The Anticancer Profile of Benzimidazolium Salts and Their Metal Complexes

Abstract

Cancer is the most lethal ailment throughout the world in the present era. The development of new anticancer remedies with minor unhealthful effects and an alternate mechanism is crucial. Benzimidazole is a distinguished heterocyclic compound and is now recognized as the privileged scaffold for new drug discovery. This chapter deals with the anticancer capability of benzimidazolium salts and their metal complexes. The benzimidazolium derivatives have been prepared by the introduction of aliphatic and aromatic groups at two nitrogen atoms of the benzimidazole ring. Other modifications include hybridization with other pharmacophores and the preparation of metal complexes. The potent derivatives presented in this review can serve as novel drug candidates against cancer.

1. Introduction

Cancer is among the most dreadful diseases and a significant cause of assassinations all over the globe. In 2018, 9.6 million expirations were because of this malady [1]. Breast cancer is the paramount form of cancer in women all over the world. In 2018, 2.3 million victims and 627,000 fatalities were reported due to breast cancer. Prostate cancer is the second most common cancer in males and 1.3 million patients were reported in 2018 [1]. It has been deduced that more than 13 million people will die due to cancer in 2030 [2]. The major risk factors associated with cancer are chronic infections, inherited mutations in genes, overweight, no physical activity, exposure to ionizing radiation, and carcinogens such as polychlorinated biphenyls, chloroform, Dichlorodiphenyltrichloroethane (DDT), and formaldehyde [3]. Treatment patterns for cancer involve radiotherapy, surgery, and drug therapy. Drug therapy includes inorganic, organic, organometallic monomers, and polymers as well as nanoparticles [4]. Drug therapy is associated with severe adverse properties such as alopecia, anemia, and infertility. There is also the development of resistance against currently available drugs [5]. Consequently, the development of new anticancer drugs affiliated with low toxicities is very significant.

Nitrogen-containing heterocycles are abundantly present in natural and synthetic drug molecules [6]. Benzimidazole is one of the most significant members of nitrogen-containing heterocycles. This heterocycle is a constituent of the structures of some natural compounds such as vitamin B12 [7]. Benzimidazole derivatives have antihypertensive [8], anti-inflammatory [9], antimicrobial [10], antiulcer [11], antiviral [12], antioxidant [13], antitumor [14], lipid modulator, and anticoagulant properties [15]. Benzimidazole derivatives have also the major therapeutic activities against cancer [16, 17, 18]. Benzimidazole is also the main pharmacophore of anticancer drugs (Figure 1) such as bendamustine (1), selumetinib (2), and galeterone (3) out of which bendamustine is approved for clinical use, while the other two are in clinical trial stages [19, 20]. The structural resemblance of benzimidazole with nucleotides makes them very vital from the biological point of view [21]. Benzimidazolium salts are 1,3-disubstituted benzimidazole derivatives and possess acidic hydrogen at position 2. Benzimidazole salts find application as a carbene precursor for the preparation of n-heterocyclic carbenes (NHC) with different metals [22]. These benzimidazolium salts and their complexes have displayed significant antimicrobial and anticancer properties [23]. This review deals with the anticancer activities of benzimidazolium salts and their metal complexes.

Figure 1.

Anticancer drugs based on benzimidazole scaffold.

2. Anticancer properties of benzimidazolium salts

Benzimidazolium salts and their anticancer capabilities have been reviewed in the following sections.

