Novel plastoquinone analogs containing benzocaine and its analogs: structure‐based design, synthesis, and structural characterization

Aminated Quinolinequinones as Privileged Scaffolds for Antibacterial Agents: Synthesis, In Vitro Evaluation, and Putative Mode of Action

Our previous studies have revealed that the aminated 1,4-quinone scaffold can be used for the development of novel antibacterial and/or antifungal agents. In this study, the aminated quinolinequinones (AQQ1-9) were designed, synthesized, and evaluated for their antimicrobial activity against a panel of seven bacterial strains (three Gram-positive and four Gram-negative bacteria) and three fungal strains. The structure-activity relationship (SAR) for the QQs was also summarized. The antibacterial activity results indicated that the two aminated QQs (AQQ6 and AQQ9) were active against Enterococcus faecalis (ATCC 29212) with a MIC value of 78.12 μg/mL. Besides, the two aminated QQs (AQQ8 and AQQ9) were active against Staphylococcus aureus (ATCC 29213) with MIC values of 4.88 and 2.44 μg/mL, respectively. The most potent aminated QQs (AQQ8 and AQQ9) were identified as promising lead molecules to further explore their mode of action. The selected QQs (AQQ8 and AQQ9) were further evaluated in vitro to assess their potential antimicrobial activity against each of 20 clinically obtained methicillin-resistant S. aureus isolates, antibiofilm activity, and bactericidal activity using time-kill curve assay. We found that the molecules prevented adhesion of over 50% of the cells in the biofilm. Molecular docking studies were performed to predict the predominant binding mode(s) of the ligands. We believe that the molecules need further investigation, especially against infections involving biofilm-forming microbes.

Exploring the Relationships between Structure and Antimicrobial Potency of Quinolinequinones

Microorganisms are responsible for hospital infections, and methicillin-resistant Staphylococcus aureus is one of them. In looking for the most effective lead structures to cope with the rise of antimicrobial (antibiotic) resistance, we evaluated the antimicrobial profile of quinolinequinones for potential antimicrobial applications. 1,4-quinone molecules fused with heteroatom have been studied extensively for many years as a source of drugs and lead structures. The aims of this study were to evaluate the antimicrobial activity of quinolinequinones against bacterial and fungal strains, and to probe for potential lead structures. For this reason, the activity of these compounds against three different strains of Candida fungi (C. albicans, C. parapsilosis, and C. tropicalis) and Gram-positive and Gram-negative pathogenic bacteria were investigated, searching for potential lead compounds. Five of nine quinolinequinones showed activity mainly against the Gram-positive strains with a minimal inhibitory concentration within the Clinical and Laboratory Standards Institute (CLSI) levels. The results revealed that quinolinequinones have significant activity against bacteria including Staphylococcus aureus and Staphylococcus epidermidis, and fungi including Candida albicans and Candida parapsilosis. QQ1, QQ2, QQ3, QQ5, and QQ6 exhibited the highest growth inhibition against two essential species of the Gram-positive strains (Staphylococcus epidermidis and Staphylococcus aureus). Among these, four molecules (QQ2, QQ3, QQ5, and QQ6) were also active against Enterococcus faecalis, the other member of the Gram-positive strains. The antifungal profile of two quinolinequinones (QQ7 and QQ8) indicated that they were as effective as the reference drug Clotrimazole against Candida albicans. The same molecules also have potential inhibitory antifungal activity against Candida tropicalis. For better understanding, the most active two quinolinequinones (QQ2 and QQ6) were examined for biofilm inhibition and a time-kill kinetic study.

In Vitro and In Silico Study to Assess Toxic Mechanisms of Hybrid Molecules of Quinone-Benzocaine as Plastoquinone Analogues in Breast Cancer Cells

We managed to obtain three different series of 2,3-dimethyl-1,4-benzoquinones, named nonhalogenated and halogenated (brominated and chlorinated) PQ analogues, via the molecular hybridization strategy. Sixteen of eighteen hybrid molecules were selected by the National Cancer Institute (NCI) of Bethesda for their in vitro antiproliferative potential against the full NCI 60 cell line panel. The hybrid molecules (BrPQ5, CIPQ1, and CIPQ3) showed good growth inhibition at 10 μM concentration, particularly against breast cancer cell lines. As per the results obtained from in vitro antiproliferative evaluation, cytotoxic activities of the hybrid molecules (BrPQ5, CIPQ1, and CIPQ3) were evaluated with an 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay in T47D and MCF7 breast cancer and human umbilical vein endothelial (HUVEC) cells. Molecules exhibited cytotoxic activity, and especially, CIPQ1 showed remarkable cytotoxic activity and good selectivity on T47D and MCF7 cells. Furthermore, CIPQ1 could inhibit cell proliferation, cause apoptotic cell death and disturb the cell cycle in T47D and MCF7 cells. Additionally, CIPQ1 caused oxidative stress induction in both cells, more so in T47D. In vitro study results indicated that the anticancer activity of CIPQ1 was more prominent in T47D cells than in MCF7 cells. The compound CIPQ1 showed a prominent binding with JNK3 in silico. Thus, the obtained hybrid molecules via the molecular hybridization strategy of two important pharmacophores could be useful in the discovery of novel antiproliferative agents, and CIPQ1 could be considered a promising drug candidate.

