Chalcone scaffolds as anti-infective agents: structural and molecular target perspectives

Chalcone Derivatives: Promising Starting Points for Drug Design

Medicinal chemists continue to be fascinated by chalcone derivatives because of their simple chemistry, ease of hydrogen atom manipulation, straightforward synthesis, and a variety of promising biological activities. However, chalcones have still not garnered deserved attention, especially considering their high potential as chemical sources for designing and developing new effective drugs. In this review, we summarize current methodological developments towards the design and synthesis of new chalcone derivatives and state-of-the-art medicinal chemistry strategies (bioisosterism, molecular hybridization, and pro-drug design). We also highlight the applicability of computer-assisted drug design approaches to chalcones and address how this may contribute to optimizing research outputs and lead to more successful and cost-effective drug discovery endeavors. Lastly, we present successful examples of the use of chalcones and suggest possible solutions to existing limitations.

Synthesis and evaluation of modified chalcone based p53 stabilizing agents

Tumor suppressor protein p53 induces cell cycle arrest and apoptotic cell death in response to various cellular stresses thereby preventing cancer development. Activation and stabilization of p53 through small organic molecules is, therefore, an attractive approach for the treatment of cancers retaining wild-type p53. In this context, a series of nineteen chalcones with various substitution patterns of functional groups including chloro, fluoro, methoxy, nitro, benzyloxy, 4-methyl benzyloxy was prepared using Claisen-Schmidt condensation. The compounds were characterized using NMR, HRMS, IR and melting points. Evaluation of synthesized compounds against human colorectal (HCT116) and breast (CAL-51) cancer cell lines revealed potent antiproliferative activities. Nine compounds displayed GI₅₀ values in the low micromolar to submicromolar range; for example (E)-1-phenyl-3-(3,4,5-trimethoxyphenyl)prop-2-en-1-one (SSE14108) showed GI₅₀ of 0.473±0.043µM against HCT116 cells. Further analysis of these compounds revealed that (E)-3-(4-chlorophenyl)-1-phenylprop-2-en-1-one (SSE14105) and (E)-3-(4-methoxyphenyl)-1-phenylprop-2-en-1-one (SSE14106) caused rapid (4 and 8-h post-treatment) accumulation of p53 in HCT116 cells similar to its induction by positive control, Nutlin-3. Such activities were absent in 3-(4-methoxyphenyl)propiophenone (SSE14106H2) demonstrating the importance of conjugated ketone for antiproliferative and p53 stabilizing activity of the chalcones. We further evaluated p53 levels in the presence of cycloheximide (CHX) and the results showed that the p53 stabilization was regulated at post-translational level through blockage of its degradation. These chalcones can, therefore, act as fragment leads for further structure optimization to obtain more potent p53 stabilizing agents with enhanced anti-proliferative activities.

Synthesis and biological evaluation of chalcone-linked pyrazolo[1,5-a]pyrimidines as potential anticancer agents

A series of pyrazolo[1,5-a]pyrimidines substituted at C5 with 1-phenylprop-2-en-1-one (6a-q) and 3-phenylprop-2-en-1-one (7a-k) was synthesized and evaluated for antiproliferative activity. Among them, 6h was found to be the most active compound against the MDA-MB-231 cell line with an IC₅₀ of 2.6 μM . The antiproliferative activity of this series of compounds ranged from 2.6 to 34.9 μM against A549 (lung cancer), MDA-MB-231 (breast cancer) and DU-145 (prostate cancer) cell lines. FACS analysis revealed that these hybrids arrest the cell cycle at the subG1 phase. Western blot analysis and an immunofluorescence assay showed the inhibition of the EGFR and STAT3 axis, which plays an important role in cell survival and apoptosis. Western blot and RT-PCR analyses that displayed an increase in apoptotic proteins such as p53, p21 and Bax and a decrease in the anti-apoptotic proteins Bcl-2 and procaspase-9 confirmed the ability of these hybrids to trigger cell death by apoptosis. Molecular docking studies described the binding of these hybrids to the ATP binding site of EGFR.

