Bioactivation and mode of action of the oxadiazine indoxacarb in insects

Abstract

DPX-MP062 (indoxacarb) is a novel oxadiazine insecticide which has good field activity against a number of pest Lepidoptera, as well as certain Homoptera and Coleoptera. Indoxacarb (discovered and developed by E.I. DuPont and Co.) is a 75%S : 25%R mixture of enantiomers at the chiral bicyclic carbon; DPX-JW062 is the corresponding racemic compound. Several species of lepidopteran larvae can rapidly metabolize ¹⁴C-JW062 to ¹⁴C-DCJW (for N-decarbomethoxyllated JW062) after ingestion, and more slowly after topical treatment; this conversion is correlated with the appearance of neurotoxic symptoms. Several sucking insects are also capable of absorbing and bioactivating indoxacarb after either dermal or oral administration, but do so much more slowly than the Lepidoptera. DCJW is a highly potent, voltage-dependent blocker of Na⁺-dependent compound action potentials when tested in a Manduca sexta larval abdominal motoneuron preparation; DPX-JW062 was much less potent in this regard. When larvae were poisoned in vivo, onset of paralysis and block of central nervous system action potentials was more rapid with DCJW than with DPX-JW062; onset of neurotoxic symptoms leads to a rapid and irreversible halt in feeding. The S-enantiomer of DCJW is active both in the motoneuron preparation in vitro and in lepidopteran larvae in vivo; however S-DPX-JW062 is active in vivo only, while the R-enantiomers of DCJW and DPX-MP062 are inactive in both. Thus, activation of the parent oxadiazines to the S-enantiomers of the N-decarbomethoxyllated metabolites, which are powerful sodium channel blockers, is the toxic mechanism of action in Lepidoptera and apparently for other pest insects as well; however, the rate of bioactivation is a critical factor in determining the speed and ultimate toxicity of this compound in different insect species. Indoxacarb's inherent activity against Lepidoptera is comparable to the most potent insecticides ever commercialized.

Introduction

Highly efficacious insecticides with novel modes of action are becoming increasingly important in agriculture as components of integrated pest management and resistance management strategies, and to replace older classes of compounds which are perceived to carry higher safety/environmental risk. Some examples of these newer insecticides are imidacloprid from Bayer AG, a highly systemic nicotinic acetylcholine receptor agonist active against many sucking and rasping insects (for review see Wollweber and Tietjen, 1999); methoxyfenozide from Rohm and Haas Co., a powerful ecdysone agonist active against Lepidoptera (Le et al., 1996); and spinosad from Dow Agrochemicals, a potent lepidopteran insecticidal natural product which has a novel excitatory action on the insect central nervous system (Thompson et al., 1995; Salgado, 1998).

The sodium channel is a well-known insecticide biochemical target which has multiple drug and insecticide binding sites, such as for DDT, pyrethroids, N-alkyl amides and certain invertebrate peptide toxins (Narahashi et al., 1995; Soderlund and Knipple, 1995). However, because of the complexity and size of the insect voltage-gated sodium channel (for example for Drosophila see Warmke et al., 1997) it is clear that opportunities remain for novel commercial insecticides to be discovered which act on this protein in new ways. The discovery of the oxadiazine DPX-MP062 (indoxacarb) by the E.I. DuPont Co. (McCann et al., 1992; Lahm et al., 1999; McCann et al., 1999) is an illustration of how this insect target can still be exploited for the benefit of crop protection.

Indoxacarb is a promising new foliar insecticide with strong field activity against Lepidoptera which attack vegetables, tree fruit, vines, cotton, corn and other crops (Harder et al., 1996). Outstanding activity has been shown against several Lepidoptera such as Heliothines, Spodoptera sp., Laspeyresia sp., Plutella sp., Trichoplusia sp., Lobesia sp., Cydia sp., Tortricids, as well as certain sucking insects such as Lygus lineolaris and Empoasca sp. and the beetle Leptinotarsa decemlineata (Harder et al., 1996; Eng, 1998; Hammes et al., 1998; Michaelides and Irving, 1998; Olszak and Pluciennik, 1998; Pluschkell et al., 1998; Brickle et al., 1999; Kharboutli et al., 1999; Sullivan et al., 1999; Andaloro et al., 2000; Sherrod et al., 2000; Pacheco and Steele, 2000). A key feature of this compound is it's novel bioactivation, then block off the insect voltage-gated sodium channel by the N-decarbomethoxyllated metabolite, first described by Wing et al. (1998). This mode of action differentiates these oxadiazines from any other commercial insecticides.

In this paper both formation of the N-decarbomethoxyllated metabolites (Fig. 1) in a variety of insects, and the profound toxic action of those metabolites in both whole insects and on lepidopteran sodium channels is documented. Several insect species can convert JW062 the racemic form or indoxacarb to their active metabolites, but their metabolic rates vary dramatically and this has a bearing on their susceptibility to indoxacarb. In addition the N-decarbomethoxyllated metabolites, but not JW062 or indoxacarb themselves, are highly potent voltage-dependent blockers of insect sodium channels, based on a combination of neurophysiological and toxicity studies. These findings, in combination with an understanding of the bioavailability of indoxacarb when sprayed in the field, help to explain the insect toxicology of this novel compound and aid in optimizing it's potential as a crop protection agent.