Benzimidazole based compounds are of great importance as anti-inflammatory and analgesic agents because of their property to inhibit cyclooxygenases (COXs), enzymes involved in biosynthesis of important inflammatory mediators called prostaglandins (Akhtar et al., 2017). Apart from the cyclooxygenases (COX), the benzimidazole derivatives interact with transient receptor potential vanilloid-1, cannabinoid receptors, bradykinin receptors, specific cytokines, and 5-lipoxygenase (5-LOX) activating protein. Thus, the compounds derived from benzimidazole moiety show the anti-inflammatory property (Veerasamy et al., 2021). Different benzimidazole derivatives with analgesic and anti-inflammatory properties are shown in Figure 5.

Mariappan et al. (Mariappan et al., 2015) synthesized a set of 2-substituted benzimidazole derivatives and reported that compounds 126–128 were found to be the most promising agents among the series displaying significant (p < 0.01) analgesic and anti-inflammatory effect at a dose level of 100 mg/kg p.o. Li et al. (Li et al., 2015) assessed the synthesized two series of 2-(piperidin-4-yl)-1H-benzo[d]imidazole derivatives for anti-inflammatory activity and found that the compound 129 exhibited the most potent inhibitory activity on nitric oxide and TNF-α production (IC₅₀ = 0.86 and 1.87 µM, respectively). Interestingly, the compound 129 also prevented 33.30 and 50.60% ear oedema in xylene-treated mice at doses of 4 and 12 mg/kg, respectively, compared to standard ibuprofen (26.77 and 39.34% inhibition at 4 and 12 mg/kg dose levels, respectively). Besides, compounds 130–132 showed the potentiality as gastroprotective lead compounds which can be developed into orally active analgesic and anti-inflammatory agents (Gaba and Mohan, 2015). Compounds 133–134, synthesized by Kumar et al. displayed significant analgesic and anti-inflammatory properties, (Kumar et al., 2015) and compound 135 showed better inhibition of acetic acid induced writhing at 20 mg/kg dose than the standard diclofenac (78.12 vs. 75%) (Datar and Limaye, 2015).

Moneer et al. (Moneer et al., 2016) synthesized a series of 5-[2-(substituted amino)-1H-benzimidazol-1-yl]-4H-pyrazol-3-ol derivatives (136–137) and investigated for in vitro cyclooxygenase inhibitory effect using Cayman’s colorimetric COX (ovine) assay. Compounds 136–137 showed remarkable in vitro cyclooxygenase inhibition (IC₅₀ on COX-1: 0.1664 and 0.2272 nM, respectively, and IC₅₀ on COX-2: 0.0370 and 0.0469 nM, respectively) and significant (p < 0.05) reduction of edema volume compared to that of standard diclofenac at all time intervals. Prajapat and Talesara (Prajapat and Talesara, 2016) synthesized several alkoxyphthalimide based benzimidazole derivatives (138–140) and Siddiqui et al. (Siddiqui et al., 2016) prepared a series of 1-{(1-(2-substituted benzyl)-1H-benzo[d]imidazol-2-yl)methyl}-3-arylthioureas (141–143). All the compounds (138–143) exerted notable anti-inflammatory effect compared to standard diclofenac.

A series of Cu(II) and Zn(II) complexes of a 2-[(1H-benzoimidazol-2-ylimino)-methyl]-6-methoxy-phenol Schiff base ligands (144–145) were synthesized from condensation of 2-aminobenzimidazole and o-vanillin (AlAjmi et al., 2016). The both compounds (144–145) at 100 mg/kg b.w. showed around 55% inhibition of inflammation compared to standard diclofenac (65.4% inhibition). Furthermore, Bukhari et al. (Bukhari et al., 2016) reported about anti-inflammatory activity of a series of benzimidazole derivatives where compound 146 was found to be a potent inhibitor of 5-LOX, COX, TNF-α, IL-6 and cytokines among the series. Figure 6 depicts a clear diagram for understanding the association of structural modifications with bioactivities of benzimidazole derivatives against inflammation.

Eswayah et al. (Eswayah et al., 2017) synthesized several N-substituted benzimidazole derivatives and concluded that compound 147 exhibited prominent analgesic activity indicated by decrease in number of writhing at 50 mg/kg dose compared to standard aspirin (17 vs. 12%). Besides, Sharma et al. (Sharma R. et al., 2017) stated that compounds 148–150 demonstrated notable reduction in edema ranging from 92.7 to 97.6% compared to the standard drugs rofecoxib and indomethacin (78.95 and 75%, respectively). Moreover, compound 151 displayed 72% analgesic activity and 67% protection of inflammation at 20 mg/kg dose in second hour in comparison with standard diclofenac (69% analgesic activity and 65% protection of inflammation) (Chikkula and Sundararajan, 2017). Similarly, Rathore et al. (Rathore et al., 2017) synthesized a series of 1-{(5-substituted-1,3,4-oxadiazol-2-yl)methyl}-2-(morpholinomethyl)-1H-benzimidazole derivatives (152–154) and reported that the compound 152 having chloro group at the ortho position of phenyl ring showed promising anti-inflammatory effect compared to standard indomethacin (74.17 ± 1.28% vs. 57.79 ± 1.71%). Besides, compounds 152–154 also produced remarkable COX-2 inhibition (IC₅₀ range = 8–13.7 µM).

By applying Mannich reaction, Sethi et al. (Sethi et al., 2017) prepared a series of N-benzimidazol-1-yl methyl-benzamide derivatives (155–159), having electron-withdrawing groups chloro and bromo at ortho position of phenyl ring (156–158) and chloromethyl group at 2-position of benzimidazole nucleus (155), asserted significant analgesic and anti-inflammatory activities compared to vehicle control group (10% DMSO, p < 0.05). Moreover, Shankar et al. (Shankar et al., 2017) synthesized a series of 2-(6-alkyl-pyrazin-2-yl)-1H-benzo[d]imidazole derivatives (160–163), where the compound 162 showed maximum selectivity towards COX-2 enzyme among the series (% inhibition 78.68 ± 0.46 and selectivity ratio 3.71) and it might be due to the presence of N-phenyl piperzine moiety in the benzimidazole nucleus. Besides, compounds 160, 161 and 163 demonstrated notable activity against COX-2 enzyme (% inhibition 71.45 ± 0.65, 76.93 ± 0.84 and 58.27 ± 0.25, respectively).

Recently, Sethi et al. (Sethi et al., 2018) synthesized two series of benzimidazole based compounds from the coupling of coumarin and benzimidazole nuclei and narrated the anti-inflammatory activity of compounds 164–165 compared to the standard indomethacin (45 vs. 48%). Brishty et al. (Brishty et al., 2020) synthesized a group of substituted benzimidazole derivatives amongst which compounds 166, 167 and 168 exhibited remarkable analgesic activity by inhibition of acetic acid induced writhing of mice compared to standard diclofenac (88.24, 84.03 and 85.71%, respectively, vs. 90.76%; p < 0.001). In continuation of the research by the group, Saha et al. (Saha et al., 2020) prepared a number of disubstituted benzimidazole derivatives. Compounds 169, 170 and 171 displayed notable analgesic property at a dose of 25 mg/kg by 88.81, 69.40 and 64.93% writhing inhibition, respectively (p < 0.05) in comparison with standard aceclofenac (88.81%). Very recently, the group reported the pharmacological investigation of some of their previously synthesized benzimidazoles (Saha et al., 2021). Compounds 170, 172 and 173 exhibited promising central analgesic potential in radiant heat tail flick method compared to standard morphine (% of elongation 58.07, 51.59, and 76.65, respectively vs. 87.17). Besides, 171, 172 and 173 displayed notable reduction in paw edema (81.75, 79.09 and 86.69%, respectively) comparable to standard aceclofenac (87.83) (Saha et al., 2021).

Many benzimidazole derivatives are known to possess potent antiulcer activity and H⁺/K⁺-ATPase inhibitory properties. During recent times, several new synthetic benzimidazole-based compounds were developed which exhibited similar or better antiulcerogenic potentials compared to the established market preparations. The benzimidazole derivatives with antiulcer activity are shown in Figure 7.

