Biological Role of Chalcones in Medicinal Chemistry

Chalcones are promising synthons and bioactive scaffolds of great medicinal interest due to their numerous pharmacological and biological activities. They are well recognized to possess antimicrobial, anticancer, antitubercular, antioxidant, anti-inflammatory, antileishmanial, and other significant biological activities. This chapter highlights recent updates and applications of chalcones as biologically, pharmacologically, and medicinally important entities.


Introduction
Chalcones (trans-1,3-diaryl-2-propen-1-ones) (1) are α,β-unsaturated ketones consisting of two aromatic rings (ring A and B) having diverse array of substituents (Figure 1). Chalcone skeleton contains two aromatic rings linked by an aliphatic three-carbon chain. The two rings of chalcone are interconnected by a highly electrophilic three-carbon α,β-unsaturated carbonyl system that assumes linear or nearly planar structure. They possess conjugated double bonds and a completely delocalized π-electron system on both the aromatic rings.
Chalcones, named so by Kostanecki and Tambor, are commonly known by different names such as benzylideneacetophenone, phenyl styryl ketone, βphenylacrylophenone α-phenyl-β-benzoylethylene, etc. and constitute the central core of biologically active heterocyclic compounds. Chalcones constitute good synthons for a variety of novel heterocycles of high therapeutic potential and good pharmaceutical profile [1,2]. Chalcones themselves are identified as interesting entities associated with several biological activities [3].
The structural modifications of the chalcone rings have led to a high degree of diversity that has proven useful for the development of new medicinal agents, and thus chalcones have become an object of continued interest in both academia and industry. The chalcones are well documented for a broad spectrum of biological activities including antimicrobial, anticancer, cytotoxic, antioxidative, antiinflammatory, antiviral, and others [4]. Currently, chalcone derivatives have been widely used for the treatment of viral disorders, cardiovascular diseases, stomach cancer, food additives, and cosmetic formulation ingredients [5]. However, much of the pharmacological potential of chalcones and their recent updates need to be understood. The purpose of this chapter is to cover and describe the recent developments, preferably after 2015 to date, the utility of chalcones as medicinally Dehydroacetic acid chalcone-1,2,3-triazole hybrids (5) were shown to possess good in vitro antimicrobial activities against Staphylococcus epidermidis, Bacillus subtilis, Escherichia coli, and Pseudomonas aeruginosa bacteria and two fungal strains, viz., Aspergillus niger and Candida albicans [11].
Talniya and Sood documented the synthesis and antibacterial activity of chalcones (11) against Bacillus subtilis bacteria and Aspergillus niger fungi by disk diffusion method [15]. The chalcones possessing o-chloro, p-chloro, and p-hydroxyl substituents showed remarkable antimicrobial activity against the screened microbes.
Oxazolidinones incorporated with chalcone hybrids (12) were evaluated for in vitro antibacterial and antifungal activities by using the serial dilution method [16]. Results showed moderate antimicrobial activities as compared with the standard drugs ciprofloxacin and linezolid.
Novel diarylsulfonylurea-chalcone hybrids (13) were evaluated by agar well diffusion method against various strains of bacteria and fungi including Bacillus subtilis, Escherichia coli, Bacillus pumilus, Staphylococcus aureus, Micrococcus luteus, Candida albicans, and Penicillium chrysogenum. Most of the compounds showed promising antibacterial and antifungal activity suggesting that the diarylsulfonylurea-chalcone hybrids can be used for the treatment of diseases caused by these microbial organisms [17].
Vanillin moiety containing chalcones (14), (15), and (16) were synthesized by the Claisen-Schmidt condensation of vanillin with different acetophenone derivatives and were studied for antimicrobial activities by using agar disk diffusion and microdilution methods [18]. The researchers found S. aureus and C. albicans to be the most sensitive strains and E. faecalis to be the least sensitive against these chalcones. The presence of halogens in chalcones increased their microbial susceptibility. The structures of some antimicrobial chalcones are shown in Figure 2.

