Methodology for the synthesis and specific activity determination of 16 alpha-[77Br]-bromoestradiol-17 beta and 16 alpha-[77Br]-11 beta-methoxyestradiol-17 beta, two estrogen receptor-binding radiopharmaceuticals

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.

Radiobromine and radioiodine for medical applications

The halogens bromine and iodine have similar chemical properties and undergo similar reactions due to their closeness in Group 17 of the periodic chart. There are a number of bromine and iodine radionuclides that have properties useful for diagnosis and therapy of human diseases. The emission properties of radiobromine and radioiodine nuclides with half-lives longer than 1 h are summarized along with properties that make radionuclides useful in PET/SPECT imaging and β/Auger therapy, such that the reader can assess which of the radionuclides might be useful for medical applications. An overview of chemical approaches that have been used to radiolabel molecules with radiobromine and radioiodine nuclides is provided with examples. Further, references to a large variety of different organ/cancer-targeting agents utilizing the radiolabeling approaches described are provided.

Optimization of the preparation of fluorine-18-labeled steroid receptor ligands 16alpha-[18F]fluoroestradiol (FES), [18F]fluoro furanyl norprogesterone (FFNP), and 16beta-[18F]fluoro-5alpha-dihydrotestosterone (FDHT) as radiopharmaceuticals

Fluorine-18-labeled steroid receptor tracers, 16α-[(18)F]fluoroestradiol (FES), [(18)F]fluoro furanyl norprogesterone (FFNP), and 16β-[(18)F]fluoro-5α-dihydrotestosterone (FDHT), are important imaging tools for studies of breast and prostate cancers using positron emission tomography (PET). The automated production of these ligands with high specific activity (SA) as radiopharmaceuticals requires modification and optimization of the currently reported methods. [(18)F]FES with high SA was synthesized in over 60% radiochemical yield (RCY) at the end of synthesis (EOS) using a small amount of precursor (1) (as low as 0.3 mg) and 1 M H2SO4 for deprotection of the intermediate (2). [(18)F]FFNP was synthesized in up to 77% RCY at EOS using the triflate precursor (4) at room temperature or in 25% RCY using the mesylate precursor (6) at 65°C. Both methods are highly reproducible and afford high SA. [(18)F]FDHT was synthesized by radiofluoride incorporation at room temperature, reduction with NaBH4 , and deprotection with HCl/acetone, giving [(18)F]FDHT in up to 75% yield (RCY). All of these methods can be easily translated to automated production. The information provided here will aid in the development of automated production of these steroid receptor tracers with high or improved yields, optimal SA, and ease of processing for research and clinical use.

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".

Synthesis and biological evaluation of two agents for imaging estrogen receptor β by positron emission tomography: challenges in PET imaging of a low abundance target

INTRODUCTION

Independent measurement of the levels of both the estrogen receptors, ERα and ERβ, in breast cancer could improve prediction of benefit from endocrine therapies. While ERα levels can be measured by positron emission tomography (PET) using 16α-[(18)F]fluoroestradiol (FES), no effective agent for imaging ERβ by PET has yet been reported.

METHODS

We have prepared the fluorine-18 labeled form of 8β-(2-fluoroethyl)estradiol (8BFEE(2)), an analog of an ERβ-selective steroidal estrogen, 8β-vinylestradiol; efficient incorporation of fluorine-18 was achieved, but required very vigorous conditions. We have examined the biodistribution of this compound, as well as of Br-041, an analog of a known non-steroidal ERβ-selective ligand (ERB-041), labeled with bromine-76. Studies were done in immature female rodents, with various pharmacological and endocrine perturbations to assess ERβ selectivity of uptake.

RESULTS

Little evidence of ERβ-mediated uptake was observed with either [(18)F]8BFEE(2) or [(76)Br]Br-041. Attempts to increase the ERβ content of target tissues were not effective and failed to improve biodistribution selectivity.

CONCLUSIONS

Because on an absolute basis level, ERβ levels are low in all target tissues, these studies have highlighted the need to develop improved in vivo models for evaluating ERβ-selective radiopharmaceuticals for use in PET imaging. Genetically engineered breast cancer cells that are being developed to express either ERα or ERβ in a regulated manner, grown as xenografts in immune-compromised mice, could prove useful for future studies to develop ER subtype-selective radiopharmaceuticals.

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.

