Blood Lecture 4

8/12/2019 Blood Lecture 4

1/82

Adelina Vlad

MD PhD, Lecturer

Hemostasis and Fibrinolysis


2/82

Hemostasis Hemostasis, from the Greek hemos (blood) and stasis

(standing), is the arrest of bleeding from an injured blood vessel

Involves well regulated interactions between components of the

vessel wall, blood platelets and plasma proteins

Subendothelial matrixWBC

Platelets

Hemostatic plug

Fibrin

Endothelial cell

RBC

WBC


3/82

Why Should We Study Hemostasis? Hemorrhage, intravascular thrombosis and embolism are clinical

manifestation of many diseases

Hemorrhage

occurs when disease or trauma damages large arteries andveins and normal hemostasis is overwhelmed

less frequently, is caused by an inherited oracquired

disorder of the hemostatic machinery itself

Thrombosis and embolism can be caused by unregulated activation of the hemostatic

system

may reduce blood flow to critical organs such as brain or the

heart


4/82

Why Should We Study Hemostasis?

Accurate diagnosis and treatment of patients with either bleeding

or thrombosis require knowledge of the physiology andpathophysiology of hemostasis


5/82

Hemostasis Initiated when trauma, surgery or disease disrupts the vascular

endothelium, exposing blood to the subendothelial connective

tissue

Achieved by:

1) Vasonstriction

2) Formation of a platelet plug

3) Formation of a blood clot as

a result of blood coagulation

4) Growth of fibrous tissue into the blood clot

Primary

hemostas is

Secondary

hemostas is


6/82

Fluido-Coagulant Equilibrium A balance normally exists between the factors that stimulate clot

formation and forces acting to delay this process

The mechanisms that prevent hemostasis from running out of

control involve anticoagulants and fibrinolytic factors

(fibrinolysis = breakdown of stable fibrin present inside a clot)


7/82

Primary Hemostasis

Vascular Factors

Platelet Plug Formation


8/82

Primary Hemostasis

Is the first physiological response to vascular injury

Occurs within seconds of injury

Important in capillaries, small arterioles, venules

Triggers secondary hemostasis (coagulation proteins)


9/82

Primary Hemostasis Vascular factors reduce blood loss from trauma through:

Local vasocon str ic t ion; results from:

1) local myogenic spasm - initiated by direct damage to the

vascular wall2) local autacoid factors t h romboxane A2 (TXA2) and

serotoninreleased by activated platelets; endothel in-1

(ET-1) produced by the endothelium in response to

thrombin

3) nervous reflexes - initiated by pain nerve impulses

Comp ression of in jured vessels by extravasation of blood

into surrounding tissues


10/82

Primary Hemostasis Platelet plug formation involves:

1) Platelet adhesion via stimulators such as thrombin

2) Platelet activation

3) Platelet aggregation


11/82

The Platelets

Resting

plateletsActivated

platelet

Blood smear with red

blood cells and platelets


12/82

Genesis

Time interval from differentiation of the stem cell to theproduction of platelets (thrombocytopoiesis) ~ 10 days

Controlled by growth inducers and differentiations inducers:

thrombopoietin, IL-6, IL-3, vitamin B12, GM-CSF

Mpl-L / TPO (megapoietin, a glycosylated hormone) is thehomeostatic regulator of platelet production; acts onmegakaryocyto-poiesis and thrombocytopoiesis


13/82

Features Disc-shaped nucleus-free fragments, 2-3 m diameter

150,000 450,000 platelets/ L in normal blood

Lifespan of about 10 days; eliminated from the circulation mainly

by the tissue macrophage system, more than 50% in the spleen


14/82

Platelet Membrane

Phospholipids - activate multiple stages in the blood-clotting

process

Platelet receptors = glycoproteins in the platelet membrane

involved in adhesion, activation and aggregation processes

Provides procoagulant surface on which coagulation

proteins can interact


15/82

Platelet Cytoplasm

Platelet cytoplasm has: Residual Golgi & ER that form a circumferential skeleton of

microtubules

synthesize enzymes, PG,TXA2

store large quantities of calcium ions

Mitochondria and enzyme systems that form ATP and ADP

Lysosomes containing heparin-cleaving enzyme

Fibrin-stabilizing factor (F XIII)

