Fluorine-18-labeled progestin ketals: synthesis and target tissue uptake selectivity of potential imaging agents for receptor-positive breast tumors

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.

Molecular imaging using PET and SPECT for identification of breast cancer subtypes

Breast cancer is a major disease with high morbidity and mortality in women. As a highly heterogeneous tumor, it contains different molecular subtypes: luminal A, luminal B, human epidermal growth factor 2-positive, and triple-negative subtypes. As each subtype has unique features, it may not be universal to the optimal treatment and expected response for individual patients. Therefore, it is critical to identify different breast cancer subtypes. Targeting subcellular levels, molecular imaging, especially PET and single photon emission computed tomography, has become a promising means to identify breast cancer subtypes and monitor treatment. Different biological processes between various subtypes, including changes correlated with receptor expression, cell proliferation, or glucose metabolism, have the potential for imaging with PET and single photon emission computed tomography radiopharmaceuticals. Receptor imaging, with radiopharmaceuticals targeting estrogen receptor, progesterone receptor, or human epidermal growth factor 2, is available to distinguish receptor-positive tumors from receptor-negative ones. Cell proliferation imaging with fluorine-18 fluorothymidine PET aids identification of luminal A and B subtypes on the basis of the correlation with the immunohistochemical biomarker Ki-67. Glucose metabolism imaging with fluorine-18 fluorodeoxyglucose PET may have potential to discriminate triple-negative subtypes from others. With increasing numbers of novel radiopharmaceuticals, noninvasive molecular imaging will be applied widely for the identification of different subtypes and provide more in-vivo information on individualized management of breast cancer patients.

PET Imaging of Steroid Hormone Receptor Expression

Steroid hormone receptor (SHR) expression and changes in SHR expression compared to basal levels, whether upregulated, downregulated, or mutated, form a distinguishing feature of some breast, ovarian, and prostate cancers. These receptors act to induce tumor proliferation. In the imaging context, total expression together with modulation of expression can yield predictive and prognostic information. Currently, biopsy for histologic assessment of SHR expression is routine for breast and prostate cancer; however, the technique is not well suited to the heterogeneous tumor environment and can lead to incorrect receptor expression assignment, which precludes effective treatment. The development of positron emission tomography (PET) radioligands to image receptor expression may overcome the difficulties associated with tumor heterogeneity and facilitate the assessment of metastatic disease.

Progesterone receptor targeting with radiolabelled steroids: an approach in predicting breast cancer response to therapy

Steroid receptors have demonstrated to be potentially useful biological targets for the diagnosis and therapy follow-up of hormonally responsive cancers. The over-expression of these proteins in human cancer cells as well as their binding characteristics provides a favourable mechanism for the localization of malignant tumours. The need for newer and more selective probes to non-invasively assess steroid receptor expression in hormone-responsive tumours has encouraged the synthesis and the biological evaluation of several steroidal derivatives labelled with positron and gamma emitters. The physiological effects of the steroid hormone progesterone are mediated by the progesterone receptor (PR). Since PR expression is stimulated by the oestrogen receptor (ER), PR status has been considered as a biomarker of ER activity and its value for predicting and monitoring therapeutic efficacy of hormonal therapy has been studied. Imaging of PR-expressing breast cancer patients under hormonal therapy may be advantageous, since the response to therapy can be more accurately predicted after quantification of both ER and PR status. Thus, ligands for PR targeting, although much less explored than ER ligands, have gained some importance lately as potential PET and SPECT tumour imaging agents. In this review, we present a brief survey of explored approaches for progesterone targeting using radiolabelled progestins as potential clinical probes to predict responsiveness to breast cancer therapy. This article is part of a Special Issue entitled "Synthesis and biological testing of steroid derivatives as inhibitors".

Molecular pathways and molecular imaging in breast cancer: an update

Breast cancer is a heterogenic cancer being characterized by a variability of somatic mutations and in particular by different receptor expressions, such as estrogen, progesterone and human epidermal receptor. These phenotype characteristics play a crucial role in determining tumour response to various chemotherapies and other treatments and in the development of resistance to therapies. Positron emission tomography (PET) as a nuclear medicine technique, has recently demonstrated the advantages in determining the severity of disease and in evaluating the efficacy of treatments in a variety of neoplasm, including breast cancer. Because this procedure is able to pinpoint molecular activity within the body, it offers the potential to identify disease in its earliest stages as well as a patient's immediate response to therapeutic interventions in a non-invasive way. In this paper we performed an extended view about the correlation between molecular factors of breast cancer and PET tracers; in particular, we focalized our attention on their possible advantages in terms of 1) early detection of primary or recurrent cancer; 2) as a guide for target therapies and 3) for the evaluation of response to specific and now-available molecular treatments.

