Effect of Prophylactic Administration of Foscarnet for HHV‐6 Encephalitis in Adult Single Cord Blood Transplantation

Background HHV‐6 encephalitis is a serious complication after allogeneic hematopoietic stem cell transplantation. Whether foscarnet prophylaxis improves outcomes after unrelated cord blood transplantation (UCBT), particularly under systemic steroid exposure, and the renal safety of foscarnet remain uncertain. Methods This retrospective cohort consisted of 156 adult patients who received single‐unit UCBT between 2005 and 2022. Foscarnet prophylaxis was defined as any use of foscarnet without encephalitis onset, including planned administration after transplantation, pre‐emptive administration after HHV‐6 DNAemia, or other indications. Primary endpoint was 60‐day incidence of HHV‐6 encephalitis, and secondary endpoints were overall survival and renal function up to 12 months. Results The 60‐day cumulative incidence of HHV‐6 encephalitis was 10.9%; 121/156 (77.6%) received prophylaxis. Incidence did not differ significantly by foscarnet exposure (9.9% vs. 14.3%; p = 0.418) and this was not significant in the multivariable analysis. A high cumulative dose of corticosteroid was associated with higher incidence (28.6% vs. 7.1%; p = 0.001). In patients with low corticosteroid exposure, the incidence of HHV‐6 encephalitis was significantly lower with foscarnet prophylaxis (4.3% vs. 14.7%; p = 0.040). Among patients who developed encephalitis, 1‐year overall survival appeared higher with prior foscarnet exposure than in those without prior exposure (72.2% vs. 20%); renal function up to 12 months was similar. Conclusions Foscarnet prophylaxis was not significantly associated with a reduced incidence of HHV‐6 encephalitis after UCBT, whereas high‐dose steroid exposure was a risk factor. Notably, a lower incidence of encephalitis with foscarnet prophylaxis was observed in the low‐dose steroid subgroup, although this finding requires confirmation in larger cohorts.