FLUORINE-18-LABELED PROGESTIN KETALS - SYNTHESIS AND TARGET TISSUE UPTAKE SELECTIVITY OF POTENTIAL IMAGING AGENTS FOR RECEPTOR-POSITIVE BREAST-TUMORS

We have studied two new fluorine-substituted progestins as potential imaging agents for progesterone-receptor-positive human breast tumors. The steroids are 16alpha,17alpha-fluoroacetophenone ketals of 16alpha,17alpha-dihydroxyprogesterone and 16alpha,17alpha,21-trihydroxy-19-norprogesterone. Synthesis of the latter compound in seven steps from 19-norandrost-4-ene-3,17-dione is reported. Both compounds demonstrate high affinity for the progesterone receptor (PgR) (52.5 and 240%, respectively, relative to R5020 = 100). The syntheses were adapted to F-18-labeling with 4'-[F-18]-fluoroacetophenone, prepared from 4'-nitroacetophenone by nucleophilic substitution with (KF)-F-18/Kryptofix. Considerable adjustment of reaction conditions was required to effect ketalization using tracer quantities of the ketone. In tissue distribution studies in estrogen-primed immature female rats, both ketals showed selective uterine uptake, which was blocked by coinjection of a saturating dose of the unlabeled progestin ORG 2058. Additionally, metabolic stability of the radiolabel was indicated by the low radioactivity levels seen in bone. Both compounds showed relatively high uptake in fat, in accord with their relative lipophilicities demonstrated by HPLC-derived octanol-water partition coefficients. The selective uterine uptake and metabolic stability of these compounds suggests that this class of PgR ligands might be promising for the selective imaging of receptor-positive tumors if derivatives of reduced lipophilicity can be prepared.