Structure-activity relationship study of alkynyl ether insecticide synergists and the development of MB-599 (verbutin)

Ionic Liquids Enhanced Alkynyl Schiff Bases Derivatives of Fipronil Synthesis and Their Cytotoxicity Studies

To obtain highly selective toxic derivatives of fipronil, a series of Schiff bases with an alkynyl group (3a–3k) were designed and synthesized from 4-ethynylbenzaldehyde (2) and 4-substituted 5-amino-N-arylpyrazole (1a–1k) via a nucleophilic addition elimination reaction in ionic liquids. Utilization of ionic liquids was demonstrated to endow the yield of each compound beyond 50%, which was enhanced over 1.5 times of the synthetic productive rates comparing the conventional method by which longer reactive time was consumed. The derivatives were characterized via nuclear magnetic resonance hydrogen spectroscopy (¹H-NMR), carbon-13 nuclear magnetic resonance spectroscopy (¹³C-NMR), and electrospray ionization high resolution mass spectrometry (ESI-HRMS). The cytotoxicity of these derivatives on Trichoplusia ni (Hi-5) cell and Spodoptera litura cell (SL cell) was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) bioassays. The results indicated that several compounds had potential cytotoxicity on Hi-5 cell, especially a 4-ethyl substituted alkynyl Schiff base derivative (3f) that was demonstrated to possess high selective toxicity to the Hi-5 cell than the SL cell. In addition, 3f exhibited comparable toxic activity to commercial fipronil on a Hi-5 cell while a little toxic effect on the SL cell, which satisfied the expectation for selective toxicity screening.

Biochemical evaluation of interactions between synergistic molecules and phase I enzymes involved in insecticide resistance in B- and Q-type Bemisia tabaci (Hemiptera: Aleyrodidae)

BACKGROUND

Metabolic resistance is an important consideration in the whitefly Bemisia tabaci, where an esterase-based mechanism has been attributed to pyrethroid resistance and over-expression of the cytochrome P450, CYP6CM1, has been correlated to resistance to imidacloprid and other neonicotinoids.

RESULTS

In vitro interactions between putative synergists and CYP6CM1, B and Q-type esterases were investigated, and structure-activity relationship analyses allowed the identification of chemical structures capable of acting as inhibitors of esterase and oxidase activities. Specifically, methylenedioxyphenyl (MDP) moieties with a polyether chain were preferable for optimum inhibition of B-type esterase, whilst corresponding dihydrobenzofuran structures were potent for the Q-esterase variation. Potent inhibition of CYP6CM1 resulted from structures which contained an alkynyl chain with a terminal methyl group.

CONCLUSIONS

Synergist candidates could be considered for field control of B. tabaci, especially to abrogate neonicotinoid resistance. © 2017 Society of Chemical Industry.

The interactions of piperonyl butoxide and analogues with the metabolic enzymes FE4 and CYP6CY3 of the green peach aphid Myzus persicae (Hemiptera: Aphididae)

BACKGROUND

Piperonyl butoxide (PBO) is a well-known insecticide synergist capable of interacting with phase 1 metabolic enzymes, specifically esterases and cytochrome P450s. In this study, structure-activity relationship analyses were used to characterise the interaction of around 30 analogues of PBO with the esterase FE4 and the P450 CYP6CY3 from insecticide-resistant Myzus persicae (Sulzer), in order to predict the synthesis of more potent inhibitors.

RESULTS

Enzyme inhibition studies were performed against esterase and oxidase activities and, together with in silico modelling, key activity determinants of the analogues were identified and optimised. Novel analogues were then designed and synthesised, some of which showed greater inhibition against both enzymatic systems: specifically, dihydrobenzofuran moieties containing an alkynyl side chain and a butyl side chain against FE4, and benzodioxole derivatives with a propyl/butyl side chain and an alkynyl ether moiety for CYP6CY3.

CONCLUSIONS

In vitro assays identified potential candidate synergists with high inhibitory potency. The in vivo confirmation of such results will allow consideration for a possible use in agriculture. © 2016 Society of Chemical Industry.

Laboratory evaluation of verbutin as a synergist of acaricides against larvae of Rhipicephalus (Boophilus) microplus (Acari: Ixodidae)

Synergistic effects of verbutin, a member of aryl alkynyl derivatives, to three commonly used acaricides were evaluated with the modified Food and Agricultural Organization Larval Packet Test (FAO-LPT) against both susceptible and resistant strains of the southern cattle tick, Rhipicephalus (Boophilus) microplus (Canestrini) (Acari: Ixodidae). These tick strains demonstrated various levels of resistance to coumaphos (2.5-8.2x), permethrin (57.9-711.7x), and amitraz (3.5-177.5x). Verbutin alone was more toxic to tick larvae than piperonyl butoxide (PBO), a conventional pesticide synergist. With synergism ratios (SRs) ranging from 1.5 to 6.0, verbutin was more potent than PBO (SR = 0.9-1.6) in synergizing coumaphos toxicity to resistant tick larvae. Similarly, verbutin (SR = 1.8-15) was also found to be more potent than PBO (SR = 0.9-2.5) in synergizing amitraz in resistant tick strains. Both verbutin and PBO significantly increased permethrin toxicity to larvae of all tick strains tested, and there was no significant difference between the two synergists. SRs ranged from 2.1 to 4.4 and from 2.1 to 3.6 for PBO and verbutin, respectively.

Esterase-mediated resistance to pyrethroids in field populations of Helicoverpa armigera (Lepidoptera: Noctuidae) from Central Africa

BACKGROUND

Evolution of pyrethroid resistance in the cotton bollworm Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) threatens continued cotton production in Central Africa. Dose-response bioassays were conducted on area-wide collection of bollworm populations from major host plants, while biochemical techniques were used to evaluate basic mechanisms underlying resistance.

RESULTS

Pyrethroid resistance is primarily associated with detoxification by enhanced esterase activity. High resistance to cypermethrin (RF = 67-1771), cross-resistance to deltamethrin (RF = 60-2972) and lack of cross-resistance to the non-ester pyrethroid etofenprox (RF = 2-10) were observed among H. armigera field populations and laboratory-selected strains. Enzymatic assays showed that esterase activity, but not oxidase content or glutathione-S-transferase activity, was positively correlated with resistance to cypermethrin. Pretreatment with piperonyl butoxide (PBO) resulted in significant synergism with cypermethrin in 6/10 field populations, but not in the laboratory-selected strain, indicating that additional mechanisms such as mixed-function oxidase (MFO) may be involved in field resistance. The absence of cross-resistance to DDT ruled out a possible target-site modification.

CONCLUSION

Knowledge of the mechanisms involved in pyrethroid resistance and the lack of cross-resistance to spinosad and indoxacarb is a key to devising new resistance management strategies aimed at restoring the efficacy of pyrethroid-based programmes.