8.3 Venous Thromboembolism & Anticoagulation Therapy (DOACs, Warfarin & Thrombosis Canada)

8.3 Venous Thromboembolism & Anticoagulation Therapy (DOACs, Warfarin & Thrombosis Canada)

Venous thromboembolism (VTE)—encompassing deep vein thrombosis (DVT) and pulmonary embolism (PE)—is the third leading cause of cardiovascular death globally after myocardial infarction and stroke. The classic pathophysiological framework governing venous thrombus formation is Virchow's Triad: endothelial injury, stasis of blood flow, and systemic hypercoagulability.

In Canadian pharmacy practice, anticoagulant stewardship is a critical clinical competency. Pharmacists must navigate the selection, dosing, laboratory assessment, drug-drug interactions, and emergency reversal of Direct Oral Anticoagulants (DOACs), Vitamin K antagonists (Warfarin), and parenteral agents in compliance with Thrombosis Canada clinical guides.

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Venous Thromboembolism (VTE): Diagnosis and Duration of Therapy

1. Clinical Presentation and Diagnostic Triage

• Deep Vein Thrombosis (DVT): Unilateral lower extremity pain, erythema, warmth, and calf swelling (measured > 3 cm difference in circumference below the tibial tuberosity). Diagnostic pathway involves calculating the Wells DVT Score:
  • Low probability --> High-sensitivity D-dimer test. A negative D-dimer safely excludes DVT (high negative predictive value > 98%).
  • High probability or positive D-dimer --> Venous compression ultrasonography (CUS) of the lower extremity.
• Pulmonary Embolism (PE): Acute-onset dyspnea, pleuritic chest pain, tachypnea, tachycardia, cough, hemoptysis, and in severe cases, syncope or hemodynamic collapse. Diagnostic testing includes Wells PE score, D-dimer, and CT Pulmonary Angiography (CTPA) (or Ventilation-Perfusion [V/Q] scan if severe renal impairment or contrast allergy).

2. Duration of Anticoagulation Therapy (Thrombosis Canada)

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Direct Oral Anticoagulants (DOACs): First-Line Pharmacotherapy

Thrombosis Canada and major international guidelines establish DOACs as preferred first-line therapy over warfarin for both non-valvular atrial fibrillation and acute VTE. In pivotal Phase III clinical trials, DOACs demonstrated non-inferiority for thromboembolism prevention and a ~50% relative risk reduction in fatal intracranial hemorrhage (ICH) compared to warfarin.

DOAC Advantages:

• Predictable pharmacokinetics and pharmacodynamics; wide therapeutic window.
• Rapid onset of full therapeutic anticoagulation (1 to 4 hours post-dose).
• Elimination half-lives of 8 to 14 hours, facilitating rapid offset before elective surgery.
• Fixed oral dosing without routine coagulation laboratory monitoring.
• Substantially fewer dietary and pharmacokinetic drug interactions than warfarin.

                      DOAC SUMMARY & DOSING COMPARISON
 ┌─────────────────────────────────────────────────────────────────────────────┐
 │ 1. APIXABAN (Eliquis) - Factor Xa Inhibitor                                 │
 │    • Acute VTE: 10 mg PO BID x 7 days, then 5 mg PO BID                     │
 │    • Extended VTE Secondary Prevention (> 6 months): 2.5 mg PO BID          │
 │    • Non-Valvular AF: 5 mg PO BID (Reduce to 2.5 mg BID if >= 2 ABC criteria)│
 │    • Renal Elimination: ~27% (Lowest among DOACs)                           │
 ├─────────────────────────────────────────────────────────────────────────────┤
 │ 2. RIVAROXABAN (Xarelto) - Factor Xa Inhibitor                              │
 │    • Acute VTE: 15 mg PO BID WITH FOOD x 21 days, then 20 mg PO daily WITH FOOD│
 │    • Non-Valvular AF: 20 mg PO daily WITH FOOD (15 mg if CrCl 30-49 mL/min) │
 │    • Renal Elimination: ~36% (active) + 30% (metabolites)                   │
 ├─────────────────────────────────────────────────────────────────────────────┤
 │ 3. DABIGATRAN (Pradaxa) - Direct Thrombin (Factor IIa) Inhibitor            │
 │    • Acute VTE: Requires 5-10 days PARENTERAL LEAD-IN (LMWH), then 150 mg BID│
 │    • Non-Valvular AF: 150 mg PO BID (110 mg BID if age >= 80)               │
 │    • Storage: Keep in original bottle with desiccant (stable 120 days)      │
 │    • Renal Elimination: ~80% (Highest among DOACs; avoid if CrCl < 30)      │
 ├─────────────────────────────────────────────────────────────────────────────┤
 │ 4. EDOXABAN (Lixiana) - Factor Xa Inhibitor                                 │
 │    • Acute VTE: Requires 5-10 days PARENTERAL LEAD-IN (LMWH), then 60 mg daily│
 │    • Non-Valvular AF: 60 mg daily; 30 mg if CrCl 30-50 (avoid if CrCl <30)  │
 │    • Renal Elimination: ~50%                                                │
 └─────────────────────────────────────────────────────────────────────────────┘

