Ecdysteroid structure

The various bioassay systems used for ecdysteroid structure-activity studies have been reviewed previously [120,150,171], Only those systems which have been used extensively and those which are recent are described below. 

The early literature on ecdysteroid structure-activity studies has been reviewed [120,150,171], The various bioassays employed vary in their complexity of performance and interpretation. A number of factors can affect the potency of a test compound to a greater or lesser extent depending on the assay system penetration, metabolism, excretion rate, sequestration and target site activity. In vivo assays will give a measure of all of these acting in concert, but not allow the individual contributions to be identified. On the other hand, cell-free receptor assays probably give a direct measure of target activity, but may bear little resemblance to the in vivo situation. 

Horn and Bergamasco [120] summarised their main conclusions regarding ecdysteroid structure-activity relationships in insect systems as follows ... 

Dinan, L., 1989, Ecdysteroid structure and hormonal activity in Ecdysone From Chemistry to Mode of Action, Koolman, J. (Ed) Thieme Verlag, Stuttgart, 345-354. 

LC-NMR can be used to identify natural products in crnde plant extracts that usually consist of complex mixtnres. The crnde natural product extracts normally contain a great nnmber of closely related and difficult-to-separate compounds. The classical separation approach may become very tedious and time-consuming. The directly conpled HPLC-NMR presents an efficient separation techniqne together with a powerfnl spectroscopic method to speed up the identification process. LC-NMR has been nsed extensively for characterization of natnral prodncts. More recently, the combination of LC-NMR and LC-MS has been further developed in this field. Eor example, Wilson et al. have nsed combined on-flow NMR and electrospray ionization MS to characterize ecdysteroids in extracts of silene otites. After reversed-phase HPLC nsing D2O in acetonitrile-dj and UV detection, the LC flow was split 95 5 for the simnl-taneous detection by NMR and MS. The peaks of interest were analyzed by stop-flow NMR to give better quality spectra for structural assignment. 

The purpose of this review is to summarise the effects, modes of action, structural diversity and biological activity of ecdysteroids and to show how ecdysteroid QSAR and molecular modelling are being applied in agriculture and medicine. 

The term ecdysteroid was originally defined as "all compounds structurally related to ecdysone" [4], However, Lafont and Horn [5] differentiated between true ecdysteroids and ecdysteroid-related compounds. True ecdysteroids possess a steroid nucleus with an A/B-cis-ring junction and a 14a-hydroxy-7-en-6-one chromophore, irrespective of their biological activity, while ecdysteroid-related compounds do not fulfil all these criteria. 

The structural variety of natural zoo-, phyto- and mycoecdysteroids is wide [78], but it does not encompass some structural features which would be desirable for certain applications or for certain SAR studies. There are a number of areas in which novel ecdysteroids would be very helpful ... 

Table 1 Biological activities of ecdysteroids and related compounds in in vivo assays. Data taken from Bergamasco and Horn [171], unless otherwise indicated. The number in brackets after the name of the ecdysteroid refers to the number allocated to it in [171], where the structures for each of these compounds is given.

The most informative analysis of ecdysteroid/receptor interaction would be by X-ray crystallography. Although it has been possible to crystallise the LBDs of several nuclear receptors and to determine their structures by X-ray crystallography (ER [208], PR [209], RAR [210], RXR [211], TR [212], USP [213] and VDR [214]), it has not yet been possible to do this for any EcR protein. This probably derives from the fact that EcR only binds ligand with high affinity when it is complexed with USP (RXR), necessitating the co-crystallisation of ligand + EcR LBD + USP LBD. Ultimately it should be possible to achieve this. In the meantime, two... 

Scheme 3). A clone of cells from the midge Cbironomus tentans was found to be resistant to the effects of ecdysteroids because they metabolized 20-hydroxyecdysone rapidly. The initial oxidation product was 20,26-dihydroxyecdysone, but this was oxidized further to 20-hydroxy-26-oxo-ecdysone (21). This aldehyde (21) then formed a tautomeric equilibrium mixture of two cyclic hemiacetals (22) and (23), which were separable, isolated, and their structures determined (Scheme 3) with the use of acetonides (Section 4.03.3.6).32 These are the first examples of ecdysteroids with side-chain hemiacetals. Although 20,26-dihydroxyecdysterone still... 

---