BACKGROUND: Teriflunomide is administered orally to treat relapsing-remitting multiple sclerosis. In this prospective pilot study, the free and total serum concentrations of teriflunomide obtained during routine health care were measured and their relationship with disease activity was evaluated. METHODS: Eighty-nine patients were included in this study. Blood samples were collected from April 2021 to February 2022, and free and total teriflunomide serum concentrations were measured. Patient assessment involved monitoring of blood counts and potential adverse effects of teriflunomide. RESULTS: In the steady-state group, total teriflunomide concentrations ranged from 14.7 to 144.2 mg/L, while free concentrations from 31.1 to 389.7 μg/L. In the non-steady-state group, the total concentration ranged from 2.2 to 59.3 mg/L, with free concentrations ranging from 6.8 to 143.5 μg/L. In the steady-state group, a significant inverse correlation was found between absolute peripheral blood lymphocyte count and both total and free teriflunomide serum concentrations. CONCLUSION: Although all patients were treated with the same dose, up to a 10-fold difference in total and free teriflunomide serum concentrations, and up to a 5-fold difference in steady-state trough concentrations were observed. This vast interindividual variability can potentially lead to toxicity or, conversely, to suboptimal therapeutic concentrations of teriflunomide, with the risk of further worsening of multiple sclerosis compensation.

Department of Clinical Pharmacology Faculty of Medicine University of Ostrava Syllabova 19 Ostrava 703 00 Czech Republic; Department of Children Neurology Department of Neurology University Hospital Ostrava 17 listopadu 1790 5 Ostrava 70852 Czech Republic

Department of Clinical Pharmacology Faculty of Medicine University of Ostrava Syllabova 19 Ostrava 703 00 Czech Republic; Department of Clinical Pharmacology Institute of Laboratory Medicine University Hospital Ostrava 17 listopadu 1790 5 Ostrava 70852 Czech Republic

Department of Neurology University Hospital Ostrava 17 listopadu 1790 5 Ostrava 70852 Czech Republic; Department of Clinical Neurosciences Faculty of Medicine University of Ostrava Syllabova 19 Ostrava 703 00 Czech Republic

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