Bromination from the macroscopic level to the tracer radiochemical level: (76)Br radiolabeling of aromatic compounds via electrophilic substitution

No-carrier-added (NCA) (76)Br labeling of 4-(5-acetoxy-7-bromobenzoxazol-2-yl)phenyl acetate, a diacetate-protected estrogen-receptor beta (ERbeta) selective ligand, was carried out successfully using [(76)Br]bromide ion. The labeling was achieved via oxidative electrophilic destannylation of an organotin precursor molecule by modification of the leaving group (from Bu(3)Sn to Me(3)Sn) and the addition of methanol to the reaction mixture. The differences between the oxidative bromination reaction under small-scale macroscopic vs tracer level radiochemical conditions were explored in terms of effective brominating agents, which depend greatly on the nature of the solvent during the radiochemical bromination, and the potential interference by trace levels of highly reactive impurities in the reaction that compete for the desired bromination at the NCA level. Our observations, and our development of experimental protocols for successful radiobromination at the tracer NCA-scale, should be applicable to the synthesis of other radiobromine-labeled organic compounds of potential interest as PET radiopharmaceuticals and radiotherapy agents.

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.

Effective specific activities determined by scintillation proximity counting for production runs of [18F]FES and 4F-M[18F]FES

INTRODUCTION

16alpha-[(18)F]Fluoro-17beta-estradiol ([(18)F]FES) and various derivatives can be used to image noninvasively the expression of estrogen receptors in breast cancer. A high specific activity is required for successful visualization of ER expression in vivo, particularly for small animal imaging. We describe a simple method for effective specific activity (ESA) measurements of ER-binding ligands.

METHODS

Scintillator-coated polystyrene microplates (FlashPlate) were coated with purified ER of the alpha subtype. [(18)F]FES and 4-fluoro-11beta-methoxy-16alpha-[(18)F]fluoroestradiol (4F-M[(18)F]FES) were prepared by stereoselective opening of their respective cyclic sulfate precursors. After decay of the radioactivity, samples at various dilutions were put in the wells of the FlashPlate along with buffer and [(3)H]estradiol. On the same FlashPlate, nonradioactive estradiol was placed at concentrations ranging from 10(-11) to 10(-6) M to provide a standard competition curve.

RESULTS

The average effective specific activities of different batches of [(18)F]FES and 4F-M[(18)F]FES were 1169 (range, 49-6251) and 4695 (range, 413-15,261) Ci/mmol, respectively.

CONCLUSION

Scintillation proximity technology allows for simple and reproducible measurements of the ESA of receptor-binding radiopharmaceutical for which purified receptors are available.

Synthesis and biodistribution of fluorine-18-labeled fluorocyclofenils for imaging the estrogen receptor

C4-[18F]Fluorocyclofenil ([18F]FCF, 6) and C3-[18F]fluoroethylcyclofenil ([18F]FECF, 9), two high-affinity nonsteroidal estrogens, were prepared and investigated as potential agents for imaging estrogen receptors (ERs) in breast tumors. Both of these compounds could be prepared conveniently from alkyl methanesulfonate precursors (5,8) by fluoride displacement reactions, and they were obtained in high radiochemical purity and radiochemical yields, with effective specific activities sufficient for in vivo biodistribution studies. While the biodistribution of [18F]FCF (6) in immature female rats showed no selective target tissue uptake, the biodistribution of [18F]FECF (9) showed selective uptake by the uterus, but this uptake could not be blocked by excess estradiol. The poor in vivo biodistribution of these otherwise high-affinity ligands arouses curiosity, and together with recent results on the biodistribution of other nonsteroidal ligands suggests that factors other than receptor binding affinity are important for in vivo imaging of estrogen target tissues and ER-positive breast tumors.

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 biological evaluation of a fluorine-18 labeled estrogen receptor-alpha selective ligand: [18F] propyl pyrazole triol

The two estrogen receptor subtypes, ERalpha and ERbeta, play important roles in breast cancer. To develop an ERalpha imaging agent, we synthesized fluoropropyl pyrazole triol (FPPT, 2), an analog of our ERalpha-selective ligand PPT. FPPT retains the high ERalpha binding selectivity of its parent PPT. We prepared [(18)F]FPPT ((18)F-2) in high specific activity, but estrogen target tissue uptake in female rats was minimal and was not displaceable by unlabeled estradiol, probably because of the lipophilicity and triphenolic nature of FPPT.

Comparison of animal models for the evaluation of radiolabeled androgens

Biodistribution of two 18F-labeled androgens and an 124I/125I-labeled androgen were studied in five androgen receptor (prostate) animal models with or lacking sex hormone binding globulin (SHBG). As models for androgen-receptor positive ovarian cancer, xenografts of three human ovarian cancer cell lines were tested in SCID mice. SHBG in the prostate model systems significantly affects the metabolism, clearance, and distribution of the radiolabeled androgens in several tissues, but ovarian cancer animal models were disappointing.

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.

