A novel class of insecticidal alkylsulfones are potent inhibitors of vesicular acetylcholine transport

Pyridine alkylsulfone derivatives typified by oxazosulfyl (Sumitomo Chemical Company Ltd.) and compound A2 (Syngenta) represent a new class of insecticides, with potent activity against several insect orders. Whilst the MOA of this class has been attributed to interaction with the voltage-gated sodium channel (VGSC), here we present strong evidence that their toxicity to insects is mediated primarily through inhibition of the vesicular acetylcholine transporter (VAChT). Alkylsulfone intoxication in insects is characterised by (i) a reduction in cholinergic synaptic transmission efficiency demonstrated by a depression of cercal afferent activity in giant-interneurone preparations of American cockroach (Periplaneta americana), (ii) selective block of cholinergic-transmission dependent post-synaptic potentials in the Drosophila giant-fibre pathway and (iii) abolition of miniature excitatory post-synaptic currents (mEPSCs) in an identified synapse in Drosophila larvae. Ligand-binding studies using a tritiated example compound ([3H]-A1) revealed a single saturable binding-site, with low nanomolar Kd value, in membrane fractions of green bottle fly (Lucilia sericata). Binding is inhibited by vesamicol and by several examples of a previously identified class of insecticidal compounds known to target VAChT, the spiroindolines. Displacement of this binding by analogues of the radioligand reveals a strong correlation with insecticidal potency. No specific binding was detected in untransformed PC12 cells but a PC12 line stably expressing Drosophila VAChT showed similar affinity for [3H]-A1 as that seen in fly head membrane preparations. Previously identified VAChT point mutations confer resistance to the spiroindoline class of insecticides in Drosophila by Gal-4/UAS directed expression in cholinergic neurones and by CRISPR gene-editing of VAChT, but none of these flies show detectable cross-resistance to this new chemical class. Oxazosulfyl was previously shown to stabilise voltage-gated sodium channels in their slow-inactivated conformation with an IC50 value of 12.3μM but inhibits binding of [3H]-A1 with approximately 5000 times greater potency. We believe this chemistry class represents a novel mode-of-action with high potential for invertebrate selectivity.

Insecticides, biologics and nematicides: Updates to IRAC's mode of action classification - a tool for resistance management

Insecticide resistance has been and continues to be a significant problem for invertebrate pest control. As such, effective insecticide resistance management (IRM) is critical to maintain the efficacy of current and future insecticides. A technical group within CropLife International, the Insecticide Resistance Action Committee (IRAC) was established 35 years ago (1984) as an international association of crop protection companies that today spans the globe. IRAC's focus is on preserving the long-term utility of insect, mite, and most recently nematode control products through effective resistance management to promote sustainable agriculture and improved public health. A central task of IRAC has been the continual development and documentation of the Mode of Action (MoA) Classification scheme, which serves as an important tool for implementing IRM strategies focused on compound rotation / alternations. Updates to the IRAC MoA Classification scheme provide the latest information on the MoA of current and new insecticides and acaricides, and now includes information on biologics and nematicides. Details for these new changes and additions are reviewed herein.

Cloning and expression of an insect Ca(2+)-ATPase from Heliothis virescens

A complementary DNA for the Tobacco Budworm, Heliothis virescens, sarco(endo)plasmic reticulum-type Ca(2+)-ATPase (HVSERCA) has been cloned and sequenced. cDNA fragments of adult rabbit fast-twitch muscle Ca(2+)-ATPase (SERCA1a) were used as heterologous probes to isolate a partial cDNA clone coding for a protein with high homology to the Ca(2+)-ATPase from Drosophila melanogaster (DRSERCA) and vertebrate ER/SR Ca2+ pumps. The entire cDNA clone contains an ORF encoding a protein of 1000 amino acids which shares the characteristic motifs of a P-type ATPase. HVSERCA shares 89% identity with DRSERCA, 80% identity with the Artemia Ca(2+)-ATPase and 72% identity with avian and mammalian SERCAs. An insect Ca(2+)-ATPase-specific polyclonal antiserum has been raised against a fusion protein containing sequence from the cytoplasmic domain of HVSERCA. Heterologous expression of the insect pump in COS-7 cells has been demonstrated by immunocytochemistry and the reticular pattern of staining is consistent with an ER localisation. However, the expressed enzyme from COS-7 cells does not appear to be active.