A series of pyrimidylthiomethyl benzimidazoles (e.g. 174) and pyrimidylsulfinylmethyl benzimidazole derivatives (e.g. 175) were developed and screened for antiulcer activity. Compounds 174–175 significantly reduced gastric acid secretion, free acidity and gastric ulcers in the pylorus-ligated rats at 10 and 30 mg/kg doses, where the sulfinyl derivative (175) was found to be more effective than thio derivative (174) (Bariwal et al., 2008). In another study, compounds 176–177 exhibited most prominent antiulcer activity against pylorus ligation-induced, aspirin induced, and ethanol induced ulcer in rat model at a dose level of 10 and 20 mg/kg compared to omeprazole (Patil et al., 2010). Besides, Reddy et al. (Reddy et al., 2011) prepared a series of 2-substituted mercaptobenzimidazole derivatives and reported that compounds 178–180 produced notable antiulcer potentiality at a dose level of 10 mg/kg comparable to omeprazole. Furthermore, compound 181 prevented H⁺/K⁺-ATPase enzymatic activity with an IC₅₀ value of 1.6 × 10^–5 M and compound 182 displayed prominent effects on inhibition of gastric lesions and gastric acid secretion in a dose dependant manner (0.3–30 mg/kg) (Tanaka et al., 2011; Yan et al., 2011).

Moreover, some benzimidazole-piperazine conjugated analogues were assessed for their in vivo antiulcer property. The 4-methoxy phenyl piperazine substituted benzimidazole derivative (183) appeared to be the most effective agent (Patil et al., 2012). Chang et al. (Chang et al., 2012) developed a series of 3,4,5-trimethoxybenzylbenzimidazole derivatives among which compound 184 (2-fluorophenyl-5-methyl-1-(3,4,5-trimethoxybenzyl) benzimidazole) emerged as the most potent inhibitor of Helicobacter pylori growth and pathogenesis of host cells. The compound specifically inhibited H. pylori adhesion and invasion of gastric epithelial cells, as revealed by in vitro H. pylori infection model. Mathew et al. (Mathew et al., 2013) synthesized a series of substituted benzimidazole derivatives (185–187) and reported that derivatives 185–187 exerted remarkable protection of ulcer (69.58, 69.56 and 67.17%, respectively) at a dose of 50 mg/kg b.w compared to omeprazole (77.37%, 2 mg/kg b.w.). Amongst a series of substituted methoxybenzyl-sulfonyl-1H-benzo[d]imidazole derivatives, compounds 188–193 appeared to be the most potent H⁺/K⁺-ATPase inhibitors compared to omeprazole (Rajesh et al., 2017). Finally, some new benzimidazole-pyrazole hybrids were evaluated for in vivo anti ulcerogenic activity using ethanol-induced gastric ulcer model in Albino rats. Compound 194 was found to be the most potent among the series with 83.1% ulcer inhibition at a dose level of 500 μg/kg (Noor et al., 2017).

Several benzimidazole derivatives have been explored through years for their capacity to act as antioxidants. Different benzimidazole derivatives with antioxidant activity are shown in Figure 8.

Two series of benzimidazole compounds made through coupling of coumarin derivatives with benzimidazole nucleus either directly or through amide linkage at 2-position were evaluated for antioxidant property (Arora et al., 2014). Compounds 195–197 demonstrated excellent antioxidant activity (IC₅₀ values 19.7, 13.9 and 1.2 µmol/L, respectively) compared to standard butylated hydroxytoluene (BHT, IC₅₀ = 23.4 µmol/L). Among the different benzimidazole derivatives with heterocyclic moieties developed by Mentese et al. (Menteşe et al., 2015), compound 198–199 with a thiophene ring exhibited remarkable antioxidant activity. Poddar et al. synthesized and evaluated the antioxidant property of substituted benzimidazoles by 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging method (Poddar et al., 2016). Compound 200 and 201 exhibited mild to moderate antioxidant potential (IC₅₀ = 400.42 and 144.84 µg/ml, respectively) in comparison with standard BHT (IC₅₀ = 51.56 μg/ml).

Among a library of N-substituted pyrazole-containing benzimidazoles, compounds 202–204 attributed prominent antioxidant activity in both DPPH and hydrogen peroxide assay supposed to the presence of benzyl substituent on imidazole nitrogen (Bellam et al., 2017). Besides, Anastassova et al. (Anastassova et al., 2018a) evaluated antioxidant property of compounds 205–206 using tert-butyl hydroperoxide (tert-BOOH) induced oxidative stress on rat hepatocytes, and reported that both compounds showed significant effect comparable to standard quercetin. Similarly, compounds 207–212 displayed notable cytoprotective effect on rat hepatocytes (Anastassova et al., 2018b).

Moreover, a library of 2-(4-nitrobenzyl)-1H-benzimidazole derivatives showed good antioxidant property where compounds 213 and 214 demonstrated prominent inhibitory effect against xanthine oxidase (IC₅₀ = 12.30 ± 0.33 μg/ml) and urease (IC₅₀ = 13.04 ± 0.89 μg/ml), respectively (Karaali et al., 2018). Furthermore, compound 215 had the most potency in the series of 2-(aryl)-6-morpholin-4-yl(or 4-methylpiperazin-1-yl)-1H-benzimidazoles (Özil et al., 2018).

Recently, Baldisserotto et al. (Baldisserotto et al., 2020) synthesized total 39 arylbenzimidazole derivatives and reported their remarkable potency against various free radicals. Figure 9 represents a general structure for describing SAR of benzimidazoles possessing antioxidant activity. The addition of cyano or carboxyl group at 5-position (R₁) of the benzimidazole nucleus was responsible for exhibiting medium to high potency against several free radicals. In contrast, the derivatives containing the 5-sulfonic acid group showed poor or no antioxidant properties. Unsubstituted 2-aromatic ring or OH, Cl, Br, 2-OH-napthyl or 4-OH-steryl substitutions enhanced activity. In another study, compound 216 showed 40–80% antioxidant potential at different concentrations (10–100 µM) (Abdelgawad et al., 2019). Amine Khodja et al. reported that compounds 217–220 exerted promising inhibition capacity against various free radicals compared to BHT (IC₅₀ (mean ± SD, µM) for DPPH assay: 40.4 ± 0.9 to 60.4 ± 1.9 vs. 70.8 ± 6.6) (Amine Khodja et al., 2020). Taha et al. (Taha et al., 2020) synthesized 20 benzimidazole derivates and found four potent antioxidant compounds 221–224 with IC₅₀ values (mean ± SEM: 22.42 ± 0.26 to 40.60 ± 0.80) comparable to standard propyl gallate (29.20 ± 1.25). Interestingly, compound 225 exhibited more inhibition (%) of DPPH-free radical than the standard antioxidant Trolox (73% ± 2.42 vs. 70% ± 0.35) (Ramos Rodríguez et al., 2020). Furthermore, compounds 166, 168, 226 and 227 displayed prominent antioxidant property with lower IC₅₀ values than the standard BHT (8.834, 7.519, 0.038 and 0.959 μg/ml, respectively vs. 14.44 μg/ml) (Brishty et al., 2020). Finally, compounds 228 and 229 demonstrated mild antioxidant potential in comparison with standard ascorbic acid (IC₅₀ = 12.25 × 10³ and 87.326 × 10³ μg/ml, respectively vs. 2.19 μg/ml) (Saha et al., 2021).

Among the anticancer drugs discovered in the recent years, different benzimidazole derivatives occupy an important place. The current review accounts the anticancer activity of benzimidazoles reported after 2013. The benzimidazole derivatives with anticancer activity are shown in Figure 10.

A series of substituted benzimidazole derivatives were evaluated for in vitro anticancer activity in human lung adenocarcinoma A549 cell line at normoxic and hypoxic conditions. Compound 230 was found to be the most cytotoxic agent with hypoxia/normoxia cytotoxic coefficient of 4.75, compared to standard tirapazamine (5.59) (Błaszczak-Świątkiewicz et al., 2014). The benzimidazole-thiazole derivatives 231–232 showed notable anticancer effect against human liver carcinoma cell line (HepG2: IC₅₀ = 0.518 and 0.578 mM) and pheochromocytoma of the rat adrenal medulla cell line (PC12: IC₅₀ = 0.309 and 0.298 mM) (Nofal et al., 2014). Compounds 233–234 with dual inhibition of Aurora A kinase and kinesin spindle protein were found to be the most prominent antitumor agents against various tested cell lines in comparison with standard drug CK0106023 (Abd El-All et al., 2015). Kalalbandi and Seetharamappa synthesized a series of 1-[(2E)-3-phenylprop-2-enoyl]-1H-benzimidazole derivatives among which compound 235 displayed notable antiproliferative activity against nine tumor subpanels, indicated by its selectivity ratios within the range of 0.79–1.53 and 0.47 to 1.69 at the GI₅₀ (growth inhibitory 50%) and TGI level, respectively (Kalalbandi and Seetharamappa, 2015). Similarly, compounds 236–238 with GI₅₀ values of 9.79, 2.58 and 3.81 µM, respectively exhibited broad spectrum antitumor activities, while the compound 236 was found to be the most potent DHFR inhibitor with IC₅₀ value of 1.05 µM (Singla et al., 2015).