1. Apixaban (Eliquis)

• Mechanism: Direct, selective, reversible inhibitor of free and clot-bound Factor Xa.
• Acute VTE Dosing: 10 mg PO BID for the first 7 days, followed immediately by 5 mg PO BID. No parenteral lead-in is required.
• Extended VTE Prophylaxis: After completing at least 6 months of therapeutic treatment for unprovoked VTE, the dose may be reduced to 2.5 mg PO BID for extended secondary prevention (AMPLIFY-EXT trial).
• Non-Valvular AF Dosing & The 'ABC' Dose-Reduction Rule:
  • Standard dose: 5 mg PO BID.
  • Dose reduction to 2.5 mg PO BID is indicated ONLY IF the patient meets at least TWO of the following THREE criteria:
    1. Age >= 80 years
    2. Body weight <= 60 kg
    3. Serum Creatinine >= 133 umol/L (1.5 mg/dL)

• Renal Handling: Only ~27% renally cleared. Among all DOACs, apixaban is the least dependent on renal elimination and is the safest option in mild-to-moderate chronic kidney disease.

2. Rivaroxaban (Xarelto)

• Mechanism: Direct, competitive Factor Xa inhibitor.
• Acute VTE Dosing: 15 mg PO BID WITH FOOD for 21 days (3 weeks), followed by 20 mg PO once daily WITH FOOD. No parenteral lead-in is required.
• Non-Valvular AF Dosing: 20 mg PO once daily WITH FOOD. In patients with moderate renal impairment (CrCl 30 to 49 mL/min), reduce dose to 15 mg PO once daily WITH FOOD.

3. Dabigatran Etexilate (Pradaxa)

• Mechanism: Oral prodrug converted by plasma and hepatic esterases to dabigatran, a direct, competitive, reversible inhibitor of free and clot-bound thrombin (Factor IIa).
• Acute VTE Protocol: CANNOT be initiated as initial monotherapy. The patient MUST receive at least 5 to 10 days of therapeutic parenteral anticoagulation (typically therapeutic LMWH such as enoxaparin 1 mg/kg SC BID or tinzaparin 175 units/kg SC daily) prior to starting oral dabigatran 150 mg PO BID (RE-COVER trials).
• Non-Valvular AF Dosing: 150 mg PO BID in patients aged < 80 years; reduced to 110 mg PO BID if age >= 80 years or age >= 75 with high bleeding risk (RE-LY trial).
• Formulation & Stability Counselling:
  • Pellets contain a tartaric acid core to provide an acidic microenvironment for intestinal absorption, frequently causing dyspepsia, heartburn, and epigastric discomfort in up to 10% of patients (advise taking with a full glass of water or food).
  • Swallow capsules whole: Capsules must never be opened, chewed, crushed, or sprinkled on food. Opening the capsule increases oral bioavailability by up to 75%, dramatically escalating fatal bleeding risk!
  • Moisture Sensitivity: Dabigatran capsules are hygroscopic. They must be dispensed and stored in their original manufacturer bottle with the desiccant-lined cap (or original blister pack) and must be discarded 120 days (4 months) after the bottle is first opened.
• Renal Handling: Approximately 80% renally eliminated (highest of all DOACs). Contraindicated in Canada if CrCl < 30 mL/min.

