PANWARFIN
Clinical safety rating: caution
Comprehensive clinical and safety monograph for PANWARFIN (PANWARFIN).
Anticoagulant that inhibits vitamin K epoxide reductase, thereby decreasing hepatic synthesis of vitamin K-dependent coagulation factors II, VII, IX, and X.
| Metabolism | Hepatic via CYP450 isoenzymes: S-enantiomer primarily by CYP2C9; R-enantiomer primarily by CYP1A2 and CYP3A4. |
| Excretion | Primarily renal as inactive metabolites; 60-92% of a dose is excreted in urine, with about 50% as the 7-hydroxywarfarin metabolite and the remainder as other metabolites. Biliary/fecal elimination accounts for approximately 10-20%. |
| Half-life | Terminal elimination half-life is 20-60 hours (mean ~40 hours). Clinically, the longer half-life allows for once-daily dosing and steady-state is achieved in 5-7 days; anticoagulant effect may persist for 2-5 days after discontinuation due to depletion of vitamin K-dependent clotting factors. |
| Protein binding | Highly bound to plasma proteins, mainly albumin, at 99% (bound fraction). Only the free (unbound) fraction is pharmacologically active; hypoalbuminemia may increase free drug and effect. |
| Volume of Distribution | Apparent volume of distribution is 0.1-0.2 L/kg (mean 0.14 L/kg). This low Vd reflects high protein binding and limited extravascular distribution; warfarin is primarily confined to the vascular space. |
| Bioavailability | Oral: 100% (completely absorbed). Intravenous: 100% (bioequivalent to oral). No other routes are clinically used. |
| Onset of Action | Oral: Anticoagulant effect (INR increase) begins 24-72 hours after a single dose; full therapeutic effect (INR in range) may require 5-7 days. Intravenous: Similar to oral; onset is determined by pharmacokinetics and clotting factor synthesis inhibition. |
| Duration of Action | Duration of anticoagulant effect is 2-5 days after cessation, corresponding to the half-lives of clotting factors (II, VII, IX, X). The effect is prolonged in liver disease or vitamin K deficiency. |
| Molecular Weight | 308.33 |
5 mg orally once daily, adjusted to maintain INR 2-3.
| Dosage form | TABLET |
| Renal impairment | No dosage adjustment required for any GFR; monitor INR closely. |
| Liver impairment | Child-Pugh A: no adjustment; Child-Pugh B: reduce dose by 50%; Child-Pugh C: contraindicated. |
| Pediatric use | 0.1-0.2 mg/kg orally once daily, adjust based on INR. |
| Geriatric use | Initiate at 2.5 mg orally once daily; monitor INR frequently due to increased sensitivity. |
| 1st trimester | Teratogenic: warfarin crosses placenta and can cause fetal hemorrhage and dysmorphogenesis; exposure during weeks 6-12 is associated with warfarin embryopathy (nasal hypoplasia, stippled epiphyses). Avoid use; use heparin or LMWH when anticoagulation required. |
| 2nd trimester | Risk of fetal intracranial hemorrhage; CNS abnormalities possible. Avoid use; use heparin or LMWH if anticoagulation needed. |
| 3rd trimester | High risk of fetal and maternal hemorrhage, particularly at delivery. Avoid use; switch to heparin or LMWH before delivery. |
Clinical note
Comprehensive clinical and safety monograph for PANWARFIN (PANWARFIN).
| Placental transfer | Warfarin crosses the placenta readily; molecular weight allows passive diffusion. Fetal plasma concentrations approximate maternal levels. Associated with fetal hemorrhage and teratogenicity. |
| Breastfeeding | Warfarin is not excreted into breast milk in clinically significant amounts; it is considered compatible with breastfeeding by the American Academy of Pediatrics. Monitor infant for bruising or bleeding if used postpartum. |
■ FDA Black Box Warning
Warfarin can cause major or fatal bleeding. Risk factors include high dose, elderly, multiple comorbidities, and certain genetic variations. Regular monitoring of INR is required.
| Serious Effects |
Pregnancy (except in patients with mechanical heart valves where risk of thromboembolism outweighs risks; use with caution and informed consent)Hemorrhagic tendencies or blood dyscrasiasRecent or planned surgery of the central nervous system or eyeMajor trauma or surgery with large open surfacesActive bleeding or peptic ulcer diseaseSevere hepatic impairment or cirrhosis with varicesMalignant hypertensionPericarditis or pericardial effusionInability to obtain regular INR monitoring
| Precautions | Hemorrhage risk; necrosis or gangrene of skin; systemic atheroemboli; calciphylaxis; need for regular INR monitoring; dose adjustments required with interacting drugs, diet, and disease states. |
| Food/Dietary | Foods high in vitamin K (e.g., kale, spinach, broccoli, Brussels sprouts, parsley, collard greens, liver) can reduce anticoagulant effect; avoid large amounts of cranberry juice, grapefruit juice, and green tea which may potentiate warfarin; maintain a consistent diet and monitor INR regularly. |
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| Lactation Rating | L2 (Safer) |
| Teratogenic Risk | First trimester: High risk of warfarin embryopathy (nasal hypoplasia, stippled epiphyses) with exposure between 6-12 weeks gestation. Second and third trimesters: Risk of central nervous system abnormalities, optic atrophy, microcephaly, and fetal hemorrhage. Fetal bleeding risk increases near term; warfarin crosses the placenta and can cause fetal intracranial hemorrhage. Contraindicated throughout pregnancy unless mechanical heart valve necessitates use. |
| Fetal Monitoring | Maternal: Frequent INR monitoring (at least weekly) with target INR tailored to indication. Fetal: Serial ultrasound to detect anomalies, especially at 18-20 weeks. Consider umbilical artery Doppler and fetal surveillance if warfarin used after 12 weeks. Evaluate for fetal hemorrhage if maternal INR >4. |
| Fertility Effects | No direct adverse effects on fertility reported. Warfarin does not affect ovarian function or spermatogenesis. However, underlying conditions requiring anticoagulation (e.g., antiphospholipid syndrome) may impair fertility. |
| Clinical Pearls | Monitor INR closely when initiating or adjusting concurrent medications; avoid NSAIDs due to increased bleeding risk; phytonadione (vitamin K1) and fresh frozen plasma are reversal agents; genetic variants in VKORC1 and CYP2C9 affect dosing. |
| Patient Advice | Take exactly as prescribed; do not change dose without consulting your doctor. · Avoid alcohol consumption; it can alter INR and increase bleeding risk. · Report signs of bleeding (unusual bruising, dark stools, blood in urine) or thrombosis (leg swelling, chest pain) immediately. · Maintain consistent intake of vitamin K-rich foods (e.g., leafy greens); do not suddenly increase or decrease consumption. · Use a soft toothbrush and electric razor to minimize bleeding risk. · Inform all healthcare providers and dentists you are taking this blood thinner. |