1. Clinical Use of Coagulation Inhibitors
Aleksandra Milosevski
Shi Yi Chen
Ya Ping Guo
Vanessa Hoi
PHM Fall 2019
Instructor: Chesa Dojo Soeandy
Coordinator: Jeffrey Henderson

2. Coagulation Cascade Overview-
When a blood vessel is damaged a series of enzymatic reactions known as the coagulation cascade are initiated. The end goal of this cascade is to create a polymer of fibrin proteins in order to form a thrombus, or a blood clot at the site of damage. This cascade is divided into 2 pathways, the extrinsic and intrinsic pathways. These pathways will merge to form the common pathway at which point inactive prothrombin will be cleaved to form active thrombin, the enzyme that is responsible for converting soluble fibrinogen into insoluble fibrin.
Silverthorn, Fig. 16.4

3. Extrinsic Pathway
Tissue Factor (III) → found in tissue underlying blood vessels
Tissue Factor (III) + Active VII Form Complex
End result → X activated
Positive feedback observed
The extrinsic pathway will start when damage to the tissues exposes tissue factor 3, a protein found in the tissue underlying the blood vessel. The tissue factor 3 protein will then activate clotting factor 7 and together will form a complex that will in turn convert inactive clotting factor 10 into active factor 10. The complex will also activate factor 9, which is needed in the intrinsic pathway.
In addition, active factor 10 will activate more factor 7 in a positive feedback fashion.
Silverthorn, Fig. 16.4

4. Intrinsic Pathway Blood vessel damage exposes collagen
Factors XII, XI, IX, VII activated
Factor X also activated
The intrinsic pathway will begin when blood vessel damage exposes collagen. This will set off a series of steps the first being collagen will convert inactive clotting factor 12 to active factor 12. Activated factor 12 will then activate factor 11. Factor 11 will activate factor 9 and factor 8. Factor 9 will then activate factor 10.
Silverthorn, Fig. 16.4

5. Common Pathway Silverthorn, Fig. 16.4
Active Factor X cleaves prothrombin → thrombin
Fibrinogen → Fibrin
Factor XIII also activated
Both the intrinsic and extrinsic pathways lead to factor 10 activation and at this point merge to form the common pathway. Factor 10 will then cleave prothrombin to form thrombin which in turn will cleave fibrinogen to fibrin. Thrombin is also responsible for activating factor 13, which crosslinks the fibrin strands in different places to form a strong mesh like network and ultimately help form the clot. To understand why and when certain anticoagulants are used clinically, it is important to know the mechanism of action and the risks and benefits associated with each, which is what we’ll talk about next.

6. Vitamin K in Coagulation Cascade
Protein carboxylation reaction activates the clotting factors
Coupled to the oxidation of vitamin K
Vitamin K is reused
Must be reduced to reactivate it via VKOR
Warfarin: Vitamin K antagonist
There are many cofactors needed in the coagulation cascade and one of them is vitamin K. It is needed in the synthesis of prothrombin, clotting factor 7, 9, 10 present in both intrinsic and extrinsic pathways.
Silverthorn, Fig. 16.4

7. Vitamin K in Coagulation Cascade
Protein carboxylation reaction activates the clotting factors
Coupled to the oxidation of vitamin K
Vitamin K is reused
Must be reduced to reactivate it via VKOR
Warfarin: Vitamin K antagonist
Inhibit VKOR
Inhibition of VKOR → inhibits reactivation of vitamin K → decreases the synthesis of clotting factors
VKOR
In order to activate the clotting factors, it needs to undergo a carboxylation rxn which is coupled to the oxidation of vitamin K
With limited vitamin K storage capacity in the body, vitamin K is reused through an oxidation-reduction cycle where the vitamin must be reduced to reactivate it by vitamin K epoxide reductase
And warfarin, one of the commonly used drug to prevent blood clot formation is a vitamin K antagonist. It reduces the regeneration of vit K by inhibiting vitamin K epoxide reductase (VKOR) in the liver
Inhibition of VKOR → inhibits reactivation of vitamin K → decreases the synthesis of these clotting factors
Katzung, Fig. 34.6

