Novel 4-aryl-pyrido[1,2-c]pyrimidines with dual SSRI and 5-HT(1A) activity. Part 5

Advances in drug design and therapeutic potential of selective or multitarget 5-HT1A receptor ligands

5-HT1A receptor (5-HT1A-R) is a serotoninergic G-protein coupled receptor subtype which contributes to several physiological processes in both central nervous system and periphery. Despite being the first 5-HT-R identified, cloned and studied, it still represents a very attractive target in drug discovery and continues to be the focus of a myriad of drug discovery campaigns due to its involvement in numerous neuropsychiatric disorders. The structure-activity relationship studies (SAR) performed over the last years have been devoted to three main goals: (i) design and synthesis of 5-HT1A-R selective/preferential ligands; (ii) identification of 5-HT1A-R biased agonists, differentiating pre- versus post-synaptic agonism and signaling cellular mechanisms; (iii) development of multitarget compounds endowed with well-defined poly-pharmacological profiles targeting 5-HT1A-R along with other serotonin receptors, serotonin transporter (SERT), D2-like receptors and/or enzymes, such as acetylcholinesterase and phosphodiesterase, as a promising strategy for the management of complex psychiatric and neurodegenerative disorders. In this review, medicinal chemistry aspects of ligands acting as selective/preferential or multitarget 5-HT1A-R agonists and antagonists belonging to different chemotypes and developed in the last 7 years (2017-2023) have been discussed. The development of chemical and pharmacological 5-HT1A-R tools for molecular imaging have also been described. Finally, the pharmacological interest of 5-HT1A-R and the therapeutic potential of ligands targeting this receptor have been considered.

Designing multi-target drugs for the treatment of major depressive disorder

INTRODUCTION

Major depressive disorders (MDD) pose major health burdens globally. Currently available medications have their limitations due to serious adverse effects, long latency periods as well as resistance. Considering the highly complicated pathological nature of this disorder, it has been suggested that multitarget drugs or multi-target-directed ligands (MTDLs) may provide long-term therapeutic solutions for the treatment of MDD.

AREAS COVERED

In the current review, recent lead design and lead modification strategies have been covered. Important investigations reported in the last ten years (2013-2022) for the pre-clinical development of MTDLs (through synthetic medicinal chemistry and biological evaluation) for the treatment of MDD were discussed as case studies to focus on the recent design strategies. The discussions are categorized based on the pharmacological targets. On the basis of these important case studies, the challenges involved in different design strategies were discussed in detail.

EXPERT OPINION

Even though large variations were observed in the selection of pharmacological targets, some potential biological targets (NMDA, melatonin receptors) are required to be explored extensively for the design of MTDLs. Similarly, apart from structure activity relationship (SAR), in silico techniques such as multitasking cheminformatic modelling, molecular dynamics simulation and virtual screening should be exploited to a greater extent.

Recent Advances in the Development of Pyrimidine-based CNS Agents

BACKGROUND

In the past few decades, considerable progress has been made in CNS drug discovery, and various new CNS agents have been developed. Pyrimidine is an important scaffold in the area of medicinal chemistry. Recently, pyrimidine-containing compounds have been successfully designed as potent CNS agents. Substantial research has been carried out on pyrimidine-bearing compounds to treat different disorders of CNS in various animal models.

METHODS

Utilizing various databases, including Google Scholar, PubMed, Science Direct, and Web of Science, the literature review was conducted. The specifics of significant articles were discussed with an emphasis on the potency of pyrimidines derivatives possessing CNS activity.

RESULTS

Recent papers indicating pyrimidine derivatives with CNS activity were incorporated into the manuscript. (46) to (50) papers included different pyrimidine derivatives as 5-HT agonist/antagonists, (62) to (67) as adenosine agonist/antagonist, (70) to (75) as anticonvulsant agents, (80) to (83) as cannabinoid receptor agonists, (102) to (103) as nicotinic and (110) as muscarinic receptor agonists. The remaining papers (113) to (114) represented pyrimidine-based molecular imaging agents.

CONCLUSION

Pyrimidine and its derivatives have been studied in detail to evaluate their efficacy in overcoming multiple central nervous system disorders. The article covers the current updates on pyrimidine-based compounds as potent CNS and molecular imaging agents and will definitely provide a better platform for the development of potent pyrimidine-based CNS drugs in the near future.

