New potent and selective A1 adenosine receptor antagonists as potential tools for the treatment of gastrointestinal diseases

Adenosine receptor antagonists: Recent advances and therapeutic perspective

Adenosine is an endogenous purine-based nucleoside expressed nearly in all body tissues. It regulates various body functions by activating four G-protein coupled receptors, A₁, A_2A, A_2B, and A₃. These receptors are widely acknowledged as drug targets for treating different neurological, metabolic, and inflammatory diseases. Although numerous adenosine receptor inhibitors have been developed worldwide, achieving target selectivity is still a big hurdle in drug development. However, the identification of specific radioligands-based affinity assay, fluorescent ligands, and MS-based ligand assay have contributed to the development of selective and potent adenosine ligands. In recent years various small heterocyclic-based molecules have shown some promising results. Istradefylline has been approved for treating Parkinson's in Japan, while preladenant, tozadenant, CVT-6883, MRS-1523, and many more are under different phases of clinical development. The present review is focused on the quest to develop potent and selective adenosine inhibitors from 2013 to early 2021 by various research groups. The review also highlights their biological activity, selectivity, structure-activity relationship, molecular docking, and mechanistic studies. A special emphsesis on drug designing strategies has been also given the manuscript. The comprehensive compilation of research work carried out in the field will provide inevitable scope for designing and developing novel adenosine inhibitors with improved selectivity and efficacy.

Design, Synthesis, and Evaluation of Novel Isothiazole-Purines as a Pyruvate Kinase-Based Fungicidal Lead Compound

Target identification is one of the most important bases for novel pesticide development; pyruvate kinase (PK) was discovered as a potent fungicide target in our previous studies. To continue the PK-based fungicide development, novel isothiazole-purine derivatives were rationally designed and synthesized. Bioassay results showed that compound 5ai displayed excellent in vitro activity against Rhizoctonia solani with an EC₅₀ of 1.5 μg/mL, which was superior to those of positive controls diflumetorim with its EC₅₀ of 19.8 μg/mL and PK-based lead YZK-C22 with its EC₅₀ of 4.2 μg/mL. Compounds 3b (5.2 μg/mL) and 3c (4.5 μg/mL) displayed better activities against Gibberella zeae with their EC₅₀s falling between 4.0 and 5.5 μg/mL, while YZK-C22 showed an EC₅₀ of 6.4 μg/mL. In addition, 5ah exhibited promising in vivo activity against Erysiphe graminis and Puccinia sorghi Schw. with 100% efficacy at 10 μg/mL and 90% efficacy at 2 μg/mL against P. sorghi Schw. Compound 5ai showed good PK inhibitory activity with an IC₅₀ of 38.8 μmol/L, and it was well docked into the active site of the target enzyme PK, which was slightly more active than YZK-C22 with its IC₅₀ of 42.4 μmol/L. Our studies discovered that isothiazole-purines were PK-based fungicidal leads deserving of further study.

Involvement of GABAA Receptors in the Anxiolytic-Like Effect of Hydroxycitronellal

Hydroxycitronellal (HC) is a monoterpene present in essential oils of aromatic plants of different species, obtained from semisynthesis of citronellal, and is widely used as a fragrance in cosmetics. The objective of this work was to evaluate the possible anxiolytic-like activity of HC and its possible mechanism of action using in vivo and in silico methodologies. Swiss male mice (Mus musculus) were treated with HC (12.5, 25, and 50 mg/kg, i.p.) and subjected to the rota rod, elevated plus maze, and open field tests. No significant impairments were observed in the rota rod tests for the motor activity of the animals treated with HC at 12.5, 25, and 50 mg/kg, i.p., indicating no myo-relaxing or sedative effects. In the elevated plus maze, HC (in the three doses) induced significant increases in the percentage of entries (respectively, 34.8%, 33.8%, and 38.6%) and in the length of stay (respectively, 49.9%, 56.1%, and 57.0%) in the open arms of the EPM, as well as the number of crossings in the open field tests. The mechanism of action of the compound's anxiolytic-like activity can be attributed to the involvement of GABA_A receptors, and this interaction was observed in in vivo and in silico studies. For HC, the results suggest anxiolytic-like effects, possibly via modulation of the GABAergic system. The use of natural products to treat anxiety can become an alternative to existing synthetic products.

