Letermovir is a human cytomegalovirus terminase inhibitor for cytomegalovirus 
infection prophylaxis in hematopoietic stem cell transplant recipients. 
Posaconazole (POS), a substrate of glucuronosyltransferase and P-glycoprotein, 
and voriconazole (VRC), a substrate of CYP2C9/19, are commonly administered to 
transplant recipients. Because coadministration of these azoles with letermovir 
is expected, the effect of letermovir on exposure to these antifungals was 
investigated. Two trials were conducted in healthy female subjects 18 to 55 
years of age. In trial 1, single-dose POS 300 mg was administered alone, 
followed by a 7-day washout; then letermovir 480 mg once daily was given for 14 
days with POS 300 mg coadministered on day 14. In trial 2, on day 1 VRC 400 mg 
was given every 12 hours; on days 2 and 3, VRC 200 mg was given every 12 hours, 
and on day 4 VRC 200 mg. On days 5 to 8, letermovir 480 mg was given once daily. 
Days 9 to 12 repeated days 1 to 4 coadministered with letermovir 480 mg once 
daily. In both trials, blood samples were collected for the assessment of the 
pharmacokinetic profiles of the antifungals, and safety was assessed. The 
geometric mean ratios (90%CIs) for POS+letermovir/POS area under the curve and 
peak concentration were 0.98 (0.83, 1.17) and 1.11 (0.95, 1.29), respectively. 
Voriconazole+letermovir/VRC area under the curve and peak concentration 
geometric mean ratios were 0.56 (0.51, 0.62) and 0.61 (0.53, 0.71), 
respectively. All treatments were generally well tolerated. Letermovir did not 
affect POS pharmacokinetics to a clinically meaningful extent but decreased VRC 
exposure. These results suggest that letermovir may be a perpetrator of 
CYP2C9/19-mediated drug-drug interactions.