Assessment of progesterone receptors in breast carcinoma by PET with 21-18F-fluoro-16α,17α-[(R)-(1'-α-furylmethylidene)dioxy]-19-norpregn-4-ene-3,20-dione

This first-in-human study was designed to evaluate the safety and dosimetry of the progesterone analog 21-(18)F-fluoro-16α,17α-[(R)-(1'-α-furylmethylidene)dioxy]-19-norpregn-4-ene-3,20-dione ((18)F-FFNP), as well the feasibility of imaging tumor progesterone receptors (PRs) by PET in breast cancer.

METHODS

Women with breast cancer underwent PET with (18)F-FFNP. Tumor (18)F-FFNP uptake was assessed semiquantitatively by determining maximum standardized uptake value and tumor-to-normal breast (T/N) activity ratio and by Logan graphical analysis. The PET results were correlated with estrogen receptor (ER) and PR status, assessed by in vitro assays of the tumor tissue. The biodistribution of (18)F-FFNP was measured in patients by whole-body PET, and human dosimetry was estimated.

RESULTS

Twenty patients with 22 primary breast cancers (16 PR-positive [PR+] and 6 PR-negative [PR-]) were evaluated. Tumor maximum standardized uptake value was not significantly different in PR+ and PR- cancers (mean ± SD, 2.5 ± 0.9 vs. 2.0 ± 1.3, P = 0.386), but the T/N ratio was significantly greater in the PR+ cancers (2.6 ± 0.9 vs. 1.5 ± 0.3, P = 0.001). In addition, there was a significant correlation between distribution volume ratio and T/N ratio (r = 0.89; P = 0.001) but not between distribution volume ratio and either PR status or standardized uptake value, likely because of small sample size. On the basis of whole-body PET data in 12 patients, the gallbladder appeared to be the dose-limiting organ, with an average radiation dose of 0.113 mGy/MBq. The whole-body dose was 0.015 mGy/MBq, and the effective dose was 0.020 mSv/MBq. No adverse effects of (18)F-FFNP were encountered.

CONCLUSION

(18)F-FFNP PET is a safe, noninvasive means for evaluating tumor PRs in vivo in patients with breast cancer. The relatively small absorbed doses to normal organs allow for the safe injection of up to 440 MBq of (18)F-FFNP.

Development of [F-18]fluorine-substituted Tanaproget as a progesterone receptor imaging agent for positron emission tomography

The level of progesterone receptors (PRs) in breast tumors can be used to guide the selection of endocrine therapies for breast cancer patients. To this end, we have prepared a fluorine-18 labeled analogue of Tanaproget, a nonsteroidal progestin with very high PR binding affinity and low affinity for androgen and glucocorticoid receptors, and have studied its tissue distribution in estrogen-primed rats to evaluate its potential for imaging PR levels by positron emission tomography. 4-[(18)F]Fluoropropyl-Tanaproget ([(18)F]9, FPTP) was prepared in three steps, within 140 min at an overall decay-corrected yield of 5% and effective specific activity of >550 Ci/mmol. In biodistribution studies, [(18)F]9 uptake was high in target tissues at both 1 and 3 h (uterus, 4.55 and 5.26%ID/g; ovary, 2.32 and 2.20%ID/g, respectively) and was cleanly blocked by coinjection of excess unlabeled compound. Uterus to blood and muscle activity ratios were 9.2 and 5.2 at 1 h and 32 and 26 at 3 h, respectively. The biodistribution of [(18)F]9 compares favorably to that of previously prepared F-18 labeled steroidal progestins, FENP and FFNP. Its high target tissue uptake efficiency and selectivity, and prolonged retention, suggest that it has excellent promise as a PET imaging agent for PR-positive breast tumors.

