Prolonged Prothrombin Time Due to Drug-Drug Interaction of Warfarin After the Change from Bosentan to Macitentan: A Case of Pharmacist Intervention in the Outpatient Clinic

A woman in her 70s who was taking warfarin 3.75 mg/day had a prothrombin time-international normalized ratio (PT-INR) within the therapeutic range. Her medication for pulmonary hypertension was changed from bosentan to macitentan. After 40 days, she developed respiratory distress, anorexia, and vomiting caused by common cold. When she visited the pharmaceutical outpatient clinic without reservation, the pharmacist suspected that bosentan discontinuation, which cancelled cytochrome P450 (CYP) 2C9 and CYP3A4 enzyme induction, and decreased vitamin K intake due to appetite loss had enhanced warfarin effect, causing PT-INR prolongation. The pharmacist requested the physician to examine the patient's PT-INR. Results showed that her PT-INR was >7. Hence, she was urgently hospitalized. Warfarin and macitentan were discontinued, and the patient's PT-INR decreased to 1.77 after the intravenous administration of vitamin K. Her appetite improved, and warfarin 2 mg/day was resumed. Additionally, when she had been administered macitentan, her hemoglobin levels decreased from 10.8 to 6.6 mg/dL. Therefore, the pharmacist and the physician during hospitalization planned to resume treatment with bosentan, but not with macitentan. The pharmacist proposed to increase the warfarin dose to 3.75 mg since the bosentan and warfarin interaction could lower PT-INR. Thereafter, the patient's PT-INR was controlled within the therapeutic range, and her hemoglobin level was 8-9 mg/dL. The patient was discharged on day 17 of admission. Thus, pharmacist intervention plays a significant role in warfarin control with consideration of drug-drug interaction in patients receiving pulmonary hypertension treatment.