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Conformational analysis in solution of β-secretase inhibitors 1 and 2 by NMR spectroscopy reveals that the hydroxyethylene isostere, an apparently flexible fragment widely used as a scissile bond replacement in aspartic protease inhibitors, exists in one predominant conformation in solution. This preferred conformation is similar to that adopted by the hydroxyethylene core of 1 in complex with β-secretase and that adopted by hydroxyethylene cores of related compounds when bound to aspartic proteases, indicating that this structural unit is preorganized in solution.

Several chiral building blocks have been obtained easily in large quantities from an epoxysulfone (9) that could be obtained in both enantiomeric forms from accessible starting materials.

A new amino acid methyl ester with a cyclopropanol has been synthesized starting from the allyl sulfone 10. The starting material, 10, could be obtained in both enantiomeric forms. The stereoselectivity of the cyclopropane formation has been studied by molecular modeling.

In pursuit of a GABAA α5-subtype-selective inverse agonist to enhance cognition, a series of 6,7-dihydro-2-benzothiophen-4(5H)-ones has been identified as a novel class of GABAA receptor ligands. These thiophenes have higher binding affinity for the GABAA α5 receptor subtype compared to the GABAA α1, α2, and α3 subtypes, and several analogues exhibit high GABAA α5 receptor inverse agonism. 6,6-Dimethyl-3-(2-hydroxyethyl)thio-1-(thiazol-2-yl)-6,7-dihydro-2-benzothiophen-4(5H)-one (43) has been identified as a full inverse agonist at the GABAA α5 receptor and is functionally selective over the other major GABAA receptor subtypes. 43 readily penetrates into the CNS to give selective occupancy of GABAA α5 receptors. In addition, 43 enhances cognitive performance in rats in the delayed ‘matching-to-place' Morris water maze test&sbd;a hippocampal-dependent memory task&sbd;without the convulsant or proconvulsant activity associated with nonselective, GABAA receptor inverse agonists.

Nonselective inverse agonists at the benzodiazepine binding site on the GABA-A chloride ion channel enhance cognitive performance in animals but cannot be used in the treatment of cognitive disorders because of anxiogenic and convulsant side effects. We have identified a novel series of GABA-A α5 receptor ligands during our search for α5 receptor inverse agonists as potential cognition enhancers. In particular, 6,6-dimethyl-3-(2-hydroxyethyl)thio-1-(thiazol-2-yl)-6,7-dihydro-2-benzothiophen-4(5H)-one (26) has been identified as a functionally selective GABA-A α5 inverse agonist.

A series of N1-arylsulfonyltryptamines were found to be potent ligands of the human serotonin 5-HT6 receptor with the 5-methoxy-1-benzenesulfonyl analogue (19) having the highest affinity. Additionally, it was discovered that a group such as 3-(3-methoxybenzyl)-1,2,4-oxadiazol-5-yl in the 2-position of the indole ring (43) can replace the arylsulfonyl substituent in the 1-position with no loss of affinity. This suggested that the binding conformation of the aminoethyl side chain at this receptor was toward the 4-position of the indole ring and was supported by the fact that the 4-(aminoethyl)indoles (45) also displayed high affinity, as did the conformationally rigid 1,3,4,5-tetrahydrobenz[c,d]indole (49). Molecular modeling showed that 19, 43, and 45 all had low-energy conformers that overlaid well onto 49. Both 19 and 49 had good selectivity over other serotonin receptors tested, with 49 also showing excellent selectivity over all dopamine receptors. In a functional adenylate cyclase stimulation assay, 19 and 49 had no agonist activity, whereas 45 behaved as a partial agonist. Finally, it was shown that 19 had good activity in the 5-HT2A centrally mediated mescaline-induced head twitch assay, which implies that it is brain-penetrant.

A new series of diterpenylquinone/hydroquinones has been prepared by Diels−Alder cycloaddition between three labdanic diterpenoids (myrceocommunic acid, methyl myrceocommunate, and myrceocommunyl acetate) and p-benzoquinone or 1,4-naphthoquinone. Influences of the quinone/hydroquinone fragment and other structural features, such as the different functionalities in the terpenic core, are considered in relation to the cytotoxicity toward neoplastic cells and the selectivity of these diterpenylnaphthoquinones/hydroquinones and anthraquinones. Several compounds showed IC50 values under the micromolar level, and four of these derivatives were evaluated at the NCI screening panel. The results showed an important selectivity toward renal cancer lines, identifying these compounds as a very promising group of antineoplastics.

