Discovery of a Fungicide Candidate Targeting Succinate Dehydrogenase via Computational Substitution Optimization

Evaluating AutoGrow4 - an open-source toolkit for semi-automated computer-aided drug discovery

INTRODUCTION

Drug discovery is a long and expensive process characterized by a high failure rate. To make this process more rational and efficient, scientists always look for new and better ways to design novel ligands for a target of interest. Among different approaches, de novo ones gained popularity in the last decade, thanks to their ability to efficiently explore the chemical space and their increasing reliability in generating high-quality compounds. Autogrow4 is open-source software for de novo drug design that generates ligands for a given target by exploiting a combination of a genetic algorithm and molecular docking calculations.

AREAS COVERED

In the present paper, the authors dissect this program's usefulness and limitations in generating new compounds from a pharmacodynamic and pharmacokinetic perspective. Specifically, this article examines all reported applications of the Autogrow code in the literature (as retrieved from the Scopus database) from the release of its first version in 2009 to the present.

EXPERT OPINION

In the hands of an expert molecular modeler, Autogrow4 is a useful tool for de novo ligand design. Its modular and open-source codebase offers many protocol customization features. The main downsides are limited control over the pharmacokinetic features of generated ligands and the bias toward high molecular weight compounds.

Discovery of a Class of Novel Succinate Dehydrogenase Inhibitors Containing a Coumarin Structure

Succinate dehydrogenase (SDH) has emerged as a highly promising target in modern agricultural research, playing a crucial role in developing environmentally friendly and efficient fungicides for combating agricultural pathogens. This study presents the discovery of a novel class of SDH inhibitors (I-III) incorporating coumarin segments achieved through an active fragment swapping and linking strategy. Fungicidal activity assays revealed that several compounds within this series demonstrate significant inhibitory effects against the tested fungal strains. Through comprehensive structure-activity relationship studies, compound N-(1-((3-butyl-4-methyl-2-oxo-2H-chromen-7-yl)oxy)propan-2-yl)-3-(difluoromethyl)-N-methoxy-1-methyl-1H-pyrazole-4-carboxamide (IIk) exhibited potent inhibitory activity against various fungal species. Notably, it demonstrated superior efficacy against S. sclerotiorum with an EC50 value of 1.14 μg/mL, outperforming the commercial control agent thifluzamide (EC50 = 4.90 μg/mL). Molecular docking simulations indicated that hydrophobic interactions serve as the primary binding mechanism between the ligand and SDH. Intriguingly, compound IIk displayed dual functionality, not only acting as an effective fungicide but also promoting the growth of wheat seedlings and Arabidopsis thaliana, resulting in increased plant biomass. Preliminary investigations into its growth-promoting mechanism suggest that IIk enhances the nitrate reductase activity, thereby facilitating plant growth.

One-Pot Synthesis of Chiral Succinate Dehydrogenase Inhibitors and Antifungal Activity Studies

In this work, a series of novel chiral succinate dehydrogenase inhibitors (SDHIs) are synthesized through a one-pot Rh-catalyzed asymmetric hydrogenation-condensation strategy. This method exhibits high efficiency (up to 1000 Ton, 94% yield over two steps), high stereoselectivity (up to 99% ee), and broad substrate scope (68 examples in total), providing a superior pathway for the synthesis of such chiral fungicides. Mechanistic studies indicate that the amino group at the 2-position of the phenyl ring acts as an activating group, enhancing the reactivity and stereoselectivity control of the reaction. Furthermore, these molecules exhibit broad-spectrum and highly effective antifungal biological activity. Notably, enantiomers show significant differences in both in vitro and in vivo fungi-inhibiting experiments. Especially, (S)-5f showcases an antifungal activity against Botrytis cinerea (EC50 = 0.48 µm) that is much higher than that of its R enantiomer (EC50 = 36.7 µm). Molecular docking calculations, molecular dynamic simulation, enzyme activity assays, and ligand-target interaction experiments demonstrate that (S)-5f (ΔGMM-PBSA = -18.86 kcal mol-1, KD = 6.04 µm) inhibits succinate dehydrogenase more effectively than its R enantiomer (ΔGMM-PBSA = -13.01 kcal mol-1, KD = 8.5 µm). Moreover, the two enantiomers have significantly different effects on spore germination and the destruction of fungal phenotype.

Discovery of Novel Quinazolinone-2-carbohydrazide Derivatives as Effective Succinate Dehydrogenase Inhibitors and Biosafety Assessment on Rice and Zebrafish

To develop potent and safe antifungal agents in agriculture, a total of 48 novel quinazolinone-2-carbohydrazide derivatives were designed and synthesized based on the pharmacophore hybridization method. The bioassay results demonstrated that many compounds exhibited powerful and broad-spectrum inhibition activities in vitro against the tested fungi. For instance, compounds E23 and F23 possessed EC50 (half-maximal effective concentration) values of 0.41 and 0.47 μg/mL against Rhizoctonia solani, comparable to that of the commercial fungicide Boscalid (EC50 = 0.49 μg/mL). Additionally, compound E23 also demonstrated pronounced antifungal effects against Verticillium dahliae, Alternaria solani, and Colletotrichum gloeosporioides with EC50 values of 0.27, 1.15, and 0.27 μg/mL, respectively. In vivo assays on rice plants revealed that this compound at 200 μg/mL exhibited nearly equipotent curative and protective effects against R. solani, as compared with the positive controls Boscalid and Carbendazim. Moreover, compound E23 had an obvious inhibition activity against fungal succinate dehydrogenase (SDH) from R. solani with the half-maximal inhibition concentration (IC50) of 11.76 μM, and the interactions between compound E23 and SDH enzyme were further confirmed by molecular docking studies. Furthermore, the presence of compound E23 also triggered obvious morphological changes of fungal mycelia and increased the conductivity and permeability of fungal cell membranes. Interestingly, the built three-dimensional quantitative structure-activity relationship (3D-QSAR) models showed that the electrostatic effect played a more important role in maintaining anti-R. solani effects of target compounds than other effects. Finally, the biosafety assessment of compound E23 demonstrated its safety toward rice and zebrafish.

