Design of radiopharmaceuticals for adrenal imaging: effect of structure and metabolism on the distribution of H-3 and F-18 labeled pregnenolones

The kinetics of the tissue distribution of pregnenolone and pregnenolone-3-acetate labeled with 3H and of 21-fluoropregnenolone and 21-fluoropregnenolone-3-acetate labeled with 3H and with 18F were studied in the rat. The maximum tissue concentration was 4.1 +- 0.5 %D/g in the adrenal glands, obtained 5 minutes after the administration of 21-fluoropregnenolone labeled with 18F. The presence of th 3-acetate group reduced the maximum adrenal concentration to 2.4 +- 0.4 %D/g. The ratio of 18F/3H concentrations were high in the adrenal glands. The data indicate that the adrenal glands contain a high concentration of 18F activity not associated with the steroid nucleus. Both 18F-labeled pregnenolones are potentially useful radiopharmaceuticals for imaging the adrenal glands and/or measuring adrenal metabolism rates