Pharmacological significance of heterocyclic 1H-benzimidazole scaffolds: a review.

Heterocyclic compounds are inevitable in a numerous part of life sciences. These molecules perform various noteworthy functions in nature, medication and innovation. Nitrogen-containing heterocycles exceptionally azoles family are the matter of interest in synthesis attributable to the way that they happen pervasively in pharmacologically dynamic natural products, multipurpose arranged useful materials also profoundly powerful pharmaceuticals and agrochemicals. Benzimidazole moiety is the key building block for several heterocyclic scaffolds that play central role in the biologically functioning of essential molecules. They are considered as promising class of bioactive scaffolds encompassing diverse varieties of activities like antiprotozoal, antihelminthic, antimalarial, antiviral, anti-inflammatory, antimicrobial, anti-mycobacterial and antiparasitic. Therefore in the present review we tried to compile the various pharmacological activities of different derivatives of heterocyclic benzimidazole moiety.

Introduction

Among heterocyclic pharmacophores, the benzimidazole ring system is quite common. These substructures are often called ‘privileged’ due to their wide recurrence in bioactive compounds [1]. Benzimidazole moiety is a fusion of benzene and imidazole ring system at the 4 and 5 positions of imidazole ring. They have properties of both acids and bases. The NH group here is highly acidic and also feebly basic. Another feature of it is that they comprise the ability to form salts. The benzimidazole moiety is useful for the development of novel medicinal compounds in pharmaceutical field. Benzimidazole is also a vital pharmacophore, a privileged sub-structure in medicinal chemistry which contributes as a key part for different natural activities [2].

Pharmacological significance of benzimidazole derivatives

Literature survey reveals that the various derivatives of benzimidazole have been synthesized for their pharmacological activities such as antimicrobial [3], anticancer [2], acetylcholinesterase [4], antiprotozoal [5], anti-inflammatory [6], analgesic [7], antihistaminic [8], antimalarial [9], antitubercular [10], anti-HIV [11] and antiviral [12]. Some of the already synthesized compounds from the above mentioned field have found very strong application in medicine praxis. The activity against bacteria, fungi and helminthes resulted their mode of action, which resulted in the blockage of microtubule in various nematode, trematode and cystode [13]. Benzimidazole-based drugs exhibit a wide range of different biological activities as a result of changing the groups on the core structure. Some marketed drugs containing benzimidazole nucleus are shown in Fig. 1.

Fig. 1

Some benzimidazoles containing medicinal preparation

Acetylcholinesterase (AChE) is a core chemical engaged with the ending of nerve signs via the hydrolysis of acetylcholine. It is an objective of medication advancement to battle the neuromuscular issue, for example, glaucoma, myasthenia gravis and Alzheimer’s disease (AD). AChE has been focused in the cure of AD, a dynamic neurodegenerative disease portrayed by neurofibrillary tangles, β-amyloid plaques and loss of focal cholinergic ability. A lack in cholinergic neurotransmission is viewed as one of the real reasons for reminiscence weaknesses in the patients with AD. One of the compelling methodologies for improving the cholinergic transmission is to utilize the inhibitors of acetylcholinesterase [4]. Parasitic ailments are as yet overall issues that deeply affect general wellbeing. Contaminations brought about by protozoa, for example, Trypanosoma cruzi, Plasmodium falciparum, Entamoeba histolytica, Leishmania Mexicana, Trichomonas vaginalis, Giardia intestinalis and helminth, for example, Taenia solium or Trichinella spiralis are overall spread ailments that influence predominantly immature nations, where tropical or template temperatures exist, yet in addition poor uncontaminated and cleanliness conditions are normal [14].

Irritation is a confined reaction of body tissues to destructive incentives or injures bringing about the arrangement of protein-rich exudates. It is a defensive reaction of the nonspecific resistant framework that expels the essential driver of cell damage; eradicate necrotic cells and tissues harmed from the incendiary procedure and commence tissue repair. The essential indications of aggravation are redness, heat, torment, swelling and loss of capacity. Reason for aggravation is physical as well chemical means, immunological responses and contamination by pathogenic life form. Aggravation can be assigned as acute and chronic. Acute irritation is described by the exudation of liquid and plasma proteins (oedema) and the development of leukocytes, particularly neutrophils. Chronic irritation is otherwise called constant aggravation, in which tissue destruction and recovering are continuing all the while, for example, tuberculosis, rheumatoid joint inflammation, constant lung infections and atherosclerosis [6].

Mosquitoes are one of the deadliest creepy crawlies in earth which generate biting irritation and also transmit lethal infections, for example, intestinal sickness, yellow fever, filariasis, chikungunya, encephalitis and dengue. Mosquitoes in the class Aedes are liable for the transmission of chikungunya, dengue, yellow fever and other pathogenic arbo-infections. Likewise, the prime vector for lymphatic filariasis is Culex quinquefasciatus, as well called southern house mosquito. Cx. quinquefasciatus ordinarily stay around human lodging and on maturing like to nibble people than different warm blooded creatures. Intestinal sickness is a mosquito-borne infectious ailment which is mostly transmitted by a contaminated female Anopheles mosquito [15].

Tuberculosis (TB), which is caused prevalently by Mycobacterium tuberculosis (Mtb), is the main source of death from a reparable irresistible ailment, and has been recognized by the World Health Organization (WHO) as one of the three need illnesses for medication innovative work [16]. Viral hemorrhagic fever is a genuine sickness portrayed by broad vascular harm and draining diathesis, fever and various organ inclusions. Various infections can cause this disorder, each with its very own creature repository, method of transmission, mortality rate, and clinical result in people [17].

Worldwide infectious disease figures have attained an alarming level following the proliferation of Gram-positive and Gram-negative multi-drug-resistant species. Patient non-compliance and the occurrence of multidrug-resistant pathogens often interfere innovative infection therapies that depend on a sustained multidrug course. Rational drug design has been shown to be very beneficial in this respect, since the biochemical basis of intrinsic and acquired resistance mechanisms is largely known [3].

One of the most commonly known gastrointestinal malignancies is colorectal tumor (CRC). Alterations in lifestyle, elevated-fat diet, physiological disillusionment and smoking are associated to pathogenesis of CRC. Approximately 25% of CRC cases were identified with early analysis metastases and at some stage of life nearly 50% of CRC patients would suffer from metastasis. The therapy results for these patients are largely unsatisfactory as normal regimens consider the possibility of homogeneous tumor mass distribution [2].