Synthesis of Novel Pyrido[1,2-c]pyrimidine Derivatives with 6-Fluoro-3-(4-piperidynyl)-1,2-benzisoxazole Moiety as Potential SSRI and 5-HT1A Receptor Ligands

Two series of novel 4-aryl-2H-pyrido[1,2-c]pyrimidine (6a–i) and 4-aryl-5,6,7,8-tetrahydropyrido[1,2-c]pyrimidine (7a–i) derivatives were synthesized. The chemical structures of the new compounds were confirmed by ¹H and ¹³C NMR spectroscopy and ESI-HRMS spectrometry. The affinities of all compounds for the 5-HT_1A receptor and serotonin transporter protein (SERT) were determined by in vitro radioligand binding assays. The test compounds demonstrated very high binding affinities for the 5-HT_1A receptor of all derivatives in the series (6a–i and 7a–i) and generally low binding affinities for the SERT protein, with the exception of compounds 6a and 7g. Extended affinity tests for the receptors D₂, 5-HT_2A, 5-HT₆ and 5-HT₇ were conducted with regard to selected compounds (6a, 7g, 6d and 7i). All four compounds demonstrated very high affinities for the D₂ and 5-HT_2A receptors. Compounds 6a and 7g also had high affinities for 5-HT₇, while 6d and 7i held moderate affinities for this receptor. Compounds 6a and 7g were also tested in vivo to identify their functional activity profiles with regard to the 5-HT_1A receptor, with 6a demonstrating the activity profile of a presynaptic agonist. Metabolic stability tests were also conducted for 6a and 6d.

Metabolic stability studies of lead compounds supported by separation techniques and chemometrics analysis

With metabolism being one of the main routes of drug elimination from the body (accounting for removal of around 75% of known drugs), it is crucial to understand and study metabolic stability of drug candidates. Metabolically unstable compounds are uncomfortable to administer (requiring repetitive dosage during therapy), while overly stable drugs increase risk of adverse drug reactions. Additionally, biotransformation reactions can lead to formation of toxic or pharmacologically active metabolites (either less-active than parent drug, or even with different action). There were numerous approaches in estimating metabolic stability, including in vitro, in vivo, in silico, and high-throughput screening to name a few. This review aims at describing separation techniques used in in vitro metabolic stability estimation, as well as chemometric techniques allowing for creation of predictive models which enable high-throughput screening approach for estimation of metabolic stability. With a very low rate of drug approval, it is important to understand in silico methods that aim at supporting classical in vitro approach. Predictive models that allow assessment of certain biological properties of drug candidates allow for cutting not only cost, but also time required to synthesize compounds predicted to be unstable or inactive by in silico models.

Structure-activity relationship and cardiac safety of 2-aryl-2-(pyridin-2-yl)acetamides as a new class of broad-spectrum anticonvulsants derived from Disopyramide

A series of 2-aryl-2-(pyridin-2-yl)acetamides were synthesized and screened for their anticonvulsant activity in animal models of epilepsy. The compounds were broadly active in the 'classical' maximal electroshock seizure (MES) and subcutaneous Metrazol (scMET) tests as well as in the 6 Hz and kindling models of pharmacoresistant seizures. Furthermore, the compounds showed good therapeutic indices between anticonvulsant activity and motor impairment. Structure-activity relationship (SAR) trends clearly showed the highest activity resides in unsubstituted phenyl derivatives or compounds having ortho- and meta- substituents on the phenyl ring. The 2-aryl-2-(pyridin-2-yl)acetamides were derived by redesign of the cardiotoxic sodium channel blocker Disopyramide (DISO). Our results show that the compounds preserve the capability of the parent compound to inhibit voltage gated sodium currents in patch-clamp experiments; however, in contrast to DISO, a representative compound from the series 1 displays high levels of cardiac safety in a panel of in vitro and in vivo experiments.

Synthesis of new 5,6,7,8-tetrahydropyrido[1,2-c]pyrimidine derivatives with rigidized tryptamine moiety as potential SSRI and 5-HT1A receptor ligands

Extended studies in the 4-aryl-pyrido[1,2-c]pyrimidine group resulted in 27 new compounds (10.1-10.27), 5,6,7,8-tetrahydropyrido[1,2-c]pyrimidine derivatives. In vitro tests (RBA) were carried out for 10.1-10.27 compounds in order to determine their affinity to 5-HT_1A receptor and SERT protein. 10.1-10.3, 10.6, 10.7, 10.16 and 10.27 compounds had high binding ability to both molecular targets (5-HT_1A K_i = 8-87 nM; SERT K_i = 8-52 nM). For these compounds (10.1-10.3, 10.6, 10.7, 10.16, 10.27) further in vitro, in vivo and metabolic stability tests were performed. In vitro studies in the extended receptor profile (D₂, 5-HT_2A, 5-HT₆ and 5-HT₇) showed their selectivity towards 5-HT_1A receptor and SERT protein. In vivo tests revealed that compounds 10.7 and 10.16 had the properties of presynaptic antagonists of the 5-HT_1A receptor. The redesign of the 2H-pyrido[1,2-c]pyrimidine residue present in the terminal part towards 5,6,7,8-tetrahydropyrido[1,2-c]pyrimidine resulted in the improved metabolic stability and enhanced affinity to both molecular targets (5-HT_1A-R and SERT) compared to the precursors.

