Background: Letermovir (LET) is a novel drug inhibiting the human cytomegalovirus virus (CMV) terminase complex. In a pivotal phase 3 trial for the prophylaxis of clinically significant CMV infection (CS-CMV) in allogenic hematopoietic cell transplantation (HCT) recipients with serological evidence of prior CMV infection, LET treatment for up to 14 weeks post-HCT significantly reduced CS-CMV through 24 weeks post-HCT. Objective: A physiologically-based pharmacokinetic (PBPK) model was developed with the intent to generate hypotheses a). to mechanistically explain the greater-than-dose-proportional pharmacokinetics behavior of LET after intravenous (IV) and oral (P.O.) dosing; b). to mechanistically explain the differences in LET exposure observed in Japanese versus Caucasian healthy volunteers (HV); c). to generate hypothesis that describe the exposure differences of LET observed in Caucasian HV and HCT recipients after oral dosing. Approaches for Model Development: Simcyp® ver.15 was utilized in the development of PBPK model for LET. The model was developed and qualified using a combination of physicochemical properties, preclinical data (in vitro and in vivo), and human ADME information followed by optimization of selected model parameters via fitting simulated plasma concentration-time profiles and PK data to the observed ones. Results: The PBPK model was not only able to capture the nonlinear PK of LET but also the magnitude of greater-than-dose-proportional increase in exposure observed across of a dose range of 120 to 720 mg IV and 120 to 480 mg P.O. of LET in Caucasian HV. The mechanism of nonlinear PK was best described by the saturation of OATP1B-mediated hepatic uptake transporter. The PBPK model also supports the hypothesis that the higher exposure observed in Japanese as compared to Caucasian HV is likely due to differences in hepatic uptake transporter OATP1B abundance and/or activities in addition to known demographic and physiological differences between these two populations such as body weight and liver mass. The PBPK model was also used to develop the hypothesis that the exposure observed in HCT recipients following the oral administration of 480 mg LET were best described by a reduced fraction absorbed. Intestinal mucositis caused by chemotherapy or radiation might be the cause for the reduced capacity of intestinal absorption and has been reported for other drugs in the literature.