The resistance mechanism of B_P225F and B_H272R mutations in succinate dehydrogenase in Botrytis cinerea

Botrytis cinerea populations resistant to succinate dehydrogenase inhibitors (SDHIs) represent a major problem for the sustainable development of modern agriculture. In the present study, the resistance mechanism of B_P225F and B_H272R mutations in B. cinerea SDH (BcSDH) resistant to SDHIs fungicides, including boscalid (BOS), penflufen (PEN), pydiflumetofen (PYD), fluopyram (FLU), and benzovindiflupyr (BEN), was uncovered. The biological assay results showed that both mutations exhibited different resistant factor (RF) for SDHIs. The molecular modeling results indicated that the B_P225F and B_H272R mutations had great effects on the conformational change of the binding pocket and the binding modes of inhibitors. For both mutations, the cation-π interaction between ligand and the residue of C_R88, playing an important contribution to the binding affinity in wild type (WT), was decreased in B_P225F and disappeared in B_H272R. It was interesting that an additional hydrogen bond (Hbond) established between inhibitors with B_R272 compensated for the reduction in binding energy that occurred with the B_H272R mutation. As a result, both mutant types (B_P225F and B_H272R) have a lower affinity when bound with SDHIs than the WT-BcSDH. The structural and mechanistic insights obtained from the present work will provide a valuable clue for designing novel SDH inhibitors to overcome drug resistance associated with B_P225F and B_H272R mutations.

Design, synthesis and antifungal activity of novel pyrazole-amide-isothiazole derivatives as succinate dehydrogenase inhibitors

To discover new fungicides to protect food safety and quality, thirty-four novel pyrazole-amide-isothiazole compounds were designed, synthesised by using scaffold hopping theory for the first time. The bioactivity of all the target compounds against five plant pathogens (Including Penicillium digitatum, Physalospora piricola, Helminthosporium maydis, Sclerotinia sclerotiorum and Botrytis cinerea) were determined, the results showed that most of the compounds exhibited certain biological activities against B. cinerea in vitro. Compounds 7-XHU-6 had better antifungal activities than fluopyram with the EC50 values were 1.02, 1.78 mg/L, respectively. Moreover, the SDH inhibiting activities results indicated that 7-XHU-6 possessed outstanding activities with an IC50 value of 0.47 mg/L which better than fluopyram (IC50 = 0.88 mg/L). Besides, the in vivo experiments indicated that compound 7-XHU-6 had excellent protection efficiency and therapeutic efficiency. In addition, molecular docking studies demonstrated that compound 7-XHU-6 (-10 kcal/mol) has superior binding energy compared to fluopyram (-8.6 kcal/mol).

Design, synthesis and antifungal activity of arylhydrazine analogs containing diphenyl ether fragments

BACKGROUND

Succinate dehydrogenase (SDH) represents a critical target in the development of novel fungicides. To address the growing issue of resistance and safeguard the economic viability of agricultural production, the pursuit of new succinate dehydrogenase inhibitors (SDHIs) has emerged as a significant focus of contemporary research.

RESULTS

In this project, 32 arylhydrazine derivatives containing diphenyl ether structural units were synthesized and evaluated for their fungicidal activities against Rhizoctonia solani, Sclerotinia sclerotiorum, Alternaria alternata, Gibberella zeae, Alternaria solani and Colletotrichum gloeosporioides. In an in vitro fungicidal activity assay, compound D6 showed significant inhibitory activity against R. solani with a half-maximum effective concentration (EC50) of 0.09 mg L-1. The in vivo fungicidal activity demonstrated that compound D6 inhibited R. solani by 95.39% in rice leaves, which was significantly better than that of boscalid (85.76%). The results of SDH enzyme assay, molecular docking simulation, mitochondrial membrane potential assay, cytoplasmic release studies and morphological observations demonstrated that the target compound D6 not only had significant SDH inhibitory activity, but also affected the membrane integrity of mycelium.

CONCLUSION

Bioactivity screening and validation of the mechanism of action indicated that compound D6 was a potentially unique SDHI, acting on SDH while also affecting cell membrane permeability, which deserved further study. © 2024 Society of Chemical Industry.

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.

Comprehensive Overview of the Amide Linker Modification in the Succinate Dehydrogenase Inhibitors

Succinate dehydrogenase inhibitors (SDHIs) have become one of the most important classes of agrochemical fungicides. According to the data from FRAC, the resistance risk for SDHIs had reached up to medium and even to high. In general, the chemical structure of SDHIs mainly contained three fragments: an acid core, a hydrophobic tail, and an amide linker, corresponding to three modification directions for each fragment. Among them, amide linker modification (ALM) has become a research hotspot for the design of novel SDHIs fungicides in recent years. We presented here a detailed review on the ALM strategy in the past decade, and some of them had entered the market. According to their chemical structures, ALM strategy were classified into four parts: (1) linked aliphatic chain between amide bond and hydrophobic tail, (2) introducing substituents to replacing hydrogen atom in the amide bond, (3) reverse extending the amide linker, and (4) changed with other bioisosteres. Moreover, the structure-activity relationship and the interaction mechanism of ALM-SDHI with SDH were discussed. This review aims to provide a global perspective on research and development of novel SDHIs, as well as suggestions for food safety management.

Development and Biological Evaluation of New Diphenyl Ether Formylhydrazide Compounds as Potent Inhibitors of Succinate Dehydrogenase

Succinate dehydrogenase (SDH), also recognized as succinate ubiquinone oxidoreductase (SQR), is considered one of the most promising targets for fungicide development, garnering significant international interest. We have focused on the development of highly effective, broad-spectrum-targeted SDH inhibitors. Using an active scaffold combining strategy, we designed and synthesized a series of novel diphenyl ether formylhydrazine derivatives, and most compounds have demonstrated broad-spectrum antifungal activity. Notably, compound M8 exhibited antifungal activity of more than 93% against four tested pathogen types at a concentration of 10 μg/mL, with an EC50 value below 0.3 μg/mL for each pathogen, outperforming boscalid. Additionally, compound M8 exhibited a control efficacy of 83% against Sclerotinia sclerotiorum on rapeseed leaves at a concentration of 200 μg/mL and demonstrated an 87% efficacy in controlling Fusarium graminearum on wheat ears when applied at 400 μg/mL. Structure-activity relationship research suggested that para-substituted benzene rings are more effective, offering stronger and more extensive antifungal potency. Further investigation, including enzyme inhibition assays, mycelial morphology observations, and molecular docking studies, suggests that the antifungal potency of M8 is due to the inhibition of its SDH activity. Therefore, our research positions compound M8 as a highly promising lead compound with broad-spectrum antifungal properties, potentially introducing a new class of fungicide.

