Novel Pyrazole Carboxamide Containing a Diarylamine Scaffold Potentially Targeting Fungal Succinate Dehydrogenase: Antifungal Activity and Mechanism of Action

Design, synthesis and structure-activity relationship of novel pyrazole-4-carboxamide derivatives

BACKGROUND

Plant diseases seriously decrease the yield and quality of agricultural crops. Fungicide treatments remain the main means of field fungi control. However, the residual activity of fungicides is rapidly reduced due to various factors in the natural environment, therefore the development of agents with novel modes of action is desirable. It is highly required to design and develop new fungicides to address the resistance issue. Designing low impact chemicals to safely and sustainably address needs of agriculture.

RESULTS

In this work, we used the highly active fluxapyroxad and flutolanil as parent structures, to design and synthesize a series of pyrazole-4-carboxamide derivatives. Some of the pyrazole-4-carboxamide derivatives exhibit fungicidal activities that are comparable to or higher than those of the commercialized fungicides fluxapyroxad and bixafen. In particular, compounds TM-1, TM-2, TM-3, TM-4, TM-5, TM-7 and TM-8 showed excellent fungicidal activities against corn rust that were 2-4 times higher than those of fluxapyroxad and bixafen. Field trial results demonstrated that at the same dosage levels, compound TM-2 exhibited comparable field control efficacy against wheat rust as compared to triadimefon and pyrazophenamide. Molecular docking simulations reveal that compound TM-2 interacts with TRP 173 of succinate dehydrogenase (SDH) through hydrogen bonding, which could explain the probable mechanism of action between compound TM-2 and the target protein.

CONCLUSION

These results indicate that compound TM-2 may be a promising fungicide candidate and provide valuable reference for further investigation. © 2024 Society of Chemical Industry.

Discovery, Optimization, and Biological Evaluation of Novel Pyrazol-5-yl-phenoxybenzamide Derivatives as Potent Succinate Dehydrogenase Inhibitors

The diphenyl ether molecular pharmacophore has played a significant role in the development of fungicidal compounds. In this study, a variety of pyrazol-5-yl-phenoxybenzamide derivatives were synthesized and evaluated for their potential to act as succinate dehydrogenase inhibitors (SDHIs). The bioassay results indicate certain compounds to display a remarkable and broad-spectrum in their antifungal activities. Notably, compound 12x exhibited significant in vitro activities against Valsa mali, Gaeumannomyces graminis, and Botrytis cinerea, with EC50 values of 0.52, 1.46, and 3.42 mg/L, respectively. These values were lower or comparable to those of Fluxapyroxad (EC50 = 12.5, 1.93, and 8.33 mg/L, respectively). Additionally, compound 12x showed promising antifungal activities against Sclerotinia sclerotiorum (EC50 = 0.82 mg/L) and Rhizoctonia solani (EC50 = 1.86 mg/L), albeit lower than Fluxapyroxad (EC50 = 0.23 and 0.62 mg/L). Further in vivo experiments demonstrated compound 12x to possess effective protective antifungal activities against V. mali and S. sclerotiorum at a concentration of 100 mg/L, with inhibition rates of 66.7 and 89.3%, respectively. In comparison, Fluxapyroxad showed inhibition rates of 29.2 and 96.4% against V. mali and S. sclerotiorum, respectively. Molecular docking analysis revealed that compound 12x interacts with SDH through hydrogen bonding, π-cation, and π-π interactions, providing insights into the probable mechanism of action. Furthermore, compound 12x exhibited greater binding energy and SDH enzyme inhibitory activity than Fluxapyroxad (ΔGcal = -46.8 kcal/mol, IC50 = 1.22 mg/L, compared to ΔGcal = -41.1 kcal/mol, IC50 = 8.32 mg/L). Collectively, our results suggest that compound 12x could serve as a promising fungicidal lead compound for the development of more potent SDHIs for crop protection.

Synthesis, Structural Characterization, and Biological Activities of 1,3,4- Thiadiazole Derivatives Containing Sulfonylpiperazine Structures

To develop novel bacterial biofilm inhibiting agents, a series of 1,3,4-thiadiazole derivatives containing sulfonylpiperazine structures were designed, synthesized, and characterized using 1H nuclear magnetic resonance (1H NMR), 13C nuclear magnetic resonance (13C NMR), and high-resolution mass spectrometry. Meanwhile, their biological activities were evaluated, and the ensuing structure-activity relationships were discussed. The bioassay results showed the substantial antimicrobial efficacy exhibited by most of the compounds. Among them, compound A24 demonstrated a strong efficacy with an EC50 value of 7.8 μg/mL in vitro against the Xanthomonas oryzae pv. oryzicola (Xoc) pathogen, surpassing commercial agents thiodiazole copper (31.8 μg/mL) and bismerthiazol (43.3 μg/mL). Mechanistic investigations into its anti-Xoc properties revealed that compound A24 operates by increasing the permeability of bacterial cell membranes, inhibiting biofilm formation and cell motility, and inducing morphological changes in bacterial cells. Importantly, in vivo tests showed its excellent protective and curative effects on rice bacterial leaf streak. Besides, molecular docking showed that the hydrophobic effect and hydrogen-bond interactions are key factors between the binding of A24 and AvrRxo1-ORF1. Therefore, these results suggest the utilization of 1,3,4-thiadiazole derivatives containing sulfonylpiperazine structures as a bacterial biofilm inhibiting agent, warranting further exploration in the realm of agrochemical development.