Exploring the Anticancer Effects of Brominated Plastoquinone Analogs with Promising Cytotoxic Activity in MCF-7 Breast Cancer Cells via Cell Cycle Arrest and Oxidative Stress Induction

Plastoquinone analogs are privileged structures among the known antiproliferative natural product-based compound families. Exploiting one of these analogs as a lead structure, we report the investigation of the brominated PQ analogs (BrPQ) in collaboration with the National Cancer Institute of Bethesda within the Developmental Therapeutics Program (DTP). These analogs exhibited growth inhibition in the micromolar range across leukemia, non-small cell lung cancer (EKVX, HOP-92, and NCI-H522), colon cancer (HCT-116, HOP-92), melanoma (LOX IMVI), and ovarian cancer (OVCAR-4) cell lines. One brominated PQ analog (BrPQ5) was selected for a full panel five-dose in vitro assay by the NCI’s Development Therapeutic Program (DTP) division to determine GI50, TGI, and LC50 parameters. The brominated PQ analog (BrPQ5) displayed remarkable activity against most tested cell lines, with GI50 values ranging from 1.55 to 4.41 µM. The designed molecules (BrPQ analogs) obeyed drug-likeness rules, displayed a favorable predictive Absorption, Distribution, Metabolism, and Excretion (ADME) profile, and an in silico simulation predicted a possible BrPQ5 interaction with proteasome catalytic subunits. Furthermore, the in vitro cytotoxic activity of BrPQ5 was assessed, and IC50 values for U-251 glioma, MCF-7 and MDA-MB-231 breast cancers, DU145 prostate cancer, HCT-116 colon cancer, and VHF93 fibroblast cell lines were evaluated using an MTT assay. MCF-7 was the most affected cell line, and the effects of BrPQ5 on cell proliferation, cell cycle, oxidative stress, apoptosis/necrosis induction, and proteasome activity were further investigated in MCF-7 cells. The in vitro assay results showed that BrPQ5 caused cytotoxicity in MCF-7 breast cancer cells via cell cycle arrest and oxidative stress induction. However, BrPQ5 did not inhibit the catalytic activity of the proteasome. These results provide valuable insights for further discovery of novel antiproliferative agents.

Promising Antibacterial and Antifungal Agents Based on Thiolated Vitamin K3 Analogs: Synthesis, Bioevaluation, Molecular Docking

In the present study, we designed and synthesized thiolated VK3 analogs (VK3a–g) along with an extensive antimicrobial study. After the evaluation of the antibacterial and antifungal activity against various bacterial and fungal strains, we presented an initial structure–activity relationship study on these VK3 analogs. In particular, four thiolated VK3 analogs exhibited superior biological potency against some Gram-positive bacterial strains, including Staphylococcus aureus (ATCC^® 29213) and Enterococcus faecalis (ATCC^® 29212). Next, all thiolated VK3 analogs were evaluated for their potential of cell growth inhibition on the NCI-60 cancer cell lines panel. This screening underlined that the thiolated VK3 analogs have no visible cytotoxicity on different cancer cell lines. The selected two thiolated VK3 analogs (VK3a and VK3b), having minimal hemolytic activity, which also have the lowest MIC values on S. aureus and E. faecalis, were further evaluated for their inhibition capacities on biofilm formation after evaluating their potential in vitro antimicrobial activity against each of the 20 clinically obtained resistant strains of Staphylococcus aureus. VK3b showed excellent antimicrobial activity against clinically resistant S. aureus isolates. Furthermore, the tested molecules showed nearly two log₁₀ reduction in the viable cell count at six hours according to the time kill curve studies. Although these molecules decreased biofilm attachment about 50%, when sub-MIC concentrations were used these molecules increased the percentage of biofilm formation. The molecular docking of VK3a and VK3b in S. aureus thymidylate kinase was conducted in order to predict their molecular interactions. VK3a and VK3b exhibited excellent lead-likeness properties and pharmacokinetic profiles that qualify them for further optimization and development. In conclusion, since investigating efficient novel antimicrobial molecules is quite difficult, these studies are of high importance, especially in the present era of antimicrobial resistance.