Application of Difluoromethyl Isosteres in the Design of Pesticide Active Molecules

Difluoromethyl (CF2H) groups have been found in many listed pesticides due to their unique physical and chemical properties and outstanding biological activity. In pesticide molecules, compared with the drastic changes brought by trifluoromethyl, difluoromethyl usually moderately regulates the metabolic stability, lipophilicity, bioavailability, and binding affinity of compounds. Therefore, difluoromethylation has become an effective means to modify the biological activity of pesticide molecules. This paper reviews the representative literatures and patents containing difluoromethyl groups in the past 10 years, and introduces the research progress. The aim is to provide an effective reference value for the study of difluoromethyl in pesticides.

Discovery of Benzothiazol-2-ylthiophenylpyrazole-4-carboxamides as Novel Succinate Dehydrogenase Inhibitors

Succinate dehydrogenase (SDH) has been considered an ideal target for discovering fungicides. To develop novel SDH inhibitors, in this work, 31 novel benzothiazol-2-ylthiophenylpyrazole-4-carboxamides were designed and synthesized using active fragment exchange and a link approach as promising SDH inhibitors. The findings from the tests on antifungal activity indicated that most of the synthesized compounds displayed remarkable inhibition against the fungi tested. Compound Ig N-(2-(((5-chlorobenzo[d]thiazol-2-yl)thio)methyl)phenyl)-3-(difluoromethyl)-1-methyl-1H-yrazole-4-carboxamide, with EC50 values against four kinds of fungi tested below 10 μg/mL and against Cercospora arachidicola even below 2 μg/mL, showed superior antifungal activity than that of commercial fungicide thifluzamide, and specifically compounds Ig and Im were found to show preventative potency of 90.6% and 81.3% against Rhizoctonia solani Kühn, respectively, similar to the positive fungicide thifluzamide. The molecular simulation studies suggested that hydrophobic interactions were the main driving forces between ligands and SDH. Encouragingly, we found that compound Ig can effectively promote the wheat seedlings and the growth of Arabidopsis thaliana. Our further studies indicated that compound Ig could stimulate nitrate reductase activity in planta and increase the biomass of plants.

Recent Advances in Design and Development of Diazole and Diazine Based Fungicides (2014-2023)

With fungal diseases posing a major threat to agricultural production, the application of fungicides to control related diseases is often considered necessary to ensure the world's food supply. The search for new bioactive agents has long been a priority in crop protection due to the continuous development of resistance against currently used types of active compounds. Heterocyclic compounds are an inseparable part of the core structures of numerous lead compounds, these rings constitute pharmacophores of a significant number of fungicides developed over the past decade by agrochemists. Among heterocycles, nitrogen-based compounds play an essential role. To date, diazole (imidazole and pyrazole) and diazine (pyrimidine, pyridazine, and pyrazine) derivatives make up an important series of synthetic fungicides. In recent years, many reports have been published on the design, synthesis, and study of the fungicidal activity of these scaffolds, but there was a lack of a comprehensive classified review on nitrogen-containing scaffolds. Regarding this issue, here we have reviewed the published articles on the fungicidal activity of the diazole and diazine families. In current review, we have classified the molecules synthesized so far based on the size of the ring.

Design, Synthesis, and Antifungal Activities of Novel Potent Fluoroalkenyl Succinate Dehydrogenase Inhibitors

The development of new fungicide molecules is a crucial task for agricultural chemists to enhance the effectiveness of fungicides in agricultural production. In this study, a series of novel fluoroalkenyl modified succinate dehydrogenase inhibitors were synthesized and evaluated for their antifungal activities against eight fungi. The results from the in vitro antifungal assay demonstrated that compound 34 exhibited superior activity against Rhizoctonia solani with an EC50 value of 0.04 μM, outperforming commercial fluxapyroxad (EC50 = 0.18 μM) and boscalid (EC50 = 3.07 μM). Furthermore, compound 34 showed similar effects to fluxapyroxad on other pathogenic fungi such as Sclerotinia sclerotiorum (EC50 = 1.13 μM), Monilinia fructicola (EC50 = 1.61 μM), Botrytis cinerea (EC50 = 1.21 μM), and also demonstrated protective and curative efficacies in vivo on rapeseed leaves and tomato fruits. Enzyme activity experiments and protein-ligand interaction analysis by surface plasmon resonance revealed that compound 34 had a stronger inhibitory effect on succinate dehydrogenase compared to fluxapyroxad. Additionally, molecular docking and DFT calculation confirmed that the fluoroalkenyl unit in compound 34 could enhance its binding capacity with the target protein through p-π conjugation and hydrogen bond interactions.

Synthesis, fungicidal activity and molecular docking of novel pyrazole-carboxamides bearing a branched alkyl ether moiety

Succinate dehydrogenase inhibitors are essential fungicides used in agriculture. To explore new pyrazole-carboxamides with high fungicidal activity, a series of N-substitutedphenyl-3-di/trifluoromethyl-1-methyl-1H-pyrazole-4-carboxamides bearing a branched alkyl ether moiety were designed and synthesized. The in vitro bioassay indicated that some target compounds displayed appreciable fungicidal activity. For example, compounds 5d and 5e showed high efficacy against S. sclerotiorum with EC50 values of 3.26 and 1.52 μg/mL respectively, and also exhibited excellent efficacy against R. solani with EC50 values of 0.27 and 0.06 μg/mL respectively, which were comparable or superior to penflufen. The further in vivo bioassay on cucumber leaves demonstrated that 5e provided strong protective activity of 94.3 % against S. sclerotiorum at 100 μg/mL, comparable to penflufen (99.1 %). Cytotoxicity assessment against human renal cell lines (239A cell) revealed that 5e had low cytotoxicity within the median effective concentrations. Docking study of 5e with succinate dehydrogenase illustrated that R-5e formed one hydrogen bond and two π-π stacking interactions with amino acid residues of target enzyme, while S-5e formed only one π-π stacking interaction with amino acid residue. This study provides a valuable reference for the design of new succinate dehydrogenase inhibitor.