Imaging progesterone receptor in breast tumors: synthesis and receptor binding affinity of fluoroalkyl-substituted analogues of tanaproget

The progesterone receptor (PR) is estrogen regulated, and PR levels in breast tumors can be used to predict the success of endocrine therapies targeting the estrogen receptor (ER). Tanaproget is a nonsteroidal progestin agonist with very high PR binding affinity and excellent in vivo potency. When appropriately radiolabeled, it might be used to image PR-positive breast tumors noninvasively by positron emission tomography (PET). We describe the synthesis and PR binding affinities of a series of fluoroalkyl-substituted 6-aryl-1,4-dihydrobenzo[d][1,3]oxazine-2-thiones, analogues of Tanaproget. Some of these compounds have subnanomolar binding affinities, higher than that of either Tanaproget itself or the high affinity PR ligand R5020. Structure-binding affinity relationships can be rationalized by molecular modeling of ligand complexes with PR, and the enantioselectivity of binding has been predicted. These compounds are being further evaluated as potential diagnostic PET imaging agents for breast cancer, and enantiomerically pure materials of defined stereochemistry are being prepared.

Evaluation of a bromine-76-labeled progestin 16alpha,17alpha-dioxolane for breast tumor imaging and radiotherapy: in vivo biodistribution and metabolic stability studies

INTRODUCTION

Progesterone receptors (PRs) are present in many breast tumors, and their levels are increased by certain endocrine therapies. They can be used as targets for diagnostic imaging and radiotherapy.

METHOD

16alpha,17alpha-[(R)-1'-alpha-(5-[(76)Br]Bromofurylmethylidene)dioxyl]-21-hydroxy-19-norpregn-4-ene-3,20-dione ([(76)Br]16alpha,17alpha-[(R)-1'-alpha-(5-bromofurylmethylidene)dioxyl]-21-hydroxy-19-norpregn-4-ene-3,20-dione (3)), a PR ligand with relative binding affinity (RBA)=65 and log P(o/w)=5.09+/-0.84, was synthesized via a two-step reaction, and its tissue biodistribution and metabolic stability were evaluated in estrogen-primed immature female Sprague-Dawley rats.

RESULTS

[(76)Br]16alpha,17alpha-[(R)-1'-alpha-(5-bromofurylmethylidene)dioxyl]-21-hydroxy-19-norpregn-4-ene-3,20-dione 3 was synthesized in 5% overall yield with specific activity being 200-1250 Ci/mmol. [(76)Br]16alpha,17alpha-[(R)-1'-alpha-(5-bromofurylmethylidene)dioxyl]-21-hydroxy-19-norpregn-4-ene-3,20-dione 3 demonstrated high PR-mediated uptake in the target tissue uterus (8.72+/-1.84 %ID/g at 1 h) that was reduced by a blocking dose of unlabeled progestin R5020, but the nonspecific uptake in blood and muscle (2.11+/-0.14 and 0.89+/-0.16 %ID/g at 1 h, respectively) was relatively high. [(76)Br]16alpha,17alpha-[(R)-1'-alpha-(5-bromofurylmethylidene)dioxyl]-21-hydroxy-19-norpregn-4-ene-3,20-dione 3 was stable in whole rat blood in vitro, but it was not stable in vivo due to the fast metabolism that occurred in the liver, resulting in the formation of a more polar radioactive metabolite and free [(76)Br]bromide. The level of free [(76)Br]bromide in blood remained high during the experiment (2.11+/-0.14 %ID/g at 1 h and 1.52+/-0.24 %ID/g at 24 h). The tissue distribution of [(76)Br]16alpha,17alpha-[(R)-1'-alpha-(5-bromofurylmethylidene)dioxyl]-21-hydroxy-19-norpregn-4-ene-3,20-dione 3 at 1 and 3 h was compared with that of the (18)F analogs, [(18)F]FFNP fluoro furanyl norprogesterone (FFNP) 1 and ketal 2.

CONCLUSION

[(76)Br]16alpha,17alpha-[(R)-1'-alpha-(5-bromofurylmethylidene)dioxyl]-21-hydroxy-19-norpregn-4-ene-3,20-dione 3 may have potential for imaging PR-positive breast tumors at early time points, but it is not suitable for imaging at later times or for radiotherapy.