Chalcone: A Privileged Structure in Medicinal Chemistry

Privileged structures have been widely used as an effective template in medicinal chemistry for drug discovery. Chalcone is a common simple scaffold found in many naturally occurring compounds. Many chalcone derivatives have also been prepared due to their convenient synthesis. These natural products and synthetic compounds have shown numerous interesting biological activities with clinical potentials against various diseases. This review aims to highlight the recent evidence of chalcone as a privileged scaffold in medicinal chemistry. Multiple aspects of chalcone will be summarized herein, including the isolation of novel chalcone derivatives, the development of new synthetic methodologies, the evaluation of their biological properties, and the exploration of the mechanisms of action as well as target identification. This review is expected to be a comprehensive, authoritative, and critical review of the chalcone template to the chemistry community.

Design of potent fluoro-substituted chalcones as antimicrobial agents

Owing to ever-increasing bacterial and fungal drug resistance, we attempted to develop novel antitubercular and antimicrobial agents. For this purpose, we developed some new fluorine-substituted chalcone analogs (3, 4, 9–15, and 20–23) using a structure–activity relationship approach. Target compounds were evaluated for their antitubercular efficacy against Mycobacterium tuberculosis H37Rv and antimicrobial activity against five common pathogenic bacterial and three common fungal strains. Three derivatives (3, 9, and 10) displayed significant antitubercular activity with IC₅₀ values of ≤16,760. Compounds derived from trimethoxy substituent scaffolds with monofluoro substitution on the B ring of the chalcone structure exhibited superior inhibition activity compared to corresponding hydroxy analogs. In terms of antimicrobial activity, most compounds (3, 9, 12–14, and 23) exhibited moderate to potent activity against the bacteria, and the antifungal activities of compounds 3, 13, 15, 20, and 22 were comparable to those of reference drugs ampicillin and fluconazole.

Synthesis and antitrypanosomal activities of novel pyridylchalcones

A library of novel pyridylchalcones were synthesised and screened against Trypanosoma brucei rhodesiense. Eight were shown to have good activity with the most potent 8 having an IC₅₀ value of 0.29 μM. Cytotoxicity testing with human KB cells showed a good selectivity profile for this compound with a selectivity index of 47. Little activity was seen when the library was tested against Leishmania donovani. In conclusion, pyridylchalcones are promising leads in the development of novel compounds for the treatment of human African trypanosomiasis (HAT).

Synthesis, cytotoxic activity and drug combination study of tertiary amine derivatives of 2′,4′-dihydroxyl-6′-methoxyl-3′,5′-dimethylchalcone

Tertiary amine derivative of DMC (2b) exhibited broad spectrum of cytotoxicity and strong synergism with Taxol® against HeLa/Tax cells.

Methoxylated 2'-hydroxychalcones as antiparasitic hit compounds

Chalcones display a broad spectrum of pharmacological activities. Herein, a series of 2'-hydroxy methoxylated chalcones was synthesized and evaluated towards Trypanosoma brucei, Trypanosoma cruzi and Leishmania infantum. Among the synthesized library, compounds 1, 3, 4, 7 and 8 were the most potent and selective anti-T. brucei compounds (EC₅₀ = 1.3-4.2 μM, selectivity index >10-fold). Compound 4 showed the best early-tox and antiparasitic profile. The pharmacokinetic studies of compound 4 in BALB/c mice using hydroxypropil-β-cyclodextrins formulation showed a 7.5 times increase in oral bioavailability.

Novel Halogenated Pyrazine-Based Chalcones as Potential Antimicrobial Drugs

Chalcones, i.e., compounds with the chemical pattern of 1,3-diphenylprop-2-en-1-ones, exert a wide range of bio-activities, e.g., antioxidant, anti-inflammatory, anticancer, anti-infective etc. Our research group has been focused on pyrazine analogues of chalcones; several series have been synthesized and tested in vitro on antifungal and antimycobacterial activity. The highest potency was exhibited by derivatives with electron withdrawing groups (EWG) in positions 2 and 4 of the ring B. As halogens also have electron withdrawing properties, novel halogenated derivatives were prepared by Claisen-Schmidt condensation. All compounds were submitted for evaluation of their antifungal and antibacterial activity, including their antimycobacterial effect. In the antifungal assay against eight strains of selected fungi, growth inhibition of Candida glabrata and Trichophyton interdigitale (formerly T. mentagrophytes) was shown by non-alkylated derivatives with 2-bromo or 2-chloro substitution. In the panel of selected bacteria, 2-chloro derivatives showed the highest inhibitory effect on Staphylococcus sp. In addition, all products were also screened for their antimycobacterial activity against Mycobacterium tuberculosis H37RV My 331/88, M. kansasii My 235/80, M. avium 152/80 and M. smegmatis CCM 4622. Some of the examined compounds, inhibited growth of M. kansasii and M. smegmatis with minimum inhibitory concentrations (MICs) comparable with those of isoniazid.

