Amenamevir Transfer into Breast Milk in Patients with Herpes Simplex Virus Infection: A Brief Report of a Prospective Multicenter Study.

Patient-initiated therapy (PIT) is a treatment approach in which patients with recurrent herpes simplex self-administer medication at the onset of prodromal symptoms. PIT, which allows patients to receive medication in advance and initiate treatment at any time, is a therapeutic approach that alleviates the burden of urgent clinic visits for acute diseases like herpes simplex and benefits many people. Furthermore, its utility is particularly high for breastfeeding women for whom clinic visits can sometimes be difficult. When PIT is used in breastfeeding patients, potential drug transfer into breast milk and infant exposure must be considered. However, no studies have evaluated the extent of amenamevir transfer into human breast milk or its safety in infants. In this study, the relative infant dose (RID) was estimated from breast milk concentrations after a single oral dose of amenamevir. In the two cases evaluated, the estimated RID values were below 1%. This preliminary data suggest low estimated transfer of amenamevir into human milk after PIT. However, because breastfeeding was interrupted and infant exposure was not evaluated, clinical safety in breastfed infants remains undetermined. Trial Registration: Japan Registry of Clinical Trials identifier: jRCTs031230591.