To investigate the pharmacokinetic/pharmacodynamic (PK/PD) characteristics of siponimod in the blood versus brain using an experimental autoimmune encephalomyelitis (EAE) mice model.

Siponimod (BAF312), is a potent, highly selective sphingosine 1-phosphate (S1P_{1, 5}) receptor modulator. Siponimod is the first oral disease-modifying therapy proven to reduce disability progression in patients with secondary progressive multiple sclerosis. Its mechanism of action involves: S1P₁-dependent retention of pathogenic lymphocytes within secondary lymphoid organs, and S1P_{1, 5}-dependent anti-inflammatory and neuroprotective effects on neural cells.

C57BL/6J mice (10-weeks old; females; n=10/group) were fed with control diet (home-made drug-free) or siponimod-loaded diet (0.2, 0.1, 0.03, 0.01, or 0.003 g/kg food weight), followed by EAE induction using recombinant rat MOG28–152. Longitudinal changes in EAE severity scores, siponimod blood and brain levels (quantified using liquid chromatography/tandem mass spectrometry), and blood lymphocyte counts were assessed at the study end (Day 28).

During the study, siponimod-loaded diet at 0.2, 0.1, 0.03, 0.01, or 0.003 g/kg achieved dose-dependent siponimod steady-state blood levels within 7, 3, 1, 0.4, or 0.2 µM range, respectively. For each blood level, siponimod brain levels were nearly 10-fold higher, and reductions in blood lymphocyte counts were all within 65–75% range. Siponimod-loaded diet at 0.2, 0.1, 0.03, 0.01, or 0.003 g/kg reduced the EAE scores by 44.9%, 45.1%, 50.4%, 72.5% and 39.3%, respectively, with optimal dose of 0.01 g/kg demonstrating maximum efficacy.

A siponimod-loaded diet successfully achieved dose-related efficacy in the EAE mice model and a loading dose of 0.01 g/kg was able to reach the blood levels within 0.4 µM range, required for maximal therapeutic efficacy. Findings also revealed good brain penetration with brain/blood exposure ratio of ~10. Further studies are underway to assess if this observation could translate into potential advantages for siponimod versus fingolimod with brain/blood exposure ratio around 30-40.