NEW YORK (AP) _ A derivative of vitamin A reversed emphysema-like abnormalities in the lungs of rats, suggesting a possible lead for a treatment.

About 2 million Americans have emphysema, mostly long-term smokers. The disease kills about 17,000 Americans a year.

``This is the first time that anyone has identified a means of reversing emphysema,'' said Dr. Claude Lenfant, director of the National Heart, Lung, and Blood Institute.

But Lenfant, whose institute paid for part of the work, said in a statement that much more basic research is needed ``before we can even begin to think about applying this to humans.''

``Until then, we caution that there is absolutely no evidence that vitamin A supplementation is useful in treating lung disorders,'' Lenfant said.

The new work is reported in the June issue of the journal Nature Medicine by Dr. Gloria De Carlo Massaro and Dr. Donald Massaro of the Georgetown University School of Medicine.

Emphysema damages the tiny lung pouches where blood picks up oxygen and gives up carbon dioxide. The pouches become unnaturally enlarged. The lungs lose elasticity and become distended, unable to expand and contract normally.

That leads to shortness of breath and later disability.

For the new experiment, rats were given a substance that attacked the walls of the pouches. That produced fewer but larger pouches as well as distended lungs, as seen in emphysema.

Some of these rats were then given doses of retinoic acid, a derivative of vitamin A. Later, researchers found that these rats grew new lung pouches of a healthy size. Their lungs were also found to be of normal size.

It's not clear how retinoic acid produced those effects, the researchers wrote.