|NCI Researchers Develop Modified Immunotoxin
for Cancer Therapy in Mouse Study
Researchers at the National Cancer Institute (NCI), part of
the National Institutes of Health (NIH), have shown in mice that
just a few modifications to a shortened version of a bacterial
toxin, called PE38, may be able to improve the efficacy of an
immunotoxin that contains PE38 as a new therapy for cancer. As
part of this new study, scientists have modified the PE38 toxin
in an effort to broaden the range of the types of tumors that
can be treated successfully. Immunotoxins, which are antibodies
linked to toxic proteins, can selectively bind to cancer cells
and kill them and immunotoxins containing PE38 have already been
used as effective treatments for certain types of leukemias and
lymphomas. The results appear in the December 15, 2006, issue
of the Journal of Immunology*.
"Immunotoxin therapy is one of a growing number of treatments
in which scientists engineer molecules to target cancer cells and
leave healthy cells unharmed," said NCI Director John Niederhuber,
M.D. "We have already witnessed the success of immunotoxin therapy
to treat hairy cell leukemia. Now, with a roadblock to further
applications apparently removed, we plan to apply this therapy
to other cancers."
By using recombinant DNA techniques, scientists in the Laboratory
of Molecular Biology at NCI's Center for Cancer Research (CCR)
combined the PE38 bacterial toxin with portions of several different
mouse antibodies. The PE38 portion was derived from a toxin secreted
by a type of bacteria that kills human cells by blocking their
ability to make proteins. The antibody fragments were chosen for
their ability to find and bind specifically to tumor cells, thereby
selectively delivering the toxin to cancer cells.
In a previous clinical trial that tested a different immunotoxin
containing PE38, over half of the participants with drug-resistant
hairy cell leukemia achieved complete remission after receiving
at least three doses of immunotoxin. However, a related immunotoxin
that also targeted solid tumors did not have similar success. Researchers
discovered that in patients with solid tumors, normal immune cells
quickly recognized the PE38 portion of the immunotoxin as foreign
and produced antibodies against it, thereby neutralizing the therapy.
In contrast, patients with leukemias, which are cancers affecting
certain white blood cells of the immune system, have a diminished
capacity to mount an immune response. As a result, the PE38-containing
immunotoxin has enough time to target and kill cancerous cells
before being attacked by the leukemia patient's immune system.
To overcome this neutralizing attack on immunotoxins when treating
patients with solid tumors, the NCI researchers, led by Ira Pastan,
M.D., identified the sites on the PE38 toxin that stimulate an
antibody response. To do this, the scientists immunized mice with
PE38 and produced 60 different types of antibodies that reacted
with the immunotoxin. Collectively, these 60 antibodies reacted
with only seven regions on PE38. Furthermore, these same seven
regions were recognized by the immune systems of patients who made
antibodies to PE38 after receiving immunotoxin treatment.
The mouse antibodies were then used to pick out exactly which
amino acids within the seven responsive regions were to blame for
the undesirable immune response. Using this information, the researchers
were able to create seven mutant immunotoxins that no longer reacted
with these antibodies, yet were still able to kill cancer cells.
Studies are now underway to incorporate these seven mutations in
a single immunotoxin molecule that should be less reactive with
the mouse's immune system.
"Figuring out which mutations abolish antibody binding might also
reveal common structural features of proteins that can elicit an
immune response," said Raffit Hassan, M.D., head of the Solid Tumor
Immunotherapy section and one of the study's authors. Hassan and
his colleagues hope that these simple changes in PE38 will reduce
a patient's chances for experiencing an immune response to PE38,
allowing the immunotoxin to be effective against solid tumors.
"We are currently testing to see whether these modifications to
the PE38 toxin can reduce the immune response in mice. If we can
diminish the immune response to this therapeutic molecule, we will
expand its use to include a wider range of human cancers, such
as ovarian cancer," said Pastan.
A diagram of the molecular process involved in this research follows:
For more information on Dr. Pastan’s research, go to http://ccr.cancer.gov/staff/staff.asp?profileid=5782.
For more information on Dr. Hassan’s research, go to http://ccr.cancer.gov/Staff/Staff.asp?profileid=7274.
For more information about cancer, please visit the NCI Web
site at http://www.cancer.gov,
or call NCI's Cancer Information Service at 1-800-4-CANCER (1-800-422-6237).
The National Institutes of Health (NIH) — The Nation's
Medical Research Agency — includes 27 Institutes and
Centers and is a component of the U.S. Department of Health and
Human Services. It is the primary federal agency for conducting
and supporting basic, clinical and translational medical research,
and it investigates the causes, treatments, and cures for both
common and rare diseases. For more information about NIH and
its programs, visit www.nih.gov.
* Onda M, Nagata S,
FitzGerald DJ, Beers R, Fisher RJ, Vincent JJ, Lee B, Nakamura
M, Hwang J, Kreitman RJ, Hassan R, Pastan I. Characterization
of the B-cell epitopes associated with a truncated form
of Pseudomonas exotoxin
(PE38) used to make immunotoxins for the treatment of cancer
of Immunology, December 15, 2006; Vol. 177, No. 12.