Comparative breast tumor imaging and comparative in vitro metabolism of 16alpha-[18F]fluoroestradiol-17beta and 16beta-[18F]fluoromoxestrol in isolated hepatocytes

16beta-[18F]Fluoromoxestrol ([18]betaFMOX) is an analog of 16alpha-[18F]fluoroestradiol-17beta ([18F]FES), a radiopharmaceutical known to be an effective positron emission tomography (PET) imaging agent for estrogen receptor-positive (ER+) human breast tumors. Based on comparisons of target tissue uptake efficiency and selectivity in a rat model, [18F]betaFMOX was predicted to be as effective an imaging agent as [18F]FES. However, in a preliminary PET imaging study with [18F]FMOX of 12 patients, 3 of whom had ER+ breast cancer, no tumor localization of [18F]betaFMOX was observed. In search for an explanation for the unsuccessful [18F]betaFMOX clinical trial, we have examined the rate of metabolism of [18F]FMOX and [18F]FES in isolated rat, baboon, and human hepatocytes. We have also studied the effect of the serum protein sex hormone-binding globulin (SHBG), which binds [18F]FES better than [18F]betaFMOX, on these rates of metabolism. Immature rat hepatocytes were found to metabolize [18F]FES 31 times faster than [18F]betaFMOX, whereas mature rat cells metabolized [18F]FES only 3 times faster, and baboon and human hepatocytes only 2 times faster than [18F]betaFMOX. In the presence of SHBG, the metabolic consumption rate for [18F]FES in mature rat hepatocytes decreased by 26%. Thus, the very favorable target tissue uptake characteristics of [18F]betaFMOX determined in the rat probably result from its comparative resistance to metabolism (vis-a-vis [18F]FES) in this species, an advantage that is strongly reflected in comparative metabolism rates in rat hepatocytes. In the baboon and human, [18F]FES is extensively protein bound and protected from metabolism, an effect that may be reflected to a degree as a decrease in the rate of metabolism of this compound in baboon and human hepatocytes relative to [18F]betaFMOX. Thus in primates, SHBG may potentiate the ER-mediated uptake of [18F]FES in ER+ tumors by selectively protecting this ligand from metabolism and ensuring its delivery to receptor-containing cells. In addition to current screening methods for 18F-estrogens that involve evaluating in vivo ER-mediated uptake in the immature female rat, studies comparing the metabolism of the new receptor ligands in isolated hepatocytes, especially those from primates or humans, may assist in predicting the potential of these ligands for human PET imaging.

Retardation of 17-oxidation of 16 alpha-[18F]fluoroestradiol-17 beta by substitution of deuterium for hydrogen in the 17 alpha position(6)

We describe the synthesis, in vitro metabolism and biodistribution of [17 alpha-2H]16 alpha-[18F]fluoroestradiol ([18F]DFES). The clinically useful breast cancer imaging agent, 16 alpha-[18F]fluoroestradiol-17 beta ([18F]FES), was deuterated at the C-17 alpha position to lower the rate of C-17 alcohol oxidation. Metabolism studies in immature female rat and mature female baboon isolated hepatocytes showed [18F]DFES being consumed ca. 2.5 times slower than [18F]FES. Biodistribution studies and time-activity curve measurements in female rats showed [18F]DFES to have superior uptake characteristics compared to [18F]FES for imaging estrogen-receptor rich targets.

Synthesis, estrogen receptor binding, and tissue distribution of [18F]fluorodoisynolic acids

Doisynolic acids, D-ring seco-steroids derived from alkaline fusion of estrones, are hormonal curiosities: Their binding affinity for the estrogen receptor is low (ca. 1-2% that of estradiol), but their in vivo potency is high and they have a long duration of action. To study the in vivo behavior of the doisynolic acids, we prepared fluorine-substituted analogs of both trans-doisynolic acid (with the natural 14 alpha-hydrogen configuration, trans-FDA) and the more active cis-doisynolic acid (with the unnatural 14 beta-hydrogen configuration, cis-FDA) from estrone and 14 beta-estrone, respectively. Modification of the D-ring haloform cleavage approach of Meyers allowed us to introduce fluorine (or fluorine-18) on the carbon atom derived from C-16 in the estrones. Fluorine substitution had little effect on the estrogen receptor binding affinity of the doisynolic acids. Tissue distribution of the fluorodoisynolic acids (trans-[18F]FDA and cis-[18F]FDA) was unusual and very different from that of typical, high-affinity ligands for the estrogen receptor. At 1-3 h in immature female rats, trans-[18F]FDA shows low and rather nonselective uptake in the principal estrogen target tissue (uterus) and slow clearance. By contrast, cis-[18F]FDA shows high uptake in nearly all tissues, with significant uterine uptake that continues to increase over the 1-6-h period. The uterine uptake of this isomer was blocked at the later times by a sufficiently high dose of unlabeled cis-FDA. After administration of the trans-[18F]FDA, a more polar metabolite slowly accumulates in the blood. The cis-[18F]FDA, however, showed no apparent metabolism, with 84% of the blood activity at 5 h assigned as the unmetabolized radioligand. After 5 h, only limited clearance from blood, liver, and kidneys has occurred. No metabolite from this isomer accumulates in the uterus. Although fluorodoisynolic acids will not be useful breast-tumor imaging agents, their behavior was found to be interesting as it deviates from that of other F-18 estrogens. Further long-term studies of cis-doisynolic acid, labeled with tritium, may be needed to explicate fully its unusual distribution properties and high in vivo activity.

