Polymerization of fibrinogen-derived fragment X and subsequent rearrangement of fibers

Review of pharmacology and physiology in thrombolysis interventions

Reperfusion therapy using thrombolytic agents has been shown to be a safe and effective treatment strategy for arterial ischemia, venous thrombosis, massive pulmonary embolism, and acute stroke. Thrombolytic agents have evolved over the course of a few decades, from nonfibrin selective to fibrin-selective agents. The development and modification of these agents have resulted in improved understanding of their pharmacologic attributes, and their effects on the complex molecular events that occur during thrombolysis goal-directed therapies. The current review focuses on the physiology and pharmacology of the thrombolytic agents that have been or are currently in use for interventional thrombolysis interventions. Attention is also given to the particular role that thrombolytic agents play in the current management of peripheral vascular disease and acute stroke.

Differentiating pharmacologic agents used in catheter-directed thrombolysis

The use of catheter-directed thrombolysis is a proven treatment for arterial ischemia, deep vein thrombosis, and severe pulmonary embolism. For arterial ischemia, thrombolysis has resulted in improved amputation-free survival and fewer subsequent surgeries to reestablish blood flow to the ischemic limb. The management of patients with thromboembolic diseases is complex, and the multiple thrombolytic drugs available to choose from compound this complexity. Although some believe the available thrombolytic agents are interchangeable, real biochemical differences exist that may prove otherwise. This article describes these pharmacologic differences and how they may affect the clinical practice of catheter-directed thrombolysis.

Fibrin structures during tissue-type plasminogen activator-mediated fibrinolysis studied by laser light scattering: relation to fibrin enhancement of plasminogen activation

The aim was to relate fibrin structure and the stimulatory effect of fibrin on plasminogen activation during t-PA-mediated fibrinolysis using Lys78-plasminogen as activator substrate. Structural studies were undertaken by static and dynamic laser light scattering, cryo transmission electron microscopy and by the measurement of conversion of fibrin to X-, Y- and D-fragments. The kinetics of plasmin formation were monitored by measurement of the rate of pNA-release from Val-Leu-Lys-pNA. The process of fibrin formation and degradation comprised three phases. In the first phase, protofibrils with an average length of about 10 times that of fibrinogen were formed. The duration of this phase decreased with increasing t-PA concentration. The second phase was characterized by a sudden elongation and lateral aggregation of fibrin fibers, most pronounced at low levels of t-PA, and by formation of fragment X-polymer. The third phase was dominated by fragmentation of fibers and by formation of Y- and D-fragments. Plasmin degraded the fibers from within, resulting in the formation of long loose bundles, which subsequently disintegrated into thin filaments with a length of less than 10 and a mass per length close to one relative to fibrinogen. Plasmin generation at high t-PA concentrations sets in just prior to (and at low t-PA concentrations shortly after) the onset of the rapid second phase of elongation and lateral aggregation of fibrin fibers. The maximal rate of plasmin formation per mol t-PA was the same at all concentrations of activator and was achieved close to the time of the peak level of fragment X-polymer. Plasmin formation ceased after formation of substantial amounts of Y- and D-fragments. At this stage the length was between 300 and 3 and the mass per length close to 1, both relative to fibrinogen. In conclusion our results indicate that (1) formation of short fibrin protofibrils is the minimal requirement for the onset of the stimulatory effect of fibrin on plasminogen activation by t-PA, (2) formation of fragment X protofibrils is sufficient to induce optimal stimulation of plasminogen activation, and (3) plasmin degrades laterally aggregated fibrin fibers from within, resulting in the conversion of the fibers into long loose bundles, which later disintegrate into thin filaments.

Structural difference between polymerized and non-polymerized fragment X, obtained by plasmin digest of fibrinogen

Fragment X (LMrFX) was obtained as low molecular weight preparations from a late stage 2 plasmin digest of human fibrinogen. The thrombin-treated LMrFX preparations, which resulted in impaired polymerization, were further subfractionated into polymerized and non-polymerized components. The fractions were examined by SDS-PAGE and immunochemical methods. In polymerized fractions, more peptide bands were observed on SDS-PAGE in the reduced state than in non-polymerized fractions. Both fractions contained a similar number of internal cleavages in the A alpha, B beta and gamma chains, which are linked by disulfide bonds. Thus, the partial deficiencies in polymerization sites of the carboxy terminal region of the gamma chain and the amino terminal portions of the B beta chain, as well as internal cleavage, were considered to participate in the impairment of the thrombin-induced polymerization of LMrFX.