Design, Synthesis, and Biological Evaluation of Indole Biphenylcarboxylic Acids as PPARγ Antagonists

Reduction in Obesity-Related Hepatic Fibrosis by SR1664

Peroxisome-proliferator-activated receptor gamma (PPARγ) is a transcription factor with adipogenic, insulin-sensitizing, and antifibrotic properties. Strong PPARγ activators, such as the thiazolidinediones, can induce unwanted effects such as edema, weight gain, and bone loss, and therefore selective modulators of PPARγ are in development. We previously reported that one selective PPARγ modulator, SR1664, reduced toxin-induced hepatic fibrosis and the activation of hepatic stellate cells (HSCs), the main collagen-producing liver cell in fibrosis. In this study, we used a high fat and high carbohydrate (HFHC) model of hepatic steatosis and fibrosis to determine the effect of SR1664. Mice were placed on a standard chow or HFHC diet for 16 weeks, with SR1664 or control treatment for the final 4 weeks. SR1664 did not alter weight gain or fasting insulin or glucose levels. The size of lipid droplets in the HFHC group was reduced by SR1664, but there was no effect on total liver triglyceride levels. The degree of fibrosis was significantly reduced by SR1664 in mice on the HFHC diet, and this was accompanied by a decrease in activated HSC. In summary, SR1664 improved insulin sensitivity and reduced fibrosis in the HFHC diet, suggesting selective PPARγ modulation is effective in obesity-related liver fibrosis.

Biological Screening and Crystallographic Studies of Hydroxy γ-Lactone Derivatives to Investigate PPARγ Phosphorylation Inhibition

PPARγ represents a key target for the treatment of type 2 diabetes and metabolic syndrome. To avoid serious adverse effects related to the PPARγ agonism profile of traditional antidiabetic drugs, a new opportunity is represented by the development of molecules acting as inhibitors of PPARγ phosphorylation by the cyclin-dependent kinase 5 (CDK5). Their mechanism of action is mediated by the stabilization of the PPARγ β-sheet containing Ser273 (Ser245 in PPARγ isoform 1 nomenclature). In this paper, we report the identification of new γ-hydroxy-lactone-based PPARγ binders from the screening of an in-house library. These compounds exhibit a non-agonist profile towards PPARγ, and one of them prevents Ser245 PPARγ phosphorylation by acting mainly on PPARγ stabilization and exerting a weak CDK5 inhibitory effect.

Molecular Modeling of Allosteric Site of Isoform-Specific Inhibition of the Peroxisome Proliferator-Activated Receptor PPARγ

The peroxisome proliferator-activated receptor gamma (PPARγ) is a nuclear receptor and controls a number of gene expressions. The ligand binding domain (LBD) of PPARγ is large and involves two binding sites: orthosteric and allosteric binding sites. Increased evidence has shown that PPARγ is an oncogene and thus the PPARγ antagonists have potential as anticancer agents. In this paper, we use Glide Dock approach to determine which binding site, orthosteric or allosteric, would be a preferred pocket for PPARγ antagonist binding, though antidiabetic drugs such as thiazolidinediones (TZDs) bind to the orthosteric site. The Glide Dock results show that the binding of PPARγ antagonists at the allosteric site yielded results that were much closer to the experimental data than at the orthosteric site. The PPARγ antagonists seem to selectively bind to residues Lys265, Ser342 and Arg288 at the allosteric binding site, whereas PPARγ agonists would selectively bind to residues Leu228, Phe363, and His449, though Phe282 and Lys367 may also play a role for agonist binding at the orthosteric binding pocket. This finding will provide new perspectives in the design and optimization of selective and potent PPARγ antagonists or agonists.