Actin, myosin, thrombosthenin - can cause the platelets to

contract and release the granules content Open membrane canalicular system

facilitates the exocytose of platelet granules

provides a large reactive surface for coagulation proteins


16/82

Granules:

Electron-dense granules contain calcium ion s, ADP,

serotonin

-Granules store several proteins, such as:

platelet factor 4 (PF4) - a heparin antagonist,

von Willebrand factor (vWf),

PDGF platelet derived growth factor, causes vascular

endothelial cells, vascular smooth muscle cells, and

fibroblasts to multiply, helping repair damaged vascular

walls

two clotting factors f ibr inogenand c lot t ing factor V

Glycogen for anaerobic glycolysis


17/82

Cross-section of an activated platelet

Cross-section of a resting platelet

Open

canalicular

system

Granules

Microtubules


18/82

Platelet Plug Formation

Adhesion

Activation

Aggregation


19/82

Platelet Adhesion Is the binding of platelets to themselves or to other components Occurs in response to vessel injury that expose platelet

receptors to ligands present in the subendothelial matrix

Platelet receptors involved in adhesion:

Glycoprotein Ib/Ia (GP Ib/Ia)

Lingand: von Willebrand factoro a glycoprotein present in plasma

o produced in endothelial cells and megacariocytes

o vWf forms a link between platelets and collagen fibers;stabilizes the interaction with collagen

Glycoprotein Ia/IIa (GP Ia/IIa) Ligand: collagen

Glycoprotein Ic/IIa (GP Ic/IIa)

Ligand: fibronectin, laminin


20/82

Platelet Adhesion


21/82

Why?

Due to:

prostacyclin and nitric

oxide released by

endothelial cells that

maintain circulating

platelets in an inactive

state

CD39 present on the

surface of endothelial

cells, an enzyme that

converts to inactive AMP

any small amounts of

ADP that might activate

platelets

Normally platelets do not adhere

to themselves, to other blood cells,or to endothelial membranes


22/82

Platelet Activation Triggered by platelet activating agents (vWf, collagen,

thrombin, epinephrine) that bind to surface receptors

signaling events: changes in the level of cyclic nucleotides,

the influx of calcium, hydrolisis of membrane PL,

phosphorylation of critical proteins

Signaling leads to:

Cytoskeletal and morphological changes formation of

finger-like filopodia

Granule release

Lysosomes heparin-cleaving enzyme

Dense granules ADP, serotonin, calcium ions

-Granules growth factors, hemostatic factors (vWf,

fibrinogen, Factor V)

Synthesis and release of TXA2 from arachidonic acid by

using cyclooxigenase (COX)


23/82

Resting and activated platelet


24/82

Platelet Aggregation Signaling molecules released by activated platelets activate

additional platelets:

ADP, binds to P2Y12 receptors on platelets

Serotonin and TXA2

vWf released by activated platelets, binds to Gp Ib/Ia

activating them and forming molecular bridges betweenplatelets

Recruitment of platelets that promotes platelet aggregation

Platelet activation

conformational change in Gp IIb/IIIa,another platelet receptor, endowing it with the capacity to bind

fibrinogen fibrinogen bridges between platelets

platelet plug formation


25/82


26/82

Adhesion

GpIIb/IIIa

Activation Pathways

GpIIb/IIIaGpIIb/IIIa Aggregation

ADP

Adrenaline Platelet

Exposed Collagen

Endothelium

vWF

COLLAGEN

GpIIb/IIIaGpIIb/IIIa AggregationGpIIb/IIIaGpIIb/IIIa Aggregation

AdhesionAdhesion

ADP

Adrenaline

THROMBIN


27/82

Aggregation


28/82

Platelet Plug Formation- Summary -

At sites of vascular injury,endothelial cells are damaged or

removed exposes collagen

fibrils, to which platelets adhere

with help from von Willebrand

factor, a blood proteinsynthesized by endothelial cells

Once activated in this way,

platelets secrete ADP and

thromboxane A2.