Rational designed based on literature survey of benzimidazole derivatives is shown in Fig. 2.

Fig. 2

Rational designed based on literature survey of benzimidazole derivatives

Reported pharmacological activities of benzimidazole derivatives

Acetyl cholinesterase inhibitory

Alpan et al. designed a class of N-{2-[4-(1H-benzimidazole-2-yl)phenoxy]ethyl} substituted amines and evaluated for its butyrylcholinesterase and acetylcholinesterase inhibitor activity. Among the synthesized derivatives, compounds 1a and 1b were found to be the most active against eeAChE and hAChE using tacrine as standard drug (Table 1, Fig. 3) [4].

Table 1

In vitro inhibition of AChE/BuChE of compounds (1a and 1b)

Comp.	IC50 ± SEM (µM)
	eeAChE	hAChE	BchE
1a	0.58 ± 0.06	3.68 ± 0.39	7.44 ± 1.51
1b	0.61 ± 0.07	0.13 ± 0.03	> 100
Tacrine	0.075 ± 0.02	0.52 ± 0.09	0.0098 ± 0.0002

Fig. 3

Molecular structures of compounds (1a–1b, 2a, 3a–3c, 4a, 5a–5c, 6a–6d, 7a–7b, 8a)

Yoon et al. synthesized a class of benzimidazoles and screened for its acetylcholinesterase and butyrylcholinesterase inhibitor activity. Compound 2a (Fig. 3) showed promising inhibitory activity with IC50 = 5.12 µM for BChE and IC50 = 8.63 µM for AChE using rivastigmine and donepezil (22.00, 7.95 µM for BChE and 50.20, 0.03 µM for AChE) as standard [18].

Antiprotozoal activity

Andrzejewska et al. synthesized two series of S-substituted 4,6-dihalogeno-2-mercapto-1H-benzimidazoles and assessed for their in vitro antiprotozoal potential towards G. intestinalis and T. vaginalis using albendazole and metronidazole as standard. Among them, compounds 3a, 3b and 3c were found to be most potent and comparable to standard drugs (Table 2, Fig. 3) [5].

Table 2

Antiprotozoal activity of benzimidazole compounds (3a–3c)

Comp.	IC50 µg/mL
	Giardia intestinalis	Trichomonas vaginalis
3a	0.006	0.021
3b	0.006	0.013
3c	0.008	0.004
Albendazole	0.010	0.422
Metronidazole	0.210	0.037

Diaz-Chiguer et al. prepared a new series of benzimidazole derivatives and evaluated in vitro (via the % of lysis of bloodstream) and in vivo for its trypanocidal activity against of Trypanosoma cruzi (NINOA and INC5). In this series, compound 4a showed significant in vitro and in vivo [INC5: 68.4 (% lysis); NINOA: 46.4 (% lysis)] trypanocidal activity (Table 3, Fig. 3) [19].

Table 3

In vitro trypanocidal activity of synthesized compound 4a

Comp.	Trypanosoma cruzi
	LC50 (mM)		CC50 (mM)	Selectivity index (SI)
	INC5	NINOA		INC5	NINOA
4a	0.32	0.014	43.2	135	3085.71
Nifurtimox	0.69	0.78	25.4	36.81	32.56
Benznidazole	0.31	0.60	23.6	76.13	39.3

Hernandez-Covarrubias et al. reported a class of benzimidazoles and evaluated for its antiprotozoal activity against G. duodenalis. All the tested compounds were found to be more active than standard metronidazole but the better activity observed with SH group compounds 5a–5c (Fig. 3) (IC50 = 18–45 µM) which exhibited considerable activity as compared to metronidazole (IC50 = 1.22 µM) [20].

Hernandez-Luis et al. synthesized a series of 2-(trifluoromethyl)-1H-benzimidazole molecules and assessed in vitro for its antiparasitic activity towards various protozoan parasites: G. intestinalis (GI), T. vaginalis (TV) E. histolytica (EH) and L. mexicana (LM) using albendazole (ABZ), mebendazole (MBZ), pentamidine as standard drugs and in vivo towards Trichinella spiralis (TS) using albendazole (ABZ), triclabendazole (TBZ) and pentamidine as standard drugs. In this class, compounds 6a, 6b and 6c exhibited good antiparasitic activity and in addition, compound 6a and 6c showed good activity against T. spiralis at adult phase and 6d possessed the good antiprotozoal potential against the muscle larvae stage (Tables 4 and 5, Fig. 3) [14].

Table 4

In vitro antiprotozoal and anthelmintic screening results

Comp.	Microbial strains (IC50 µg/mL)
	GI	EH	TV	LM	TS (% reduction, 0.18 µM)	TS (% reduction, 0.37 µM)	TS (% reduction, 1.80 µM)
6a	0.030	0.009	0.016	24.00	54 ± 2	62 ± 2	80 ± 3
6b	0.063	0.019	0.110	4.10	44 ± 2	48 ± 3	67 ± 2
6c	0.005	0.019	0.086	13.78	43 ± 3	50 ± 2	65 ± 3
ABZ	0.037	56.6	1.592	a	58.6 ± 2	61.9 ± 3	67 ± 6
MTZ	1.228	0.350	0.216	b	b	b	b
Pentamidine	b	b	b	2.421	b	b	b

aNo effect

bNot determined

Table 5

Percentage of adult and muscle larvae load reduction in T. spiralis

Comp.	Adult phase	Muscle larvae stage
	50 mg/kg	75 mg/kg	75 mg/kg
6a	a	58	46
6c	69	80	40
6d	b	36	64
ABZ	62	73	63
MTZ	41	7	25
Alpha	28	a	24
Control	0	0	0

aNot determined

bNo reduction

Kopanska et al. reported a series of 1H-benzimidazole analogues and assessed for its in vitro antiprotozoal activity against Acanthamoeba castellanii and compared with chlorhexidine as reference. The screening results indicated that compounds 7a and 7b were found most efficient in reducing the figure of trophozoites and cysts (Table 6, Fig. 3) [21].

