Iodohexestrols. I. Synthesis and photoreactivity of iodinated hexestrol derivatives

The quest for improving the management of breast cancer by functional imaging: The discovery and development of 16α-[18F]fluoroestradiol (FES), a PET radiotracer for the estrogen receptor, a historical review

INTRODUCTION

16α-[18F]Fluoroestradiol (FES), a PET radiotracer for the estrogen receptor (ER) in breast cancer, was the first receptor-targeted PET radiotracer for oncology and is continuing to prove its value in clinical research, antiestrogen development, and breast cancer care. The story of its conception, design, evaluation and use in clinical studies parallels the evolution of the whole field of receptor-targeted radiotracers, one greatly influenced by the research and intellectual contributions of William C. Eckelman.

METHODS AND RESULTS

The development of methods for efficient production of fluorine-18, for conversion of [18F]fluoride ion into chemically reactive form, and for its rapid and efficient incorporation into suitable estrogen precursor molecules at high molar activity, were all methodological underpinnings required for the preparation of FES. FES binds to ER with very high affinity, and its in vivo uptake by ER-dependent target tissues in animal models was efficient and selective, findings that preceded its use for PET imaging in patients with breast cancer.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

Comparisons between ER levels measured by FES-PET imaging of breast tumors with tissue-specimen ER quantification by IHC and other methods show that imaging provided improved prediction of benefit from endocrine therapies. Serial imaging of ER by FES-PET, before and after dosing patients with antiestrogens, is used to determine the efficacious dose for established antiestrogens and to facilitate clinical development of new ER antagonists. Beyond FES imaging, PET-based hormone challenge tests, which evaluate the functional status of ER by monitoring rapid changes in tumor metabolic or transcriptional activity after a brief estrogen challenge, provide highly sensitive and selective predictions of whether or not there will be a favorable response to endocrine therapies. There is sufficient interest in the clinical applications of FES that FDA approval is being sought for its wider use in breast cancer.

CONCLUSIONS

FES was the first PET probe for a receptor in cancer, and its development and clinical applications in breast cancer parallel the conceptual evolution of the whole field of receptor-binding radiotracers.

PET Imaging Agents (FES, FFNP, and FDHT) for Estrogen, Androgen, and Progesterone Receptors to Improve Management of Breast and Prostate Cancers by Functional Imaging

Many breast and prostate cancers are driven by the action of steroid hormones on their cognate receptors in primary tumors and in metastases, and endocrine therapies that inhibit hormone production or block the action of these receptors provide clinical benefit to many but not all of these cancer patients. Because it is difficult to predict which individuals will be helped by endocrine therapies and which will not, positron emission tomography (PET) imaging of estrogen receptor (ER) and progesterone receptor (PgR) in breast cancer, and androgen receptor (AR) in prostate cancer can provide useful, often functional, information on the likelihood of endocrine therapy response in individual patients. This review covers our development of three PET imaging agents, 16α-[18F]fluoroestradiol (FES) for ER, 21-[18F]fluoro-furanyl-nor-progesterone (FFNP) for PgR, and 16β-[18F]fluoro-5α-dihydrotestosterone (FDHT) for AR, and the evolution of their clinical use. For these agents, the pathway from concept through development tracks with an emerging understanding of critical performance criteria that is needed for successful PET imaging of these low-abundance receptor targets. Progress in the ongoing evaluation of what they can add to the clinical management of breast and prostate cancers reflects our increased understanding of these diseases and of optimal strategies for predicting the success of clinical endocrine therapies.

Study of the rat liver S-adenosylmethionine synthetase active site with 8-azido ATP

The active site of rat liver S-adenosylmethionine synthetase was studied using 8-azido ATP, a photolabile analogue of ATP. Both forms of the enzyme, tetramer and dimer, could be labelled by using concentrations of the analogue similar to the KmATP values for each form, 350 microM and 1 mM respectively. Labelling of both S-adenosylmethionine synthetase forms with 8-azido [alpha-32P]ATP, followed by tryptic digestion and purification by HPLC, afforded one specifically labelled peptide in each case. Identification of the labelled peptide by amino acid analysis and peptide sequencing, and comparison with the enzyme sequence, indicated that the same peptide (267-286) was modified in both enzyme forms. The results are discussed on the basis of the high degree of similarity that this peptide shows in all the known S-adenosylmethionine synthetase sequences.

