Bis-neonicotinoid insecticides: Observed and predicted binding interactions with the nicotinic receptor

Small-scale field evaluation of the entomological efficacy and the residual activity of Fludora® Fusion WP-SB indoor residual spraying against Anopheles culicifacies s.l. in Gujarat, India

OBJECTIVES

To evaluate the entomological efficacy and the residual activity of indoor residual spraying with Fludora^® Fusion 562.5 WP-SB, a combination formulation containing clothianidin, a neonicotinoid and deltamethrin, a pyrethroid, against the main rural malaria vector, Anopheles culicifacies s.l., in India in a small-scale trial.

METHODS

In three study villages, suitable households were randomly allocated to five treatments: Fludora^® Fusion 562.5 WP-SB (target dose 225 mg active ingredient AI/m² ); clothianidin 70 WG (target dose 200 mg AI/m² ); K-Othrine 250 WG (deltamethrin, target dose 25 mg AI/m² ); Ficam VC 80 WP-SB (bendiocarb, target dose 400 mg AI/m² ) and unsprayed control. Insecticides were sprayed by hand compression sprayers with control flow valves and 8002E nozzles. Post-spray cone bioassays were done on insecticide-treated walls using a colonised, deltamethrin-resistant strain of An. culicifacies. Mosquitoes were collected from treated rooms by different methods. The insecticide content on filter papers collected from the sprayed walls was determined by chemical assay to assess the spray quality.

RESULTS

The ratios of applied to target doses of insecticides were within 0.84 to 1.4, showing a good spray quality. The cone bioassays revealed residual action lasting 7 months for all insecticides without significant differences in mortality between different surfaces treated nor between the four treatment arms (P > 0.05). Considering all entomological parameters such as indoor resting density, excito-repellency, blood-feeding inhibition and delayed mortality, the overall efficacy of Fludora^® Fusion WG-SB was equal or better compared with other insecticides.

CONCLUSIONS

Fludora^® Fusion showed overall equal or better efficacy than deltamethrin and bendiocarb alone against a pyrethroid-resistant malaria vector population and can be considered as an alternative product for management of pyrethroid resistance in malaria vectors.

The binding properties of cycloxaprid on insect native nAChRs partially explain the low cross-resistance with imidacloprid in Nilaparvata lugens

BACKGROUND

Neonicotinoids, such as imidacloprid, are selective agonists of insect nicotinic acetylcholine receptors (nAChRs) used to control Nilaparvata lugens, a major rice insect pest. High imidacloprid resistance has been reported in N. lugens both in the laboratory and in the field. Cycloxaprid (CYC), an oxa-bridged cis-nitromethylene neonicotinoid, showed high insecticidal activity against N. lugens and low cross-resistance in imidacloprid-resistant strains and field populations.

RESULTS

Binding studies demonstrated that imidacloprid has two binding sites with different affinities (K_d = 3.18 ± 0.43 pm and 1.78 ± 0.19 nm) in N. lugens nAChRs. CYC was poor at displacing [³ H]imidacloprid at its high-affinity binding site (K_i = 159.38 ± 20.43 nm), but quite efficient at the low-affinity binding site (K_i = 1.27 ± 0.35 nm). These data showed that CYC had overlapping binding sites with imidacloprid only at its low-affinity binding site. Therefore, the low displacement ability of CYC against imidacloprid binding at its high-affinity site could partially explain the low cross-resistance of CYC in imidacloprid-resistant populations.

CONCLUSION

The high insecticidal activity, low cross-resistance and different binding properties on insect nAChRs of CYC show that it is a potential insecticide for the control of N. lugens and related insect pests, especially ones with high resistance to neonicotinoids. © 2018 Society of Chemical Industry.

Village-scale (Phase III) evaluation of the efficacy and residual activity of SumiShield® 50 WG (Clothianidin 50%, w/w) for indoor spraying for the control of pyrethroid-resistant Anopheles culicifacies Giles in Karnataka state, India

OBJECTIVE

There is an urgent need to test and incorporate new molecules with promising efficacy and novel mode of action to control insecticide-resistant mosquito vectors for disease control. We tested a new compound, clothianidin (SumiShield 50 WG), for its efficacy as an indoor residual spray (IRS) for the control of pyrethroid-resistant Anopheles culicifacies (Diptera: Culicidae) in comparison with pirimiphos methyl (Actellic CS) as a positive control.