Table 6

Reduction in viability of A. castellanii trophozoites and cysts

Comp.	Concentrations [µmol/L]	% of survivors			Percentage content of particular stages
		Trophozoites	Cysts	Total	Trophozoites	Cysts
7a	5.5	23.3 ± 2.0	15.0 ± 2.3	22.5 ± 2.0	93.4 ± 8.0	6.6 ± 1.0
	11.1	41.2 ± 2.8	76.0 ± 9.7	44.5 ± 3.5	83.6 ± 5.7	16.4 ± 2.1
7b	5.2	26.5 ± 2.3	19.0 ± 3.4	25.8 ± 2.4	92.7 ± 7.9	7.3 ± 1.3
	7.9	22.0 ± 1.8	121.0 ± 12.6	31.6 ± 2.9	62.5 ± 5.2	37.5 ± 3.9
Chlorohexidine	4.4	23.4 ± 0.7	11.0 ± 1.6	22.3 ± 0.8	95.3 ± 2.9	4.7 ± 0.7
	11.0	24.2 ± 1.1	31.0 ± 4.8	24.8 ± 2.6	88.4 ± 3.9	11.6 ± 1.8

Mavrova et al. synthesized novel derivatives of thieno[2,3-d]pyrimidin-4(3H)-ones and screened for their in vitro antiparasitic activity against Trichinella spiralis using albendazole (as standard drug). Among them, compound 8a showed good antiparasitic activity. The significance results of the active compound shown in Table 7 and Fig. 3 [22].

Table 7

Antihelmintic activity of compound 8a against Trichinella spiralis

Comp.	Efficacy (%) after 24 h	Efficacy (%) after 48 h
8a	95.05	95.05
Albendazole	10.6	14.8

Navarrete-Vazquez et al. synthesized a sequence of 2-(trifluoromethyl)-1H-benzimidazoles along with various bioisosteric substituents at 5- and 6-position (–Cl, –F, –CF3, –CN) and examined for its in vitro antiprotozoal activity towards the protozoa T. vaginalis and G. intestinalis using metronidazole and albendazole as reference. In this series, compound 9a showed most promising activity than metronidazole against G. intestinalis and compound 9b found more active against T. vaginalis than the reference drugs. The compound 9b as well displayed modest antimalarial activity against D6 and W2 strains of Plasmodium falciparum (Table 8, Fig. 4) [23].

Table 8

IC50 (μM) of synthesized compounds (9a and 9b)

Comp.	G. intestinalis	T. vaginalis	P. falciparum
			D6	W2
9a	0.107 ± 0.017	3.314 ± 0.130	> 20	> 20
9b	0.672 ± 0.020	0.232 ± 0.021	5.98 ± 0.25	6.12 ± 0.32
Metronidazole	1.226 ± 0.125	0.236 ± 0.016	–	–
Albendazole	0.038 ± 0.003	3.390 ± 0.125	> 20	>20

Fig. 4

Molecular structures of compounds (9a–9b, 10a–10c, 11a–11b, 12a–12b, 13a, 14a, 15a, 16a–16b, 17a–17b, 18a)

Marquez-Navarro et al. developed new derivatives of benzimidazole moiety and examined for their in vivo antiprotozoal activity toward Hymenolepis nana adult and in vitro toward Toxocara canis larvae. In vitro screening results indicated that compound 10a showed significant activity toward T. canis whereas compounds 10b and 10c showed the good in vivo results against H. nana and compared to standard albendazole (Table 9, Fig. 4) [24].

Table 9

Antihelmintic screening results

Comp.	C log P	T. canis J2 larvae in vitro relative mobility (%)			H. nana in vivo adult reduction (%)
		0.18 µM	1.8 µM	18 µM	50 mg/kg
10a	3.23	40	40	30	–
10b	–	–	–	–	97
10c	–	–	–	–	96
Albendazole	3.01	80	40	40	83

Oh et al. synthesized a novel class of 2,3-dihydroimidazo[1,2-a]benzimidazole and screened for its anti-leishmanial and anti-trypanosomal activities towards Leishmania donovani and Trypanosoma cruzi using miltefosine, benznidazole and amphotericin B as standard. Compounds 11a and 11b showed promising antiprotozoal activity (Tables 10 and 11, Fig. 4) [25].

Table 10

In vitro anti-leishmanial screening results

Comp.	Leishmania donovani
	Amastigote form			Promastigote form
	EC50 (μM)	CC50 (μM)	SI	EC50 (μM)
11a	3.05	> 50	> 16.4	1.25
11b	5.29	39.7	7.5	1.48
Miltefosine	4.83	18.9	3.91	11.1
Amphotericin B	0.25	7.57	30.2	0.22

CC cytotoxicity, EC half maximal effective concentration, SI selective index (EC50/CC50)

Table 11

In vitro anti-trypanosomal screening results

Comp.	Trypanosoma cruzi
	EC50 (μM)	CC50 (μM)	SI
11a	1.10	36.5	33.2
11b	2.10	18.8	8.95
Benznidazole	20.7	> 50	> 2.42

Palomares-Alonso et al. developed new substituted benzimidazoles and assessed for their cysticidal activity against Taenia crassiceps cysts (ORF and WFU strain) using albendazole sulfoxide as control drug. Among them, compounds 12a and 12b displayed superior cysticidal activity (Table 12, Fig. 4) [26].

Table 12

Cysticidal activity against T. crassiceps (ORF and WFU strains)

Comp.	Cysts mortality (%)
	ORF strain		WFU strain
	0.28 µM	1.70 µM	0.28 µM	1.70 µM
12a	41 ± 4.6	68 ± 7	22.6 ± 2.3	26 ± 4
12b	37 ± 6.1	62 ± 8	6.3 ± 2.3	16.7 ± 3
Albendazole sulfoxide	46 ± 5	88 ± 7	25 ± 2.3	35 ± 2.3

Perez-Villanueva et al. synthesized a new class of 2-{[2-(1H-imidazol-1-yl)ethyl]-sulfanyl}-1H-benzimidazole derivatives and assessed for its in vitro antiprotozoal activity against protozoa G. intestinalis, T. vaginalis and E. histolytica using metronidazole and albendazole as standard drugs. Among them, compound 13a showed highest activity against G. intestinalis (Table 13, Fig. 4) [27].

