Retrospective review of lymphocyte changes when switching between fumarates in patients with multiple sclerosis

BACKGROUND: When treating multiple sclerosis, lymphopenia is an established side effect of fumaric acids. Research demonstrating the impact of switching fumarates on lymphocyte counts is scarce. The objective of this study is to characterize the incidence and severity of lymphopenia in patients transitioning from dimethyl fumarate (DMF) to monomethyl fumarate (MMF) or diroximel fumarate (DRF). METHODS: A single center, retrospective review of adult medical records at the Vanderbilt Multiple Sclerosis Clinic who filled prescriptions for DMF then either DRF or MMF between January 1st, 2019 and February 28th, 2023. RESULTS: Patients who switched to DRF experienced a greater median percent change in absolute lymphocyte count (ALC) [45% decrease (IQR -95 to -14)] compared to those who switched to MMF [6% increase (IQR -4 to 17)]. Patients who switched to MMF, on average, had an ALC at follow-up 1.8 times higher than those switched to DRF (95% CI 1.4 to 2.3; p < 0.001). Additionally, patients switched to DRF were more likely to experience grade 2 or 3 lymphopenia compared to patients switched to MMF. CONCLUSION: Our study demonstrated that patients switched to DRF had a significantly lower ALC at follow-up and were more likely to have lymphopenia compared to patients switched to MMF.