METHODS

Ten villages were selected, five each for IRS with clothianidin (300 mg AI/m² ) and pirimiphos methyl (1000 mg AI/m² ) in Almatti Dam catchment area in Karnataka state, India. Entomological parameters were monitored in these sprayed villages using standard methods. Assessment of quality of spray was performed by analysing the insecticide content in the filter paper samples collected from sprayed houses. Perceptions of spray men and inhabitants were recorded post-spray on safety of these molecules.

RESULTS

The mean applied to target ratio of content was 1.7 (n = 29) for clothianidin and 1.8 (n = 50) for pirimiphos methyl on filter paper samples analysed. Residual activity (≥80% mortality in exposed mosquitoes) after 24 h post-exposure of SumiShield WG was 5 months and increased to 6 months when the holding period was extended to 120 h and that of Actellic CS was 3 months at 24-h holding period and extended to 4 months at 120-h extended holding period. The mean densities of An. culicifacies in both arms fell drastically post-spray. In light trap collections, density of mosquitoes collected indoors was lower than outdoors in both arms indicating effectiveness of IRS. SumiShield WG was more efficacious in reducing the per-structure density than Actellic CS. The proportion of nulliparous mosquitoes was higher than that of parous mosquitoes during post-spray collections in both arms. The majority of adverse events reported were transitory and subsided without medication.

CONCLUSION

Indoor residual spraying with SumiShield WG was found effective, operationally feasible and safe, and it is effective for up to 6 months.

Flupyrimin: A Novel Insecticide Acting at the Nicotinic Acetylcholine Receptors

A novel chemotype insecticide flupyrimin (FLP) [N-[(E)-1-(6-chloro-3-pyridinylmethyl)pyridin-2(1H)-ylidene]-2,2,2-trifluoroacetamide], discovered by Meiji Seika Pharma, has unique biological properties, including outstanding potency to imidacloprid (IMI)-resistant rice pests together with superior safety toward pollinators. Intriguingly, FLP acts as a nicotinic antagonist in American cockroach neurons, and [³H]FLP binds to the multiple high-affinity binding components in house fly nicotinic acetylcholine (ACh) receptor (nAChR) preparation. One of the [³H]FLP receptors is identical to the IMI receptor, and the alternative is IMI-insensitive subtype. Furthermore, FLP is favorably safe to rats as predicted by the very low affinity to the rat α4β2 nAChR. Structure-activity relationships of FLP analogues in terms of receptor potency, featuring the pyridinylidene and trifluoroacetyl pharmacophores, were examined, thereby establishing the FLP molecular recognition at the Aplysia californica ACh-binding protein, a suitable structural surrogate of the insect nAChR. These FLP pharmacophores account for the excellent receptor affinity, accordingly revealing differences in its binding mechanism from IMI.

Initial stage of the degradation of three common neonicotinoids: theoretical prediction of charge transfer sites

Tautomerization of acetamiprid gives alternatives to search new pathways for its degradation in water.

Adulticidal & larvicidal efficacy of three neonicotinoids against insecticide susceptible & resistant mosquito strains

BACKGROUND & OBJECTIVES

Due to ever growing insecticide resistance in mosquitoes to commonly used insecticides in many parts of the globe, there is always a need for introduction of new insecticides for the control of resistant vector mosquitoes. In this study, larvicidal and adulticidal efficacies of three neonicotinoids (imidacloprid, thiacloprid and thiamethoxam) were tested against resistant and susceptible populations of Anopheles stephensi Liston 1901, Aedes (Stegomyia) aegypti Linnaeus, and Culex quinquefasciatus Say (Diptera: Culicidae).

METHODS

Laboratory-reared mosquito species were used. Insecticide susceptibility tests were done using standard WHO procedures and using diagnostic dosages of insecticide test papers and larvicides. Adulticidal efficacy of candidate insecticides was assessed using topical application method and larval bioassays were conducted using standard WHO procedure.

RESULTS

The results of topical application on 3-5 day old female mosquitoes indicated that resistant strain of An. stephensi registered lower LC 50 values than the susceptible strain. Among the three insecticides tested, thiacloprid was found more effective than the other two insecticides. Culex quinquefasciatus registered lowest LC 50 for imidacloprid than the other two mosquito species tested. In larval bioassays, the LC 50 values registered for imidacloprid were in the order of Cx. quinquefasciatus < An. stephensi (SS) < An. stephensi (RR) < Ae. aegypti. In case of thiacloprid, the order of efficacy (LC 50 ) was Cx. quinquefasciatus < An. stephensi (SS) < An. stephensi (RR), whereas in case of thiamethoxam, the larvicidal efficacy was in the order of An. stephensi (RR) < An. stephensi (SS) < Cx. quinquefasciatus.

