Thesis: Ticlopidine may be part of the solution for thrombotic occlusions of coronary-artery stents as well as hemorrhagic and vascular complications of intensive anticoagulation therapy.
Over a century ago, the German pathologist Rudolf Virchow presented a hypothesis that three factors; vessel injury, altered blood flow, and changes in blood coagulability, were responsible for vascular thrombosis. This concept has been the central principle to explain venous and arterial thromboembolism. Thromboembolism is the obstruction of a blood vessel with thrombotic material carried by the blood stream from the site of origin to plug another vessel. Venous thrombi are not the focus, as they contain few platelets and abundant fibrin, thus, they can be effectively treated with anticoagulants such as heparin. On the other hand, arterial thrombi are composed largely of platelets and form at sites of vascular injury and high blood flow. Recently, cardiologists have concluded that myocardial infarctions are caused by platelet-rich thrombi that form at sites of rupture or crevice of atherosclerotic plaque (Handin). These findings have lead to the increase use and research of antiplatelet drugs such as Ticlopidine.
Development of antiplatelet agents for treatment of patients with vascular disease has been a scientifically complex and slow process. When evaluation of ticlopidine was initiated in the United States in 1979, several platelet aggregation inhibitory drugs had been studied in a large multicenter clinical trial, the Aspirin Myocardial Infarction Study (AMIS), for prevention of nonfatal and fatal secondary myocardial infarction (MI) and death. Aspirin, together with dypyridamole, was also examined in a study similar to AMIS, the Persantine Aspirin Reinfarction Study (PARIS), for prevention of the same events, and sulfinpyrazone was studied in the Anturane Reinfarction Trial (ART) to prevent myocardial deaths. Although none of these trials achieved acceptable statistical significance, both studies involving aspirin displayed trends toward prevention of reinfarction and death. Ticlopidine may be part of the solution for thrombotic occlusions of coronary-artery stents as well as hemorrhagic and vascular complications of intensive anticoagulation therapy. Ticlopidine is a new specific and potent antiplatelet drug that inhibits platelet aggregation induced by adenosine diphosphate and by thrombin.
Since ticlopidine acts specifically on platelets it is important to understand what they are and how they function. Platelets are not really cells at all, but chips of cells about two to three micrometers in diameter. They have no nuclei and originate as pinched-off cytoplasmic fragments of large cells in the bone marrow. The outer surface of the platelet is called the glycocalyx. The plasma membrane contains thirty or more glycoproteins, of which some of the major ones are glycoproteins Ib, IIb/IIIa complex, I, IIa, and IV. Microtubles are located beneath the membrane and give the platelet its structural support. Between the membrane and the microtubules are contractile microfilaments, which are primarily composed of actin and mysosin. There is a canalicular system that is open to the outside of the platelet. The dense tubular system is the site of arachidonic acid metabolism and provides small amounts of calcium to the resting platelet. Mitochondria, glycogen, alpha granules, dense bodies, lysosomes, and peroxisomes are all present within the platelet cytoplasm. Dense bodies are composed of ATP, ADP, calcium, serotonin, and several other components.
Platelets play a central role in hemostasis, is the process that retains the blood within the vascular system during periods of injury. When vascular injury occurs, platelets function in both primary hemostasis (platelet adhesion, secretion, aggregation) and in secondary hemostasis (coagulation). As the result of a vascular injury platelets come in contact with the subendothelium (collagen, fibronectin) and adhere to portions of it. This is known as platelet adhesiveness and most likely occurs because of the presence of von Willebrand factor (vWf) being deposited on the injured tissues. The von Willebrand factor is present in the plasma and subendothelium and binds to glycoprotein sites (Ib and IIb/IIIa complex) on the platelet membrane. Following activation, the platelet undergoes a shape change most probably caused by contraction of the microtubules. The platelet changes from a disk-shape to a spherical shape with the extrusion of numerous pseudopods. At the same time, the platelet granules move to the center of the platelet and fuse with the open canalicular system connected to the outside of the platelet. This way the contents of the granules (ADP, serotonin, beta thromboglobulin, platelet factor 4, vWf, platelet-derived growth factor, etc.) are extruded to the outside. Simultaneously with platelet release, platelet stimulating agents (collagen, ADP, epinephrine, thrombin) bind to the platelets, causing them to adhere to one another. This process is known as platelet aggregation (Brown).