Functional Differentiation of the Succinate Dehydrogenase Subunit SdhC Governs the Sensitivity to SDHI Fungicides, ROS Homeostasis, and Pathogenicity in Fusarium asiaticum

Succinate dehydrogenase (SDH) is an integral component of the tricarboxylic acid cycle (TCA) and respiratory electron transport chain (ETC), targeted by succinate dehydrogenase inhibitors (SDHIs). Fusarium asiaticum is a prominent phytopathogen causing Fusarium head blight (FHB) on wheat. Here, we characterized the functions of the FaSdhA, FaSdhB, FaSdhC1, FaSdhC2, and FaSdhD subunits. Deletion of FaSdhA, FaSdhB, or FaSdhD resulted in significant growth defects in F. asiaticum. The FaSdhC1 or FaSdhC2 deletion mutants exhibited substantial reductions in fungal growth, conidiation, virulence, and reactive oxygen species (ROS). The FaSdhC1 expression was significantly induced by pydiflumetofen (PYD). The ΔFaSdhC1 mutant displayed hypersensitivity to SDHIs, whereas the ΔFaSdhC2 mutant exhibited resistance against most SDHIs. The transmembrane domains of FaSdhC1 are essential for regulating mycelial growth, virulence, and sensitivity to SDHIs. These findings provided valuable insights into how the two SdhC paralogues regulated the functional integrity of SDH, ROS homeostasis, and the sensitivity to SDHIs in phytopathogenic fungi.

Design, Synthesis, Antifungal Activity, and Molecular Docking Studies of Novel Chiral Isoxazoline-Benzofuran-Sulfonamide Derivatives

Succinate dehydrogenase (SDH) is one of the most important molecular targets for the development of novel fungicides. With the emerging problem of resistance in plant fungal pathogens, novel compounds with high fungicidal activity need to be developed, but the study of chiral pesticides for the inhibition of highly destructive plant pathogens has been rarely reported in recent years. Therefore, a series of novel chiral isoxazoline-benzofuran-sulfonamide derivatives were designed to investigate potential novel antifungal molecules. The chiral target compound 3a was cultured as a single crystal and confirmed using X-ray diffraction. All the target compounds were tested for antifungal activity, and compounds 3c, 3i, 3s, and 3r were found to have significant antifungal effects against S. sclerotiorum with EC50 values of 0.42 mg/L, 0.33 mg/L, 0.37 mg/L, and 0.40 mg/L, respectively, which were superior to the commercial fungicide fluopyram (EC50 = 0.47 mg/L). The IC50 value of compound 3i against the SDH of S. sclerotiorum was 0.63 mg/mL, which was further demonstrated by enzyme activity assays. Scanning electron microscopy showed that 3i had a significant inhibitory effect on S. sclerotiorum. In addition, the fluorescence quenching analysis assay indicated that compound 3i had a similar effect with the positive control fluopyram. Molecular docking exhibited that target compounds with chiral configuration had better affinity than racemic configuration, and 3i possessed stronger action than fluopyram, which was in keeping with the in vitro test results. These results would provide a basis and reference for the development of novel chiral fungicides.

Design, synthesis, antifungal activity and molecular docking of novel pyrazole-4-carboxamides containing tertiary alcohol and difluoromethyl moiety as potential succinate dehydrogenase inhibitors

BACKGROUND

Resistance problems with the long-term and frequent use of existing fungicides, and the lack of structure diversity of traditional pyrazole-4-carboxamide succinate dehydrogenase inhibitors, it is highly required to design and develop new fungicides to address the resistance issue.

RESULTS

Different from previous pyrazole-4-carboxamide succinate dehydrogenase inhibitors by breaking the norm of difluoromethyl at the C-3 position of pyrazole and introducing a tertiary alcohol group at the C-3 position, 27 novel pyrazole-4-carboxamide derivatives were designed, synthesized and characterized by proton (1 H) nuclear magnetic resonance (NMR), carbon-13 (13 C) NMR, fluorine-19 (19 F) NMR and high-resolution electrospray ionization mass spectrometry (HR-ESI-MS). The crystal structures of compounds A14 and C5 were analyzed by single crystal X-ray diffraction. Their in vitro antifungal activities were evaluated against phytopathogen Fusarium graminearum, Botrytis cinerea, Phytophthora capsica, Sclerotinia sclerotiorum, Thanatephorus cucumeris. The results displayed that most of them exhibited significant antifungal activities against S. sclerotiorum at 50 mg/L, the half maximal effective concentration (EC50 ) data of A8 and A14 were 3.96 and 2.52 mg/L, respectively. Their in vivo antifungal activities were evaluated against Pseudoperonospora cubensis, Puccinia sorghi Schw, Colletotrichum gloeosporioides, F. graminearum, Erysiphe graminis, Thanatephorus cucumeris, the control efficacies of A6, B3, C3, and C6 against E. graminis reached 100% at a concentration of 400 mg/L. The molecular docking results showed that the binding mode of the target compounds containing tertiary alcohols were similar to that of fluxapyroxad in succinate dehydrogenase. In addition, tertiary alcohols were involved in the formation of hydrogen bonds.

CONCLUSION

The excellent in vitro and in vivo inhibitory activities of novel pyrazole-4-carboxamide derivatives against succinate dehydrogenase were reported for the first time, and they could be used as the potential lead compounds. © 2023 Society of Chemical Industry.

Design, Synthesis, and Fungicidal Activities of Novel N-(Pyrazol-5-yl)benzamide Derivatives Containing a Diphenylamine Moiety

To accelerate the development of novel fungicides, a variety of N-(pyrazol-5-yl)benzamide derivatives with a diphenylamine moiety were designed and synthesized using a pharmacophore recombination strategy based on the structure of pyrazol-5-yl-aminophenyl-benzamides. The bioassay results demonstrated that most of the target compounds had excellent in vitro antifungal activities against Sclerotinia sclerotiorum, Valsa mali, and Botrytis cinerea. In particular, compound 5IIIh exhibited remarkable activity against S. sclerotiorum (EC50 = 0.37 mg/L), which was similar to that of fluxapyroxad (EC50 = 0.27 mg/L). In addition, compound 5IIIc (EC50 = 1.32 mg/L) was observed to be more effective against V. mali than fluxapyroxad (EC50 = 12.8 mg/L) and comparable to trifloxystrobin (EC50 = 1.62 mg/L). Furthermore, compound 5IIIh demonstrated remarkable in vivo protective antifungal properties against S. sclerotiorum, with an inhibition rate of 96.8% at 100 mg/L, which was close to that of fluxapyroxad (99.6%). Compounds 5IIIc (66.7%) and 5IIIh (62.9%) exhibited good in vivo antifungal effects against V. mali at 100 mg/L, which were superior to that of fluxapyroxad (11.1%) but lower than that of trifloxystrobin (88.9%). The succinate dehydrogenase (SDH) enzymatic inhibition assay was conducted to confirm the mechanism of action. Molecular docking analysis further revealed that compound 5IIIh has significant hydrogen-bonding, π-π, and p-π conjugation interactions with ARG 43, SER 39, TRP 173, and TYR 58 in the binding site of SDH, and the binding mode was similar to that of the commercial fungicide fluxapyroxad. All of the results suggest that compound 5IIIh could be a potential SDH inhibitor, offering a valuable reference for future studies.