Carbon-11-labeled estrogens as potential imaging agents for breast tumors

We have prepared two estrogens labeled with carbon-11, 17 alpha-[11C] methylestradiol and 11 beta-ethyl-17 alpha-[11C]methylestradiol, at a specific activity of 300-1000 Ci/mmol (11.1-37 TBq/mmol), and we have determined their in vivo biodistribution in immature female rats. Both compounds accumulated selectively in two target tissues, the uterus and ovaries, reaching levels of 3.5-4.9%ID/g at 20 min and 4.6-6.6%ID/g at 40 min; uterus-to-blood ratios reached 12-23. Uterine uptake showed a saturation dependence with the amount of injected mass, and was displaced by unlabeled estradiol, indicating that this uptake was receptor mediated. These results suggest that these compounds may be useful in estrogen receptor-based imaging of breast tumors.

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.

Effect of 11 beta-substituents on the regioselective chlorination of estrogens with 2,3,4,5,6,6-hexachloro-2,4-cyclohexadienone

The reaction of 11 beta-substituted estrogens with 2,3,4,5,6,6-hexachloro-2,4-cyclohexadienone affords exclusively ortho-substituted monochlorinated products including a major 4-chloro and a minor 2-chloro derivative. In the absence of an 11 beta-substituent, regioselectivity is lost, resulting in a mixture of 10 beta- and ortho-chlorinated products.

Oxohexestrol derivatives labeled with fluorine-18. Synthesis, receptor binding and in vivo distribution of two non-steroidal estrogens as potential breast tumor imaging agents

We have prepared two non-steroidal estrogens in the 2-oxohexestrol series labeled with the positron-emitting radionuclide fluorine-18, 1-fluoro-5-oxohexestrol (4) and 1-fluoro-2-oxohexesterol (5). We anticipated that the polar ketone function at the interior of these ligands would reduce their level of non-specific binding, which might increase the selectivity of their uptake in vivo. The two compounds were prepared by total synthesis: compound 4 was prepared in fluorine-18 labeled form by [18F]fluorine ion displacement on a suitably protected methanesulfonate precursor followed by deprotection under acidic hydrogenolytic conditions; the isomer 5 was prepared from a protected alpha-keto trifluoromethanesulfonate precursor with deprotection under basic conditions as the final step. The binding affinity of these hexestrol derivatives for the estrogen receptor was determined by competitive radiometric binding assays at 0 and 25 degrees C, and their lipophilicity (as octanol-water partition coefficients, log P values) and non-specific binding were estimated. The log P values determined by a reversed phase HPLC method were higher, relative to estradiol, than those calculated by the fragment method of Rekker. In tissue distribution studies in immature (50 g) rats, both of these compounds showed selective uptake in estrogen target tissues. At 1 h, activity in the uterus reached the level of 2.5-3.0% of the injected dose per gram tissue, with uterus-to-blood and uterus-to-muscle ratios of 14-20 and 8-14, respectively. The uptake efficiency and selectivity of these fluoro-oxohexestrols in principal estrogen target tissues is less than that of fluorine-18 labeled steroidal estrogens we have prepared previously, but their receptor-mediated uptake in certain secondary target tissues is substantial. The specific and non-specific components of target tissue uptake of these two compounds appear to be directly related to their non-specific binding and their binding selectivity.