Among the derivatives identified by Liu et al. (Liu et al., 2015), compound 239 showed notable inhibition of PI3K-AKT-mTOR pathway having GI₅₀ values in the range of 0.07–0.41 μmol/L against most of the tested cancer cell lines, signifying its potential to be used as anticancer agent. Sontakke and co-workers (Sontakke et al., 2015) synthesized 2-anthryl benzimidazole derivatives (240–242) bearing hydrogen, benzoyl and carboxyl substituents, respectively at 5^th position. Compounds 241 and 242 showed anticancer potency against MCF-7 cell lines (IC₅₀: 16.18 and 19.21 µM, respectively) and HL-60 cell lines (IC₅₀: 15.15 and 18.29 µM, respectively) followed by compound 240 (MCF-7 IC₅₀ = 20.48 µM and HL-60 IC₅₀ = 23.23 µM). In another study, compound 243 displayed most notable activity against HeLa cell lines (IC₅₀ = 05.34 ± 1.2 µM) compared to standards doxorubicin and 5-flurouracil (IC₅₀ = 03.56 ± 2.7 and 02.78 ± 2.6 µM, respectively). On the other hand, compound 244 showed marked activity against Hep3B cell line with an IC₅₀ value of 11.10 ± 1.1 µM (Varshney et al., 2015).

Bhambra et al. (Bhambra et al., 2016) synthesized a library of fluoroaryl benzimidazole derivatives (245–248) among which compounds 246–248 demonstrated inhibition against K-562 and MCF-7 cell lines in micromolar range. Compounds 246 and 248 were reported as activators of caspases, which play important role in apoptosis of cancerous cells. Bramhananda Reddy et al. designed and synthesized a library of benzimidazole fused ellipticine derivatives (249–253) and delineated antiproliferative potential against human cancer cell lines Zr-75–1, HeLa, MCF-7 and A-549 with GI₅₀ values of <0.1–34.6 µM, compared to standard etopoxide (GI₅₀ = 0.2–3.08 µM) (Bramhananda Reddy et al., 2016). Sharma et al. (Sharma P. et al., 2017) designed and synthesized a series of benzimidazole bearing thiazolidinedione derivatives, and proved remarkable cytotoxicity of these compounds (254–256) towards PC-3, HeLa, A549 and HT1080 cancer cell lines with IC₅₀ values of 0.096–0.63 µM. In a different study, Compound 257 exhibited potent antiproliferative effect against MFC cells IC₅₀ (mean ± SD) value of 25.72 ± 3.95 μM (Wang et al., 2017). Triazole containing 4,5,6,7-tetrabromo-1H-benzimidazole derivatives 258–259 bearing carboxyl substituent manifested the most prominent inhibitory effect against protein kinase 2 (CK2) with binding affinity value in the range of 1.96–0.91 µM (Chojnacki et al., 2017). Compounds 260–264 exerted antiproliferative property in MTT assay against five human cancer cell lines, breast (T47D), lung (NCl H-522), liver (HepG2), colon (HCT-15) and ovary (PA-1) with IC₅₀ (mean ± SD) value of 7.5 ± 0.3 to 14.6 ± 0.4 μM (Kumar et al., 2018). Upon assessment of antiproliferative property against four cancer cell lines (HeLa, MCF-7, A549 and DU-145 alongside normal HEK-293 cell line), Baig et al. (Baig et al., 2018) enumerated noteworthy IC₅₀ (1.08 µM) of derivative 265 against A549 cell line. Compounds 266–268 displayed prominent cytotoxic activity against A549 and MCF-7 cancer cell lines in comparison with standard drug cisplatin with IC₅₀ values of 0.03–0.06 μM. The presence of 2,4-dichlorobenzylidene (266), 2-nitrobenzylidene (267) and 2-methoxybenzylidene moiety (268) contributed for notable cytotoxic activity of these compounds (Acar Çevik et al., 2018). The oxetanyl substituted compound 269 exhibited cytotoxicity towards a wide range of cancer cell types, e. g. lung, prostate and ovarian cancers with prominent activity against highly aggressive cancer lines (IC₅₀ = 0.9–3.8 µM) (Cheong et al., 2018). Besides, Ibrahim et al. synthesized 2-substituted-5-nitro-benzimidazole derivative 270 as dual inhibitors of c-Met and VEGFR-2 kinases which is important therapeutic target in the treatment of lung (IC₅₀ 2.19 ± 0.09 against A549) and colorectal (IC₅₀ 10.97 ± 0.09 µM against HCT116) cancers (Ibrahim et al., 2018).

Recently, Djemoui et al. (Djemoui et al., 2020) synthesized several triazole-benzimidazole-chalcone hybrid compounds 271–274 and narrated potential anti-proliferative property of these derivatives against two breast cancer (T47-D and MDA-MB-231) and one prostate cancer cell line (PC3) compared to standard Doxorubicin. It is mentionable here that the chloro substituent 274 at the chalcone ring proliferated the anticancer effects. In a distinct research, a total of 24 new molecules containing benzimidazole group, arene, and alkyl chain-bearing cyclic moieties were synthesized, where the compound 275 impeded the growth of MCF-7 and human ovarian carcinoma (OVCAR-3) cell lines manifesting superior effects to standard cisplatin (IC₅₀ (mean ± SD, µM): 8.91 ± 0.07 vs. 11.7 ± 0.12 and 10.76 ± 0.12 vs. 16.04 ± 0.74, respectively) (Hsieh et al., 2019). A copper (II) complex (276) of benzimidazole derivatives showed excellent potency at 72 h post treatment against prostate cancer cell line (DU145) with IC₅₀ 10 µM (Kacar et al., 2020). Besides, two zinc (II) complexes with 2-[2-(benzimidazol-2-yl)-phenyl]-1-methyl-benzimidazole and 1,2-bis(1-methyl-benzimidazol-2-yl)-benzene exerted both dose and time dependent cytotoxicity against breast cancer cell lines (MB-MDA-231) (Su et al., 2019). In another study (Yılmaz et al., 2019), 18 complexes of zinc (II) and cobalt (II) containing 1-benzyl and 2-phenyl moieties showed in vitro potency against human prostate (DU-145) and human ovarian (A-2780) cancer cell lines. Notably, compounds 277–280 at a concentration of 0.1 µM exhibited superior activity against A-2780 cell line in comparison with standard docetaxel. Jian-Song et al. (Jian-Song et al., 2019) synthesized a spectrum of unconventional BZD derivatives and reported their in vitro anticancer property, particularly against three genre of cell lines (MGC-803, PC-3, MCF-7). Notably, compound 281 inhibited predominately of all the three cancer cell lines compared to 5 Fluorouracil (IC₅₀ (mean ± SD, µM): 1.02 ± 0.03 vs. 6.82 ± 1.17, 3.34 ± 0.09 vs. 18.42 ± 1.73, and 5.40 ± 0.51 vs. 17.11 ± 2.94, respectively). The structural modifications of the compound 281 might significantly influence its anti-proliferative property that has been illustrated in Figure 11. In another study (Suk et al., 2019), a benzimidazole derivative carrying a pyrrolidine side chain (282) significantly suppressed sorafenib-resistant cell lines growth in xenograft model by inhibiting the phosphorylation of AKT, p70S6 and the downstream molecule RPS6, has unlocked another milestone in the treatment of hepatocellular carcinoma. Yeong et al. (Yeong et al., 2019) synthesized several new benzimidazole derivatives 283–287 and screened them against sirtuin cancer lines (SIRT1, SIRT2, and SIRT3). Among them, compound 284 elicited significant inhibition of SIRT1-3 compared to tenovin-6 (IC₅₀ (mean ± SD, µM): 7.7 ± 1.4 vs. 42.10, 5.6 ± 1.3 vs. 25.6, and 9.8 ± 2.0 vs. 82.65). Moreover, among 37 synthesized molecules, compound 288 exerted the most inhibition of angiogenesis (79%), and HUVEC and HepG2 cell lines (IC₅₀: 1.47 and 2.57 mM, respectively), and VEGFR-2 kinase inhibition (Yuan et al., 2019).

