Oxidant-protease interaction in the lung. Prospects for antioxidant therapy

In inflammatory lung disorders, oxidants and proteases complement each other in their potential to destroy lung parenchyma. It is therefore rational to combine therapeutic strategies aimed at augmenting the antiproteolytic defenses of the lung in diseases such as emphysema with antioxidant strategies. In the healthy lung, the oxidant burden is balanced by the local antioxidant defenses. However, both an increased oxidant burden and/or decreased antioxidant defenses may reverse the physiologic oxidant-antioxidant balance in favor of oxidants, leading to lung injury. This concept points to an obvious therapeutic strategy: augmentation of the antioxidant screen of the lung to prevent oxidant-mediated tissue damage. Studies using reduced glutathione (GSH), the major pulmonary antioxidant, as a model therapeutic agent demonstrated that GSH can be administered directly to the respiratory epithelial surface by aerosol and is fully functional as an antioxidant both in vitro and in vivo. In pulmonary diseases such as idiopathic pulmonary fibrosis or following HIV infection, GSH aerosol therapy not only normalized deficient pretherapy GSH levels in the lung, but was capable of favorably influencing cellular events such as oxidant release by pulmonary inflammatory cells. The same was true for oral antioxidant therapy with N-acetylcysteine, a glutathione precursor. These results suggest that it is possible to use antioxidants to reverse the imbalance between oxidants and antioxidants at the site of oxidant injury to prevent the progressive tissue damage in lung disorders characterized by high oxidant states. Antioxidants, alone and in combination with antiproteases, merit further long-term studies for clinical therapy.