Synthesis and structure–activity relationships of fibrate-based analogues inside PPARs

α-Halocarbonyls as a Valuable Functionalized Tertiary Alkyl Source

This review introduces the synthetic organic chemical value of α-bromocarbonyl compounds with tertiary carbons. This α-bromocarbonyl compound with a tertiary carbon has been used primarily only as a radical initiator in atom transfer radical polymerization (ATRP) reactions. However, with the recent development of photo-radical reactions (around 2010), research on the use of α-bromocarbonyl compounds as tertiary alkyl radical precursors became popular (around 2012). As more examples were reported, α-bromocarbonyl compounds were studied not only as radicals but also for their applications in organometallic and ionic reactions. That is, α-bromocarbonyl compounds act as nucleophiles as well as electrophiles. The carbonyl group of α-bromocarbonyl compounds is also attractive because it allows the skeleton to be converted after the reaction, and it is being applied to total synthesis. In our survey until 2022, α-bromocarbonyl compounds can be used to perform a full range of reactions necessary for organic synthesis, including multi-component reactions, cross-coupling, substitution, cyclization, rearrangement, stereospecific reactions, asymmetric reactions. α-Bromocarbonyl compounds have created a new trend in tertiary alkylation, which until then had limited reaction patterns in organic synthesis. This review focuses on how α-bromocarbonyl compounds can be used in synthetic organic chemistry.

Anti-Inflammatory, Antioxidant, and WAT/BAT-Conversion Stimulation Induced by Novel PPAR Ligands: Results from Ex Vivo and In Vitro Studies

Activation of peroxisome proliferator-activated receptors (PPARs) not only regulates multiple metabolic pathways, but mediates various biological effects related to inflammation and oxidative stress. We investigated the effects of four new PPAR ligands containing a fibrate scaffold—the PPAR agonists (1a (αEC50 1.0 μM) and 1b (γEC50 0.012 μM)) and antagonists (2a (αIC50 6.5 μM) and 2b (αIC50 0.98 μM, with a weak antagonist activity on γ isoform))—on proinflammatory and oxidative stress biomarkers. The PPAR ligands 1a-b and 2a-b (0.1–10 μM) were tested on isolated liver specimens treated with lipopolysaccharide (LPS), and the levels of lactate dehydrogenase (LDH), prostaglandin (PG) E2, and 8-iso-PGF2α were measured. The effects of these compounds on the gene expression of the adipose tissue markers of browning, PPARα, and PPARγ, in white adipocytes, were evaluated as well. We found a significant reduction in LPS-induced LDH, PGE2, and 8-iso-PGF2α levels after 1a treatment. On the other hand, 1b decreased LPS-induced LDH activity. Compared to the control, 1a stimulated uncoupling protein 1 (UCP1), PR-(PRD1-BF1-RIZ1 homologous) domain containing 16 (PRDM16), deiodinase type II (DIO2), and PPARα and PPARγ gene expression, in 3T3-L1 cells. Similarly, 1b increased UCP1, DIO2, and PPARγ gene expression. 2a-b caused a reduction in the gene expression of UCP1, PRDM16, and DIO2 when tested at 10 μM. In addition, 2a-b significantly decreased PPARα gene expression. A significant reduction in PPARγ gene expression was also found after 2b treatment. The novel PPARα agonist 1a might be a promising lead compound and represents a valuable pharmacological tool for further assessment. The PPARγ agonist 1b could play a minor role in the regulation of inflammatory pathways.

The Small Molecule PPARγ Agonist GL516 Induces Feeding-Stimulatory Effects in Hypothalamus Cells Hypo-E22 and Isolated Hypothalami

PPARγ agonists are implicated in the regulation of diabetes and metabolic syndrome and have therapeutic potential in brain disorders. PPARγ modulates appetite through its central effects, especially on the hypothalamic arcuate nucleus (ARC). Previous studies demonstrated that the small molecule GL516 is a PPARγ agonist able to reduce oxidative stress and apoptosis with a potential neuroprotective role. Herein, we investigated the effects of GL516, in vitro and ex vivo, on the levels of hypothalamic dopamine (DA) and serotonin (5-HT). The gene expressions of neuropeptide Y, CART, AgRP, and POMC, which play master roles in the neuroendocrine regulation of feeding behavior and energy balance, were also evaluated. HypoE22 cells were treated with H₂O₂ (300 μM) for 2 h e 30’ and with different concentrations of GL516 (1 nM-100 µM). The cell viability was evaluated after 24 and 48 h of culturing using the MTT test. DA and 5-HT levels in the HypoE22 cell supernatants were analyzed through HPLC; an ex vivo study on isolated hypothalamic specimens challenged with scalar concentrations of GL516 (1–100 µM) and with pioglitazone (10 µM) was carried out. The gene expressions of CART, NPY, AgRP, and POMC were also determined by a quantitative real-time PCR. The results obtained showed that GL516 was able to reduce DA and 5-HT turnover; moreover, it was effective in stimulating NPY and AgRP gene expressions with a concomitant reduction in CART and POMC gene expressions. These results highlight the capability of GL516 to modulate neuropeptide pathways deeply involved in appetite control suggesting an orexigenic effect. These findings emphasize the potential use of GL516 as a promising candidate for therapeutical applications in neurodegenerative diseases associated with the reduction in food intake and stimulation of catabolic pathways.