Anticancer chalcones
Cancer is a widely spreading disease all over the world, necessitating the need to develop new anticancer agents [19]. Anticancer or antineoplastic drugs are those that are effective in the treatment of malignant or cancerous disease. Increasing recurrence of mammalian tumors and severe side effects of chemotherapeutic agents reduce the clinical efficiency of a large variety of commonly used anticancer agents, and thus, there is always a constant need to develop alternative or synergistic anticancer drugs with minimal side effects [20].
The treatment of cancer is a complicated process as the drugs used target human cells and albeit cells that have undergone genetic changes and are dividing at a fast and uncontrolled rate. However, only a few anticancer drugs can differentiate between normal tissue cells and cancer cells to a large extent. Thus, there is always a constant need to develop alternative or synergistic anticancer drugs with minimal side effects. This part of the present chapter highlights significant and recent developments in chalcones used as anticancer agents: Sulfonylpiperazines linked with [1,3]dioxolo[4,5-g]chromenones (17) were synthesized by the aldol condensation and evaluated as antioxidants against DPPH, ABTS, as well as antiproliferative agents against non-cancer MDCK cell lines [21].
The design, synthesis, and antitumor potential of chalcones (18) were studied against human breast adenocarcinoma MCF-7 cells in a concentration-dependent manner [22]. They triggered significant changes in cell morphology and biochemical/molecular parameters and revealed the apoptosis inductor nature of the titled compounds and their application as promising alternatives for the treatment of neoplasia, especially in terms of drug resistance development.
Novel anthraquinone-chalcone hybrids (19) possessing amide functionality were synthesized, then characterized, and reported for good cytotoxic potential against K562, Jurkat, and HL-60 leukemia cell lines [23]. An apoptosis is an important phenomenon, which affects many diseases, such as cancer and Alzheimer's disease. Chalcones (20) induced apoptosis of human hepatic and lung cancer cells and inhibited cancer cell migration and invasion [24].
The bis-chalcone derivatives (21) were studied for their ability to inhibit xanthine oxidase and growth inhibitory activity against MCF-7 and caco-2 human cancer cell lines in vitro. The bis-chalcone with fluoro group at the 2 nd or 2, 5 th position of B-ring was found to be a potent inhibitor of the enzyme possessing IC 50 values in the low micromolar range. The activities of the compounds were found to be around seven times higher than the standard allopurinol [25].
Chalcones ( Some novel Pt(IV) complexes of chalcone analogs (30) were synthesized and evaluated for antiproliferative activity by using MTT assay. The in vitro evaluation revealed that all Pt(IV) complexes showed good activity against the three human cancer cells [31]. The overexpression of the CYP1 class of enzymes is associated with the development of human carcinomas. The pyridine-4-yl series of chalcones (31) were synthesized and screened for the inhibition of CYP1 isoforms in Sacchrosomes TM and live human HEK293 cells. The chalcones bearing tri-alkoxy groups on nonheterocyclic ring displayed selective inhibition of the CYP1A1 enzyme with IC 50 values less than 70 nM [32].
The pyrazolic chalcone analogous compounds (32) were synthesized and evaluated as potential chemotherapeutic agents for the treatment of hepatocellular carcinoma [33]. Some of the screened compounds exhibited potent cytotoxic activity against all the cancer cell lines tested and had good cytotoxic activities.
DNA ligases play a crucial role in causing cancer. Gupta et al. reported the inhibition DNA ligases resulting in DNA nick-sealing activity followed by the antiproliferative activity of the indole-chalcone based benzopyran chalcones (33) on cancer cells [34].
Indolizine-chalcone hybrids (34) were synthesized and studied for apoptosis and anticancer effect on human lymphoma cells by S. Park and coworkers [35].
Prenyl and geranyl group-bearing chalcones (35) were synthesized by using regioselective iodination followed by the Suzuki coupling reaction and studied for in vitro anticancer activity against human tumor cell line K562 by MTT assay. Morphology changes revealed that the chalcone derivatives inhibited the proliferation of K562 cells by inducing apoptosis [36]. Leukemia is a hematologic malignancy with poor prognosis in humans. Diprenylated chalcone (36) was studied as a new potential antileukemia agent [37].