Fluorine-18 labeling of small molecules: the use of 18F-labeled aryl fluorides derived from no-carrier-added [18F]fluoride as labeling precursors

The favourable long-half life, the ease of production and the low energy of the emitted positron make 18F an ideal radionuclide for PET imaging. Radiochemistry of 18F basically relies on two distinctive types of reactions: nucleophilic and electrophilic reactions. All syntheses of 18F-labeled radiotracers are based on either [18F]fluoride ion or [18F]fluorine gas as simple primary labeling precursors which are obtained directly from the cyclotron. They can be applied either directly to the radiosynthesis or they can be transformed into more complex labeling precursors enabling the multi-step build-up of organic tracer molecules. The topic of this review is a survey on the application of several 18F-labeled aryl fluorides as building blocks derived from no-carrier-added (n.c.a.) [18F] fluoride to build up small monomeric PET radiotracers at high specific radioactivity by multi-step synthesis procedures.

Bromine- and Iodine-Substituted 16α,17α-Dioxolane Progestins for Breast Tumor Imaging and Radiotherapy: Synthesis and Receptor Binding Affinity

Progesterone receptors (PRs) are present in many breast tumors, and their levels are increased by certain endocrine therapies. We describe the synthesis and PR binding affinities of a series of bromine- and iodine-substituted 16alpha,17alpha-dioxolane progestins, some of which, when appropriately radiolabeled, are potential agents for diagnostic imaging of PR-positive breast tumors using positron emission tomography (PET) and for radiotherapy. These compounds were synthesized from halogenated furanyl, phenyl, and thiophenyl aldehydes and a progestin 16alpha,17alpha,21-triol (5) in the presence of HClO4 or Sc(OTf)3 in high yields under optimized conditions. A new reagent, perfluoro-1-butanesulfonyl fluoride (PBSF), was used to convert the C-21 OH to F in high yields. The relative binding affinities (RBAs) of the most promising compounds for the PR (RBA of R5020 = 100) were 16alpha,17alpha-[(R)-1'-alpha-(5-bromofurylmethylidene)dioxyl]-21-hydroxy-19-norpregn-4-ene-3,20-dione (endo-6; RBA = 65 and moderate lipophilicity), 21-fluoro-16alpha,17alpha-[(R)-1'-alpha-(5-iodofurylmethylidene)dioxyl]-19-norpregn-4-ene-3,20-dione (endo-14; RBA = 40) and 21-fluoro-16alpha,17alpha-[(S)-1'-beta-(4-iodophenylmethylidene)dioxyl]-19-norpregn-4-ene-3,20-dione (exo-16; RBA = 34).

Synthesis and Activity of C11-Modified Wortmannin Probes for PI3 Kinase

The key role played by PI3 kinase in cancer, hormone action, and a host of other biological functions suggests that specific inhibitors whose disposition could be ascertained in vivo would be useful in biological research or, potentially, for imaging PI3K in a clinical setting. Wortmannin (Wm, 1) is an inhibitor of PI3 kinase with high specificity for this enzyme. We synthesized three modified Wm probes, a biotinylated Wm (7a), a 4-hydroxy-3-iodophenylated Wm, which was obtained both unlabeled (7b) and labeled with (125)I (8), and a fluoresceinated Wm (7c), through modification at C-11, and evaluated their inhibitive activity as inhibitors of PI3 kinase. Biotinylated (7a) and 4-hydroxy-3-iodophenylated Wm's (7b) had IC(50)s for PI3K of 6.11 and 11.02 nM, respectively, compared to an IC(50) for Wm of 1.63 nM. Fluoresceinated Wm (7c) lost considerably more activity than the other derivatives, with an IC(50) of 64.9 nM. The (125)I labeled 4-hydroxy-3-iodophenylated Wm (8) could be detected after reaction with an immunoprecipitate of PI3 kinase. The activity of these reporter Wm's is discussed in relationship to earlier findings on the pharmacological activity of Wm derivatives and the ability of inhibitors to fit into the ATP pocket of PI3 kinase.