Table 13

Antiprotozoal screening results

Comp.	Microbial strains IC50 (µM)
	T. vaginalis	G. intestinalis	E. histolytica
13a	0.0761 ± 0.0094	0.0083 ± 0.0023	0.0298 ± 0.0047
Meteronidazole	0.2360 ± 0.0160	1.2260 ± 0.1250	0.3798 ± 0.1461
Albendazole	1.5905 ± 0.0113	0.0370 ± 0.0030	56.5334 ± 18.8445

Sondhi et al. synthesized pyrimido[1,6-a]benzimidazoles and assessed for their in vitro antiamoebic activity by microdilution method against E. histolytica. In this series, compound 14a (Fig. 4) showed best IC50 value 1.82 µM as compared to metronidazole which showed IC50 value 1.22 µM [28].

Torres-Gomez et al. reported some benzimidazole pentamidine compounds and assessed for their in vitro antiprotozoal activity against L. Mexicana, E. histolytica, Giardia lamblia, T. vaginalis and Plasmodium berghei using pentamidine and metronidazole (as reference drugs). Among the reported compounds, compound 15a showed good activity against G. lamblia, E. histolytica, L. mexicana and T. vaginalis and comparable to standard pentamidine (Table 14, Fig. 4) [29].

Table 14

Antiprotozoal screening results

Comp.	Microbial strains (IC50 µ/M)
	T. vaginalis	G. lamblia	E. histolytica	L. mexicana	P. berghei
15a	0.164	0.435	0.109	34.641	0.712
Pentamidine	3.815	4.079	11.801	a	9.568
Meteronidazole	0.286	1.286	0.771	–	–

– Not tested

aCell damage, due to cytophatic effect caused by pentamidine

Velazquez-Lopez et al. reported some new benzimidazole derivatives and evaluated for their in vitro antiprotozoal activity against T. cruzi epimastigotes INC-5 and NINOA using reference drug (nifurtimox). Among the synthesized compounds, compound 16a showed potent activity towards the T. cruzi epimastigote INC-5 strain while compound 16b found active against the NINOA strain and comparable to nifurtimox (Table 15, Fig. 4) [30].

Table 15

In vitro susceptibility of bloodstream epimastigote

Comp.	IC50 INC-5 (μM)	IC50 NINOA (μM)	CC50 (μM)
16a	28.672 ± 0.602	98.799 ± 1.990	134.580 ± 1.995
16b	186.230 ± 4.103	56.967 ± 0.961	90.436 ± 1.426
Nifurtimox	50.750 ± 0.839	89.804 ± 1.138	131.503 ± 0.490

Anti-inflammatory activity

Achar et al. prepared a class of 2-methylaminobenzimidazole compounds and screened in vivo for its analgesic (acetic acid induced writhing in mice) and anti-inflammatory activities (carrageenan induced paw oedema in rats). Among them, compounds 17a and 17b were displayed considerable analgesic and anti-inflammatory activities in comparison to reference nimesulide (Tables 16, 17 and 18, Fig. 4) [6].

Table 16

Analgesic screening results

Comp.	Mean values (X ± SE)	(%) Protection
Control	300 ± 1.55	–
17a	5.6 ± 1.85	81.33
17b	3.3 ± 1.66	89.00
Nimesulide	–	100

Table 17

Anti-inflammatory screening results

Comp.	Paw oedema thickness (mm)
	30 m (X ± SE)	OedemaInhibition (%)	60 m (X ± SE)	OedemaInhibition (%)
Control	1.3 ± 0.05	–	1.5 ± 0.03	–
17a	1.1 ± 0.03	15.3	1.1 ± 0.00	26.6
17b	1.2 ± 0.03	7.6	1.1 ± 0.06	26.6
Nimesulide	1.1 ± 0.05	15.3	1.1 ± 0.00	26.6

Table 18

Anti-inflammatory screening results

Comp.	Paw oedema thickness (mm)
	120 m (X ± SE)	OedemaInhibition (%)	180 m (X ± SE)	OedemaInhibition (%)
Control	1.7 ± 0.03	–	1.8 ± 0.03	–
17a	1.1 ± 0.03	41.1	1.0 ± 0.03	44.4
17b	1.4 ± 0.03	17.6	1.5 ± 0.05	16.6
Nimesulide	1.0 ± 0.00	41.1	1.1 ± 0.00	44.4

El-Feky et al. designed novel fluorinated quinoline incorporated benzimidazoles and evaluated for their in vivo anti-inflammatory activity by carrageenin induced edema bioassay method in rats using celecoxib. Among them, compound 18a demonstrated the highest anti-inflammatory activity and exhibited best binding profiles into the COX-2 binding site as compared to celecoxib. The significance result of the active compound is shown in Table 19, Fig. 4 [31].

Table 19

Anti-inflammatory screening results

Comp	Anti-inflammatory activity
	Protection at 50 mg/kg dose (%)
18a	55
Celecoxib	50

Gaba et al. reported phenylsulfonyl substituted benzimidazoles and evaluated in vivo for their anti-inflammatory activity (carrageenan-induced paw edema in rats) and analgesic activity (acetic acid-induced writhing test in mice), respectively. Among them, compounds 19a, 19b, 19c and 19d showed significant reduction in edema and compared to standard drug indomethacin and protection in the number of writhes produced by acetic acid, and comparable to the reference drug acetyl salicylic acid (Tables 20 and 21, Fig. 5) [7].

Table 20

Anti-inflammatory screening results

Comp.	Edema at 3 h (%, mean ± SEM)	Reduction in edema (%)
19a	68.66 ± 72.99	31.34
19b	67.16 ± 73.06	32.84
19c	65.67 ± 73.78	34.33
19d	62.69 ± 73.27	37.31
Control	100.00 ± 73.59	0.00
Indomethacin	52.23 ± 74.27	47.76

Table 21

Analgesic screening results

Comp.	No. of writhes in 15 min (%, Mean ± SEM)	Protection (%)
19a	32.33 ± 73.62	54.03
19b	33.17 ± 73.39	52.84
19c	32.67 ± 73.57	53.55
19d	29.83 ± 72.45	57.58
Control	70.33 ± 73.01	0.00
Acetyl salicylic acid	25.67 ± 71.45 6	3.51

Fig. 5

Molecular structures of compounds (19a–19d, 20a–20b, 21a–21c, 22a–22b, 23a–23d)

Jesudason et al. reported a class of N-Mannich bases of benzimidazole compounds and screened for its analgesic activity by the acetic acid induced writhing method using Wistar albino mice and anti-inflammatory activity by the formalin-induced paw edema method on Wistar albino rats by plethysmography. In this series, compound 20a exhibited similar results to paracetamol and compound 20b showed more potent than diclofenac (Tables 22 and 23, Fig. 5) [32].