Antitubercular chalcones
Tuberculosis (TB), caused by the acid-fast gram-positive bacillus, Mycobacterium tuberculosis, remains the leading source of bacterial infectious disease [40]. M. tuberculosis establishes an infection through an invasion of alveolar macrophages. The Mycobacterium tuberculosis encodes for more than 60 adenylating enzymes, mainly tRNA synthetases, acyl-AMP ligases, etc. [41]. Currently, the treatment of TB employs four first-line drugs, isoniazid, rifampin, pyrazinamide, and ethambutol, which must be administered in the body daily for a 2-month intensive phase. However, for susceptible TB strains, this therapy is 95% effective. The emergence of multidrug-resistant (MDR) strains, defined as resistant to isoniazid and rifampin, requires the use of less effective and more toxic second-line TB drugs. Herein we discuss some recent updates in the application of chalcones against tuberculosis: New sulfonamide-bearing chalcones (40) were synthesized by the Claisen-Schmidt condensation and were reported as excellent antituberculosis hits showing low selectivity, being equally inhibitory to M. tuberculosis and mammalian T3T cells [42].
Gomes et al. studied antitubercular activities of chalcones (41) and (42). The chalcones showed good selectivity towards M. tuberculosis with low cytotoxicity against Vero cells and thus possess promising antitubercular potential [43].
Spirochromone annulated chalcone conjugates (43) were documented for antitubercular activity against Mycobacterium tuberculosis H37Rv strain. Molecular docking studies performed against the receptors revealed MTB phosphotyrosine phosphatase B protein as the most probable target based on the high bindingaffinity scores [44].
Babu et al. studied chalcones containing nitrophenyl moieties (44) for antitubercular activity using MABA assay and antibacterial and antifungal activities by cup plate method. Molecular docking study predicted the inhibition of thymidine kinase of the Mycobacterium tuberculosis [45]. Anti-tubercular chalcones are depicted in Figure 5.

Antioxidant chalcones
Antioxidants are the compounds that inhibit the oxidation process. These substances can prevent or slow damage to cells caused by free radicals. Oxidation is a chemical reaction that generates free radicals, thereby leading to chain reactions which may damage the cells of organisms and hence responsible for oxidative stress resulting in chronic diseases such as heart diseases, stroke, cancer, arthritis, respiratory diseases, Parkinson's disease, and other inflammatory conditions [46].
Cao et al. documented a series of 4 0 -OH-flurbiprofen-chalcone hybrids (45) and evaluated them as potential multifunctional agents for the treatment of Alzheimer's disease. Besides, the compounds were reported for good antioxidant activities, MAO inhibitions, biometal chelating abilities, and in vitro anti-neuroinflammatory activities [47].
Selenoenzymes and nuclear factor erythroid 2-related factor 2 (Nrf2)-regulated phase II enzymes constitute the main components of cellular redox and antioxidant systems giving information about multiple interrelations involved in the oxidation processes. Chalcones (46) were proved to interfere with the biosynthesis of Nrf2regulated selenoenzymes [48].
El-Sayed et al. documented the antioxidant activity of chalcones (47) [49]. The chalcone derivatives (48) were synthesized by the Claisen-Schmidt condensation with KOH in ethanol at room temperature under sonication conditions and screened for antioxidant potential by Polo et al. [50].
The chalcones (49) were studied as potent antioxidants by Tajammal and coworkers. These compounds have lower IC 50 values than the Trolox and ascorbic acid standards [51].
A series of chalcone (50) analogs were designed, synthesized, and screened for antioxidant activities. The chalcone was found as a promising anti-ischemic stroke drug candidate, providing novel dual-antioxidant mechanism strategies and concepts for oxidative stress-related disease treatment [52]. The prenylated chalcones (51) were reported for good antioxidant activity [53]. Figure 6 represents the structures of antioxidant chalcones.

Anti-inflammatory chalcones
Anti-inflammatory drugs are the drugs which are used to reduce pain and inflammation. In other words, these are pain-relieving drugs. These drugs work mainly by inhibiting the cyclooxygenase enzymes, COX-1 and COX-2, that produce prostaglandins [54]. Herein we discuss some of the efforts for the development of chalcone-based heterocycles as effective anti-inflammatory compounds: Indole-based chalcones (52) were synthesized and evaluated for in vitro COX-1 and COX-2 inhibitory activity [55].
Zhang et al. identified methoxy chalcones (55) as a potential candidate for treating acute inflammatory diseases [57].
Pyrazole-and morpholine-containing chalcones (56) were reported for antiinflammatory activity by Gadhave and Uphade. The anti-inflammatory activity performed by carrageenan-induced rat paw edema method showed good potency of some of the tested compounds as compared with the standard diclofenac drug [58].
Nurkenov et al. studied the in vitro anti-inflammatory effect of chalcones (57) to inhibit the lipopolysaccharide-induced production of anti-inflammatory cytokine interleukin-6 and tumor necrosis factor [59].
The imidazole containing chalcone molecule (58) demonstrated noteworthy anti-inflammatory activity as compared with the standard drug, indomethacin [60].