2.1 Hybrid molecules containing benzimidazolium salts

Molecular hybridization has become an effective approach for new drug discovery. In molecular hybridization, two or more pharmacophores are linked to each other to produce the new molecules [24, 25]. Yang et al. synthesized hybrid molecules in which benzimidazolium salts were linked to trimethoxy phenyl chalcones. Compound 4 (Figure 2) demonstrated excellent anticancer potential against leukemia (HL-60), breast carcinoma (MCF-7), and colon carcinoma (SW480) cell lines presenting IC₅₀ values of 0.83, 1.57, and 2.92 μM, respectively, which is 5–11-folds higher than the standard drug cisplatin. In compound 4, 2-naphthylmethyl substituent is attached to benzimidazole nitrogen. The superior activity of these salts was related to the high solubility of benzimidazolium salts. Benzimidazole derivatives with substituents at positions 5, 6 showed greater activity as compared to unsubstituted benzimidazoles [26]. Karatas et al. reported a series of hybrid molecules in which coumarin was attached to the substituted benzimidazolium chlorides. Anticancer screening by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay showed that these compounds have the potential to discontinue the cell cycle for human prostate (PC-3) and ovarian (A2780) cancer cells. Specifically, compound 5 (IC₅₀ = 44.5 μM) showed some inhibitory potential against PC-3 at the dose of 1 μM [27]. Next year, Wang et al. produced the 3-benzyl coumarin imidazolium salts using the hybrid molecular strategy. The anticancer activity evaluation showed that compound 6 is one of the most active derivatives having IC₅₀ values ranging from 2.04 to 4.51 μM. The best activity was displayed versus the MCF-7 cell line having an IC₅₀ value of 2.04 μM. The 5, 6-dimethyl-substituted benzimidazole hybrids exhibited prominent activity as compared to unsubstituted benzimidazoles or imidazoles. These molecules showed selectivity for MCF-7 and SW-480 cancer cell lines. Compound 6 also showed the apoptosis in liver carcinoma (SMMC-7721) cell line [28]. Deng et al. linked benzimidazolium salts with steroidal molecules such as cholesterol, dehydroepiandrosterone, and diosgenin. Anticancer activities were carried out against HL-60, A-549, SMMC-7721, SW480, and MCF-7 cancer cell lines. Diosgenin-imidazolium salts displayed higher activity, and compound 7 was the most effective having IC₅₀ values from 0.44 to 0.79 μM against different cell lines. Compound 7 disrupted the cell cycle in G1/G0 stage and showed apoptosis in the SMMC-7721 cell line. Structure-activity relationship (SAR) studies showed that 5, 6-dimethyl-substituted benzimidazolium salts showed excellent anticancer activity. Attachment of 2-bromobenzyl with 2-naphthyl methyl at position 3 of benzimidazole also amplified the activity [29]. Brazilin is a natural compound and possesses extensive bioactivities such as anti-inflammatory, anticancer, and antioxidant [30]. Huang et al. connected aza-brazilin with imidazolium salts to produce hybrid molecules. Anticancer activity was evaluated against A549, SMMC-7721, MCF-7, and SW480 cell lines. Derivative 8 appeared as the most active (IC₅₀ = 0.35 μM) against the MCF-7 cell line and displayed more potency as compared to cisplatin. Derivatives having a 5,6-dimethylbenzimidazole ring displayed prominent activity. The introduction of electron-withdrawing groups on the aza-brazilin nucleus produced more active derivatives. Derivatives having an alkyl chain as linker groups produce higher potency as compared to acyl chains [31]. Zhou et al. also used a hybrid molecular strategy to conjugate N-substituted tetrahydro-β-carbolines with imidazolium salts. Compound 9 exhibited prominent activity with IC₅₀ values ranging from 2.61 to 17.13 μM against five different cancer cell lines. Most prominent activity was achieved against MCF-7 (IC₅₀ = 2.79 μM) and SW-480 (IC₅₀ = 9.46 μM) cancer cell lines. Compound 9 carries a naphthyl methyl scaffold at position 3 of benzimidazole. Compound 9 also showed the phenomenon of apoptosis in the MCF-7 cell line as well as inhibited the cell cycle in the G1 phase [32].

Figure 2.

Hybrid molecules comprising benzimidazole salts and natural compounds.

Xu et al. coupled a three-substituted indole ring with imidazolium salts to produce new hybrid molecules. Upon evaluation of anticancer activity, compound 10 (Figure 3) showed prominent performance against MCF-7 (IC₅₀ = 3.19 μM), A549 (IC₅₀ = 3.51 μM), SW480 (IC₅₀ = 11.57 μM), and SMMC-7721 (IC₅₀ = 3.60 μM) cancer cell lines. SAR studies showed that 5,6-dimethyl-substituted benzimidazole derivatives showed prominent activity as compared to unsubstituted benzimidazoles. Compound 10 carries a naphthyl acyl ring at position 3 of the benzimidazole nucleus. Further studies showed that compound 10 is capable of inducing apoptosis and caused cell cycle blockage in the S phase [33]. Li et al. synthesized carbazole and imidazolium salts using the molecular hybridization technique. Replacement of the imidazole ring by the benzimidazole increased the anticancer selectivity against a particular cell line. Among the benzimidazolium salts, derivative 11 showed excellent activity against the HeLa cell line (IC₅₀ = 0.02 μM) as compared to standard drug cisplatin (IC₅₀ = 13.61 μM) [34]. Wang et al. synthesized imidazolium salts and dibenzofuran comprising hybrid molecules. The estimation of anticancer activity against five cancer cell lines showed that 2-methylbenzimidazolium and dibenzofuran hybrid molecular salts are more active as compared to individual molecules. 2-Methyl-substituted benzimidazolium salts showed higher activity as compared to unsubstituted and 5,6-dimethylbenzimidazolium salts. Compound 12 expressed prominent activity (IC₅₀ = 0.64–1.47 μM) against MCF-7, A549, SW480, HL-60, and SMMC-7721 cancer cell lines. Most prominent activity was observed against MCF-7 (IC₅₀ = 0.64 μM) and SW-480 (IC₅₀ = 0.88 μM) cell lines. Compound 12 carries a 4-methoxy phenacyl substituent at position 3 of the benzimidazole nucleus. Replacement of 4-methoxy phenacyl substituent by 2-naphthylacyl also produced potent derivatives [35]. Zhang and co-workers synthesized hybrid molecules comprising 2,3,6,7-tetrahydrobenzodifuran and imidazolium salts. Compound 13 appeared as the most active derivative against five cancer cell lines having an IC₅₀ value less than 4.34 μM. Compound 13 showed selectivity for A549, SMMC-7721, and SW-480 cancer cell lines. Some derivatives of this series also exhibited the phenomenon of apoptosis and seized cell cycle in the G1 stage [36]. Zhou et al. manufactured hexahydropyrrolo[2,3b]indole-1H-imidazolium salts as anticancer agents. The nitrogen at position 3 of the benzimidazole ring was linked to 2-bromobenzyl and 2-naphthyl methyl scaffolds. Compound 14 emerged as the most active derivative against HL-60, MCF-7, SMMC-7721, A549, and SW-480 cell lines having an IC₅₀ value less than 2.68 μM. The introduction of the N-benzyl group at the indole nitrogen also increased the activity [37].

