Anti-thrombotic Dr. Roane

Question 1

How do platelets become activated?

Answer 1

Willebrand Factor

Plasma/tissue/platelet protein binding to platelets and collagen

Question 2

What is released upon platelet activation?

Answer 2

-ADP -> activates more platelets
-TXA2 (thromboxane) -> activates more platelets
-serotonin (increases muscle tone -> vasoconstriction)

-> Attraction and activation of other platelets

Question 3

Function of fibrin

Answer 3

-coating aggregated platelets
-binding platelets together in a plug

Question 4

How does the platelet activation cascade get deactivated?

Answer 4

-by undamaged adjacent cells
-PGI2 (prostacyclin) and NO (nitric oxide

Question 5

Intrinsic Pathway

Clotting pathway

Answer 5

12 - 11 - 9 - 10 - Prothrombin to Thrombin

Vessel damage -> Collagen exposed
Collagen on 12: 12 to 12a
12a activates 11: 11 to 11a
11a activates 9: 9 to 9a
9a activates 10: 10 to 10a
10a activates: Prothrombin: to Thrombin

Thrombin: positive loop to 5, 8, and 11

Question 6

Extrinsic pathway

Clotting pathway

Answer 6

Vessel damage -> subendothelial exposure to blood -> Tissue factor activates 7: 7 to 7a
7a activates 9: 9 to 9a
9a activates 10: 10 to 10a
10a activates: Prothrombin: to Thrombin

7a also activates 10 directly: 10 to 10a

Thrombin: positive loop to 5, 8, and 11

(8 helps 9, 5 helps 10)

Question 7

What is the role of Thrombin?

Answer 7

-Thrombin can also activate Fibrin:
Fibrinogen to loose Fibrin

-Thrombin activates 13: 13 to 13a
13a activates loose fibrin: loose fibrin to stabilized fibrin (bind platelets together)

-Thrombin acting in a positive loop to further stimulate the clotting cascade

Question 8

Function of Antithrombin III

Answer 8

-turns off the intrinsic pathway (12-11-9-10): acts on 12a, 11a, 9a and 10a

-turns off the common pathway: 10a and Thrombin

Question 9

Where is Antithrombin III synthesized?

Answer 9

-in the liver
-naturally occurring
plasma protein
-Human recombinant ATIII is
available as the drug

Question 10

What is the indirect thrombin inhibitor?

Answer 10

Heparin (naturally polysaccharid), variable lenght of chains

-Unfractionated heparin (UFH): all chain
-Low molecular weight heparin (LMWH) - short chain

Question 11

Which form of heparin is known to be very short?

Answer 11

Fondaparinux (Arixtra)

Question 12

How does Heparin work?

Answer 12

it attaches Antithrombin III to Thrombin

-Antithrombin turns Thrombin off and terminates the clotting pathway

Question 13

half-life, onset, formulation

Answer 13

-injectable
-short-halflife
-quick onset
-very potent (can also be turned off quickly)

Question 14

What do all heparin forms have in common?

Answer 14

all contain a penta(5)saccharide sequence

Question 15

How are the heparin forms different?

Answer 15

-vary in the length of the GAG tail

-UFH: has a long GAG + penta(5)saccharide
long links Antithrombin and thrombin
links antithrombin and factor 10a (also 12a, 11a, 9a)

-LMWH: has a short GAG + penta(5)saccharide
links antithrombin and 10a

-Fondaparinux has no GAG, only the penta(5)saccharide
links antithrombin and 10a

Question 16

How is the short form of heparin different in therapy?

Answer 16

they only link ATIII with 10a, so not as potent, lower risk of causing bleeding

Question 17

Low molecular weight heparin drugs

Answer 17

-parin
Enoxaparin, Dalteparin, Tinzaparin

-short form of heparin
-activity at Xa and Thrombin 2a?
-t1/2 of 4h, renal clearance !!!

Question 18

Side effects

Answer 18

-Bleeding
-platelet activation and clotting (paradox)
-Thrombocytopenia (low number of platelets bc destroyed by immune cells or used up heparin-induced thrombocytopenia)
-LMWH can cause kidney damage: contraindicated in GFR < 30 ml/min

Question 19

What are the antidotes of heparin?

Answer 19

-for UFH: Protamine sulfate

-for Fondaparinux: PCC (prothrombin complex concentrate)

Question 20

Which form of heparin is preferred to spare the kidneys?