Akkoc et al. (Akkoç et al., 2020) synthesized three benzimidazole derivatives 289–291 and reported the most promising anti-breast cancer feature of compound 291 compared to standard cisplatin (IC₅₀ (mean ± SD, µM): 1.26 ± 0.85 vs. 5.77 ± 0.40). In another study, Atmaca et al. (Atmaca et al., 2020) disclosed significant cytotoxicity of compound 292 against breast cancer (MCF-7), prostate cancer (DU-145), and lung cancer (H69AR) with IC50 values of 17.8 ± 0.24, 10.2 ± 1.4 and 49.9 ± 0.22 μg/ml, respectively, compared to 5-Fluorouracil. Compounds 293–294 showed stronger anticancer property against HepG2 (IC₅₀: 26.62 and 20.29, respectively) and DLD-1 cells (IC₅₀: 21.29 and 19.23 µM, respectively) than cisplatin (IC₅₀: 30.38 and 60.79 µM) (Caymaz et al., 2020). Particularly, compound 295–296 demonstrated potent anti-breast cancer effect by obstructing MCF-7 cell growth compared to standard cisplatin (IC₅₀: 0.016 ± 0.001 and 0.018 ± 0.001 vs. 0.020 ± 0.009 µM). Besides, compound 295 displayed efficiency for impeding estrogen-dependent breast cancer by inhibiting aromatase enzyme with IC₅₀ 0.032 ± 0.042 µM, compared to IC₅₀ 0.024 ± 0.001 µM for letrozole (Acar Çevik et al., 2020). Furthermore, compound 297 blocked neddylation process with superior anticancer property compared to candesartan cilexetil (IC₅₀ 5.51 vs 16.43 mM) (Chen et al., 2021). Notably, compound 298 displayed excellent effect in the treatment of lung cancer by inhibiting A-549 and NCI-H460 cell lines growth with IC₅₀ level of 0.63 ± 0.21 μM and 0.99 ± 0.01 μM, respectively, compared to 5-Fluorouracil (IC₅₀ (μM): 1.69 ± 0.90 and 3.20 ± 0.50, respectively). Apart from, the compound 298 significantly suppressed the breast cancer cell lines MCF-7 and MDA-MB-23 with IC₅₀ (μM) values, comparable to 5-Fluorouracil (1.3 ± 0.18 vs. 2.80 ± 0.12, and 0.94 ± 0.02 vs. 0.79 ± 0.09, respectively) (Sridhar Goud et al., 2020).

Meguid et al. (El-Meguid et al., 2020) synthesized an array of novel 6-benzoyl benzimidazole derivatives where most of the compounds exhibited promising anticancer activity with safety profile. Remarkably, compounds 299–300 exhibited superior inhibition of EGFR, HER2, PDGFR-β and VEGFR2, in comparison to erlotinib that opened several promising fighting tools against cervical cancer. In a distinct study, compounds 301–304 strongly prevented breast cancer cell lines manifesting IC₅₀ values of 5.70 9.55, 5.58 and 6.84 μg/ml, respectively compared to standard doxurubucin (IC₅₀ at 4.17 μg/ml) (Nashaat et al., 2020). Furthermore, Sireesha et al. (Sireesha et al., 2021) synthesized a library of hybrid β-carbolines 305–311 that were found to be effective against various cancer cell lines where compounds 306–307 exerted higher in vitro efficacy than the reference etoposide to prevent breast (IC₅₀ against MCF-7: 0.092 ± 0.001 and 0.81 ± 0.062, vs. 2.11 ± 0.024), lung (IC₅₀ against A549: 0.72 ± 0.042 and 1.90 ± 0.88, vs. 3.08 ± 0.135), colon (IC₅₀ against Colo-205: 0.34 ± 0.071 and 0.41 ± 0.12, vs. 0.13 ± 0.017), and ovarian (IC₅₀ against A2780: 1.23 ± 0.55 and 1.80 ± 0.59, vs. 1.31 ± 0.27) cancers. Similarly, Srour et al. (Srour et al., 2020) synthesized a series of benzimidazole derivatives 312–318 and reported promising anticancer potential of 312–316 against breast cancer (IC₅₀ against MCF-7: 5.96–11.91 μM, vs. IC₅₀ of erlotinib; 4.15 μM). Besides, compounds 312, 314, 316, 317 and 318 showed significant cytotoxicity against epidermal growth factor receptor tyrosine kinase with IC₅₀ values of 71.67–152.59 nM compared to IC₅₀ of standard erlotinib 152.59 nM. Finally, Yamani et al. (Yamani et al., 2021) applied scaffolds hybridization technique to formulate a total of 24 pyrazole-benzimidazole derivatives for blocking fibroblast growth factor receptors (FGFRs). Amongst the derivatives, compound 319 selectively inhibited FGFR (1–4) with IC₅₀ values of 0.75, 0.50, 3.05, and 87.90 nM, respectively. Due to acceptable safety and pharmacokinetic profiles along with in vivo anti-tumor potency, the compound 319 is now undergoing with an open-label, multicenter, dose-escalation phase I clinical trial for assessing the safety and tolerability against the adults patients with bladder, gastric, and squamous cell lung cancers (NCT04149691) (Yamani et al., 2021).

Compounds containing heterocyclic moieties, such as pyrrole, imidazole and benzimidazole have been reported to demonstrate excellent antitubercular properties (Wang et al., 2015). Benzimidazole scaffold has been on target of the scientists for producing novel antitubercular agents. Different benzimidazole derivatives with antitubercular property are shown in Figure 12.

Warekar et al. (Warekar et al., 2016) synthesized a series of 4-(4-nitro-phenyl)-2-phenyl-1,4-dihydro-benzo[4,5]imidazo[1,2-a]pyrimidine-3-carboxylic acid ethyl ester derivatives and reported that compounds 320–321 appeared to be the most promising antitubercular agents among the series with minimum inhibitory concentration (MIC) value of 25 μg/ml against Mycobacterium tuberculosis H37Rv strain. Compound 322 also displayed good activity against same strain under aerobic conditions, as indicated by its IC₉₀ and MIC values (77 µg/ml and >100 μM, respectively) (Mantu et al., 2016). In a different study (Anguru et al., 2017), compounds 323–328 exhibited good activity against M. tuberculosis H37Rv strain while compound 325 emerged as the most promising agent with MIC value of 16 μg/ml.

A number of substituted fluorobenzimidazole derivatives (329–330) were synthesized and evaluated for in vitro antimycobacterial property against pathogenic M. tuberculosis H37Rv strain (ATCC 27294) using MABA method. Compounds 329–330 exhibited notable antitubercular activity against H37Rv strain and their activity was contributed by the incorporation of methylenedioxyphenyl moiety at 2- and 6-position of benzimidazole ring (Nandha et al., 2017). Harika et al. (Harika et al., 2017) synthesized a series of 2-substituted benzimidazole derivatives (331–333) using condensation of o-phenylenediamine with different aliphatic, aromatic, fatty acids, and amino acids, and depicted remarkable antitubercular property against M. tuberculosis H37Rv strain compared to reference drugs pyrazinamide, streptomycin and ciprofloxacin. In addition, Yeong et al. (Yeong et al., 2017) designed two series of benzimidazole derivatives among which compound 334 having trifluoromethyl group displayed antimycobacterial effect against both M. tuberculosis H37Rv strain and the drug-resistant-tuberculosis strain. Compounds 335–336, synthesized by Prasad and Sundararajan, exhibited notable antitubercular activity against M. tuberculosis H37Rv strain with MIC value of 3.9 µg/ml compared to the standard isoniazid (Prasad and Sundararajan, 2017).