Figure 3.

Hybrid molecules of benzimidazole salts with synthetic molecules.

2.2 Benzimidazolium salts having aromatic and aliphatic substituents

Akkoc et al. reported benzimidazolium salts screened for anticancer potential against human embryonic kidney (HEK-293 T), human colon epithelial colorectal adenocarcinoma (DLD-1), and human breast epithelial adenocarcinoma (MDA-MB-231) cancer cell lines by using MTT assay. Palladium (Pd) metal complexes were also prepared and found inactive against these cells lines having IC₅₀ values over 100 μM. The naphthalen-1-yl-methyl incorporated benzimidazolium chloride 15 (Figure 4) (IC₅₀ = 26.09 μM) showed most cytotoxicity against DLD-1 cell line [38]. In an additional study, Akkoc and his coworkers reported a novel series of benzimidazolium salts containing cyanobenzyl, nitrophenyl, and N-methylphthalimide substitutions. All salts were tested for their cytotoxicity against DLD-1 and human breast cancer (MDA-MB-231) cell lines. Compounds 16 and 17 demonstrated exceptional activity against MDA-MB-231 cell lines with IC₅₀ values of 1.26 and 2.01 μM and were considerably better than cisplatin (IC₅₀ = 5.77 μM). Derivatives 16 and 17 contain anthracene and naphthalene rings, respectively, attached with the nitrogen of benzimidazole. The N¹ and N³ substituents produced a prominent effect on anticancer activity [39]. In 2019, Akkoc extended his previous finding of cytotoxicity of benzimidazolium salts. In this regard, 2-hydroxyethyl-containing benzimidazolium salts along with respective Pd-complexes were prepared and their anticancer capacity was noted against human cancer cell lines. Surprisingly, compound 18 exhibited notable activity against MDA-MB-231 (IC₅₀ = 7.59 μM) and DLD-1 (IC₅₀ = 39.51 μM) cell lines in comparison with their Pd-complexes [40].

Figure 4.

Benzimidazole salts having aromatic substituents.

Lin et al. synthesized 1,3-bis-naphthyl-substituted benzimidazolium bromides and estimated for activity against MDA-MB-468 as well as PC-3 cell lines. As compared to the standard drug tamoxifen (IC₅₀ = 22.5 μM), compound 19 (Figure 5) was found as an active agent (IC₅₀ = 9.7 μM) against MDA-MB-468. The presence of the naphthyl group was vital for the activity of these derivatives [41]. Wright et al. synthesized naphthalene-substituted imidazolium salts and evaluated the anticancer performance against non-small-cell lung cancer (NSCLC) cell lines. The anticancer activity was evaluated by the MTT assay. Compound 20, the benzimidazolium salt, displayed IC₅₀ values of 3, 4, and 5 μM against NCI-H460, NCI-H1975, and HCC-827, respectively. Compound 20 carries naphthyl rings at both nitrogens of benzimidazole [42]. Stromyer et al. synthesized benzimidazole salts having triphenylphosphonium group. The nitrogen atoms of benzimidazole were linked with naphthyl methyl groups. Compound 21 revealed marvelous activity against bladder cancer cell lines RT4, RT112, UMUC3, and SW780. This compound also showed apoptosis by causing mitochondrial damage. The drug causes a rapid and irreversible effect against bladder cancer [43]. Shelton et al. synthesized N,N-bis-arylmethyl-substituted benzimidazolium salts via cyclization of o-phenylenediamine or 2-(2-(2-methoxy ethoxy)ethoxy)acetic acid with 2-(chloromethyl) quinolone or 2-(bromomethyl)-naphthalene followed by alkylation and quaternization. Various hydrophilic and hydrophobic groups were added at both nitrogen atoms of benzimidazole. Insight into in vitro cytotoxicity of synthesized salts, compound 22 showed adequate activity against NSCLC cancer cells having IC₅₀ values ranging between 1 and 7 μM comparable to the standard drug, cisplatin. This compound bears quinoline and naphthalene rings to both nitrogen atoms of benzimidazole. The presence of ether linkage at position two increased the hydrophilicity of this compound [44].

Figure 5.

Benzimidazole salts having naphthalene and quinoline rings.

