Benzimidazole Derivatives as Scaffolds for Antimicrobial Drug Discovery: Advances, Structure–Activity Relationships, and Therapeutic Potential

Bacteria are unique among the prokaryotes in that so many of them are normal flora that colonize the host without causing infection. New species and new variants of familiar species continue to be discovered, particularly as we intrude into new ecosystems. Among the top causes of mortality in the world, lower respiratory infection is the third most common and diarrhea is the sixth. Both are often caused by bacteria. Tuberculosis is the seventh most common cause of death. Antimicrobial resistance is a global health crisis, with pathogens such as Staphylococcus aureus, Escherichia coli, and Candida albicans developing resistance to conventional drugs. An ideal antimicrobial agent acts at a target site that is present in the infecting organism but not the host cells. Four major sites in the bacterial cell can be targeted by antibiotics because they are sufficiently different from human cells. These are the cell wall, the cell membrane, the nucleic acid synthetic pathway, and the ribosome. Antibacterial agents, or antibiotics, are typically products of other microorganisms, elaborated by them in order to compete for space and resources. Some bacteria are innately resistant to certain classes of antibiotics, either because they lack the target or are impermeable to the drug. Others are innately susceptible but develop resistance by one of a growing variety of mechanisms. Resistant strains of bacteria have a selective advantage, surviving in the presence of antibiotics, and can spread throughout the host and even be transferred to other hosts. This phenomenon is important where antibiotic use is common, such as in hospitals or in congregate housing such as nursing homes.

1. Benzimidazoles

Benzimidazole is an organic heterocyclic aromatic compound is a benzene ring is joint to 4 & 5 location of an imidazole ring with bicyclic nature. The nucleus is synonymously referred as benzoglyoxalines and 1,3-benzodiazoles and has amphoteric properties (both acidic & basic). The -NH functional group in benzimidazoles shows weak basic nature & strong acidic nature and has the capacity to form salts¹. 

Figure 1. Benzimidazole

The very first benzimidazole (2, 5 or 2, 6-dimethylbenzimidazole) was prepared in 1872 by Hoebrecker² through reduction of 2-nitro-4-methylacetanilide. Several years later, Ladenburg obtained the same compound by refluxing 3, 4-diamino toluene with acetic acid.

Scheme 1

Though all seven positions in the benzimidazole nucleus can be substituted with a variety of chemical entities, but most of the biologically active benzimidazole based compounds bear functional groups at 1, 2 and/or 5(or 6) positions. Accordingly, the compounds may be mono-, di- or tri-substituted derivatives of the nucleus.

Table 1. Some clinically used benzimidazole compounds

The general method for synthesis of 2-substituted benzimadazoles involves the reaction between 1, 2-phenylene diamine and a carboxylic acid or an acid chloride or nitrile in the presence of strong acid catalyst³ or with aldehydes in the presence of oxidants⁴.

Scheme 2

2. Pharmacological actions of benzimidazoles

The enormous potentiality of benzimidazole-based compounds in medicinal chemistry has led to a lot of work being directed towards the feasible prolific applications of benzimidiazole derivatives in diverse areas. In a recent study, Kumar et al. (2024)⁵ performed the synthesis of benzimidazole derivatives. All the compounds were characterized by UV, IR, 1H NMR, mass spectral data and CHN elemental analysis. The synthesized derivatives were screened for analgesic and anti-inflammatory activities. All the compounds showed significant effect at 100 mg/kg p.o. and the experimental data are statistically significant at p < 0.01 level.

The design and synthesis of benzimidazole?oxadiazole derivatives as new inhibitors for vascular endothelial growth factor receptor?2 (VEGFR?2) was reported by Cevik et al. (2024).⁶ The designed members were assessed for their in vitro anticancer activity against three cancer cell lines and two normal cell lines; A549, MCF?7, PANC?1, hTERT?HPNE and CCD?19Lu.

A series of benzo[d]imidazole?amide containing 1,2,3?triazole?N?arylacetamide derivatives were synthesized and evaluated them for their inhibitory activity against α?glucosidase by Yousefnejad et al. (2023)⁷. In vitro α?glucosidase inhibition assay demonstrated that more than half of the title compounds with IC50 values in the range of 49.0–668.5 μM.

The anti-diabetic activities of some new heterocyclic compounds based on benzimidazole-2-carboxaldehyde thiosemicarbazone was reported by Abas et al. (2022).⁸ Oral administration of new synthesized benzimidazole derivatives compounds ameliorated all biochemical parameters (ALT, AST ALB, T.Bili, urea, creatinine, CK-MB and LDH) and enhanced activity of antioxidant enzymes.

Several arylated benzimidazoles derivatives were synthesized by Akande et al. (2021)⁹ and screened for α-amylase inhibitory, 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and 2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activities. In vitro screening results revealed that all molecules demonstrated signi?cant α-amylase inhibition with IC50 values of 1.86 ± 0.08 to 3.16 ± 0.31 μM.

Seventeen derivatives of 2- mercaptobenzimidazole bearing sulfonamide were synthesized by Hussain et al. (2021)¹⁰. All the compounds were screened for their α- amylase inhibitory potential and displayed a variable degree of α- amylase activity having IC 50 values ranging between 0.90 ±0.05 to 11.20 ±0.30 μM when compared with the standard drug acarbose having IC 50 value 1.70 ± 0.10 μM.

Wang et al. (2018)¹¹ synthesized a series of chrysin benzimidazole derivatives and studied their anticancer activity. Among synthesized compounds, compound showed the most potent anti-proliferative activity against MFC cells with IC50 values of 25.72 ± 3.95 μM.

In an attempt to design ACE inhibitors, Abdulaziz Hammad et al. (2017)¹² designed series of benzimidazole derivatives as ACE inhibitor. From molecular docking study and in silico toxicity study, they found compound 2-(2-(butylthio)-5-methoxy-1H-indol-1-yl)-1-(2-nitrophenyl) ethan-1-one as an equipotent ACE inhibitor with respect to lisinopril as a standard drug.

5-methanesulphonamido benzimidazole derivatives were synthesized by Ratika Sharma et al. (2017)¹³ and tested in carrageenan induced rat paw edema model as anti-inflammatory agents. Among tested compounds, three showed maximum (92.73%, 95.64 % and 97.62% respectively) reduction in edema and were also non-ulcer genic at the tested doses.

A series of benzimidazole derivatives was synthesized and tested for antimicrobial activity by N.S. El-Gohary et al. (2017)¹⁴. Among the tested two compounds showed good activity toward S. aureus with MIC value 0.524 µg/ml and 0.684 µg/ml respectively, whereas one compound with MIC value 0.489 µg/ml exhibited remarkable activity toward B. cereus. One compound was found the most active antifungal analog toward C. albicans with MIC value 0.262 µg/ml.

In another study, L Ravithej Singh et al. (2017)¹⁵ synthesized new coumarin–benzimidazole derivatives and evaluated them for their antibacterial activity. They found that the compound showed better antibacterial activity against P. aeruginosa with (MIC 3.12 μg/ml).

Olayinka O. Ajani et al. (2016)¹⁶ synthesized 2-substituted benzimidazole derivatives and evaluated their antimicrobial activity against gram positive bacteria (S. aureus, P. vulgaris and S. faecalis) and gram-negative bacterial strains (K. pneumoniae, Pseudomonas aeruginosa and Escherichia coli) by zone of inhibition method.

Table 2. Structure–Activity Relationship (SAR) of Benzimidazole Derivatives for antimicrobial action