Molecular and neuroimmune pharmacology of S1P receptor modulators and other disease-modifying therapies for multiple sclerosis

Neurological disorders are medically defined as diseases of the nervous system, including stroke, Alzheimer's disease and other dementias, Parkinson's disease, multiple sclerosis (MS), epilepsy, brain tumors, and others. They are the leading cause of disability-adjusted life-years (DALYs) and one of the major causes of death (Collaborators, 2019; Kang et al., 2022), and thus the market for neurological disorders is continuously growing. Among neurological diseases, MS is unique in that it involves peripheral immune cells, both B and T cells, that attack the central nervous system (CNS) (Frohman, Racke, & Raine, 2006; Nakahara, Maeda, Aiso, & Suzuki, 2012; Noseworthy, Lucchinetti, Rodriguez, & Weinshenker, 2000). Clinical presentation is variable over time and includes optic neuritis, dizziness, ataxia, sensory loss, urinary incontinence, fatigue, depression, cognitive decline, and so on. Currently, the diagnosis of MS is standardized by the McDonald criteria (Kihara, 2019; McDonald et al., 2001; Thompson et al., 2018). Based on the pattern of progression and symptom changes over time, MS can be grouped into relapsing remitting MS (RRMS), secondary progressive MS (SPMS), and primary progressive MS (PPMS) (Frohman et al., 2006; Nakahara et al., 2012; Noseworthy et al., 2000). RRMS is the most common form of MS, with unpredictable, repeated cycles of exacerbations and remissions. Most cases of RRMS enter a progressive phase with relapses but no remission, i.e., SPMS. PPMS is defined prospectively or retrospectively by the accumulation of disability for >1 year without remission. Clinically isolated syndrome (CIS) is a single episode of MS symptoms with MRI features that may or may not be indicative of future MS. Epidemiology shows differences in gender (female:male = 2– 3:1 (Compston & Coles, 2002)), geography (high latitude > equator (Tao, et al., 2016)), and ethnicity (White, Black> Hispanic > Asian (Langer-Gould, Gonzales, Smith, Li, & Nelson, 2022)). The prevalence of MS in the United States (U.S.) is estimated to be approximately 1 million people and 2.3 million people worldwide (Lane, Ng, Poyser, Lucas, & Tremlett, 2022). MS is characterized by plaques (lesions) in the CNS, which can be detected by magnetic resonance imaging (MRI) (Aslam et al., 2022). MS lesions in the white matter are histopathologically characterized by myelin loss (demyelination) and/or inflammation and are classified as preactive (normal myelin density and morphology with clusters of activated microglia), active (demyelination with myelin-laden macrophages), chronic active (demyelinated and hypocellular center with macrophages at the edge of the lesions), or chronic inactive (complete demyelination without macrophages/microglia) (Jonkman et al., 2015). The exact cause of MS is still unknown, but it is thought to be a combination of genetic and environmental factors. Genetic predisposition, including human leukocyte antigen (HLA) genes and interleukin 2 or 7 receptor (IL2R, IL7R) genes, have been identified as risk genes (Goris, Vandebergh, McCauley, Saarela, & Cotsapas, 2022). Environmental factors, including exposure to the Epstein-Barr (EB) virus and vitamin D deficiency have been associated with increased MS risk (Bjornevik, Munz, Cohen, & Ascherio, 2023; Janousek et al., 2022). Although no drug can completely cure the disease, medications have been developed to prevent relapses and slow down the progression of MS, known as disease-modifying therapies (DMT).

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