Synthesis and evaluation of potential radioligands for the progesterone receptor

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Abstract
Several steroidal analogues were synthesized as potential .gamma.-emitting radioligands for the progesterone receptor. Each of these compounds was tested as an inhibitor of the specific binding of [3H]-17.alpha.,21-dimethyl-19-nor-4,9-pregnadiene-3,20-dione (R5020) to the progesterone receptor in rabbit urine cytosol. R5020 is a well-known progestin with high affinity for the receptor. Of the compounds synthesized, aromatic N-substituted C-17 steroidal carboxamides inhibited the binding only poorly. Three compounds, 16.alpha.-iodo-4-estren-17.beta.-ol-3-one, 17.alpha.-[2(E)-iodovinyl]-4-estren-17.beta.-ol-3-one, and 17.alpha.-[2(Z)-iodovinyl]-4-estren-17.beta.-ol-3-one were excellent competitors, each having a Ki less than or equal to that of the natural progestin, progesterone. Since similar iodinated analogues of estrogens have been shown to be extremely stable both in vivo and in vitro, these compounds are potentially useful ligands for the progesterone receptor.