Synthesis, nematocidal activity and SAR study of novel difluoromethylpyrazole carboxamide derivatives containing flexible alkyl chain moieties

A series of novel difluoromethylpyrazole carboxamides derivatives were synthesized by introduction of flexible alkyl chain. Nematicidal bioassay results showed that some of them exhibited good control efficacy against M. incognita, which indicated that these difluoromethylpyrazole carboxamides derivatives might be potential novel lead compounds for discovery new nematicides. The nematicidal activity was affected by the substituted position in the molecule, especially the substitution group on the alkyl chain. It was found that the compound 6-9 and 6-23 possess about 50% inhibition effect against M. incognita even at 5.0 and 1.0 mg L^-1. Meanwhile, greenhouse field trial showed the nematicidal activity of compound 6-9 is a litter weaker than that of Abamectin. The mammalian toxicology results indicated that compound 6-9 was a low-toxicity and low-sensitive compound. In conclusion compound 6-9 is a potential candidate for further development. In addition, the molecular docking simulations revealed that compounds 6 with a flexible NHCOO show its binding affinities for the acetylcholine receptor (AChR), which may provide useful information for further design novel nematicides.

Bistacrine derivatives as new potent antimalarials

Linking two tacrine molecules results in a tremendous increase of activity against Plasmodia in comparison to the monomer. This finding prompted the synthesis of a library of monomeric and dimeric tacrine derivatives in order to derive structure-activity relationships. The most active compounds towards chloroquine sensitive Plasmodium strain 3D7 and chloroquine resistant strain Dd2 show IC50 values in the nanomolar range of concentration, low cytotoxicity and target the cysteine protease falcipain-2, which is essential for parasite growth.

Unambiguous Identification of β-Tubulin as the Direct Cellular Target Responsible for the Cytotoxicity of Chalcone by Photoaffinity Labeling

Chalcone is a simple and potentially privileged structure in medicinal chemistry with a diverse repertoire of biological activities, among which cytotoxicity is of particular interest. The sharp structure-activity relationship (SAR) for chalcone's cytotoxicity suggests structure-specific target interactions. Despite the numerous putative targets proposed, evidence for direct target interactions in cells is unavailable. In this study, guided by the sharp cytotoxic SAR, we developed a cytotoxic chalcone-based photoaffinity labeling (PAL) probe, (E)-3-(3-azidophenyl)-1-[3,5-dimethoxy-4-(prop-2-yn-1-yloxy)phenyl]-2-methylprop-2-en-1-one (C95; IC50 : 0.38±0.01 μm), along with two structurally similar non-cytotoxic probes. These probes were used to search for the direct cellular target responsible for chalcone's cytotoxicity through intact cell-based PAL experiments, in which β-tubulin was identified to specifically interact with the cytotoxic probe (i.e., C95) but not the non-cytotoxic probes. A set of phenotypical and biochemical assays further reinforced β-tubulin as the cytotoxic target of chalcones. Peptide mass quantitation by mass spectrometric analysis revealed one peptide potentially labeled by C95, providing information on chalcone's binding site on β-tubulin.

Effects of phloretin on oxidative and inflammatory reaction in rat model of cecal ligation and puncture induced sepsis

BACKGROUND

Sepsis is a debilitating systemic disease and described as a severe and irregular systemic inflammatory reaction syndrome (SIRS) against infection. We employed CLP (Cecal Ligation and Puncture) model in rats to investigate anti-inflammatory and antioxidant effects of phloretin, as a natural antioxidant agent, and its protective effect on liver tissue damage caused by sepsis.

METHODS

Male Wistar albino rats were randomly divided into three groups: sham group, CLP induced sepsis group and phloretin treated CLP group. Sepsis was induced by CLP method. 50 mmol/kg Phloretin was administered intraperitoneally in two equal doses immediately after surgery.