These molecules bind to receptorson circulating platelets they

become activated and recruite

into the growing platelet plug.


29/82

Tests for Primary Hemostasis Bleeding time: skin incision time to stop bleeding; global

screen of platelet role in hemostasis

Platelet count; Mean Platelet Volume (MPV) indicates theuniformity of size of a platelet population

vWf assays: vWf Ag, vWf:RCof - measure the amount and

function of vWf; assesses function of VWF ligand in itsinteraction with platelet Gp Ib/Ia receptor

Platelet aggregometry: in vitro full platelet aggregation inresponse to various stimulation - ADP, epinephrine, collagen,thrombin, ristocetin; predominantly assesses function of plateletglycoprotein IIb/IIIa receptor

Membrane glycoproteins: Gp Ib/Ia, Gp IIb/IIIa measured usingmonoclonal antibodies and flow cytometry

Platelet granule content: electron microscopy


30/82

Antiplatelet Drugs Acetylsalicylic acid (aspirin)

Target: COX-1 and -2, irreversibly inactivated by acetylation

Effect: prevents TXA2 synthesis impairment of platelet secretionand aggregation

Thienopyridines (ticlopidine, clopidogrel)

Target: platelet ADP receptors (P2Y12), selectively and irreversiblyinhibited

Effect: prevents ADP-induced platelet activation and aggregation

Gp IIb/IIIa antagonists (abciximab, eptifibatide, tirofibam)

Target: Gp IIb/IIIa receptors

Effect: prevents platelet aggregation

Antiplatelet therapy reduces overall mortality from vasculardisease by 15% and nonfatal vascular events by 30%


31/82

theroscleroticlaque


32/82

Secondary Hemostasis

Blood Clot Formation


33/82

Secondary Hemostasis Blood clot a semisolid mass made of platelets and a fibrin

network that entraps erythrocytes, leukocytes and serum

Thrombus an intravascular blood clot; can be red, when fibrin

predominates (thrombus of the venous circulation) or white,

when a higher proportion of platelets is present (thrombus of

arterial circulation)

Primary and secondary hemostasis are related events:

activated platelets release some clotting factors

several clotting factors (thrombin, fibrinogen) play a role in

platelet-plug formation

they may occur in parallel or in the absence of the other

Secondary hemostasis is also called blood coagulation


34/82

Red blood cellsentrapped in the fibrin

network

Platelets, red blood cells andfibrin


35/82

Secondary Hemostasis Mechanism:

Coagulation proteins work in concert to generate prothrombin

activator orprothrombinase (step 1)

Prothrombinase acts on prothrombin, a plasma protein, to

form thrombin (step 2)

Thrombin converts fibrinogen to fibrin (step 3);

fibrin consolidates the platelet plug made in primaryhemostasis such that a fibrin clot (secondary hemostatic

plug) is formed


36/82

Classic Model Two distinct events can induce blood clotting:

The contact of blood with the vascular endothelial collagen or

with the surface of an activated platelet triggers the intrinsic

pathway or con tact phase of coagulat ion; the trigger is

inside the vascular system

The interaction of blood with material from damaged cell

membranes (tissue factor) activates the extrinsic pathway or

t issue factor d ependent pathway; the trigger is outside the

vascular system


37/82

Classic Model In response to the triggering event, an inactive plasma

proenzyme is converted to a reactive enzyme that, in turn,

converts another proenzyme to its active form

= the waterfall or cascadeconcept

! The cascades do not occur in the fluid phase of the blood, but

at the membrane of activated platelets (intrinsic pathway)

or

at a tissue factor that is membrane bound (extrinsic pathway)