Discovery of 4-benzyloxy and 4-(2-phenylethoxy) chalcone fibrate hybrids as novel PPARα agonists with anti-hyperlipidemic and antioxidant activities: Design, synthesis and in vitro/in vivo biological evaluation

In the current work, a series of novel 4-benzyloxy and 4-(2-phenylethoxy) chalcone fibrate hybrids (10a-o) and (11a-e) were synthesized and evaluated as new PPARα agonists in order to find new agents with higher activity and fewer side effects. The 2-propanoic acid derivative 10a and the 2-butanoic acid congener 10i showed the best overall PPARα agonistic activity showing E_max% values of 50.80 and 90.55%, respectively, and EC₅₀ values of 8.9 and 25.0 μM, respectively, compared to fenofibric acid with E_max = 100% and EC₅₀ = 23.22 μM, respectively. These two compounds also stimulated carnitine palmitoyltransferase 1A gene transcription in HepG2 cells and PPARα protein expression. Molecular docking simulations were performed for the newly synthesized compounds to study their predicted binding pattern and energies in PPARα active site to rationalize their promising activity. In vivo, compounds 10a and 10i elicited a significant hypolipidemic activity improving the lipid profile in triton WR-1339-induced hyperlipidemic rats, including serum triglycerides, total cholesterol, LDL, HDL and VLDL levels. Compound 10i possessed better anti-hyperlipidemic activity than 10a. At a dose of 200 mg/kg, it demonstrated significantly lower TC, TG, LDL and VLDL levels than that of fenofibrate at the same dose with similar HDL levels. Compounds 10i and 10a possessed atherogenic indices (CRR, AC, AI, CRI-II) like that of fenofibrate. Additionally, a promising antioxidant activity indicated by the increased tissue reduced glutathione and plasma total antioxidant capacity with decreased plasma malondialdehyde levels was demonstrated by compounds 10a and 10i. No histopathological alterations were recorded in the hepatic tissue of compound 10i (200 mg/kg).

A Photohormone for Light-Dependent Control of PPARα in Live Cells

Photopharmacology enables the optical control of several biochemical processes using small-molecule photoswitches that exhibit different bioactivities in their cis- and trans-conformations. Such tool compounds allow for high spatiotemporal control of biological signaling, and the approach also holds promise for the development of drug molecules that can be locally activated to reduce target-mediated adverse effects. Herein, we present the expansion of the photopharmacological arsenal to two new members of the peroxisome proliferator-activated receptor (PPAR) family, PPARα and PPARδ. We have developed a set of highly potent PPARα and PPARδ targeting photohormones derived from the weak pan-PPAR agonist GL479 that can be deactivated by light. The photohormone 6 selectively activated PPARα in its trans-conformation with high selectivity over the related PPAR subtypes and was used in live cells to switch PPARα activity on and off in a light- and time-dependent fashion.

Investigation of the effect of different linker chemotypes on the inhibition of histone deacetylases (HDACs)

Histone Deacetylases (HDACs) are among the most attractive and interesting targets in anticancer drug discovery. The clinical relevance of HDAC inhibitors (HDACIs) is testified by four FDA-approved drugs for cancer treatment. However, one of the main drawbacks of these drugs resides in the lack of selectivity against the different HDAC isoforms, resulting in severe side effects. Thus, the identification of selective HDACIs represents an exciting challenge for medicinal chemists. HDACIs are composed of a cap group, a linker region, and a metal-binding group interacting with the catalytic zinc ion. While the cap group has been extensively investigated, less information is available about the effect of the linker on isoform selectivity. To this aim, in this work, we explored novel linker chemotypes to direct isoform selectivity. A small library of 25 hydroxamic acids with hitherto unexplored linker chemotypes was prepared. In vitro tests demonstrated that, depending on the linker type, some candidates selectively inhibit HDAC1 over HDAC6 isoform or vice versa. Docking calculations were performed to rationalize the effect of the novel linker chemotypes on biologic activity. Moreover, four compounds were able to increase the levels of acetylation of histone H3 or tubulin. These compounds were also assayed in breast cancer MCF7 cells to test their antiproliferative effect. Three compounds showed a significant reduction of cancer proliferation, representing valuable starting points for further optimization.

