I-labeled 19-iodinated and 6 beta-iodomethyl-19-nor steroids: effect of structural modification on the adrenal...

19-Iodinated and 6 beta-iodomethyl-19-nor derivatives of cholesterol and 17-ketosteroid labeled with 131I were tested in rats to determine the critical structural features required for maximal adrenal uptake. The introduction of the 17-keto group in place of the 17 beta-side chain of cholesterol caused most of the radioactivity to be taken up by the thyroids. Fluorination at the C-3 position had deleterious effects on the adrenal concentration and led to the loss of adrenal specificity. A beta-hydroxy group at the C-3 position is substantially required for adrenal uptake.