Aims Letermovir, a cytomegalovirus (CMV) DNA terminase complex inhibitor, is a substrate of ABCB1 (P‐glycoprotein; P‐gp), organic anion transporting polypeptide (OATP)1B1/3, UDP‐glucuronosyltransferase (UGT)1A1, UGT1A3 and possibly ABCG2 (breast cancer resistance protein; BCRP). A study was conducted to evaluate the effects of itraconazole, a prototypic ABCB1/ABCG2 inhibitor, on letermovir pharmacokinetics (PK) and the effects of letermovir on itraconazole PK. Methods In an open‐label, fixed‐sequence study in 14 healthy participants, 200?mg oral itraconazole was administered once daily for 4?days. Following a 10‐day washout, 480?mg oral letermovir was administered once daily for 14?days (Days 1–14) and then coadministered with 200?mg itraconazole once daily for 4 days (Days 15–18). Intensive PK sampling was performed for letermovir and itraconazole. PK and safety were evaluated. Results Letermovir geometric mean ratio (GMR; 90 confidence interval CI) for area under the concentration–time curve from time 0 to 24?h (AUC0–24) was 1.33 (1.17, 1.51) and for maximum concentration (Cmax) was 1.21 (1.05, 1.39) following administration with/without itraconazole. Itraconazole GMR (90 CI) for AUC0–24 was 0.76 (0.71, 0.81) and for Cmax was 0.84 (0.76, 0.92) following administration with/without letermovir. Coadministration of letermovir with itraconazole was generally well tolerated. Conclusions The increase in letermovir exposure with coadministration of itraconazole is likely predominantly due to inhibition of intestinal ABCB1 and potentially ABCG2 transport. The mechanism for the decrease in itraconazole exposure is unknown. The modest changes in letermovir and itraconazole PK are not considered clinically meaningful.
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