Photohormones Enable Optical Control of the Peroxisome Proliferator-Activated Receptor γ (PPARγ)

Photopharmacology aims at the optical control of protein activity using synthetic photoswitches. This approach has been recently expanded to nuclear hormone receptors with the introduction of "photohormones" for the retinoic acid receptor, farnesoid X receptor, and estrogen receptor. Herein, we report the development and profiling of photoswitchable agonists for peroxisome proliferator-activated receptor γ (PPARγ). Based on known PPARγ ligands (MDG548, GW1929, and rosiglitazone), we have designed and synthesized azobenzene derivatives, termed AzoGW1929 and AzoRosi, which were confirmed to be active in cell-based assays. Subsequent computer-aided optimization of AzoRosi resulted in the photohormone AzoRosi-4, which bound and activated PPARγ preferentially in its light-activated cis-configuration.

Sulfonimide and Amide Derivatives as Novel PPARα Antagonists: Synthesis, Antiproliferative Activity, and Docking Studies

An agonist-antagonist switching strategy was performed to discover novel PPARα antagonists. Phenyldiazenyl derivatives of fibrates were developed, bearing sulfonimide or amide functional groups. A second series of compounds was synthesized, replacing the phenyldiazenyl moiety with amide or urea portions. Final compounds were screened by transactivation assay, showing good PPARα antagonism and selectivity at submicromolar concentrations. When tested in cancer cell models expressing PPARα, selected derivatives induced marked effects on cell viability. Notably, 3c, 3d, and 10e displayed remarkable antiproliferative effects in two paraganglioma cell lines, with CC₅₀ lower than commercial PPARα antagonist GW6471 and a negligible toxicity on normal fibroblast cells. Docking studies were also performed to elucidate the binding mode of these compounds and to help interpretation of SAR data.

Structural Basis for PPARα Activation by 1H-pyrazolo-[3,4-b]pyridine Derivatives

Small-molecule agonism of peroxisome proliferator-activated receptor α (PPARα), a ligand-activated transcriptional factor involved in regulating fatty acid metabolism, is an important approach for treating dyslipidemia. Here, we determined the structures of the ligand-binding domain (LBD) of PPARα in complex with 1H-pyrazolo[3,4-b]pyridine-4-carboxylic acid derivatives, which were recently identified as PPARα-selective activators with markedly different structures from those of the well-known PPARα agonists fibrates. The crystal structures of the complexes showed that they form a canonical hydrogen-bond network involving helix 12 in the LBD, which is thought to be essential for PPARα activation, as also observed for fibrates. However, the phenyl side chain of the compounds occupies a small cavity between Ile272 and Ile354, which is rarely accessed by fibrates. This unique feature may be essential for subtype selectivity and combine with the well-characterized binding mode of fibrates to improve activity. These findings demonstrate the advantage of using 1H-pyrazolo-[3,4-b]pyridine as a skeleton of PPARα agonists and provide insight into the design of molecules for treating dyslipidemia.

Druggability profile of stilbene-derived PPAR agonists: determination of physicochemical properties and PAMPA study

PPAR agonists represent a new therapeutic opportunity for the prevention and treatment of neurodegenerative disorders, but their pharmacological success depends on favourable pharmacokinetic properties and capability to cross the BBB. In this study, we assayed some PPAR agonists previously synthesized by us for their physicochemical properties, with particular references to lipophilicity, solubility and permeability profiles, using the PAMPA. Although tested compounds showed high lipophilicity and low aqueous solubility, the results revealed a good overall druggability profile, encouraging further studies in the field of neurodegenerative diseases.

PPAR Ligands Containing Stilbene Scaffold

Peroxisome proliferator-activated receptors (PPARs) are transcriptional factors which belong to the ligand-activated nuclear receptor superfamily. They are ubiquitously expressed throughout the body. So far, three major subtypes have been identified, PPARα, PPARβ/δ and PPARγ. They are crucial for lipid and glucose metabolism and are also involved in the regulation of several types of tumors, inflammation, cardiovascular diseases and infertility. The importance of these transcription factors in physiology and pathophysiology has been largely investigated. Synthetic PPAR ligands are widely used in the treatment of dyslipidemia (e.g. fibrates - PPARα activators) or in diabetes mellitus (e.g. thiazolidinediones - PPARγ agonists) while a new generation of dual agonists reveals hypolipemic, hypotensive, antiatherogenic, anti-inflammatory and anticoagulant action. Many natural ligands, including polyphenolic compounds, influence the expression of these receptors. They have several health-promoting properties, including antioxidant, anti-inflammatory, and antineoplastic activities. Resveratrol, a stilbene polyphenol, is a biological active modulator of several signaling proteins, including PPARs. Given the enormous pharmacological potential of resveratrol, stilbene-based medicinal chemistry had a rapid increase covering various areas of research. The present review discusses ligands of PPARs that contain stilbene scaffold and summarises the different types of compounds on the basis of chemical structure.

