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"It's a totally different approach to treating asthma, and it's the first biotechnology drug for asthma," said Shelley Schneiderman Ducker, a spokeswoman for the drug's maker, Genentech Inc. of South San Francisco, Calif. "It works much earlier in the allergic cascade than any other product on the market."

The ideal patients would be asthmatics who "are inadequately controlled on inhaled steroids," Ducker said. "They still live in fear of asthma attacks."

"It's for severe asthmatic patients who aren't responding to traditional therapies, meaning inhaled steroids and allergy injections and who may even require oral steroids to keep them out of the hospital," added Dr. Jonathan Field, assistant director of pediatric and adult allergy and asthma at New York University Medical Center.

"It is a second-line treatment, recommended only after first-line treatments have failed," the FDA said in a statement. "It is not approved for children under the age of 12; and it is known to work and approved only for patients with moderate to severe, allergy-related asthma."

Some 17 million American adults and children suffer from asthma, 60 percent of them from allergic asthma, which basically involves the vicissitudes of an over-responsive immune system.

When the body detects something it is allergic to, IgE antibodies send signals to "mast" cells all over the body to release histamines, prostaglandins, and other chemicals to counter the attacks. Sometimes this is normal, but other times the antibodies react against substances that are completely benign. "Your immune system has set up a response against something harmless," Field said.

In these cases, it's no longer the allergen that's the problem. It's the allergic reaction itself, which can be severe enough to kill a person.

Xolair (generic name omalizumab) prevents IgE antibodies from sending messages to the mast cells so these cells never get the signal to release the chemicals which cause the reaction.

The drug will be given by injection about once a month in a doctor's office, Field said.

Dosage depends on the patient's weight, according to Ducker, but half of those in the clinical trials used two vials per month.

The drug is expected to be available in 20 days, she said.

While hundreds of thousands of asthma sufferers no doubt consider the drug to be nothing less than a miracle, it won't come cheaply.

Ducker would not state specifically how much the drug will cost, but because it's a bioengineered medication it's expected to be expensive. Still, she said, "I think we'll be priced in line," and that "there will be receptivity" from health insurers to cover the cost for the patients who most need it.

Other reports have put the likely price at $1,000 per month or $10,000 per year.

And, in truth, no one knows how insurance companies are going to react. "If you look at reimbursement, not all 500,000 people are going to get it or it's not going to be easy," said Mo Mayrides, director of public policy at the Asthma and Allergy Foundation of America in Washington, D.C.

"My understanding is it's a very powerful drug, not a drug that would be prescribed in the ordinary course of events. And because it is so new, I'm not sure that companies have not really had a chance to figure out how it fits into the overall payment schedule," said Larry Akey, spokesman for the Health Insurance Association of America, also in Washington, D.C.

Regardless of who is likely to pay for it, the drug has created quite a buzz among doctors and patients. Mayrides said he has received a number of calls from asthma sufferers in recent days.

And Field has lined up patients to get the drug once it's available. One such a patient, a 40-year-old man with severe asthma since childhood, has tried all the available traditional medicines, including inhaled steroids and allergy shots. Yet he's still ending up in the hospital's intensive care unit and requires high-potency oral steroids. The man has osteoporosis because of all the steroids, and he suffers from cataracts. "He's had all these complications of asthma and the treatment just hasn't worked. It hasn't been enough for him," Field said. "Once we get set up to be a center, he will be one of the first people. For him, it's a godsend."

More information

For more on allergic asthma, visit the American Academy of Allergy, Asthma and Immunology. Meanwhile, Genentech has more on Xolair.