26-[125I]iodoponasterone A is a potent ecdysone and a sensitive radioligand for ecdysone receptors.

The effects of ecdysone, the steroid molting hormone of arthropods, are of considerable interest both to insect physiologists and to those studying steroid-regulated gene expression. Yet progress in understanding ecdysone receptors has been inhibited by the lack of a suitable highly radioactive hormone analog with high affinity for the receptor. Here we report that the synthetic ecdysteroid 26-iodoponasterone A is one of the most active ecdysones known, inducing half-maximal morphological transformation in Drosophila Kc167 cells when present at 0.5 nM. 26-[125I]Iodoponasterone A can be prepared at a specific activity of 2175 Ci/mmol (1 Ci = 37 GBq) by reaction of the precursor 26-mesylinokosterone with carrier-free Na125I. The radiolabeled material binds to Kc167 cell ecdysone receptors specifically and with affinity (Kd ca. 3.8 X 10(-10) M). Thus, 26-[125I]iodoponasterone A appears to be a superior radioligand for ecdysone receptors on grounds both of affinity and of specific activity. Its ready availability should greatly facilitate studies of these receptors.