Highly Active Small Aminated Quinolinequinones against Drug-Resistant Staphylococcus aureus and Candida albicans

Two subseries of aminated quinolinequinones (AQQs, AQQ1-16) containing electron-withdrawing group (EWG) or electron-donating group (EDG) in aryl amine moiety were successfully synthesized. Antimicrobial activity assessment indicates that some of the AQQs (AQQ8-10 and AQQ12-14) with an EDG in aryl amine exhibited strong antibacterial activity against Gram-positive bacterial strains, including Staphylococcus aureus (ATCC^® 29213) and Enterococcus faecalis (ATCC^® 29212). In contrast, AQQ4 with an EWG in aryl amine displayed excellent antifungal activity against fungi Candida albicans (ATCC^® 10231) with a MIC value of 1.22 μg/mL. To explore the mode of action, the selected AQQs (AQQ4 and AQQ9) were further evaluated in vitro to determine their antimicrobial activity against each of 20 clinically obtained resistant strains of Gram-positive bacteria by performing antibiofilm activity assay and time-kill curve assay. In addition, in silico studies were carried out to determine the possible mechanism of action observed in vitro. The data obtained from these experiments suggests that these molecules could be used to target pathogens in different modes of growth, such as planktonic and biofilm.

Discovery and structure-activity relationships of the quinolinequinones: Promising antimicrobial agents and mode of action evaluation

In our pursuit of developing the novel, potent, and selective antimicrobial agents, we managed to obtain the quinolinequinone for their antimicrobial profile with minimal inhibitory concentrations (MICs) determined against a panel of seven bacterial strains (three gram-positive and four gram-negative bacteria) and three fungi. The structure-activity relationship (SAR) for the quinolinequinone class of antimicrobials was determined. Interestingly, QQ1, QQ4, QQ6-9, QQ12, and QQ13 displayed equal antibacterial potential against S. aureus (MIC = 1.22 mg/L), respectively, to the standard positive control Cefuroxime-Na. QQ10 had the best inhibitory activity with the MIC value of 1.22 mg/L (fourfold more potent compared to reference standard Clotrimazole) against Candida albicans. On the other hand, while QQ10 is not too effective against gram-positive bacteria as much as the other analogs, QQ10 was the most effective quinolinequinones against fungi. Selected quinolinequinones were further evaluated for the mode of action, using in vitro antibiofilm activity, bactericidal activity by using time-kill curve assay, antibiofilm activity, and potential antimicrobial activity against each of 32 clinically obtained resistant strains of Gram-positive Bacteria. The results also revealed that the QQ14 had specific antifungal activity against fungi in particular C. albicans. Our results clearly showed that quinolinequinones are much more active in the inhibition of the biofilm attachment process than the inhibition of mature biofilm formation. Thus, as treatment options are narrowing for Methicillin-resistant Staphylococcus spp., Vancomycin-resistant Staphylococcus spp. daily, the quinolinequinones reported herein display promise as the lead candidates for further clinical applications against serious infections.

Exploration of brominated Plastoquinone analogs: Discovery and structure-activity relationships of small antimicrobial lead molecules

In the fight with the antimicrobial resistance, our continuous effort to find quinone analogs with higher inhibitory activity has previously led us to the promising Plastoquinone analogs. The 1,4-quinone moiety substituted with alkoxy substituent(s) plays an important role in the field of antimicrobial and anticancer drug discovery and development. Thus, an extensive series of 1,4-quinones, substituted in different positions with a variety of alkoxy substituents, has been designed, synthesized, and evaluated for their antimicrobial activity. Here, we describe the synthesis of brominated Plastoquinone analogs (BrPQ1-15) based on the dimethyl-1,4-quinone scaffold by employing two different paths. We also present here the in vitro antimicrobial activity of these analogs (BrPQ1-15) against a panel of pathogenic organisms. These studies resulted in several new selective antibacterial inhibitors and gave valuable insights into the structure-activity relationships. Among all the analogs studied, two analogs BrPQ1 with a methoxy substituent and BrPQ14 with a cyclic dioxy stand out as the most promising antibacterial molecules against Staphylococcus aureus and Staphylococcus epidermidis. Afterwards, two analogs were selected for a further investigation for biofilm evaluation. Finally, molecular docking studies for BrPQ1 and BrPQ14 with probable target S. aureus PNPase (5XEX) and predictive ADMET studies were also carried out.