suppressor gene is a pro-apoptotic
protein kinase, it should be possible to restore normal
apoptotic function to tumor cells by importing the appropri-
ate kinase activity, leading to cell death [9]. Such is the
reasoning behind the development of immunokinases, that
is, targeted fusion proteins comprising a tumor-specific
targeting moiety and an apoptosis-inducing protein kinase
[6]. The targeting moiety may be an antibody or antibody
fragment (hence immunokinase) but the same term is used
when the targeting moiety is a ligand specific for a receptor
preferentially displayed on the surface of tumor cells.
Targeting is necessary because the unregulated delivery of
a pro-apoptotic kinase would result in indiscriminate cell
death; also, the target must be an internalizing receptor to
ensure the kinase is delivered into the cytoplasm, allowing it
to interact with the apoptotic machinery. Downregulation of
pro-apoptotic DAP kinase family members such as DAPK1
and DAPK2 have been described for different tumors in the
process of oncogenic transformation [10,11]. Therefore, to
increase specificity of targeted drugs, we developed a new
dual-target concept based on the targeted restoration of a
missing or downregulated tumor suppressor protein in tumor
surface antigen-positive cells [6,7]. In this chapter, we

21.1 INTRODUCTION

Specific pathophysiological properties of tumor cells, par-
ticularly the aberrant expression of surface and intracellular
markers, make it possible to explore rational strategies for
immunological targeting of cancer cells without harming
the surrounding healthy tissue. Therefore, successful
immunotherapy depends on the identification of reliable
tumor-associated molecular targets. Therapeutic fusion pro-
teins can be rationally engineered to take advantage of these
significant changes. They consist of a major component that
binds specifically to a disease-specific cell-surface target
molecule and another that confers selective cytotoxicity in
terms of induction of apoptosis by substantial inhibition of
protein biosynthesis (immunotoxins) [1], degradation of
RNA molecules (immunoRNases) [2,3], proteolytic cleav-
age of multiple apoptosis-related cytoplasmic protein sub-
strates (Granzyme B fusion proteins) [4,5], or functional