Miscellaneous applications of chalcones
Besides the above-discussed applications, chalcones are useful for miscellaneous applications. Some of them are mentioned as follows: Leishmania is a genus of trypanosomes responsible for the disease leishmaniasis. Leishmaniasis is spread through sand flies of the genus Phlebotomus, primary hosts being the vertebrates. The chalcone (60) was evaluated against 29 promastigotes of Leishmania donovani exhibiting low toxicity against mammalian cells [62].
A series of new chalcone-rivastigmine hybrids (61) was designed, synthesized, and evaluated in vitro for the ability to inhibit human acetylcholinesterase and butyrylcholinesterase. Results showed that these compounds exhibited selective activity in micro-and submicromolar ranges as compared with the standard rivastigmine and thus the compounds can serve as the lead ones for the treatment of Alzheimer's disease [63].
Sang et al. reported AChE/BChE inhibitory, MAO-A/MAO-B inhibitory, and antioxidant activities of chalcone-O-carbamate derivatives (62). Results revealed that the compounds show highly selective BChE inhibitory activity with IC50 values of 1-3 mM range [64]. Some 1,3,4-oxadiazole/thiadiazole-chalcone conjugates (63) were synthesized and evaluated for in vitro and in vivo antiviral activities against TMV. These conjugates have low binding constant values which were comparable to the standard ningnanmycin [65].
Amide tethered 7-chloroquinoline-chalcone bifunctional hybrids (64) were synthesized and employed as antimalarial agents against the resistant strain of Plasmodium falciparum. Methoxy substituent at the para position of ring B on chalcones and longer alkyl chain lengths significantly improved the antiplasmodial profiles of the chalcone derivatives [66].
The halogenated 1-tetralone or 6-amino-1-tetralone chalcone derivatives (65) were synthesized and evaluated for inhibitory effects against ROS production in LPS-stimulated RAW 264.7 macrophages. The structure-activity relationship revealed that amino moiety at the sixth position of 1-tetralone chalcones plays an important role for greater ROS inhibitory potency [67].
Chalcone derivatives (66) were studied for hepatoprotective ability, and the results were compared with the standard hepatoprotective drug silymarin. The experimental results were supported by a molecular docking study [68].
Triazole-linked 4-aminoquinoline-chalcone/-N-acetylpyrazoline conjugates (67) were synthesized and evaluated for antiplasmodial activities against cultured chloroquine-resistant strain. The activities were found to be dependent on the length of the alkyl chain as well as on the presence of methoxy substituents on the chalcone rings [69].
Chalcone analogs (68) were synthesized and evaluated for cytotoxic effects in human hepatoma HepG2 cells. The percentage of apoptotic cells was significantly higher in the compounds than that in the control cells [70].
The oxygenated chalcones (69) were found to inhibit monoamine oxidases, and the lead compounds were found to be nontoxic at 200 μg/mL in normal rat spleen cells [71].
Hameed et al. studied the quinoline-based chalcone compounds (70) as reverse transcriptase inhibitors. Bromo-and chloro-substituted chalcones exhibited a high degree of inhibition against the reverse transcriptase [72].
Sashidhara et al. documented the antiulcer activity of some novel quinolinechalcone hybrids (73) in various ulcer models in Sprague Dawley rats. Additional studies including in vitro metabolic stability and in vivo pharmacokinetics showed their potential to act as an orally active and safe candidate for the development of an antiulcer agent [74].
Pyrene ring-bearing chalcone (74) was studied as a sensitive and highly selective sensor for the detection of aluminum (Al 3+ ) ions by fluorimetric studies by Suresh et al. The chalcone was found to be useful for the electrosorptive removal of Al 3+ ion and several other biological applications including the bio-imaging of bacterial cells [75].
Chalcones (75) were reported for potent antimalarial activities against Plasmodium falciparum using Rieckmann's method. Allyloxy, hydroxy, and alkoxy functional groups increased the antimalarial activity of the chalcone derivatives [76].
Human African trypanosomiasis is an infectious disease that affects the lives of people living in rural areas of Africa. Beteck et al. studied the antitrypanosomal activities of indanone-based chalcone analogs (76) by screening against T.b. brucei [77]. The structures of chalcones having miscellaneous activities are depicted in Figure 8.

Conclusion
Chalcones and their analogs possess significant biological activities including antimicrobial, anticancer, antitubercular, antioxidant, anti-inflammatory, antileishmanial, enzyme inhibitory, and miscellaneous applications and hence acquire a unique place in medicinal chemistry. The growing interest of synthetic organic, pharmacological, and medicinal chemists towards chalcones and their derivatives will be continued in the future also. This chapter is expected to provide a stimulus for researchers to design, synthesize, and carry out further investigation on the pharmacological effects of new chalcone derivatives for different biological activities.