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

17.8.1
Immune Responses and Dose-limiting Toxicity
17.8 Improving the Therapeutic Window of Recombinant Immunotoxins
As with any cytotoxic agent, side effects such as non-specific toxicity and immunogenicity
can occur when multiple injections of immunotoxins are given. One class of
side effects is due to inappropriate targeting of the immunotoxin to normal cells because
of the poor specificity of the antibody. In addition, the toxin or the Fv portion
of the antibody can bind non-specifically to various tissues. For example, in mice,
which usually do not contain target antigens, liver damage occurs when large
amounts of immunotoxins are given [179]. Molecular modeling combined with sitedirected
mutagenesis may help in the design of new versions of the toxin with decreased
toxicity caused by non-specific binding.
The development of neutralizing antibodies usually occurs after 10 days and limits
the therapeutic application of immunotoxins to this 10-day period [4]. Recent data
from clinical trials indicate that patients with solid tumors develop antibodies much
more readily then those with hematologic tumors. It is speculated that some hematologic
tumors may be associated with less immunogenicity than others. For example,
none of 14 patients with chronic lymphocytic leukemia treated with LMB-2 or
BL22 have shown any evidence of antibodies [4, 122, 128].
Several approaches have been taken to reduce immunogenicity. One is to make
small molecules, which appear to be less immunogenic; another is to use immunosuppressive
agents such as deoxyspergualin or CTLA-4±Ig [180, 181], an inhibitor of
the co-stimulation pathways required for T cell help and activation through the
CD28/CTLA-4±CD80/CD86 complex. Another approach is to use the anti-CD20
mAb, Rituximab, which induces B cell depletion in the majority of patients and is itself
non-immunogenic [182].
The dose-limiting toxicity of many immunotoxins is vascular leak syndrome (VLS).
Recent studies indicate that recombinant toxins, including those containing mutated
forms of PE, produce VLS in rats and that inflammation, which can be suppressed
by steroids or non-steroidal anti-inflammatory agents, mediates the VLS. VLS can
also be mediated indirectly by the activation of endothelial cells and/or macrophages
via cytokines such as tumor necrosis factor-a and interferon-g. The activated cells
produce NO which then can mediate oxidative damage to the endothelial cells and
result in increased permeability [183].
Some studies demonstrate direct endothelial cell damage caused by binding the
toxin to the cells. The direct damage to the cell is mediated by the enzymatic activity
of the toxin [181, 184, 185], while others show an indirect damage that is
mediated by binding of the targeting moiety. For example, experiments with human
umbilical vein endothelial cells exposed to LMB-1 (antibody conjugate with
truncated PE38) indicated that the mAb B3 rather than PE38 was binding to the
LeY antigen on endothelial cells [184]. Recent experiments using an in vivo model
composed of human neonatal foreskin xenografts in SCID immunodeficient mice
identified a 3-amino-acid motif present in protein toxins and in IL-2 that causes
VLS without other toxin activity [186±188]. Thus,VLS can be blocked in future trials
with anti-inflammatory agents to block cytokine action, or by mutations or peptide
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