1,3-diphenylpropan-1-ones as allosteric modulators of α7 nACh receptors with analgesic and antioxidant properties

Recent Advances in the Discovery of Nicotinic Acetylcholine Receptor Allosteric Modulators

Positive allosteric modulators (PAMs), negative allosteric modulators (NAMs), silent agonists, allosteric activating PAMs and neutral or silent allosteric modulators are compounds capable of modulating the nicotinic receptor by interacting at allosteric modulatory sites distinct from the orthosteric sites. This survey is focused on the compounds that have been shown or have been designed to interact with nicotinic receptors as allosteric modulators of different subtypes, mainly α7 and α4β2. Minimal chemical changes can cause a different pharmacological profile, which can then lead to the design of selective modulators. Experimental evidence supports the use of allosteric modulators as therapeutic tools for neurological and non-neurological conditions.

Targeting α7 nicotinic acetylcholine receptors for chronic pain

Despite rapid advances in the field of chronic pain, it remains extremely challenging in the clinic. Pain treatment strategies have not improved for decades as opioids remain the main prescribed drugs for chronic pain management. However, long-term use of opioids often leads to detrimental side effects. Therefore, uncovering the mechanisms underlying the development and maintenance of chronic pain may aid the discovery of novel therapeutics to benefit patients with chronic pain. Substantial evidence indicates downregulation of α7 nicotinic acetylcholine receptors (α7 nAChR) in the sciatic nerve, dorsal root ganglia, and spinal cord dorsal horn in rodent models of chronic pain. Moreover, our recent study and results from other laboratories demonstrate that potentiation of α7 nAChR attenuates pain behaviors in various murine models of chronic pain. This review summarized and discussed the preclinical evidence demonstrating the therapeutic potential of α7 nAChR agonists and allosteric modulators in chronic pain. This evidence indicates that potentiation of α7 nAChR is beneficial in chronic pain, mostly by alleviating neuroinflammation. Overall, α7 nAChR-based therapy for chronic pain is an area with great promise, but more research regarding its detailed mechanisms is warranted.

Application of Amino Acids in the Structural Modification of Natural Products: A Review

Natural products and their derivatives are important sources for drug discovery; however, they usually have poor solubility and low activity and require structural modification. Amino acids are highly soluble in water and have a wide range of activities. The introduction of amino acids into natural products is expected to improve the performance of these products and minimize their adverse effects. Therefore, this review summarizes the application of amino acids in the structural modification of natural products and provides a theoretical basis for the structural modification of natural products in the future. The articles were divided into six types based on the backbone structures of the natural products, and the related applications of amino acids in the structural modification of natural products were discussed in detail.

Natural Polyhydroxy Flavonoids, Curcuminoids, and Synthetic Curcumin Analogs as α7 nAChRs Positive Allosteric Modulators

The α7 nicotinic acetylcholine receptor (α7 nAChR) is a ligand-gated ion channel that is involved in cognition disorders, schizophrenia, pain, and inflammation. Allosteric modulation of this receptor might be advantageous to reduce the toxicity in comparison with full agonists. Our previous results obtained with some hydroxy-chalcones, which were identified as positive allosteric modulators (PAMs) of α7 nAChR, prompted us to evaluate the potential of some structurally related naturally occurring flavonoids and curcuminoids and some synthetic curcumin analogues, with the aim of identifying new allosteric modulators of the α7 nAChR. Biological evaluation showed that phloretin, demethoxycurcumin, and bis-demethoxicurcuming behave as PAMs of α7 nAChR. In addition, some new curcumin derivatives were able to enhance the signal evoked by ACh; the activity values found for the tetrahydrocurcuminoid analog 23 were especially promising.

Characterization of Novel Synthetic Polyphenols: Validation of Antioxidant and Vasculoprotective Activities

Antioxidant compounds, including polyphenols, have therapeutic effects because of their anti-inflammatory, antihypertensive, antithrombotic and antiproliferative properties. They play important roles in protecting the cardiovascular and neurological systems, by having preventive or protective effects against free radicals produced by either normal or pathological metabolism in such systems. For instance, resveratrol, a well-known potent antioxidant, has a counteracting effect on the excess of reactive oxygen species (ROS) and has a number of therapeutic benefits, like anti-inflammatory, anti-cancer and cardioprotective activities. Based on previous work from our group, and on the most frequent OH substitutions of natural polyphenols, we designed two series of synthetically accessible bis-polyhydroxyphenyl derivatives, separated by amide or urea linkers. These compounds exhibit high antioxidant ability (oxygen radical absorbance capacity (ORAC) assay) and interesting radical scavenging activity (RSA) values (2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and α,α-diphenyl-β-picrylhydrazyl (DPPH) tests). Some of the best polyphenols were evaluated in two biological systems, endothelial cells (in vitro) and whole aorta (ex vivo), highly susceptible for the deleterious effects of prooxidants under different inflammatory conditions, showing protection against oxidative stress induced by inflammatory stimuli relevant in cardiovascular diseases, i.e., Angiotensin II and IL-1β. Selected compounds also showed strong in vivo antioxidant properties when evaluated in the model organism Saccharomyces cerevisiae.

