Small molecules

Small molecules as the ligands for different biological targets

In the past years, we have been interested particularly in the synthesis and study of ligands bearing imidazo[4,5-b]pyridine and imidazo[4,5-c]pyridine as the core heterocycles. This long-term project started by the development of high-throughput synthetic approach leading to the mentioned imizadopyridines starting from immobilized amines, with 2,4-dichloro-3-nitroypridine and aldehydes as the joint building blocks (ACS Comb. Sci.  2014, 16(10) 558-565). Subsequently, we have been using this method for the rapid synthesis of the focused chemical libraries of variously substituted imidazopyridines for their biological evalutation (see below). Furthermore, we later developed an alternative synthetic pathways with application of the traditional solution-phase synthesis.

Imidazopyridines targeting the central nervous system (CNS)

Motivation for our long-term research in this area is based on the fact that imidazopyridines act as potent ligands of diverse molecular targets localized in CNS. Various imidazopyridines can be powerful modulators of several diseases associated with CNS dysfunction including Alzheimer’s disease, Parkinson’s disease, schizophrenia, depression or sleeping disorders. In our recent review (Eur. J. Med. Chem. 2019, 181, 111569), we summarized the imidazopyridine-based ligands and their therapeutic importance.

Taking this fact in account, we recently applied a scaffold-hopping approach to design a novel N-benzyl imidazo[4,5-b] and imidazo[4,5-c]pyridine derivatives which have been identified as potent and selective 5-HT6R neutral antagonists (Eur. J. Med. Chem. 2018, 144, 716-729). We evaluated the in vitro affinity of synthesized compounds for 5-HT6Rs, functional profile and selectivity panel, followed by ADMET and pharmacokinetic properties for the most promising derivative. Finally, the pro-cognitive activity of selected compound has been evaluated in the novel object recognition (NOR) task in rats by measuring its ability to reverse drug-induced memory deficits.

To further investigate conformational effects of the 5-HT6 receptor, a focused library of N-benzenesulfonyl imidazo[4,5-b]pyridine and imidazo[4,5-c]pyridine derivatives was designed (J. Med. Chem. 2021, in press). The study identified compound 7e (1-[(3-chlorophenyl)sulfonyl]-2-ethyl-4-(piperazin-1-yl)-1H-imidazo[4,5-c]pyridine), which acts as a potent 5-HT6 receptor neutral antagonist at Gs, and does not impact neurite growth, in a process controlled by Cdk5 signaling pathways. Molecular dynamic simulations highlighted intramolecular constraints and receptor conformational changes. In cell-based assays, neutral antagonists (compound 7e and CPPQ), but not inverse agonists (SB-258585, intepirdine and PZ-1444), displayed glioprotective properties against 6-OHDA- and doxorubicin- induced cytotoxicity. Notably, both observed effects suggest that targeting the activated conformational state of 5-HT6R with neutral antagonists implicates the protecting properties of astrocytes. Additionally, compound 7e abolished scopolamine-induced memory deficits in the novel object recognition (NOR) task. In view of these findings, compound 7e is a molecular probe that can be used to further understand the functional outcomes of the different conformational states of the 5-HT6 receptor.

Except for ligands of 5-HT6R, our investigation of focused libraries of imidazopyridines identified hit compounds which inhibited phosphodiesterase 4 (PDE4) with IC50 values comparable to rolipram and displayed different inhibitory potency against phosphodiesterase 7 (PDE7). The best compound showed a beneficial effect in all the studied animal models of inflammatory and autoimmune diseases (concanavalin A-induced hepatitis, lipopolysaccharide-induced endotoxemia, collagen-induced arthritis, and MOG35-55-induced encephalomyelitis). In addition, compound showed a favorable pharmacokinetic profile after intraperitoneal administration; it was characterized by a fast absorption from the peritoneal cavity and a relatively long terminal half-life in rats. It was found to penetrate brain barrier in mice. The performed experiments sheds light on the impact of PDE7A inhibition for the efficacy of PDE4 inhibitors in these disease conditions.

Imidazopyridines as the kinases inhibitors

In comparison to other purine bioisosteres, imidazo[4,5-b]pyridine and imidazo[4,5-c]pyridine are relatively unexplored core heterocycles included in the ligands of protein-kinases. Inspired by this fact, we designed imidazopyridines as inhibitors of Bruton’s tyrosine kinase (BTK) (Eur. J. Med. Chem. 2021, 211, 113094). Two alternative synthetic routes for the simple preparation of desired compounds with variable substitutions at the N1, C4, C6 positions were introduced with readily available building blocks. Further, the developed synthetic approach was feasible for isomeric compounds bearing imidazo[4,5-b]pyridine scaffolds. In contrast to expectations based on previous studies, the imidazo[4,5-c]pyridine inhibitor exhibited a significantly higher activity against BTK compared to its imidazo[4,5-b]pyridine isomer. An inherent SAR study in the series of imidazo[4,5-c]pyridine compounds revealed a remarkably high tolerance of C6 substitutions for both hydrophobic and hydrophilic substituents. Preliminary cellular experiments indicated selective BTK targeting in Burkitt lymphoma and mantle cell lymphoma cell lines. The inhibitors could thus serve as starting points for further development, eventually leading to BTK inhibitors that could be used after ibrutinib failure.

In the next study, we designed and prepared five isosteres in which the imidazo[1,2-a]pyrazine scaffold of entospletinib was altered to pyrazolo[3,4-d]pyrimidine, pyrrolo[3,2-d]pyrimidine, imidazo[4,5-b]pyridine, imidazo[4,5-c]pyridine and purine (Eur. J. Med. Chem. 2020, 204, 112636). The last two isosteres were the most potent SYK inhibitors, with IC50 values in the mid-nanomolar range. Importantly, three compounds also inhibited BTK more effectively than did entospletinib. Further experiments then showed that BCR signalling was suppressed in Ramos cells by the potent compounds. Preliminary kinase inhibition screening also revealed LCK and SRC as additional targets. Our results further support the hypothesis that multikinase targeting compounds could produce more robust responses in the treatment of B lymphoid neoplasms.