Table 22

Analgesic screening results and LD50

Comp.	Dose (mg/kg)	% Protection	LD50 (mg/kg)
20a	20	32.2	175
	40	47.49
Paracetamol	100	47.76	–

Table 23

Anti-inflammatory screening results

Comp.	Dose (mg/kg)	% Reduction of edema
		30 min	60 min	90 min	120 min
20b	40	48	56	59	62
Diclofenac	50	48	65	64	65

Mariappan et al. developed some 2-substituted benzimidazole molecules and screened for their in vivo anti-inflammatory and analgesic activities using pentazocine as standard. Among the synthesized derivatives, compounds 21a, 21b, 21c showed significant analgesic and anti-inflammatory activity (Tables 24 and 25, Fig. 5) [33].

Table 24

Analgesic activities of benzimidazole compounds 21a-c via Tail-flick method

Comp.	(Mean ± SEM) tail withdrawing time in second
	0 h	1 h	2 h	3 h	4 h
Control (0.5% CMC)	1.56 ± 0.16	2.16 ± 0.16	2.33 ± 0.21	2.66 ± 0.21	2.82 ± 0.72
Pentazocine	2.16 ± 0.16	8.5 ± 0.34	11.33 ± 0.21	10.16 ± 0.30	10.83 ± 0.30
21a	2.0 ± 0.25	3.0 ± 0.25	4.16 ± 0.33	10.5 ± 0.22	9.83 ± 0.33
21b	2.0 ± 0.25	4.33 ± 0.21	3.73 ± 0.30	8.63 ± 0.21	10.03 ± 0.30
21c	2.0 ± 0.25	6.51 ± 0.21	7.83 ± 0.30	9.73 ± 0.21	9.25 ± 0.30

Table 25

Anti-inflammatory activities of benzimidazole compounds 21a-c by carrageenan-induced rat paws edema method

Comp.	(Mean ± SEM) tail withdrawing time in second					Inhibition (%)
	0 h	1 h	2 h	3 h	4 h	4 h
Control (0.5% CMC)	0.14 ± 0.01	0.23 ± 0.01	0.24 ± 0.02	0.25 ± 0.01	0.25 ± 0.01	–
Pentazocine	0.14 ± 0.01	0.12 ± 0.01	0.12 ± 0.01	0.10 ± 0.01	0.09 ± 0.01	64
21a	0.14 ± 0.02	0.12 ± 0.02	0.11 ± 0.01	0.11 ± 0.02	0.10 ± 0.01	60
21b	0.15 ± 0.02	0.15 ± 0.01	0.13 ± 0.01	0.13 ± 0.01	0.10 ± 0.01	60
21c	0.14 ± 0.01	0.13 ± 0.01	0.12 ± 0.01	0.10 ± 0.11	0.09 ± 0.02	64

Paramashivappa et al. synthesized a class of substituted benzimidazoles and assessed for its human cyclooxygenase-2 (COX-2) and cyclooxygenase-1 (COX-1) enzyme inhibition activity in human whole blood assay using rofecoxib as reference. In this series, compound 22a and 22b were found as most active agents (Table 26, Fig. 5) [34].

Table 26

Inhibitory effect on COX-2 and COX-1 activity in human whole blood assay

Comp.	COX-2 IC50 µM	COX-1 IC50 µM	COX-1/COX-2
22a	1	384	384
22b	1.06	> 500	> 470
Rofecoxib	0.057	11.4	200

Ravindernath et al. designed new benzo[d]imidazolyl tetrahydropyridine carboxylates and evaluated for their anti-inflammatory activity by the Carrageenan-induced paw edema test in rats using diclofenac sodium as a reference drug for comparison. All synthesized compounds (23a–23d) displayed appreciable activity. The significance results of the active compounds are shown in Table 27, Fig. 5 [35].

Table 27

Anti-inflammatory screening results

Comp.	Time
	1 h	2 h	3 h	4 h
23a	0.78 ± 0.022	1.45 ± 0.057	0.5 ± 0.027	0.08 ± 0.003
23b	0.55 ± 0.0389	1.583 ± 0.045	0.616 ± 0.0315	0.3 ± 0.023
23c	0.64 ± 0.011	1.4 ± 0.038	0.31 ± 0.024	0.31 ± 0.024
23d	0.82 ± 0.030	1.76 ± 0.07	0.58 ± 0.03	0.1 ± 0.002
Control	0.90 ± 0.04	1.60 ± 0.018	2.38 ± 0.02	3.25 ± 0.03
Diclofenac sodium	0.95 ± 0.03	1.72 ± 0.03	0.60 ± 0.03	0.60 ± 0.02

Sondhi et al. synthesized pyrimido[1,6-a]benzimidazoles and tested in vitro for their anti-inflammatory and analgesic activities using carrageenin induced paw oedema model. Among the synthesized compounds, compound 24a (Fig. 6) displayed superior anti-inflammatory (46%) and mild analgesic activity (50%) using ibuprofen as standard (51% and 75%) [28].

Fig. 6

Molecular structures of compounds (24a, 25a–25c, 26a, 27a, 28a, 29a, 30a–30b, 31a–31g, 32a–32b, 33a and 34a)

Sondhi et al. developed a class of benzimidazole acridine derivatives and tested for its anti-inflammatory, analgesic and kinase (CDK-1, CDK-5 and GSK-3) inhibition activities using ibuprofen as standard. Among the series, compound 25a displayed considerable activity against kinase while compounds 25b and 25c displayed significant anti-inflammatory and analgesic activities (Table 28, Fig. 6) [36].

Table 28

Anti-inflammatory, analgesic and kinase inhibition activities

Comp.	Anti-inflammatory activity (%)	Analgesic activity (%)	Kinase IC50 (µM)
			CDK-1	CDK-5	GSK-3
25a	–	–	7.4	4.6	42
25b	31.4	60	–	–	–
25c	35.8	50	–	–	–
Ibuprofen	38.8	50	–	–	–

Vicini et al. synthesized benzimidazole tetrazolyl- and carboxyl-derivatives and screened for their anti-inflammatory and antipyretic activities in rat paw oedema and rat Escherichia coli derived LPS-induced pyrexia along with antinociceptive property examined in writhing and hot plate tests in mice. Among them, compound 26a (1H-benzimidazol-2-yl) acetic acid showed central analgesic activity. The significance results of the active compounds are shown in Table 29, Fig. 6 [37].