4. Edoxaban (Lixiana)

• Mechanism: Direct Factor Xa inhibitor.
• Acute VTE Protocol: Similar to dabigatran, edoxaban requires a 5 to 10 day initial parenteral anticoagulation lead-in (Hokusai-VTE trial) prior to starting edoxaban 60 mg PO once daily.
• Dose Reduction Criteria: Reduce dose to 30 mg PO once daily if creatinine clearance is 30 to 50 mL/min, body weight is 60 kg or less, or the patient takes certain P-glycoprotein inhibitors (cyclosporine, dronedarone, erythromycin, ketoconazole). The Canadian monograph does not recommend edoxaban when CrCl is below 30 mL/min. Thrombosis Canada's clinical guide and the European and US labels instead allow 30 mg down to a CrCl of 15 mL/min, so read which source an item cites; with no source given, use the Canadian monograph.

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Warfarin (Coumadin) Pharmacotherapy

Warfarin remains the anticoagulant of choice for patients with mechanical prosthetic heart valves, moderate-to-severe rheumatic mitral stenosis, severe antiphospholipid antibody syndrome (triple-positive), and end-stage renal disease on hemodialysis.

1. Mechanism of Action

Warfarin inhibits the enzyme Vitamin K Epoxide Reductase Complex 1 (VKORC1), blocking the recycling of oxidized vitamin K to its active hydroquinone form. This prevents the vitamin K-dependent gamma-carboxylation of glutamic acid residues on:

• Procoagulant Clotting Factors: Factor II (Prothrombin), Factor VII, Factor IX, and Factor X.
• Natural Anticoagulant Regulatory Proteins: Protein C and Protein S.

                    COAGULATION FACTOR HALF-LIVES
 ┌───────────────────────────┬──────────────┬──────────────────────────────────┐
 │ Clotting Factor / Protein │ Half-life    │ Functional Significance          │
 ├───────────────────────────┼──────────────┼──────────────────────────────────┤
 │ Factor VII                │ ~ 6 hours    │ Drops first; elevates INR early  │
 │ Protein C (Anticoagulant) │ ~ 8 hours    │ Depletes early -> HYPERCOAGULABLE│
 │ Protein S (Anticoagulant) │ ~ 30 hours   │ Intermediate depletion           │
 │ Factor IX                 │ ~ 24 hours   │ Moderate clearance               │
 │ Factor X                  │ ~ 36 hours   │ Slower clearance                 │
 │ Factor II (Prothrombin)   │ ~ 60-72 hrs  │ TRUE antithrombotic protection   │
 └───────────────────────────┴──────────────┴──────────────────────────────────┘

2. The Bridging Mandate and Transient Hypercoagulable State

Because Protein C has a very short half-life (~8 hours), its levels plummet rapidly during the first 24 to 48 hours of warfarin therapy. In contrast, Factor II (prothrombin) has a half-life of 60 to 72 hours and remains functionally active for several days. This mismatch creates a transient prothrombotic / hypercoagulable state during the first 2 to 3 days of therapy, which can precipitate catastrophic microvascular thrombosis and warfarin-induced skin necrosis.

3. Therapeutic INR Targets

• Non-Valvular Atrial Fibrillation, DVT, PE, Bioprosthetic Valves, Aortic Mechanical Valve (low risk): INR 2.0 to 3.0 (Target: 2.5).
• Mechanical Mitral Valve, High-Risk Mechanical Aortic Valve, or Antiphospholipid Syndrome with Arterial Thrombi: INR 2.5 to 3.5 (Target: 3.0).

4. Significant Warfarin Drug-Drug Interactions

Warfarin is administered as a racemic mixture. The S-enantiomer is 3 to 5 times more potent than the R-enantiomer and is cleared almost exclusively via hepatic CYP2C9. The R-enantiomer is cleared via CYP1A2 and CYP3A4.