38/82

Waterfall Scheme of Coagulation

Extrinsic

Pathway

IntrinsicPathway


39/82

NAME ALETERNATE NAME PROPERTIES

Factor I Fibrinogen

Factor Ia FibrinFactor II Prothrombin Synthesis in liverrequires

Vitamin K

Factor IIa Thrombin

Factor III (cofactor) Tissue factor

Tissue thromboplastin

An integral membrane

glycoprotein

Receptor for Factor VIIaFactor IV Ca2+

Factor V Labile factor

Proaccelerin

Accelerator globulin

Synthesized by liverand stored in

platelets

Factor Va (cofactor) Heterodimer held together by a

single Ca2+ ionHighly homologous to Factor VIIIa

Factor VII Stable factor

Serum prothrombin

conversion accelerator

(SPCA)

Proconvertin

Synthesis in liver requires

Vitamin K

PROCOAGULANT FACTORS


40/82


41/82

Intrinsic Pathway Three plasma proteins form a complex on vascular endothelial

collagen or on the surface of activated platelets: Hageman factor

(XII), HMWK and prekallikrein (PK)

HMWK is a product of platelets that may help anchoring XII to

trigger surfaces; after binding to HMWK, XII slowly activates to

XIIa XIIa with HMWK as an anchor activates XI to XIa and converts

PK to kallilkrein

Kallikrein accelerates the conversion of XII to XIIa = positive

feedback

! Patients with deficit of XII, HMWK or PK have apparently normal

hemostasis an alternative mechanism for activation of XI may

exist


42/82

Contact Activation

Trigger SurfaceXII

FXIIa

Thrombin Generation

XIa

XII

FXIIa

FXI

HK

PK

HK

FXIIa

Kallikrein

XII

Kallikrein

FXIIa

PK

HK


43/82

Intrinsic Pathway

XIa bound on HMWK activates IX to IXa

IXa, together with Xaand th rombin(downstream activated

factors from the cascade = positive feedback), activate VIII to

VIIIa, a cofactor in the next reaction

IXa, VIIIa, calcium ions (provided on the spot by activated

platelets), and phospholipids form a complex called tenase

Tenase convert X to Xa


44/82


45/82

Extrinsic Pathway

The extrinsic pathway is a cascade of protease reaction initiatedby factors that are outside the vascular system

Tissue factor, TF (tissue thromboplastin or factor III) is

a membrane protein expressed by nonvascular cells

a receptor for factor VIIa

TF-Bearing Cell

TF VIIa

Xa

XCa++

After a vessel injury, VII

comes in contact with TF that

activates it to VIIa

TF + VIIa + calcium ions =

= a complex analogous to

tenase that cleaves X to Xa


46/82

Common Pathway

Xa produced by either intrinsic or extrinsic pathway is the firstprotease of the common pathway

Thrombin activates V (provided by activated platelets) to

cofactor Va (positive feedback)

Xa + Va + calcium ions + phospholipids = prothrombinase

Prothrombinase form thrombin by cleaving prothrombin

Thrombin

Main effect: catalyse the proteolysis of soluble plasma

fibrinogen to form soluble fibrin monomers that polymerize insoluble fibrin polymers gel that traps blood cells

Activates factor XIII, released by activated platelets, to XIIIa

covalent crosslinking of the fibrin polymers mesh called

stable fibrin


47/82

Intrinsic pathway Extrinsic pathway


48/82

Thrombin - other effects

Positive feedback at upstream levels of the cascade

Can cathalyze the formation of new thrombin from

prothrombine

Activates VIII and V, accelerate the actions of Factors

IX, X, XI, XII

Blood clot continues to grow until bleeding is stopped

Paracrine actions linked to hemostasis

Stimulates endothelial cells to produce NO, PGI2, ADP,vWf, tissue plasmin activator

Activates platelets, a process that may lead to more

thrombin formation (positive feedback)


