Cancer Immune Therapy Edited by G. Stuhler and P. Walden ...

350 17 Immunotoxins and Recombinant Immunotoxins in Cancer Therapy
17.2
First-and Second-Generation Immunotoxins
First-generation immunotoxins were made in the early 1970s and were composed of
cancer-specific monoclonal antibodies (mAbs) to which native bacterial or plant toxins
were chemically conjugated. The understanding of toxin structure±function
properties, and the advancement in recombinant DNA technology and antibody engineering
lead to important breakthroughs in the late 1980s to construct second-generation
recombinant immunotoxins that are composed of recombinant antibody
fragments derived from cancer-specific antibodies or phage-display libraries and
truncated forms of toxins. These molecules are produced in large amounts, needed
for preclinical and clinical studies, in bacteria and feature better clinical properties.
As shown in Fig. 17.1, first- and second-generation immunotoxins contain toxins
that have their cell-binding domains either mutated or deleted to prevent them from
binding to normal cells, and are either chemically conjugated or fused to a ligand or
an antibody specific for cancer cells.
First-generation immunotoxins, composed of whole antibodies chemically conjugated
to toxins, demonstrated the feasibility of this concept. Cancer cells cultured in
vitro could be killed under conditions in which the immunotoxin demonstrated low
toxicity towards cultured normal cells. Clinical trials with these agents had some success;
however, they also revealed several problems, such as non-specific toxicity towards
some normal cells, difficulties in production and, particularly for the treatment
of solid tumors, poor tumor penetration due to their large size.
Second-generation immunotoxins have overcome many of these problems. Progress
in the elucidation of the toxin's structure and function combined with the techniques
of protein engineering facilitated the design and construction of recombinant molecules
with a higher specificity for cancer cells and reduced toxicity to normal cells. At
the same time, advances in recombinant DNA technology and antibody engineering
enabled the generation of small antibody fragments. Thus, it was possible to decrease
the size of immunotoxins significantly and to improve their tumor-penetration potential
in vivo. The development of advanced methods of recombinant protein production
enabled the large-scale production of recombinant immunotoxins of high purity and
quality for clinical use in sufficient quantities to perform clinical trials.
Another strategy to target cancer cells is to construct chimeric toxins in which the
engineered truncated portion of the toxin [Pseudomonas exotoxin (PE) or diphtheria
toxin (DT)]gene is fused to cDNA encoding growth factors or cytokines. These include
transforming growth factor (TGF)-b [18], insulin-like growth factor (IGF)-1
[19], acidic and basic fibroblast growth factor (FGF) [20], IL-2 [21], IL-4 [22], and IL-6
[23]. These recombinant toxins (oncotoxins) are designed to target specific tumor
cells that over-express these receptors (Fig. 17.1).
In the following sections we will summarize the rationale and current knowledge on
the design and application of second-generation recombinant Fv±immunotoxins,
which utilize recombinant antibody fragments as the targeting moiety. Recent results
of clinical trials are summarized. We will also discuss the powerful new technologies
for selecting new antibodies with unique specificities and improved properties.