Novel Phenyldiazenyl Fibrate Analogues as PPAR α/γ/δ Pan-Agonists for the Amelioration of Metabolic Syndrome

The development of PPARα/γ dual or PPARα/γ/δ pan-agonists could represent an efficacious approach for a simultaneous pharmacological intervention on carbohydrate and lipid metabolism. Two series of new phenyldiazenyl fibrate derivatives of GL479, a previously reported PPARα/γ dual agonist, were synthesized and tested. Compound 12a was identified as a PPAR pan-agonist with moderate and balanced activity on the three PPAR isoforms (α, γ, δ). Moreover, docking experiments showed that 12a adopts a different binding mode in PPARγ compared to PPARα or PPARδ, providing a structural basis for further structure-guided design of PPAR pan-agonists. The beneficial effects of 12a were evaluated both in vitro, on the expression of PPAR target key metabolic genes, and ex vivo in two rat tissue inflammatory models. The obtained results allow considering this compound as an interesting lead for the development of a new class of PPAR pan-agonists endowed with an activation profile exploitable for therapy of metabolic syndrome.

PPAR-γ agonist GL516 reduces oxidative stress and apoptosis occurrence in a rat astrocyte cell line

AIMS

The worldwide increase in aging population is prevalently associated with the increase of neurodegenerative diseases. Peroxisome Proliferator-Activated Receptors (PPARs) are ligand-modulated transcriptional factors which belong to the nuclear hormone receptor superfamily which regulates peroxisome proliferation. The PPAR-γ is the most extensively studied among the three isoforms and the neuroprotective effects of PPAR-γ agonists have been recently demonstrated in a variety of preclinical models of neurological disorders. The aim of the study is to biologically evaluate the neuroprotective effects of new PPAR-γ selective agonists in an in vitro model.

MAIN METHODS

CTX-TNA2 rat astrocytes were treated with G3335, a PPAR-γ antagonist, to simulate the conditions of a neurological disorder. Newly synthetized PPAR-γ selective agonists were added to the cell culture. Cytotoxicity was assessed by MTT assay, catalase activity was investigated by a colorimetric assay, Reactive Oxygen Species (ROS) production and apoptosis occurrence were measured by flow cytometry. Western blotting were performed to measure the levels of protein involved in the apoptotic pathway.

KEY FINDINGS

Four PPAR-γ agonists were selected. Among them, the GL516, a fibrate derivative, showed low cytotoxicity and proved effective in restoring the catalase activity, reducing ROS production and decreasing the apoptosis occurrence triggered by the G3335 administration. The effects of this molecule appear to be comparable to the reference compound rosiglitazone, a potent and selective PPAR-γ agonist, mainly at prolonged exposure times (96 h).

SIGNIFICANCE

Based on recent evidence, hypofunctionality of the PPAR-γ in glial cells could be present in neurodegenerative diseases and could participate in pathological mechanisms through peroxisomal damage. The fibrate derivative PPAR-γ agonist GL516 emerged as the most promising molecule of the series and could have a role in preventing the pathophysiology of neurodegenerative disorders.

The Positive Regulation of eNOS Signaling by PPAR Agonists in Cardiovascular Diseases

Increasing evidence shows that activation of peroxisome proliferator-activated receptors (PPARs) plays an essential role in the regulation of vascular endothelial function through a range of mechanisms, including non-metabolic. Among these, the PPAR-mediated activation of endothelial nitric oxide synthase (eNOS) appears to be of considerable importance. The regulated and sustained bioavailability of nitric oxide (NO) in the endothelium is essential to avoid the development of cardiovascular diseases such as hypertension or atherosclerosis. Therefore, a deeper understanding of the different effects of specific PPAR ligands on NO bioavailability could be useful in the development of novel or multi-targeted PPAR agonists. In this review, we report the most meaningful and up-to-date in vitro and in vivo studies of the regulation of NO production performed by different PPAR agonists. Insights into the molecular mechanisms of PPAR-mediated eNOS activation are also provided. Although findings from animal studies in which the activation of PPARα, PPARβ/δ, or PPARγ have provided clear vasoprotective effects have been promising, several benefits from PPAR agonists are offset by unwanted outcomes. Therefore, new insights could be useful in the development of tissue-targeted PPAR agonists with more tolerable side effects to improve treatment options for cardiovascular diseases.