Intracellular actions of steroid hormones and their therapeutic value, including the potential of radiohalosteroids against ovarian cancer

The biological activities of steroid hormones are effected via intracellular receptors. The receptors are part of a ligand-activated family of transcription regulator proteins that are critical for steroid-regulated cell differentiation. With recombinant cDNA technology, yeast and cultured animal cells can be made to express mammalian cDNA steroid receptors from cDNA clones that contain deletions and substitutions. Among the leading problems addressed in these models is the characterization of sequences that promote association or interaction with other transcription regulating molecules, including oncogene products. Recently it has been found that heat shock proteins may serve not only to stabilize the receptor proteins but also to precondition the activation imparted by ligand binding. Aberrant receptor proteins can be found in ovarian cancer. Whether aberrant receptor proteins are associated with transformation in general or with a variable clinical response to steroidal or anti-steroidal therapy is not known. Even after chemotherapy, steroid receptors are expressed in the metastases of ovarian cancers seen clinically, and they may have potential uses for localization and treatment of receptor-rich cancers. Radioligand pharmaceuticals appropriate for imaging or for site-directed radiocytotoxicity can be sequestered to the nuclei of receptor-rich cancers. Initial clinical imaging and therapy trials with such pharmaceuticals have been approved and begun. In the use of halogenated estrogen radiopharmaceuticals, liver metabolism and enterohepatic recirculation are important considerations. Ascites prolongs retention of a radiohalogenated estrogen in the abdominal cavity. Distant metastases have been localized with [123I]-estrogen in breast cancer patients in pre-operative procedures. Receptor-mediated cytotoxicity occurs when estrogen receptor radioligand pharmaceuticals that are Auger electron emitters are used in vitro.

Biologic activity of the iodoestrogens and their use in breast cancer

Several iodinated estrogens, 6-iodoestra-1,3,5(10), 6-tetraene-3, 17 beta-diol (4) 16 beta-iodo-estradiol (3) were shown to displace 3H-estradiol (1b) from the uterine cytosol receptor by a competitive type of inhibition. The three compounds translocated the cytosol receptor to the nucleus in vitro and increased mouse uterine weight in vivo. In all tests the relative activities were 16 alpha-(2a) greater than 16 beta-(3a) greater than 6-(4a). When the compounds were made with [125I] the 16 alpha-[125I]-iodoestradiol (2b) bound with high affinity, Kd = 0.4 x 10(-10), to the 8S cytosol receptor. No high affinity binding could be demonstrated for the 6-[125I]-iodoestratetraene (4b). In in vivo experiments following the administration of 16 alpha-[125]-iodoestradiol (2b) to rats, high levels of radioactivity were found in the uterus, liver and thyroid but only in the liver and thyroid following administration of 6-[125I]-iodoestratetraene (4b). After administration of (2b) to rats with DMBA-induced mammary tumors, no tumor concentration of radioactivity could be detected by imaging. When (4b) was administered similarly, radioactivity could be detected in some of the tumors by imaging. The radioactivity was associated with non-specific 4S proteins.

Interaction of 16 alpha-[125I]iodo-estradiol with estrogen receptor and other steroid-binding proteins

This communication describes the synthesis of 16 alpha-[125I]iodo-estradiol (125I-E2) with specific activities greater than 1000 Ci/mmol (1 Ci = 3.7 x 10(10) becquerels). We show that it binds to the same specific estrogen receptor sites as does [3H]estradiol and that it does so with an affinity that is indistinguishable from that for the latter steroid. This is true for receptor obtained from calf uterus and from a pool of human mammary carcinomas. There is no significant binding of 125 I-E2 to the testosterone-estradiol-binding globulin of human plasma. 125I-E2 also binds tightly to anti-estradiol antibodies raised against estradiol derivatized at carbons 3, 6, and 17. Finally, we show that unlabeled I-E2 is an active estrogen in vivo as demonstrated by its ability to increase uterine weight in ovariectomized rats.

Iodine-125--labeled estradiol: a gamma-emitting analog of estradiol that binds to the estrogen receptor

17 beta-[16 alpha-125I]Iodoestradiol has been synthesized by exchange of 16 beta-bromoestradiol with Na125I. The iodinated product is readily separated from the bromo reactant by column chromatography. It concentrates in the rat uterus in vivo and binds avidly and specifically to the uterine estrogen receptor in vitro.