Mode of action of the ecdysone agonist tebufenozide (RH-5992), and an exclusion mechanism to explain resistance to it

Spruce budworm larvae (Chotistoneura fumiferana) upon ingesting tebufenozid e (RH-5992) stop feeding and go into a precocious, incomplete molt, leading eventually to death. Like 20-hydroxyecdysone (20E), tebufenozide also acts at the receptor level and transactivates the expression of up-regulated ge nes but, because of its persistence, the down-regulated genes that are norm ally expressed in the absence of 20E are not expressed. While tebufenozide is lepidopteran-specific, an analog, RH-5849, is effective on dipterans. Th is is reflected in the respective effects of the two compounds on Cf-203 (C . fumiferana - 203), a lepidopteran cell line and Dm-2 (Drosophila melanoga ster - 2), a dipteran cell line. Cf-203 cells accumulated [C-14]tebufenozid e and expressed CHR3 (Choristoneura hormone receptor 3), but Dm-2 cells exc luded the material and did not express DHR3 (Drosophila hormone receptor 3) . Using yeast ABC (ATP binding cassette) transporter mutants, we determined that PDR5 (pleiotropic drug resistance 5) was responsible for the exclusio n. We discovered recently that older instars of the white-marked tussock mo th (Orgyia leucostigma) are resistant to tebufenozide, perhaps as a result of such an exclusion system. We are currently cloning PDR5 (pleiotropic dru g resistance 5), which is an essential step in studying the resistance mech anism. (C) 2001 Society of Chemical Industry.