Anticancer Activity of Stilbene-Based Derivatives

Stilbene is an abundant structural scaffold in nature, and stilbene-based compounds have been widely reported for their biological activity. Notably, (E)-resveratrol and its natural stilbene-containing derivatives have been extensively investigated as cardioprotective, potent antioxidant, anti-inflammatory, and anticancer agents. Starting from its potent chemotherapeutic activity against a wide variety of cancers, the stilbene scaffold has been subject to synthetic manipulations with the aim of obtaining new analogues with improved anticancer activity and better bioavailability. Within the last decade, the majority of new synthetic stilbene derivatives have demonstrated significant anticancer activity against a large number of cancer cell lines, depending on the type and position of substituents on the stilbene skeleton. This review focuses on the structure-activity relationship of the key compounds containing a stilbene scaffold and describes how the structural modifications affect their anticancer activity.

Deciphering the Roles of Thiazolidinediones and PPAR in Bladder Cancer

The use of thiazolidinedione (TZD) therapy in type II diabetic patients has proven useful in the lowering of blood glucose levels. However, recent investigations have shown that there may be potential health concerns associated, including the risk of developing bladder cancer as well as complications in the cardiovasculature. TZDs are ligands for the nuclear receptor PPARγ, and activation causes lipid uptake and insulin sensitization, both of which are critical processes for diabetic patients whose bodies are unable to utilize insulin effectively. Several studies have shown that PPARγ/TZDs decrease IGF-1 levels and, thus, reduce cancer growth in carcinomas such as the pancreas, colon, liver, and prostate. However, other studies have shed light on the potential of the receptor as a biomarker for uroepithelial carcinomas, particularly due to its stimulatory effect on migration of bladder cancer cells. Furthermore, PPARγ may provide the tumor-promoting microenvironment by de novo synthesis of nutrients that are needed for bladder cancer development. In this review, we closely examine the TZD class of drugs and their effects on PPARγ in patient studies along with additional molecular factors that are positive modulators, such as protein phosphatase 5 (PP5), which may have considerable implications for bladder cancer therapy.

Different binding and recognition modes of GL479, a dual agonist of Peroxisome Proliferator-Activated Receptor α/γ

Peroxisome Proliferator-Activated Receptors (PPARs) are ligand-dependent transcription factors that control various functions in human organism, including the control of glucose and lipid metabolism. PPARγ is a target of TZD agonists, clinically used to improve insulin sensitivity whereas fibrates, PPARα ligands, lower serum triglyceride levels. We report here the structural studies of GL479, a synthetic dual PPARα/γ agonist, designed by a combination of clofibric acid skeleton and a phenyldiazenyl moiety, as bioisosteric replacement of stilbene group, in complex with both PPARα and PPARγ receptors. GL479 was previously reported as a partial agonist of PPARγ and a full agonist of PPARα with high affinity for both PPARs. Our structural studies reveal different binding modes of GL479 to PPARα and PPARγ, which may explain the distinct activation behaviors observed for each receptor. In both cases the ligand interacts with a Tyr located at helix 12 (H12), resulting in the receptor active conformation. In the complex with PPARα, GL479 occupies the same region of the ligand-binding pocket (LBP) observed for other full agonists, whereas GL479 bound to PPARγ displays a new binding mode. Our results indicate a novel region of PPARs LBP that may be explored for the design of partial agonists as well dual PPARα/γ agonists that combine, simultaneously, the therapeutic effects of the treatment of insulin resistance and dyslipidemia.

Blocking the peroxisome proliferator-activated receptor (PPAR): an overview

Peroxisome proliferator-activated receptors (PPARs) have been studied extensively over the last few decades and have been assessed as molecular targets for the development of drugs against metabolic disorders. A rapid increase in understanding of the physiology and pharmacology of these receptors has occurred, together with the identification of novel chemical structures that are able to activate the various PPAR subtypes. More recent evidence suggests that moderate activation of these receptors could be favorable in pathological situations due to a decrease in the side effects brought about by PPAR agonists. PPAR partial agonists and antagonists are interesting tools that are currently used to better elucidate the biological processes modulated by this family of nuclear receptors. Herein we present an overview of the various molecular structures that are able to block each of the PPAR subtypes, with a focus on promising therapeutic applications.