Titration of the in vivo uptake of 16 alpha-[18F]fluoroestradiol by target tissues in the rat: competition by tamoxifen, and implications for quantitating estrogen receptors in vivo and the use of animal models in receptor-binding radiopharmaceutical development

We have measured in vivo the uptake of 16 alpha-[18F]estradiol (FES) by target tissues in the immature rat at increasing dose levels (obtained by dilution of [18F]FES with unlabeled estradiol). This was done to examine the binding capacity of target tissues in vivo and to determine whether the uptake in receptor-rich tissues was flow limited, as this has implications concerning the appropriateness of using receptor-rich tissues in experimental animals as models for FES uptake by receptor-poor breast tumors in humans. We also wanted to establish the dose level of the anti-estrogen tamoxifen required to block target tissue uptake of FES. We found that in untreated rats, specific uptake in the uterus saturated at c. 180 pmol/g, in the ovary at c. 54 pmol/g and in the muscle at c. 2 pmol/g. At an intermediate dose of tamoxifen (570 micrograms/kg), uptake saturated at somewhat lower levels, and at a high tamoxifen dose (1710 micrograms/kg), yet lower specific uptake was evident. In the FES titrations at low dose levels of FES, both the uterus and the ovaries, but not the muscle, showed characteristics of flow-limited uptake, i.e. the uptake-to-dose ratio reached a maximum level. This flow limitation suggests that only when receptor levels are sufficiently low will the FES uptake be related to receptor concentration. While receptor-rich tissues such as the rat uterus will show this flow limitation, the receptor concentration in most primary and metastatic human breast tumors is sufficiently low, so that the uptake should parallel receptor content. In in vivo distribution studies, target tissues (or tumors) with low receptor content will be more fully saturated and ligand more readily displaced. Also, uptake by secondary target tissues (i.e. those with a lower content of estrogen receptor, such as muscle, thymus and kidney) may be better models for assessing the effectiveness of new breast tumor imaging agents than uptake by receptor-rich tissues.

Radiolabeled steroidal estrogens in cancer research

It has now been more than 30 years since the laboratory verification of the localization of estrogen in certain animal tissues. Much has been learned since that time regarding the details of this process, including the presence of specific receptors for these hormones in target tissues, the mechanism of ligand binding, the association of the ligand-receptor complex with unique chromatin sequences, and the activation of transcription. A concrete use of this knowledge has been the exploitation of these receptors as a targeting mechanisms for radiopharmaceuticals. This is an exciting area that encompasses both diagnosis and therapy. This review will summarize the in vitro and in vivo data obtained from evaluation of the many compounds that have been examined as radiolabeled receptor ligands, and will also discuss the chemistry necessary for their preparation. In particular, relative binding affinity values for relevant compounds will be tabulated, grouped according to molecular class. For those materials for which biodistributions have been performed, uterine (target), liver (nontarget, clearance), and, when available, tumor tissue uptake values are presented. These data should provide a reminder of what has been accomplished, and should serve as a working reference for those engaged in the pursuit of new candidates for these applications.

A synthesis of 7 alpha-substituted estradiols: synthesis and biological evaluation of a 7 alpha-pentyl-substituted BODIPY fluorescent conjugate and a fluorine-18-labeled 7 alpha-pentylestradiol analog

In an effort to assist in the preparation of ligands for the study of the estrogen receptor (ER), we have developed a new synthesis of 7 alpha-substituted estradiols. The key step in the synthesis involves a copper-catalyzed, alpha-selective, 1,6-conjugate addition of 4-pentenyl magnesium bromide to a suitably protected 6-dehydrotestosterone derivative. Desaturation and then reductive aromatization of the resulting 7 alpha-pentenyl androgen gave the 7 alpha-pentenylestradiol in good yields. The alpha-stereoselectivity of this addition in the testosterone series, compared with the 19-nortestosterone series, is significantly improved by the presence of the C-19 methyl group, which shields the beta face from attack. A key intermediate was functionalized further by substitution with fluorine-18 to provide a potential imaging agent for positron emission tomography, and by conjugation with a BODIPY (Molecular Probes Inc., Eugene, OR, USA) fluorophore to make a fluorescent probe for the estrogen receptor. The synthesis and biological evaluation of these analogs is presented, as well as a discussion of the improvements in the synthetic procedure.

16 beta-[18F]fluoromoxestrol: a potent, metabolically stable positron emission tomography imaging agent for estrogen receptor positive human breast tumors

16 beta-[18F]Fluoromoxestrol (beta FMOX, 1) is a highly selective, metabolically stable estrogen with potential as a receptor imaging agent. It demonstrates receptor-mediated uptake in the immature rat in the estrogen receptor-rich primary target tissues, uterus and ovaries, as well as, in receptor-poor secondary target tissues, muscle, thymus and kidneys; uptake in the uterus is nearly four times that of the clinically useful 16 alpha-[18F]fluoroestradiol (FES), most likely due to the extended lifetime of the labeled beta FMOX in the blood afforded by its relatively slow metabolism. In vivo and in vitro studies demonstrate a nearly four-fold decrease in metabolism rate between beta FMOX and FES. Dosimetry studies indicate radiation absorbed doses comparable to FES. beta FMOX possesses desirable imaging characteristics and may prove to be a clinically useful imaging agent.

