Recommended foscarnet dose is not associated with improved outcomes in cytomegalovirus salvage therapy.

Background: Cytomegalovirus (CMV) infection causes significant morbidity and mortality in transplant recipients. Ganciclovir and valganciclovir have proven efficacy but are limited by resistance and toxicity, whereas foscarnet typically retains activity when CMV has become resistant to other antivirals. Foscarnet dosing used in practice may be discordant with what is recommended in product labeling, as the result of an unconventional dosing nomogram or prescriber preference; however, it is unknown how discordant foscarnet dosing affects outcomes.

Objective: Our purpose was to characterize the relationship between initial foscarnet dosing intensity (relative to product labeling) and key effectiveness and safety endpoints.

Study Design: This single-center, retrospective study included immunosuppressed adults with CMV viremia who received foscarnet between January 2012-July 2017. Subjects were divided into low dose (LD) and non-low dose (NLD) groups, according to foscarnet dose intensity. The primary endpoint was time-to-CMV eradication. Secondary endpoints included time-to-CMV clearance, acute kidney injury, hematologic toxicity, and mortality.

Results: Of 87 subjects, 38 met inclusion. Primary immunosuppression reasons were solid organ (63%) or hematopoietic cell transplant (29%). Seventeen and 21 subjects were in the LD and NLD groups, respectively. Median time-to-CMV eradication was 17 days (LD group) versus 13 days (NLD group), p = 0.823. Median time-to-CMV clearance was also non-significant (p = 0.505). There was no association between initial foscarnet dosing intensity and acute kidney injury, hematologic toxicity, or mortality (24% in both groups).

Conclusions: These findings suggest outcomes may be sensitive to other factors and underscore the need for further studies to improve understanding of foscarnet dosing in immunosuppressed patients.