The major target of platelet inhibitors has been the platelet glycoprotein IIb/IIIa complex, which is a member of the integrin family of adhesion receptors that mediates platelet aggregation. Glycoprotein IIb/IIIa is selectively expressed on the surface of platelets in an inactive form. Exposure to agonists such as ADP or thrombin activates the complex so that it binds fibrinogen, linking platelets together into large aggregates (Handin).
Ticlopidine is an antiplatelet agent with a unique mechanism of action different from the presently known agents. Aspirin is the only FDA approved platelet-inhibiting drug that is now available for prevention of stroke, myocardial infarction, or vascular death. At it is currently used clinical doses, ticlopidine's inhibitory effects are most readily demonstrated using ADP as the agonist. In addition, ticlopidine acts irreversibly on platelets, a characteristic that maintains inhibition of platelet function despite an occasional missed dose. Ticlopidine is a platelet aggregation inhibitor that is structurally different from other known antithrombotic agents. Many studies have been performed in an attempt to understand ticlopidine's mechanism of action; however, the molecular mechanism is still not known. Ticlopidine is unusual, since it is not active when added directly to platelet suspensions. Ticlopidine is essentially inactive when added directly to platelet rich in plasma suspension and oral administration results in irreversible inhibition. This suggests that ticlopidine depends on metabolic processing to be activated and that this metabolism permanently alters the platelet (Handin). Since it was reported that a circulating metabolite in the plasma of ticlopidine treated subjects inhibited aggregation of platelets from untreated individuals, experiments have been carried out to investigate this possibility.
When platelets from untreated individuals were resuspended in plasma from subjects dosed for seven days with ticlopidine, no inhibition of platelet aggregation was observed, indicating that circulating, stable metabolites of ticlopidine do not directly inhibit platelet aggregation. When platelets from treated individuals were resuspended in plasma from either group, inhibition of ADP induced aggregation was found. This study shows that ticlopidine's effect is permanently associated with the platelet.
The only identified metabolite of ticlopidine that exhibits antiplatelet effects is 2-hydroxy ticlopidine (2-HT). It significantly inhibits platelet aggregation, but similar to ticlopidine, it requires oral administration to exhibit its full antiplatelet effects. Like ticlopidine, 2-HT is inactive in vitro and has not been detected in plasma of rats, mice, rhesus monkeys, baboons, or humans given oral doses of ticlopidine. Metabolism of ticlopidine to 2-HT may represent an initial metabolic step that results in formation of an active metabolite (Hass).
A large multicenter randomized trial entitled "Ticlopidine Aspirin Stroke Study," (TASS) compared patients who were given 250 mg of ticlopidine twice daily with patients who were given 650 mg of aspirin twice daily. The study analyzed the baseline characteristics of the 3,034 on-treatment patients in TASS, and the carotid lesion characteristics of the 1,188 patients with carotid arteriography for carotid distribution qualifying symptoms, to determine those factors which might differentiate response to ticlopidine versus aspirin. Patients in this study consisted of both men and women and were followed for two to six years. In the overall population of patients, ticlopidine produced a 12% reduction in the risk of stroke or death and a 21% reduction in the risk of subsequent fatal or nonfatal stroke compared with aspirin. Ticlopidine significantly reduced the risk of fatal or nonfatal stroke in both women and men compared with aspirin. This advantage over aspirin also occurred in patients with previous stroke or history of cardiac disease (Grotta).
Another randomized study compared antiplatelet and anticoagulant therapy after the placement of coronary-artery stents. A stent is a mold for keeping a skin graft in place, made of Stent's mass or some acrylic or dental compound. After successful placement of coronary-artery stents, 257 patients were randomly selected to receive antiplatelet therapy (ticlopidine plus aspirin) and 260 to receive anticoagulant therapy (intravenous heparin, phenprocoumon, and aspirin). The study identified a high level of surface expression of the inducible fibrinogen receptor on platelets as a strong independent predictor of thrombosis in the stented vessel, whereas monitoring of anticoagulation was not predictive. These findings suggested that effective inhibition of platelet function may be superior to anticoagulation therapy in preventing stent occlusions. The data obtained in this study showed that , compared with anticoagulant therapy, antiplatelet therapy was associated with a lower rate of noncardiac events, including hemorrhagic and peripheral vascular complications. Antiplatelet therapy was also associated with a lower rate of cardiac events, such as death, myocardial infarction, and the need for repeated intervention. Also associated with anitplatelet therapy was a lower incidence of thrombotic occlusion of the stented vessels. The most notable finding of the study was the significantly lower rate of cardiac events in the group that received antiplatelet therapy, which translated into a 75% reduction in the risk of such events (Schomig).