49/82

Thrombin positive feedback at upstream levels of the cascade


50/82

Actual Concept: Coagulation as a

Connected Diagram Intrinsic and extrinsic pathways seem to be connected during

in vivo hemostasis

Clinical evidence suggest that initiation of physiologic

coagulation depends largely on the extrinsic pathway:

factors IX and X can be activated by the TF - VIIa - calcium

ions complex

TF can be exposed inside the vessels by peripheral blood

cells, particularly leukocytes, and activated endothelial

cells; both of them do not express normally TF activity on their

surface

TF-VIIa is inhibited by TFPI (tissue factor pathway inhibitor)

susta ined activation of X by IXa and VIIIa on the intrinsic

pathway becomes critical for normal hemostasis; XI is activated

to XIa mainly by the positive feedback effect of thrombin


51/82

Initiation Phase


52/82

Amplification Phase


53/82

Propagation Phase

C l i P h


54/82

Contact Tissue Factor + VII

XIIIa

XIII

Thrombin

Fibrin(strong)

Fibrinogen Fibrin(weak)

IX

XI

XIa

IXa

XaVa

XIIa

Prothrombin

TF-VIIa

(Prothrombinase)

PL

PL

(Tenase)

VIIIa

PL

X

Intrinsic Pathway

HKa

Extrinsic Pathway

Common Pathway

TF Pathway

Coagulation Pathways


55/82

TF-VIIa

TFPI

TFPI

Questions left unanswered by

the classic view of hemostasis:

Why factor XII deficiency does notcause bleeding?

Why do deficiencies in factors VIII

or IX (hemophilia) produce dramatic

bleeding even though the extrinsic

pathway stays intact?

Answers:

Initial activation of IX by TF-VIIa

complex compensates for

deficiencies in XII

TF-VIIa is inhibited by TFPI (tissue

factor pathway inhibitor)sustained activation of X by IXa and

VIIIa becomes critical for normal

hemostasis

coagulation as a connected

diagram


56/82

Secondary Hemostasis


57/82

Clot Retraction

The fibrin fibers adhere to damaged surfaces of blood vessels the blood clot becomes adherent to any vascular opening

After a clot is formed, it begins to contract:

activation of platelet thrombosthenin, actin, and myosin

molecules cause strong contraction of the platelet spiculesattached to the fibrin

the fluid expressed is called serum because fibrinogen and

most of the other clotting factors have been removed

as the clot retracts, it becomes more tight and stable;

furthermore, the edges of the broken blood vessel are pulledtogether


58/82

Coagulation: A Balancing Act

The cardiovascular system maintains a precarious balancebetween two pathological states: inadequate clotting that would

lead to blood loss, and overactive clotting that would cause

thrombosis and cessation of blood flow


59/82

Only small amounts of each coagulation factor is activated theclot does not propagate beyond the site of injury

Blood fluidity is maintained by

The flow of blood

The adsorption of coagulation factors to surfaces and their

trapping in the emerging clot

Paracrine factors

Anticoagulant factors

How is Clotting Limited?


60/82

Produced by endothelial cells

Prostacyclin (PGI2)

Inhibits platelet activation

Promotes vasodilation and thus blood flow

Nitric oxide

Following thrombine stimulation

Inhibits platelet adhesion and aggregation via cGMP

Paracrine Factors


61/82

Anticoagulant Factors

Tissue factor pathway inhibitor (TFPI) Plasma protein that binds to TF-VIIa complex and blocks the

protease activity of VIIa

Linked to the endothelial cell membrane where it maintains an

antithrombotic surface

Antithrombin III (AT III)

Binds to and inhibits Xa and thrombin

Its activity is much increased by heparan sulfate (present on most

cells surface) and heparin (released by mast cells and basophils)


62/82

Thrombomodulin

Produced by endothelial cells, present at their surface

Binds thrombin, removing it from the circulation

Binds protein C

Protein C

Binds to the thrombomodulin-thrombin complex gets activated by

thrombin protein Ca, a protease (! Ca means protein C activated,

not calcium)

