Search

To test whether a turnlike arrangement is involved in the bioactive conformation of CCK4 analogues upon CCK1 receptor recognition, we describe the preparation of two series of CCK4 derivatives, in which the central dipeptide Met-Asp has been replaced by recognized β-turn mimetics {(2S,5S,11bR)- and (2R,5R,11bS)-2-amino-5-carboxy-3-oxo-2,3,5,6,11,11b-hexahydro-1H-indolizino[8,7-b]indole (IBTM) and β-turn dipeptide, 2-oxo-7-thio-1-azabicyclo[4.3.0]nonane (BTD)}. The incorporation of the indolizinoindole IBTM type II β-turn mimetic is preferred over its type II‘ counterpart for efficient CCK1 receptor recognition, while BTD derivatives were completely inactive. The structure−conformation−activity relationship study in the IBTM series has shown some essential requirement of these CCK4 derivatives to favorably interact with CCK1 receptors:  (a) the adoption of turnlike conformations, (b) the presence of an l-Phe residue and a C-terminal carboxamide moiety, and (c) the indole ring of the IBTM skeleton. Moreover, the existence of π−π interactions between the phenyl ring of d-Phe residues and the indole ring of IBTM framework is detrimental for binding affinity. A series of potent and selective CCK1 receptor antagonists, exemplified by compounds 8a and 8b, emerges among these IBTM-containing derivatives.

With the aim of reversing selectivity or antagonist/agonist functionality in the 5-(tryptophylamino)-1,3-dioxoperhydropyrido[1,2-c]pyrimidine-derived potent and highly selective CCK1 antagonists, a series of 4-benzyl and 4-methyl derivatives have been synthesized. Whereas the introduction of the benzyl group led, in all cases, to complete loss of the binding affinity, the incorporation of the methyl group gave a different result depending on the stereochemistry of the 1,3-dioxoperhydropyrido[1,2-c]pyrimidine scaffold. Thus, the introduction of the methyl group into the (4aS,5R)-diastereoisomers, giving a (4S)-configuration, produced a 3-fold increase in the CCK1 binding potency and selectivity. However, the same structural manipulation in the opposite (4aR,5S)-stereochemistry, leading to a (4R,4aR,5S)-configuration, produced reversal of the selectivity for CCK1 to the CCK2 receptors. The replacement of the Boc group at the tryptophan moiety by a 2-adamantyloxycarbonyl group also contributed to that reversal. The resulting compounds displayed moderate CCK2 antagonist activity in rat and human receptors, and a very small partial agonist effect on the production of inositol phosphate in COS-7 cells transfected with the wild-type human CCK2 receptor.

It has been proposed that 5-HT1A receptor antagonists augment the antidepressant efficacy of selective serotonin (5-HT) reuptake inhibitors. In a search toward new and efficient antidepressants, 1-(aryl)-3-[4-arylpiperazin-1-yl]-1-propane molecular hybrids were designed, synthesized, and evaluated for 5-HT reuptake inhibition and 5-HT1A receptor affinity. The design was based in coupling structural moieties related to inhibition of serotonin reuptake, such as benzo[b]thiophene derivatives to arylpiperazines, typical 5-HT1A receptor ligands. In binding studies, several compounds showed affinity at the 5-HT transporter and at 5-HT1A receptors. Molecular modeling studies predicted the pharmacophore elements required for high affinity binding and the features that enable to discriminate between agonist, partial agonist, or antagonist action at 5-HT1A receptors and 5-HT transporter inhibition. Solvent interactions in desolvation prior to the binding step along with enthalpy and enthropy compensations might be responsible to explain agonist, partial agonist, and antagonist character. Hydrogen-bonding capability seems to be important to break hydrogen interhelical hydrogen bonds or alternatively to form other bonds upon ligand binding. Partial agonists and antagonists are unable to do this as the full agonist, which interacts closely by long-range forces or directly. The compounds showing the higher affinity at both the 5-HT transporter (Ki < 50 nM) and the 5-HT1A receptors (Ki < 20 nM) were further explored for their ability to stimulate [³⁵S]GTPγS binding or to antagonize 8-hydroxy-2-di-n-propylamino-tetralin (8-OH-DPAT)-stimulated [³⁵]GTPγS binding to rat hippocampal membranes, an index of agonist/antagonist action at 5-HT1A receptors, respectively. Compound 8g exhibited agonist activity (EC50 = 30 nM) in this assay, whereas compounds 7g and 8h,i behaved as weak partial agonists and 7h−j and 8j,l antagonized the R(+)-8-OH-DPAT-stimulated GTPγS binding. Functional characterization was performed by measuring the antagonism to 8-OH-DPAT-induced hypothermia in mice.