The Role of Peroxisome Proliferator-Activated Receptor Gamma and Atherosclerosis: Post-translational Modification and Selective Modulators

Atherosclerosis is the hallmark of cardiovascular disease (CVD) which is a leading cause of death in type 2 diabetes patients, and glycemic control is not beneficial in reducing the potential risk of CVD. Clinically, it was shown that Thiazolidinediones (TZDs), a class of peroxisome proliferator-activated receptor gamma (PPARγ) agonists, are insulin sensitizers with reducing risk of CVD, while the potential adverse effects, such as weight gain, fluid retention, bone loss, and cardiovascular risk, restricts its use in diabetic treatment. PPARγ, a ligand-activated nuclear receptor, has shown to play a crucial role in anti-atherosclerosis by promoting cholesterol efflux, repressing monocytes infiltrating into the vascular intima under endothelial layer, their transformation into macrophages, and inhibiting vascular smooth muscle cells proliferation as well as migration. The selective activation of subsets of PPARγ targets, such as through PPARγ post-translational modification, is thought to improve the safety profile of PPARγ agonists. Here, this review focuses on the significance of PPARγ activity regulation (selective activation and post-translational modification) in the occurrence, development and treatment of atherosclerosis, and further clarifies the value of PPARγ as a safe therapeutic target for anti-atherosclerosis especially in diabetic treatment.

A promising in silico protocol to develop novel PPARγ antagonists as potential anticancer agents: Design, synthesis and experimental validation via PPARγ protein activity and competitive binding assay

Peroxisome proliferator-activated receptor gamma (PPARγ), a member of the nuclear receptor superfamily is an excellent example of targets that orchestrates cancer, inflammation, lipid and glucose metabolism. We report a protocol for the development of novel PPARγ antagonists by employing 3D QSAR based virtual screening for the identification of ligands with anticancer properties. The models are generated based on a large and diverse set of PPARγ antagonist ligands by the HYPOGEN algorithm using Discovery Studio 2019 drug design software. Among the 10 hypotheses generated, Hypotheses 2 showed the highest correlation coefficient values of 0.95 with less RMS deviation of 1.193. Validation of the developed pharmacophore model was performed by Fischer's randomization and screening against test and decoy set. The GH score or goodness score was found to be 0.81 indicating moderate to a good model. The selected pharmacophore model Hypo 2 was used as a query model for further screening of 11,145 compounds from the PubChem, sc-PDB structure database, and designed novel ligands. Based on fit values and ADMET filter, the final 10 compounds with the predicated activity of ≤ 3 nM were subjected for docking analysis. Docking analysis revealed the unique binding mode with hydrophobic amino acid that can cause destabilization of the H12 which is an important molecular mechanism to prove its antagonist action. Based on high CDocker scores, Cpd31 was synthesized, purified, analyzed and screened for PPARγ competitive binding by TR-FRET assay. The biochemical protein binding results matched the predicted results. Further, Cpd31 was screened against cancer cells and validated the results.

Virtual screening and biological evaluation of PPARγ antagonists as potential anti-prostate cancer agents

The peroxisome proliferator-activated receptor gamma (PPARγ) was identified as an oncogene and it plays a key role in prostate cancer (PC) development and progression. PPARγ antagonists have been shown to inhibit PC cell growth. Herein, we describe a virtual screening-based approach that led to the discovery of novel PPARγ antagonist chemotypes that bind at the allosteric pocket. Arg288, Lys367, and His449 appear to be important for PPARγ antagonist binding.

Rational Multitargeted Drug Design Strategy from the Perspective of a Medicinal Chemist

The development of multitarget-directed ligands (MTDLs) has become a widely focused research topic, but rational design remains as an enormous challenge. This paper reviews and discusses the design strategy of incorporating the second activity into an existing single-active ligand. If the binding sites of both targets share similar endogenous substrates, MTDLs can be designed by merging two lead compounds with similar functional groups. If the binding sites are large or adjacent to the solution, two key pharmacophores can be fused directly. If the binding regions are small and deep inside the proteins, the linked-pharmacophore strategy might be the only way. The added pharmacophores of second targets should not affect the binding mode of the original ones. Moreover, the inhibitory activities of the two targets need to be adjusted to achieve an optimal ratio.

