Pharmacokinetic analysis driven letermovir dosing in pediatric hematopoietic cell transplantation patients with...

Reactivation of cytomegalovirus (CMV) in CMV-seropositive patients after haploidentical T-cell receptor αβ⁺ /CD19⁺ depleted hematopoietic cell transplant (HCT) is common. Due to delayed CMV-specific immune reconstitution, patients may require prolonged antiviral therapy, including secondary prophylaxis (SP). We present our clinical experience with the off-label use of letermovir for SP in a severely immunocompromised 2-year-old toddler with refractory pre-B-cell ALL and bilateral retinitis caused by resistant CMV (A594V UL97 mutation) following a haploidentical TCRαβ⁺ /CD19⁺ depleted HCT.

The patient underwent measurement of two separate sets of letermovir serum concentrations, drawn at pre-dose, 1 and 4 h (and 8 h during the second therapeutic drug monitoring) post-dose. Pharmacokinetic parameters, including AUC_0-24 were calculated, and dose adjustment was performed based on the drug level.

While receiving oral letermovir 240 mg once daily without cyclosporine, the observed AUC_0-24 was high (75 815 ng h/mL) with a C_min of 209 ng/mL. The dose was reduced by 25% to 180 mg once daily. Despite the dose reduction, both AUC_0-24 and C_min values further increased to 119 095 ng h/L and 959 ng/mL, respectively. The patient continued oral letermovir 180 mg once daily for about 3 months, with adequate viral suppression (CMV viral load in plasma <150 IU/mL) and no recurrent CMV end-organ disease or adverse events.

Given limited options for anti-CMV therapy in young children with resistant CMV, letermovir could be considered as an alternative antiviral for SP. Further studies are warranted to evaluate the pharmacokinetics of letermovir in pediatric allogeneic HCT recipients.