Quinolone appended aryl diamines: Design, synthesis and in vitro antibacterial evaluation

Abstract

Tuberculosis (tb) is a communicable disease that has led to considerable mortality rates throughout history. the cause of tb is mycobacterium tuberculosis (mtb), a pathogenic bacterium. The World Health Organisation (WHO) reports that Mtb infects almost 2 billion people globally, and that it imposes a great burden on societies, healthcare systems, and the economies of many countries, especially in developing countries. The WHO’s Global Tuberculosis Report 2022 states that in 2020, 10.1 million people became ill from TB and 1.5 million died, of which 214 000 had a human immunodeficiency virus (HIV) co-infection. In 2021, it was reported that there were 10.6 million people who became ill from TB and 1.6 million who died, of which 187 000 were people who had lived with HIV. Being HIV-positive and being co-infected with TB is particularly debilitating; HIV-positive patients have a suppressed immune system and may develop severe TB disease more easily. The recent coronavirus disease 2019 (COVID-19) pandemic has also had negative consequences for worldwide efforts to control the spread of TB, owing to the suspension of many TB health service providers due to movement restrictions. TB therapy regimens consist of two phases, namely the intensive phase and the continuation phase, which last for a minimum of six months. Second-line drugs are utilised when first-line drugs fail to eradicate Mtb. First-line drugs include isoniazid (INH), rifampicin (RIF), ethambutol (EMB), and pyrazinamide (PYZ). Mtb possesses high levels of antibiotic resistance, which may be intrinsic or acquired. Unfortunately, TB therapy compliance rates are low due to the lengthy regimens and high toxicity of the treatments. Low compliance rates also aid further development of drug resistance in Mtb. Recent studies have shown that the TB crisis is worsening due to a lack of novel antitubercular agents and the emergence of drug-resistant Mtb strains. Antibacterial resistance is prevalent to an even higher degree in pathogens known as the ESKAPE pathogens. ESKAPE is an acronym for the scientific nomenclature of six increasingly antibiotic-resistant (ABR) bacteria. These ABR bacteria are Enterococcus faecium (E. faecium), Staphylococcus aureus (S. aureus), Klebsiella pneumoniae (K. pneumoniae), Acinetobacter baumannii (A. baumannii), Pseudomonas aeruginosa (P. aeruginosa), and Enterobacter spp. Escherichia coli (E. coli) is a bacterium that also exhibits antibiotic resistance, albeit to a lesser extent than the ESKAPE pathogens. Nevertheless, E. coli quickly gains resistance through various mechanisms. In 2017, the WHO announced that the ESKAPE pathogens, along with six other bacteria, urgently require novel antibiotics. The WHO described three classes of bacteria (critical, high, medium) according to their need for novel antibiotics. Carbapenem-resistant A. baumannii (CRAB), carbapenem-resistant P. aeruginosa (CRPA), extended spectrum β-lactamase (ESBL) K. pneumoniae, carbapenem-resistant K. pneumoniae (CRKP), and carbapenem-resistant Enterobacteriacae (CRE) were classified as critical. Vancomycin-resistant Enterococcus (VRE), methicillin-resistant S. aureus (MRSA), and vancomycin-resistant S. aureus (VRSA) were classified as high priority. The ESKAPE pathogens are able to ‘escape’ the inhibitory effects of various antibiotics. Around 15.5% of nosocomial infections are caused by ABR bacteria, of which the ESKAPE bacteria are the main pathogens. A survey performed in 2011 in the United States reported a total of 722 000 cases of nosocomial infections, with 75 000 deaths being nosocomial-related. Another survey conducted in 2002 revealed that approximately 1.7 million cases of all reported bacterial cases of infection were nosocomial, which resulted in 99 000 deaths annually. There are a limited amount of treatments for ESKAPE pathogen infections due to their ever-rising antibiotic resistance and the lack of novel antibacterials in the market. Combination therapy, such as trimethoprim (TMP) and sulfamethoxazole (SMX) combinations, is often effective against some ESKAPE pathogens. However, current treatments are not adequate to address the antibiotic resistance emergency, and novel treatments or strategies need to be investigated. Molecular hybridization (MH) is a type of combination therapy in which two pharmacologically active molecules are linked together to simultaneously exert two different mechanisms of action. This study proposes the utilisation of this strategy by linking a quinolone ring to a 2,4-diamino-1,3,5-triazine scaffold to generate novel hybrid compounds. Quinolones are privileged scaffolds with pharmacological applications in various fields of medicine. Quinolones exert their antibacterial effects by inhibiting specific enzymes, namely deoxyribonucleic acid (DNA) gyrase, topoisomerase II, and topoisomerase IV. This impedes nucleic acid synthesis and causes chromosomal breakage, resulting in bacterial cell death. Quinolones comprise more than 70% of novel antibiotics currently in use. This implies that quinolones have adequate drug-like properties. Moreover, quinolones are synthetically tractable; they can be synthesised via various synthetic procedures. It has been reported that 2,4-diamino-1,3,5-triazine derivatives elicit activity against certain bacteria, such as E. coli and Mtb. They achieve this activity by acting as potent inhibitors of the dihydrofolate reductase (DHFR) enzyme. Target compounds 5d, 5i-5l, 6c-6i, and 8a-8d were evaluated in vitro for antitubercular, antibacterial, and antifungal activity. The green-fluorescent protein (GFP) reporter strain of Mtb was cultured in middlebrook 7H9 media supplemented with 10% albumin-dextrose-catalase (ADC), glucose (GLU), and 0.05% Tween-80 (Tw). Antitubercular activity was reported in micro molar (μM) as the minimum concentration required to inhibit 90% of the bacterial population (MIC90), and hit compounds were compounds with an MIC90 value of <10 μM. RIF was used as the reference for this assay. Six compounds demonstrated antitubercular activity and were considered hit compounds, with MIC90 values ranging from <0.244 μM–7.625 μM, namely 6c, 6d, 6g, 6h, 8a, and 8c. Three compounds demonstrated submicromolar MIC90 values, namely 6c, 8a, and 8c. Compound 8c showed the highest activity, with an MIC90 value of <0.244 μM. All the compounds tested in this study did not exhibit activity against bacteria or fungi. The majority of the compounds showed acceptable cLogP values that ranged from 0.064 to 4.099, and molecular weights less than 500 daltons (Da), indicating adequate drug-like properties. These compounds warrant further investigation as antimycobacterial agents.