Ligand and structure-based computational designing of multi-target molecules directing FFAR-1, FFAR-4 and PPAR-G as modulators of insulin receptor activity

Multi-agent therapies are an important treatment modality in many diseases based on the assumption that combining agents may result in increased therapeutic benefit by overcoming the mechanism of resistance and providing superior efficiency. Extensively validated 3D pharmacophore models for free fatty acid receptor-1 (FFAR-1), free fatty acid receptor-4 (FFAR-4), and peroxisome proliferator-activated receptor-G (PPAR-G) was developed. The pharmacophore model for FFAR-1 (r² = 0.98, q² = 0.90) and PPAR-G (r² = 0.89, q² = 0.88) suggested that one hydrogen bond acceptor, one hydrogen bond donor, three aromatic rings, and two hydrophobic groups arranged in 3D space are essential for the binding affinity of FFAR-1 and PPAR-G inhibitors. Similarly, the pharmacophore model for FFAR-4 (r² = 0.92, q² = 0.87) suggested that the presence of a hydrogen bond acceptor, one negative atom, two aromatic rings, and three hydrophobic groups plays a vital role in the binding of an inhibitor of FFAR-4. These pharmacophore models allowed searches for novel FFAR-1, PPAR-G, and FFAR-4 triple inhibitors from multi-conformer 3D databases (Asinex). Finally, the twenty-five best hits were selected for molecular docking, to study the interaction of their complexes with all the proteins and final binding orientations of these molecules. After molecular docking, ten hits have been predicted to possess good binding affinity as per the Molecular Mechanics Generalized Born Surface Area (MM-GBSA) calculation for FFAR-1, FFAR-4, and PPAR-G which can be further investigated for its experimental in-vitro/in-vivo anti-diabetic activities.Communicated by Ramaswamy H. Sarma.

Structure optimization of a new class of PPARγ antagonists

Peroxisome proliferator-activated receptor gamma (PPARγ) modulators have found wide application for the treatment of cancers, metabolic disorders and inflammatory diseases. Contrary to PPARγ agonists, PPARγ antagonists have been much less studied and although they have shown immunomodulatory effects, there is still no therapeutically useful PPARγ antagonist on the market. In contrast to non-competitive, irreversible inhibition caused by 2-chloro-5-nitrobenzanilide (GW9662), the recently described (E)-2-(5-((4-methoxy-2-(trifluoromethyl)quinolin-6-yl)methoxy)-2-((4-(trifluoromethyl)benzyl)oxy)-benzylidene)-hexanoic acid (MTTB, T-10017) is a promising prototype for a new class of PPARγ antagonists. It exhibits competitive antagonism against rosiglitazone mediated activation of PPARγ ligand binding domain (PPARγLBD) in a transactivation assay in HEK293T cells with an IC₅₀ of 4.3 µM against 1 µM rosiglitazone. The aim of this study was to investigate the structure-activity relationships (SAR) of the MTTB scaffold focusing on improving its physicochemical properties. Through this optimization, 34 new derivatives were prepared and characterized. Two new potent compounds (T-10075 and T-10106) with much improved drug-like properties and promising pharmacokinetic profile were identified.

Design, synthesis, and evaluation of potent novel peroxisome proliferator-activated receptor γ indole partial agonists

Peroxisome Proliferator-Activated Receptor γ (PPARγ) is a nuclear receptor important for glucose homeostasis and insulin sensitivity. The anti-diabetic drugs thiazolidinediones improve insulin sensitivity by blocking PPARγ phosphorylation at S273; however, their full agonism on PPARγ also causes significant unwanted side effects. The indole derivative UHC1 displays insulin-sensitizing effect by acting as a partial agonist through the inhibition of PPARγ S273 phosphorylation, but without full agonist-associated side effects; however, its potency leaves much to be desired. Herein we report the design and synthesis of potent indole analogs as partial PPARγ agonists via the structure-activity relationship studies. Our studies revealed that vanillylamine and piperonyl benzylamine at Site 1 are favored to bind PPARγ with either biphenyl or 3-trifluoromethyl benzyl group at Site 2. In particular, compound WO91A with vanillylamine at Site 1 displays highly potent PPARγ binding affinity (IC₅₀ = 16.7 nM), over 30-fold more potent than the parental compound UHC1, yet with less side effect-associated transactivation activity.