Recently, Ashok et al. (Ashok et al., 2018) synthesized a series of indole-benzimidazole-based 1,2,3-triazole hybrids (337–339) by conventional and microwave-assisted methods and evaluated for in vitro antitubercular activity against M. tuberculosis H37Rv strain. The derivatives 337–339 showed prominent antitubercular activity with MIC values in the range of 3.125–6.25 μg/ml. Compound 338 was the most potent among all (MIC = 3.125 μg/ml) which was likely due to the presence of nitro group at ortho position of phenyl ring. Compound 340 displayed notable activity (MIC = 0.05 μg/ml) and emerged as a promising antitubercular agent among the reported series (Shaikh et al., 2018). Compound 341 showing 67.56, 53.45, and 47.56% inhibition against mycobacterial enzymes isocitrate lyase, pantothenate synthetase and chorismate mutase, respectively appeared to be the most potent antitubercular agent among the series (Yadav et al., 2018). Very recently, compound 342 exerted excellent potency against M. tuberculosis H37Rv with IC₅₀ value of 0.78 mg/ml compared to standard ethambutol (IC₅₀ 1.56 mg/ml) (Sirim et al., 2020). Gobis et al. (Gobis et al., 2015) have prepared a series of compounds containing novel 2-(2-phenalkyl)-1H-benzo[d]imidazole where the compound bearing methyl groups at the benzimidazole system and phenethyl substituent at the C-2 position with electronegative chlorine atom at the phenyl ring exhibited prominent tuberculostatic property against Mycobacterium tuberculosis strains with MIC values ranging from 0.8 to 1.6 μg/ml (Figure 13).

The exploration of benzimidazole nucleus to discover new structural features required for the optimization of novel antiprotozoal agents is of utmost importance. Benzimidazole derivatives with antileishmanial, antimalarial, and antiprotozoal activities against different species are shown in Figure 14.

A number of marketed antihypertensive drugs comprise benzimidazole moiety, Candesartan cilexetil and Telmisartan are two major examples (Figure 2). Categorically they are the antagonists of angiotensin II receptor playing important role in managing hypertension (Keri et al., 2015). In recent years, a number of scientists have conducted research to prepare benzimidazole based novel antihypertensive agents which provided similar or even better efficacy than the conventional types of antihypertensive drugs. Different benzimidazole derivatives with antihypertensive activity are shown in Figure 16.

Sharma et al. (Sharma et al., 2010) synthesized a series of substituted benzimidazole derivatives and evaluated for their property as angiotensin II receptor antagonists or sartans using invasive (direct) method in Wister rats. Compounds 379 and 380 appeared to be the most prominent antihypertensive agents from the series compared to standard losartan. Kusumoto et al. (Kusumoto et al., 2011) reported about a novel, long-lasting and potent AT₁ blocker azilsartan medoxomil and its active metabolite azilsartan (381) and investigated its pharmacological profile in rat and dog models. Oral administration of 0.1–1 mg/kg azilsartan medoxomil in spontaneously hypertensive rats (SHRs) and renal hypertensive dogs demonstrated better effect in reduction of blood pressure at all doses compared to standard drug olmesartan medoxomil. Abou-Seri et al. (Abou-Seri et al., 2011) synthesized a series of 2-alkoxy-4-aryl-6-(1H-benzimidazol-2-yl)-3-pyridinecarbonitrile derivatives. All compounds in the series displayed significant vasodilation properties. Compounds 382–385 showed most prominent activity (IC₅₀ = 0.145, 0.202, 0.210, and 0.214 mM, respectively) compared to standard prazosin hydrochloride (IC₅₀ = 0.487 mM).

Datani et al. (Datani et al., 2012) synthesized a series of twenty novel 5-nitro benzimidazole analogues and screened them for ex vivo vasorelaxant property in rat aorta rings pre-contracted with phenylephrine. Compounds 386–390 exhibited prominent vasorelaxant activity with EC₅₀ value less than 30 μM. Among a new set of 5-nitro benzimidazole derivatives, compound 391 emerged as the most active agent against AT₁ with IC₅₀ value of 1.03 ± 0.26 nM (Zhu et al., 2014). The presence of butyl chain on 2-position of benzimidazole moiety (391) helped the derivative to interact and bind tightly with lipophilic pocket of the receptor. Several benzimidazole derivatives containing indazole moiety were synthesized by Lamotte et al. (Lamotte et al., 2014) among which compound 392 displayed potent AT₁ receptor antagonism as indicated by IC₅₀ value (0.006 mM).

Two series of nitric oxide (NO) releasing benzimidazole derivatives were synthesized by coupling benzimidazole biphenyl skeleton with nitro ester and furoxan NO-donor moieties, where compounds 393–394 were reported to possess comparable activity to positive control losartan (Zhang et al., 2015). Hao et al. (Hao et al., 2015) designed and synthesized a series of 4′-[(benzimidazol-1-yl)methyl]biphenyl-2-sulphonamide derivatives and reported that compound 395 was found to be the most potent AT₁ and Endothelin ET_A receptor antagonist with IC₅₀ values 28 and 10 nM, respectively. Upon the identification of a new series of benzimidazole oxazolidinediones as mineralocorticoid receptor (MR) antagonists by high-throughput screening, compound 396 showed similar efficacy as standard drug spironolactone at a dose of 100 mg/kg (p.o.) in rat natriuresis model (Yang et al., 2015).

Bao et al. (Bao et al., 2017) synthesized a series of benzimidazole derivatives which reduced blood pressure in dose-dependent manner in spontaneously hypertensive rats. Among the series, compound 397 exhibited long-lasting efficiency in decreasing blood pressure, with a maximal lowered response of 35.82 ± 6.20 mmHg at 5 mg/kg and 55.98 ± 4.74 mmHg at 10 mg/kg. The compound also showed potent affinity towards AT₁ receptor compared to standard telmisartan with IC₅₀ value of 1.13 ± 1.68 nM. In a separate study, Khan et al. (Khan et al., 2018) synthesized a series of 2-phenyl substituted benzimidazoles and assessed antihypertensive activity of these derivatives by using tail cuff method and confirmed excellent antihypertensive property of compound 398 in spontaneously hypertensive rats compared to standard losartan. A very recent report (Yang et al., 2020) ascertained the promising pulmonary hypotensive effect of compound 399 with excellent pharmacokinetic profile in comparison with tadalafil. Finally, compounds 400–401 showed superior inhibition of AT1 receptor (IC₅₀ (mean ± SEM): 0.8 ± 0.1 and 2.3 ± 0.7, respectively) than the both standard losartan and telmisartan (Wu et al., 2020).

Several benzimidazole based compounds have displayed promising antidiabetic activity by acting as targets of varied stages of carbohydrate metabolism and some of them have been marketed for the treatment of type 2 diabetes. The structure of benzimidazole derivatives with antidiabetic activity reported within recent years are shown in Figure 17.

Several 2-(pyridine-2-yl)-1H-benzimidazole derivatives were synthesized by Ishikawa et al. (Ishikawa et al., 2009) among which compound 402 emerged as the most potent and metabolically stable glucokinase activator. Besides, the compound proved its oral glucose lowering efficacy in rat oral glucose tolerance test (OGTT) model. A new series of 4-thiazolidinones and 1,3,4-oxadiazoles bearing 2-mercapto benzimidazole moiety were prepared by Shingalapur et al. (Shingalapur et al., 2010) among which compounds 403–406 produced notable results in OGTT model. All the potent derivatives contained hydroxyl group which might have contributed for their antidiabetic property. Some 2-benzoyl benzimidazole derivatives were assessed for antidiabetic and lipid-lowering effects (Ushiroda et al., 2011). Compound 407 appeared to be an effective peroxisome proliferator-activated receptor (PPARγ) partial agonist. Shaikh et al. (Shaikh et al., 2012) reported one-pot synthesis of carbonyl-amide linkage based benzimidazole derivatives (408–413) via Passerini reaction and demonstrated their antidiabetic potential using rat OGTT model compared to the standard drug glibenclamide.

Diacylglycerol O-acyltransferase-1 (DGAT-1) is an enzyme which catalyzes the formation of triglycerides from diacylglycerol and Acyl-CoA and thereby plays an important role in intestinal fat absorption. DGAT-1 has emerged as an important therapeutic target for the management of different metabolic disorders, such as obesity and diabetes (Birch et al., 2010). Kwak et al. (Kwak et al., 2013) prepared some benzimidazole derivatives as inhibitors of DGAT-1 which contained a phenylcyclohexyl acetic acid group in benzimidazole moiety. Among the derivatives, compound 414 displayed potent in vivo antidiabetic potential in a 4-week study with diet-induced obesity (DIO) mouse model.

