U.S. military officials are expressing enthusiasm about an experimental drug that they say could protect the health of troops, police officers and emergency medical personnel who respond to terrorist attacks involving nuclear weapons or radiation-spewing "dirty bombs."

The drug being developed by Hollis-Eden Pharmaceuticals Inc. of San Diego appears to offer significant protection from radiation sickness, which would kill many more people in nuclear attacks than the initial blast, military officials and experts said.

"We want it on the fast track," said Navy Adm. James A. Zimble, a top military health official who is president of the Uniformed Services University of the Health Sciences in Bethesda. "We've been very encouraged by the very positive results" of tests on animals, he added.

Experts cautioned that more research needs to be done to prove the drug's effectiveness and safety when administered to humans. The vast majority of new drugs that appear promising in animal studies never gain approval for humans. But radiation specialists said tests on this drug with mice, dogs and monkeys suggest that it will work in people and will not prove toxic.

Since the 1950s, military researchers have scrutinized thousands of compounds in a search for something that could protect troops in a nuclear war zone, but have failed. This drug is the first to hold such promise, said Mark H. Whitnall, a top researcher at the Armed Forces Radiobiology Research Institute in Bethesda, which is working closely with Hollis-Eden on the drug.

"My personal opinion is any agency dealing with emergency response to terrorist events should be interested" in the new drug, Whitnall said.

U.S. military officials are encouraged by results from animal studies that appear to demonstrate that the drug, called HE-2100, offers protection when administered before radiation exposure as well as a few hours after exposure, or even later. This suggests it could be given to military personnel or firefighters when it is known that they will be entering a radiated zone or as they are leaving one. Currently, there is no safe medicine to give people after they are exposed to dangerous levels of radiation, experts in the field said.

Radiation severely compromises the body's immunity to disease, so most fatalities caused by a nuclear explosion or dirty bomb blast would come from infections, including influenza and pneumonia, beginning a week to six weeks after detonation, medical experts said. A dirty bomb is a conventional explosive attached to radioactive material, which is spread when the device goes off.

HE-2100 buttresses the immune system, in particular the infection-fighting powers of bone marrow, which is most vulnerable to radiation. The drug protects the bone marrow's ability to continue creating infection-fighting cells called neutrophils even after radiation exposure. The loss of too many of these cells brings on a condition called neutropenia, which leads to infections and possibly death.

HE-2100 stimulates neutrophil production by causing cells that become neutrophils to mature and to be released into the bloodstream.

In a Hollis-Eden study completed earlier this year, monkeys that received a near-lethal radiation dose and did not receive the drug suffered severe neutropenia 50 percent of the time over the next 21 days. By contrast, monkeys given HE-2100 about three hours after being dosed with radiation, and then for the following seven days, suffered the same effects only 9 percent of the time. None of the monkeys suffered ill effects from HE-2100, the firm said.

By staving off radiation-related infection and illness in the weeks after a nuclear event, HE-2100 can, it appears, "bring people over that hump in time, where, without it, they would die," said David Grdina, a professor of radiation and cellular oncology at the University of Chicago.

"There are definitely applications for homeland security in this drug," Grdina said. Even so, Hollis-Eden is pursuing the drug's development through the U.S. military, as it has for several years, rather than switching to the Department of Homeland Security.

Some civilian officials say developing medical protections against radiation is less of a priority than working on cures for bioterror agents, which they view as the gravest current terrorist threat.

In any case, Whitnall added that the fact that HE-2100 has shown such encouraging results in tests involving three different species suggests it will be successful in humans.

"It protects against radiation damage; there's no doubt about that," said William McBride, a radiation biologist at the University of California in Los Angeles. "The question is: How much can you give a person" before it proves toxic? "It seems non-toxic so far. It's encouraging they got it into primates."

The only other drug that has been shown to protect animals from radiation, ethyol, must be given before radiation exposure, and can be highly toxic. Hollis-Eden and military officials said HE-2100 is the only compound on the horizon that has its potential.

The firm estimates that an eight-day course of the drug would cost as much as $100. Military officials said the idea is to stockpile enough doses across the country to treat both first responders and as many people in the general population as could be radiated in an attack.

The Food and Drug Administration has decided the firm can seek approval of the drug under a new set of streamlined procedures for substances believed to protect people in nuclear-biological-chemical attacks. Because it would be unethical to irradiate human beings to determine whether the drug works, the FDA says the company can rely on the animal studies to show its efficacy, and give the HE-2100 to people to test for adverse reactions.

The drug is being developed in injection form, but Hollis-Eden is looking into developing it in pill form.

Other drugs also protect humans in radioactive crises, but only from a limited range of radioactive isotopes. Potassium iodide pills, if given within hours of a radioactive event, can protect the thyroid gland, which is extremely sensitive to radiation damage. The World Health Organization recommends that the drug be stockpiled in homes near nuclear power plants.

A compound called Prussian blue also can be used to treat people who receive high doses of the radioactive element cesium, which terrorism experts say could be disseminated in a dirty bomb. Only one small European firm makes the drug, and the FDA has asked U.S. companies to apply to manufacture it.

But for now, military officials see the greatest possible benefit in HE-2100. "We're a long way from having a product," Zimble said, "but we think we can protect troops with it."