Design and Synthesis of Novel Diphenyl Ether Carboxamide Derivatives To Control the Phytopathogenic Fungus Sclerotinia sclerotiorum

Sclerotinia stem rot (SSR) caused by the phytopathogenic fungus Sclerotinia sclerotiorum has led to serious losses in the yields of oilseed rape and other crops every year. Here, we designed and synthesized a series of carboxamide derivatives containing a diphenyl ether skeleton by adopting the scaffold splicing strategy. From the results of the mycelium growth inhibition experiment, inhibition rates of compounds 4j and 4i showed more than 80% to control S. sclerotiorum at a dose of 50 μg/mL, which is close to that of the positive control (flubeneteram, 95%). Then, the results of a structure-activity relationship study showed that the benzyl scaffold was very important for antifungal activity and that introducing a halogen atom on the benzyl ring would improve antifungal activity. Furthermore, the results of an in vitro activity test suggested that these novel compounds can inhibit the activity of succinate dehydrogenase (SDH), and the binding mode of 4j with SDH was basically similar to that of the flutolanil derivative. Morphological observation of mycelium revealed that compound 4j could cause a damage on the mycelial morphology and cell structure of S. sclerotiorum, resulting in inhibition of the growth of mycelia. Furthermore, in vivo antifungal activity assessment of 4j displayed a good control of S. sclerotiorum (>97%) with a result similar to that of the positive control at a concentration of 200 mg/L. Thus, the diphenyl ether carboxamide skeleton is a new starting point for the discovery of new SDH inhibitors and is worthy of further development.

Design, Synthesis, and Antifungal Activity of N-(alkoxy)-Diphenyl Ether Carboxamide Derivates as Novel Succinate Dehydrogenase Inhibitors

Succinate dehydrogenase (SDH, EC 1.3.5.1) is one of the most promising targets for fungicide development and has attracted great attention worldwide. However, existing commercial fungicides targeting SDH have led to the increasingly prominent problem of pathogen resistance, so it is necessary to develop new fungicides. Herein, we used a structure-based molecular design strategy to design and synthesize a series of novel SDHI fungicides containing an N-(alkoxy)diphenyl ether carboxamide skeleton. The mycelial growth inhibition experiment showed that compound M15 exhibited a very good control effect against four plant pathogens, with inhibition rates of more than 60% at a dose of 50 μg/mL. A structure-activity relationship study found that N-O-benzyl-substituted derivatives showed better antifungal activity than others, especially the introduction of a halogen on the benzyl. Furthermore, the molecular docking results suggested that π-π interactions with Trp35 and hydrogen bonds with Tyr33 and Trp173 were crucial interaction sites when inhibitors bound to SDH. Morphological observation of mycelium revealed that M15 could inhibit the growth of mycelia. Moreover, in vivo and in vitro tests showed that M15 not only inhibited the enzyme activity of SDH but also effectively protected rice from damage due to R. solani infection, with a result close to that of the control at a concentration of 200 μg/mL. Thus, the N-(alkoxy)diphenyl ether carboxamide skeleton is a new starting point for the discovery of new SDH inhibitors and is worthy of further investigation.

Discovery of Novel Pyrazol-5-yl-benzamide Derivatives Containing a Thiocyanato Group as Broad-Spectrum Fungicidal Candidates

In an effort to promote the development of new fungicides, a series of 48 novel N-(1-methyl-4-thiocyanato-1H-pyrazol-5-yl)-benzamide derivatives A1-A36 and B1-B12 were designed and synthesized by incorporating a thiocyanato group into the pyrazole ring, and their fungicidal activities were evaluated against Sclerotinia sclerotiorum, Valsa mali, Botrytis cinerea, Rhizoctonia solani, and Phytophthora capsici. In the in vitro antifungal/antioomycete assay, many of the target compounds exhibited good broad-spectrum fungicidal activities. Among them, compound A36 displayed the best antifungal activity against V. mali with an EC50 value of 0.37 mg/L, which was significantly higher than that of the positive controls fluxapyroxad (13.3 mg/L) and dimethomorph (10.3 mg/L). Meanwhile, compound B6 exhibited the best antioomycete activity against P. capsici with an EC50 value of 0.41 mg/L, which was higher than that of azoxystrobin (29.2 mg/L) but lower than that of dimethomorph (0.13 mg/L). Notably, compound A27 displayed broad-spectrum inhibitory activities against V. mali, B. cinerea, R. solani, S. sclerotiorum, and P. capsici with respective EC50 values of 0.71, 1.44, 1.78, 0.87, and 1.61 mg/L. The in vivo experiments revealed that compounds A27 and B6 presented excellent protective and curative efficacies against P. capsici, similar to that of the positive control dimethomorph. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses showed that compound B6 could change the mycelial morphology and severely damage the ultrastructure of P. capsici. The results of the in vitro SDH enzymatic inhibition experiments indicated that compounds A27 and B6 could effectively inhibit the activity of P. capsici SDH (PcSDH). Furthermore, molecular docking analysis demonstrated significant hydrogen bonds and Pi-S bonding between the target compounds and the key amino acid residues of PcSDH, which could explain the probable mechanism of action. Collectively, these studies provide a valuable approach to expanding the fungicidal spectrum of pyrazol-5-yl-benzamide derivatives.

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.

