Acquired resistance to treatment with arvin

New Anticoagulants for the Cardiovascular Patient

Manuscript received May 9, 1994; accepted July 7, 1994. Address correspondence and reprint requests to Dr. Jeanine M. Walenga, Department of Pathology, Loyola University Med ical Center, 2160 South First Avenue, Maywood, IL 60153, U.S.A. Summary: Invasive cardiac procedures and cardiac sur gery, which have become a significant component of health care, provide a particular case scenario where there are special considerations given to control bleeding and to have available alternative anticoagulants for pa tients unable to tolerate heparin (e.g., heparin-induced thrombocytopenia). We describe a comprehensive proto col for blood preservation in cardiac surgery incorporat ing the patient's medical history, autologous transfusions, and intraoperative techniques of heparin and protamine titration and dosing. Furthermore, various new anticoag ulants under clinical development are discussed, particu larly hirudin, hirulog, iloprost, ancrod, Lomoparan, low- molecular-weight heparins, 1-deamino-8-D-arginine- vasopressin (DDAVP), and aprotinin. These approaches should benefit the patient with difficult to control periop erative bleeding, heparin-induced thrombocytopenia, al lergic reactions to protamine, platelet dysfunction, and poor response to heparin (e.g., due to antithrombin III deficiency).

Defibrinogenating enzymes

The venoms from 3 snakes have been shown to induce defibrinogenation: ancrod from the venom of Calloselasma rhodostoma (formerly known as Agkistrodon rhodostoma), batroxobin from the venom of Bothrops atrox moojeni, and crotalase from the venom of Crotalus adamanteus. The purified fractions of ancrod, batroxobin, and crotalase possess coagulant, proteolytic and esterolytic properties, although their primary mechanism of action is a proteolytic effect on circulating fibrinogen. Ancrod cleaves only the A-fibrinopeptides, but not the B-fibrinopeptides, from fibrinogen; this contrasts with thrombin, batroxobin and crotalase, which cleave both fibrinopeptides A and B. Within minutes of administration of ancrod or batroxobin, there is a significant reduction in plasma fibrinogen levels, and these remain exceedingly low with repeated administration (once or twice daily). The rapid fall in plasma fibrinogen levels is accompanied by a slightly delayed but marked rise in the level of fibrinogen-fibrin degradation products. Plasminogen levels are decreased and blood viscosity is reduced, but formed elements in the circulating blood remain unaltered. Ancrod and batroxobin have been investigated in patients with stroke, deep-vein thrombosis, myocardial infarction, peripheral arterial thrombosis, priapism, and sickle-cell crisis; crotalase has not been administered to humans. However, results have been difficult to interpret, and additional well designed trials are needed to better define the optimum role of ancrod and batroxobin in the management of these conditions. Overall, treatment is well tolerated and serious adverse events are infrequent. In the coagulation laboratory, ancrod, batroxobin and crotalase may be used as reagents to perform coagulation studies on specimens of blood that contain heparin. These venom fractions can be substituted for thrombin in performing the thrombin time and in removing fibrinogen from plasma for accurate determination of fibrinogen-fibrin degradation products.

Ancrod improves survival in murine systemic lupus erythematosus

The effect of ancrod, a defibrinating agent, on murine lupus glomerulonephritis in the male BXSB mouse was studied to determine the relationship between macrophage procoagulant activity (PCA), fibrin deposition and glomerulonephritis. Marked renal disease and fibrin deposition were noted by three months of age in control mice, whereas little or no disease was seen in ancrod treated mice until five months of age. Similar high titers of anti-DNA antibodies and renal deposition of IgG were seen in both groups of mice. PCA rose with age in both ancrod treated and untreated mice, although it was significantly higher in control animals than in the ancrod treated group. Furthermore, ancrod therapy resulted in a decrease in plasma PCA inducing activity (PIF) and a decrease in the effectiveness of PIF to induce PCA in peritoneal macrophages in vitro. No mortality was observed in the 20 ancrod treated mice, whereas 10 of 20 control animals died. We conclude that defibrination with ancrod delays the development of renal fibrin deposition and glomerulonephritis and improves survival in BXSB mice. This was associated with a decrease in plasma PCA inducing activity and with an inhibitory effect on PCA induction. These results suggest that PCA contributes to injury in murine lupus glomerulonephritis by promoting fibrin deposition.

Hemodynamic effects of slow and rapid defibrination with defibrizyme, the thrombin-like enzyme from venom of the timber rattlesnake

The hemodynamic response to slow and rapid defibrination was sutdied in anesthetized beagle dogs, with the following results: 1. Slow defibrination was a benign procedure that had little or no effect on the hemodynamic variables studied. 2. Rapid defibrination induced statistically significant decreases in cardiac output, stroke volume, and mean aortic arterial pressure. 3. Bradycardia, a drop in mean left v"ntricular pressure, cardiac and minute work indices, an increase in pulmonary artery pressure, and a drastic rise in pulmonary and systemic vascular resistances were also observed. Although physiologically apparent, these changes were not statistically significantly different from control levels. 4. Pulmonary capillary wedge pressure, left ventricular end-disatolic pressure, arterial pH, and blood gases were not altered by rapid defibrination. 5. In view of the similarities between the hemodynamic changes observed after rapid defibrination and acute myocardial ischemia, the role of decreasing fibrinogen concentrations and blood viscosity in aucte myocardial infarction and the sudden death syndrome is questioned.

Further experience in the use of ancrod (Arvin) to prevent thrombosis on prosthetic heart valves

Intravenous administration of ancrod produces hypofibrinogenaemia, which may be prolonged by further doses. Hypofibrinogenaemia, maintained for a week, was used to prevent thrombosis on prosthetic heart valves in the calf.

Thirteen calves were used in the study. Three of these were used to determine dose schedules and to test antibody formation. The calf was found to have hyperactive fibrinogenesis, and doses higher than previously reported were required to maintain prolonged hypofibrinogenaemia. Antibodies were detected in the calves treated for prolonged periods and the phenomenon was probably related to the massive dosage required.

In 10 calves, the tricuspid valve was replaced with a polypropylene valve, using cardiopulmonary bypass. Two of these calves, used as controls and not treated with ancrod, showed massive thrombus formation on the valves. The remaining eight calves were treated with ancrod on different dose schedules. In four calves consistent low fibrinogen levels were not achieved. In the other four, treated by a continuous infusion of ancrod, 8-10 units/kg of body weight per day supplemented further by twice daily intravenous injections of 8·0 units/kg, it was possible to maintain sufficiently low fibrinogen levels and to prevent thrombus formation on the valves.

In man, hypofibrinogenaemia is more easily maintained and antibody formation is less likely with the small dosage needed.