This article is available online at http://www.jlr.org chaperone therapy, bone marrow transplantation, or gene therapy) ( 1 ). Enzyme replacement therapy is clinically approved for lysosomal storage diseases with peripheral manifestations but is limited by the absence of distribution of infused recombinant enzyme into the central nervous system (CNS) and by the frequent development of auto-antibodies to the protein in patients who carry null mutations. The second strategy involves synthesis inhibition therapy ( 2 ). Synthesis inhibition is a more recent therapeutic approach and has focused on identifying small molecule inhibitors of GCS. Two classes of these inhibitors have been described, including imino sugars and analogs of D-threo -1-phenyl-2-decanoylamino-3-morpholino-propanol (PDMP) ( 3 ). The imino sugar N-butyldeoxynojirimycin (NBDNJ) is limited by its micromolar level inhibitory activity and limited specifi city against the synthase. The latter trait is associated with a high level of untoward effects resulting from secondary sites of action unrelated to glycolipid synthesis inhibition. These effects most notably include diarrhea, weight loss, and tremor, and limit its approved use in the United States ( 4 ). However, one distinct advantage of NBDNJ over the PDMP-based homologs reported to date is its ability to distribute into the CNS.The active lead compound that is currently in clinical trials and is structurally related to PDMP is N- Two general strategies for the treatment of lysosomal storage diseases exist. The fi rst strategy includes the replacement or restoration of the defective or absent catabolizing enzyme (e.g., the infusion of recombinant enzyme, Abbreviations: BBB, blood-brain barrier; CNS, central nervous system; GlcCer, glucosylceramide; GCS, glucosylceramide synthase; HBA, number of hydrogen bond acceptors; HBD, number of hydrogen bond donors; MDR1, P-glycoprotein; MDR/WT, the ratio of MDR-MDCKII IC 50 divided by WT-MDCKII IC 50 ; MW, molecular weight; NBDNJ, Nbutyldeoxynojirimycin; PDMP, D-threo -1-phenyl-2-decanoylamino-3-morpholino-propanol; RotB, rotatable bonds; TPSA, topological polar surface area; WT, wild-type.