Discovery of Benzothiazolylpyrazole-4-Carboxamides as Potent Succinate Dehydrogenase Inhibitors through Active Fragment Exchange and Link Approach

Succinate dehydrogenase (SDH) is an attractive target for developing green fungicides to manage agricultural pathogens in modern agriculture research. Herein, in this work, we report the discovery of benzothiazolylpyrazole-4-carboxamides I-III as potent SDH inhibitors using active fragment exchange and link approach. The results of the fungicidal activity assays showed that some of the synthesized compounds exhibited excellent inhibition against the tested fungi. Systematic structure-activity relationship studies led to the discovery of compound Ip, N-(1-((4,6-difluorobenzo[d]thiazol-2-yl)thio)propan-2-yl)-3-(difluoromethyl)-N-methoxy-1-methyl-1H-pyrazole-4-carboxamide, which showed higher fungicidal activity against Fusarium graminearum Schw (EC50 = 0.93 μg/mL) than the commercial fungicides thifluzamide (EC50 > 50 μg/mL) and boscalid (EC50 > 50 μg/mL). The molecular simulation studies suggested that hydrophobic interactions were the primary driving forces between ligands and SDH. Promisingly, we found that Ip could stimulate the growth of wheat seedlings and Arabidopsis thaliana and increase the biomass of the treated plants. Preliminary studies on the plant growth promoter mechanism of Ip indicated that it could increase nitrate reductase activity in planta, that, in turn, stimulates the growth of plants.

Bioactivity-Guided Subtraction of MIQOX for Easily Available Isoquinoline Hydrazides as Novel Antifungal Candidates

The discovery of novel and easily available leads provides a convincing solution to agrochemical innovation. A bioassay-guided scaffold subtraction of the previous "Chem-Bio Model" isoquinoline-3-oxazoline MIQOX was conducted for identifying the easily available isoquinoline-3-hydrazide as a novel antifungal scaffold. The special and practical potential of this model was demonstrated by a phenotypic antifungal bioassay, molecular docking, and cross-resistance evaluation. A panel of antifungal leads (LW2, LW3, and LW11) was acquired, showing much better antifungal performance than the positive controls. Specifically, compound LW3 exhibited a broad antifungal spectrum holding EC₅₀ values as low as 0.54, 0.09, 1.52, and 2.65 mg/L against B. cinerea, R. solani, S. sclerotiorum , and F. graminearum, respectively. It demonstrated a curative efficacy better than that of boscalid in controlling the plant disease caused by B. cinerea. The candidate LW3 did not show cross-resistance to the extensively used succinate dehydrogenase inhibitor (SDHI) fungicides and can efficiently inhibit resistant B. cinerea strains. The molecular docking of compound LW3 is quite different from that of the positive controls boscalid and fluopyram. This progress highlights the practicality of isoquinoline hydrazide as a novel model in fungicide innovation.

Exploration of Fungicidal Activity and Mode of Action of Ferimzone Analogs

Lead discovery and molecular target identification are important for developing novel pesticides. Scaffold hopping, an effective approach of modern medicinal and agrochemical chemistry for a rational design of target molecules, is aiming to design novel molecules with similar structures and similar/better biological performance. Herein, 24 new ferimzone derivatives were designed and synthesized by a scaffold-hopping strategy. In vitro bioassays indicated that compound 5o showed similar potency to ferimzone against Cercospora arachidicola and 2-fold higher potency than ferimzone against Alternaria solani. Compounds 5q, 6a, and 6d displayed fungicidal activity with EC₅₀ values ranging from 1.17 to 3.84 μg/mL against Rhizoctonia solani, and compounds 5q and 6a displayed 1.6-1.8-fold higher activity than ferimzone against Fusarium graminearum. The in vivo bioassays at 200 μg/mL indicated that compound 5q was more potent than ferimzone against Pyricularia oryzae (90% vs 70% efficacy, respectively). Density functional theory (DFT) calculations elucidated the structure-energy relationship. Although the mode of action of ferimzone is still unclear, studies suggested that compound 5q significantly inhibited the growth and reproduction of R. solani, and its energy metabolism pathways (e.g., starch, sucrose, lipids, and glutathione) were seriously downregulated after a 5q treatment.

Discovery of aromatic 2-(3-(methylcarbamoyl) guanidino)-N-aylacetamides as highly potent chitinase inhibitors

Chitinases are important glycoside hydrolases that are closely related to bacterial pathogenesis, fungal cell wall remodelling, and insect moulting. Consequently, chitinases have become attractive targets for therapeutic drugs and pesticides. In this study, we designed and synthesised a series of novel chitinase inhibitors based on the N-methylcarbamoylguanidinyl group of the natural product argifin. The most active compound 8h showed strong inhibitory activity against the group I chitinases HsChit1, SmChiB, and OfChi-h, with IC₅₀ values of 0.19 µM, 4.2 nM, and 25 nM, respectively. Binding mode studies revealed that the compound 8h formed π-π stacking/hydrophobic interactions at +1 or +2 subsite of chitinases. In addition, a key hydrogen bond net was formed between the pharmacophore N-methylcarbamoylguanidinyl and key residues at the -1 subsite. Together, the findings of this study provide novel insights into the development of potent small-molecule chitinase inhibitors using a combination of planar structures and N-methylcarbamoylguanidinyl.

Design, Synthesis, and Fungicidal Evaluation of Novel N-Methoxy Pyrazole-4-Carboxamides as Potent Succinate Dehydrogenase Inhibitors

Succinate dehydrogenase (SDH, EC 1.3.5.1, also known as complex II) has been recognized as one of the most promising targets of fungicides. Here, based on the binding mode of pydiflumetofen with SDH, the carbon-carbon double bond was introduced into the chemical of pydiflumetofen and then produced the target compounds 6a-6o. The enzymatic inhibitory activity and structure-activity relationship (SAR) study showed that the 2-position and 4-position in terminal benzene were positive to increasing activity. Furthermore, fungicidal activity results in greenhouses indicated that compound 6o showed good control effects against wheat powdery mildew (WPM), cucumber powdery mildew (CPM), and southern corn rust (SCR), showing its broad-spectrum property. Especially, compound 6o exhibited 95 and 75% control effects against CPM and SCR at 6.25 mg/L, respectively, which are better than pydiflumetofen (80% control effects against CPM and 15% against SCR), indicating its potency that is worthy of further development.