Synthesis of the 7alpha-cyano-(17alpha,20E/Z)-[125I]iodovinyl-19-nortestosterones: potential radioligands for androgen and progesterone receptors

We report the preparation of the 7alpha-cyano derivative of the isomeric (17alpha,20E/Z)-[125I]iodovinyl-19-nortestosterones (IVNT) together with their binding affinity for the androgen receptor (AR) and their biodistribution in two different animal models. The cyano group was introduced at the 7alpha-position by hydrocyanation of 4,6-estradien-17beta-ol-3-one with diethylaluminum cyanide. Selective protection of the A-ring enone system as the dienol ether followed by ethynylation and deprotection under base and acid hydrolysis condition gave 7alpha-cyano-17alpha-ethynyl-19-nortestosterone. The stannyl derivatives were prepared by addition of tri-n-butylstannyl hydride and converted stereospecifically to the corresponding [125I]iodovinyl analog using [125I]NaI and H2O2. The [125I]iodovinylsteroids were intravenously administered to male rats and estrogen-primed immature female rats and tissue uptake was measured up to 6h post-injection. Co-administration of NLP-004 or ORG-2058, highly selective ligands for the progesterone receptor, to the female rats did not affect uterus uptake of the 125I-ligands. However co-injection of testosterone to DES-primed male rats induced a marked increase in prostate uptake of the 20Z-isomer of 7alpha-cyano-[125I]-IVNT. The relative binding affinity (RBA) of either 7alpha-cyano-(17alpha,20E/Z)-IVNT isomer for the AR is low (RBA=4 and 3, respectively, versus 100 for 5alpha-dihydrotestosterone (DHT)), suggesting the absence of a possible role of the AR in the localization process. These findings contrast previously reported data for the analogous 7alpha-methyl-[125I]-IVNT where co-administration of testosterone was shown to result in a 50% drop in prostate uptake. These data indicate that the addition of an electron withdrawing 7alpha-cyano group to 123I-labeled nortestosterone derivatives does not improve their potential to serve as SPECT agents for the imaging of AR densities in the prostate.

An efficient route for the preparation of a 21-fluoro progestin-16 alpha,17 alpha-dioxolane, a high-affinity ligand for PET imaging of the progesterone receptor

Two different synthetic routes were explored for the synthesis of fluoro furanyl norprogesterone (FFNP) 1, a high-affinity ligand for the progesterone receptor (PgR) that is being developed as a PET imaging agent for PgR-positive breast cancer. Both approaches proceed through a key intermediate, triol 5. The first approach, starting from keto-ketal 2, employed a dioxenyl group as a synthon for installing a corticosteroid side chain in keto-alcohol 4. The second approach, starting from propargylic acetate 12b, involved the application of a two-step method, a Pd(II)-catalyzed oxidative rearrangement followed by a base-catalyzed acetate rearrangement of the intermediate unsaturated acetate 13b, to generate the requisite corticosteroid side chain in keto-acetate 14b. This intermediate was further elaborated to the final product 1 via efficient dihydroxylation with potassium permangnate, furan acetalization with scandium triflate, and mesylation and fluorination reactions. The palladium-catalyzed route is considerably more efficient than the dioxene approach for the synthesis of key intermediate triol 5, and the scandium triflate-catalyzed acetalization, in particular, led to a considerable improvement in the overall yield of the endo furan acetal alcohol 16a. This route provides a major improvement in the overall yield of the final progestin target, FFNP 1.

Radiochemical synthesis and tissue distribution of Tc-99m-labeled 7alpha-substituted estradiol complexes

The diagnosis and staging of breast cancer could be improved by the development of radiopharmaceutical imaging agents that provide a noninvasive determination of the estrogen receptor (ER) status of tumor cells. Agents labeled with (99m)Tc would be especially valuable in this regard. In attempting to achieve this goal, we synthesized four (99m)Tc-labeled 7alpha-substituted estradiol complexes. One complex utilizes the "3+1" mixed ligand design to introduce the Tc metal, whereas the other three took advantage of the cyclopentadienyltricarbonylmetal (CpTM) design. The Tc moieties were attached to the 7alpha position of estradiol with a hexyl tether, a monoether tether, or a polyether tether. The corresponding rhenium compounds have binding affinities for the ER of 20-45% compared with estradiol. Radiochemical yields of the (99m)Tc-labeled compounds ranged from approximately 15% for the CpT-Tc complexes to 95% for the 3 + 1 inorganic complex. Tissue distribution studies in immature female rats showed low nonreceptor-mediated uptake in the target organs and high uptake in nontarget organs such as the liver and fat. These complexes represent the first time that estradiol has been labeled at the 7alpha position with (99m)Tc and provide a further refinement of our understanding of ligand structure-binding affinity correlations for the ER.

