Oxidative/Nitrosative stress and the pathobiology of chronic obstructive pulmonary disease.

The understanding of the pathobiology of Chronic Obstructive Pulmonary Disease (COPD) has undergone a major change in the past three decades. The classical 'protease-antiprotease' hypothesis still holds true, nevertheless, the sequence of the biochemical events which lead to the protease/antiprotease imbalance have been unraveled. For instance, tobacco smoke, a primary risk factor for COPD, contains a plethora of reactive Oxygen/Nitrogen Species (ROS/RNS) that serve to initiate the oxidant/antioxidant imbalance in the respiratory tract of chronic smokers, a phenomenon that is amplified if certain other risk factors co-exist (e.g. a genetic deficiency of the major antiproteases, a suboptimal antioxidant defense system, airway hyper responsiveness etc.). The inflammatory response that ensues as a result of the initial occult exogenous oxidative/ nitrosative stress becomes a secondary endogenous source of ROS/RNS. This perpetuates the ongoing lung damage, even though the primary insult may no longer be present (abstinence). Depletion of the pulmonary antioxidants, damage to the local antiprotease protective screen, a decreased immune response, hypersecretion of mucus, superadded infections, oxygen therapy-induced oxidant production, etc. are some of the critical factors which account for the oxidative/ nitrosative stress-mediated pulmonary as well as extrapulmonary features of COPD. In the light of the recent developments, remarkable efforts are being made, either to develop novel therapeutic strategies or to improve the existing ones, which are aimed at treating different aspects of the disease. Thus, it is reasonable to recommend antioxidants as a useful adjunct to the more conventional treatment options, keeping in view the 'oxidant/antioxidant' hypothesis as a unifying theme for the 'protease/antiprotease' theory of COPD.