The objective of Johns Hopkins Drug Discovery’s neutral sphinghomyelinase 2 (nSmase2) project is to optimize a small molecule inhibitor for the treatment of Alzheimer’s disease (AD). NSMase2 catalyzes the hydrolysis of sphinghomyelin (SM) to phosphorylcholine and ceramide. Production of ceramide through nSMase2 upregulation has been associated with diverse cellular processes ranging from apoptosis (1) to modulation of synaptic plasticity (2) to manufacturing of ceramide-rich exosomes (3). While transient nSMase2 upregulation is part of normal brain functioning, chronic upregulation of the enzyme has been implicated in the pathogenesis of various neurodegenerative diseases associated with excess ceramide including Alzheimer’s disease (AD) (4-9), HIV-associated neurocognitive disorders (HAND) (1, 10), multiple sclerosis (MS) (11) and amyotrophic lateral sclerosis (ALS) (12). NSMase2 is predominantly expressed in the brain and it is this isoform, rather than nSMase 1 or 3, that has been implicated in neurodegenerative disease (13, 14). Post mortem AD brains exhibit abnormal increases in endogenous ceramide (5, 8, 15) and ceramide has been shown to be an integral component of exosomes (3). Recent experimental results suggest that elevation of ceramide-rich exosomes in the brain is involved in AD progression (16, 17), that these exosomes are involved in tau propagation upon secretion from activated microglia (16) and that inhibition of exosome manufacturing through pharmacological inhibition of nSMase2 prevents tau propagation (16). In another recent, independent study using an AD mouse model, ceramide-enriched exosomes exacerbated AD pathology and cognitive deficits while nSMase2 deficiency ameliorated AD pathology and improved cognition (18). The results provide a compelling rationale for the potential use of nSMase2 inhibitors for the treatment of AD.
While nSmase2 has emerged as an important player in AD etiology, the current armamentarium of nSMase2 inhibitors is inadequate to develop potential treatments. Currently available inhibitors have many limitations including low potency (IC50’s in µM level), poor solubility, no brain penetration and their chemical structures have not been amenable to optimization into drug-like molecules (19, 20). In order to address these limitations, we screened several chemical libraries (~ 500,000 compounds) which led to the identification of several hits belonging to different chemical series. One of these hits was significantly improved for potency and selectivity and we now have systematically synthesized several analogs with IC50 < 100 nM that are being further optimized for microsomal stability and brain penetration. Selected optimized compounds will be evaluated in the recently published AD model (16), as well as characterized in models of HAND (2, 10) and multiple sclerosis (21).
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