Medicinal chemistry of P2 and adenosine receptors: Common scaffolds adapted for multiple targets

Prof. Geoffrey Burnstock originated the concept of purinergic signaling. He demonstrated the interactions and biological roles of ionotropic P2X and metabotropic P2Y receptors. This review paper traces the historical origins of many currently used antagonists and agonists for P2 receptors, as well as adenosine receptors, in early attempts to identify ligands for these receptors - prior to the use of chemical libraries for screening. Rather than presenting a general review of current purinergic ligands, we focus on common chemical scaffolds (privileged scaffolds) that can be adapted for multiple receptor targets. By carefully analyzing the structure activity relationships, one can direct the selectivity of these scaffolds toward different receptor subtypes. For example, the weak and non-selective P2 antagonist reactive blue 2 (RB-2) was derivatized using combinatorial synthetic approaches, leading to the identification of selective P2Y2, P2Y4, P2Y12 or P2X2 receptor antagonists. A P2X4 antagonist NC-2600 is in a clinical trial, and A3 adenosine agonists show promise, for chronic pain. P2X7 antagonists have been in clinical trials for depression (JNJ-54175446), inflammatory bowel disease (IBD), Crohn's disease, rheumatoid arthritis, inflammatory pain and chronic obstructive pulmonary disease (COPD). P2X3 antagonists are in clinical trials for chronic cough, and an antagonist named after Burnstock, gefapixant, is expected to be the first P2X3 antagonist filed for approval. We are seeing that the vision of Prof. Burnstock to use purinergic signaling modulators, most recently at P2XRs, for treating disease is coming to fruition.

Anti-inflammatory activity of novel thiosemicarbazone compounds indole-based as COX inhibitors

BACKGROUND

In this article, a series of 20 new thiosemicarbazone derivatives containing indole were synthesized and evaluated for their anti-inflammatory potential.

METHODS

The compounds were obtained through a synthetic route of only two steps, with yields that varied between 33.6 and 90.4%, and characterized by spectroscopic and spectrometric techniques.

RESULTS

An initial screening through the lymphoproliferation assay revealed that compounds LT76, LT81, and LT87 were able to inhibit lymphocyte proliferation, with CC₅₀ of 0.56 ± 0.036, 0.9 ± 0.01 and 0.5 ± 0.07 µM, respectively, better results than indomethacin (CC₅₀ > 12 µM). In addition, these compounds were able to suppress the in-vitro production of TNF-α and NO, in addition to stimulating the production of IL-4. Reinforcing in-vitro assays, the compounds were able to inhibit COX-2 similar to Celecoxib showing greater selectivity for this isoform (LT81 SI: 23.06 versus Celecoxib SI: 11.88). Animal studies showed that compounds LT76 (64.8% inhibition after 6 h), LT81 (89% inhibition after 6 h) and LT87 (100% inhibition after 4 h) were able to suppress edema in mice after inoculation carrageenan with greater potency than indomethacin, and immunohistochemistry revealed that the groups treated with LT76, LT81 and LT87 reduced the expression of COX-2, similar or better results when compared to indomethacin. Complementarily, in-silico studies have shown that these compounds have a good pharmacokinetic profile, for respecting the parameters of Lipinski and Veber, showing their good bioavailability.

CONCLUSIONS

These results demonstrate the potency of thiosemicarbazone derivatives containing indole and confirm their importance as scaffolds of molecules with notorious anti-inflammatory activity.

Medicinal Chemistry and Therapeutic Potential of Agonists, Antagonists and Allosteric Modulators of A1 Adenosine Receptor: Current Status and Perspectives

Adenosine is a purine nucleoside, responsible for the regulation of a wide range of physiological and pathophysiological conditions by binding with four G-protein-coupled receptors (GPCRs), namely A1, A2A, A2B and A3 adenosine receptors (ARs). In particular, A1 AR is ubiquitously present, mediating a variety of physiological processes throughout the body, thus represents a promising drug target for the management of various pathological conditions. Agonists of A1 AR are found to be useful for the treatment of atrial arrhythmia, angina, type-2 diabetes, glaucoma, neuropathic pain, epilepsy, depression and Huntington's disease, whereas antagonists are being investigated for the treatment of diuresis, congestive heart failure, asthma, COPD, anxiety and dementia. However, treatment with full A1 AR agonists has been associated with numerous challenges like cardiovascular side effects, off-target activation as well as desensitization of A1 AR leading to tachyphylaxis. In this regard, partial agonists of A1 AR have been found to be beneficial in enhancing insulin sensitivity and subsequently reducing blood glucose level, while avoiding severe CVS side effects and tachyphylaxis. Allosteric enhancer of A1 AR is found to be potent for the treatment of neuropathic pain, culminating the side effects related to off-target tissue activation of A1 AR. This review provides an overview of the medicinal chemistry and therapeutic potential of various agonists/partial agonists, antagonists and allosteric modulators of A1 AR, with a particular emphasis on their current status and future perspectives in clinical settings.

Approaches for designing and discovering purinergic drugs for gastrointestinal diseases

INTRODUCTION

Purines finely modulate physiological motor, secretory, and sensory functions in the gastrointestinal tract. Their activity is mediated by the purinergic signaling machinery, including receptors and enzymes regulating their synthesis, release, and degradation. Several gastrointestinal dysfunctions are characterized by alterations affecting the purinergic system.