Bansode et al. carried out the synthesis of ferrocene-linked ionic liquids by incorporating long alkyl chains. Anticancer activity was evaluated against MCF-7 by using sulforhodamine B assay. These ferrocene-quaternized azolium salts showed significant cytotoxic potential against MCF-7 and 1-(ferrocenylmethyl)-3-tetradecylbenzimidazolium bromide 23 (Figure 6) was found to be most potent (GI₅₀ = 0.016 μM) as compared to standard drug doxorubicin (GI₅₀ = 0.018 μM). Derivatives in this followed the Lipinski rule of five and showed excellent pharmacokinetic properties [45]. Kucukbay et al. synthesized N, N-disubstituted benzimidazolium bromides and evaluated anticancer activity against PC-3 and ovarian (A2780) cancer cell lines. Derivatives 24-26 presented prominent activity against PC-3 and A2780 cancer cell lines having IC₅₀ values in micromolar concentration. These compounds bear a 4-methoxyphenyl ethyl group at the benzimidazole nitrogen [46]. Haque et al. prepared a collection of bis-benzimidazolium salts and evaluated against human colon cancer (HCT-116). All compounds showed superior activity than the reference drug, fluorouracil. Derivatives having N-methylene phenyl substituents presented prominent activities. The highest anticancer potential was observed in the case of derivative 27 (IC₅₀ = 0.2 μM) remarkably better than reference drug, 5-fluorouracil (IC₅₀ = 19.2 μM) [47]. Noor ul Huda et al. synthesized bis-NHC benzimidazolium salts and evaluated them as antimicrobial and anticancer agents. Compound 28 exhibited prominent activity against HCT-116 cancer cell lines showing 75% inhibition at 1 mg/ml as determined by using sulforhodamine Β assay. Compound 28 contains a lipophilic alkyl chain and the lipophilicity of the alkyl chain was linked to the increased activity of this derivative [48].

Figure 6.

Bis-benzimidazolium and ferrocene-linked benzimidazolium salts.

2.3 Benzimidazolium silver metal complexes

Cisplatin is the first metal-based drug used for the cure of cancer [49]. The serendipitous discovery of cisplatin stimulated the search for new metal-based anticancer agents. Silver (Ag) salts have been used as antimicrobial agents for purification of drinking water and wound healing [50, 51]. Based on its antimicrobial property, silver has also been explored as an anticancer agent. N-Heterocyclic carbenes (NHC) are a prominent family of organometallic ligands.

2.3.1 Bis-benzimidazolium silver metal complexes

Iqbal and his coworkers performed a detailed study to reduce the risk of malignant neoplasm and reported novel binuclear benzimidazolium salt and corresponding Ag (I) NHC complex. Compound 29 (Figure 7)presented prominent activity (IC₅₀ = 1.7 μM) against HCT-116 cell line. This compound also showed significant inhibition of inflammatory cytokines such as tumor necrosis factor-alpha and interleukin in human macrophages. Compound 29 showed apoptotic activity via inhibition of the caspase pathway. Photomicrographs of the cell treated with compound 29 showed deposition of silver in cells [52]. Gadhayeb et al. carried out the synthesis of mono- and bis-NHC complexes having palladium (Pd) and silver metals. The anticancer activity was evaluated out against HCT-116 cell line. Compounds 30 and 31 exhibited prominent activities having IC₅₀ values of 12.3 ± 1.2 μM and 10.6 ± 1.8 μM, respectively. The mono-NHC showed more activity as compared to bis-NHC. The increased activity of compound 31 could be due to the increased release of silver from the mono-NHC complex. These compounds contain a butyl chain at the benzimidazole nitrogen [53]. Following this principle, Sarhan et al. reported benzimidazolium-acridine-based salts and metal complexes with pronounced biological potential. Specifically, compound 32 can be considered an excellent in vitro anticancer agent against MCF-7 (IC₅₀ = 21 μM) and selectivity index of 3.6. Therefore, Ag-NHC complexes demonstrated prominent activity [54]. A series of 5-methyl benzimidazole-based n-heterocyclic carbene (NHC) salts and their silver (I)-complexes were prepared by Habib et al. The Ag (I)-benzimidazolium complexes showed dominant activity against human breast cancer (MDA-MB-231) and colon cancer (HCT-116) as compared to NHC salts. Compound 33 showed promising activity against MDA-MB-231 (IC₅₀ = 4.2 ± 0.24 μM, SI = 7.63) and HCT-116 (IC₅₀ = 7.43 ± 0.23 μM SI = 4.33) as compared to reference drug (IC₅₀ = 8.20 ± 0.14 μM and 5.5 ± 0.34 μM) against these cell lines respectively. Compound 33 carries pentyl chains at both nitrogen atoms of the benzimidazole core. Derivatives having a longer alkyl chain were found more active. These molecules showed dose-dependent cytotoxicities and apoptosis by mitochondrial pathways [55]. A range of substituted Ag(I)-benzimidazolium carbene complexes were reported by Atif et al. and in vitro anticancer studies were carried out against MCF-7, HCT 116, and erythromyeloblastoid leukemia (K-562) cell lines. Promising anticancer activity was shown by 34 (IC₅₀ = 0.31 μM) against the K-562 cell line. Compound 34 is a bis-benzimidazole silver complex and carries propyl groups at both nitrogen atoms of benzimidazole rings [56].

