Mending a torn screen in the lung.

Mending a torn screen in the lung

Replacing a protective protein thatstops potentially harmful enzymes may slow lung-tissue destruction in a hereditary disease associated with emphysema, suggest recent human trials at the National Heart, Lung, and Blood Institute (NHLBI) in Bethesda, Md. But researchers caution that deciding whether the replacement therapy would be clinically useful in large numbers of patients will require years of testing.

Destruction of the lung's alveoli (sacsin which oxygen and carbon dioxide are exchanged between blood and inhaled air) causes the shortness of breath characteristic of emphysema, which affects millions in the United States. Scientists believe an imbalance between enzymes called proteases and molecules that inhibit those proteases is responsible for the progressive breakdown of tissue, much of it due to the protease elastase. When there is a shortage of the liver-produced blood protein called alpha1-antitrypsin, elastase is free to attack the connective tissue in alveolar walls. Although a healthy pair of lungs contains 300 million alveoli, once any are lost, they are gone forever.

Nearly all cases of emphysema can belinked directly to smoking, says Ronald G. Crystal, chief of NHLBI's pulmonary research, who described the latest findings at a briefing last week. But 2 percent--an estimated 20,000 to 40,000 patients--of the U.S. emphysema cases are people who have inherited from both parents the gene for alpha1-antitrypsin deficiency.

Serum levels of the protein in patientswith alpha1-antitrypsin deficiency are as low as 10 to 15 percent of normal, thus exposing their lungs to abnormally high elastase activity; 85 percent of those with this deficiency eventually die from emphysema. "The obvious approach to therapy would to be reestablish that protective screen,' says Crystal.

In the NHLBI study, described in theApril 23 NEW ENGLAND JOURNAL OF MEDICINE, 21 patients were given alpha1-antitrypsin commercially purified from pooled donor blood. (Researchers elsewhere are making synthetic forms of the protein for similar use.) Because the protein does not remain long in the body, weekly influsions are required.

Data prove the "biochemical efficacy'of the replacement treatment, with minimal side effects, says Crystal. In other words, infusion of the protein apparently maintains a patient's blood concentrations of the protein at near normal. Also, samples taken of lung fluid indicate that alpha1-antitrypsin levels there are essentially normal, as well as the anti-elastase capacity of the protein found in the lung.

This particular treatment, whichwould be a preventive measure rather than a cure, is limited to the hereditary disease, according to Crystal. Nevertheless, he says that learning about this relatively uncommon type of emphysema "charts the directions to research that has implications for the garden variety of the disease.'

Photo: A scanning electron micrograph showsthe destructive erosion of the lung's air sacs seen in emphysema patients.