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Severe interaction between ritonavir and acenocoumarol.
Josep M Llibre1, Joan Romeu, Estibaliz López
1Internal Medicine Department, Hospital Sant Jaume, Barcelona, Spain. jmllibre@bcgest.scs.es
The Annals of Pharmacotherapy
|March 29, 2002
Summary
Ritonavir (RTV) significantly reduces the anticoagulant effect of acenocoumarol, necessitating dose increases that proved ineffective. This drug interaction requires careful consideration for patients on oral anticoagulants.
Area of Science:
- Pharmacology and Drug Interactions
- Clinical Pharmacy
- HIV Therapeutics
Background:
- Ritonavir (RTV) is a protease inhibitor used in HIV therapy.
- Acenocoumarol is an oral anticoagulant used to prevent thrombosis.
- Potential drug interactions between RTV and acenocoumarol require investigation.
Observation:
- A patient on acenocoumarol experienced a dramatic decrease in anticoagulant effect after initiating RTV.
- Increased acenocoumarol dosage failed to restore therapeutic anticoagulation levels.
- RTV administration was discontinued due to the unmanageable interaction.
Findings:
- Ritonavir acts as an inducer of hepatic CYP1A2, CYP1A4, and CYP2C9/19 enzymes.
- This induction leads to accelerated metabolism of acenocoumarol.
- The interaction results in a clinically significant reduction in acenocoumarol's anticoagulant efficacy.
Implications:
- Ritonavir severely diminishes the effectiveness of acenocoumarol.
- This interaction necessitates avoiding ritonavir in patients receiving acenocoumarol.
- Clinicians should be aware of this interaction when prescribing RTV to patients on oral anticoagulants.