Design, Synthesis, Inhibitory Activity, and Molecular Modeling of Novel Pyrazole-Furan/Thiophene Carboxamide Hybrids as Potential Fungicides Targeting Succinate Dehydrogenase

To discover new fungicides targeting succinate dehydrogenase (SDH), 36 new furan/thiophene carboxamides containing 4,5-dihydropyrazole rings were designed, synthesized, and characterized. The crystal structure of compound 5l was determined with the X-ray diffraction (XRD) of single crystals. The antifungal activity of these compounds was studied against Botrytis cinerea, Pyricularia oryzae, Erysiphe graminis, Physalospora piricola, and Penicillium digitatum. Bioassay results were that most compounds had obvious inhibitory activity at 20 μg/mL. Compounds 5j, 5k, and 5l possessed outstanding inhibitory activity against B. cinerea. Their EC₅₀ values were 0.540, 0.676, and 0.392 μg/mL, respectively. They owned better effects than fluxapyroxad (EC₅₀ = 0.791 μg/mL). In the meantime, the inhibitory activity of 16 compounds was evaluated against SDH. It turned out that these compounds displayed excellent activity. The IC₅₀ values of compounds 5j, 5k, and 5l reached 0.738, 0.873, and 0.506 μg/mL, respectively, whereas the IC₅₀ value of fluxapyroxad was 1.031 μg/mL. The results of molecular dynamics (MD) simulation showed that compound 5l possessed a stronger affinity to SDH than fluxapyroxad.

Identification of novel nematode succinate dehydrogenase inhibitors: Virtual screening based on ligand-pocket interactions

To discover new nematicidal succinate dehydrogenase (SDH) inhibitors with novel structures, we conducted a virtual screening of the ChemBridge library with 1.7 million compounds based on ligand-pocket interactions. The homology model of Caenorhabditis elegans SDH was established, along with a pharmacophore model based on ligand-pocket interactions. After the pharmacophore-based and docking-based screening, 19 compounds were selected for the subsequent enzymatic assays. The results showed that compound 1 (ID: 7607321) exhibited inhibitory activity against SDH with a determined IC₅₀ value of 19.6 μM. Structural modifications and nematicidal activity studies were then carried out, which provided further evidence that compound 1 exhibited excellent nematicidal activity. Molecular dynamics simulations were then conducted to investigate the underlying molecular basis for the potency of these inhibitors against SDH. This work provides a reliable strategy and useful information for the future design of nematode SDH inhibitors.

Discovery of N-Methoxy-(biphenyl-ethyl)-pyrazole-carboxamides as Novel Succinate Dehydrogenase Inhibitors

Succinate dehydrogenase (SDH) inhibitor is one of the research hotspots for the development of fungicides. Herein, we describe the design and synthesis of N-methoxy-(biphenyl-ethyl)-pyrazole-carboxamide derivatives with enhanced fungicidal activity by employing fragment combination strategy. The SDH enzymatic activity was evaluated for 24 title compounds, and compound 7s was identified as the highest activity against porcine SDH with an IC₅₀ value of 0.014 μM, 205-fold greater than that of fluxapyroxad. Furthermore, the greenhouse experiments showed that compound 7u exhibited potent fungicidal activity against wheat powdery mildew with an EC₅₀ value of 0.633 mg/L, higher activity than fluxapyroxad and benzovindiflupyr. The computational results showed that the fluorine atom substituted on the pyrazole ring formed an extra dipolar-dipolar interaction with C_S42 and then increased the van der Waals interaction between the compound and SDH. The structural and mechanistic insights obtained from the present work will provide a valuable clue to developing novel SDH inhibitors.

Design, Synthesis, and Biological Evaluation of Novel Pyrazol-5-yl-benzamide Derivatives Containing Oxazole Group as Potential Succinate Dehydrogenase Inhibitors

A series of pyrazol-5-yl-benzamide derivatives containing the oxazole group were designed and synthesized as potential SDH inhibitors. According to the results of the bioassays, most target compounds displayed moderate-to-excellent in vitro antifungal activities against Valsa mali, Sclerotinia scleotiorum, Alternaria alternata, and Botrytis cinerea. Among them, compounds C13, C14, and C16 exhibited more excellently inhibitory activities against S. sclerotiorum than boscalid (EC₅₀ = 0.96 mg/L), with EC₅₀ values of 0.69, 0.26, and 0.95 mg/L, respectively. In vivo experiments on rape leaves and cucumber leaves showed that compounds C13 and C14 exhibited considerable protective effects against S. sclerotiorum than boscalid. SEM analysis indicated that compounds C13 and C14 significantly destroyed the typical structure and morphology of S. scleotiorum hyphae. In the respiratory inhibition effect assays, compounds C13 (28.0%) and C14 (33.9%) exhibited a strong inhibitory effect on the respiration rate of S. sclerotiorum mycelia, which was close to boscalid (30.6%). The results of molecular docking indicated that compounds C13 and C14 could form strong interactions with the key residues TRP O:173, ARG P:43, TYR Q:58, and MET P:43 of the SDH. Furthermore, the antifungal mechanism of these derivatives was demonstrated by the SDH enzymatic inhibition assay. These results demonstrate that compounds C13 and C14 can be developed into novel SDH inhibitors for crop protection.

Bioisosterism and Scaffold Hopping in Modern Nematicide Research

The application of agrochemicals is critical to global food safety. Nowadays, environmentally friendly green agrochemicals are the trend in field crop protection. The research and development of nematicides absorbed more attention as a typical representation of agrochemicals. This review describes the origin of recently commercialized nematicides, the application of bioisosterism and scaffold hopping in the discovery and optimization of agrochemicals, especially nematicides, and novel bioisosteric design strategies for the identification of fluensulfone analogues. Pesticide repurposing, high-throughput screening, computer-aided drug design, and incorporation of known pharmacophoric fragments have been the most successful approach for the discovery of new nematicides. As outlined, the strategies of bioisosteric replacements and scaffold hopping have been very successful approaches in the search for new nematicides for sustainable crop protection. In the exploration of novel fluensulfone analogues with nematicidal activity, bioisosteric replacement of sulfone by amide, chain extension by insertion of a methylene group, and reversal of the amide group have proven to be successful approaches and yielded new and highly active fluensulfone analogues. These attempts might result in compounds with an optimal balance of steric, hydrophobic, electronic, and hydrogen-bonding properties and contribute to deal with the complex problem during the research and development of new nematicides. Further ideas are also put forward to provide new approaches for the molecular design of nematicides.

