Effects of Heterocyclic Aromatic Substituents on Binding Affinities at Two Distinct Sites of Somatostatin Receptors. Correlation with the Electrostatic Potential of the Substituents

Abstract

In our continuing program exploring glucose-based peptidomimetics of somatostatin (SRIF-14), we sought to improve the water solubility of our glycosides. This led to insights into the nature of the ligand binding sites at the SRIF receptor. Replacement of the C4 benzyl substituent in glucoside (+)-2 with pyridinylmethyl or pyrazin-2-ylmethyl congeners increased water solubility and enhanced affinity for the human SRIF subtype receptor 4 (sst4). We attribute this effect to hydrogen bond formation. The pyridin-3-ylmethyl substituent at C4, when combined with the imidazol-4-ylmethyl group at C2, generated (-)-19, which has the highest affinity of a glucose-based peptidomimetic at a human SRIF receptor to date (K(i) 53 +/- 23 nM, n = 6 at sst4). The C4 heterocyclic congeners of glucosides bearing a 1-methoxy substituent rather than an indole side chain at the anomeric carbon, such as (+)-16, also provided information about the Trp(8) binding pocket. We correlated the SARs at both the C4 and the Trp(8) binding pockets with calculations of the electrostatic potentials of the diverse C4 aromatic substituents using Spartan 3-21G(*) MO analysis. These calculations provide an approximate analysis of a molecule's ability to interact within a receptor binding site. Our binding studies show that benzene and indole rings, but not pyridinylmethyl nor pyrazin-2-ylmethyl rings, can bind the hydrophobic Trp(8) binding pocket of sst4. The Spartan 3-21G(*) MO analysis reveals significant negative electrostatic potential in the region of the pi-clouds for the benzene and indole rings but not for the pyridinylmethyl or pyrazin-2-ylmethyl congeners. Our data further demonstrate that the replacement of benzene or indole side chains by heterocyclic aromatic rings typified by pyridine and pyrazine not only enhances water solubility and hydrogen bonding capacity as expected, but can also profoundly diminish the ability of the pi-cloud of the aromatic substituent to interact with side chains of an aromatic binding pocket such as that for Trp(8) of SRIF-14. Conversely, these calculations accommodate the experimental findings that pyrazin-2-ylmethyl and pyridinylmethyl substituents at C4- of C1-indole-substituted glycosides afford higher affinities at sst4 than the C4-benzyl group of (+)-2. This result is consistent with the high electron density in the plane of the heterocycle depicted in Figure 6 which can accept hydrogen bonds from the C4 binding pocket of the receptor. Unexpectedly, we found that the 2-fluoropyridin-5-ylmethyl analogue (+)-14 more closely resembles the binding affinity of (+)-8 than that of (+)-2, thus suggesting that (+)-14 represents a rare example of a carbon linked fluorine atom acting as a hydrogen bond acceptor. We attribute this result to the ability of the proton to bind the nitrogen and fluorine atoms simultaneously in a bifurcated arrangement. At the NK1 receptor of substance P (SP), the free hydroxyl at C4 optimizes affinity.

Carbohydrate-Based Scaffolds in Drug Discovery

“…Hirschmann and co-workers were the first to employ sugar scaffolds as peptide mimetics during their investigation on somatostatin (somatotropin release inhibiting factor, SRIF) mimetics [1,[26][27][28]. This work formed a landmark achievement in the field of templated drug design and development.…”

“…This mirrors the improvement of hSSTR-activity observed when a phenylalanine of an active cyclic hexapeptide is replaced with a histidine, providing further parallelism between the peptides and their sugar-based mimetics. More recently, a comprehensive SAR study of the congeners of compound 19 was accomplished in the same laboratory [26]. A series of compounds containing various heterocyclic rings at the 4 position, including imidazole, pyrazine and various pyridine derivatives, were prepared systematically and their affinities towards somatostatin receptors examined.…”

Syntheses of Peptidomimetics Based on Pyranose and Polyhydroxylated Piperidine Scaffolds

“…Subsequent alkylation with N-monomethoxytrityl (MMTr)-protected 4-chloromethylimidazole followed by Staudinger reduction, removal of the indole sulfonamide and acidic removal of the monomethoxytrityl group in the presence of a dithiol scavenger gave 11. Hirschmann and co-workers have more recently incorporated heterocyclic aromatic substituents onto the 2-and 4-OH of pyranosides structurally related to 2 and 3 [19]. This was synthetically more demanding than preparation of 3 but has led to the identification of a novel β-Dglucopyranoside based ligand 11 with enhanced affinities (53 nM) at human SRIF (somatostatin) receptor subtypes (sst4 and sst2).…”

Ferrier–Nicholas pyranosidic cations: application to diversity-oriented synthesis

“…In this context, carbohydrates because of their conformational rigidity and the stereo-defined display of their hydroxyl groups were early recognized as valuable substrates for the attainment of substitutional (appendage), stereochemical, and functional group diversity [5,6]. Thus, after some seminal contributions by Smith, Nicolaou, Hirschmann and co-workers on d-glucose based peptidomimetics of somatostatin [7][8][9], reports appeared that focused on the use of pyranose cores for the incorporation of different functional groups. Sofia and co-workers reported the preparation of pyranose templates with three sites of diversification aiming to provide the minimal requirements needed for pharmacophoric molecular recognition [10].…”