Agent protects cells from lethal effects of radiation even if given after exposure

May 7, 2007

No drugs exist to protect the public from the high levels of radiation that could be released by a "dirty" bomb or nuclear explosion. Such excessive exposure typically causes death within weeks as the radiation kills blood cells vital to clotting and fighting infection, along with the stem cells needed to replenish their supply. But now researchers at Washington University School of Medicine in St. Louis report they have developed an agent that protects cells from the lethal effects of radiation, regardless of whether it is given before or after exposure.

Using this agent in mice, the investigators found that the treatment helped shield rapidly dividing cells that are most vulnerable to radiation-induced death, providing proof in principle that it is possible to fend off radiation damage, according to a study published in the April issue of Biochemical and Biophysical Research Communications.

Current treatments for severe radiation exposure, also called acute radiation syndrome, are limited to drugs that boost the production of blood cells and platelets, but this approach is futile if underlying stem cells are also killed off. Moreover, there are no available treatments that can be given after exposure to limit damage to cells.

"We are using an entirely different approach," says Clayton Hunt, Ph.D., of the Department of Radiation Oncology. "Rather than ramp up the production of blood cells, we are trying to prevent radiation-induced cell death from occurring in the first place."

The researchers developed the agent by attaching a portion of the Bcl-xL protein already known to block cell death - a snippet called BH4 - to the HIV protein TAT, which can deftly carry other molecules into cells. They gave the agent intravenously to mice exposed to 5 Grays of radiation. In humans, this level of exposure would cause a sharp drop in blood cells, leaving individuals with an increased risk of infection and bleeding.

They found the treatment helped protect rapidly dividing T cells and B cells in the spleen - immune system cells that are prone to radiation damage - whether it was given 30 minutes before radiation exposure or 30 minutes afterward.

As part of the research, the investigators monitored the levels at which old or damaged cells in the spleen were dying, a process called apoptosis. In a group of control mice that were not exposed to radiation, the researchers determined that 4.7 percent of T cells and 5.1 percent of B cells in the spleen were undergoing apoptosis. This level is considered normal as cells naturally die and are replaced by new ones. After the mice received 5 Grays of whole body radiation, apoptosis increased to 15.6 percent of T cells and 38.7 percent of B cells.

But when the researchers gave TAT-BH4 to the mice prior to whole body radiation, levels of apoptosis dropped significantly, to 8.6 percent of T cells and 16.9 percent of B cells. In mice given TAT-BH4 after radiation exposure, the proportion of cells undergoing apoptosis dropped even further, to 5.7 percent of T cells and 12.3 percent of B cells.

The Washington University approach appears to halt apoptosis by targeting pathways within cells that are far removed, or downstream, from the initial radiation insult. In particular, BH4 is thought to block a release of the electrical charge across the membrane of mitochondria - the powerhouses of cells - a key event in initiating cellular self-destruction. "This gives us a window of opportunity to treat patients and still prevent cells from undergoing programmed cell death," said Richard Hotchkiss, M.D., professor of anesthesiology, medicine and surgery. "We have a lot more work to do, but we are encouraged by these early findings."

Follow-up data suggest that TAT-BH4 is still effective when it is given to irradiated mice one hour after exposure, and the researchers plan further studies to determine how long after exposure the agent can prevent radiation-induced apoptosis.

In the past several years, the federal government has devoted increasing resources to the development of countermeasures that protect the public from chemical, biological, radiological or nuclear attack. TAT-BH4 may one day be a viable candidate because theoretically it could be given after radiation exposure, administered in pill form, and synthesized and stored in large quantities - all properties that would be desirable for treating large groups of individuals exposed to high levels of radiation, Hotchkiss said.

The researchers contend that developing such a drug would be less challenging than finding a way to protect healthy cells from radiation therapy aimed at destroying cancer cells. "In radiation therapy, you want to give a dose of radiation to a tumor and reduce the exposure to surrounding, healthy tissues," Hunt said. "This is difficult because a drug has to distinguish between tumor and normal tissue. But with people exposed to a large dose of radiation over the entire body, you want to protect all the cells in the body. To me, that is an easier problem to solve."

Source: Washington University School of Medicine