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Abstract Background Cytomegalovirus (CMV) is a common cause of morbidity after allogeneic haematopoietic cell transplantation (alloHCT). Pre‐emptive therapy (PET) with valganciclovir (VGC) is associated with haematological toxicity. Methods We included alloHCT patients from 2018 to 2021 where letermovir (LTV) was used for CMV PET because of cytopenias. Results Ten patients were included. Six received VGC prior to LTV. VGC was commenced at median d42, given for median 40 days. LTV was commenced at median d90, given for median 54 days. At commencement of antiviral, CMV viral load was higher for VGC at 3.7 log 10 IU/mL, compared to LTV at 2.9 log 10 IU/mL. Viral load reduction occurred at 0.18 log 10 IU/mL per week for VGC, compared to 0.17 log 10 IU/mL per week for LTV. There was no clinically significant CMV viremia after stopping LTV. Cytopenias improved on LTV. Conclusion LTV was effective in controlling CMV viremia when it was given at a lower starting CMV viral load and later post alloHCT than VGC. Further study is required of LTV as upfront PET following alloHCT. image