Answer 20

-UFH (long chain form) bc of the short half-life

Question 21

MOA of Warfarin

Answer 21

-factors 7, 9,10 (extrinsic), and 2 (Prothrombin) need the reduced form of Vitamin K and carboxylation to be activated

-Warfarin blocks the enzyme VKORC1: reduction of Vitamin K

-Warfarin is a competitive inhibitor of Vitamin K, with enough Vitamin K intake, the potency of warfarin is lowered

Question 22

Warfarin pharmacokinetic profile

Answer 22

100% oral bioavailability
99% protein bound to albumin (watch out for other protein binders -> displace warfarin from albumin and increases warfarin’s potency -> bleeding)
-long half-life, long onset
-racemic mixture: L-isomer is 4x potent

Question 23

What is the antidote of warfarin?

Answer 23

-PCC (prothrombin complex concentrate)
-also the antidote of Fondaparinux

Question 24

What are the available clotting tests?

Answer 24

-PT: prothrombin time (12sec): evaluates the extrinsic and common pathway (so it contains the tissue factor)

-INR: international normalized ratio: more accurate and useful for patients taking Vitamin K antagonists (like warfarin) -> bc Vitamin K needed to build Prothrombin

-aPTT: activated partial thrombin time (30-45 sec): evaluates the intrinsic and common pathway
more sensitive to patients on heparin
-> bc heparin binds to the factors in the intrinsic pathway and thrombin

Question 25

Name the oral Factor Xa inhibitors

Answer 25

-Rivaroxaban (Xarelto) -Apixaban (Eliquis) -Edoxaban (Savaysa) -don't need monitoring (like heparin or warfarin) -Most seen in DVT prevention

Question 26

What is the antidote for Factor Xa inhibitors?

Answer 26

-Andexanet alfa (recombinant gene product) -injected -decoy binding site for Rivaroxaban, Apixaban, Edoxaban -> the drugs will bind andexanet alfa instead of Factor Xa

Question 27

Direct thrombin inhibitor (factor 2a)

Answer 27

-block the thrombin catalytic site and the substrate binding site on thrombin -> blocks the conversion of Fibrinogen to loose fibrin and to stabilized fibrin -Hirudin -Bivalirudin -quick onset, short-acting, safer than heparin

Question 28

What are the large molecule thrombin inhibitors?

Answer 28

-Hirudin -Bivalirudin -quick onset, short-acting, safer than heparin

Question 29

What are the small molecule thrombin inhibitors?

Answer 29

-Argatroban, IV (contraindicated in hepatic dysfunction) -Dabigatran, PO: slow onset long half-life - renal elimination

Question 30

What is the reversal agent Dabigatran?

Answer 30

Idarucizumab

Question 31

Thrombolytics

Answer 31

-TA: tissue plasminogen activator -convert Plasminogen to Plasmin (Clotbuster, cuts clots) -Streptokinase -Urokinase -Altepase (t-PA) -Reteplase -Tenecteplase

Question 32

Where are thrombolytics used?

Answer 32

-to remove clots -MI -ischemic stroke

Question 33

Platelet activation

Answer 33

-exposure of collagen -> GPV1 protein on moving platelets bind to collagen Activation of: -COX-1 -> TxA2 (thromboxane): recruits other platelets -GPIIb/IIa: activates fibrinogen receptor for platelet clotting -ADP: stimulates GPIIb/IIa and TxA2 in other recruited platelets -Serotonin: vasoconstriction

Question 34

What is the receptor on platelets that respond to soluble thrombin (IIa)?

Answer 34

PAR-1/PAR-4 -thrombin binds to PAR-1/PAR-4 for platelet recruiting and activation via TxA2 -blocked by the drug: Vorapaxar

Question 35

Which molecules inhibit platelet aggregation?

Answer 35

-PGI2 (prostacyclin) -NO (nitric oxide) -from adjacent endothelial cells

Question 36

Antiplatelet drugs

Answer 36

-Aspirin (blocks irreversibly COX-1 -> no platelet recruiting) -Vorapaxar (Zontivity) (blocks thrombin from binding to thrombin receptor PAR-1/PAR-4 -> no platelet recruiting via TxA2) -Clopidogrel (prodrug CYP2C19) -Prasugrel (prodrug) -Cangrelor (short-acting) -Ticagrelor (reversible, AE: dyspnea) -> blocking ADP receptor irreversibly -Abciximab -Eptifibatide -Tirofiban -> Blocking GPIIa/IIb for platelet fusion

Question 37

Why are high doses of aspirin harmful?

Answer 37

-because it not only blocks COX-1 but also inhibits PGI2 (prostacyclin) which stops the platelet cascade

Question 38

Which drug is used to stop bleeding? MOA

Answer 38

-Amino caproic acid (Amicar) -used esp after surgery -binds to Plasminogen, inhibits conversion to active Plasmin (inhibits the "clot cutter")

Question 39

Treatment of the inherited bleeding disorder Hemophilia

Answer 39

Hemophilia A (missing factor 8) Hemophilia B (missing factor 9) -administration of the missing factor