Figure 7.

Mono and binuclear bis-benzimidazolium silver metal complexes.

Achar et al. reported a novel series based on benzimidazolium salts linked with coumarin heterocycle, and their silver cationic bis-NHC and Ag neutral mono-NHC were synthesized. The anticancer activity was evaluated by sulforhodamine assay. Complex 35 (Figure 8) exhibited moderate activity against the A549 cell line having an IC₅₀ value of 8.3 ± 0.40 μM. Compound 35 is the bis-NHC-coordinated Ag hexafluorophosphate (PF6) salt. The mono-NHC coordinates Ag acetate complexes showed inferior activity with IC₅₀ values greater than 10 μM. Bis-NHC complexes also showed prominent antibacterial activity [57]. Yasar et al. also worked out to yield novel zwitterionic-sulfonated benzimidazolium salts and their Ag-(I) complexes by following a reported synthetic approach. Anticancer activity was assessed against human cervix carcinoma (HeLa), human adenocarcinoma (HT29), and mouse fibroblast (L929) cancer cell lines. Silver complexes were found most active as compared to their salts. Compound 36 (IC₅₀ = 11 ± 1 μM) showed higher potency against HT29 cancer cell line as compared to cisplatin (IC₅₀ = 42 ± 6 μM). Compound 36 was found to be the least toxic (IC₅₀ = 126 ± 3 μM) against non-cancer L929 cell lines [58]. Karlık et al. prepared a series of aqua-bis-benzimidazole Ag (I) p-toluene sulfonate complexes via a multistep approach and investigated their anticancer properties against human colorectal (Caco-2) and MCF-7 cancer cell lines. Salts were found to be ineffective against these cell lines. Benzimidazolium Ag complex 37 (IC₅₀ value of 9 ± 3 μM) showed excellent activity against the Caco-2 cell line but was inactive against the MCF-7 cell line. Derivatives 37 carries an o-chloro-substituted benzyl group attached with the nitrogen of benzimidazole and this substituent was found more effective at this position as compared to o-methyl and p-methyl analogues [59]. Fatima et al. successfully explored the cytotoxic effect of different alkyl chains on benzimidazolium-based Ag complexes. It was observed that compound 38 containing longer n-alkyl chains showed the best cytotoxic potential against HCT-116 (IC₅₀ = 0.02 μM) as compared to 5-fluorouracil (IC₅₀ = 10.2 μM) as a standard drug. The incorporation of silver ions and elongation of the alkyl chain amplified the anticancer activity [60].

Figure 8.

Bis-benzimidazolium silver metal complexes having alkyl chain, heterocyclic and aromatic scaffold.

Similarly, Haque et al. prepared Ag(I) complexes containing nitrile-functionalized benzimidazolium salt as an active agent against HCT-116. Among all synthesized complexes, compound 39 (IC₅₀ = 14.9 ± 0.8 μM) showed the highest cytotoxicity as compared to fluorouracil (IC₅₀ = 5.2 ± 0.3 μM). The activity was linked to the nitrile group (Figure 9) at the meta position of the benzyl group causing the weak electron-withdrawing effect [61]. Early on, Haque et al. also synthesized silver metal complexes containing benzimidazolium ligand. These silver-based benzimidazolium complexes are capable of slow release of the silver ion at the cancerous cell, affecting cell morphology. Complex 40 (IC₅₀ = 1.20 ± 0.3 μM), a binuclear silver entity, expressed superior activity as compared to fluorouracil (IC₅₀ = 5.2 ± 0.3 μM) against HCT-116. Complex 40 was found to be the least active (IC₅₀ = 103 ± 2.3 μM) against the HT29 cell line. Therefore, the HT-29 cell line was found to be resistant to this complex [62]. Hussaini et al. successfully formulated silver (I)-benzimidazolium carbenes. A series of propylene-linked bis-benzimidazolium salts having different alkyl chains and respective binuclear silver complexes were prepared. Complexes showed dose-dependent cytotoxicities. Their cytotoxic studies against the MCF-7 cell line revealed compound 41 as the most active (IC₅₀ = 7 ± 1 μM) complex as compared to tamoxifen (IC₅₀ = 11 ± 2 μM). The presence of the propyl chain at the benzimidazole nucleus was found to be optimum for anticancer activity [63]. Later on, in 2018, Hussaini and his coworkers reported benzimidazolium salts having aliphatic nitrile group and their Ag (I)-benzimidazolium carbenes complexes. All of these NHC complexes showed good cytotoxicities with IC₅₀ values in the range of 7.0–12.9 μM against the MCF-7 cell line. Compound 42 (IC₅₀ = 7 ± 1.06 μM) was found to be the most potent and it carries a pentyl chain attached to the benzimidazole nitrogen. Cytotoxicities of these compounds increase as the alkyl chain length expands [64].

Figure 9.

Bis-benzimidazolium silver metal complexes containing aliphatic nitriles, aromatic nitriles, benzyl and alkyl chains.