8. Warfarin: Vitamin K antagonist
Clinical Use:
Prophylaxis and/or treatment of venous thrombosis
Atrial fibrillation
Prosthetic heart valves
Pharmacokinetics:
Completely absorbed after oral administration
Biotransformed in liver and excreted by the kidney
Very little Warfarin is excreted unchanged in the urine → elimination by metabolism
8-12h delay in the action of warfarin
Peak anticoagulant effect may be delayed 3-4 days
Half-life: 20-60h
Over 99% of the drug is bound to plasma albumin
The primary medical use for warfarin is for prevention and treatment of venous thrombosis and it is a long-term therapy for all patients with mechanical heart valves. Since it is metabolized by the liver, patients with renal impairment can take this medication if their liver is healthy.
Warfarin is completely absorbed after oral administration but there is an 8 to 12 hour delay in the action of warfarin and peak anticoagulant effect may be delayed 3-4 days.
The effective half-life of warfarin ranges from 20-60h b/c over 99% of the drug is bound to plasma albumin.

9. Adverse Effects & Warnings/Precautions
Excessive bleeding
Reversed by stopping the drug and administering vitamin K, fresh-frozen plasma
Narrow therapeutic index
Requires frequent monitoring
Dosing is affected by polymorphism of the CYP2C9 enzyme and VKOR
Dose is individualized
Can cross placenta
Cause birth malformations and fatal hemorrhage in fetus
Up to 160 drugs would have potential drug interaction with warfarin such as acetaminophen and amoxicillin (CYP2C9, VKOR)
46 reported to have interactions such as atorvastatin and ranitidine
Warfarin can have dangerous side effects and interactions. One of the side effects is excessive bleeding. But for warfarin, excessive anticoagulant effect and bleeding can be reversed by administering vitamin K or fresh-frozen plasma that contains all coagulation factors.
Warfarin has a very narrow therapeutic index and inherited polymorphisms in CYP2C9 and VKORC1 can have significant effects on warfarin dosing therefore dosing must be individualized and requires routine monitoring,
pregnant women should avoid taking this medication since warfarin can cross the placenta readily and cause hemorrhagic disorders in fetus.
Since up to 160 drugs would have potential drug interactions with warfarin and vitamin K intake will interfere with the effectiveness of warfarin, patients on this medication will have dietary restrictions and drug interaction monitoring.

10. Direct Factor Xa Inhibitors (Rivaroxaban, Apixaban, Edoxaban)
Activation of Factor-X to Factor-Xa (FXa)
FXa and FVa form prothrombinase complex
Converts prothrombin to thrombin
Ultimately increases clot formation
Silverthorn, Fig. 16.4
Perzborn, Fig. 1
Coagulation cascade exhibits amplification
FXa causes an explosive burst of thrombin and clot formation
The next drug is a novel oral anticoagulant
Activation of Factor-X is the start of the common pathway. FXa and FVa form the prothrombinase complex, converting prothrombin to thrombin and increasing clot formation. Factor Xa plays a pivotal role and causes an explosive burst in thrombin due to amplification of the coagulation cascade.
Wheeless, 2015

11. Direct Factor Xa Inhibitors (Rivaroxaban, Apixaban, Edoxaban)
Activation of Factor-X to Factor-Xa (FXa)
FXa and FVa form prothrombinase complex
Converts prothrombin to thrombin
Ultimately increases clot formation
Silverthorn, Fig. 16.4
Perzborn, Fig. 1
Coagulation cascade exhibits amplification
FXa causes an explosive burst of thrombin and clot formation
Inhibition of FXa terminates thrombin generation and decreases coagulation.
Inhibition of FXa terminates thrombin generation, thus decreasing coagulation.
Wheeless, 2015

12. Binds free FXa and FXa bound in Prothombinase.
Rivaroxaban is a highly selective, competitive, direct, antithrombin-independent Factor Xa inhibitor.
Binds free FXa and FXa bound in Prothombinase.
This is exactly what rivaroxaban does; it is a highly selective, competitive, and direct antithrombin-independent Factor-Xa inhibitor. It binds free and prothrombinase-bound FXa.
Perzborn, Fig. 2

