Molecular interaction of novel benzothiazolyl triazolium analogues with calf thymus DNA and HSA-their biological investigation as potent antimicrobial agents

Highlights

• Novel imidazo [2,1-b] benzothiazolyl triazolium analogues were synthesized and assessed for antimicrobial activity.

• Interaction mode between calf thymus DNA and compound 5q was explored by spectroscopic techniques.

• Good molecular docking score revealed the interaction of compound 5q with HSA at sub-domain IIA binding site.

• Cytotoxic investigation was carried out on four different cancerous cell lines by MTT Assay.

Abstract

The binding behaviour between calf thymus DNA and synthesized benzothiazolyl triazolium derivatives as potent antimicrobial agents was explored by means of spectroscopic applications together with molecular docking study at the sub-domain IIA, binding site I of human serum albumin (HSA). Most of the synthesized derivatives presented significant antimicrobial inhibition when compared with the clinical Norfloxacin, Chloromycin, and Fluconazole. In particular, compound 5q presented efficient anti-Bacillus subtilis, anti-Escherichia coli, anti-Salmonella typhi, and anti-Psuedomonas aeruginosa activity with low MIC values of 2–8 μg/mL which were relatively superior to the reference drugs. The preliminarily investigation of interaction studies with calf thymus DNA demonstrated that the most active compound 5q could effectively intercalate into DNA to form 5q-DNA complex. Further investigations revealed that human serum albumin could effectively transport compound 5q while molecular modelling studies with good docking score showed that hydrophobic interactions as well as hydrogen bonds played a significant role in the interaction of compound 5q with HSA. In addition, the cytotoxic investigation carried out on four different cancerous cell lines (3 human cell lines and 1 murine cell lines) by MTT assay presented that compound 5n is active against MDA cell lines with IC₅₀ values less than 100 μg/mL.

Introduction

Antibiotic resistance that hasten by the abuse of clinical antimicrobial medicines has been a critical worldwide menace for public health. A spectacular intensity of pathogenic bacteria has escalated globally over the past few years as a result of developed resistance to antibiotics [1]. Consequently most of the infections triggered by resistant microorganisms remained irresponsive to the traditional treatment while antibiotics even have lost their control in few cases [2]. Antibiotic modification is considered as one of the most significant resistance mechanisms towards the β-lactam and aminoglycoside classes of antibacterial drugs. A World Health Organization (WHO) article published in April 2014 stated that “This severe menace would no longer be a prediction for the future, which is happening in all regions of the world and has the ability to affect anyone of any age. Consequently, antibiotics would no longer be useful to people to treat infections henceforth this recently is a major threat to public health’’ [3]. On the other hand, similar to bacteria, even some fungi no longer respond to the antifungal medications that are intended to treat them and this budding phenomenon is a predominant concern for invasive infections [4]. Moreover, some species of Candida are developing increasing resistance to first-line and second-line antifungal medications, specifically, Fluconazole and echinocandins [5]. Consequently, in order to prevail over these circumstances, there is demanding requirement to develop novel antimicrobial agents that can endure microbial resistance [6,7].

Interestingly, triazole derivatives like Miconazole, Econazole, followed by Ketoconazole, Fluconazole, and Itraconazole, were earlier reported as significant clinical candidates for treating human fungal infections. These azole antifungal drugs restrict the fungal growth by inhibiting an enzyme lanosterol 14 α-demethylase which is essential for the conversion of lanosterol to ergosterol. Exhaustion of ergosterol in fungal membrane interrupts the structure and various other metabolic functions of fungal membrane, thereby restricting the fungal growth [8]. In particular, 1,2,3-triazoles are previously reported for possessing a huge extent of pharmacological properties such as antimicrobial [9], antimalarial [10], antiviral [11], antimycobacterial [12], and anticonvulsant [13]. 1,2,3-triazoles are well known for their high aromatic stability, dipole moment (5D), and own dynamic ability to form hydrogen bonds, dipole-dipole and π-π stacking interactions which facilitate them to bind with the biological targets effortlessly thus enhancing their solubility [[14], [15], [16]]. Consequently, triazoles have been exploited to enhance the pharmacokinetic abilities of the desired drug.

This study aims to synthesize hybrid molecules through the combination of different chemical entities with the intention of achieving potent antibacterial and antifungal lead compounds. Benzothiazole derivatives continue to be one of the most versatile classes of compounds against several microbial infections, thus, are valuable substructures for extended molecular exploration. A great deal of research has revealed that benzothiazolyl derivatives possess significant potential as antimicrobial drugs [17]. Structural modifications on benzothiazoles resulted in numerous derivatives which were further studied for their diverse biological properties, like antimicrobial [18], anti-inflammatory [19], anticonvulsant [20], immunosuppressive [21], antiallergenic [22], and antitumor activities [23]. For instance, some of the clinically active 1,2,3 triazole and benzothiazole analogues were represented in Fig. 1. Fused imidazo heterocycles like imidazo[2,1-b] benzothiazole containing ring-junction nitrogen atom play a prominent role in the area of medicinal chemistry [24]. Consequently, owing to such eminence and incidence of fused imidazole frameworks in the field of main medicinal chemistry, an efficient protocol for the development of this motif is desirable.

In addition, human serum albumin (HSA), a well renowned transport protein possesses various significant pharmacological and physiological properties [25]. The chief function of HSA is the transport of drug to the binding site, when drugs bind reversibly to it. Apparently, the study of small bioactive molecule or drug interaction with deoxyribonucleic acid (DNA) and HSA protein not only can serve as a valuable source to understand about drug distribution, transportation, absorption along with drug metabolism but also to intend and screen drug motifs [26]. Interaction of small molecules to DNA has the ability to impact various biological functions that involve DNA participation such as transcription and replication [27]. In the present scenario, there is a demanding attention towards the study of binding behaviour between bioactive molecules and DNA to explore rational design and development of novel and effective drugs. While the interaction between drugs and HSA can bring about a change in the overall metabolism, efficacy, and distribution of drugs. A number of newly invented drugs were proved to be ineffective as they possess too high or too low affinity towards this protein. Henceforth, based on the preliminary anti microbial assay, pharmacokinetic properties and transportation of highly active compounds were studied by evaluating their binding affinity with DNA and HSA [28].

In view of the above aspects, it was of immense consideration to synthesize triazolium analogues of imidazo [2,1-b] bezothiazole that were more likely to possess enhanced drug-resistance along with improved antimicrobial properties. In the current work, the YM-201627 (Fig. 1) drug like benzothiazole pharmacophore is taken as structural back bone and linked with biologically active 1,2,3-triazolium bearing substituted aliphatic and aromatic moieties that possess significant physiological properties to achieve the ideal of potent antimicrobial agents. The design idea is represented in Fig. 2.

Reasonably, different aliphatic chains with various lengths and other substitutions like chloro, fluoro, and nitro groups on the phenyl ring at R₂, R_3, and R₄ positions were further modified based on the preliminary active screening (Scheme 1).