To further define the pharmacophore of the potent and selective 5-(tryptophyl)amino-1,3-dioxoperhydropyrido[1,2-c]pyrimidine-based CCK1 receptor antagonists the electronic and topographic properties of the central 1,3-dioxoperhydro-pyrido[1,2-c]pyrimidine scaffold have been modified. With this aim, the 1- and 3-oxo groups have been replaced by the thioxo- and deoxi-analogues, and the fused piperidine ring has been contracted to the corresponding pyrrolidine moiety. The results of the evaluation of the new analogues as CCK receptor ligands, in rat pancreas and cerebral cortex preparations, showed that, whereas replacement of oxygen with sulfur is allowed, reduction of the 1- or 3-oxo groups or the contraction of the fused piperidine ring lead to the complete loss of binding affinity at CCK1 receptors. The thioxo-analogues 5a, 8a, 12a, and 12b showed functional CCK1 antagonist activity, inhibiting the CCK-8-stimulated amylase release from pancreatic acinar cells. The 1-thioxo analogue 5a, with subnanomolar affinity (IC50 = 0.09 × 10^-9 M), was found to be the most potent and selective compound within the family of 5-(tryptophyl)amino-1,3-dioxoperhydropyrido[1,2-c]pyrimidine-based CCK1 antagonists.

In a search toward new and efficient antidepressants, 1-aryl-3-(4-arylpiperazin-1-yl)propane derivatives were designed, synthesized, and evaluated for 5-HT reuptake inhibition and 5-HT1A receptor antagonism. This dual pharmacological profile should lead, in principle, to a rapid and pronounced enhancement in serotoninergic neurotransmission and consequently to a more efficacious treatment of depression. The design was based on coupling structural moieties related to inhibition of serotonin reuptake, such as γ-phenoxypropylamines, to arylpiperazines, typical 5-HT1A ligands. In binding studies, several compounds showed affinity at the 5-HT transporter and 5-HT1A receptors. Antidepressant-like activity was initially assayed in the forced swimming test with those compounds with Ki < 200 nM in both binding studies. Functional characterization was performed by measuring the intrinsic effect on rectal temperature in mice and also the antagonism to 8-OH-DPAT-induced hypothermia. The most efficacious compounds (12f, 23gE, 28a, and 28b) were further explored for their ability to antagonize 8-OH-DPAT-induced inhibition of forskolin-stimulated cAMP formation in a cell line expressing the 5-HT1A receptor. Furthermore, the antidepressant-like properties of 12f, 28a, and 28b, which exhibited 5-HT1A receptor antagonistic property in the latter study, were also evaluated in the learned helplessness test in rats. Among these three compounds, 28b (1-benzo[b]thiophene-3-yl)-3-[4-(2-methoxyphenyl)-1-ylpropan-1-ol) showed the higher affinity at both the 5-HT transporter and 5-HT1A receptors (Ki = 20 nM in both cases) and was also active in the other pharmacological tests. Such a pharmacological profile could lead to a new class of antidepressants with a dual mechanism of action and a faster onset of action.