13. Indications and Clinical Use
Rivaroxaban 10 mg, 15 mg, and 20 mg is indicated for:
prevention of venous thromboembolic events (VTE) in patients after total hip or knee replacement surgery
treatment of VTE like deep vein thrombosis (DVT) and pulmonary embolism (PE), and prevention of recurrence
prevention of stroke and systemic embolism in patients with non-valvular atrial fibrillation (AF)
No data to support safety and efficacy in patients with prosthetic heart valves
Rivaroxaban 2.5 mg in combination with 75 mg-100 mg acetylsalicylic acid is indicated for:
prevention of stroke, myocardial infarction, and cardiovascular death
prevention of acute limb ischemia and mortality in coronary artery disease (CAD)
Indications and Clinical uses for 10, 15, and 20 mg doses are similar to warfarin which include the prevention and treatment of VTE events like DVTs and PEs, especially after total knee or hip replacement surgeries; as well as prevention of stroke and systemic embolism in patients with non-valvular AF. Unlike warfarin, there have been no studies to support safety and efficacy in patients with prosthetic heart valves.
2.5 mg doses in combination with aspirin was recently indicated for: prevention of stroke, myocardial infarction, and cardiovascular death in patients with coronary artery disease.

14. Pharmacokinetics of Rivaroxaban
Elimination
Approximately 1/3 of dose eliminated as unchanged active drug by kidneys
30% via active renal secretion, transporters involved are P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP)
6% via glomerular filtration
Approximately 2/3 of dose metabolically deactivated
CYP 3A4/5 and CYP2J2 via hepatic oxidative biotransformation
CYP-independent hydrolysis of amide bonds
Resulting metabolites eliminated renally and via the hepatobiliary route
Half-life: 5–9 hours (shorter than warfarin)
Absorption, Bioavailability, Protein Binding, Distribution
High Oral Bioavailability (Foral):
2.5 mg, 5 mg, 10 mg (with or without food) or 20 mg (with food): 80% - 100%
20 mg (without food): 66%
Peak plasma level (t max): 2 – 4 hours
High reversible plasma protein binding
approximately 92–95 % in vitro
mainly bound to serum albumin
Volume of distribution at steady state = 50 L (0.62 L/kg)
low-to-moderate affinity to peripheral tissues
Here are some pharmacokinetic properties: Main things I wanted to point out were that rivaroxaban has high oral bioavailability and plasma protein binding. It has a rapid onset of action and shorter half-life than warfarin.
It’s elimination contrasts warfarin’s, in that 1/3 of a dose is excreted as unchanged active drug by the kidneys, via P-glycoprotein and breast cancer resistance protein.
The rest is metabolically deactivated by CYP 3A4/5 and CYP2J2 via hepatic oxidative biotransformation and CYP- independent hydrolysis

15. Pros & Cons CONS PROS no standardized test should monitoring be needed
wide therapeutic window and predictable pharmacokinetics
fixed-dose oral administration regimens that do not require monitoring
limited known drug interactions and no known dietary restrictions
Andexanet, a FXa decoy, recently approved as antidote
CONS
no standardized test should monitoring be needed
expensive
PROS of using Rivaroxaban are that:
It has a wide therapeutic window and predictable pharmacokinetics
It follows fixed-dose oral administration regimens that do not require monitoring
It has limited known drug interactions and no known dietary restrictions and
Andexanet, a FXa decoy has also been recently approved as an antidote
Some CONS are that:
There exists no standardized test should monitoring be needed and it is expensive