Protein Ca + protein S (cofactor) inactivates Va and VIIIa

Protein S

Cofactor for protein C


63/82

NAME ALTERNATE NAMES PROPERTIES

TFPI Tissue factor pathway

inhibitor

Protease inhibitor produced by

endothelial cells

Linked to the cell membrane

Antithrombin III AT III A plasma protein

Inhibits Factor Xa and thrombin, and

probably also Factors XIIa, XIa, and

IXa

Heparan and heparin enhance theinhibitory action

Thrombomodulin (cofactor) Glycosaminoglycan on surface of

endothelial cell

Binds thrombin and promotes

activation of protein C

Protein C Anticoagulant protein C

Autoprothrombin IIA

A plasma protein

Synthesis in liver requires Vitamin KProtein S (cofactor) A plasma protein

Synthesis in liver requires Vitamin K

Cofactor for protein C

ANTICOAGULANT FACTORS


64/82


65/82


66/82

Routine Coagulation Assays

Prothrombin Time (PT) Activated Partial Thromboplastin Time (APTT)

Quantitative Fibrinogen (FIB)

Thrombin Time (TT)

Assays for specific coagulation factors

Factors assessed by a PT-based test system: FVII, FV, FX,

and FII

Factors assessed by an APTT-based test system: FXII, FXI,

FIX, and FVIII

Tests for fibrinogen


67/82

International Normalized Ratio,


68/82

Anticoagulant Drugs

Used to prevent and treat thrombosis in medical and surgicalpatients

Heparin (unfractioned heparin UFH, low-molecular-weight

heparin LMWH, heparinoids):

AT III binds to heparin binding and inactivation of Xa andthrombin; heparin is destroyed in the blood by an enzyme

known as heparinase (1.5 4 hours)

Direct thrombin inhibitors (DTI): hirudin, lepirudin, argatroban,

bivalirudin, xilemagatran

Coumarin derivatives: bishydroxycoumarin (dicumarol),

warfarin (coumadin)

Inhibit vitamin K synthesis of hypofunctional prothrombin,

factor VII, IX, X, protein C and protein S


69/82

Hemostasis: A Delicate Balance


70/82

Time Frame for Hemostasis


71/82

Fibrinolysis

Fibrinolysis is the breakdown of stable fibrin; starts immediatelyafter the formation of the definitive hemostatic plug

Begins with the conversion of plasminogen (plasma protein) to

plasmin, catalysed by tissue-type plasminogen activator (t-

PA) or urokinse-type plasminogen activator (u-PA)

t-PA:

serine protease produced by endothelial cells

fibrin accelerates the conversion of plasminogen to plasmin

u-PA or urokinase

serine protease present in plasma

the proteolysis occurs when u-PA attaches to urokinase

plasminogen activator receptor (u-PAR) on the cell surface


72/82

Fibrinolysis

Plasminogen

Plasmin

Fibrin, fibrinogen

Activation

Extrinsic: t-PA, urokinase

Intrinsic: factor XIIa, HMWK, kallikrein

Exogenous: streptokinase

Fibrin(ogen)degradation products


73/82

Fibrinolysis Regulators

Catecholamines and bradykinin increase t-PA levels Serine protease inhibitors (serpins):

Plasminogen activator inhibitor-1 (PAI-1) inhibits t-PA

Plasminogen activator inhibitor-2 (PAI-2) inhibits u-PA

2-Antiplasmin (

2-AP), inactivates plasmin when is not bound

to fibrin the presence of a clot (fibrin) promotes thebreakdown of the clot (fibrinolysis)


74/82

FIBRINOLYTIC FACTORSNAME ALTERNATE NAMES PROPERTIES

Tissue-type plasminogen

activator

t-PA A serine protease that catalyzes hydrolysis of

plasminogen at the junction between the N-

terminal heavy chain and C-terminal light chain

N terminus contains two loop structures called

kringles

Urokinase-type

plasminogen activator

u-PA A serine protease

Urokinase-type

plasminogen activatorreceptor

u-PAR Binds to and required for the activity of u-PA

Plasminogen Single-chain plasma glycoprotein with large N-

terminal and small C-terminal domain.