To improve our knowledge of the bioactive conformation of CCK1 antagonists, we previously described that replacement of the α-MeTrp residue of dipeptoids with the (2S,5S,11bR)-2-amino-3-oxohexahydroindolizino[8,7-b]indole-5-carboxylate (IBTM) skeleton, a probed type II‘ β-turn mimetic, led to restricted analogues (2S,5S,11bR,1‘S)- and (2S,5S,11bR,1‘R)-2-(benzyloxycarbonyl)amino-5-[1‘-benzyl-2‘-(carboxy)ethyl]carbamoyl-3-oxo-2,3,5,6,11,11b-hexahydro-1H-indolizino[8,7-b]indole, 1a,b, showing high binding affinity and selectivity for CCK1 receptors. In this report, we describe the synthesis and binding profile of new analogues of compounds 1 designed to explore the importance of the C-terminal residue and of the type of β-turn on the receptor binding affinity and selectivity. Structure−affinity relationship studies show that a C-terminal free carboxylic acid and an S configuration of the Phe and βHph residues are favorable for CCK1 receptor recognition. Moreover, selectivity for this receptor subtype is critically affected by the β-turn type. Thus, while compounds 15a and 16a, containing the (2S,5S,11bR)- and (2R,5R,11bS)-IBTM frameworks, respectively, are both endowed with nanomolar affinity for CCK1 receptors, restricted dipeptoid derivative 15a, incorporating the type II‘ IBTM mimetic, shows approximately 6-fold higher CCK1 selectivity than analogue 16a, with the type II mimetic. From these results, we propose that the presence of a β-turn-like conformation within the peptide backbone of dipeptoids could contribute to their bioactive conformation at the CCK1 receptor subtype. Concerning functional activity, compounds 15a and 16a behave as CCK1 receptor antagonists.

Analogues of the previously reported potent and highly selective CCK1 receptor antagonist (4aS,5R)-2-benzyl-5-(N-Boc-tryptophyl)amino-1,3-dioxoperhydropyrido-[1,2-c]pyrimidine (2a) were prepared to explore the structural requirements at the Boc-tryptophan domain for CCK1 receptor affinity. Structural modifications of 2a involved the Trp side chain, its conformational freedom, the Boc group, and the carboxamide bond. Results of the CCK binding and in vitro functional activity evaluation showed three highly strict structural requirements:  the type and orientation of the Trp side chain, the H-bonding acceptor carbonyl group of the carboxamide bond, and the presence of the Trp amino protection Boc. Replacement of this acid-labile group with 3,3-dimethylbutyryl or tert-butylaminocarbonyl conferred acid stability to analogues 14a and 15a, which retained a high potency and selectivity in binding to CCK1 receptors, as well as an in vivo antagonist activity against the acute pancreatitis induced by caerulein in rats. Oral administration of compounds 14a and 15a also produced a lasting antagonism to the hypomotility induced by CCK-8 in mice, suggesting a good bioavailability and metabolic stability.

The synthesis and stereochemical structure−activity relationships of a new class of potent and selective non-peptide cholecystokinin-A (CCK-A) receptor antagonists based on the 1,3-dioxoperhydropyrido[1,2-c]pyrimidine skeleton are described. The most potent member of this series of eight diastereoisomers, (4aS,5R)-2-benzyl-5-[N-[(tert-butoxycarbonyl)-l-tryptophyl]amino]-1,3-dioxoperhydropyrido[1,2-c]pyrimidine, displays nanomolar CCK-A receptor affinity and higher than 8000-fold potency at the CCK-A than at the CCK-B receptor. As CCK-A antagonist, this compound inhibits the CCK-8-evoked amylase release from pancreatic acinar cells at a low concentration, similar to that of the typical antagonist Devazepide. Highly strict stereochemical requirements for CCK-A receptor binding and selectivity have been found. The l-Trp and the 4a,5-trans disposition of the bicyclic perhydropyrido[1,2-c]pyrimidine are essential for binding potency and selectivity.