Introduction Teriflunomide is an immunomodulator known to decrease the proliferation of stimulated lymphocytes via inhibition of dihydro-orotate dehydrogenase. Lymphocyte/neutrophil counts were assessed in pooled data from one phase 2 and three phase 3 (TEMSO, TOWER, and TOPIC) placebo-controlled studies.

Methods Patients were randomized to receive once-daily teriflunomide 14mg (n=1002), 7mg (n=1045), or placebo (n=997). Blood samples were collected throughout the studies.

Results Mean baseline lymphocyte and neutrophil counts were similar across groups. Small decreases in mean lymphocyte and neutrophil counts occurred within the first 12 weeks (lymphocytes) or 6 weeks (neutrophils) of treatment, and stabilized within the normal range for most patients thereafter. Patients with neutrophil counts <1×10^9^/L were to discontinue treatment; 11 (1.1%; 14 mg), 8 (0.8%; 7 mg), and 1 (0.1%; placebo) patients discontinued due to neutropenia or neutrophil count decrease as per protocol requirements. Neutropenia was reported as a serious adverse event (SAE) in 7 (0.7%; 14 mg), 2 (0.2%; 7 mg), and 3 (0.3%; placebo) patients; there were no lymphopenia SAEs. No link between neutrophil or lymphocyte count decreases and infection was observed.

Conclusions These data demonstrate that teriflunomide has small, reversible effects on lymphocyte/neutrophil counts, with no increase in infection risk observed. (Study supported by Genzyme, a Sanofi company).