Scientists at Ohio University Edison Biotechnology Institute have
used a nonviral gene expression system, invented and patented by Ohio
University several years ago, to eliminate human cancer cells in
animals. The investigators reported achieving a 60% tumor regression
rate with this system without the potential dangers usually
associated with conventional viral gene therapy techniques.
The universitys scientists are collaborating on the project
with Progenitor Inc., a California-based biotechnology company that
has licensed the T7 gene expression system used in the studies. If
further tests of the method prove as successful as early studies,
human trials could begin in the next year or two.
Nonviral Gene Therapy Vector
The same gene therapy protocol was employed widely in the research of
human cancer in animal models. Cellular expression of a herpes
simplex virus-thymidine kinase (HSV-TK) gene was stimulated in
tumors, and then tumor-bearing animals were treated with ganciclovir
(Cytovene). This approach resulted in permanent elimination of 30% of
While similar tests of the HSV-TK/ganciclovir protocol have suggested
that it effectively kills cancer cells, previous studies used viral
vectors to deliver the HSV-TK gene to the cells. Since viral vectors
are not transient, the potential exists for viruses to infect healthy
cells and cause mutations that could become part of the genome.
Accordingly, scientists have been looking for a nonviral delivery
system to use with gene therapy treatments, says Xiao Chen, PhD, lead
author of the study and an assistant professor of clinical research
at Ohio Universitys Edison Biotechnology Institute and College
of Osteopathic Medicine.
The work was published in a March 1998 issue of Human Gene Therapy.
"Viral gene therapy vectors are very efficient in delivery of
therapeutic genes to targeted cells, but there are safety concerns
associated with the viral vectors," says Dr. Chen. "Results
of our studies of the T7 system suggest that it may be useful as an
alternative to viral vectors for cancer therapy and other transient
gene therapies or in biologic applications that require temporary but
rapid and efficient gene expression."
Benefits of T7 Gene Therapy
Ohio University scientists have long suspected that the T7 gene
expression system would produce tumor reduction rates similar to
those achieved with viral vectors. These new studies confirm that
suspicion, says Thomas Wagner, PhD, distinguished professor of
molecular and cellular biology at Ohio University and principal
investigator of the T7 project at Edison Biotechnology Institute. The
benefit to using T7 for gene therapy, says Dr. Wagner, is that the
genes expressed via this method are transient and do not become part
of the cells chromosomes. Also, T7 allows genes to be expressed
outside of the nucleus of a cell, which means that it can be used on
cells that are not dividing.
"We think this is why we had such positive results of total and
permanent regression of the tumors we studied," says Dr. Wagner.
"While most cancer cells are dividing, oncologists tell us that
in the core of many tumors there are large populations of nondividing
cells. These cells may be the source of the recurrence of the cancer."
The T7 system, however, allowed expression of the HSV-TK gene in all
cells in the tumor, even those that were not dividing. The high level
of gene expression, another benefit to the T7 system, is due to its
design, says Dr. Wagner. While other gene therapy techniques use only
DNA genetic material, the T7 method uses DNA prebound with T7 RNA
polymerase--a protein that causes the gene to begin producing RNA
immediately--an important step in protein synthesis within a cell.
Enhancing the Gene Expression System
At Progenitor Inc, scientists are conducting animal studies with T7
to measure its effectiveness on brain and skin cancers. While these
studies have been successful, the scientists are also trying to
improve efficiency of the technology for commercial purposes, says
Ralph Snodgrass,PhD, coauthor of the study and vice-president of
research and chief scientific officer at Progenitor.
Clinical trials will likely be conducted sometime in the future, but
before trials can begin, scientists want to enhance the expression
system to allow for a higher rate of tumor regression. In some mice,
tumor regression was partial and in others it was complete. Dr.
Snodgrass believes that the problem may lie in the way that the DNA
is delivered to the tumor cells.
"In some tumors, you might only need gene expression in 10% of
the cells to be effective. But in other tumors, you might need
expression in 25% of the cells," he says. "Some cells take
up the DNA very easily but others may need some help." The
possibility of using different lipids or other molecules to improve
the delivery efficiency and consistency of the T7 system is also
It is suggested that when clinical trials do begin, the following
therapeutic strategy might be employed. First, physicians would
perform a computed tomographic (CT) scan of the tumor to identify its
exact location. Next, an automated injection system would introduce
the T7 protein package into every segment of the tumor. Drs. Chen and
Wagner have found that a series of injections increases tumor
regression, so the patient would likely require several rounds of
Once the HSV/TK genes began to express in the cell, the patient would
receive intravenous doses of ganciclovir. Tumor regression should
become evident in a matter of days, says Dr. Snodgrass. Unlike
chemotherapy, which is not selective in the cells it kills, genetic
therapy could potentially destroy only the diseased cells.