Synthesis and Binding Affinities of Novel Re-Containing 7alpha-Substituted Estradiol Complexes: Models for Breast Cancer Imaging Agents

The diagnosis and staging of breast cancer could be improved by the development of imaging radiopharmaceuticals that provide a noninvasive determination of the estrogen receptor status in the tumor cells. Toward this goal, we have synthesized a number of novel Re-containing 7alpha-substituted estradiol complexes. The introduction of the 7alpha side chain involves the alkylation of tetrahydropyranyloxy-protected 6-keto estradiol. The methods used to introduce the rhenium metal involve "3 + 1" and "4 + 1" mixed ligand complexes (2a-c and 5, respectively), tricarbonyl dithioether complexes (3), and the cyclopentadienyltricarbonylmetal organometallic system (4ab, 6, 7). These complexes showed binding affinities for the estrogen receptor (as high as 45% for the "3 + 1" complex 2c) when compared to the native ligand estradiol. The polarity of some complexes (4ab) was modified to improve biodistribution properties by introducing (poly)ether linkages into the 7alpha side chain (6, 7). These complexes provide a further refinement of our understanding of ligand structure-binding affinity correlations for the estrogen receptor, and they furnish the synthetic groundwork for the synthesis of the analogous Tc-99m complexes for evaluation as breast tumor imaging agents.

Integrated "3+1" oxorhenium(V) complexes as estrogen mimics

The diagnosis and staging of breast cancer could be improved by the development of imaging radiopharmaceuticals that provide a noninvasive determination of the estrogen receptor (ER) status of tumor cells. Toward this goal, we have synthesized a number of integrated "3+1" oxorhenium(V) complexes designed to mimic estradiol and a class of nonsteroidal estrogens, the tetrahydrochrysenes (THC). The monodentate component of the estradiol mimic is a p-hydroxyphenethyl thiol ligand with ethyl substituents at the benzylic and homobenzylic positions. Model complexes of this ligand were easily made, but steric hindrance of the secondary thiol prevented the formation of the complex with the disubstituted ligand. The three "3+1" oxorhenium(V) complexes prepared to mimic the THC class mimics represent the first pyridinedithiol rhenium complexes of their kind to be made. These complexes are quite stable to air and moisture. The target tridentate ligand was prepared from chelidamic acid, and the VT NMR of the rhenium complex displays interesting fluxional behavior. The binding affinities of these complexes for the estrogen receptor are low, and their lipophilicities are rather high. Nevertheless, our findings provide a further refinement of our understanding of ligand structure-binding affinity correlations for the estrogen receptor.

New iodinated progestins as potential ligands for progesterone receptor imaging in breast cancer. Part 1: Synthesis and in vitro pharmacological characterization

Five putative iodinated progesterone receptor (PR) binding ligands were synthesized and evaluated as potential imaging agents for PR-positive human breast tumours. Two compounds (E- and Z-17-hydroxy-21-iodo-19-nor-17alpha-pregna-4,20-dien-3-one; E- and Z-IPG1) were previously described, but are re-evaluated. The other three were novel compounds: two nortestosterone analogues derived from ORG 3236 (E- and Z-13-ethyl-17-hydroxy-21-iodo-11-methylene-18,19-dinor-17alpha-pre gna-4,20-diene-3-one; E- and Z-IPG2) and one norprogesterone analogue derived from ORG 2058 (21-[4-iodophenoxy]-16alpha-ethyl-19-norpregn-4-ene-3, 20-dione; IPG3). The E-iodovinyl nortestosterone compounds were obtained by a new route of synthesis. Competitive binding studies were performed to determine their binding affinities for the PR in three types of tissue (human MCF-7 breast tumour cells and rat uterine and mammary tumour tissue) and for the androgen receptor (AR) in human MCF-7 breast tumour cells, as well as for the sex hormone-binding globulin (SHBG) and corticosteroid-binding globulin (CBG) in human plasma. All four 17alpha-iodovinyl nortestosterone derivatives displayed high binding affinity for the human PR, that of Z-IPG1 and E- and Z-IPG2 being even higher than that of ORG2058. Their affinities for the rat PR were somewhat lower, especially those of both E-isomers. The affinity of IPG3 was lower for both the human and rat PR. The nortestosterone derivatives also showed AR binding, the relative binding affinities ranging from 4.3 to 17.0% as compared with 5alphaDHT. Additionally, neither of these steroids displayed any significant binding to either SHBG or CBG in human plasma. We conclude that the in vitro binding properties of all four 17alpha-iodovinyl nortestosterone derivatives warrant evaluation of the distribution characteristics of their 123I-labelled analogues to determine their usefulness as PR imaging agents.