Table 29

Analgesic effect of compound 26a against acetic acid induced writhing in mice

Comp.	ID50 (mg/kg os)	Maximal inhibition % mean ± SEM
26a	> 200	42 ± 15
Acetaminophen	208	90 ± 17

Wang et al. prepared a class of benzimidazole compounds and assessed for its in vitro H1 antihistamine activity. Among them, compound 27a found to display excellent activity to reduce mast cell degranulation, moderate anti-PAF activity and decreased potency on hERG as compared to standard astermizole and desloratadine (Table 30, Fig. 6) [8].

Table 30

Antihistamine, receptor binding and anti-PAF activities

Comp.	Anti H1 activity ileum IC50 (µmol/L)	H1 receptor binding IC50 (µmol/L)	PAF-induced plateletAggregation IC50 (µmol/L)
27a	0.00794	0.000881	78
Desloratadine	0.0721	0.00588	130
Astermizole	0.0453	0.004	ND

Yang et al. designed new benzimidazoles and then assessed for their in vitro phosphodiesterase 10A (PDE10A) inhibitor activity. From the newly developed compounds, compound 28a (Fig. 6) showed good IC50 = 3.73 ± 0.60 nM along with selectivity (> 1000-fold) for PDE10A [38].

Antimalarial activity

Bandyopadhyay et al. synthesized new thiophosphorylated and phosphorylated benzimidazole derivatives and examined for their antimalarial activity toward Aedes albopictus and Culex quinquefasciatus for mosquito larvicidal properties at different concentration. Compound 29a (Fig. 6) found most active toward Ae. albopictus (LC50—6.42 and 5.25 mg/L at 24 and 48 h) and Cx. Quinquefasciatus (LC50—7.01 and 3.88 mg/L) using temephos as positive control (2.85 ± 2.64, 2.8 ± 3.04 toward Ae. Albopictus and for Cx. Quinquefasciatus 3.04 ± 2.31, 3.55 ± 2.45) [15].

Camacho et al. synthesized a class of N′-substituted-2-(5-nitrofuran or 5-nitrothiophen-2-yl)-3H-benzo[d]imidazole-5-carbohydrazides and investigated in vitro for its efficiency to inhibit β-hematin formation (IβHS), hemoglobin hydrolysis and then in vivo in rodent Plasmodium berghei for its antimalarial efficacy. Compounds 30a and 30b showed good antimalarial activity via inhibition of β-hematin formation and as proficient as chloroquine (Table 31, Fig. 6) [9].

Table 31

Antimalarial activity of benzimidazole derivatives (30a and 30b)

Comp.	IβHS	IC50 (µM)	IGP	% P	SD
30a	95.43 ± 0.58	8.43	0	4.02 ± 0.45	17 ± 1.26
30b	75.76 ± 0.99	11.10	14.08 ± 0.88	1.8 ± 0.49	18.8 ± 2.05
Leupeptin	–	–	91.62 ± 0.69	–	–
Pepstatin	–	–	95.45 ± 0.66	–	–
Chloroquine	94.19 ± 0.36	–	24.12 ± 1.16	1.3 ± 0.3	> 30
Saline Solution	–	–	–	21.8 ± 2.31	11.66 ± 1.66

Divatia et al. synthesized novel thiosemicarbazones containing benzimidazole nucleus and evaluated for their in vitro antimalarial activity towards P. falciparum by minimum inhibitory concentration using chloroquine and quinine as standards. Among them, compounds 31a, 31b, 31c, 31d, 31e, 31f and 31g showed excellent antimalarial activity. From structure activity relationship study it was observed that compounds having electron withdrawing groups (EWG) (chloro, fluoro and iodo) showed promising activity (Table 32, Fig. 6) [39].

Table 32

Antimalarial activity of benzimidazole derivatives 30a and 31 g

Comp.	Minimum inhibitory concentration (IC50 µg/mL)
31a	0.023
31b	0.003
31c	0.012
31d	0.025
31e	0.005
31f	0.26
31 g	0.15
Quinine	0.268
Chloroquine	0.020

Toro et al. reported ferrocenyl and cyrhetrenyl benzimidazoles and evaluated for their in vitro antimalarial activity against chloroquine susceptible-strain (3D7) and the chloroquine resistant-strain (W2) of Plasmodium falciparum. A better activity was observed for the compounds 32a–32b (Fig. 6) (IC50 = 10.4–26.5 μM) than its ferrocenyl analog 1-Fe-(H, NO2) (IC50 = 23.9–48.0 μM) [40].

Anti-mycobacterial/antitubercular activity

Camacho et al. synthesized some novel N′-substituted-2-(5-nitrofuran or 5-nitrothiophen-2-yl)-3H-benzo[d]imidazole-5-carbohydrazide compounds and investigated for their antitubercular potency against multidrug resistant MDR-MTB and MTB H37Rv strains. Compounds 33a (Fig. 6) exhibited good anti-mycobacterial activity (MIC = 12.5 µg/mL) against sensitive M. tuberculosis H37Rv and MDR-MTB (MIC = 6.25 µg/mL) strains and compared to isoniazid (MIC = 0.063 µg/mL) and rifampin (MIC = 32 µg/mL) [9].

Gong et al. reported a new series of substituted benzimidazole derivatives and investigated for their antitubercular potency against M. tuberculosis in a replicating state (R-Mtb), a physiologically-induced non-replicating state (NR-Mtb). Among the synthesized derivatives, compound 34a (Fig. 6) (NR-Mtb: MIC90 = 0.20 µg/mL; R-Mtb: MIC90 < 0.049 µg/mL) [16].

Desai et al. reported a class of benzimidazole containing 2-pyridones compounds and evaluated for its antimycobacterial potential towards M. tuberculosis H37Rv strain in Middlebrook 7H12 medium by microplate alamar blue assay (MABA) using isoniazid as a reference drug. In this series, compounds 35a, 35b, 35c, 35d and 35e (Fig. 7) exhibited good activity with MIC values (2.76–20.4 µM) as compared to isoniazid with MIC value (0.24 µM) [10].