2.3.2 Benzimidazolium silver and gold metal complexes

Akkoc et al. reported a series of silver- and palladium-based metal complexes with benzimidazolium ligand. This attempt was made in search of the non-platinum antitumor drugs due to the observed side effects of cisplatin and nedaplatin. However, silver complex 43 (Figure 10) showed promising in vitro cytotoxic potential against DLD-1 (IC₅₀ = 12.41 μM), MDA-MB-231 (IC₅₀ = 11.98 μM) cancer cell lines, and HEK-239 (IC₅₀ = 4.2 μM) non-cancer cell lines. Therefore, the silver complex was found more potent than the palladium complex [65]. Sahin et al. employed the conventional technique to synthesize n-allyl-substituted benzimidazolium-based carbene and corresponding Ag-(I) complexes. The first step was to obtain n-alkylated benzimidazole and later followed by salt formation. Silver dioxide was used to obtain the allyl-linked benzimidazolium Ag-(I) complex. Among all synthesized compounds, derivatives 44 (IC₅₀ = 1.41 μM) and 45 (IC₅₀ = 1.21 μM) showed the best in vitro anticancer potential against DU-145 and MCF-7, and MDA-MB-231 (IC₅₀ < 1 μM) cancer cell lines. Derivatives 44 and 45 also exhibited some degree of selectivity [66]. Ozdemir et al. carried out the synthesis of silver and gold (Au) NHC-propyl sulfonate complexes. Silver complexes presented dominant activity as compared to gold salts. Compounds 46 (IC₅₀ = 2.32 ± 0.089 μM) and 47 (IC₅₀ = 9.31 ± 0.95 μM) presented prominent in vitro activities against adenocarcinoma (HEP3B) cancer cell lines. Complexes 46 and 47 are silver and gold complexes, respectively. These complexes possess a dimethoxyethyl group at the benzimidazole nitrogen. Replacement of dimethoxyethyl group by diethoxyethyl group produced less active derivatives [67].

Figure 10.

Benzimidazolium silver and gold metal complexes.

Similarly, the synthetic work of Cevik Yildiz and his coworkers resulted in the formation of novel benzimidazolium salt. These benzimidazolium salts were used as a ligand to obtain corresponding novel Ag (I)- benzimidazolium complexes. Cytotoxic studies of salts and complexes were carried out against MCF-7, MDA-MB-23, DU-145 by MTT assay. Compound 48 (Figure 11) displayed prominent activity (IC₅₀ < 1 μM) against breast cancer MCF-7 and MDA-MB-23 cell lines. Compound 49 also showed prominent activity (IC₅₀ < 1 μM) against the MCF-7 cell line than standard drug. These compounds showed concentration-dependent killing and also showed selectivity for cancer cell lines [68]. Aktas et al. carried out the synthesis of 2-morpholine ethyl-substituted benzimidazolium salts and their Ag-NHC complexes. Compounds 50 (IC₅₀ = 6.59 μM) and 51 (IC₅₀ = 6.56 μM) exhibited prominent activity against MCF-7 cell line. Ag-NHC complexes presented prominent activity as compared to benzimidazolium salts. Compounds 50 and 51 carry methyl and tetra-methyl benzyl groups at the benzimidazole nitrogen [69]. A series of di-isopropylamine ethyl benzimidazolium salts have been reported by Kızrak et al. These salts were further used as a precursor for the syntheses of corresponding silver and gold benzimidazolium carbene complexes. Resultant derivatives displayed prominent activity against the human brain (SHSY5Y) cell line and compounds 52, 53 were most prominent presenting IC₅₀ values of 5.23 and 4.74 μM, respectively. Compounds 52 and 53 are silver and gold complexes, respectively. Therefore, the gold complex was found more potent than silver complexes. Compound 52 was also found significant against HEP3B (IC₅₀ = 6.19 ± 1.09 μM) and HTC-116 (IC₅₀ = 8.44 ± 1.07 μM) cancer cell line [70]. Sahin-Bolukbasi and Sahin synthesized two new benzimidazolium salts and reacted with silver dioxide (Ag₂O) to obtain Ag-(I) benzimidazolium complexes. Evaluation of the anticancer potential of all these compounds reflected the higher cytotoxicity (IC₅₀ < 1 μM) of 54 against DU-145, MCF-7, and MDA-MB-231. This compound carries 2-methyl propenyl and p-isopropyl benzyl substituents at the N¹ and N³ positions of benzimidazole ring [71]. Early on, Sahin-Bolukbasi et al. synthesized unsymmetrical benzimidazolium salt and respective Ag-(I) complexes. All of the synthesized compounds were subjected to cytotoxic evaluation. Higher cytotoxicity was noted for benzimidazolium-based Ag complexes as compared to benzimidazolium salts, and particularly 55 and 56 were most active (IC₅₀ < 1 μM) against MCF-7, MDA-MB-231 cancer cell lines. These compounds also demonstrated activity against DU-145 cell line having IC₅₀ values of 6.02 ± 0.30 μM and 5.16 ± 0.33 μM, respectively. The ortho-substituted benzyl group proved more active as compared to meta- and para-substituted benzyl groups. And in ortho-substituted derivatives methyl group was found more potent as compared to the chlorine atom [72].