16. Adverse Reactions/Contraindications/Warnings
increased risk of bleeding which can result in weakness, paleness, dizziness, or swelling and secondary complications like renal failure
Contraindications
no data to support use in children; pregnant or nursing women; patients with prosthetic heart valves; patients with clinically significant bleeding (ulcers, cerebral infarction etc.)
Warnings and Precautions
concomitant use with strong inhibitors of CYP3A4 and P-glycoprotein (ketoconazole, itraconazole, posaconazole, ritonavir)
concomitant use with any other hemostasis-affecting drugs (non-steroidal anti-inflammatory drugs, acetylsalicylic acid, platelet aggregation inhibitors or selective serotonin reuptake inhibitors, and serotonin norepinephrine reuptake inhibitors)
hepatic disease and renal impairment; must first determine creatinine clearance in all patients
Furthermore, Adverse Reactions include increased risk of bleeding which can result in weakness, paleness, dizziness, or swelling and secondary complications like renal failure
There is no data to support use in children, and pregnant, or nursing women; and those with clinically significant bleeding
Lastly, warning and precautions should be taken for patients who concomitantly use strong inhibitors of CYP3A4 and P-glycoprotein or any other hemostasis-affecting drugs; AND in those with hepatic disease and renal impairment: you must first determine creatinine clearance in all patients

17. Heparin Mixture of mucopolysaccharides Co-factor of anti-thrombin
Target: Thrombin
The last coagulation inhibitor is heparin. It is a mixture of mucopolysaccharides that are naturally produced in our body to stop blood clot. It works with anti-thrombin to inactivate thrombin within the common pathway. To understand heparin, we must first understand anti-thrombin.
Silverthorn, Fig. 16.4

18. Heparin: Mechanism of Action
Enhance Anti-Thrombin Activity by 1000 times
Inhibit thrombin
Inhibit factor X
Anti-Thrombin
Slow
Anti-thrombin itself can bind to thrombin and inactivate it. Therefore, stop the blood from clotting. Without heparin, this reaction only happens slowly
Silverthorn, Fig. 16.4

19. Heparin: Mechanism of Action
Enhance Anti-Thrombin Activity by 1000 times
Inhibit thrombin
Inhibit factor X
Heparin
Anti-Thrombin
with heparin, the reaction is accelerated by 1000 times. Because heparin can bind to anti-thrombin, induce a conformational change, and expose its active site for a rapid reaction
Silverthorn, Fig. 16.4

20. Heparin: Mechanism of Action
Enhance Anti-Thrombin by 1000 times
Inhibit thrombin
Inhibit factor X
Heparin
Anti-Thrombin
Besides, heparin can also work with anti-thrombin to inactivate factor ten.
Silverthorn, Fig. 16.4

21. Heparin: Clinical Use Administration and onset Half-life: 90 mins
IV: immediate
Deep subcutaneous: might be delayed mins
No oral: Negatively charged and large
Half-life: 90 mins
Commonly used for
Cardiac surgery
Prevention of venous thromboembolism
The onset of action is immediate after IV injection but can be delayed following subcutaneous injection. Due to the negative charge and large molecular size, it cannot pass through the oral route. Heparin can be used in some cardiac surgery to prevent venous thromboembolism

22. Heparin: pros and cons Advantages Disadvantages Safe for pregnancy
Antidote available: Protamine
Protamine: positively charged
Heparin: negatively charged
Adverse effect
Bleeding
Platelet deficiency
Monitoring required
Effect depending on [anti-thrombin]
No anti-thrombin, no action
One of the advantages is that it is safe to be used during pregnancy because it cannot pass through placenta. There is also an antidote available. Protamine is a positively charged protein. It strongly binds to the negatively charged heparin and inactivates it. The downside of heparin would be the possible adverse effects. It could cause bleeding and platelet deficiency. Therefore, patients must be closely monitored after administration of heparin.
Heparin has another problem too. It must work with anti-thrombin III to inhibit coagulation. If the patient has low anti-thrombin III level, its effect would be limited.

23. Heparin: LMWH Low molecular weight heparin (LMWH)
Fragmented heparin
Weaker anticoagulation
CANNOT inactivate thrombin
CAN inactivate Factor X c
Heparin c
Anti-Thrombin
Heparin can also come in a fragmented form. It is called low molecular weight heparin (LMWH). The reason to cut it down is to get weaker anticoagulation.
Silverthorn, Fig. 16.4

24. Heparin: LMWH only Low molecular weight heparin (LMWH)
Fragmented heparin
Weaker anticoagulation
CANNOT inactivate thrombin
CAN inactivate Factor X c
only
Heparin
Anti-Thrombin
The shorter version cannot stabilize the anti-thrombin - thrombin complex and therefore cannot inactivate thrombin. However, it can still inactivate factor ten.
Silverthorn, Fig. 16.4

