1-Azahomocubane

Highly strained cage hydrocarbons have long stood as fundamental molecules to explore the limits of chemical stability and reactivity, probe physical properties, and more recently as bioactive molecules and in materials discovery. Interestingly, the nitrogenous congeners have attracted much less attention. Previously absent from the literature, azahomocubanes, offer an opportunity to investigate the effects of a nitrogen atom when incorporated into a highly constrained polycyclic environment. Herein disclosed is the synthesis of 1-azahomocubane, accompanied by comprehensive structural characterization, physical property analysis and chemical reactivity. These data support the conclusion that nitrogen is remarkably well tolerated in a highly strained environment.

Semi-synthetic ecdysteroids as gene-switch actuators: synthesis, structure-activity relationships, and prospective ADME properties

The ligand-inducible, ecdysteroid receptor (EcR) gene-expression system can add critical control features to protein expression in cell and gene therapy. However, potent natural ecdysteroids possess absorption, distribution, metabolism and excretion (ADME) properties that have not been optimised for use as gene-switch actuators in vivo. Herein we report the first systematic synthetic exploration of ecdysteroids toward modulation of gene-switch potency. Twenty-three semi-synthetic O-alkyl ecdysteroids were assayed in both a natural insect system (Drosophila B(II) cells) and engineered gene-switch systems in mammalian cells using Drosophila melanogaster, Choristoneura fumiferana, and Aedes aegypti EcRs. Gene-switch potency is maintained, or even enhanced, for ecdysteroids methylated at the 22-position in favourable cases. Furthermore, trends toward lower solubility, higher permeability, and higher blood-brain barrier penetration are supported by predicted ADME properties, calculated using the membrane-interaction (MI)-QSAR methodology. The structure-activity relationship (SAR) of alkylated ecdysteroids indicates that 22-OH is an H-bond acceptor, 25-OH is most likely an H-bond donor, and 2-OH and 3-OH are donors and/or acceptors in network with each other, and with the EcR. The strategy of alkylation points the way to improved ecdysteroidal actuators for switch-activated gene therapy.

Effect of ecdysone receptor gene switch ligands on endogenous gene expression in 293 cells

Regulated gene expression may substantially enhance gene therapy. Correlated with structural differences between insect ecdysteroids and mammalian steroids, the ecdysteroids appear to have a benign pharmacology without adversely interfering with mammalian signaling systems. Consequently, the ecdysone receptor-based gene switches are attractive for application in medicine. In the present study, the effect of inducers of ecdysone receptor switches on the expression of endogenous genes in HEK 293 cells was determined. Four ligand chemotypes, represented by a tetrahydroquinoline (RG-120499), one amidoketone (RG-121150), two ecdysteroids [20-hydroxyecdysone (20E) and ponasterone A (Pon A)], and four diacylhydrazines (RG-102240, RG-102277, RG-102398 and RG-100864), were tested in HEK 293 cells. The cells were exposed to ligands at concentrations of 1 microm (RG-120499) or 10 microm (all others) for 72 h and the total RNA was isolated and analyzed using microarrays. Microarray data showed that the tetrahydroquinoline ligand, RG-120499 caused cell death at concentrations > or = 10 microm. At 1 microm, this ligand caused changes in the expression of genes such as TNF, MAF, Rab and Reprimo. At 10 microm, the amidoketone, RG-121150, induced changes in the expression of genes such as v-jun, FBJ and EGR, but was otherwise noninterfering. Of the two steroids tested, 20E did not affect gene expression, but Pon A caused some changes in the expression of endogenous genes. At lower concentrations pharmacologically relevant for gene therapy, intrinsic gene expression effects of ecdysteroids and amidoketones may actually be insignificant. A fortiori, even at 10 microm, the four diacylhydrazine ligands did not cause significant changes in expression of endogenous genes in 293 cells and therefore should have minimum pleiotropic effects when used as ligands for the ecdysone receptor gene switch.

Non-genomic ecdysone effects and the invertebrate nuclear steroid hormone receptor EcR--new role for an "old" receptor?

The ecdysteroids (Ec), invertebrate steroid hormones, elicit genomic but also non-genomic effects. By analogy to vertebrates, non-genomic responses towards Ec may be mediated not only by distinct membrane-integrated but also by membrane-associated receptors like the classical nuclear ecdysteroid receptor (EcR) of arthropods. This is supported by a comparison of physiological properties between invertebrate and vertebrate steroid hormone systems and recent findings on the subcellular localization of EcR. The measured or predicted high degree of conformational flexibility of both Ec and the ligand binding domain (LBD) of EcR give rise to a conformational compatibility model: the compatibility between conformations of the cognate receptor's ligand binding domain and structures or conformations of the ligand would determine their interaction and eventually the initiation of genomic versus non-genomic pathways. This model could also explain why specific non-genomic effects are generally not observed with non-steroidal agonists of the bisacylhydrazine group.