Transcriptome analysis reveals the mechanism of antifungal peptide epinecidin-1 against Botrytis cinerea by mitochondrial dysfunction and oxidative stress

The marine antifungal peptide epinecidin-1 (EPI) have been shown to inhibit Botrytis cinerea growth, while the molecular mechanism have not been explored based on omics technology. This study aimed to investigate the molecular mechanism of EPI against B. cinerea by transcriptome technology. Our findings indicated that a total of 1671 differentially expressed genes (DEGs) were detected in the mycelium of B. cinerea treated with 12.5 μmol/L EPI for 3 h, including 773 up-regulated genes and 898 down-regulated genes. Cluster analysis showed that DEGs (including steroid biosynthesis, (unsaturated) fatty acid biosynthesis) related to cell membrane metabolism were significantly down-regulated, and almost all DEGs involved in DNA replication were significantly inhibited. In addition, it also induced the activation of stress-related pathways, such as the antioxidant system, ATP-binding cassette transporter (ABC) and MAPK signaling pathways, and interfered with the tricarboxylic acid (TCA) cycle and oxidative phosphorylation pathways related to mitochondrial function. The decrease of mitochondrial related enzyme activities (succinate dehydrogenase, malate dehydrogenase and adenosine triphosphatase), the decrease of mitochondrial membrane potential and the increase content of hydrogen peroxide further confirmed that EPI treatment may lead to mitochondrial dysfunction and oxidative stress. Based on this, we speculated that EPI may impede the growth of B. cinerea through its influence on gene expression, and may lead to mitochondrial dysfunction and oxidative stress.

Design, Synthesis, Antifungal Activity, and Action Mechanism of Pyrazole-4-carboxamide Derivatives Containing Oxime Ether Active Fragment As Succinate Dehydrogenase Inhibitors

The dearomatization at the hydrophobic tail of the boscalid was carried out to construct a series of novel pyrazole-4-carboxamide derivatives containing an oxime ether fragment. By using fungicide-likeness analyses and virtual screening, 24 target compounds with theoretical strong inhibitory effects against fungal succinate dehydrogenase (SDH) were designed and synthesized. Antifungal bioassays showed that the target compound E1 could selectively inhibit the in vitro growth of R. solani, with the EC50 value of 1.1 μg/mL that was superior to that of the agricultural fungicide boscalid (2.2 μg/mL). The observations by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) demonstrated that E1 could reduce mycelial density and significantly increase the mitochondrial number in mycelia cytoplasm, which was similar to the phenomenon treated with boscalid. Enzyme activity assay showed that the E1 had the significant inhibitory effect against the SDH from R. solani, with the IC50 value of 3.3 μM that was superior to that of boscalid (7.9 μM). The mode of action of the target compound E1 with SDH was further analyzed by molecular docking and molecular dynamics simulation studies. Among them, the number of hydrogen bonds was significantly more in the SDH-E1 complex than that in the SDH-boscalid complex. This research on the dearomatization strategy of the benzene ring for constructing pyrazole-4-carboxamides containing an oxime ether fragment provides a unique thought to design new antifungal drugs targeting SDH.

Inhibitory Activity Against Rhizoctonia Solani and Chemical Composition of Extract from Moutan Cortex

Rice sheath blight (RSB), caused by Rhizoctonia solani, is a significant disease of rice. The negative effects of chemical fungicides have created an urgent need for low-toxicity botanical fungicides. Our previous research revealed that the ethanol crude extract of Moutan Cortex (MC) exhibited superior antifungal activity against R. solani at 1000 μg/mL, resulting in a 100 % inhibition rate. The antifungal properties were mainly found in the petroleum ether extract. However, the active ingredients of the extract are still unclear. In this study, gas chromatography-mass spectrometry (GC-MS) was utilised for the analysis of its chemical components. The mycelium growth rate method was utilized to detect the antifungal activity. The findings indicated that paeonol constituted the primary active component, with a content of more than 96 %. Meanwhile, paeonol was the most significant antifungal active ingredient, the antifungal activity of paeonol (EC50=44.83 μg/mL) was much higher than that of β-sitosterol and ethyl propionate against R. solani. Observation under an optical microscope revealed that paeonol resulted in abnormal mycelial morphology. This study provided theoretical support for identifying monomer antifungal compounds and developing biological fungicides for R. solani.