Pentameric Ligand-Gated Ion Channels as Pharmacological Targets Against Chronic Pain

Chronic pain is a common detrimental condition that affects around 20% of the world population. The current drugs to treat chronic pain states, especially neuropathic pain, have a limited clinical efficiency and present significant adverse effects that complicates their regular use. Recent studies have proposed new therapeutic strategies focused on the pharmacological modulation of G-protein-coupled receptors, transporters, enzymes, and ion channels expressed on the nociceptive pathways. The present work intends to summarize recent advances on the pharmacological modulation of pentameric ligand-gated ion channels, which plays a key role in pain processing. Experimental data have shown that novel allosteric modulators targeting the excitatory nicotinic acetylcholine receptor, as well as the inhibitory GABA_A and glycine receptors, reverse chronic pain-related behaviors in preclinical assays. Collectively, these evidences strongly suggest the pharmacological modulation of pentameric ligand-gated ion channels is a promising strategy towards the development of novel therapeutics to treat chronic pain states in humans.

1-(2',5'-Dihydroxyphenyl)-3-(2-fluoro-4-hydroxyphenyl)-1-propanone (RGM079): A Positive Allosteric Modulator of α7 Nicotinic Receptors with Analgesic and Neuroprotective Activity

Acetylcholine α7 nicotinic receptors are widely expressed in the brain, where they are involved in the central processing of pain as well as in neuropsychiatric, neurodegenerative, and inflammatory processes. Positive allosteric modulators (PAMs) show the advantage of allowing the selective regulation of different subtypes of acetylcholine receptors without directly interacting with the agonist binding site. Here, we report the preparation and biological activity of a fluoro-containing compound, 1-(2',5'-dihydroxyphenyl)-3-(2-fluoro-4-hydroxyphenyl)-1-propanone (8, RGM079), that behaves as a potent PAM of the α7 receptors and has a balanced pharmacokinetic profile and antioxidant properties comparable or even higher than well-known natural polyphenols. In addition, compound RGM079 shows neuroprotective properties in Alzheimer's disease (AD)-toxicity related models. Thus, it causes a concentration-dependent neuroprotective effect against the toxicity induced by okadaic acid (OA) in the human neuroblastoma cell line SH-SY5Y. Similarly, in primary cultures of rat cortical neurons, RGM079 is able to restore the cellular viability after exposure to OA and amyloid peptide Aβ_1-42, with cell death almost completely prevented at 10 and 30 μM, respectively. Finally, compound RGM079 shows in vivo analgesic activity in the complete Freund's adjuvant (CFA)-induced paw inflammation model after intraperitoneal administration.

Activation of the cholinergic anti-inflammatory pathway by GTS-21 attenuates cisplatin-induced acute kidney injury in mice

Acute kidney injury (AKI) is the most common side effect of cisplatin, a widely used chemotherapy drug. Although AKI occurs in up to one third of cancer patients receiving cisplatin, effective renal protective strategies are lacking. Cisplatin targets renal proximal tubular epithelial cells leading to inflammation, reactive oxygen species, tubular cell injury, and eventually cell death. The cholinergic anti-inflammatory pathway is a vagus nerve-mediated reflex that suppresses inflammation via α7 nicotinic acetylcholine receptors (α7nAChRs). Our previous studies demonstrated the renoprotective and anti-inflammatory effects of cholinergic agonists, including GTS-21. Therefore, we examined the effect of GTS-21 on cisplatin-induced AKI. Male C57BL/6 mice received either saline or GTS-21 (4mg/kg, i.p.) twice daily for 4 days before cisplatin and treatment continued through euthanasia; 3 days post-cisplatin mice were euthanized and analyzed for markers of renal injury. GTS-21 significantly reduced cisplatin-induced renal dysfunction and injury (p<0.05). GTS-21 significantly attenuated renal Ptgs2/COX-2 mRNA and IL-6, IL-1β, and CXCL1 protein expression, as well as neutrophil infiltration after cisplatin. GTS-21 blunted cisplatin-induced renal ERK1/2 activation, as well as renal ATP depletion and apoptosis (p<0.05). GTS-21 suppressed the expression of CTR1, a cisplatin influx transporter and enhanced the expression of cisplatin efflux transporters MRP2, MRP4, and MRP6 (p<0.05). Using breast, colon, and lung cancer cell lines we showed that GTS-21 did not inhibit cisplatin’s tumor cell killing activity. GTS-21 protects against cisplatin-AKI by attenuating renal inflammation, ATP depletion and apoptosis, as well as by decreasing renal cisplatin influx and increasing efflux, without impairing cisplatin-mediated tumor cell killing. Our results support further exploring the cholinergic anti-inflammatory pathway for preventing cisplatin-induced AKI.

Amino acid and peptide prodrugs of diphenylpropanones positive allosteric modulators of α7 nicotinic receptors with analgesic activity

α7 Nicotinic acetylcholine receptors (nAChRs) are ion channels implicated in a number of CNS pathological processes, including pain and psychiatric, cognitive and inflammatory diseases. Comparing with orthosteric agonism, positive allosteric modulation of these channels constitutes an interesting approach to achieve selectivity versus other nicotinic receptors. We have recently described new chalcones and 1,3-diphenylpropanones as positive allosteric modulators (PAMs) of α7 nAChRs, which proved to have good analgesic activities but poor pharmacokinetic properties. Here we report the preparation of amino acid and peptide derivatives as prodrugs of these modulators with the aim of improving their in vivo biological activity. While the valine derivative showed very short half life in aqueous solutions to be considered a prodrug, Val-Val and Val-Pro-Val are suitable precursors of the parent 1,3-diphenylpropanones, via chemical and enzymatic transformation, respectively. Compounds 19 (Val-Val) and 21 (Val-Pro-Val), prodrugs of the 2',5',4-trihydroxy-1,3-diphenylpropan-1-one 3, showed significant antinociceptive activity in in vivo assays. The best compound, 21, displayed a better profile in the analgesia test than its parent compound 3, exhibiting about the same potency but long-lasting effects.