Background: Diroximel fumarate (DRF) and dimethyl fumarate (DMF) are similar disease-modifying therapies (DMTs) that reduce disease activity in patients with relapsing-remitting multiple sclerosis (MS). We expect that patients on DRF would experience a similar incidence and severity of lymphopenia, given that it is a well-documented side effect of DMF treatment.

Methods: We utilized linear mixed-effects models to test for differences in white blood cell count (WBC), absolute lymphocyte count (ALC), absolute CD3+ count, absolute CD4+ count, and absolute CD8+ count over time in clinically stable patients with MS on DMF who switched to DRF.

Results: Twenty-two patients with MS who were clinically stable on DMF switched to DRF. Linear mixed-effects models showed a decrease in ALC when switching medications (β = -225.70, p < 0.040). In addition, the models showed a decrease in absolute CD8+ counts after switches from DMF to DRF (β = -85.59, p = 0.034).

Conclusions: Patients with MS who are stable on DMF and switch to DRF may experience worsening of lymphopenia and lower absolute CD8+ counts, which may increase their risk of opportunistic infections. These findings indicate that close lymphocyte subset monitoring is clinically important when switching patients with MS from DMF to DRF.