An update about the crucial role of stereochemistry on the effects of Peroxisome Proliferator-Activated Receptor ligands

Peroxisome Proliferator-Activated Receptors (PPARs) are ligand-activated transcription factors that govern lipid and glucose homeostasis playing a central role in cardiovascular disease, obesity, and diabetes. These receptors show a high degree of stereoselectivity towards several classes of drugs. This review covers the most relevant findings that have been made in the last decade and takes into consideration only those compounds in which stereochemistry led to unexpected results or peculiar interactions with the receptors. These cases are reviewed and discussed with the aim to show how enantiomeric recognition originates at the molecular level. The structural characterization by crystallographic methods and docking experiments of complexes formed by PPARs with their ligands turns out to be an essential tool to explain receptor stereoselectivity.

Elusive Conformational Dynamics of PPARγ Inactivation Tied Down by Chemical Cross-Linking

In this issue of Structure, Zheng et al. (2018) have described the dynamics of PPARγ in complex with a non-agonist by exploring its solution-phase conformational landscape through chemical cross-linking in combination with a multitude of different treatment conditions, including their new synthetic anti-diabetic non-agonist, revealing the physical mechanism of PPARγ inactivation.

Recent Advances in the Catalytic Synthesis of Imidazolidin-2-ones and Benzimidazolidin-2-ones

2-Imidazolidinone and its analogues are omnipresent structural motifs of pharmaceuticals, natural products, chiral auxiliaries, and intermediates in organic syntheses. Over the years, continuous efforts have been addressed to the development of sustainable and more efficient protocols for the synthesis of these heterocycles. This review gives a summary of the catalytic strategies to access imidazolidin-2-ones and benzimidazolidin-2-ones that have appeared in the literature from 2010 to 2018. Particularly important contributions beyond the timespan will be mentioned. The review is organized in four main chapters that identify the most common approaches to imidazolidin-2-one derivatives: (1) the direct incorporation of the carbonyl group into 1,2-diamines, (2) the diamination of olefins, (3) the intramolecular hydroamination of linear urea derivatives and (4) aziridine ring expansion. Methods not included in this classification will be addressed in the miscellaneous section.

Preparation of spiro[imidazolidine-4,3′-indolin]-2′-imines via copper(i)-catalyzed formal [2 + 2 + 1] cycloaddition of 3-diazoindolin-2-imines and triazines

We report a facile and efficient synthesis of spiro[imidazolidine-4,3′-indolin]-2′-imines via a copper(i)-catalyzed cascade reaction of 3-diazoindolin-2-imines with 1,3,5-triazines.

Chemical Crosslinking Mass Spectrometry Reveals the Conformational Landscape of the Activation Helix of PPARγ; a Model for Ligand-Dependent Antagonism

Peroxisome proliferator-activated receptors (PPARs) are pharmacological targets for the treatment of metabolic disorders. Previously, we demonstrated the anti-diabetic effects of SR1664, a PPARγ modulator lacking classical transcriptional agonism, despite its poor pharmacokinetic properties. Here, we report identification of the antagonist SR11023 as a potent insulin sensitizer with significant plasma exposure following oral administration. To determine the structural mechanism of ligand-dependent antagonism of PPARγ, we employed an integrated approach combining solution-phase biophysical techniques to monitor activation helix (helix 12) conformational dynamics. While informative on receptor dynamics, hydrogen/deuterium exchange mass spectrometry and nuclear magnetic resonance data provide limited information regarding the specific orientations of structural elements. In contrast, label-free quantitative crosslinking mass spectrometry revealed that binding of SR11023 to PPARγ enhances interaction with co-repressor motifs by pushing H12 away from the agonist active conformation toward the H2-H3 loop region (i.e., the omega loop), revealing the molecular mechanism for active antagonism of PPARγ.

