The Gladstone Connection

Heart disease continues to be the major cause of death in the United States. The key lesion in heart disease—the atherosclerotic plaque—is composed of lipoproteins, cholesterol, and macrophages. For many years, the common belief was that plaques grow slowly over time and eventually cause the complete obstruction of coronary arteries, leading to myocardial infarctions, or “heart attacks.”

We now know this is not the case. Myocardial infarctions are actually caused by the sudden rupture of a plaque, followed by the formation of a blood clot, or thrombus, that rapidly occludes the artery. It is the thrombus, rather than the plaque, that blocks the artery.

What is a thrombus and how does it form? Understanding these questions was the focus of scientists in the thrombosis and platelet group of the Gladstone Institute of Cardiovascular Disease. Their basic research formed the basis for the development of a major drug to control thrombosis.

Thrombosis is a natural process that is necessary to control bleeding. The process is usually initiated when blood encounters a foreign substance, such as skin or the connective tissue around the outside of blood vessels. A thrombus is made of proteins derived from the blood and small cells known as platelets. One of these proteins is fibrin, a fine meshwork formed when the protein fibrinogen is cleaved by thrombin. Thrombin itself is generated in response to an injury, such as the rupture of a blood vessel. However, fibrin alone is not sufficient to control bleeding. Newly formed clots require platelets to become stabilized and permanently stop bleeding. Normally, platelets quickly bind to exposed fibrin and form large aggregates that strengthen the clot.

Careful control of the clotting process is critical. Unfortunately, the process can go awry. For example, when the “injury” is the rupture of a plaque in a coronary artery, the same mechanisms that provided life-saving hemostasis can cause a myocardial infarction.

When an atherosclerotic plaque ruptures inside an artery, the flowing blood “identifies” the newly exposed connective tissue as foreign, and a series of events rapidly ensues. First, thrombin is generated as the cascade of coagulation proteins in the blood is activated, and fibrin begins forming. Next, platelets bind to collagen, a component of the blood vessel wall that is now in direct contact with flowing blood, and begin to aggregate. Finally, thrombin acts directly on the platelets to enhance aggregation. Specialized proteins on the surface of the platelets, known as glycoproteins IIb and IIIa (GPIIb-IIIa), bind the blood protein fibrinogen to form a bridge between two activated platelets. The formation of many such bridges results in large platelet aggregates.

Since the Institutes were established, scientists at Gladstone have actively studied the molecular mechanisms that regulate platelet activation and their role in thrombus stabilization. Their research has provided seminal insight into the process and facilitated the discovery of Integrilin, a important drug for the treatment of heart disease.

The basis for that drug was the discovery of GPIIb-IIIa by Gladstone investigator Dr. David R. Phillips. Additional work by his research group revealed details of the mechanism of fibrinogen binding to the GPIIb-IIIa complex. Dr. Leslie Parise showed that the GPIIb-IIIa complex is both necessary and sufficient to bind fibrinogen. Dr. Laurence Fitzgerald cloned and sequenced the gene for GPIIIa. Dr. Israel Charo found that endothelial cells express surface proteins very similar to GPIIb-IIIa.

We now know that platelet GPIIb-IIIa and the GPIIb-IIIa-like proteins on endothelial cells are members of a large family of adhesion receptors known as the integrins. Integrins control cell-cell and cell-matrix interactions in virtually all organs of the body, including the blood, and are critical in other processes, such as inflammation, defense against infectious pathogens, and tumor metastasis. The pharmaceutical industry has had considerable interest in developing integrin antagonists for therapeutic use.

    In 1988, Drs. Phillips, Fitzgerald, and Charo left Gladstone to start Cor Therapeutics, a biotechnology company dedicated to developing GPIIb-IIIa antagonists. Taking advantage of advances in our understanding of GPIIb-IIIa biology made at Gladstone, Cor Therapeutics successfully developed a series of potent and specific GPIIb-IIIa antagonists. These peptide-based antagonists effectively blocked platelet aggregation in test-tube experiments and prevented life-threatening thrombus formation when administered to laboratory animals.

In 1990, just two years after the company was founded, Cor Therapeutics began Phase I clinical trials in healthy human volunteers to establish the safety of Integrilin, a potent and specific antagonist of GPIIb-IIIa. In 1998, Integrilin received FDA approval for use in angioplasty—an invasive procedure performed to reduce or eliminate blockages in arteries—and treating the acute coronary syndrome. Marketed by Millennium Pharmaceuticals, Integrilin has annual sales in excess of $300 million.