Design, synthesis, and evaluation of chalcone derivatives as antibacterial agents and monoamine oxidase inhibitors

Abstract

Antibiotics have been successfully used to treat bacterial infections since the discovery of penicillin in the early 20th century. However, the misuse of these compounds led to the widespread development of antimicrobial resistance in bacterial strains resulting in a drastic increase in morbidity and mortality. This combined with the decrease in the discovery and development of novel antibiotics emphasises the need for novel antibiotic lead compounds. Drug repurposing is a drug discovering technique which entails identifying novel indications for drugs that were originally designed for a different indication. Globally, Parkinson's disease (PD) is the second most common neurodegenerative disorder with a rising incidence mostly attributed to the increase in aging populations and industrialisation. Levodopa is the first-choice treatment for patients with PD, although its efficacy decreases as the disease progresses with patients commonly experiencing wearing-off periods. It is evident that more novel avenues of symptomatic treatment are needed. Chalcones are simple natural chemical scaffolds with various biological effects including antibacterial and anti-Parkinson activities. Nitrocatechol-chalcones have previously been shown to target not only catechol O-methyl transferase (COMT), but also monoamine oxidase (MAO), both targets for neurodegenerative diseases. Furthermore, chalconecontaining compounds have been shown to have antibacterial activity against a wide variety of bacteria, however, there is no known antibacterial target. This study aimed to determine whether chalcone-containing compounds with known MAO activity possess antibacterial properties, starting with the screening of these derivatives against Acinetobacter baumannii, Enterobacter aerogenes, Klebsiella pneumoniae, and Staphylococcus aureus using the broth microdilution method. The in vitro data were utilised to create and validate a common feature pharmacophore model by determining the rank score, enrichment factor (EF), and area under the receiver operating characteristic - area under the curve (ROC-AUC). The pharmacophore model aided in the design of novel compounds of which the antibacterial activity was determined against the relevantpathogens. A checkerboard assay was performed to indicate a possible antibacterial mode of action. Lastly, the novel derivatives were screened for activity against MAO-A and MAOB and their mode of inhibition was determined via Lineweaver-Burke plots. In the first article for this study, it was found that previously synthesised nitrocatecholchalcone and nitrocatechol-pyrazoline derivatives with activity against MAO-A, MAO-B and/or COMT showed activity against S. aureus with compounds 2a, 2b and 1b (1 - 2 µg/ml) (article 1) having activity comparable to that of tetracycline. A common feature pharmacophore model was created (rank score: 84.02, maximum fit value: 4), which was able to identify active chalcone and pyrazoline derivatives out of a test set (EF10%: 6.7, ROCAUC: 0.85). The pharmacophore model highlighted that four hydrogen bond acceptors are important for antibacterial activity. Utilising the pharmacophore data, novel nitrocatecholchalcone and pyrazoline derivatives were synthesised after which the minimum inhibitory concentration (MIC) of each derivative was determined against both the drug-susceptible and resistant S. aureus (MRSA) strains. The most active compounds were 3i (0.5 µg/ml) and 3c (0.5 µg/ml) (article 1) against the susceptible and resistant strains respectively, which were more active than tetracycline. The most active pyrazoline derivatives were 4a and 4c(article 1), with activities of 16 µg/ml and 32 µg/ml against the susceptible and resistant strains, respectively. In the second article for this study, novel guaiacol-chalcone and nitroguaiacol-chalcone derivatives were synthesised and their antibacterial activity determined against S. aureus, MRSA, K. pneumoniae, A. baumannii, Pseudomonas aeruginosa and K. aerogenes. The most active derivative was nitroguaiacol-chalcone derivative 2c (8 µg/ml) (article 2), with activity against the susceptible S. aureus and MRSA strains. The checkerboard assay demonstrated that the nitroguaiacol-chalcone derivative (2a) (article 2) displayed a synergistic effect when combined with chloramphenicol, yielding a fractional inhibitory concentration index (FICI) of 0.45 ± 0.09. This finding suggests that the antibacterial mechanism of action of 2a may also involve targeting bacterial protein synthesis. A pharmacophore model was developed and validated (max. fit: 4, rank score: 52.34, EF10%: 8, ROC-AUC: 0.95 ± 0.02). It indicated four features that are important for antibacterial activity i.e., ring aromatic, a hydrophobic moiety and two hydrogen bond acceptors.In the third article of this study, the novel chalcone and pyrazoline derivatives synthesised for both articles 1 and 2 were evaluated for MAO inhibition by determining their half-maximal inhibitory concentration (IC50) values. Overall, the chalcone derivatives were more potent inhibitors of MAO-B compared to MAO-A and the pyrazoline derivatives had comparable MAO-A and MAO-B activity. The guaiacol-chalcone derivative, 3c (article 2) had the most potent inhibition against MAO-B with an IC50 of 0.06 ± 0.01 μM, which is more potent than the reference inhibitor safinamide (IC50 = 0.12 ± 0.02 μM), however, showed less potent inhibition against MAO-A with an IC50 of 0.49 ± 0.03 μM. The guaiacol-chalcone derivatives, 3b and 3f (article 2) exhibited inhibitory action comparable to safinamide with an IC50-value of 0.12 ± 0.01 μM against MAO-B. The Lineweaver-Burk graphs indicated that compound 3c (article 2) competitively inhibited both MAO-A and MAO-B. A pharmacophore model was developed and subsequently validated, showing a maximum fit of 4, a rank score of 62.24, EF10% of 7.5, and a ROC-AUC of 1.00 ± 0.002. This pharmacophore model suggested that two hydrophobic features, along with one hydrogen bond donor and one hydrogen bond acceptor, are important for MAO inhibitory activity.