Figure 11.

Benzimidazolium silver metal complexes.

2.4 Selenium-based benzimidazolium salts and complexes

Selenium (Se) is very important for the human body and its deficiency can lead to cancer, diabetes, and cardiovascular diseases [73]. Selenium is present in some food and drinks in traces [74]. Recently, selenium has been incorporated in new anticancer agents due to its low toxicity. Kamal et al. applied a green synthetic approach to obtain novel benzimidazolium salts and Se-based benzimidazolium-heterocyclic carbenes. The in vitro anticancer potential was evaluated against RGC-5, Hela, MCF-7, and mouse melanoma (B16F10) cancer cell lines using fluorouracil as a standard drug. Prominent anticancer activity was observed against Hela and RCG cell lines. Among benzimidazolium salts (Figure 12), compounds 57 (IC₅₀ = 0.04 ± 0.31 μM) and 58 (IC₅₀ = 0.24 ± 0.22 μM) displayed prominent activity against Hela cell line. The corresponding Se-NHC-adducts compounds 59 (IC₅₀ = 0.11 ± 0.20 μM) and 60 (IC₅₀ = 4.3 ± 0.11 μM) were found most active against Hela cell line. Compound 59 was also effective against RCG cell line (IC₅₀ = 9.16 ± 0.27 μM). Molecular docking investigation of compounds 59 and 60 with epidermal growth factor (EGF), vascular endothelial growth factor (VEGF), and cyclooxygenase (COX-1) displayed strong interactions [75]. Similarly, green synthesis of benzimidazolium salts and di-Se-N-heterocyclic carbene complexes was carried out by Iqbal et al. The benzimidazolium salts 61 (IC₅₀ = 3.94 μM) and respective Se-NHC 62 (IC₅₀ = 3.49 μM) displayed prominent activity against HCT-116 cell line as compared to fluorouracil (IC₅₀ = 4.9 μM). Both compounds possess benzyl groups at positions 1 and 3 of the benzimidazole nucleus. Compounds 61 and 62 also showed some degree of apoptosis by the mitochondrial pathway. Further pro-apoptotic evaluation for HCT-116 even at low concentration due to a strong release of selenium metal for DNA interaction was successfully reported [76].

Figure 12.

Benzimidazolium salts and selenium metal complexes.

2.5 Ruthenium complexes based on benzimidazolium salts

Organic compounds having ruthenium (Ru) metal are also being used as anticancer agents. Akkoc et al. synthesized methylpyridine-linked benzimidazolium salt and corresponding Ru (II) complexes containing benzimidazolium ligand. The antiproliferative assay revealed that 63 (Figure 13) showed DNA binding as well as the best cytotoxic potential against MCF-7 (IC₅₀ = 23.8 μM), Caco-2 (IC₅₀ = 18.0 μM) cell lines, respectively. Compound 63 carries a hexamethyl phenyl ring at position 2 of the benzimidazole ring. Compound 63 showed electrostatic and hydrophobic interaction with DNA and presented a binding affinity of −13.779 kcal/mol [77]. Omar et al. synthesized benzotriazole-functionalized palladium and ruthenium complexes. Comparison of cytotoxic studies revealed that ruthenium complexes are better than palladium complexes against MCF-7 and Caco-2 cancer cell lines. Although the activity displayed by these compounds was not prominent, these compounds were found inert for normal cell line L-929. Compound 64 (IC₅₀ = 90 μM) is one member of this series [78]. Lam and his coworkers aimed for the novel benzimidazolium-based Ru complexes with halogen ligands for exploration of cytotoxic potential against HCT-116, SiHa, and NCI-H460 cell lines. Promising cytotoxic potential was observed in case of 65 against HCT-116 (IC₅₀ = 6.2 ± 0.4 μM), SiHa (IC₅₀ = 8.4 ± 0.2 μM), and NCI-H460 (IC₅₀ = 7.8 ± 1.0 μM) cancer cell lines. Replacement of ruthenium by osmium (Os) also presented equal cytotoxicity against HCT-116 cell line [79].

Figure 13.

Ruthenium complexes.