25. Heparin: LMWH vs. unfractionated
More predictable pharmacokinetic
No monitoring is required
Different onset and half-life: not interchangeable
Unfractionated
Low molecular weight
Onset
Immediate after IV;
20-60 after deep subcutaneous injection
3-6 hours after subcutaneous injection
Half-life
About 90 mins
Varies between agents:
Eg. Dalteparin 3-4 hours
This selective inactivation allows its pharmacokinetics to be more predictable. Therefore, no monitoring is required, although bleeding is still a possible adverse effect. We also need to keep in mind that the onset and half-life are both very different then the unfractionated heparin. They are not interchangeable.

26. Summary Warfarin Direct Factor Xa Inhibitor (Rivaroxaban) Heparin
In summary, anticoagulants have no direct effect on an established thrombus, nor do they reverse ischemic tissue damage. However, once a thrombus has occurred, the goal of anticoagulant treatment is to prevent further extension of the formed clot and prevent complications.
Today we talked about 3 of many anticoagulants, which we have summarized in this table.

27. Summary Drug Class Examples Use Mechanism of Action Pros Cons
Vitamin K Antagonist
Warfarin
Prevention and treatment of venous thromboembolic events (deep vein thrombosis, pulmonary embolism)
Vitamin K antagonist
Decreases the synthesis of clotting factor II (prothrombin) VII, IX, X
Cheap
Antidote available (vitamin K, fresh-frozen plasma)
Narrow therapeutic index → requires frequent monitoring
Can cross placenta → cause birth malformations and fatal hemorrhage in fetus
Require dietary restrictions and drug interaction monitoring
Direct Factor Xa Inhibitor
Rivaroxaban
Apixaban
Edoxaban
Prevention of stroke
Prevention of heart attacks
People with atrial fibrillation
People with mechanical heart valves (warfarin)
Competitive, direct, antithrombin-independent inhibition of Factor Xa
Inhibits cleavage of prothrombin to thrombin to terminate thrombin burst and therefore decrease coagulation
Wide therapeutic window, predictable pharmacokinetics
No monitoring required
Limited drug interactions and dietary restrictions and
Antidote available (Andexanet, FXa decoy)
No standardized test should monitoring be needed
Expensive
Increased risk of bleeding and secondary complications
Not for use in children, pregnant women, those with renal or hepatic impairment
Heparin
Unfractionated heparin, LMWH
In cardiac surgery (heparin)
Enhanced Anti-thrombin inhibition of thrombin and factor X
Safe for pregnancy
Antidote available (Protamine, cationic peptide)
Monitoring required
Increased risk of bleeding and platelet deficiency
Antithrombin-dependent

28. References
CPS [Internet]. Ottawa (ON): Canadian Pharmacists Association; c2016 [updated 2018 SEP 04; cited 2019 SEP 16]. Coumadin [product monograph]. Available from: or
CPS [internet]. Ottawa (ON): Canadian Pharmacists Association; c2016 [updated 2016 DEC; cited 2019 SEP 16]. Heparin: unfractionated. Available from: or
CPS [internet]. Ottawa (ON): Canadian Pharmacists Association; c2016 [updated 2015 JUN; cited 2019 SEP 16]. Heparin: low molecular weight. Available from: or
CPS [Internet]. Ottawa (ON): Canadian Pharmacists Association; c2016 [updated 2018 SEP 18; cited 2019 SEP 16]. Xarelto [product monograph]. Available from: or
Katzung, B.G. (2018). Basic and Clinical Pharmacology, 14th Edition. McGraw Hill Education.
Perzborn Elisabeth, Roehrig Susanne, Straub Alexander, Kubitza Dagmar, Mueck Wolfgang, and Laux Volker. “Rivaroxaban: A New Oral Factor Xa Inhibitor” Arteriosclerosis, Thrombosis, and Vascular Biology (2010): 376–381. Web. 16 Sep
Silverthorn, D.U. (2012). Human Physiology: An Integrated Approach, 8th Edition. Pearson Education.