Synthesis, radiolabeling and tissue distribution of 11 beta-fluoroalkyl- and 11 beta-fluoroalkoxy-substituted estrogens: target tissue uptake selectivity and defluorination of a homologous series of fluorine-18-labeled estrogens

We have synthesized six estrogens substituted at the 11 beta-position with a fluoroalkyl or fluoroalkoxy substituent. These compounds bind to the estrogen receptor with moderate to high affinity, with the fluoroalkyl analogs being higher affinity binders than the fluoroalkoxy ones. All of these fluorine-substituted estrogens were prepared in fluorine-18-labeled form, with high radiochemical purity and at effective specific activities (15.4-50.4 TBq/mmol; 415-1362 Ci/mmol) adequate for biodistribution studies. In immature female rats, five of the six fluoroestrogens showed selective uptake by the uterus, with uterine uptake as a percent of the injected dose per gram being 4-9% at 1 h, and uterus-to-blood or uterus-to-muscle ratios being 10-40. Selective uterine uptake was eliminated by co-administration of a blocking dose of unlabeled estradiol. The only compound that did not show selective uterine uptake was 11 beta-fluoropropoxyl estradiol; its rapid metabolism and its low affinity for the estrogen receptor, particularly at 25 degrees C, may account for its lack of specific uptake. The level of bone activity, an index of metabolic defluorination, shows that the defluorination rates of these six estrogens are a complex function of structure and functionality. Least prone to defluorination is 11 beta-(2-fluoroethoxy)estradiol and most prone is 11 beta-(2-fluoroethyl)estradiol. The extent of defluorination of the remaining compounds shows weak evidence for the protective effect of a heteroatom-substituted beta to the site of metabolism (the CH bonds on the fluorine-bearing carbon atom). The binding affinity, tissue distribution and metabolism of these 11 beta-fluoroalkyl- and fluoroalkoxy-substituted estrogens further our understanding of the behavior of fluorine-18-labeled estrogens as potential imaging agents for estrogen receptor-positive breast cancer.

Fluorine-substituted corticosteroids: synthesis and evaluation as potential receptor-based imaging agents for positron emission tomography of the brain

We have prepared eight fluorine-substituted corticosteroids representing ligands selective for Type I and Type II corticosteroid receptor subtypes as potential imaging agents for corticosteroid receptor-containing regions of the brain. Receptor binding affinity assays show that fluorine substitution for hydroxyl or hydrogen in these steroids generally results in some reduction in affinity, with the result that the absolute affinity of these fluorine-substituted ligands for receptor is less than that typical for steroid hormones that show receptor-based, target selective uptake in vivo. Five of these compounds were prepared in fluorine-18 labeled form by a simple sulfonate ester displacement reaction, and their tissue distribution was studied in the adrenalectomized rat. There is no selective accumulation nor selective retention of the Type I selective corticosteroids (18F-RU 26752, 21-[18F]fluoroprogesterone, 21-[18F]fluoro-11 beta-hydroxyprogesterone) in either the brain, or other target tissues (pituitary, kidney, liver). The Type II selective corticosteroids (18F-RU 28362, 18F-triamcinolone acetonide) show uptake into the hippocampus which can be partially blocked by a competing ligand; in target tissues outside the brain, the blocking is more complete. All of the 18F-labeled compounds show considerable defluorination, evident as high bone activity levels. These results, coupled with earlier findings in the literature, suggest that radiolabeled corticosteroid receptor ligands with both greater metabolic stability and higher receptor binding affinity and selectivity are needed for imaging corticosteroid receptors in the hippocampus.

Estrogen platinum-diamine complexes: preparation of a non-steroidal estrogen platinum-diamine complex labeled with platinum-191 and a study of its binding to the estrogen receptor in vitro and its tissue distribution in vivo

