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Difficulty: Very hardPharmacologic Interventions

A 4545-year-old man comes to the emergency department due to a 22-day history of worsening left leg swelling and pain. He was diagnosed with active pulmonary tuberculosis three weeks ago and is currently in the intensive phase of treatment with rifampin, isoniazid, pyrazinamide, and ethambutol. He also has a history of HIV, for which he takes dolutegravir 50 mg50\text{ mg} twice daily, tenofovir alafenamide, and emtricitabine. His viral load is undetectable, and his CD4 count is 450/mm3450/\text{mm}^3. Vital signs are within normal limits. Physical examination reveals unilateral edema, erythema, and tenderness of the left calf. Laboratory studies show a serum creatinine of 0.9 mg/dL0.9\text{ mg/dL} and normal liver function tests. A duplex ultrasound of the left lower extremity demonstrates a noncompressible, acute thrombus in the left popliteal and femoral veins. Which of the following is the most appropriate pharmacologic intervention for managing this patient's deep vein thrombosis?

  1. A
    Oral apixaban
  2. Subcutaneous enoxaparinAnswer
  3. C
    Oral dabigatran
  4. D
    Oral clopidogrel
  5. E
    Intravenous alteplase

Answer

Subcutaneous enoxaparin
The correct answer is subcutaneous enoxaparin. This patient requires immediate systemic anticoagulation for an acute proximal deep vein thrombosis. While direct oral anticoagulants (DOACs) are typically first-line therapy for uncomplicated venous thromboembolism, this patient is actively taking rifampin for tuberculosis. Rifampin is a profound inducer of both the cytochrome P450 3A4 (CYP3A4) enzyme and the intestinal P-glycoprotein (P-gp) efflux transporter. Apixaban, rivaroxaban, and dabigatran heavily rely on one or both of these pathways for absorption and metabolism. Concomitant use of rifampin rapidly clears these drugs, resulting in highly subtherapeutic levels and exposing the patient to therapeutic failure (e.g., clot extension, fatal pulmonary embolism). Enoxaparin, a parenteral low-molecular-weight heparin, avoids these specific metabolic pathways and is eliminated via the kidneys, making it the safest and most reliable anticoagulant in this clinical scenario.

Step-by-Step Solution

1
Identify the primary diagnosis requiring management.
The patient has an acute proximal deep vein thrombosis requiring immediate systemic anticoagulation.
Duplex ultrasound confirmed an occlusive thrombus in the popliteal and femoral veins.
2
Review the patient's current medication list for clinically significant interactions.
The patient is taking rifampin for tuberculosis, which is a profound inducer of both hepatic cytochrome P450 3A4 (CYP3A4) and the P-glycoprotein (P-gp) efflux transporter.
Antimicrobial therapy for tuberculosis often involves drugs with severe pharmacokinetic interactions.
3
Evaluate standard oral anticoagulant options against the metabolic profile of the patient.
Standard direct oral anticoagulants (apixaban, rivaroxaban, dabigatran) rely heavily on CYP3A4 and/or P-gp for absorption and metabolism.
Co-administration with rifampin will result in highly subtherapeutic anticoagulant levels and a massive risk of therapeutic failure.
4
Select a pharmacologic agent that bypasses these induced metabolic pathways.
Low-molecular-weight heparin (enoxaparin) is administered parenterally and cleared renally, avoiding CYP450 and P-gp pathways.
This provides predictable, therapeutic anticoagulation without interference from rifampin.

Key Concept

Management of venous thromboembolism in patients taking potent CYP450 and P-glycoprotein inducers
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Pharmacologic Interventions

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