Anticoagulants: Warfarin to DOACs
Anticoagulants prevent thrombus formation by targeting different points in the coagulation cascade. The shift from warfarin to direct oral anticoagulants has simplified management for many patients.
## Coagulation Cascade Overview
The coagulation cascade converges on thrombin (Factor IIa), which converts fibrinogen to fibrin, forming the structural basis of a clot. Anticoagulants target different points: vitamin K-dependent factor synthesis (warfarin), thrombin directly (dabigatran), or Factor Xa (rivaroxaban, apixaban, edoxaban).
## Warfarin
Warfarin inhibits vitamin K epoxide reductase (VKORC1), preventing regeneration of the reduced form of vitamin K needed for gamma-carboxylation of clotting factors II, VII, IX, and X. Because it blocks synthesis of new factors rather than inactivating existing ones, full anticoagulant effect takes 5-7 days.
**Monitoring:** International Normalized Ratio (INR), target typically 2.0-3.0 for atrial fibrillation and VTE (2.5-3.5 for mechanical heart valves).
**Challenges:** Narrow therapeutic index, extensive drug and food interactions (CYP2C9, CYP3A4, vitamin K-rich foods), genetic polymorphisms in VKORC1 and CYP2C9 causing variable dosing requirements. Requires frequent blood tests.
**Reversal:** Vitamin K (phytonadione) for non-emergent reversal. Four-factor prothrombin complex concentrate (4F-PCC) or fresh frozen plasma for urgent bleeding.
## Direct Oral Anticoagulants (DOACs)
| DOAC | Target | Half-Life | Renal Clearance | Reversal Agent |
|------|--------|-----------|----------------|----------------|
| Dabigatran | Thrombin (IIa) | 12-17 h | 80% | Idarucizumab |
| Rivaroxaban | Factor Xa | 5-9 h | 33% | Andexanet alfa |
| Apixaban | Factor Xa | 12 h | 27% | Andexanet alfa |
| Edoxaban | Factor Xa | 10-14 h | 50% | Andexanet alfa |
DOACs offer predictable pharmacokinetics, fixed dosing, no routine monitoring, rapid onset (2-4 hours), fewer food interactions, and lower intracranial hemorrhage risk compared to warfarin.
## Clinical Indications
- **Non-valvular atrial fibrillation** -- DOACs are preferred over warfarin for stroke prevention (DOAC trials: RE-LY, ROCKET-AF, ARISTOTLE, ENGAGE AF).
- **Venous thromboembolism** -- Treatment and extended prophylaxis of DVT and PE. Apixaban and rivaroxaban allow single-drug approaches without initial heparin bridging.
- **Mechanical heart valves** -- Warfarin remains the ONLY option. Dabigatran was tested (RE-ALIGN trial) and showed increased thrombosis and bleeding.
## Choosing Between Agents
- **Renal impairment** -- Apixaban has the lowest renal dependence and is preferred in CKD. Avoid dabigatran with CrCl <30 mL/min.
- **GI bleeding risk** -- Apixaban has the lowest GI bleeding rate among DOACs. Dabigatran and rivaroxaban are associated with higher GI bleeding.
- **Cost and adherence** -- DOACs are more expensive. Twice-daily dosing (dabigatran, apixaban) vs once-daily (rivaroxaban, edoxaban) affects adherence.
## Key Takeaways
- DOACs have replaced warfarin as first-line for non-valvular AF and VTE in most patients.
- Warfarin remains essential for mechanical heart valves and antiphospholipid syndrome.
- Renal function must be assessed before and during DOAC therapy.
- Specific reversal agents exist: idarucizumab for dabigatran, andexanet alfa for factor Xa inhibitors.