Total Enantioselective Synthesis and In Vivo Biological Evaluation of a Novel Fluorescent BODIPY α‐Galactosylceramide

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Abstract
Natural killer T (NKT) cells are a distinct subset of mature lymphocytes endowed with features of activated and regulatory T cells. α-Galactosylceramides (α-GalCers), the synthetic prototype of which is KRN7000, are the only natural reagents recognised by the T-cell receptor of NKT cells. The α-GalCer-activated NKT cells promptly release IFNγ and IL-4 (IFN=interferon; IL=interleukin) and undergo apoptotic death within hours. In mice, activated NKT cells are responsible for antitumour activity and protection against autoimmune diseases. KRN7000 can thus be considered as the root of a family of novel immunoregulatory drugs. To get insights into the in vivo behaviour of α-galactosylceramides, an original fluorescent derivative has been prepared by following a convergent synthetic scheme. This strategy allows the introduction of different acyl chains, carbohydrate residues and various labels in the final steps of the synthesis. The fluorescent BODIPY probe derived from a versatile glycolipid precursor is as active as KRN7000 for inducing apoptosis of liver NKT cells. Fluorescence was detected in peritoneal macrophages and splenic antigen-presenting cells, in Kupffer-like cells in the liver, but not in lymphocytes.