New Gene Therapy Method Used in Mice Could Increase Effectiveness of Stem Cell Transplants

A study by a University of Nebraska Medical Center researcher has shown

that gene therapy can kill tumor cells that otherwise are returned to cancer

patients following high-dose chemotherapy, while keeping the good cells

intact.

In a report published in December in Cancer Gene Therapy, James Talmadge,

Ph.D., announced that his group was able to purge tumor cells without affecting

the stem cells needed to make the high-dose chemotherapy patient healthy

again.

“When cancer patients turn to transplantation for treatment, oftentimes

the stem cells that are reinserted into their body aren’t completely cancer-free,”

said Dr. Talmadge, professor of pathology and microbiology at the UNMC

and principal investigator on the study. “Treating stem cells with adenovirus

protects the numbers of healthy cells replaced in the body and reduces

cancer cell numbers to levels researchers never have seen before.”

Dr. Talmadge’s group used adenovirus vectors, a common virus naturally

found in the lungs, to carry a gene to breast cancer cells. The gene, p53,

inhibits tumor growth. The adenovirus vectors, or delivery systems, used

in these studies were replication incompetent, meaning they are unable

to cause infection.

In adult stem cell transplantation, a large number of stem cells are

removed from the body before a patient is given a high-level dosage of

chemotherapy. While the cells are outside the body, they can be treated

with a process called “purging” to reduce the number of tumor cells contaminating

the stem cell product. Generally, purging kills most of the tumor cells

and as many as 30 to 50 percent of the stem cells, a problem Dr. Talmadge

and his collaborators were able to overcome using adenovirus gene therapy.

“It is clear that the cells within the stem cell products used in transplants

can cause cancer, based on retrovirus studies reported previously by other

researchers,” Dr. Talmadge said. “At this time, there isn’t a good, clear

way to purge stem cells completely of cancer. Either you miss some tumor

cells or you lose healthy stem cells, which is a problem because reinserting

healthy cells in the point of transplant protocol.”

Dr. Talmadge, working with Canji, Inc., a biotechnology company in San

Diego that is affiliated with Schering-Plough Corporation, sought to find

a way to limit tumor cells from being re-inserted in humans. Through his

research, Dr. Talmadge found that incubating stem cells with adenovirus

for a four-hour period kills tumor cells without having an impact on stem

cells.

“In a further test we re-inserted these human steam cells into a mouse

without an immune system,” Dr. Talmadge said. “The human hematopoietic

stem cells in those mice grew, while the tumor cell levels were reduced

to all-time lows.”

Hematopoietic cells are found in bone marrow and help red blood cells,

white blood cells and platelets regenerate after stem cell transplants.

Normally, these cells are weakened severely by chemotherapy, causing cancer

patients to become anemic, neutropenic and to develop life-threatening

infections.

In this study, scientists purged the stem cell products used to rescue

patients following high-dose chemotherapy of the contaminating tumor cells.

These stem cells were then used to treat severe combined immunodeficient

mice and allow them to generate human blood cells. The unique aspect of

these studies was that adult human stem cells were used to reconstitute

these myelosuppressed mice, demonstrating that the human stem cells remained

fully functional. Earlier in vitro studies examining this dilemma were

encouraging, but in those experiments it was not possible to determine

if the purged stem cells were unable to produce all of the blood elements

needed for recovery following high-dose chemotherapy.

To prove that the hematopoietic cells were regenerating after transplant,

Dr. Talmadge took hematopoietic cells out of the mice and used them to

rescue another group of mice, again with no immune system. Multiple tests

on mice produced the same results, proving that these cells were self-renewing

and would not exhaust themselves following transplantation and so could

continue to produce cells throughout the lifespan of the transplanted patient,

in this case a mouse.

Initial testing focused on breast cancer, but Dr. Talmadge said other

types of cancer, such as ovarian cancer, have been targeted for further

studies. The Nature Publishing Group published the research in Cancer Gene

Therapy, a month scientific journal highlighting the latest cancer and

gene therapy discoveries.

Clinical trials won’t be starting any time soon, Dr. Talmadge said.

Adenovirus vectors received an undeservedly bad reputation from an incident

in Philadelphia, he said, and transplantation for breast cancer has yet

to demonstrate a significant prolongation of survival. Dr. Talmadge said

that as the use of adenovirus in gene therapy becomes better accepted,

he hopes human trials will be initiated.