LXR␣ and -␤ are nuclear receptors that regulate the metabolism of several important lipids, including cholesterol and bile acids. Previously, we have proposed that LXRs regulate these pathways through their interaction with specific, naturally occurring oxysterols, including 22(R)-hydroxycholesterol, 24(S)-hydroxycholesterol, and 24(S),25-epoxycholesterol. Using a ligand binding assay that incorporates scintillation proximity technology to circumvent many of the problems associated with assaying extremely hydrophobic ligands, we now demonstrate that these oxysterols bind directly to LXRs at concentrations that occur in vivo. To characterize further the structural determinants required for potent LXR ligands, we synthesized and tested a series of related compounds for binding to LXRs and activation of transcription. These studies revealed that position-specific monooxidation of the sterol side chain is requisite for LXR high-affinity binding and activation. Enhanced binding and activation can also be achieved through the use of 24-oxo ligands that act as hydrogen bond acceptors in the side chain. In addition, introduction of an oxygen on the sterol B-ring results in a ligand with LXR␣-subtype selectivity. These results support the hypothesis that naturally occurring oxysterols are physiological ligands for LXRs and show that a rational, structure-based approach can be used to design potent LXR ligands for pharmacologic use.Nonsteroidal nuclear hormone receptors are ligand-activated transcription factors that regulate an array of signaling pathways. Several members of this protein family, including the vitamin D receptor, retinoic acid receptor, and peroxisome proliferator-activated receptor, are primary regulators in nutrient metabolism and are known to be associated with a variety of metabolic disorders (1). These receptors function by forming obligate heterodimers with the retinoid X receptor (RXR) and binding specific DNA sequences (response elements) within the promoters of the genes they regulate. Upon binding ligand, the heterodimerized receptor undergoes a conformational change that results in enhanced transcription of the target gene. Thus far, all known native ligands for nuclear receptors are small hydrophobic compounds, many of which are the products of lipid metabolism (1).The liver X receptors (LXRs) were originally identified as orphan members of the nuclear receptor superfamily because their ligands were unknown. Like other receptors in the family, LXRs heterodimerize with RXR and bind to specific response elements (LXREs) characterized by direct repeats separated by four nucleotides (2-4). Two genes (␣ and ␤) are known to encode LXR proteins. LXR␣ is expressed most highly in the liver and to a lesser extent in the kidney, small intestine, spleen, and adrenal gland (2, 4). In contrast to the restricted expression pattern of LXR␣, LXR␤ is ubiquitously expressed (5). In a comprehensive screen for LXR ligands we observed that the naturally occurring oxysterols 24(S)-hydroxycholesterol [24(S)-H...

[formula: see text] A novel catalytic enantioselective Strecker synthesis of chiral alpha-amino nitriles and alpha-amino acids is described and analyzed with regard to the possible mechanistic basis for stereoselectivity. Key features of the enantioselective process include (1) the use of the chiral bicyclic guanidine 1 as catalyst and (2) the use of the N-benzhydryl substituent on the imine substrate.

Recent evidence suggests that blocking aberrant hedgehog pathway signaling may be a promising therapeutic strategy for the treatment of several types of cancer. Cyclopamine, a plant Veratrum alkaloid, is a natural product antagonist of the hedgehog pathway. In a previous report, a seven-membered D-ring semisynthetic analogue of cyclopamine, IPI-269609 (2), was shown to have greater acid stability and better aqueous solubility compared to cyclopamine. Further modifications of the A-ring system generated three series of analogues with improved potency and/or solubility. Lead compounds from each series were characterized in vitro and evaluated in vivo for biological activity and pharmacokinetic properties. These studies led to the discovery of IPI-926 (compound 28), a novel semisynthetic cyclopamine analogue with substantially improved pharmaceutical properties and potency and a favorable pharmacokinetic profile relative to cyclopamine and compound 2. As a result, complete tumor regression was observed in a Hh-dependent medulloblastoma allograft model after daily oral administration of 40 mg/kg of compound 28.