• Potentiators (Markedly INCREASE INR & Bleeding Risk):
  • The 'FAB-4' / 'F-A-M-S' Group: Fluconazole (and azole antifungals), Amiodarone, Metronidazole, and Sulfamethoxazole/Trimethoprim (Septra/Bactrim). These drugs potently inhibit CYP2C9, precipitating dramatic INR surges into the 6.0 to 15.0+ range within days.
  • Broad-spectrum antibiotics (cephalosporins, fluoroquinolones, macrolides) deplete gut flora that synthesize vitamin K2 and inhibit warfarin metabolism.
• Inhibitors / Inducers (Markedly DECREASE INR & Precipitate Clots):
  • Potent CYP inducers: Rifampin, Carbamazepine, Phenobarbital, Phenytoin, and St. John's Wort. These accelerate CYP2C9 and CYP3A4 transcription, causing therapeutic failure and subtherapeutic INRs (< 1.5).
• Dietary Vitamin K: Foods high in vitamin K (dark green leafy vegetables: spinach, kale, collard greens, broccoli, Brussels sprouts) competitively counteract warfarin. Patients should maintain a consistent dietary vitamin K intake rather than strictly abstaining.

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Managing Elevated INR, DOAC Reversal & Emergency Bleeding

Clinical guidelines from Thrombosis Canada and the American College of Chest Physicians (CHEST) outline structured protocols for elevated INRs and anticoagulant-induced hemorrhage:

1. Warfarin: Managing Elevated INR in Non-Bleeding Outpatients

2. Emergency Reversal of Warfarin in Major / Life-Threatening Bleeding

In any patient with major, life-threatening hemorrhage (e.g., intracranial hemorrhage, massive gastrointestinal bleeding) or requiring emergency surgery within 6 hours, regardless of the exact INR value:

1. Withhold warfarin immediately.
2. Administer 4-Factor Prothrombin Complex Concentrate (4F-PCC: Octaplex or Beriplex) at a dose of 25 to 50 units/kg IV (or fixed-dose protocols 1000 to 2000 units). 4F-PCC contains concentrated human clotting factors II, VII, IX, and X, restoring normal hemostasis within 10 to 15 minutes.
3. Administer Vitamin K1 (phytonadione) 5 to 10 mg via slow IV infusion (infused over 20 to 30 minutes to minimize anaphylactoid risk). Because the infused 4F-PCC factors have half-lives of 6 to 24 hours, IV vitamin K1 is mandatory to stimulate ongoing endogenous hepatic clotting factor synthesis.
4. Clinical Note: Fresh Frozen Plasma (FFP) is obsolete and second-line; it requires hours to thaw and crossmatch, and infuses large fluid volumes (1 to 2 L) that frequently precipitate acute pulmonary edema (TACO).

3. Emergency Reversal of Direct Oral Anticoagulants (DOACs)

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Clinical Case Scenario: Managing Suspected Drug-Drug Interaction on Warfarin

A 72-year-old female with a mechanical mitral valve replacement is maintained on warfarin 4 mg daily with a stable therapeutic INR between 2.5 and 3.5 (last INR was 2.8 two weeks ago). She visits a walk-in clinic for severe dental pain and facial swelling and is diagnosed with an acute odontogenic abscess. The walk-in physician prescribes metronidazole 500 mg PO TID and amoxicillin 500 mg PO TID for 7 days. She presents the prescriptions to her community pharmacy.

Pharmacist Clinical Assessment & Intervention:

1. Identify the Severe Drug Interaction: Metronidazole is a potent inhibitor of hepatic CYP2C9. Co-administering metronidazole with warfarin drastically reduces the clearance of S-warfarin, typically causing the INR to surge to > 7.0 to 10.0 within 3 to 5 days, posing a severe risk of fatal intracranial or gastrointestinal hemorrhage.
2. Prescriber Collaboration: The pharmacist immediately contacts the dentist/physician. For dental abscess, amoxicillin-clavulanate (or clindamycin in penicillin-allergic patients) can provide adequate anaerobic and gram-positive coverage without the extreme CYP2C9 inhibition seen with metronidazole.
3. If Metronidazole Cannot Be Avoided: If metronidazole must be used, the pharmacist must establish an aggressive monitoring plan: empirically reduce the warfarin dose by 30% to 50% starting on Day 1 of antibiotic therapy, check an INR at Day 3 and Day 5, and readjust dosing based on results.