Killing the very hungry caterpillar

George Lahm tells the story of his quest for an insecticide.

One of the major challenges for the 21st century is to maintain successfully the world food supply. Crops need protecting from harmful insects but the ability of these insects to become quickly resistant to man-made protection agents is one reason why the hunt is always on for new products. Many old products are being reviewed, either because they are toxic or have some other environmental impact. The ultimate goal is to find the safest possible form of crop protection.

Caterpillars from the Lepidoptera order are some of the most voracious crop feeders known. They have a greedy appetite for cotton, soya bean and alfalfa and are also known to be partial to other leafy and fruity vegetables, including cabbage, peppers and tomatoes. And if they are still hungry, they have been known to feed on vines and tree fruit. An infestation of these pernicious insects can destroy an entire crop in a matter of days. Indoxacarb, sold by DuPont under the trade names of Steward? and Avaunt?, is the latest product and as far as I am aware is leading the field in controlling this pest. In the countries of West Africa, in particular, indoxacarb has been having a real impact on the lives of farmers.

Only hours after ingesting indoxacarb, insects stop feeding, followed quickly by their paralysis and death. Indoxacarb has a new mode of action that blocks the insect’s sodium channel. Crucially, this causes the feeding cessation which in turn provides rapid crop protection and stops further damage. As well as killing the damaging insects, indoxacarb does no harm to other, non-target arthropods, especially beneficial insects. Consequently, the important ecological balance provided by these beneficial insects is maintained. Indoxacarb also has a short environmental half-life and is safe to humans, allowing the Environmental Protection Agency to classify it as a reduced risk pesticide.

We at DuPont were intrigued by this family of chemicals. Not only were they strong insecticides but they also showed great potential for a new mode of action. We anticipated that pyrazolines might be able to address some of the issues required for new crop protection agents, and so began our quest to discover other chemical classes based on pyrazolines that would solve the problems of toxicity and environmental stability.

In 1986 we started to look at tethered pyrazolines. At first, we wanted to understand what effects restraining the geometry of the pyrazoline leads had on their conformational properties. Data from molecular models and x-ray crystallography suggested that the pyrazoline insecticides had a relatively planar shape held together by an intramolecular hydrogen bond and an extended pi chromophore. Based on this analysis we designed a series of molecules that were tethered so that their rotation would be restricted. Different tether groups were investigated until we reached the optimum system, based on the 4-chromanone ring system, like D-001.

Initially we screened a battery of representative insects with a selection of the analogues. The same group of chemicals was also used for a preliminary look at their soil half-lives. D-001 showed outstanding insecticidal effects, and at much better rates than many commercial standards. This promising activity was confirmed when field tests showed that D-001 was excellent at controlling a wide range of chewing pests from the Lepidoptera order. While the soil half-life was less than ideal it was certainly in the right range to justify searching for improved analogues. Unfortunately, a major disappointment followed with the discovery that D-001 had a propensity for bioaccumulation that was far beyond the acceptable limits. We put this down to a poor metabolic profile that resulted in accumulation in fatty tissue. Obviously the news was frustrating, but it prompted us to include in our discovery programme a metabolic assay to let us identify early on which of the analogues had a potential for bioaccumulation. This was an essential tool given the large number of metabolically stable analogues we later uncovered. As far as we know this was the first instance in an agricultural discovery programme that an assay like this was used in first tier screens.

What emerged were some very interesting results proving that there was an important relationship between molecular shape and biological activity. For example, the related molecule D-002 was very active as an insecticide on caterpillars, equaling the rates of commercial standards. Conversely, D-003 was also strongly active as an insecticide, but surprisingly there was a spectrum shift involving lesser activity on the caterpillars and greater activity against beetles. Both compounds proved their promise in field tests with D-002 showing excellent caterpillar control at rates less than 50 g/ha and D-003 showing excellent beetle control at rates as low as 10 g/ha. We also saw a dramatic difference in the metabolic profile for these two compounds. On one hand D-002 proved to be relatively stable with a long metabolic half-life and high levels of fat accumulation, but on the other hand D-003 showed rapid metabolic clearance and a very favourable profile. This was in fact a very positive discovery and suggested that careful optimisation in conjunction with our early metabolic screens might lead us to compounds with desirable metabolic traits.

Armed with the knowledge that both restrained and acyclic ’pyrazolines’ retained good levels of activity we began to understand the importance of molecular shape and the spatial relationship of functionality. We hoped that pyridazines, such as D-004, would show high activity because their spatial profile was similar to both restrained and acyclic predecessors. And as we hoped, the pyridazines were excellent insecticides. We had prepared 4000 or more compounds and from all of those D-004 was the most active we had discovered. In the laboratory Lepidoptera were effectively controlled with rates of well under 1 ppm and field use rates were superior to standards.

In the spring and summer of 1990, Stephen McCann, a member of our pyrazoline discovery team, was helping to synthesise the field sample and experienced first-hand the difficulties with the chemistry. To avoid any more tricky synthesis he suggested trying oxadiazines, such as DPX-JT333, as a good alternative to the pyridazine structures. His suggestion was based on two key assumptions. The first was that the proposed compounds should be more susceptible to degradation, both metabolically and in the soil because they contained a more fragile heterocyclic ring. These issues plagued many of our compounds so we were always looking for improvements on this front. The second assumption was borne out of a real practical need to make the synthesis easier. McCann was relying on the anticipation that it would be more convenient to prepare oxadiazines, especially if the key cyclisation step could be achieved. His hunch paid off and the findings were excellent. DPX-JT333 was exceptionally active, with a short soil half-life and a much improved metabolic profile. Thankfully, the new compound was also much easier to prepare.

From the pioneering work of Mulder and Wellinga in 1972 it took 28 years for the successful introduction of a sodium channel blocker. The five-year span in DuPont Discovery covering our part of the indoxacarb story was filled with great excitement. It was a stepwise process, filled with many ups and downs, and involved numerous small discoveries along the path. Critical breakthroughs in the discovery of new chemistry and associated insecticidal activity were complemented with upfront testing across a host of toxicologically significant parameters. The complex nature of the molecule also required significant process and manufacturing breakthroughs. Ultimately the balance that we achieved in the complex relationship between all of these issues allowed us to bring a powerful and important product to the market with great success.

Acknowledgements

George Lahm is a research fellow at EI DuPont, Agricultural Products, Stine-Haskell Research Center, Newark, DE, US.