Anti-inflammatories for cardiovascular disease

Cardiovascular disease is the main cause of death in industrialized countries. Atherosclerosis develops mainly in large and medium-sized arteries, producing ischemic events in the heart, brain, or extremities, and may lead to infarction (1). Therapy in patients with coronary artery disease is designed to alleviate the symptoms of angina and to reduce myocardial infarction. This has been partially accomplished with the use of coronary angioplasty. However, despite the use of new technologies, including the use of stents, restenosis continues to be a major clinical problem in the management of the disease. Restenosis can be defined as a recurrent narrowing of blood vessel that occurs after angioplasty.

At the cellular level, and probably because of the vascular injury, several cell types are involved in the restenosis process. Platelets, macrophages, polymorphonuclear leukocytes, and smooth muscle cells become major players at the site of endothelial injury. These activated cells promote an inflammatory response with the synthesis of vasoactive molecules. These include cytokines and several types of growth factors, including platelet-derived growth factor, fibroblast growth factor, and vascular endothelial growth factor. This inflammatory response stimulates the migration and proliferation of smooth muscle cells in the area, leading to the restenosis event. In this issue of PNAS, Fisher and colleagues (2) report a marked reduction in neointimal formation in animals treated with the nonsteroidal anti-inflammatory drug (NSAID) sulindac but not with aspirin (acetylsalicylic acid). They used a new animal model in which a femoral artery injury is introduced and neointimal formation is analyzed 4 weeks later. To further resemble the human condition in which high plasma concentration of cholesterol is one of the main risk factors for atherosclerosis, they performed the studies in hyperlipidemic, apolipoprotein-E-deficient (apoE −/−) mice, a well-known animal model of atherosclerosis (3). This report is extremely important. It generates a series of important questions and possible applications to other cardiovascular diseases such as atherosclerosis, transplant atherosclerosis, and restenosis after angioplasty. Is it possible to use anti-inflammatory agents such as sulindac for the prevention of vascular restenosis after angioplasty and related procedures? Why did aspirin fail to demonstrate an effect in this model? Is this activity unique to sulindac and thus, what is the molecular mechanism by which sulindac exerts its pharmacological activity? Sulindac and aspirin are two NSAIDs used for the management of inflammatory conditions such as arthritis. There are more than 20 NSAIDs in the U.S. market. All of them are inhibitors of cyclooxygenases, the enzymes that metabolize arachidonic acid into prostaglandins (PGs). Sulindac is a prodrug and a relatively weak inhibitor, whereas its sulfide metabolite is very potent inhibitor of cyclooxygenase (4). Two isoforms of cyclooxygenase, COX-1 and COX-2, are responsible for the synthesis of PGs. COX-1 is the constitutive form of the enzyme present in many tissues, including the normal vasculature, and the PGs that are produced by COX-1 play a role in normal physiology of the gastrointestinal tract, the kidney, and platelets (5). In contrast, COX-2 expression is restricted under normal conditions but it is rapidly induced in the context of inflammation and other pathological conditions such as atherosclerosis (6). In atherosclerotic tissue, COX-2 is expressed in medial smooth muscle cells and endothelial cells, including those of the vasa vasorum, a clear indication that this enzyme is up-regulated in the context of the atherosclerotic inflammatory response. Another piece of evidence to indicate that COX-2 activity present within the lesion in macrophages, smooth muscle cells, and endothelial cells might be the primary target of sulindac is that the increased synthesis of PGs (which has been observed in animal models of atherosclerosis as well as in patients with severe atheroma) have a diverse array of biological responses. These include the promotion of monocyte adhesion, induction of cell differentiation, proliferation, and promotion of angiogenesis. For example, neointimal formation must include neovascularization within the tissue, and therefore inhibitors of angiogenesis are effective in blocking neointimal formation. This has been demonstrated in ApoE-deficient mice in which two angiogenesis inhibitors, endostatin and TNP-470, inhibited plaque growth and intimal neovascularization (7). Similar results would be expected with inhibitors of COX-2 because they are potent antiangiogenic agents (8). If this mechanism is correct, inhibition of COX-2 by sulindac may explain the remarkable effect seen in inhibiting neointimal formation after arterial injury.

It is very well known that chronic use of NSAIDs is associated with serious side effects, including ulcers, bleeding, and perforations in approximately 2–4% per year in chronic NSAID users (9). These serious complications that limit their use in chronic conditions have been substantially reduced with the development of specific COX-2 inhibitors (10). Thus, it would be very important to demonstrate whether COX-2-specific inhibitors can achieve similar efficacy in this model. It is also important to indicate that sulindac has demonstrated chemopreventive activity in animal models of cancer prevention, and it has shown to reduce polyp formation in patients with familial adenomatous polyposis (FAP). All of these pharmacological activities of sulindac have been reproduced with the specific COX-2 inhibitor celecoxib (11).

Understanding the role of COX-2 inhibitors during restenosis might also help to explain why another inhibitor of COX-1 and COX-2, such as aspirin, failed to demonstrate an effect in this animal model. One possible explanation is that aspirin was tested at doses that are very effective in blocking platelet aggregation, e.g., because of the inhibition of platelet COX-1. But these doses might not be sufficient to inhibit the COX-2 enzyme in macrophages and other cells that can overcome the initial inhibition of COX activity by generating new enzyme. In fact, Vane and colleagues (12) demonstrated that aspirin has a differential effect on platelet COX-1 as compared with the peripheral site of inflammation. Aspirin irreversibly acetylates the active site of COX-1 in platelets and, because they have no machinery to make new enzyme, the inhibitory effect lasts for the life of the platelet—up to 10 days. In contrast, inflammatory cells are actively producing COX-2 and, because of the short (15-min) half-life of aspirin and its conversion to salicylate, it is necessary to generate much more salicylate to substantially inhibit the synthesis of PGs at the inflamed site. While Fisher and colleagues (2) clearly demonstrated the inhibition of aggregation by aspirin in the study, inhibition of tissue PG synthesis at the site of restenosis was not determined and probably was not significant at the dose of aspirin used.

Another possibility is that sulindac exerts its effects by a mechanism that is independent of COX activity. This mechanism could include inhibition of proliferation, induction of apoptosis, inhibition of peroxisome proliferator-activated receptor-δ, or increased formation of intracellular ceramide leading to the induction of apoptosis among other effects (13, 14). Interestingly, all of these COX-independent mechanisms have been postulated not only for sulindac but also for other NSAIDs such as aspirin and for the new specific COX-2 inhibitors. Furthermore, most of these effects have been observed in cells in vitro at concentrations that are not possible to achieve in vivo.

Despite these caveats, the data with sulindac preventing restenosis clearly indicate the importance of inflammation during this process. There is now clear evidence that atherosclerosis is not a degenerative disease of aging, but rather a result of a chronic inflammatory condition with very important clinical consequences (15). The availability of new scientific tools to investigate cardiovascular disease and the new information regarding its link to inflammation suggest that novel drugs that are effective at blocking inflammatory processes should be considered for the prevention and treatment of this and other chronic diseases.