Synthesis and biological activities of inositol 1,4,5-trisphosphate mimics related to xylopyranosides

Both d- and l-Glucose Polyphosphates Mimic d-myo-Inositol 1,4,5-Trisphosphate: New Synthetic Agonists and Partial Agonists at the Ins(1,4,5)P3 Receptor

Chiral sugar derivatives are potential cyclitol surrogates of the Ca²⁺-mobilizing intracellular messenger d-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃]. Six novel polyphosphorylated analogues derived from both d- and l-glucose were synthesized. Binding to Ins(1,4,5)P₃ receptors [Ins(1,4,5)P₃R] and the ability to release Ca²⁺ from intracellular stores via type 1 Ins(1,4,5)P₃Rs were investigated. β-d-Glucopyranosyl 1,3,4-tris-phosphate, with similar phosphate regiochemistry and stereochemistry to Ins(1,4,5)P₃, and α-d-glucopyranosyl 1,3,4-tris-phosphate are full agonists, being equipotent and 23-fold less potent than Ins(1,4,5)P₃, respectively, in Ca²⁺-release assays and similar to Ins(1,4,5)P₃ and 15-fold weaker in binding assays. They can be viewed as truncated analogues of adenophostin A and refine understanding of structure-activity relationships for this Ins(1,4,5)P₃R agonist. l-Glucose-derived ligands, methyl α-l-glucopyranoside 2,3,6-trisphosphate and methyl α-l-glucopyranoside 2,4,6-trisphosphate, are also active, while their corresponding d-enantiomers, methyl α-d-glucopyranoside 2,3,6-trisphosphate and methyl α-d-glucopyranoside 2,4,6-trisphosphate, are inactive. Interestingly, both l-glucose-derived ligands are partial agonists: they are among the least efficacious agonists of Ins(1,4,5)P₃R yet identified, providing new leads for antagonist development.

Chemistry of xylopyranosides

Xylose is one of the few monosaccharidic building blocks that are used by mammalian cells. In comparison with other monosaccharides, xylose is rather unusual and, so far, only found in two different mammalian structures, i.e. in the Notch receptor and as the linker between protein and glycosaminoglycan (GAG) chains in proteoglycans. Interestingly, simple soluble xylopyranosides can not only initiate the biosynthesis of soluble GAG chains but also function as inhibitors of important enzymes in the biosynthesis of proteoglycans. Furthermore, xylose is a major constituent of hemicellulosic xylans and thus one of the most abundant carbohydrates on Earth. Altogether, this has spurred a strong interest in xylose chemistry. The scope of this review is to describe synthesis of xylopyranosyl donors, as well as protective group chemistry, modifications, and conformational analysis of xylose.

Conformational and inframolecular studies of the protonation of adenophostin analogues lacking the adenine moiety

Four adenophostin analogues lacking the adenine moiety were subjected to 31P- and 1H-NMR titrations in order to determine the acid-base behaviour of the individual ionisable groups of the molecules and the complex interplay of intramolecular interactions resulting from the protonation process. For the two trisphosphorylated compounds, the curve pattern of the phosphorus nuclei corresponds to the superimposition of the titration curves of a monophosphorylated polyol and a polyol carrying two vicinal phosphates, suggesting that the two phosphate moieties behave independently. Also, the general shape of 1H-NMR titration curves of the studied compounds is very close to that of adenophostin A, indicating that the adenine moiety does not specifically interact with the phosphorylated sugar moieties. The curves show, however, that both trisphosphorylated compounds adopt slightly different preferential conformations which could contribute to explain the difference in their affinity for Ins(1,4,5)P3 receptor. Their macroscopic as well as the microscopic protonation constants are higher than those of adenophostin A, indicating that the adenine moiety plays a base-weakening effect on the phosphate groups. Further analysis of the microscopic protonation constants confirms that the compound whose conformation is the closest to that of adenophostin A also shows the highest biological activity. The two bisphosphorylated analogues studied behave very similarly, suggesting that the deletion of the hydroxymethyl group on the pentafuranosyl ring only weakly influences the protonation process of the phosphate groups that bear the glucopyranose moiety.

Synthesis and Ca2+-Mobilizing Activity of Purine-Modified Mimics of Adenophostin A: A Model for the Adenophostin−Ins(1,4,5)P3Receptor Interaction

The synthesis of a series of adenophostin A analogues modified at C-6 and C-2 of adenine is described. The target compounds were synthesized by a convergent route involving a modified Vorbrüggen condensation of either 6-chloropurine or 2,6-dichloropurine with a protected disaccharide, yielding two versatile intermediates capable of undergoing substitution with a range of nucleophiles. The new analogues showed a range of abilities to mobilize Ca(2+) from the intracellular stores of permeabilized hepatocytes and are among the first totally synthetic compounds to approach the activity of adenophostin A. In agreement with the biological results, docking studies of adenophostin A using the recently reported X-ray crystal structure of the type 1 Ins(1,4,5)P(3) receptor binding core suggested that, in likely binding modes of adenophostin A, the area around N(6) may be relatively open, identifying this region of the adenophostin A molecule as a promising target for further elaboration. The docking results also point to specific interactions involving residues within the binding domain of the Ins(1,4,5)P(3) receptor that may be involved in the molecular recognition of the adenophostins.