We have prepared in radiolabeled form (platinum-191) a non-steroidal estrogen platinum-diamine complex (Pt-diamine complex) that is reported to have selective cytostatic activity in estrogen receptor positive mouse mammary tumors. We then studied the interaction of this metal radiolabeled complex with the estrogen receptor in vitro and its distribution in immature rats in vivo. The radiolabeled complex was prepared by incubation of the non-steroidal estrogen diamine with [191Pt](II)Cl(-2)4 (t 1/2 = 2.96 days, sp. act. 7.54 Ci/mmol) in dimethylformamide (DMF)/H2O, followed by purification by HPLC. The final radiolabeled product coeluted with an authentic standard of the unlabeled Pt-diamine complex, with a retention time distinct from those of the precursor diamine and chloroplatinate. In competitive radiometric receptor binding assays with rat uterine estrogen receptor, samples of the unlabeled diamine and Pt-diamine complex have apparent binding affinities of 53 +/- 3% and 32 +/- 11%, respectively, relative to estradiol (RBA = 100% as standard). However, attempts to observe the binding of the 191Pt-diamine complex with the estrogen receptor were complicated by a very high level of non-receptor binding, an irreversible binding to proteins in the receptor preparation, and a degradation of the platinum complex that, in part, releases the diamine. As a result, it is difficult to be certain whether the binding affinity measured for the Pt-diamine complex in the competitive binding assays is due to the complex itself, or whether it arises from diamine released upon degradation of the complex. In tissue distribution studies in immature female rats, much of the 191Pt-diamine complex was deposited in the liver; there was no evidence of selective uptake of this compound by estrogen target tissues. Thus, it is not clear, from these studies, that the observed bioactivity of this complex arises from the interaction of the Pt complex or the diamine ligand with the estrogen receptor.

Preparation and evaluation of 17-ethynyl-substituted 16 alpha-[18F]fluoroestradiols: selective receptor-based PET imaging agents

We have prepared and studied six new analogs of 16 alpha-fluoroestradiol (FES): 17 alpha- and 17 beta-ethynyl-FES (7 [FEES] and 7a), and the 11 beta-ethyl (8 and 8a) and 11 beta-methoxy (9 and 9a) derivatives, novel estrogen receptor-based PET imaging agents. The relative binding affinity (RBA) for the estrogen receptor (ER) versus FES is increased for 7, 9 and 9a but decreased for 7a, 8 and 8a. All six analogs have been labeled in the 16 alpha position with 18F by the nucleophilic displacement of the corresponding 16 beta-trifluoromethanesulfonate with nBu4N18F. Subsequent ethynylation with lithium trimethylsilylacetylide yielded the FEES analogs (total synthesis time: 120 min; effective specific activity: 200-2400 Ci/mmol). Selective uptake in the uterus was high for [18F]7, [18F]8, [18F]9 and [18F]9a (% ID/g values at 1 h: 11.2, 12.9, 9.9 and 8.3, respectively), while uptake was effectively blocked by coinjection of an excess of unlabeled estradiol. The FEES analogs, [18F]7, [18F]8 and [18F]9, exhibited the highest selectivity, in terms of target (uterus)-to-blood ratios, ever seen amongst estrogen radiopharmaceuticals, 154, 145 and 169, respectively. The analogs [18F]7a and [18F]8a displayed no uptake in the uterus, consistent with their low RBAs. Metabolism studies revealed that most of the uterine activity is unmetabolized while the blood exhibits a rapid and subsequently sustained mixture of metabolites. The muscle shows a metabolic profile intermediate to the uterus and blood. These analogs provide an array of desirable characteristics for the optimal PET imaging of ER-rich target tissues.

Radiotoxicity of 17 alpha-[125I]iodovinyl-11 beta-methoxyestradiol in MCF-7 human breast cancer cells

Therapeutic strategies for human breast cancer using 125I-labeled steroid hormones are clinically attractive in light of the estrogen dependence of many human breast cancers and the favorable microdosimetry resulting from 125I decay. We determined the uptake specific estrogen receptor binding and radiotoxicity of 17 alpha-[125I]iodovinyl-11 beta-methoxyestradiol (125IVME2) in vitro using cultured MCF-7 human breast carcinoma cells. 125IVME2 rapidly enters MCF-7 cells and reaches a plateau in the presence of competing 10(-7) M 17 beta-estradiol. In the absence of competitor, uptake is substantially greater before reaching a plateau. Efflux of 125IVME2 from cells incubated in the absence of estradiol decreases to levels corresponding to specific binding. Under equilibrium conditions and in the absence of competitor, 125IVME2 binds to both specific and nonspecific sites but, in the presence of excess 17 beta-estradiol, the observed binding is nonspecific. 125IVME2 is cytotoxic to exponentially growing MCF-7 cells and produces a survival curve typical of those observed for [125I]iododeoxyuridine and 16 alpha-[125I]iodoestradiol.