Evaluation of Solution and Solid-Phase Approaches to the Synthesis of Libraries of α,α-Disubstituted-α-acylaminoketones

Solid phase, solution, and hybrid approaches to the synthesis of small focused libraries of alpha,alpha-disubstituted-alpha-acylaminoketones have been explored. Solution and hybrid approaches that used support-bound reagents and scavenger resins were the most productive.

Highly flexible ligand binding pocket of ecdysone receptor: a single amino acid change leads to discrimination between two groups of nonsteroidal ecdysone agonists

The insect steroid hormone 20-hydroxyecdysone works through a ligand-activated nuclear receptor, the ecdysone receptor (EcR), which plays critical roles in insect development and reproduction. The EcR has been exploited to develop insecticides to control pests and gene switches for gene regulation. Recently reported crystal structures of the EcR protein show different but partially overlapping binding cavities for ecdysteroid (ECD) and diacylhydrazine (DAH) ligands, providing an explanation for the differential activity of DAH ligands in insects. 1-Aroyl-4-(arylamino)-1,2,3,4-tetrahydroquinoline (THQ) ligands were recently discovered as ecdysone agonists. Mutagenesis of the EcR (from Choristoneura fumiferana, CfEcR) ligand binding domain followed by screening in a reporter assay led to the identification of CfEcR mutants, which responded well to THQ ligands but poorly to both ECD and DAH ligands. These mutants were further improved by introducing a second mutation, A110P, which was previously reported to cause ECD insensitivity. Testing of these V128F/A110P and V128Y/A110P mutants in a C57BL/6 mouse model coactivator interaction assay and in insect cells showed that this mutant EcR is activated by THQ ligands but not by ECD or DAH ligands. The CfEcR and its V128F/A110P mutant were used to demonstrate simultaneous regulation of two reporter genes using THQ and DAH ligands.

Superimposition evaluation of ecdysteroid agonist chemotypes through multidimensional QSAR

The EC50 values for a training set of 66 ecdysteroids and 97 diacylhydrazines were measured in the ecdysteroid-responsive Drosophila melanogaster BII cell line, a prototypical homologous inducible gene expression system. Each of eight superimposition hypotheses for the folded diacylhydrazine conformation was evaluated and ranked on the basis of CoMFA and 4D-QSAR Q2 values for the training set and R2 values for a 52-member test set comprising randomly-chosen diacylhydrazines and chronologically-chosen ecdysteroids for which data became available after model construction. Both 4D-QSAR and CoMFA rate a common superimposition as the preferred one. Two additional superimpositions, with somewhat weaker 4D-QSAR and CoMFA consensus, nonetheless share several important topological features. The resultant QSAR models address the question of relative binding orientation of the two ligand families and can be useful as a virtual screen for new chemotypes.

Optimization of alpha-acylaminoketone ecdysone agonists for control of gene expression

Fifteen new alpha-acylaminoketones were prepared by four different routes in an initial effort to optimize the potency of these compounds as ecdysone agonists. The compounds were assayed in mammalian cells expressing the ecdysone receptors from Bombyx mori (BmEcR) and Choristoneura fumiferana (CfEcR) for their ability to cause expression of a reporter gene downstream of an ecdysone response element. A new alpha-acylaminoketone was identified which had activity equal to that of the standard dibenzoylhydrazine ecdysone agonist GS()-E in the assay based on CfEcR.

Synthesis and biological activities of turkesterone 11alpha-acyl derivatives

Turkesterone is a phytoecdysteroid possessing an 11alpha-hydroxyl group. It is an analogue of the insect steroid hormone 20-hydroxyecdysone. Previous ecdysteroid QSAR and molecular modelling studies predicted that the cavity of the ligand binding domain of the ecdysteroid receptor would possess space in the vicinity of C-11/C-12 of the ecdysteroid. We report the regioselective synthesis of a series of turkesterone 11alpha-acyl derivatives in order to explore this possibility. The structures of the analogues have been unambiguously determined by spectroscopic means (NMR and low-resolution mass spectrometry). Purity was verified by HPLC. Biological activities have been determined in Drosophila melanogaster B(II) cell-based bioassay for ecdysteroid agonists and in an in vitro radioligand-displacement assay using bacterially-expressed D. melanogaster EcR/USP receptor proteins. The 11alpha-acyl derivatives do retain a significant amount of biological activity relative to the parent ecdysteroid. Further, although activity initially drops with the extension of the acyl chain length (C2 to C4), it then increases (C6 to C10), before decreasing again (C14 and C20). The implications of these findings for the interaction of ecdysteroids with the ecdysteroid receptor and potential applications in the generation of affinity-labelled and fluorescently-tagged ecdysteroids are discussed.