The two hormones cholecystokinin and gastrin share the same C-terminal sequence of amino acids, namely Gly²⁹-Trp³⁰-Met³¹-Asp³²-Phe³³-NH2. Nevertheless, this congruence has not precluded using this structure to develop selective ligands for either CCK1 or CCK2 receptors. Manipulation of the hydrophobic residues at positions 31 and 33 gave a series of CCK1 tripeptide antagonists, typified by N-t-BOC-Trp-2-Nal-Asp-2-(phenyl)ethylamide (pKB 6.8 ± 0.3). Molecular modeling was used to identify the bioactive conformation of these CCK1-selective compounds and prompted the design of new peptoid structures. We aimed to maintain the conformation of the parent series by exploiting patterns of hydrogen-bonding and π-stacking interactions present in the original molecule, rather than introducing additional covalent bonds. The prototype, N-(succinyl-d-Asp-2-phenylethylamido)-l-Trp-2-(2-naphthyl)ethylamide, was a potent and selective CCK1 antagonist (pKB 7.2 ± 0.3). Furthermore, the new series showed patterns of biological activity that mirrored those of the parent tripeptides. These compounds contain elements of both peptide primary and secondary structure and represent a novel approach to designing peptidomimetics. Interesting results were obtained from comparing models of a representative tripeptide CCK1 antagonist with a conformation of CCK30-33 that others have proposed to be responsible for its activity at the CCK2 receptor. The results suggest that CCK1 and CCK2 receptors recognize enatiomeric dispositions of the Trp³⁰ indole, Asp³² carboxylic acid, and C-terminal phenyl groups arrayed about a common backbone configuration. This “functional chirality” may underpin the mechanism by which these closely related receptor systems bind CCK30-33 and explain patterns of selectivity observed with optical isomers of a number of peptoid and nonpeptide ligands.

Several 5-HT1D/1B receptor agonists are now entering the marketplace as treatments for migraine. This paper describes the development of selective h5-HT1D receptor agonists as potential antimigraine agents which may produce fewer side effects. A series of 3-[3-(piperidin-1-yl)propyl]indoles has been synthesized which has led to the identification of 80 (L-772,405), a high-affinity h5-HT1D receptor full agonist having 170-fold selectivity for h5-HT1D receptors over h5-HT1B receptors. L-772,405 also shows very good selectivity over a range of other serotonin and nonserotonin receptors and has excellent bioavailability following subcutaneous administration in rats. It therefore constitutes a valuable tool to delineate the role of h5-HT1D receptors in migraine. Molecular modeling and physical properties have been utilized to postulate the binding conformation of these compounds in the receptor cavity.

It has previously been reported that a 3-(3-(piperazin-1-yl)propyl)indole series of 5-HT1D receptor ligands have pharmacokinetic advantages over the corresponding 3-(3-(piperidin-1-yl)propyl)indole series and that the reduced pKa of the piperazines compared to the piperidines may be one possible explanation for these differences. To investigate this proposal we have developed versatile synthetic strategies for the incorporation of fluorine into these ligands, producing novel series of 4-fluoropiperidines, 3-fluoro-4-aminopiperidines, and both piperazine and piperidine derivatives with one or two fluorines in the propyl linker. Ligands were identified which maintained high affinity and selectivity for the 5-HT1D receptor and showed agonist efficacy in vitro. The incorporation of fluorine was found to significantly reduce the pKa of the compounds, and this reduction of basicity was shown to have a dramatic, beneficial influence on oral absorption, although the effect on oral bioavailability could not always be accurately predicted.

The design, synthesis, and biological activity of a series of high-affinity, basic ligands for the cholecystokinin-B receptor are described. The compounds, which incorporate a piperidin-2-yl or a homopiperidin-2-yl group attached to C5 of a benzodiazepine core structure, are substantially more basic (e.g., 9d, pKa = 9.48) than previously reported antagonists based on 5-amino-1,4-benzodiazepines (e.g., 5, pKa = 7.1) and have improved aqueous solubility. In view of their basicity, it would be tempting to speculate that the present series of compounds might be binding to the CCK-B receptor in their protonated form. Compounds such as 9d,e and 10d showed high affinity for this receptor (IC50 < 2.5 nM) and very good selectivity over CCK-A (CCK-A/CCK-B > 2000), even as the racemates. Additionally, a significantly improved in vivo half-life was observed for a selection of compounds compared to the clinical candidate L-365,260 (1).