Acute relapse after initiation of Siponimod in a patient with secondary progressive MS.

Sphingosine 1-phosphate (S1P) is a signaling molecule that binds to five G protein-coupled receptors (Proc Natl Acad Sci USA 108:751-756, 2011). Modulation of these receptors has been associated with pleiotropic biological effects in the immune, cardiovascular, and central nervous systems (CNS). The functional S1P receptor antagonist fingolimod was the first member of this class of pharmacotherapeutics to be approved for treatment of relapsing multiple sclerosis (MS). Siponimod is currently in clinical trial in patients with secondary progressive (SP) MS, a clinical trial for which there is an unmet need for disease-modifying agents. 10 weeks into the trial, the patient awoke with blurry vision in his left eye, and was subsequently diagnosed with an acute optic neuritis. Despite discontinuation of siponimod and treatment with pulse corticosteroids, the patient did not regain visual function in the affected eye. This is the first report of disease reactivation shortly after initiating siponimod in a patient with SPMS. This case illustrates that the known changes in lymphocyte numbers and composition in the CNS associated with S1P receptor antagonism during the SPMS disease stage may have adverse outcomes in some patients during treatment initiation, and that close clinical and paraclinical monitoring is advised.