Chronic obstructive pulmonary disease (COPD) is a multifactorial disorder that is associated with an influx of macrophages, neutrophils and T-lymphocytes to the lungs and the extracellular release of several proteases, including serine (proteinase 3, elastase, cathepsin G), cysteine (cathepsin S) and metallo- (macrophage metalloelastase, MMP-12) proteases, leading to a protease/antiprotease imbalance. The pathogenesis of COPD arises from an imbalance between the levels of proteases and their endogenous protein inhibitors. Poor regulation of the activity of the released proteases results in the degradation of elastin, the major component of lung connective tissue. Agents capable of suppressing the activity of these enzymes are of potential therapeutic value. Furthermore, the availability of highly selective inhibitors of these enzymes can help delineate the precise role the aforementioned proteases play in COPD. The work described in this thesis is focused on the use of the benzisothiazolin-3-one scaffold in the design of selective inhibitors of PR 3. The synthesized inhibitors are intended to interact with the S' subsites of the enzyme. The results of these studies have demonstrated that highly selective inhibitors of PR 3 over HLE can be realized using this heterocyclic scaffold.