Fig. 7

Molecular structures of compounds (35a–35e, 36a–36d, 37a, 38a–38g)

Kalalbandi et al. developed a novel class of 1-[(2E)-3-phenylprop-2-enoyl]-1H-benzimidazoles and assessed for its antitubercular activity towards M. tuberculosis H37Rv by microplate alamar blue assay. Among them, compounds 36a, 36b, 36c and 36d (Fig. 7) (MIC = 3.12, 6.25, 3.12 and 1.6 µg/mL, respectively) showed excellent in vitro activity against H37Rv strain as compared to pyrazinamide, streptomycin and rifampicin having MIC = 3.12, 6.25 and 0.12 µg/mL, respectively [41].

Park et al. synthesized some new 2,5,6-trisubstituted benzimidazoles and assessed for their antitubercular potential against drug sensitive Mtb H37Rv strain using microplate alamar blue assay. Compound 37a (Fig. 7) displayed the best potency with the MIC value (0.63 µg/mL) against Mtb H37Rv [42].

Ramprasad et al. synthesized some imidazo[2,1-b] [1, 3, 4] thiadiazole-benzimidazole compounds and evaluated for their in vitro antituberculosis potency against M. tuberculosis H37Rv strain by agar dilution method using standard drugs ethambutol, isoniazid and pyrazinamide for comparison which showed the values in the range of 3.125–50.0 µg/mL. Among the synthesized compounds, compounds 38a, 38b, 38c, 38d, 38e, 38f and 38g (Fig. 7) showed potent anti-tubercular activity with MIC value (3.125 µg/mL) and comparable to standard ethambutol (MIC = 3.13 µg/mL) [43].

Ranjith et al. developed a class of positional isomers having benzimidazole moiety and evaluated for its antitubercular potency against M. smegmatis (MS), M. tuberculosis H37Rv (MTB), M. fortuitum (MF) and MDR-TB strains using isoniazid and rifampicin as standards. Among the synthesized derivatives, compounds 39a, 39b and 39c displayed significant activity against M. tuberculosis H37Rv (Table 33, Fig. 8) [44].

Table 33

Antitubercular screening results

Comp.	Screening results, MIC (µg/mL)
	MTB	MS	MF	MDR-TB
39a	0.625	10	10	6.25
39b	0.625	1.25	10	6.25
39c	0.625	1.25	10	6.25
Isoniazid	0.7	50	12.5	12.5
Rifampicin	0.5	1.5	1.5	25

Fig. 8

Molecular structures of compounds (39a–39c, 40a–40e, 41a, 42a–42b, 43a–43c, 44a–44b)

Shingalapur et al. synthesized some novel 5-(nitro/bromo)-styryl-2-benzimidazole compounds and evaluated for their in vitro anti-tubercular activity towards M. tuberculosis H37 Rv by alamar blue assay using streptomycin (100% inhibition) as reference. Among them, compounds 40a, 40b, 40c, 40d and 40e showed significant antitubercular activity (Table 34, Fig. 8) [45].

Table 34

Antitubercular activity {MIC (µg/mL)}

Comp.	M. tuberculosis H37 Rv
40a	> 7.25 (45)
40b	> 7.25 (83)
40c	> 7.25 (54)
40d	> 7.25 (63)
40e	> 7.25 (76)

Yoon et al. prepared some new benzimidazole derivatives and evaluated for their antimycobacterial potency against M. tuberculosis H37Rv strain using BacTiter-Glo™ microbial cell viability (BTG) assay using six standard drugs (rifampicin, cycloserine, pyrimethamine, isoniazid, amikacin and ethambutol). In this series, compound 41a was found to be the highly potent agent as compared to standard drugs (Table 35, Fig. 8) [46].

Table 35

Antimycobacterial activity of benzimidazole derivative 41a

Comp.	M. tuberculosis H37Rv
	Alamar blue			BTG
	IC50 (µM)	IC90 (µM)	MIC (µM)	IC50 (µM)	IC90 (µM)	MIC (µM)
41a	16.14	44.46	100	11.52	16.53	50
Amikacin	0.12	0.14	0.16	0.07	0.12	0.16
Cycloserine	24.76	28.01	100	23.55	26.38	100
Ethambutol	3.45	> 200	NA	1.50	1.64	6.25
Isoniazid	0.19	> 5	NA	0.13	0.20	0.31
Pyrimethamine	25.09	28.00	100	24.27	46.37	100
Rifampicin	0.02	0.02	0.16	0.02	0.03	0.04

Antiviral activity

Cheng et al. synthesized some novel benzimidazoles and demonstrated for their antiviral activity against Coxsackie virus B3 in VERO cells. Among the synthesized derivatives, compounds 42a and 42b (Fig. 8) showed potent selective activity with IC50 values (1.43 and 0.54 µg/mL) as compared to ribavirin (RVB) with IC50 value and eminent selective index (411.7 µg/mL and > 2.42) [47].

Fonseca et al. synthesized benzimidazole compounds incorporated into a hydrophenanthrene and naphthalene skeleton and screened for their in vitro antiviral activity against several RNA and DNA viruses. Among them, compounds 43a, 43b and 43c (Fig. 8) displayed good activity against VZV and CMV replication and comparable to that of acyclovir and ganciclovir (Table 36) [48].

Table 36

Antiviral screening results of the synthesized compounds (43a-c)

Comp.	Antiviral potency IC50 (µg/mL)
	CMV	VZV
43a	> 0.2	0.2–0.5
43b	1.1–3.2	0.6–2.8
43c	1.0–1.2	0.8–1.4
Acyclovir	–	0.3–3.0
Ganciclovir	0.9–1.5	–

Hwu et al. developed some new benzimidazole derivatives bearing coumarin ring and evaluated for their antiviral activity against the hepatitis C virus. Among the synthesized derivatives, compounds 44a and 44b (Fig. 8) were found to be most active and showed EC50 values (3.4 µM and 4.1 µM) [49].

Li et al. synthesized a class of novel benzimidazoles and assessed for their hepatitis B virus inhibition activity. Among them, compounds 45a and 45b showed outstanding anti-HBV potency and comparable to lamivudine and adefovir (Table 37, Fig. 9) [50].

