Whole blood concentrations of fingolimod and its pharmacologically active metabolite fingolimod phosphate obtained during routine health care of patients with multiple sclerosis.

Background: Fingolimod is a first-in-class, orally administered drug indicated for the treatment of relapsing-remitting multiple sclerosis. It acts as an immunomodulator, is classified as a "disease-modifying therapy", and its main mechanism of action is the modulation of sphingosine-1-phosphate receptors. In this prospective pilot study, whole blood concentrations of fingolimod and fingolimod phosphate obtained during routine health care were measured. In this study, we aimed to determine whether therapeutic monitoring of fingolimod and fingolimod phosphate concentrations can help personalise pharmacotherapy for patients with multiple sclerosis.

Method: The study group consisted of 73 patients treated with oral fingolimod (0.5 mg) once daily. Blood samples were collected between July 2021 and January 2022. The whole blood concentrations of fingolimod and fingolimod phosphate were analysed using ultra-high-performance liquid chromatography-tandem mass spectrometry. The relationship between the measured concentrations and the absolute peripheral blood lymphocyte count was evaluated.

Results: Fingolimod concentrations ranged from 0.61 to 6.21 µg/L, and fingolimod phosphate concentrations from 0.48 to 4.28 µg/L. Significantly higher concentrations of the active metabolite, fingolimod phosphate, and a significantly higher fingolimod phosphate/fingolimod concentration ratio were observed in women. The sum of fingolimod and fingolimod phosphate concentrations was significantly higher in the subgroup of patients with a lower absolute peripheral blood lymphocyte count.

Conclusion: Although all patients were treated with the same dose of fingolimod (0.5 mg orally daily), a 10-fold difference was observed in the achieved whole blood concentrations of fingolimod and fingolimod phosphate. This wide inter-individual variability may lead to potential toxicity or suboptimal concentrations, with the risk of further deterioration in the clinical condition of patients with multiple sclerosis. Therefore, fingolimod is a suitable candidate for therapeutic drug monitoring, including monitoring patient adherence to treatment.