Synthesis and SAR of alpha-acylaminoketone ligands for control of gene expression

A lead discovery library and a follow-up focused library of alpha-acylaminoketones were designed based on known dibenzoylhydrazine ecdysone agonists, including GS(TM)-E. The compounds were assayed in mammalian cells expressing the ecdysone receptor from Bombyx mori for their ability to cause expression of a reporter gene downstream of an ecdysone response element. The most potent alpha-acylaminoketones were comparable to GS(TM)-E in this assay.

Ecdysteroid 7,9(11)-dien-6-ones as potential photoaffinity labels for ecdysteroid binding proteins

Three ecdysteroid 7,9(11)-dien-7-ones (dacryhainansterone, 25-hydroxydacryhainansterone and kaladasterone) were prepared by dehydration of the corresponding 11a-hydroxy ecdysteroids (ajugasterone C, turkesterone and muristerone A, respectively). The biological activities of the dienones in the Drosophila melanogaster B(II) cell bioassay, which reflect the affinity for the ecdysteroid receptor complex, showed that the dienones retain high biological activity. Irradiation at 350 nm of the ecdysteroid dienones (100 nM) with bacterially-expressed dipteran and lepidopteran ecdysteroid receptor proteins (DmEcR/DmUSP or CfEcR/CfUSP), followed by loading with [(3)H]ponasterone A revealed that irradiation of dacryhainansterone or kaladasterone resulted in blocking of >70% of the specific binding sites. Thus, ecdysteroid dienones show considerable potential as photoaffinity analogues for ecdysteroid binding proteins.

4D-QSAR analysis of a set of ecdysteroids and a comparison to CoMFA modeling

The ecdysteroid-responsive Drosophila melanogaster B(II) cell line is a prototypical homologous inducible gene expression system. A training set of 71 ecdysteroids, for which the -log(EC(50)) potencies in the ecdysteroid-responsive B(II) cell line were measured, was used to construct 4D-QSAR models. Four nearly equivalent optimum 4D-QSAR models, for two modestly different alignments, were identified (Q(2) = 0.76-0.80). These four models, together with two CoMFA models, were used in consensus modeling to arrive at a three-dimensional pharmacophore. The C-2 and C-22 hydroxyls are identified as hydrogen-bond acceptor sites which enhance activity. A hydrophobic site near C-12 is consistent with increasing activity. The side-chain substituents at C-17 are predicted to adopt semiextended "active" conformations which could fit into a cylinder-shaped binding pocket lined largely with nonpolar residues for enhanced activity. A test set of 20 ecdysteroids was used to evaluate the QSAR models. Two 4D-QSAR models for one alignment were identified to be superior to the others based on having the smallest average residuals of prediction for the prediction set (0.69 and 1.13 -log[EC(50)] units). The correlation coefficients of the optimum 4D-QSAR models (R(2) = 0.87 and 0.88) are nearly the same as those of the best CoMFA model (R(2) = 0.92) determined for the same training set. However, the cross-validation correlation coefficient of the CoMFA model is less significant (Q(2) = 0.59) than those of the 4D-QSAR models (Q(2) = 0.80 and 0.80).

An extensive ecdysteroid CoMFA

The ecdysteroid agonist activity of 71 HPLC-purified ecdysteroids was measured in the Drosophila melanogaster BII tumorous blood cell line assay. The resultant log(ED50) values, spanning almost 6 orders of magnitude, were used to construct a comparative molecular field analysis (CoMFA) model in which conformations were selected by homology to the crystal structure of ecdysone. Model A was constructed by utilization of the region-focused electrostatic indicator field (q2 = 0.631, r2 = 0.903, 5 components, 4 outliers). Model B made use of region-focused electrostatic and steric indicator fields along with MlogP (q2 = 0.694, r2 = 0.892, 5 components, 4 outliers). The model and its underlying bioassay data support a pharmacophore hypothesis in which ecdysteroid binding is understood to be due principally to the summation of localized interactions from approximately six specific loci. This is in contrast to previous structure-activity relationship hypotheses which are formulated in terms of the presence or absence of essential functional groups, without which ecdysteroid receptor affinity would be completely absent. The present CoMFA model is utilized to predict the activities of heretofore unknown ecdysteroids.