New iodinated progestins as potential ligands for progesterone receptor imaging in breast cancer. Part 2: In vivo pharmacological characterization

On the basis of the observed high selective binding to both the human and rat progesterone receptor (PR) in vitro, three 17alpha-iodovinyl-substituted nortestosterone derivatives, i.e., the Z-isomer of 17alpha-iodovinyl-19-nortestosterone (Z-IVNT; Z-IPG1) and both the stereoisomers of 17alpha-iodovinyl-18-methyl-11-methylene-19-nortestosterone (E- and Z-IPG2), were selected for radio-iodination and subsequently evaluated as potential radioligands for PR imaging in human breast cancer. Their target tissue uptake, retention, and uptake selectivity were studied in female rats. The distribution studies revealed that PR-mediated uptake in the uterus and ovaries could only be demonstrated for Z-[123I]IPG2. The target tissue uptake selectivity was, however, low, with the highest uterus-to-nontarget tissue uptake ratios observed at 2-4 h postinjection (p.i.), being 4.4, 1.8, and 7.4 for the uterus-to-blood, -fat, and -muscle ratio, respectively. For Z-[123I]IPG2, distribution was also studied in dimethylbenzanthracene (DMBA)-induced mammary tumour-bearing rats and in normal rabbits. Mammary tumour uptake of Z-[123I]IPG2 in the mammary tumour-bearing rat was also found to be PR-specific. In rabbits, higher selective target tissue uptake of Z-[123I]IPG2 was observed than in rats, resulting in uterus-to-blood, -fat, and -muscle ratios of 6.6, 2.2, and 21.3 at 2-4 h p.i., respectively. In conclusion, Z-[123I]IPG2, which displayed high binding affinity for both the human and rat PR in vitro, showed specific PR-mediated target tissue uptake in rats and rabbits in vivo, the uptake selectivity being highest in the latter. Because the binding characteristics appeared to vary between species, a pilot study in breast cancer patients may be needed to decide whether Z-[123I]IPG2 can be of potential use as PR imaging agent in breast cancer.

Facile synthesis of [16,16,17-2H3]-testosterone, -epitestosterone and their glucuronides and sulfates

The analysis of trace components in complex biological matrices requires the use of reliable internal standards. For the gas chromatography/mass spectrometry (GC/MS) and high performance liquid chromatography/mass spectrometry (HPLC/MS) analyses, the stable isotope-labelled analogues of the analyte molecules are the most appropriate internal standards. In this work high-yield synthetic procedures for stably labelled and isotopically pure [16,16,17-2H3]-testosterone and- epitestosterone are reported. Synthetic methodologies for the glucuronidation and sulfation were established with the commercially available epitestosterone. Structure characterization of 4-androsten-17 alpha-ol-3-one methyl-2',3',4'-tri-O-acetyl-beta-D-glucuronate was made by two-dimensional nuclear magnetic resonance (COSY). Subsequently glucuronidation of [16,16,17-2H3]-testosterone and sulfation of [16,16,17-2H3]-epitestosterone were carried out at greater than 60% yield. However, the yield from the glucuronidation of epitestosterone was not as high. Electrospray mass spectrometry of four conjugates: testosterone sulfate, epitestosterone sulfate, testosterone glucuronide and epitestosterone glucuronide was carried out in the positive ion mode at a number of orifice voltages (50-95 V). Studies of the collisionally induced dissociation at both the interface and in the collision cell (MSMS) confirmed that the glycosidic bond of epitestosterone glucuronide was more labile than that of testosterone glucuronide. Use of the deuterated internal standards is reported to demonstrate the direct analysis of the steroid conjugates by HPLC/MS.