The α-glucosidase inhibitors are an important class of antidiabetic agents because of their property to reduce the postprandial glucose level in type-2 diabetic patients (Özil et al., 2016). Mobinikhaledi et al. (Mobinikhaledi et al., 2015) synthesized several new benzimidazole derivatives from amino acids in the presence of phosphorus oxychloride (POCl₃). Compounds 415 and 416, upon evaluation of yeast and rat intestinal α-glucosidases inhibitory effect produced most notable results. The IC₅₀ values for compound 416 against yeast and rat intestinal α-glucosidases were reported as 9.1 and 36.7 μM, respectively and thus it appeared to be the most potent benzimidazole among the series. Özil and co-workers (Özil et al., 2016) prepared a series of bis-benzimidazole derivatives by the reaction of o-phenylenediamine and 4-nitro-o-phenylenediamine with oxalic acid using both conventional and microwave techniques. Compounds 417–420 demonstrated prominent α-glucosidase inhibition with IC₅₀ values of 0.54 ± 0.01, 0.44 ± 0.04, 1.24 ± 0.05 and 0.49 ± 0.01 µM, respectively compared to standard acarbose (IC₅₀ 13.34 ± 1.26 µM).

Some novel 1,3,4-thiadiazole substituted 2-methyl benzimidazole derivatives were synthesized by conventional methods among which compound 421 exhibited significant in vitro antidiabetic property (Nair et al., 2016). A series of hybrid benzimidazole-thiourea derivatives were synthesized by Zawawi et al. (Zawawi et al., 2017) and evaluated for α-glucosidases inhibitory potential. Compounds 422–423 displayed significant inhibitory properties with IC₅₀ values of 50.57 ± 0.81 and 35.83 ± 0.66 µM, respectively. Besides, Singh et al. (Singh et al., 2018) synthesized a library of N-substituted-benzimidazolyl linked para substituted benzylidene derivatives. Compound 424 bearing 2,4-thiazolidinedione group at 4-position of phenyl ring exhibited pronounced in vitro α-amylase and α-glucosidase inhibitory properties (IC₅₀ = 0.54 ± 0.01 µM) and appeared as a promising antidiabetic agent from the series.

Recently, Bharadwaj et al. (Bharadwaj et al., 2018) have developed several novel benzimidazole derivatives (425–430) and reported excellent antidiabetic activity by applying α-glucosidase inhibitory method with a range of IC₅₀ = 0.66 ± 0.05 to 3.79 ± 0.46 μg/ml compared to standard IC₅₀ value of acarbose (1460.28 ± 244.365). Besides, two potent AMP-activated protein kinase (AMPK) activators (431–432) with multi-target antidiabetic property were reported against LPS-activated murine peritoneal macrophages (Babkov et al., 2019). In another study, two molecules, 1,3-disubstituted-benzimidazole-2-imine (433) and 1,3-thiazolo[3,2-a]benzimidazolone derivative (434), exerted dual effects against dipeptidyl peptidase-IV (DPP-IV) and xanthine oxidase (XO) enzymes with IC₅₀ values less than 200 μM (Tomovic et al., 2020). Finally, Kanwal et al. (Kanwal et al., 2020) prepared a library of benzimidazole-benzothiazine hybrid molecules by Gabriel–Colman rearrangement of methyl 2-(1,1-dioxido-3-oxobenzo[d]isothiazol-2(3H)-yl) acetate, and reported that compounds 435–436 exhibited potential antidiabetic property by inhibiting Ecto-nucleotide pyrophosphatases/phosphodiesterases 1 (ENPP1) 1 and -3.

Different substituted benzimidazole derivatives have been explored for several years for their anticoagulant activity and potential use in clinical practice. Benzimidazole derivatives acting as anticoagulants are shown in Figure 18.

Thrombin (fIIa) is a multifunctional serine protease responsible for the proteolytic cleavage of fibrinogen. Inhibition of thrombin is a crucial mechanism for inhibition of coagulation. Benzimidazole moiety serves as a suitable template for placing a wide range of substituents required for interaction with thrombin (Hauel et al., 2002). Hauel et al. (Hauel et al., 2002) designed and synthesized a series of new benzimidazole derivatives bearing structural similarity to α-NAPAP (N-alpha-(2-naphthylsulfonylglycyl)-4-amidinophenylalanine piperidide), a benzamidine-based powerful inhibitor of thrombin, trypsin and other serine proteases. With the addition of ethyl ester and hexyloxycarbonyl carbamide hydrophobic side chains, improved pharmacokinetic profile was obtained leading to the invention of orally absorbed prodrug, 437 (Dabigatran etexilate). The prodrug 437 reached clinical trials and its active form 438 (Dabigatran) was discovered with excellent thrombin inhibitory potency and tolerability.

Recently, Ren et al. (Ren et al., 2016) designed a series of benzimidazole derivatives and tested them for thrombin inhibitory effects. Compound 439 with IC₅₀ value of 3.11 ± 0.21 nM appeared to be a potent thrombin inhibitor exhibiting better activity than the standard argatroban (IC₅₀ 9.88 ± 2.26 nM). A series of 1,2,5-trisubstituted benzimidazole fluorinated derivatives (440–442) were also evaluated for in vitro inhibitory activity against thrombin (Yang et al., 2016). Compounds 440–442 with IC₅₀ values of 2.26 ± 0.38, 1.54 ± 0.09 and 3.35 ± 0.87 nmol/L, respectively showed improved result compared to argatroban (IC₅₀ 9.88 ± 2.26 nmol/L), thus showing their potential as thrombin inhibitors. A library of benzimidazole derived 1,3,4-oxadiazole derivatives (443–445) were assessed for ex vivo anticoagulant activity by determining the effect of compounds in increasing prothrombin time (PT) and activated partial thromboplastin time (aPTT) (Vishwanathan and Gurupadayya, 2015). Compounds 443–445 displayed significant increase in PT (32 ± 0.7, 36 ± 0.5 and 41 ± 0.4 s, respectively) compared to standard drug acenocoumarol (48 ± 0.5 s). The compounds, however, caused a slight increase in aPTT in comparison with the reference drug, unfractionated heparin (500 IU/kg).

Factor IXa (fIXa), an important coagulation factor, is a useful target for developing potent and selective antithrombotic agents. A research involving pharmacophore modelling of a new series of benzimidazole analogues presented the chemical features necessary for designing fIXa inhibitors and showed that benzimidazole derivatives have the potential to be developed into effective antithrombotic agents (Kumbhar et al., 2017). Compound 446 was found to be the most active compound from the series, indicated by fIXa binding affinity (K_i) value of 0.016 µM.

Recently, Zhang et al. (Zhang et al., 2020) designed and synthesized ten novel dabigatran derivatives (447–456) with high docking score. The study uncovered that all the compounds showed more than 50% in vitro thrombin inhibitory property at 1 mg/ml concentration, where the IC₅₀ values of compounds 447, 450 and 456 were 1.92, 2.17 and 1.54 nM, respectively, comparable to the IC₅₀ value of positive control dabigatran (1.20 nM). The derivatives 425 and 428, previously reported in this review for antidiabetic property, exerted anticoagulant activity by augmenting the clotting duration. However, only compound 425 exhibited excellent inhibition (93.4%) of epinephrine-induced platelet aggregation (Bharadwaj et al., 2018).

Epilepsy is one of the most prevalent and serious neurological disorders, and recurrent seizures or convulsions are its characteristic syndrome. Around one-third of patients in the world show poor response to currently available antiepileptic drugs (Keri et al., 2015). In search of novel clinically effective anticonvulsant medications, benzimidazole nucleus has recently been explored by scientists with promising results. The benzimidazole derivatives with anticonvulsant property are shown in Figure 19.