RESULTS

It was observed that blood urea nitrogen (BUN) and tumor necrosis factor alpha (TNF-α) levels were dramatically increased in the CLP induced sepsis group (43.88 ± 1.905 mg/dl, 37.63 ± 1.92, respectively) when compared to the sham group. Moreover, tissue Glutathione (GSH) and liver nuclear factor ĸB (NF-ĸB p65) transcription factor values were higher in CLP induced sepsis group. This elevation was considerably reduced in the phloretin treated CLP group. No significant differences were observed in serum creatinine and creatinine phosphokinase levels.

CONCLUSIONS

The present study suggested that phloretin, as a natural protective agent, act against tissue damages introduced following the experimental sepsis induced model, likely caused by free oxygen radicals.

Synthesis and bioevaluation of substituted chalcones, coumaranones and other flavonoids as anti-HIV agents

A series of chalcone, flavone, coumaranone and other flavonoid compounds were screened for their anti HIV-1 activity in two cell culture models using TZM-bl and PM1 cells. Within the systems evaluated, the most promising compounds contained either an α- or β-hydroxy-carbonyl motif within their structure (e.g., 8 and 9). Efficacious substituents were identified and used to design new HIV inhibitors with increased potency and lower cytotoxicity. Of the scaffolds evaluated, specific chalcones were found to provide the best balance between anti-HIV potency and low host cell toxicity. Chalcone 8l was shown to inhibit different clinical isolates of HIV in a dose-dependent manner (e.g., IC50 typically⩽5μM). Inhibition of HIV infection experiments using TZM-bl cells demonstrated that chalcone 8l and flavonol 9c had IC50 values of 4.7μM and 10.4μM, respectively. These insights were used to design new chalcones 8o and 8p. Rewardingly, chalcones 8o and 8p (at 10μM) each gave >92% inhibition of viral propagation without impacting PM1 host cell viability. Inhibition of viral propagation significantly increased (60-90%) when PM1 cells were pre-incubated with chalcone 8o, but not with the related flavonol 9c. These results suggested that chalcone 8o may be of value as both a HIV prophylactic and therapy. In summary, O-benzyl-substituted chalcones were identified as promising anti-HIV agents for future investigation.

An easy approach to dihydrochalcones via chalcone in situ hydrogenation

Dihydrochalcones are polyphenols that exhibit a diversity of bioactivities, namely anti-inflammatory, antimicrobial and antiviral. We have explored the synthetic access to such molecular entities, and describe now an easy and scalable approach based on reduction of the olefinic double bond of chalcone precursors via in situ hydrogenation with the system Et3SiH-Pd/C in very high yield. The intermediate chalcones were synthesized also by a simple and efficient microwave-assisted Claisen–Schmidt condensation of aromatic aldehydes with acetophenones, conveniently protected with ethoxymethyl ether, if required. Chalcones were obtained as single reaction product in high yield in 2–3 h, while under conventional conditions at room temperature the reaction was carried out with completion only after 24 h. In addition, microwave irradiation has proven very efficient for deprotection of ethoxymethyl ether with iron chloride in only 10 min and very high yield.

Hepatoprotective effect of trans-Chalcone on experimentally induced hepatic injury in rats: inhibition of hepatic inflammation and fibrosis

The current study investigated the hepatoprotective effect of trans-Chalcone in carbon tetrachloride (CCl4) and paracetamol (PCM) induced liver damage in rats. Administration of CCl4 and PCM (1 mL/kg, i.p., 3 days, and 2 g/kg, p.o., single dose, respectively) produced hepatic injury. Ponderal changes (percent change in body mass and relative liver mass) and biochemical parameters (serum ALT, AST, ALP, bilirubin) were estimated. The markers of oxidative and nitrosative stress (TBARS, reduced GSH, nitrite and nitrate), hepatic fibrosis (TGF-β1, collagen content), hepatic inflammation (TNF-α), and histopathological study were evaluated. trans-Chalcone (5, 10, and 20 mg/kg, i.p.) was found to be beneficial as demonstrated by significant reversal of liver histology by perceptible reduction of inflammatory cell infiltration with regenerative changes in hepatocytes. Improvement in percent change in body mass and significant reduction in relative liver mass were observed. Marked reduction in serum levels of ALT, AST, ALP, and bilirubin were noted. Decreases in TBARS and nitrites and nitrates and increases in reduced GSH levels were noted. Hepatic fibrosis and inflammation were significantly decreased. The findings indicate a novel hepatoprotective role for trans-Chalcone by improving hepatic injury by possible actions such as anti-oxidant, anti-nitrosative, anti-fibrotic, and anti-inflammatory. Hence, it can be used as promising hepatoprotective agent.