D-myo-inositol-1,4,5-trisphosphate and adenophostin mimics: importance of the spatial orientation of a phosphate group on the biological activity

Three different routes for the synthesis of heterocyclic analogues of the second messenger D-myo-inositol-1,4,5-trisphosphate (InsP(3)) and the natural adenophostins, starting from allyl D-xyloside are described. The two diastereoisomers at C-2 of new compounds, which we named xylophostins, were obtained. The preliminary biological studies shows that the presence of the adenine residue has a beneficial effect on the affinity for the receptor. The low potency of one of the two diastereoisomeric compounds shows that the configuration of the carbon bearing the non-vicinal phosphate group is an important requirement for a high affinity to the receptor. These results provide evidence for the existence of a binding pocket for the adenine ring nearby the InsP(3) binding site. The consequence of these stabilizing interactions should be to place the phosphate group in a suitable position to perfectly mimic InsP(3) in the more active diastereoisomer. Obviously, in the other diastereoisomer, the phosphate cannot accommodate the same orientation, thus explaining the low affinity. The existence of such a binding pocket for adenine is in line with the high potency of adenophostins.

Total synthesis of nucleobase-modified adenophostin A mimics

The adenophostins exhibit approximately 10-100 times higher receptor binding and Ca2+ mobilising potencies in comparison with the natural second messenger D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. Despite many synthetic attempts to determine the minimal structural requirement for this unusual behaviour of the adenophostins, few related simplified analogues displaying higher activity than that of Ins(1,4,5)P3 have been reported. However, biological evaluation of such analogues has revealed that one of the key factors for the enhanced biological activity is the adenine moiety. To further understand the effect that the adenine base has upon the activity of the adenophostins, congeners in which this functionality is replaced by uracil, benzimidazole, 2-methoxynaphthalene, 4-methylanisole and 4-methylnaphthalene using the common intermediate 1,2-di-O-acetyl-5-O-benzyl-3-O-(3,4-di-O-acetyl-2,6-di-O-benzyl-alpha-D-glucopyranosyl)-ribofuranose have been synthesised using a base replacement strategy. The synthesis of the uracil and benzimidazole analogues was achieved using the Vorbrüggen condensation procedure. The 1'-C-glycosidic analogues were prepared using Friedel-Crafts type C-aryl glycosidation reactions. Phosphate groups were introduced using the phosphoramidite method with subsequent removal of all-benzyl protecting groups by catalytic hydrogenation or catalytic hydrogen transfer. Apart from one analogue with an alpha-glycosidic linkage all compounds were more potent than Ins(1,4,5)P3 and most tended more towards adenophostin in activity. These analogues will be valuable tools to unravel the role that the adenine moiety plays in the potent activity of the adenophostins and demonstrate that this strategy is effective at producing highly potent ligands.

Bicyclic analogues of D-myo-inositol 1,4,5-trisphosphate related to adenophostin A: synthesis and biological activity

The high affinity of adenophostin A for 1D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] receptors may be related to an alteration in the position of its 2'-phosphate group relative to the corresponding 1-phosphate group in Ins(1,4,5)P(3). To investigate this possibility, two bicyclic trisphosphates 9 and 10, designed to explore the effect of relocating the 1-phosphate group of Ins(1,4,5)P(3) using a novel fused-ring system, were synthesized from myo-inositol. Biological evaluation of 9 and 10 at the Ins(1,4,5)P(3) receptors of hepatocytes showed that both were recognized by hepatic Ins(1,4,5)P(3) receptors and both stimulated release of Ca(2+) from intracellular stores, but they had lower affinity than Ins(1,4,5)P(3). This finding may be explained by considering the three-dimensional structures of 9 and 10 in light of recent studies on the conformation of adenophostin A.