INTERPRETATION & CONCLUSIONS

The present study indicated that insecticide resistant strains of mosquito species tested showed more susceptibility to the three neonicotinoids tested, and the possibility of using neonicotinoids for the control of resistant mosquitoes should be explored.

Azobenzene Modified Imidacloprid Derivatives as Photoswitchable Insecticides: Steering Molecular Activity in a Controllable Manner

Incorporating the photoisomerizable azobenzene into imidacloprid produced a photoswitchable insecticidal molecule as the first neonicotinoid example of remote control insecticide performance with spatiotemporal resolution. The designed photoswitchable insecticides showed distinguishable activity against Musca both in vivo and in vitro upon irradiation. Molecular docking study further suggested the binding difference of the two photoisomers. The generation of these photomediated insecticides provides novel insight into the insecticidal activity facilitating further investigation on the functions of insect nicotinic acetylcholine receptors and opens a novel way to control and study insect behavior on insecticide poisoning using light.

Design, synthesis, crystal structure, insecticidal activity, molecular docking, and QSAR studies of novel N3-substituted imidacloprid derivatives

Three novel series of N3-substituted imidacloprid derivatives were designed and synthesized, and their structures were identified on the basis of satisfactory analytical and spectral ((1)H NMR, (13)C NMR, MS, elemental analysis, and X-ray) data. Preliminary bioassays indicated that all of the derivatives exhibited significant insecticidal activities against Aphis craccivora, with LC50 values ranging from 0.00895 to 0.49947 mmol/L, and the insecticidal activities of some of them were comparable to those of the control imidacloprid. Some key structural features related to their insecticidal activities were identified, and the binding modes between target compounds and nAChR model were also further explored by molecular docking. By comparing the interaction features of imidacloprid and compound 26 with highest insecticidal activity, the origin of the high insecticidal activity of compound 26 was identified. On the basis of the conformations generated by molecular docking, a satisfactory 2D-QSAR model with six selected descriptors was built using genetic algorithm-multiple linear regression (GA-MLR) method. The analysis of the built model showed the molecular size, shape, and the ability to form hydrogen bond were important for insecticidal potency. The information obtained in the study will be very helpful for the design of new derivatives with high insecticidal activities.

Discovery of a novel series of phenyl pyrazole inner salts based on fipronil as potential dual-target insecticides

A series of novel phenyl pyrazole inner salt derivatives based on fipronil were designed and synthesized in the search for dual-target insecticides. These compounds were designed to target two families of nicotinic acetylcholine receptors and γ-aminobutyric acid receptors. The insecticidal activities of the new compounds against diamondback moth (Plutella xylostella) were evaluated. The results of bioassays indicated that most of the inner salts showed moderate to high activities, of which the phenyl pyrazole inner salts containing quinoline had excellent biological activity. Previous structure-activity relationship studies revealed that a suitable structure of the quaternary ammonium salts was critical for the bioactivity of phenyl pyrazole inner salts, which contribute to exposing the cationic nitrogen to bind to the receptor (for instance, nicotinic acetylcholine receptors) and possibly interact with the receptor via hydrogen bonding and cooperative π-π interaction. The present work demonstrates that the insecticidal potency of phenyl pyrazole inner salts holds promise for the development new dual-target phenyl pyrazole insecticides.

Clearance of ingested neonicotinoid pesticide (imidacloprid) in honey bees (Apis mellifera) and bumblebees (Bombus terrestris)

BACKGROUND

Bees in agricultural landscapes are exposed to dietary pesticides such as imidacloprid when they feed from treated mass-flowering crops. Concern about the consequent impact on bees makes it important to understand their resilience. In the laboratory, the authors therefore fed adult worker bees on dosed syrup (125 μg L(-1) of imidacloprid, or 98 μg kg(-1)) either continuously or as a pulsed exposure and measured their behaviour (feeding and locomotory activity) and whole-body residues.

RESULTS

On dosed syrup, honey bees maintained much lower bodily levels of imidacloprid than bumblebees (<0.2 ng versus 2.4 ng of imidacloprid per bee). Dietary imidacloprid did not affect the behaviour of honey bees, but it reduced feeding and locomotory activity in bumblebees. After the pulsed exposure, bumblebees cleared bodily imidacloprid after 48 h and recovered behaviourally.

