Ecdysteroid mimics

Figure 3. Ecdysteroid titers of female 5th-stage 0. fasciatus treated topically with compounds. Circles, thiolcarbamate 5 treated squares, JH-mimic AI3-63604 treated triangles, acetone treated (controls).

A number of plants synthesize ecdysteroids and sequester them in much greater concentrations than do insects (Fig. 23.19). It is probable that these compounds serve as defensive systems in the plant much in the same way as juvenile hormone mimics. For activity, a cw-A,B-ring fusion, a 6-keto-7-ene group, a full sterol side chain with a 22/ -oxygen function, an oxygen function at C-3, and oxygen groups at C-14 and 2p are necessary. However, not all insects may respond in an identical manner (Camps, 1991). 

For the last two decades, in order to develop insecticides with selective toxicity, efforts to identify mimics (agonists) of 20-hydroxyecdysone, the insect molting hormone, have always led to the use of ecdysteroids or closely related steroidal analogs which are not commercially cost-effective. Recently, a new class of insect growth regulators, the 1,2-diacyl-l-alkylhydrazines, has been discovered and shown to be the first nonsteroidal agonists of 20-hydroxyecdysone. 

In contrast, no insecticide has been developed to interfere specifically with the process associated with the steroidal insect molting hormone, 20-hydroxyecdysone. Although active ecdysteroids can be obtained from plant and animal sources (S), the main reason for the failure to develop them as insecticides has been that (a) their structures are too complex to produce economically, (b) their hydrophilic nature prohibits their penetration into insect cuticle, and (c) insects have powerful mechanisms to eliminate ecdysones between molts. One approach to solve these problems is to synthesize a simple molecule which can mimic ecdysone, but with appropriate chemical and transport properties, and acceptable metabolic stability. 

Insect defensive compounds are usually eflFective at short distance and their toxicity or repellency is not sufficient for them to have found any industrial application. Venoms can be powerful, but usually require injection. Of the insect hormones, ecdysteroids (Chapter 7) have not yet found practical appUcation, but there are several examples, in special circumstances, of very effective use of juvenile hormone mimics. 

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