Xylopyranoside-based agonists of D-myo-inositol 1,4,5-trisphosphate receptors: synthesis and effect of stereochemistry on biological activity

The synthesis of a series of tetrahydrofuranyl alpha- and beta-xylopyranoside trisphosphates, designed by excision of three motifs of adenophostin A is reported. The synthetic route features improved preparations of allyl alpha-D-xylopyranoside and its 2-O-benzyl ether, and gives access to four diastereoisomeric trisphosphates, which show a range of abilities to mobilise Ca2+ from the intracellular stores of hepatocytes. A comparison of the potencies of the four trisphosphates provides useful information relating to the effects of stereochemical variation on the recognition of carbohydrate-based trisphosphates by D-myo-inositol 1,4,5-trisphosphate receptors. 1-O-[(3'S,4'R)-3-hydroxytetrahydrofuran-4-yl] alpha-D-xylopyranoside 3,4,3'-trisphosphate (8) is the most active member of the series with a potency close to Ins(1,4,5)P3; a beta-linked analogue, 1-O-[(3'R,4'S)-3-hydroxytetrahydrofuran-4-yl] beta-D-xylopyranoside 3,4,3'-trisphosphate, is ca. 20-fold weaker than Ins(1,4,5)P3, and the other compounds are much less active. While no compound attained a potency close to that of adenophostin A, we believe that 8 represents the minimal structure for potent Ca2+-releasing activity in this type of carbohydrate-based analogue.

Contribution of the adenine base to the activity of adenophostin A investigated using a base replacement strategy

Syntheses of 3'-O-alpha-D-glucopyranosyl-1-beta-D-ribofuranosidoimidazole 2',3'', 4''-trisphosphate (7) and 3'-O-alpha-D-glucopyranosyl-9-beta-D-ribofuranosidopurine 2',3'',4''- trisphosphate (8), two analogues of the superpotent 1D-myo-inositol 1,4,5-trisphosphate receptor agonist adenophostin A (2), are described. 5-O-Benzyl-1, 2-O-isopropylidene-alpha-D-ribofuranose was prepared by an improved route from 1,2-O-isopropylidene-alpha-D-xylofuranose and was coupled with 3,4-di-O-acetyl-2,6-di-O-benzyl-D-glucopyranosyl dimethyl phosphite to give 3',4'-di-O-acetyl-2',5, 6'-tri-O-benzyl-3-O-alpha-D-glucopyranosyl-1, 2-O-isopropylidene-alpha-D-ribofuranose. Removal of the isopropylidene acetal and subsequent acetylation gave the central disaccharide 1,2,3',4'-tetra-O-acetyl-2',5, 6'-tri-O-benzyl-3-O-alpha-D-glucopyranosyl-D-ribofuranose. Vorbrüggen condensation with activated imidazole or purine gave the required beta-substituted derivatives which were further elaborated to 7 and 8, respectively. Radioligand binding assays to hepatic InsP(3) receptors and functional assays of Ca(2+) release from permeabilized hepatocytes gave a rank order of potency of the ligands 2 approximately 8 > 7 approximately Ins(1,4,5)P(3) indicating that the N(6)-amino group of 2 is of little importance for activity and that a minimum of a two-fused-ring nucleobase is required for activity to exceed that of Ins(1,4,5)P(3). The role of the adenine base in the activity of the adenophostins is discussed. This general method should facilitate ready access to nucleobase-modified adenophostin analogues for SAR studies.

C-glycoside based mimics of D-myo-inositol 1,4,5-trisphosphate

Epimeric C-glycoside based polyphosphates, alpha- and beta-D-glucopyranosylmethanol 3,4,1'-trisphosphates (8 and 9) were prepared from D-glucose. The key intermediate, allyl 2,6-di-O-benzyl-alpha-D-glucopyranoside, was prepared in five steps (67% yield) from allyl alpha-D-glucopyranoside without the need for chromatography. Compounds 8 and 9 were shown to be full agonists at the Ins(1,4,5)P3 receptors of permeabilised hepatocytes, but with markedly different potencies. Such C-glycoside analogues are worthy of further development as Ins(1,4,5)P, receptor ligands.

Simplification of adenophostin A defines a minimal structure for potent glucopyranoside-based mimics of D-myo-inositol 1,4,5-trisphosphate

The synthesis of 1-O-[(3S,4R)-3-hydroxytetrahydrofuran-4-yl]-alpha-D-glucopyranosid e 3,4,3'-trisphosphate (7), a novel Ca2+ mobilising agonist at the Ins(1,4,5)P3 receptor, designed by excision of two motifs of adenophostin A is reported, defining a potential minimal structure for potent glucopyranoside-based agonists of Ins(1,4,5)P3 receptors.

New developments in the molecular pharmacology of the myo-inositol 1,4,5-trisphosphate receptor

Receptor-mediated activation of phospholipase C to generate inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] is a ubiquitous signalling pathway in mammalian systems. A family of three IP3 receptor subtype monomers form functional tetramers, which act as effectors for Ins(1,4,5)P3, providing a ligand-gated channel that allows Ca2+ ions to move between cellular compartments. As IP3 receptors are located principally, although not exclusively, in the endoplasmic reticular membrane, Ins(1,4,5)P3 is considered to be a second messenger that mobilizes Ca2+ from intracellular stores. Ca2+ store mobilization by Ins(1,4,5)P3 can be shown to contribute to a variety of physiological and pathophysiological phenomena, and therefore the IP3 receptor represents a novel, potential pharmacological target. In this article, Rob Wilcox and colleagues review recent developments in IP3 receptor pharmacology, with particular emphasis on ligand molecular recognition by this receptor-channel complex. The potential for designing non-inositol phosphate-based agonists and antagonists is also discussed.