Table 37

Antiviral activity results of the synthesized compounds (45a–45b)

Comp.	IC50 (µM)	CC50 (µM)	SI
45a	0.70	192	274
45b	0.70	86	123
Lamivudine	0.38	> 1000	> 2632
Adefovir	1.7	57	34

Fig. 9

Molecular structures of compounds (45a–45b, 46a, 47a, 48a–48b, 49a, 50a–50b, 51a–51b, 52a, 53a–53b, 54a–54b)

Luo et al. developed few novel benzimidazole compounds and evaluated for their anti-hepatitis B virus (HBV) activity and cytotoxicity in HepG 2.2.15 cells. In this study, compound 46a showed significant antiviral activity using lamivudine as reference (Table 38, Fig. 9) [51].

Table 38

Antiviral activity results of the synthesized compounds 46a

Comp.	IC50 (µM)	CC50 (µM)	SI
46a	< 0.41	33.3	81.2
Lamivudine	5	0.16	3.13

Miller et al. designed a series of N-substituted benzimidazoles as CXCR4 antagonists. In this series, compound 47a (Fig. 9) exhibited promising antiviral activity having IC50 of 2 nM, a 1000-fold cytotoxicity window and a twofold protein shift. A modification in side chain and stereochemical optimization led to significantly enhancement in potency and protein shift to afford compounds with low nanomolar anti-HIV activity [52].

Monforte et al. synthesized some novel N1-aryl-2-arylthioacetamido-benzimidazoles and screened for their human immunodeficiency virus type-1 (HIV-1) inhibitor activity. In this series, compounds 48a and 48b were found as the most active compounds with no toxicity (Table 39, Fig. 9) [11].

Table 39

Anti-RT and anti-HIV-1 activities, cytotoxicity and selectivity index in MT-4 cells

Comp.	IC50 (µM)	EC50 (µM)	CC50 (µM)	SI
48a	0.12 ± 0.035	0.04 ± 0.01	> 221.59	> 5540
48b	0.18 ± 0.018	0.06 ± 0.02	≥ 235.64	≥ 3927
Nevirapine	2.55 ± 0.93	0.19 ± 0.06	> 15.02	> 79
Efavirenz	0.032 ± 0.009	0.006 ± 0.0001	> 1056	> 6.34

Starcevic et al. synthesized 2-substituted-5-amidino-benzimidazoles and assessed for their in vitro inhibitory activity against GMK cell line and HeLa cell line by MTT assay. From this series, compound 49a showed prominent activity against all four types of viruses with no cytotoxicity (Table 40, Fig. 9) [12].

Table 40

Antiviral activity EC50 (µM)

Comp.	HeLa		GMK
	Adenovirus 5	Herpesvirus 1	Coxsackievirus B5	Echovirus 7
49a	5.9	30	3.5	5

Zhang et al. reported some new benzimidazole derivatives and screened for their anti-Coxsackie virus B3 (CVB3) activity in VERO cells. In this series, compounds 50a and 50b (Fig. 9) exhibited better inhibitory activity with IC50 values (5.30 and 1.06 µg/mL) together with good selective indexes (12.1 and 7.5) than those of ribavirin (RBV) with IC50 value 353.33 [53].

Anticancer activity

In this study, Tahlan et al. developed a new class of benzimidazole benzamide compounds and demonstrated for its anticancer activity against cancer cell line (HCT116) by SRB method and compared to standard drugs (5-fluorouracil). From the synthesized derivatives, compound 51a and 51b (Fig. 9) showed the significant anticancer activity (Table 41) [3].

Table 41

Anticancer activity results of synthesized compounds (51a and 51b)

Comp.	Cancer cell line (IC50 = μM)
	HCT116
51a	5.85
51b	4.53
5-Fluorouracil	9.99

Designed and synthesized a novel series of benzimidazole derivatives by Tahlan et al. and evaluated for its anticancer potency towards cancer cell line (HCT116) by SRB assay. In this series, compound 52a (Fig. 9) was found to be most promising anticancer compound. The significant result of the most active compound is shown in Table 42 [2].

Table 42

Anticancer activity results of synthesized compound (52a)

Comp.	Cancer cell line (IC50 = μM)
	HCT116
52a	4.12
5-Fluorouracil	7.69

Antimicrobial activity

Novel class of benzimidazole Schiff base derivatives has been synthesized by Tahlan et al. and evaluated for their antimicrobial activity against Gram positive and Gram negative bacterial and fungal species by tube dilution method. In this series, compounds 53a and 53b (Fig. 9) displayed potent antifungal activity against A. niger and C. albicans. The significant result of the active compounds is shown in Table 43 [54].

Table 43

Antimicrobial results of compounds (53a–53b)

Comp.	Microbial strains (MIC = µM/mL)
	Bacterial strains					Fungal strains
	S. aureus	E. coli	B. subtilis	P. aeruginosa	S. enterica	C. albicans	A. niger
53a	9.62	9.62	2.41	2.41	4.81	2.41	1.20
53b	5.82	2.91	5.82	5.82	5.82	1.46	2.91
Cefadroxil	1.72	1.72	1.72	1.72	1.72	–	–
Fluconazole	–	–	–	–	–	2.04	2.04

Tahlan et al. synthesized a class of benzimidazole Schiff base derivatives and screened for its antimicrobial activity toward selected microbial species. From the series compounds 54a and 54b (Fig. 9) exhibited promising antimicrobial activity towards bacterial and fungal species. The significant result of the active compounds is shown in Table 44 [55].

Table 44

Antimicrobial results of compounds (54a–54b)

Comp.	Microbial strains (MIC = µM/mL)
	Bacterial species					Fungal species
	B. subtilis	P. aeruginosa	E. coli	S. typhi	K. pneumoniae	C. albicans	A. niger
54a	1.28	1.28	1.28	2.55	5.11	5.11	2.55
54b	0.68	0.68	2.72	2.72	5.44	5.44	2.72
Cefadroxil	1.73	3.46	3.46	0.86	3.46	–	–
Fluconazole	–	–	–	–	–	4.08	4.08

Conclusion

The present review based on reported heterocyclic benzimidazole derivatives which displayed the significant biological potentials in medicinal chemistry. Benzimidazole moiety is the key building block for several heterocyclic scaffolds that play central role in the biologically functioning of essential molecules and are surprisingly effective with their restraint movement and favorable selectiveness. The present review article is based on various reported pharmacological activities of heterocyclic 1H-benzimidazole derivatives. The review article shows the pharmacological activities of the reported synthesized benzimidazole derivatives in medicinal field. We hope this paper may be helpful in the development of new derivatives of benzimidazole based on medicinal chemistry and as well as designing of new drug molecule in future.