Betulinic acid is a PPARγ antagonist that improves glucose uptake, promotes osteogenesis and inhibits adipogenesis

PPAR antagonists are ligands that bind their receptor with high affinity without transactivation activity. Recently, they have been demonstrated to maintain insulin-sensitizing and antidiabetic properties, and they serve as an alternative treatment for metabolic diseases. In this work, an affinity-based bioassay was found to be effective for selecting PPAR ligands from the dried extract of an African plant (Diospyros bipindensis). Among the ligands, we identified betulinic acid (BA), a compound already known for its anti-inflammatory, anti-tumour and antidiabetic properties, as a PPARγ and PPARα antagonist. Cell differentiation assays showed that BA inhibits adipogenesis and promotes osteogenesis; either down-regulates or does not affect the expression of a series of adipogenic markers; and up-regulates the expression of osteogenic markers. Moreover, BA increases basal glucose uptake in 3T3-L1 adipocytes. The crystal structure of the complex of BA with PPARγ sheds light, at the molecular level, on the mechanism by which BA antagonizes PPARγ, and indicates a unique binding mode of this antagonist type. The results of this study show that the natural compound BA could be an interesting and safe candidate for the treatment of type 2 diabetes and bone diseases.

Synthesis of fused indoline heterocycles via dearomatization of indoles with α-bromohydrazones: a systematic study on the substrates

An efficient metal-free dearomatization of indoles with α-bromohydrazones is reported. Various fused indoline heterocycles, which are potentially biologically active, were achieved in good yields (up to 94%) under mild conditions.

PPARG Post-translational Modifications Regulate Bone Formation and Bone Resorption

The peroxisome proliferator-activated receptor gamma (PPARγ) regulates osteoblast and osteoclast differentiation, and is the molecular target of thiazolidinediones (TZDs), insulin sensitizers that enhance glucose utilization and adipocyte differentiation. However, clinical use of TZDs has been limited by side effects including a higher risk of fractures and bone loss. Here we demonstrate that the same post-translational modifications at S112 and S273, which influence PPARγ pro-adipocytic and insulin sensitizing activities, also determine PPARγ osteoblastic (pS112) and osteoclastic (pS273) activities. Treatment of either hyperglycemic or normoglycemic animals with SR10171, an inverse agonist that blocks pS273 but not pS112, increased trabecular and cortical bone while normalizing metabolic parameters. Additionally, SR10171 treatment modulated osteocyte, osteoblast, and osteoclast activities, and decreased marrow adiposity. These data demonstrate that regulation of bone mass and energy metabolism shares similar mechanisms suggesting that one pharmacologic agent could be developed to treat both diabetes and metabolic bone disease.

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PPARγ S273 regulates osteoclast differentiation and insulin sensitivity

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PPARγ S112 regulates osteoblast and adipocyte differentiation

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PPARγ and PPARα regulate osteocyte activities of bone formation and turnover

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SR10171, a PPARγ inverse agonist and PPARα weak agonist, is anabolic for bone

Diabetes is a condition with compromised energy balance and is associated with bone fractures. Some treatment options for diabetes sensitize the patient to insulin via targeting the transcription factor PPARγ. PPARγ is also key regulator of bone formation and bone resorption. Anti-diabetic drugs TZDs target PPARγ protein and this leads to bone loss and increase in fractures in postmenopausal women. Bone mass and energy metabolism share similar regulating pathways, and here we demonstrate a new class of insulin sensitizers that is a selective modulator of PPARγ activity; resulting in a pharmacologic agent that can be beneficial for both diabetes and metabolic bone disease.