Disaccharide polyphosphates based upon adenophostin A activate hepatic D-myo-inositol 1,4,5-trisphosphate receptors

The glyconucleotides adenophostin A and B are the most potent known agonists at type 1 inositol trisphosphate [Ins(1,4,5)P3] receptors, although their stuctures differ markedly from that of Ins(1,4,5)P3. Equilibrium competition binding with [3H]Ins(1,4,5)P3 and unidirectional 45Ca2+ flux measurements were used to examine the effects of adenophostin A in hepatocytes, which express predominantly type 2 Ins(1,4,5)P3 receptors. Both Ins(1,4,5)P3 (Kd = 8.65 +/- 0.98 nM) and adenophostin A (Kd = 0.87 +/- 0.20 nM) bound to a single class of [3H]Ins(1,4,5)P3-binding site and each fully mobilized the same intracellular Ca2+ pool; although, adenophostin A (EC50 = 10.9 +/- 0.7 nM) was more potent than Ins(1,4,5)P3 (EC50 = 153 +/- 11 nM). Working on the assumption that it is the phosphorylated glucose component of the adenophostins that mimics the critical features of Ins(1,4,5)P3, we synthesized various phosphorylated disaccharide analogs containing this structure. The novel disaccharide-based analogs, sucrose 3,4,3'-trisphosphate [Sucr(3,4,3')P3], alpha,alpha'-trehalose 3,4,3',4'-tetrakisphosphate [Trehal(3,4,3',4')P4], alpha,alpha'-trehalose 2,4,3', 4'-tetrakisphosphate [Trehal(2,4,3',4')P4], and methyl 3-O-(alpha-d-glucopyranosyl)-beta-d-ribofuranoside 2,3', 4'-trisphosphate [Rib(2,3',4')P3], were all able to mobilize the same intracellular Ca2+ pool as Ins(1,4,5)P3 and adenophostin A; although, none was as potent as adenophostin A. The rank order of potency of the analogs, adenophostin A > Ins(1,4,5)P3 approximately Rib(2,3',4')P3 > Trehal(2,4,3',4')P4 > Glc(2',3,4)P3 approximately Trehal(3,4,3',4')P4 > Sucr(3,4,3')P3, was the same in radioligand binding and functional assays of hepatic Ins(1,4,5)P3 receptors. Both Rib(2,3',4')P3, which was as potent as Ins(1,4,5)P3, and Trehal(2,4,3',4')P4 bound with significantly higher affinity ( approximately 27 and approximately 3-fold, respectively) than the only active carbohydrate agonist of Ins(1,4,5)P3 receptors previously examined [Glc(2',3,4)P3]. We conclude that phosphorylated disaccharides provide novel means of developing high-affinity ligands of Ins(1,4,5)P3 receptors.

A Ca(2+)-mobilising carbohydrate-based polyphosphate: synthesis of 2-hydroxyethyl alpha-D-glucopyranoside 2',3,4-trisphosphate

Two routes to a glucose-based mimic of the second messenger 1D-myo-inositol 1,4,5-trisphosphate related to adenophostin A are described. Fischer glycosidation of D-glucose with allyl alcohol in the presence of a strong cation-exchange resin gave a 7:3 alpha: beta-anomeric mixture of allyl glucopyranosides (5ab) from which the pure alpha anomer 5a was isolated by crystallisation. Treatment of 5ab with 1.05 equiv of dibutyltin oxide followed by 2.1 equiv of benzoyl chloride gave allyl 2,6-di-O-benzoyl-alpha-D-glucopyranoside, which was converted in 3 steps into allyl 2,6-di-O-benzyl-3,4-O-isopropylidene-alpha-D-glucopyranoside (4). Alternatively, treatment of 5a with 2.5 equiv of dibutyltin oxide followed by benzyl bromide gave allyl 2,6-di-O-benzyl-alpha-D-glucopyranoside (9) which was also converted into 4. Compound 4 was elaborated to the phosphorylation precursor 2-hydroxyethyl 2,6-di-O-benzyl-alpha-D-glucopyranoside (12) in a convenient one-pot reaction, and 12 was phosphorylated and deblocked to afford 2-hydroxyethyl alpha-D-glucopyranoside 2',3,4-trisphosphate. The 2,6-di-O-benzyl derivative 9 was converted in high yield into 2,6-di-O-benzyl-3,4-di-O-(p-methoxybenzyl)-D-glucopyranose, a useful intermediate for the synthesis of adenophostin A and related compounds.