Discovery of Pyrazole-5-yl-amide Derivatives Containing Cinnamamide Structural Fragments as Potential Succinate Dehydrogenase Inhibitors

To promote the development of novel agricultural succinate dehydrogenase inhibitor (SDHI) fungicides, we introduced cinnamamide and nicotinamide structural fragments into the structure of pyrazol-5-yl-amide by carbon chain extension and scaffold hopping, respectively, and synthesized a series of derivatives. The results of the biological activity assays indicated that most of the target compounds exhibited varying degrees of inhibitory activity against the tested fungi. Notably, compounds G22, G28, G34, G38, and G39 exhibited excellent in vitro antifungal activities against Valsa mali with EC50 values of 0.48, 0.86, 0.57, 0.73, and 0.87 mg/L, respectively, and this result was significantly more potent than boscalid (EC50 = 2.80 mg/L) and closer to the specialty control drug tebuconazole (EC50 = 0.30 mg/L). Compounds G22 and G34 also exhibited excellent in vivo protective and curative effects against V. mali at 40 mg/L. The SEM and TEM observations indicated that compounds G22 and G34 may affect normal V. mali mycelial morphology as well as the cellular ultrastructure. Molecular docking analysis results indicated that G22 and boscalid possessed a similar binding mode to that of SDH, and detailed SDH inhibition assays validated the feasibility of the designed compounds as potential SDH inhibitors. Compounds G22 and G3 were selected for theoretical calculations, and the terminal carboxylic acid group of this series of compounds may be a key region influencing the antifungal activity. Furthermore, toxicity tests on Apis mellifera l. revealed that compounds G22 and G34 exhibited low toxicity to A. mellifera l. populations. The above results demonstrated that these series of pyrazole-5-yl-amide derivatives are promising for development as potential low-risk drug-resistance agricultural SDHI fungicides.

Design, Synthesis, Insecticidal Activity, and SAR of Aryl Isoxazoline Derivatives Containing Pyrazole-5-carboxamide Motif

It is important to develop new insecticides with a new mode of action because of increasing pesticide resistance. In this study, a series of novel aryl isoxazoline derivatives containing the pyrazole-5-carboxamide motif were designed and synthesized. Their structures were confirmed by 1H NMR, 13C NMR, and HRMS. Bioassays indicated that the 24 compounds synthesized possessed excellent insecticidal activity against Mythimna separate and no activity against Aphis craccivora and Tetranychus cinnabarinus. Among these aryl isoxazoline derivatives, 3-(5-(3,5-dichlorophenyl)-5-(trifluoromethyl)-4,5-dihydrozol-3-yl)-N-(4-fluorophenyl)-1-methyl-1H-pyrazole-5-carboxamide (IA-8) had the best insecticidal activity against M. separate, which is comparable with the positive control fluralaner. The molecular docking results of compound IA-8 and fluralaner with the GABA model demonstrated the same docking mode between compound IA-8 and positive control fluralaner in the active site of GABA. Molecular structure comparisons and ADMET analysis can potentially be used to design more active compounds. The structure-activity relationships are also discussed. This work provided an excellent insecticide for further optimization.

Design, Synthesis, and Study of the Dual Action Mode of Novel N-Thienyl-1,5-disubstituted-4-pyrazole Carboxamides against Nigrospora oryzae

Due to the single target but extensive application of commercialized succinate dehydrogenase inhibitors (SDHIs), resistance problems have gradually become apparent in recent years. To solve this problem, a series of novel N-thienyl-1,5-disubstituted-1H-4-pyrazole carboxamide derivatives were designed and synthesized in this work based on the active skeleton 5-trifluoromethyl-4-pyrazole carboxamide. The bioassay results indicated that some target compounds exhibited excellent in vitro antifungal activities against the eight phytopathogenic fungi tested. Among them, the EC₅₀ values of T₄, T₆, and T₉ against Nigrospora oryzae were 5.8, 1.9, and 5.5 mg/L, respectively. The in vivo protective and curative activities of 40 mg/L T₆ against rice infected with N. oryzae were 81.5% and 43.0%, respectively. Further studies revealed that T₆ not only significantly inhibited the growth of N. oryzae mycelia but also effectively hindered spore germination and germ tube elongation. Morphological studies using scanning electron microscopy (SEM), fluorescence microscopy (FM), and transmission electron microscopy (TEM) found that T₆ could affect the mycelium membrane integrity by increasing cell membrane permeability and causing peroxidation of cellular lipids, and these results were further verified by measuring the malondialdehyde (MDA) content. The IC₅₀ value of T₆ against succinate dehydrogenase (SDH) was 7.2 mg/L, lower than that of the commercialized SDHI penthiopyrad (3.4 mg/L). Further, ATP content detection and the results after docking T₆ and penthiopyrad suggested that T₆ was a potential SDHI. These studies demonstrated that active compound T₆ could both inhibit the activity of SDH and affect the integrity of the cell membrane at the same time via a dual action mode, which is different from the mode of action of penthiopyrad. Thus, this study provides a new idea for a strategy to delay resistance and diversify the structures of SDHIs.