The use of Ticlopidine has been widely accepted by the medical and scientific community. However, reports in the medical literature suggest that ticlopidine may be associated with adverse side effects. This drug has been marketed in the United States since October 1991, however; based on reports since then, the labeling of ticlopidine has been changed. The product labeling that was approved in 1991 included a boxed warning that stated neutropenia occurred in 2.4% and severe neutropenia and/or agranulocytosis occurred in 0.8% of stroke patients who received ticlopidine in premarketing clinical trials. Neutropenia is a decrease in the number of neutrophilic leukocytes in the blood. Leukocytes are important cells of the immune system which help protect the body against foreign substances. Agranulocytosis is a symptom complex characterized by marked decrease in the number of granulocytes and by lesions of the throat and other mucous membranes, of the gastrointestinal tract, and of the skin. A granulocyte is a nongranular leukocyte. Since then new reports have surfaced associating ticlopidine with the same and new side effects. As a result of these reports the warning section of the ticlopidine labeling was updated in early 1995 to add to aplastic anemia, pancytopenia, thrombocytopenia, and immune thrombocytopenia (Wysowksi). Aplastic anemia is a form of anemia generally unresponsive to specific antianemia therapy, in which the bone marrow may not necessarily be acellular or hypoplastic but fails to produce adequate numbers of peripheral blood elements. Pancytopenia is a deficiency of all cell elements of the blood. Thrombocytopenia is a decrease in the number of blood platelets. It is obvious that although this drug may contain possible side effects, the decision to use this drug depends entirely on the individual's medical condition. It was also mentioned in the literature that it was observed that the majority if the time the benefits of using this drug out weighed the possible side effects.
A major challenge of molecular genetics will be elucidation of the relation between the inherited predisposition to thrombosis and its clinical consequences. One of the dilemmas is most illustrated with protein C deficiency. Protein C is a vitamin K dependent protein, that acts as an anticoagulant. The majority of individuals with heterozygous protein C deficiency, as well as the heterozygous parents of infants with homozygous protein C deficiency and neonatal purpura fulminans, are free of thrombotic complications. This finding indicates that protein C deficiency is an autosomal recessive disease. Yet a strong association has been found between heterozygous protein C deficiency and thrombosis among members of families with a clinical thrombotic predisposition. Scientist are only at the earliest stages of understanding hypercoagulable states on a molecular level. There are numerous candidates for prothrombotic mutations that have not yet been linked to a predisposition to thrombosis. In some areas, progress is hindered by the lack of readily accessible tissue for testing, as is likely to be the case for a variety of primary vascular abnormalities that can potentially cause thrombosis (Schafer).
Ticlopidine is now internationally recognized for its efficacy as a major drug that will help protect patients at high vascular risk against thousands of fatal or nonfatal occlusive events each year. Beyond the improved vascular prognosis that ticlopidine should provide to many patients, its already long scientific and clinical history has also demonstrated its usefulness as a biological and therapeutic tool to better understand the role of platelets in the mechanisms of arterial thrombosis.
Grotta, J.C., "Prevention of stroke with ticlopidine: Who benefits," Neurology, Jan. 1, 1992. pp. 111-115.
Handin, Robert I., "Platelets and coronary artery disease," The New England Journal of Medicine, April 25, 1996 Vol. 334 No. 17, pp. 11261126-1128.
Hass, William K., Ticlopidine, Platelets and Vascular Disease, Springe-Verlag New York, 1993.
Schafer, Andrew., "Hypercoagulable states: molecular genetics to clinical practice," The Lancet, Dec. 24, 1994 Vol. 343, pp. 1739-1744.
Schomig, Albert, "A randomized comparison of antiplatelet and anticoagulant therapy after the placement of coronary-artery stents," The New England Journal of Medicine, April 25, 1996 Vol. 334 No. 17, pp. 1084-1089.
Wysowksi, Diane K., "Blood dyscrasias and hematologic reactions in ticlopidine users," JAMA, Sept. 25, 1996 Vol. 276, No. 12, pp. 952.