A library of 2-mercaptobenzimidazole derivatives were evaluated for anticonvulsant activity using maximal electroshock seizure (MES) model. The synthesized compounds displayed anticonvulsant property at a dose of 20 mg/kg (i.p.) compared to standard phenytoin and compounds 457–458 appeared to be the most potent of all (Anandarajagopal et al., 2010). A series of 4-thiazolidinones and 1,3,4-oxadiazoles bearing 2-mercaptobenzimidazole moiety were assessed for in vivo anticonvulsant activity (Shingalapur et al., 2010). Compounds 459–462 emerged as the most promising anticonvulsants in MES model.

Siddiqui and co-workers described the synthesis of several 1-{(1-(2-substituted benzyl)-1H-benzo[d]imidazol-2-yl)methyl}-3-arylthioureas which have been mentioned earlier for their analgesic potential. The research group in a previous study reported about the anticonvulsant and cytotoxic effects of the same series of compounds. Compounds 463–465 were found to possess potent anticonvulsant property in comparison with standard drug phenytoin (Siddiqui and Alam, 2010). A series of nitro-benzimidazole derivatives were synthesized by Jain et al. and screened for anticonvulsant activity using MES and subcutaneous pentylenetetrazole (scPTZ) models. Compound 466 displayed the most promising result in inhibiting convulsion induced in mice by both methods (Jain et al., 2010). Bhrigu et al. synthesized a library of 2-[(1-substituted phenylethylidine) hydrazine]-N-phenyl-1H-benzo[d]imidazole-1-carbothioamides (467–471) from the reaction of 2-mercaptobenzimidazole with hydrazine hydrate, substituted acetophenones and phenylisothiocyanate. Compounds 467–471 were found to be active compounds in MES and scPTZ models, and devoid of neurotoxicity (Bhrigu et al., 2012).

A series of benzimidazole substituted semicarbazones were synthesized and tested for anticonvulsant activity using MES model. Compound 472 at a dose of 50 mg/kg (i.p.) appeared to be the most potent among the derivatives (Rajak, 2015). In another study, some oxadiazole bearing benzimidazoles and several derivatives of 2-[2-(phenoxymethyl)-1H-benzimidazol-1-yl]-N0-[(Z)-phenylmethylidene] acetohydrazide (473–474) were evaluated employing MES and scPTZ methods. Compounds 473–474 were found to be effective anticonvulsant agents (Shaharyar et al., 2016). A new series of hybrid benzimidazole containing pyridazinones were developed to assess anticonvulsant property. Compound 475 emerged as an effective and safe anticonvulsant in both MES and scPTZ models. The compound also exhibited notable increase in GABA level (1.7-fold) compared to control which was attributed to its good binding property with the GABA_A receptor (Partap et al., 2017). Recently, Sahoo et al. (Sahoo et al., 2019) synthesized several benzimidazole derivatives and disclosed that compounds 476–480 exhibited notable anticonvulsant potency (around 70–80%) in comparison to standard drug phenytoin.

Neurological disorders comprise the disorders of central and peripheral nervous system. Alzheimer disease (AD), dementia, stroke, Parkinson’s disease, multiple sclerosis, brain tumor etc. are the most common diseases affecting a large number of people. AD is a chronic neurodegenerative disorder and the most common form of dementia manifested by loss of memory, language, cognitive functions, behavior and emotion. The factors which mostly contribute in the development of AD include the levels of acetylcholine (ACh) and deposits of neurotoxic amyloid-β-peptide (Aβ) (Akhtar et al., 2017). A number of substituted benzimidazole derivatives have been developed for the management and treatment of neurological diseases. The benzimidazole derivatives explored recently as neuro-protective agents are shown in Figure 20.

Khan et al. synthesized a series of 6-nitrobenzimidazole derivatives and screened their phosphodiesterase inhibitory property. Among them, compounds 481 (IC₅₀ = 1.5 ± 0.043 µM) and 482 (IC₅₀ = 2.4 ± 0.049 µM) were found to possess better activity than the standard EDTA (IC₅₀ = 274 ± 0.007 µM). The presence of 2,3-dihydroxyphenyl (481) and 4-hydroxyphenyl (482) moiety might have contributed for their notable property (Khan et al., 2012). Hu et al. designed and synthesized a series of keto-benzimidazole derivatives (483–484) as potent and selective inhibitors of Phosphodiesterase 10A (PDE10A). Compound 484 showed 55% receptor occupancy (RO) of PDE10A at 30 mg/kg p.o. in rat brain. Further research led to the identification of compound 484, with improvements in in vivo efficacy (57% RO in rats at 10 mg/kg p.o.), rat clearance and oral bioavailability (Hu et al., 2013). Hamaguchi et al. (Hamaguchi et al., 2013) prepared a series of benzimidazole derivatives as PDE10A inhibitors with reduced CYP1A2 inhibition among which the compound 485 appeared to be the most prominent.

Tamura and co-workers synthesized a series of benzimidazole derivatives for neuropeptide Y (NPY) receptor antagonistic activity which is an important pharmacological target for the treatment of different neurodegenerative disorders. Compound 486 appeared to be the most promising agent (Tamura et al., 2012a). In another study by the same research group (Tamura et al., 2012b), compound 487 was reported to show high NPY Y5 receptor binding affinity along with good absorption, distribution, metabolism and elimination (ADME) profile resulting in notable in vivo efficacy as NPY Y5 receptor antagonist.

Kim et al. evaluated a series of 1-heteroaryl-2-aryl-1H-benzimidazole derivatives as inhibitors of c-Jun N-terminal kinases (JNK3), a group of degenerative signal transducers and potential target of neurodegenerative diseases, e.g. Alzheimer’s and Parkinson’s diseases. Majority of the compounds exhibited high affinity (K_d = 10 µM–46 nM) to JNK3 when investigated through SPR, JNK3 kinase assay and cell-viability of human neuroblastoma cells. The most potent compound 488 showed notable cell protective effect (IC₅₀ = 1.09 µM) against toxicity induced by anisomycin (Kim et al., 2013).

Human Presequence Protease (hPreP) is a mitochondrial metalloprotease capable of degrading amyloid-β peptide and increasing its proteolysis in human neuronal cells. Identification of potential agonists of hPreP is of great importance for Alzheimer’s drug design. Compounds 489–490 showed marked enhancement of hPreP-mediated proteolysis of Aβ, pF1β and fluorogenic-substrate V and thus presented great potential in the treatment of Alzheimer’s disease (Vangavaragu et al., 2014).

Among the therapeutic agents used for the treatment of traumatic brain injury (TBI), positive allosteric modulators (PAMs) of metabotropic glutamate receptor 5 (mGluR5) are important. He et al. (He et al., 2015) designed and synthesized a series of acyl-2-aminobenzimidazole derivatives based on the chemical structure of a well-known mGluR5 PAM called 3,3’-difluorobenzaldazine (DFB). The compounds were tested for binding affinity to transmembrane domain of mGluR5 using nitric oxide (NO) production assay, among which compound 491 (IC₅₀ = 6.4 µM) was found to be around 20 times more potent than DFB (IC₅₀ = 136 µM) (He et al., 2015).

Zhu et al. synthesized a series of 2-aminobenzimidazole derivatives (492–494) under microwave irradiation and assessed their acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) inhibitory activities. Compounds 492–494 displayed more than 25-fold more selectivity towards BuChE than AChE (Zhu et al., 2013). In another study, the compounds (495–496) showed selective inhibition on BChE and appeared to be the most potent BuChE inhibitors (IC₅₀ = 5.18 and 5.22 µM, respectively) (Ozadali-Sari et al., 2017). Besides, compound 497 was found to be the most potent AChE inhibitor (IC₅₀ = 0.93 ± 0.04 µM) with marked selectivity ratio (13.68) (Alpan et al., 2017). Similarly, compound 498 carrying a methyl group at 5-position of benzimidazole ring and 2-fluorobenzyl group connected to 1,2,3-triazole system was found to be the most potent inhibitor of AChE displaying 84% inhibition at 100 μM concentration (Faraji et al., 2017). The nitrophenyl piperazine substituted derivative 499 showed 57.25 and 77.92% inhibition of Rho-associated protein kinase II (ROCK II) enzyme at a concentration of 0.5 and 1 mM, respectively and exerted prominent IOP lowering effect (51.56%) compared to the standard fasudil (Abbhi et al., 2017).