CONCLUSION

The differential behavioural resilience of the two species can be attributed to the observed differential in bodily residues. The ability of bumblebees to recover may be environmentally relevant in wild populations that face transitory exposures from the pulsed blooming of mass-flowering crops.

Pyrrole- and dihydropyrrole-fused neonicotinoids: design, synthesis, and insecticidal evaluation

Versatile pyrrole- and dihydropyrrole-fused neonicotinoids were obtained from cyclic and non-cyclic nitroeneamines. Anhydrous aluminum chloride (AlCl₃) exhibited high catalytic selectivity for the synthesis of the titled etherified compounds at room temperature and the eliminated products under reflux conditions. The target molecules have been identified on the basis of satisfactory analytical and spectral [¹H and ¹³C nuclear magnetic resonance (NMR), high-resolution mass spectrometry (HRMS), and X-ray] data. All synthesized compounds have been screened for insecticidal activity. The preliminary insecticidal activity results showed that some of the aimed compounds displayed excellent insecticidal activity against cowpea aphids (Aphis craccivora).

Design and synthesis of heterocyclic enamine derivatives based on skeleton of neonicotinoids

Novel heterocyclic enamine derivatives based on skeleton of neonicotinoids were designed and prepared by the substitution reaction between heterocyclic enamines with intermediates of neonicotinoids, which provides readily accessible neonicotinoid analogs for potential bioactivity assay.

Design, synthesis, crystal structure analysis, and insecticidal evaluation of phenylazoneonicotinoids

On the basis of research of the proposed modes of action between neonicotinoids and insect nicotinic acetylcholine receptor (nAChR), a series of phenylazoneonicotinoids were designed and synthesized to further promote the π-π interaction between molecule and amino acid residues. The target compounds have been identified on the basis of satisfactory analytical and spectral ((1)H NMR, (13)C NMR, HRMS, and X-ray) data. The preliminary results revealed that tiny differences in substitutes resulted in different configurations and great bioactivity variations. Some compounds with electron-donating groups on positions 2 and 6 of the phenyl ring presented higher insecticidal activity than imidacloprid against cowpea aphids ( Aphis craccivora ). The impressive crystal structure of the excellent insecticidal activity compound 9q clearly proved that the functional electronegative pharmacophore was approximately vertical to the methyleneimidazolidine plane. The differences in the mode of interaction on nAChR of typical compounds 9h and 9q remain unclear.

cis-Configuration: a new tactic/rationale for neonicotinoid molecular design

Resistance development and limited lepidopteran activities call for the discovery of "super-neonicotinoids" solving these problems. Compounds with the cis-configuration offer an opportunity for further optimization. Fixing the nitro group in the cis-configuration provided a new approach for neonicotinoid molecular design. Introductions of the heterocycle or a bulky group are two synthesis concepts to fix the cis-configuration of the nitro group. The design, synthesis, bioactivity, and preliminary modes of action of five types of cis-neonicotinoids are reviewed. cis- and trans-neonicotinoids have some differences in bioactivities and modes of action. This study focused, especially, on the reaction diversities of nitromethylene analogues of imidacloprid with various aldehydes.

Neonicotinoid metabolism: compounds, substituents, pathways, enzymes, organisms, and relevance

Neonicotinoids are one of the three principal insecticide chemotypes. The seven major commercial neonicotinoids are readily biodegraded by metabolic attack at their N-heterocyclylmethyl moiety, heterocyclic or acyclic spacer, and N-nitroimine, nitromethylene, or N-cyanoimine tip. Phase I metabolism is largely dependent on microsomal CYP450 isozymes with situ selectivity in hydroxylation, desaturation, dealkylation, sulfoxidation, and nitro reduction. Cytosolic aldehyde oxidase is a nitroreductase for some neonicotinoids. Phase II metabolism involves methylation, acetylation, and formation of glucuronide, glucoside, amino acid, and sulfate- and glutathione-derived conjugates. Some neonicotinoids act as proinsecticides with metabolism to more potent nicotinic agonists. Pest resistance is more commonly due to synergist-reversible CYP450 detoxification than to nAChR mutants or variants. Metabolites in some cases contribute to mammalian hepatotoxicity and carcinogenesis and in others to enhanced plant vigor and stress shields. These relationships explain much of neonicotinoid comparative toxicology and provide the basis for continued and improved safety and effectiveness of this chemotype.