Synthesis and organ distribution of [18F]fluoro-Org 6141 in the rat: a potential glucocorticoid receptor ligand for positron emission tomography

For the synthesis of [18F]Fluoro-Org 6141 via a nucleophilic substitution reaction with 18F-, the tosyl group was chosen as the leaving group because of its stability and excellent leaving group ability. The biodistribution of the high affinity and moderate lipophilicity (log P = 2.66, calculated value) ligand [18F]Fluoro-Org 6141 (specific activity 8.2 to 37 TBq/mmol, yield 10% at EOB) was examined in sham adrenalectomized (sADX) and adrenalectomized (ADX) male Wistar rats. Two days after ADX or sADX, the animals were anesthetized and 0.37 to 1.85 MBq of [18F]Fluoro-Org 6141 was administered intravenously. Kinetics of 18F activity uptake were monitored for 3 h using a stationary double-headed positron emission tomography (PET) camera, and the biodistribution was assessed by ex vivo determination of radioactivity in several tissues and different brain areas. One hour after injection of the radioligand, the bladder, kidney, liver, trachea, and bone of sADX animals contained more concentration on a wet weight basis than blood. Three hours post injection, radioactivity was retained in bladder, trachea, and bone. The accumulation of radioactivity in brain corresponded to the concentration of activity in the blood within the first hours after injection. ADX animals showed a higher uptake of 18F activity in spleen, testes, and brain areas (hippocampus and brainstem) but a lower uptake in bone than sADX rats. PET scans suggested that 18F activity uptake in the brain had not yet reached a maximum at this interval. Although [18F]Fluoro-Org 6141 is not useful for PET studies of glucocorticoid receptors (GRs), the results obtained with this compound indicate a synthetic strategy suitable for the synthesis of high-affinity radioligands for GRs.

6 alpha-[18F]fluoroprogesterone: synthesis via halofluorination-oxidation, receptor binding and tissue distribution

We have evaluated 6 alpha-[18F]fluoroprogesterone as a potential imaging agent for progesterone receptor (PgR)-positive breast cancer. 6 alpha-Fluoroprogesterone (1) was obtained via halofluorination of the C-5 double bond in pregnenolone, followed by oxidation of the 3 beta-OH group, elimination of HBr from C-4,5, and epimerization at the C-6 center. The relative binding affinity (RBA) of 6 alpha-fluoroprogesterone (1) to PgR is 11 (R5020 = 100), and its binding selectivity index (BSI, i.e. the ratio of the RBA to the non-specific binding, NSB) is 14.4; these values are similar to those of progesterone. 17 alpha-Acetoxy-6 alpha-fluoroprogesterone (2) was also prepared by the same method, but was not used for fluorine-18 labeling studies because its binding affinity for PgR is very low (0.9). The synthesis of 1 was adapted to fluorine-18 labeling and although the overall radiochemical yield was low (decay-corrected, 0.3%), progestin [18F]1 was obtained in moderately high effective specific activity (147 Ci/mmol). In vivo distribution studies using estrogen-primed immature female rats showed that 6 alpha-fluoroprogesterone ([18F]1) has low uterine uptake, low target tissue selectivity, and high fat uptake, presumably due to its low RBA and BSI. High uptake in bone, which indicates extensive metabolic defluorination, suggests that the C-6 position of steroids may not be a good site for fluorine-18 labeling.

Synthesis of C-6 fluoroandrogens: evaluation of ligands for tumor receptor imaging

Seven androgens, substituted with fluorine at C-6, were prepared as potential imaging agents for androgen receptor-positive prostate tumors and were evaluated in vitro in terms of their lipophilicity and their relative binding affinities (RBA, relative to R 1881 = 100) for the androgen receptor and for sex steroid binding protein. Introduction of a fluorine atom into the C-6 position of an androgen generally decreases binding affinity to the androgen receptor, except in the two cases: 6 alpha-fluoro-19-nor-testosterone (RBA = 41.6 versus 30.6 for the unsubstituted steroid) and 6 alpha-fluorotestosterone (RBA = 8.9 versus 6.6). Receptor binding of the C-6 fluoro-androgens is also stereospecific, showing higher binding affinities for the alpha-epimers compared to the corresponding beta-epimers (4:1-15:1). Binding affinity to sex steroid binding protein is the lowest with 19-nor-testosterone, which is also the least lipophilic androgen studied. Based on the binding properties of compounds in this series, 6 alpha-fluoro-19-nor-testosterone appears to have the most promise as a tumor imaging agent.