Enzymes are proteins that catalyze chemical reactions and are involved in almost all metabolic pathways in a living cell. They speed up the conversion of a substrate into a new product. Enzymes are specific for a given molecular substrate and are classified by the mechanism by which they act on the substrate. Actelion is currently working on several different enzyme targets in its research programs, most of them being soluble intracellular proteins.

One such class is the aspartic proteinases. This class of enzyme promotes chemical reactions inside and outside cells. There are more than 50 known aspartic proteinases, of which at least nine are currently known to exist in humans. Although knowledge of their precise physiological roles is still emerging, they have been implicated in cancer, as well as in inflammatory, degenerative, and cardiovascular diseases. In addition, aspartic proteinases play a vital role in organisms that cause infections, including parasites, fungi, and retroviruses such as HIV.