Discovery of imidacloprid and further developments from strategic molecular designs

The invention of imidacloprid, the most important neonicotinoid insecticide, was initiated by replacement of the framework of nithiazine with an imidazolidine ring. Through the finding of 1-(6-chloro-3-pyridylmethyl)-2-nitromethyleneimidazolidine, imidacloprid was invented. At the same time cyanoiminothiazolidinyl neonicotinoid thiacloprid was discovered. These products possess pronounced systemic properties and improved photostability in addition to supreme insecticidal ability. Crystal structure analysis led to the drug-receptor interaction model consisting of the guanidine (amidine) part conjugated to a powerful electron-withdrawing group bearing an H-bond accepting tip such as NO(2) or CN, and the chloronicotinyl group enhances the binding to the receptor. The QSAR study not only supports the key pharmacophore but also clarifies the crucial involvement of the phamacokinetic factors in the insecticidal activity. A concept for strategic and rational design led to the discovery of alkylene-tethered bis-imidacloprid derivatives with unexpected systemic insecticidal property and the unique binding mechanism revealing the second cavity in the neonicotinoid receptor.

Furan-2,5-dimethylene-tethered bis-imidacloprid insecticide conferring high potency

Bis-imidacloprid (bis-IMI) analogues with suitable alkylene spacers have plant-systemic insecticidal properties. The alkylene-tethered bis-IMI binds in a unique mode to the insect nicotinic acetylcholine receptor (nAChR) wherein the chloropyridine moieties are embraced by two distinct and distant domains. The heptamethylene spacer optimally bridges these two subsites, yet the linker itself binds in a relatively nonspecific manner. This investigation examines the hypothesis that a bis-IMI analogue with a heteroaromatic tether, which undergoes specific interaction(s) with the newly recognized receptor cavity, may enhance the potency relative to those of the alkylene-tethered derivatives. Remarkably, a novel bis-IMI with a furan-2,5-dimethylene fulcrum showed highest receptor potency and insecticidal activity among the analogues with various chemotype spacers. The nAChR structural model, simulating the binding site interactions of the furan-2,5-dimethylene-tethered bis-IMI, reveals that the furan ring is nestled in a hydrophobic pocket, consisting of three aromatic amino acids, and is stabilized via hydrogen bonding.

Divalent and oxabridged neonicotinoids constructed by dialdehydes and nitromethylene analogues of imidacloprid: design, synthesis, crystal structure, and insecticidal activities

A series of divalent and oxabridged neonicotinoids were synthesized by reactions of nitromethylene analogues of imidacloprid and dialdehydes, and their structures were confirmed by (1)H NMR, (13)C NMR, high-resolution mass spectroscopy, and X-ray diffraction analysis. The bioassays indicated that some of them were endowed with excellent insecticidal activities against cowpea aphid ( Aphis craccivora ), armyworm ( Pseudaletia separata Walker), and brown planthopper ( Nilaparvata lugens ). Divalent neonicotinoid 6 and oxabridged 8a had higher activities than imidacloprid against cowpea aphids and armyworm; furthermore, the activity of 8a was 40.4-fold higher than that of imidacloprid against imidacloprid-resistant brown planthopper.

Identification of critical elements determining toxins and insecticide affinity, ligand binding domains and channel properties

Insect nicotinic acetylcholine receptors have been objects of attention since the discovery of neonicotinoid insecticides. Mutagenesis studies have revealed that, although the detailed subunit composition of insect nicotinic acetylcholine receptors subtypes eludes us, the framework provided by mutagenesis analysis makes a picture of the subunits involved in the ligand binding and channel properties. In fact, many residues that line the channel or bind to the ligand seemed to be strongly conserved in particular in the N-terminal extracellular region and the second transmembrane domain which constitutes the ion-conducting pathway supporting the flux of ions as well as their discrimination. In fact, the positions are carried by loops B and C, respectively, which contain amino acids directly contributing to the acetylcholine binding site. Mutation ofthese residues accounts for insect resistance to neonicotinoid insecticides such as imidacloprid or a loss ofspecific binding. The discovery of the same mutation at homologous residues in different insect species or its conservation raises the intriguing question of whether a single mutation is essential to generate a resistance phenotype or whether some subunit confer insensitivity to ligand. Consequently, recent finding using information from Torpedo marmorata al subunit and soluble Aplysia californica and Lymnae stagnalis acetylcholine bindingproteins from crystallization suggest that insect nAChR subunits had contributing amino acids in the agonist site structure which participate to affinity and pharmacological properties of these receptors. These new range of data greatly facilitate the understanding of toxin-nAChR interactions and the neonicotinoid binding and selectivity.