Warfarin Sodium / Acenocoumarol

Mechanism

Mechanism of Action

VKAs are oral anticoagulants that act by indirectly interfering with the hepatic synthesis of biologically active coagulation factors.

Cellular Physiology: Inhibition of Coagulation-Factor Carboxylation

1. Coagulation factors II (Thrombin), VII, IX, and Xand regulatory proteins Protein C and Protein Srequire a post-translational reaction for function: γ-carboxylation of glutamate (Glu) residues to γ-carboxyglutamate (Gla) in their amino-terminal domains.
2. Hepatic carboxylase catalyses γ-carboxylation using reduced vitamin K (vitamin K hydroquinone) as an essential cofactor.
3. During the reaction, reduced vitamin K is oxidized to vitamin K epoxide. To maintain the cycle, the enzyme Vitamin K Epoxide Reductase (VKORC1) must catalyze reduction of vitamin K epoxide back to its active form.
4. Enzymatic inhibition: Warfarin and acenocoumarol competitively inhibit subunit 1 of the VKORC1. This depletes intracellular reduced vitamin K, leading to hepatic secretion of nonfunctional (undercarboxylated) factors known as PIVKAs (Proteins Induced by Vitamin K Absence), which cannot bind phospholipid surfaces through calcium ions.

Pharmacokinetics

Pharmacokinetics

• Route of administration: Oral administration (rapid, complete absorption).
• Bioavailability (F): > 95%. Peak plasma concentration reached in 4 hours for warfarin.
• Distribution: Small apparent volume of distribution (approximately 0.14 L/kg). High plasma protein binding, predominantly albumin (99%), allows displacement interactions.
• Metabolism: Predominantly hepatic. Warfarin is a racemic mixture of R and S enantiomers. The S-warfarin is 3 to 5 times more potent than than R-warfarin and is selectively metabolized in the liver by the enzyme CYP2C9 of cytochrome P450. The R-enantiomer is metabolized by CYP1A2 and CYP3A4.
• Excretion: Almost exclusively renal elimination of inactive metabolites formed by hepatic metabolism.
• Half-life (t_1/2): Warfarin approximately 36 to 42 hours; acenocoumarol has a shorter half-life of 8 to 12 hours.

Indicators and dose

Indications

• Treatment and secondary prevention of DVT and PE.
• Prevention of systemic thromboembolic events in patients with nonvalvular or valvular atrial fibrillation.
• Mandatory treatment of choice in patients with prosthetic mechanical heart valves of any location.
• Prevention of recurrent embolism in patients with acute myocardial infarction complicated by left ventricular mural thrombus.

Security

Contraindications

Absolute

• Confirmed pregnancy (except in high-risk mechanical valve carriers in specific weeks under monitoring).
• Clinically relevant active hemorrhagic diathesis.
• Recent major CNS, spinal cord or eye surgery.
• Lack of compliance or inability of the patient to undergo periodic laboratory monitoring (INR).
• Uncontrolled malignant arterial hypertension.

Relative

• Advanced liver disease with prolongation of basal hemostasis.
• High risk of repetitive traumatic falls (elderly, dementia).
• Chronic concomitant use of potent NSAIDs.

Adverse effects (ADRs)

Interactions

• Warfarin enhancers (Increase in INR and risk of bleeding): Inhibitors of the metabolism of CYP2C9 (Amiodarone, Metronidazole, Fluconazole, Sulfamethoxazole-Trimethoprim, Ciprofloxacin), displacement from albumin binding (NSAIDs, salicylates), reduced intestinal bacterial synthesis of vitamin K (broad-spectrum antibiotics such as third-generation cephalosporins).
• Inhibitors of the effect (Decrease in INR and risk of thrombosis): Hepatic enzyme inducers (rifampicin, carbamazepine, phenytoin and St John's wort) and increased dietary vitamin K intake from leafy green vegetables such as kale, spinach and broccoli.

Dosing and Monitoring

Vitamin K antagonist dosing is individualized and adjusted according to the International Normalized Ratio (INR), which measures prothrombin time prolongation.

• Standard therapeutic INR target: 2.0 to 3.0 for most indications, such as AF or DVT.
• Goal for mechanical valves: 2.5 to 3.5 for selected mitral mechanical prostheses or older-generation valves; use the relevant valve-specific guidance.
• Warfarin starting dose: Common initial oral dose is 5 mg daily in suitable adults. High loading doses > 10 mg can increase bleeding and early protein C depletion; individualize initiation and any indication for heparin bridging.

Clinical

Clinical Pearl: Pre-treatment Genotyping

The variability in warfarin dose response is largely determined by single nucleotide polymorphisms (SNP) in the CYP2C9 genes (*2 and *3 alleles that associate slowed clearance) and VKORC1 (haplotype A that confers hypersensitivity to inhibition). Patients carrying non-wild-type variants of CYP2C9 require drastic reductions in the maintenance dose (up to 50-70% of the standard dose) to avoid initial catastrophic overdose.

Pregnancy and Breastfeeding

FDA Category X. It is highly teratogenic. It easily crosses the placenta and produces warfarin embryopathy syndrome if administered during the first trimester (weeks 6 to 12), characterized by nasal hypoplasia, calcified chondrodysplasia punctata, mental retardation and CNS malformations. It can induce fetal cerebral hemorrhage in utero in the third trimester. Totally contraindicated in pregnancy, except in exceptional situations of very high-risk mitral mechanical valves where the maternal benefit outweighs the extreme fetal risk (weeks 14 to 34 only). Compatible with breastfeeding; warfarin is not actively excreted in human milk and does not induce alterations in the infant's coagulation levels.