2.6 Miscellaneous metal complexes

As mentioned earlier, NHC-carbenes are highly reactive species depending upon the nature of the ligand and transition metal used; consequently, different transition metals have been coordinated with benzimidazolium ligand for a better cytotoxic effect. Troung et al. studied the cytotoxicity of rhodium (Rh)- and iridium (Ir)-based benzimidazolium complexes. A series of rhodium and iridium complexes were prepared and evaluated for anticancer potential against HCT-116, NCI-H460, SiHa, SW480 human cancer cell lines. Compounds 66 and 67 (Figure 14) advertised notable activity against HCT-116 (IC₅₀ = 7.4 ± 0.5 μM, 11 ± 0.1 μM), NCI-H460 (IC₅₀ = 11 ± 1 μM, 23 ± 3 μM), SiHa (IC₅₀ = 10 ± 1 μM, 19 ± 1 μM), SW (IC₅₀ = 5.8 ± 1 μM, 19 ± 1 μM) cancer cell lines. Compounds 66 and 67 are Ir and Rh metal complexes, respectively. These compounds contain benzyl groups at both nitrogen atoms of benzimidazole. Studies of the mode of action of rhodium complexes are based on the fact that instead of interaction with DNA, it accumulates in the cytoplasm [80]. Zhao et al. synthesized naphthyl NHC-Rh complexes by incorporating a hydroxy alkyl chain at benzimidazole nitrogen. Upon evaluation against MCF-7 cell line, compounds 68 (IC₅₀ = 0.38 μM), 69 (IC₅₀ = 0.45 μM), and 70 (IC₅₀ = 0.72 μM) showed excellent activity as compared to standard drug paclitaxel (IC₅₀ = 1.38 μM). Therefore, the introduction of the hydroxyl group and alkyl group at the benzimidazole nitrogen is beneficial for the activity [81]. Sanchez-Mora et al. reported the formation of two benzimidazolium-based Ir(I) complexes as new cytotoxic agents. The benzimidazole ring was substituted by benzyl and pentafluorobenzyl groups. The benzyl-substituted derivative presented prominent activity and compound 71 was found to be the strongest agent against PC-3 (IC₅₀ = 10.6 ± 0.9 μM) and SKLU-7 (IC₅₀ = 10.4 ± 1.5 μM) cell lines. This compound showed less toxicity for normal cell line COS-7 [82]. Choo et al. synthesized pyridine-functionalized Pd-based imidazolium and benzimidazolium carbenes. Imidazolium Pd complex showed prominent activity against cancer cell lines. Benzimidazolium Pd complex 72 is also an excellent candidate for anticancer studies. Generally, NHC-Pd complexes create covalent bonding with DNA resulting in cross-linking of guanine base [83]. Early on in 2012, Sivaram and his coworker were able to synthesize new gold (I) and gold (III) complexes bearing benzimidazolium ligand. These complexes were mono-, homo-bis-, and hetero-bis-benzimidazole NHC. The hetero-bis-benzimidazole complexes are nonclassical pyrazole-derived NHC. Complexes 73 (IC₅₀ = 0.284 ± 0.11 μM) and 74 (IC₅₀ = 0.24 ± 0.01 μM) exhibited prominent inhibitory action against NSCLC (NCI-H1666) cell line. These complexes are isopropyl-substituted homo-bis and hetero-bis NHC complexes, respectively [84]. Rehm et al. synthesized benzimidazole platinum complexes having different alkyl chains such as methyl, ethyl, butyl, and octyl chains. The bis-phosphane complexes showed the excellent anticancer activity against seven cancer cell lines. Compound 75 appeared as the most effective against different cancer cell lines having IC₅₀ values placing from 0.10 to 0.30 μM. Complex 75 displayed prominent activity (IC₅₀ = 0.10 ± 0.01 μM) against multidrug-resistant strains of MCF-7. Cell cycle analysis of some complexes indicated that they produced cell blockage in the G1 stage [85].

Figure 14.

Benzimidazole metal complexes with gold, rhodium, iridium, and palladium.

3. Conclusion

The pharmacological properties of benzimidazolium salts have attracted the attention of medicinal chemists. The resemblance of benzimidazole scaffold with purine bases establishes it biologically significant. Benzimidazolium salts have demonstrated promising activities against various cancer cell lines. Benzimidazolium salts derivatives have been prepared by the functionalization of two nitrogen atoms in the imidazole ring along with the preparation of hybrid molecules, and metal complexes. The hybridization of benzimidazole salts with natural compounds such as chalcones and steroids exhibited prominent activities (IC₅₀ < 1 μM) against various cancer cell lines. Some hybrids compounds also showed the phenomenon of apoptosis. Compounds carrying alkyl chains and aromatic rings at benzimidazole nitrogen showed pronounced activity. The introduction of phenyl, naphthalene, anthracene, and quinoline rings at the benzimidazole nitrogens through methylene groups intensified the anticancer activity. The 5,6-dimethyl-substituted benzimidazole derivatives were also found more active as compared to unsubstituted benzimidazole rings. In the case of silver metal complexes, bis-benzimidazolium complexes exhibited exceptional activity against colon cancer (IC₅₀ < 1 μM) cell line. But silver metal complexes presented less selectivity indices. Mono-benzimidazolium metal complexes proved more active against breast cancer (IC₅₀ < 1 μM) cell lines. In some derivatives, the introduction of a long alkyl chain at the benzimidazole nitrogen is beneficial for the augmentation of anticancer activity. The activity of selenium metal complexes was almost equivalent to their respective salts. While the halogen-substituted ruthenium benzimidazole metal complexes showed moderate activity. Rhodium, platinum, and gold complexes have also shown encouraging anticancer activities and are excellent candidates for future investigations. The in vivo investigations of potent compounds mentioned in this chapter could lead to further developments in the field.