A case report: pitfalls in antibacterial therapy with rifampicin for mechanical valve endocarditis-the king of drug interactions.

Background: Rifampicin is a strong inducer of the hepatic cytochrome P450 (CYP) family and is known to interact with many clinical drugs. However, to our knowledge, no case of worsening heart failure (HF) due to the interaction between rifampicin and HF drugs has been reported.

Case summary: A 32-year-old female, who had undergone intracardiac repair for an incomplete atrioventricular septal defect with dextrocardia and prosthetic valve replacements for right and left atrioventricular valve regurgitation, presented as an outpatient. Her medications included tolvaptan 15 mg and warfarin 1.25 mg. She had a slight fever and Osler nodes at her fingers. Blood culture bottles grew methicillin-resistant Staphylococcus epidermidis, and several vegetations were observed on the right atrioventricular mechanical valve with a transoesophageal echocardiogram. She was diagnosed with prosthetic valve endocarditis and treated with antibiotic agents including rifampicin. After a week, she developed systemic oedema and had a marked decrease in prothrombin time-international normalized ratio (PT-INR). Rifampicin was promptly discontinued due to a strong suspicion of a drug-drug interaction. Consequently, both her congestion and the PT-INR stabilized, and she was discharged after 8 weeks of antibiotic treatment.

Discussion: The introduction of rifampicin induces CYP family members such as CYP3A4 and CYP2C9. Warfarin is metabolized by CYP2C9 and tolvaptan is also metabolized by CYP3A4, resulting in a notable reduction of their blood levels when co-administered with rifampicin. The clinical challenges arising from interactions between HF drugs and rifampicin can be categorized into two main groups: worsening HF and thrombotic complications. Clinicians should remain vigilant and informed about these potential issues.