AREAS COVERED

The authors provide an overview on the purinergic receptor signaling machinery, the molecules and proteins involved, and a summary of medicinal chemistry efforts aimed at developing novel compounds able to modulate the activity of each player involved in this machinery. The involvement of purinergic signaling in gastrointestinal motor, secretory, and sensory functions and dysfunctions, and the potential therapeutic applications of purinergic signaling modulators, are then described.

EXPERT OPINION

A number of preclinical and clinical studies demonstrate that the pharmacological manipulation of purinergic signaling represents a viable way to counteract several gastrointestinal diseases. At present, the paucity of purinergic therapies is related to the lack of receptor-subtype-specific agonists and antagonists that are effective in vivo. In this regard, the development of novel therapeutic strategies should be focused to include tools able to control the P1 and P2 receptor expression as well as modulators of the breakdown or transport of purines.

In vitro activity, ultrastructural studies and in silico pharmacokinetic properties of indol-3-yl-thiosemicarbazones derivatives and analogues against juvenile and adult worms of S. mansoni

The present work aimed to carry out in vitro biological assays of indol-3-yl derivatives thiosemicarbazones (2a-e) and 4-thiazolidinones (3a-d) against juvenile and adult worms of S. mansoni, as well as the in silico determination of pharmacokinetic parameters for the prediction of the oral bioavailability of these derivatives. All compounds were initially screened at a concentration of 200 μM against S. mansoni adult worms and the results evidenced the good activity of compounds 2b, 2d and 3b, which caused 100% mortality after 24, 48 and 72 h, respectively. Subsequent studies with these same compounds revealed that compound 2b was able to reduce the viability of the parasites by 85% and 83% at concentrations of 200 and 100 μM, respectively. In relation to the juvenile worms, all compounds (2b, 2d and 3b) were able to cause mortality, but compound 2b demonstrated better activity causing 100% mortality in 48 h. Additionally, it was possible to observe reduction in the viability of juvenile worms of 85%, 81% and 64% at concentrations of 200, 100 and 50 μM, respectively. Several ultrastructural damages were observed when adult and juvenile S. mansoni worms were exposed to compound 2b (200 μM) that was characterized by extensive destruction by the integument, which may justify the mortality rate of cultured parasites. In the DNA interaction assay, fragmentation of the genetic material of adult worms when treated with compound 2b (200 μM) was evidenced, indicating the apoptosis process as mechanism of parasite death. Regarding pharmacokinetic properties, all derivatives are according to the required parameters, predicting good oral bioavailability for the studied compounds. The results presented in this study reveal the good activity of compound 2b in both adult and juvenile worms of S. mansoni, pointing this compound as promising in the development of further studies on schistosomicidal activity.

Topoisomerase inhibition and albumin interaction studies of acridine-thiosemicarbazone derivatives

In the present study, acridine-thiosemicarbazones (ATD) derivatives were tested for their interaction properties with BSA through UV-Vis absorption and fluorescence spectroscopic studies. Both hyperchromic and hypochromic effects, as well as red or blue shifts were demonstrated after the derivatives were added to the BSA. Values for the binding constant (K_b) ranged from 1.62 × 10⁴ to 8.71 × 10⁵ M^-1 and quenching constant (K_SV) from 3.46 × 10² to 7.83 × 10³ M^-1 indicating a good affinity to BSA protein. Complementary, two compounds were selected to assess their inhibition activity against topoisomerase IIα enzyme, of which derivative 3a presented the best result. Moreover, to evaluate protein-ligand interactions, as well as the antitopoisomerase potential of these compounds, tests of molecular modeling were performed between all compounds using the albumin and Topoisomerase IIα/DNA complex. Finally, in silico studies showed that all derivatives used in this research displayed good oral bioavailability potential.

New A2A adenosine receptor antagonists: a structure-based upside-down interaction in the receptor cavity

Adenosine receptor antagonists are generally based on heterocyclic core structures presenting substituents of various volumes and chemical-physical profiles. Adenine and purine-based adenosine receptor antagonists have been reported in literature. In this work we combined various substituents in the 2, 6, and 8-positions of 9-ethylpurine to depict a structure-affinity relationship analysis at the human adenosine receptors. Compounds were rationally designed trough molecular modeling analysis and then synthesized and evaluated at radioligand binding studies at human adenosine receptors. The new compounds showed affinity for the human adenosine receptors, with some derivatives endowed with low nanomolar K_i data, in particular at the A_2AAR subtype. The purine core proves to be a versatile core structure for the development of novel adenosine receptor antagonists with nanomolar affinity for these membrane proteins.