Synthesis of novel pyrido[1,2-c]pyrimidine derivatives with rigidized tryptamine moiety as potential SSRI and 5-HT1A receptor ligands

The study enabled obtaining a number of new derivatives of 4-aryl-pyrido[1,2-c]pyrimidine 9.1-9.27 having conformationally restricted tryptamine moiety. In vitro studies (RBA) have shown that derivatives 9.1, 9.2, 9.4, 9.7, 9.9, 9.14 and 9.27 exhibit high affinity to molecular targets 5-HT_1A receptor and SERT protein. In general, compounds with an unsubstituted or a para-substituted benzene ring of the pyrido[1,2-c]pyrimidine residue in the terminal part were characterized by higher binding ability, which can be justified by the greater flexibility of the structure. For the selected compounds 9.1, 9.7, 9.9 and 9.27, further in vitro, in vivo and metabolic stability tests were performed. The in vitro studies in the extended receptor profile (D₂, 5-HT_2A, 5-HT₆ and 5-HT₇) indicated their selectivity toward the 5-HT_1A receptor and SERT protein. The in vivo studies (8-OH-DPAT-induced hypothermia in mice, FST) revealed that the compound 9.1 has the properties of presynaptic agonist of the 5-HT_1A receptor, and compound 9.7 demonstrated the properties of a presynaptic antagonist of the 5-HT_1A receptor. Metabolic stability studies, in turn, showed that compounds 9.1, 9.7 and 9.9, having an unsubstituted indole residue, were more resistant to biotransformation reactions of the first pass phase than was compound 9.27 containing a 5-methoxy-substituted indole residue. The obtained results allowed further optimization of the structure.

Revealing vilazodone's binding mechanism underlying its partial agonism to the 5-HT1A receptor in the treatment of major depressive disorder

It has been estimated that major depressive disorder (MDD) will become the second largest global burden among all diseases by 2030. Various types of drugs, including selective serotonin reuptake inhibitors (SSRIs), serotonin-norepinephrine reuptake inhibitors (SNRIs), and serotonin receptor partial agonist/reuptake inhibitors (SPARIs), have been approved and become the primary or first-line medications prescribed for MDD. SPARI was expected to demonstrate more enhanced drug efficacy and a rapid onset of action as compared to SSRI and SNRI. As one of the most famous SPARIs, vilazodone was approved by the FDA for the treatment of MDD. Because of the great clinical importance of vilazodone, its binding mechanism underlying its partial agonism to the 5-HT_1A receptor (5-HT_1AR) could provide valuable information to SPARIs' drug-like properties. However, this mechanism has not been reported to date; consequently, the rational design of new efficacious SPARI-based MDD drugs is severely hampered. To explore the molecular mechanism of vilazodone, an integrated computational strategy was adopted in this study to reveal its binding mechanism and prospective structural feature at the agonist binding site of 5-HT_1AR. As a result, 22 residues of this receptor were identified as hotspots, consistently favoring the binding of vilazodone and its analogues, and a common binding mechanism underlying their partial agonism to 5-HT_1AR was, therefore, discovered. Moreover, three main interaction features between vilazodone and 5-HT_1AR have been revealed and schematically summarized. In summary, this newly identified binding mechanism will provide valuable information for medicinal chemists working in the field of rational design of novel SPARIs for MDD treatment.

Novel 1-(2-pyrimidin-2-yl)piperazine derivatives as selective monoamine oxidase (MAO)-A inhibitors

In the present study, a new series of 2-[4-(pyrimidin-2-yl)piperazin-1-yl]-2-oxoethyl 4-substituted piperazine-1-carbodithioate derivatives (2a-n) were synthesized and screened for their monoamine oxidase A and B inhibitory activity. The structures of compounds were elucidated using spectroscopic methods and some physicochemical properties of new compounds were predicted using Molinspiration and MolSoft programs. Compounds 2-[4-(pyrimidin-2-yl)piperazin-1-yl]-2-oxoethyl 4-(4-nitrophenyl)piperazine-1-carbodithioate (2j) and 2-[4-(pyrimidin-2-yl)piperazin-1-yl]-2-oxoethyl 4-benzhydrylpiperazine-1-carbodithioate (2m) exhibited selective MAO-A inhibitory activity with IC₅₀=23.10, 24.14 µM, respectively. Some of the biological results were found in accordance with the obtained in silico data based on Lipinski’s fule of five.