Recently, compounds 500–501 were reported as promising AChE and BuChE inhibitors (IC₅₀ = 0.14 and 0.22 μM, respectively), highly neuroprotective against hydrogen peroxide mediated toxicity, metal chelators, and free radical scavengers in the treatment of Alzheimer’s disease (Sarıkaya et al., 2018). Similarly, compounds 502–503 demonstrated duel effects as anti-Alzheimer agents by beta amyloid cleavage enzyme-1 (BACE1) inhibition and neuroprotection (Gurjar et al., 2020). In a 6-hydroxydopamine (6-OHDA)-induced oxidative stress in vitro model assay, compound 504 exerted neuroprotective action compared to melatonin, and reduced superoxide anion radical and hypochlorite level in a luminol-dependent chemiluminescent assay (Anastassova et al., 2020). In another study, compound 505 displayed significant anti-neuroinflammatory property (IC₅₀ = 5.07 mM to prevent nitric oxide generation) and 65.7% hBACE1 inhibition. Besides, the compound (505) increased glutathione (GSH) level, reduced ROS production, and subsequently opened a door for further development to be established as a promising treatment option against Alzheimer’s disease (Fang et al., 2019). Finally, compound 506 exhibited promising neuroprotective role on SH-SY5Y cells by preserving the synaptosomal viability and minimizing GST level compared to the standards melatonin and rasagiline (Anastassova et al., 2021).

Several other classes of benzimidazole derivatives have been prepared by different scientists during the last few years. Some of these novel benzimidazole based compounds are shown in Figure 21.

Saify et al. (Saify et al., 2014) synthesized a series of 2-(2’-pyridyl) benzimidazole derivatives and evaluated their urease inhibitory property. Compound 507 displayed significant urease inhibition compared to the standard thiourea (IC₅₀ = 19.22 ± 0.49 µM and 21.00 ± 0.01 µM, respectively) and twofold more activity than another standard acetohydroxamic acid (IC₅₀ = 42.00 ± 1.26 µM). Several 1-alkylbenzimidazoles and 1,3-dialkyl benzimidazolium salts were evaluated for their tyrosinase inhibitory activity among which compound 508 emerged as the most potent derivative (IC₅₀ = 0.31 mM) (Karatas et al., 2014).

A series of 2-pyridylbenzimidazole derivatives were designed and synthesized as Cannabinoid 1 (CB1) receptor agonists amongst which compound 509 appeared to be the most potent with a binding affinity value (K_iCB1) of 0.53 nM (Mella-Raipán et al., 2013). The same research group synthesized a series of N-acyl-2,5-dimethoxyphenyl-1H-benzimidazole derivatives and assessed them for human CB1 receptor binding affinity using competitive binding assays (Espinosa-Bustos et al., 2015). Compound 510, carrying 3-nitrophenyl group in the aroyl region of benzimidazole moiety was found to be the most promising agent (K_i = 1.2 nM). More recently, they synthesized a new series of benzimidazole derivatives which displayed more selectivity towards human Cannabinoid 2 (CB2) receptor than that towards CB1 receptor (Romero-Parra et al., 2016). Compound 511 demonstrated the best binding affinity (K_i = 0.08 µM), high selectivity index (K_iCB1/K_iCB2 >125) and low toxicity.

Lim et al. synthesized a series of 2-heteroaryl substituted benzimidazole derivatives bearing piperidinylphenyl acetamide group at 1-position and screened them for their effect on melanin-concentrating hormone (MCH), an attractive target for developing anti-obesity agents. Compound 512 displayed prominent MCH receptor 1(MCH-R1) binding affinity (IC₅₀ = 1 nM), as well as low human ether-a-go-go-related gene (hERG) binding affinity thereby ensuring low risks of cardiovascular diseases, metabolic stability and preferable pharmacokinetic profile (Lim et al., 2013). Igawa et al. synthesized several 1-(1H-benzimidazol-6-yl) pyridin-2(1H)-one compounds among which compound 513 showed most prominent antagonistic property against MCH-R1 (Igawa et al., 2016).

Mochizuki and co-workers (Mochizuki et al., 2016) assessed several benzimidazole analogues for in vivo corticotropin releasing factor type 1 (CRF1) receptor antagonistic property. Compound 514 with an electron withdrawing cyano group at the 4-position of benzimidazole nucleus emerged as the most promising among the series. In continuation with their research to develop CRF1 receptor antagonists, compound 515 was reported as a potent CRF1 binding inhibitor (IC₅₀ = 4.1 nM) and in vitro CRF1 antagonist (IC₅₀ = 44 nM) (Mochizuki et al., 2017). More recently they designed a series of 1,2,3,4-tetrahydropyrimido[1,2-a]benzimidazole analogues as novel CRF1 receptor antagonists. Compound 516 displayed very prominent CRF1 receptor binding activity (IC₅₀ = 58 nM) and oral bioavailability (F = 68% in rat model) indicating its potential in developing clinically effective CRF1 receptor antagonist in the future (Kojima et al., 2018).

A library of small molecules were tested for their property as human gonadotropin releasing hormone (GnRH) receptor antagonists. The derivative 517 (K_i = 13.8 nM) exhibited highest affinity towards human GnRH receptor and emerged as the most prominent GnRH receptor antagonist (Fjellaksel et al., 2017). Furthermore, a recent study reported several benzimidazole analogs (518–521) as intraocular pressure (IOP) reducer in the treatment of dexamethasone-induced ocular hypertension. Among these, compound 521 exerted the best anti-glaucoma action by the maximum IOP reduction of 22.32% from baseline following single drop administration (0.1%) (Marcus et al., 2019).

Maltsev et al. (Maltsev et al., 2020) evaluated a series of Diazepino[1,2-a]benzimidazole derivatives for their possible psychotropic properties. Compound 522 at a dose of 2.34 mg/kg showed prominent anxiolytic, antidepressant and anticonvulsant activities in both rat and mice models. Vasil’ev et al. (Vasil’ev et al., 2017) investigated a series of imidazo[1,2-a]benzimidazole derivatives using corazole-induced seizure model. The compounds 523–526 displayed notable anticonvulsive activity in comparison with standard valproic acid. The prominent activity is attributed to the presence of a 4-fluoro substituent in the 2-position and a dialkylaminoalkyl or cycloalkylaminoalkyl substituent in the 9-position of the fused ring.

Change et al. (Chang et al., 2017) investigated the antiplatelet activity of some novel saccharide-based benzimidazole derivatives. Compound 527 exhibited concentration-dependent inhibitory property against thrombin (0.01 U/ml) and collagen (1 µg/ml)-induced human platelet aggregation in an in vitro model. Its inhibitory effect might be attributed to the presence of 1-imidazolyl moiety at one end carrying a long chain of three sugar moieties. A series of benzimidazole derivatives bearing a sterically hindered phenolic group in their structures were evaluated for in vitro antiplatelet activity using the Adenosine diphosphate (ADP)-induced platelet aggregation model of rabbit’s plasma (Spasov et al., 2020). Compound 528 showed notable antiplatelet activity by exceeding the standard acetylsalicylic acid by 21.8 times. In the in vivo study of inhibition of intravascular platelet aggregation, the same compound displayed 1.5 times superior activity than acetylsalicylic acid and slightly inferior activity than another standard clopidogrel.

Idris and his co-workers (Idris et al., 2019) reported that benzimidazole derivatives 529 and 530 decreased repeated morphine administration-induced thermal hyperalgesia and tactile allodynia as well as the expression of spinal Tumor Necrosis Factor-alpha (TNF-α) in Balb-c mice. The compounds displayed PPARγ agonist activity, indicating their potential in the management of morphine-induced paradoxical pain. Akhtar et al. (Akhtar et al., 2021) evaluated the role of benzimidazole derivative 531 in managing nalbuphine-induced tolerance in cisplatin-induced neuropathic pain in mice. Cisplatin, a platinum-based anticancer medication, is often coupled with Nalbuphine, an opioid analgesic used for treating acute and chronic pain. The compound 531 attenuated the tolerance to the analgesic activity of nalbuphine developed due to its long-term use, as well as TNF-α expression in the spinal cord of mice.

Moreover, Raka et al. (Raka et al., 2021) reported a series of substituted benzimidazole derivatives and found a notable inhibitory property of compound 532 against the acetylcholinesterase enzyme with an IC₅₀ value of 29.64 μg/ml compared to the standard drug donepezil (IC₅₀ = 9.54 μg/ml).