Drug Class: Vitamin K Inhibitor

Question 1

Example of vitamin K inhibitor

Answer 1

WARFARIN

Question 2

Mode of action of WARFARIN + conditions used in

Answer 2

• Inhibits production of active clotting factors

Used in prevention of DVT, PE, stroke

• In renal failure
• Significantly obese patients

Question 3

Side effects of WARFARIN

Answer 3

Bleeding

Question 4

Drug-drug interactions of WARFARIN

Answer 4

• Interaction with other drugs via CYP450
  ↑ warfarin activity
• Aspirin, Cotrimoxazole, Trimethoprim = decrease albumin binding= ↑INR
• Cimetidine, erythromycin= inhibit degradation by inhibiting CYP450
• Oral antibiotics= interfere with Vit K synthesis -> decrease synthesis of clotting factors
  ↓ warfarin activity
• Barbiturate, phenytoin, alcohol= induction of CYP450
• Vitamin K= promote clotting factor synthesis
• Cholestyramine= reduce absorption
  ↑↑ bleeding
• NSAIDs/ Aspirin= inhibition of platelets/prostaglandins
• Heparin metabolites= inhibition of clotting factors

Question 5

Contraindications of WARFARIN

Answer 5

Narrow therapeutic range 
-	International normalised ratio (INR) 
	-	Normal INR 1 
	-	Therapeutic INR 2.5-4
Avoid in pregnancy (esp. 1st-3rd trimesters)
-	TERATOGNEIC (chondrodysplasia) 
-	Retroplacental and fetal intracerebral BLEEDING 
	-	Replace with subcutaneous/IV heparin