Discovery of (5-(Benzylthio)-4-(3-(trifluoromethyl)phenyl)-4H-1,2,4-triazol-3-yl) Methanols as Potent Phytoene Desaturase Inhibitors through Virtual Screening and Structure Optimization

Phytoene desaturase (PDS) is not only an important enzyme in the biosynthesis of carotenoids but also a promising target for herbicide discovery. However, in recent years, no expected PDS inhibitors with new scaffolds have been reported. Hence, a solution for developing PDS inhibitors is to search for new compounds with novel chemotypes based on the PDS protein structure. In this work, we integrated structure-based virtual screening, structure-guided optimization, and biological evaluation to discover some PDS inhibitors with novel chemotypes. It is noteworthy that the highly potent compound 1b, 1-(4-chlorophenyl)-2-((5-(hydroxymethyl)-4-(3-(trifluoromethyl)phenyl)-4H-1,2,4-triazol-3-yl)thio)ethan-1-one, exhibited a broader spectrum of post-emergence herbicidal activity at 375-750 g/ha against six kinds of weeds than the commercial PDS inhibitor diflufenican. Surface plasmon resonance (SPR) assay showed that the affinity of our compound 1b (K_D = 65.9 μM) to PDS is slightly weaker but at the same level as diflufenican (K_D = 38.3 μM). Meanwhile, determination of the phytoene content and PDS mRNA quantification suggested that 1b could induce PDS mRNA reduction and phytoene accumulation. Moreover, 1b also caused the increase of reactive oxygen species (ROS) and the change of ROS-associated enzyme activity in albino leaves. Hence, all these results indicated the feasibility of PDS protein structure-based virtual screen and structure optimization to search for highly potent PDS inhibitors with novel chemotypes for weed control.

Novel [1,2,4]-Triazolo[3,4-b]-[1,3,4]thiadizoles as Potent Pyruvate Kinase Inhibitors for Fungal Control

To discover novel target-based fungicidal candidates, a molecular design model was established with a three-dimensional (3D) structure of Rhizoctonia solani pyruvate kinase (RsPK) simulated with the AlphaFold 2 and YZK-C22 as a fungicidal lead. A series of novel [1,2,4]triazolo[3,4-b][1,3,4]thiadiazole derivatives were rationally designed, synthesized, evaluated for their fungicidal performance, and validated for their mode of action. The in vitro bioassays with R. solani indicated that compounds 5g, 5o, and 5z with an EC₅₀ value ranging from 1.01 to 1.54 μg/mL displayed higher fungicidal activity than the positive control YZK-C22 with its EC₅₀ of 3.14 μg/mL. Especially, 5o exhibited high potency and a broad spectrum against Alternaria solani, Botrytis cinerea, Cercospora arachidicola, Physalospora piricola, R. solani, and Sclerotinia sclerotiorum with its EC₅₀ value falling between 1.54 and 13.10 μg/mL. Like all positive controls, 5g, 5o, and 5z showed excellent in vivo growth inhibition against Pseudoperonospora cubensis at 200 μg/mL. Even though the PK enzymatic inhibition assay showed that 5o was approximately 2.6 times less active than YZK-C22 (IC₅₀: 29.14 vs 11.15 μg/mL, respectively), the similar fluorescence quenching patterns of RsPK by 5o and YZK-C22, and the docking results of interactions between RsPK and 5o or YZK-C22 implied that they might share the similar binding site in the RsPK active pocket. Our studies suggested that 5o could be used as a potent fungicidal lead for further optimization. The results of comparative molecular field analysis (CoMFA) provided a direction for further molecular design.

Rational Design, Synthesis, and Biological Evaluation of Fluorine- and Chlorine-Substituted Pyrazol-5-yl-benzamide Derivatives as Potential Succinate Dehydrogenase Inhibitors

To develop novel succinate dehydrogenase inhibitors (SDHIs), two series of novel N-4-fluoro-pyrazol-5-yl-benzamide and N-4-chloro-pyrazol-5-yl-benzamide derivatives were designed and synthesized, and their antifungal activities were evaluated against Valsa mali, Sclerotinia sclerotiorum, FusaHum graminearum Sehw, Physalospora piricola, and Botrytis cinerea. The bioassay results showed that some of the target compounds exhibited good antifungal activities in vitro against V. mali and S. sclerotiorum. Remarkably, compound 9Ip displayed good in vitro activity against V. mali with an EC₅₀ value of 0.58 mg/L. This outcome was 21-fold greater than that of fluxapyroxad (12.45 mg/L) and close to that of the commercial fungicide tebuconazole (EC₅₀ = 0.36 mg/L). In addition, in vivo experiments proved that compound 9Ip has good protective fungicidal activity with an inhibitory rate of 93.2% against V. mali at 50 mg/L, which was equivalent to that of the positive control tebuconazole (95.5%). The results of molecular docking indicated that there were obvious hydrogen bonds and p-π interactions between compound 9Ip and succinate dehydrogenase (SDH), which could explain the probable action mechanism. In addition, the SDH enzymatic inhibition assay was carried out to further prove its mode of action. Our studies suggest that compound 9Ip could be a fungicidal lead to discover more potent SDHIs for crop protection.

Reversible Regulation of Succinate Dehydrogenase by Tools of Photopharmacology

Succinate dehydrogenase (SDH) is extremely important in metabolic function and biological processes. Modulation of SDH has been reported to be a promising therapeutic target to SDH mutations. Current measures for the regulation of SDH are scarce, and precise and reversible modulation of SDH still remains challenging. Herein, a powerful tool for reversible optical control of SDH was proposed and evaluated utilizing the technology of photopharmacology. We reported photochromic ligands (PCLs), azobenzene-pyrazole amides (APAs), that exert light-dependent inhibition effects on SDH. Physicochemical property tests and biological assays were conducted to demonstrate the feasibility of modulating SDH. In this paper, common agricultural pathogens were used to develop a procedure by which our PCLs could reversibly and precisely control SDH utilizing green light. This research would help us to understand the target-ligand interactions and provide new insights into modulation of SDH.

Comprehensive Overview of Carboxamide Derivatives as Succinate Dehydrogenase Inhibitors

Up to now, a total of 24 succinate dehydrogenase inhibitors (SDHIs) fungicides have been commercialized, and SDHIs fungicides were also one of the most active fungicides developed in recent years. Carboxamide derivatives represented an important class of SDHIs with broad spectrum of antifungal activities. In this review, the development of carboxamide derivatives as SDHIs with great significances were summarized. In addition, the structure-activity relationships (SARs) of antifungal activities of carboxamide derivatives as SDHIs was also summarized based on the analysis of the structures of the commercial SDHIs and lead compounds. Moreover, the cause of resistance of SDHIs and some solutions were also introduced. Finally, the development trend of SDHIs fungicides was prospected. We hope this review will give a guide for the development of novel SDHIs fungicides in the future.