Therapeutic potential of chalcones as cardiovascular agents

Cardiovascular diseases are the leading cause of death affecting 17.3 million people across the globe and are estimated to affect 23.3 million people by year 2030. In recent years, about 7.3 million people died due to coronary heart disease, 9.4 million deaths due to high blood pressure and 6.2 million due to stroke, where obesity and atherosclerotic progression remain the chief pathological factors. The search for newer and better cardiovascular agents is the foremost need to manage cardiac patient population across the world. Several natural and (semi) synthetic chalcones deserve the credit of being potential candidates to inhibit various cardiovascular, hematological and anti-obesity targets like angiotensin converting enzyme (ACE), cholesteryl ester transfer protein (CETP), diacylglycerol acyltransferase (DGAT), acyl-coenzyme A: cholesterol acyltransferase (ACAT), pancreatic lipase (PL), lipoprotein lipase (LPL), calcium (Ca(2+))/potassium (K(+)) channel, COX-1, TXA2 and TXB2. In this review, a comprehensive study of chalcones, their therapeutic targets, structure activity relationships (SARs), mechanisms of actions (MOAs) have been discussed. Chemically diverse chalcone scaffolds, their derivatives including structural manipulation of both aryl rings, replacement with heteroaryl scaffold(s) and hybridization through conjugation with other pharmacologically active scaffold have been highlighted. Chalcones which showed promising activity and have a well-defined MOAs, SARs must be considered as prototype for the design and development of potential anti-hypertensive, anti-anginal, anti-arrhythmic and cardioprotective agents. With the knowledge of these molecular targets, structural insights and SARs, this review may be helpful for (medicinal) chemists to design more potent, safe, selective and cost effective chalcone derivatives as potential cardiovascular agents.

Licochalcone B induces apoptosis of human oral squamous cell carcinoma through the extrinsic- and intrinsic-signaling pathways

Licochalcone B (Lico B), which belongs to the retrochalcone family, is isolated from the roots of Chinese licorice. Lico B has been reported to have several other useful pharmacological properties, such as anti-inflammatory, antibacterial, antioxidant, antiulcer, anticancer, and anti-metastasis activities. We elucidated the underlying mechanism by which Lico B can induce apoptosis in oral squamous cell carcinoma (OSCC). Our results showed that exposure of OSCC cells (HN22 and HSC4) to Lico B significantly inhibited cell proliferation in a time- and concentration-dependent manner. Lico B caused cell cycle arrest at G1 phase along with downregulation of cyclin D1 and upregulation of p21 and p27 proteins. Lico B also facilitated the diffusion of phospholipid phosphatidylserine (PS) from inner to outer leaflets of the plasma membrane with chromatin condensation, DNA fragmentation, accumulated sub-G1 population in a concentration-dependent manner. Moreover, Lico B promoted the generation of reactive oxygen species (ROS), which, in turn, can induce CHOP, death receptor (DR) 4 and DR5. Lico B treatment induced downregulation of anti-apoptotic proteins (Bid and Bcl-xl and Mcl-1), and up-regulation of pro-apoptotic protein (Bax). Lico B also led to the loss of mitochondrial membrane potential (MMP), resulting in cytochrome c release. As can be expected from the above results, the apoptotic protease activating factor-1 (Apaf-1) and survivin were oppositely expressed in favor of apoptotic cell death. This notion was supported by the fact that Lico B activated multi-caspases with cleavage of poly (ADP-ribose) polymerase (PARP) protein. Therefore, it is suggested that Lico B is a promising drug for the treatment of human oral cancer via the induction of apoptotic cell death.

Synthesis of N-aryl 2-quinolinones via intramolecular C(sp2)–H amidation of Knoevenagel products

An efficient synthesis of N-aryl 2-quinolinones via K₂S₂O₈-mediated intramolecular C(sp²)–H amidation of Knoevenagel products was developed.