N terminus contains five kringles

Plasmin Fibrinolysin A serine protease

Plasminogen activator

inhibitor-1

PAI-1 A serpin (serine protease inhibitor)

In plasma and platelets

Forms 1:1 complex with t-PA in blood

Plasminogen activator

inhibitor-2

PAI-2 A serpin (serine protease inhibitor)

Detected only in pregnancy

2-Antiplasmin

2-AP A serpin (serine protease inhibitor)

Forms 1:1 complex with plasmin in blood


75/82

Fibrinolysis Cascade

The injured tissues and vascular endothelium release slowlyt-PA and u-PA that enter the fibrin clot and convert

plasminogen into plasmin

Plasmin proteolitically cleaves stable fibrin to fibrin

degradation products, cleared by the macrophages

Plasmin digests some other protein coagulants such asfibrinogen, Factor V, Factor VIII, prothrombin, and Factor XII

(sometimes even causing hypocoagulability of the blood)

Plasmin can degrade fibrinogen, but the reaction remains

localized because

t-PA and u-PA activate plasminogen more effectively when it is

adsorbed to fibrin clots

any free plasmin is complexed with 2-AP

endothelial cells release PAI-1 that blocks t-PA


76/82

Breakdown of Fibrin(ogen)

Following fibrin(ogen)cleavage D, E

fragments are liberated

Plasmin acting on

stable fibrin (covalently

cross-linked fibrinpolymers) releases D-

dimers plasma D-

dimer assay = test of

fibrin degradation

Fibrin(ogen) degradation products have anticoagulant andantiplatelet actions, enhancing the net antithrombotic effect of

fibrinolysis


77/82

Tests for Fibrinolysis

Plasminogen activity assay Euglobulin lysis time; diluted whole blood clot lysis

Plasma fibrinogen

Fibrin split products assay

Plasma D-Dimer assay


78/82

Fibrinolytic Agents

Most currently available thrombolytic agents are plasminogenactivators (PA)

Convert patient plasminogen to plasmin which acts on fibrin

within a thrombus

Additionally can breakdown fibrinogen (fibrinogenolysis)

Therapeutic doses of PA overwhelm PAI-1 and 2-antiplasmin

Beneficial effect is reduction of thrombus size (thrombolysis)

Negative effect is that hemostatic plugs are also lysed

Most commonly used agents are: Streptokinase (SK), Alteplase(tPA), Reteplase (r-PA), and Tenecteplase (TNK-tPA)


79/82

Fibrinolytic Agents

Streptokinase, obtained from cultures of beta-hemolyticstreptococci; complexed with plasminogen can convert other

plasminogen molecules to plasmin; it is not fibrin selective

(produces lysis of fibrinogen and fibrin)

u-PA, native, obtained from human fetal kidney cell cultures;

recombinant; it is not fibrin selective rt-PA, recombinant t-PA, with some fibrinogen selectivity

t-PA variants: recombinant PA (reteplase, r-PA), TNK-rt-PA, with

increased resistance to PAI-1 and a better fibrin selectivity

Newer non-t-PA fibrinolytics: more potent and fibrin-selective

PAs, or direct fibrinolytic (not PA) agents (alfimiprase)


80/82


81/82

Thrombosis: Balance is Disrupted


82/82

Summary

Primary hemostasis, a platelet-dependent process, forms plateletplugs when a vessel is injured

Secondary hemostasis, a coagulation factor-dependent process,

begins with Tissue Factor exposure

Small amounts of thrombin are generated via FXa formation

by the TF:FVIIa complex (Extrinsic Pathway) Sustained thrombin generation depends on FXa formation via

FIXa and FVIIIa-mediated complexes on an activated platelet

surface

Amount of stable fibrin generated dictates bleeding or

thrombotic risk

Documents

Blood Lecture 4