Target tissue uptake selectivity of three fluorine-substituted progestins: potential imaging agents for receptor-positive breast tumors

We have studied three new fluorine-substituted progestins (1-3) as potential imaging agents for progesterone receptor (PgR)-positive human breast tumors. Two of these are fluorine-substituted analogs of the potent progestin R5020 (promegestone), derived from (21S)-hydroxy R 5020 (RU 27987) and (21R)-hydroxy R 5020 (RU 27988), known metabolites of R 5020, which have affinities for PgR that are 116 and 4%, respectively (relative to R 5020 = 100%). These precursors were protected as their 3,3-dioxolane derivatives and converted to the 21-trifluoromethanesulfonate derivatives. Fluoride ion displacement, followed by acid-catalyzed deprotection, furnished in good yield the epimeric fluoroanalogs, (21S)- and (21R)-fluro R 5020 (1 and 2, affinities for PgR, 11 and 45%, respectively). These compounds were also prepared in 18F labeled form by the same route, in 14-32% overall radiochemical yield (decay corrected; synthesis time 90 min; sp. act. 370-1060 Ci/mmol). In tissue distribution studies in estrogen-primed immature rats, uterus-to-muscle ratios were 4.3 at 1 h for the 21S-epimer and 1.1 for the 21R-epimer, paralleling their relative binding affinities. Considerable metabolic defluorination was observed. The third fluorine-substituted progestin, DU 41165, has a novel retroprogesterone (9 beta, 10 alpha) structure, substituted with fluorine at C-6; its binding affinity is 145% relative to R 5020, and it was prepared in tritium-labeled form by acetylation of DU 41231, the 17 alpha-hydroxy precursor, with [3H]acetic anhydride. In estrogen-primed immature rats, this compound shows uterus-to-muscle ratios of 15 at 1 h, and 18-71 between 2 and 6 h, suggesting that compounds in this retroprogesterone series may be very promising candidates for selective imaging of PgR-positive tissues and tumors.

Binding of 16 alpha-[18F]fluoro-17 beta-estradiol to alphafetoprotein in Sprague-Dawley female rats affects blood levels

To examine the relationship between blood levels of 16 alpha-[18F]fluoro-17 beta-estradiol(18F-ES) and serum alphafetoprotein (AFP) concentration, we undertook a study in which serum from various aged (20-33 days old) Sprague-Dawley female rats injected with 18F-ES was analyzed for both blood activity levels and AFP. There is a strong positive correlation between serum AFP concentration and 18F-ES blood levels (r = 0.914, P less than 0.001), suggesting that the binding of 18F-ES by AFP has a significant effect on blood activity levels. The AFP concentration and ultimately the AFP-18F-ES binding is dependent on the age and weight of the rat: younger, as well as low weight rats exhibited high AFP concentrations and consequently increased 18F-ES blood activity. The rats most suitable for comparative studying of labeled estrogens are 25-28 days of age and weigh a minimum of 50-55 g. Thus, the use of the immature rat model to compare labeled estrogens requires a careful consideration of possible interference from blood binding proteins (i.e. AFP), as well as potential receptor binding competition from endogenous estrogens produced during the estrous cycle. Comparable consideration of blood binding proteins (sex steroid binding protein, SBP) and endogenous estrogens must be made in human studies, as well.

Characterization of the uptake of 16 alpha-([18F]fluoro)-17 beta-estradiol in DMBA-induced mammary tumors

In order to investigate possible correlations between the uptake of 16 alpha-([18F]fluoro)-17 beta-estradiol (18F-ES) by 7,12-dimethylbenz(a) anthracene (DMBA)-induced tumors in rats and the estrogen receptor (ER) content of these tumors, a comprehensive study was performed in which the tissue distribution of 18F-ES was measured in tumor-bearing rats, together with simultaneous measurements of blood volume (by technetium-labeled red blood cells) and blood flow (by iodoantipyrine infusion). In addition, the time course of 18F-ES metabolism and the tissue distribution of the metabolites was studied. Metabolism of 18F-ES is very rapid, and after 2 h, most of the activity in blood and nontarget tissues is due to metabolites; target tissue activity, however, is due mainly to unmetabolized compound. Most of the circulating activity, both 18F-ES and its metabolites, is strongly associated with macromolecules or cells, and while the metabolites are not taken up selectively by target tissues, they do enter nontarget tissues. Tumor blood volume and blood flow vary widely, but not in a way that appears related to tumor necrosis. The uptake of 18F-ES by the uterus and DMBA-induced mammary tumors of adult rats reaches maximum levels (ca 0.35 and 0.10% I.D./g X kg, respectively) at early times (0-1 h), and drops slowly thereafter. The uterus to nontarget or tumour to nontarget tissue ratios, however, start low and continue to increase, reaching maximum levels (ca 20 and 15, respectively) at 2-3 h. There does not, however, appear to be a simple relationship between tumor uptake (either as % I.D./g X kg or tumor to nontarget ratio) measured at a single 3 h time point and tumor ER content, even considering differences in tumor blood flow. This suggests that an estimation of tumor ER content will require the application of more complex pharmacodynamic models that involve the measurement of the complete profile of receptor lignad uptake, retention, and washout from target to nontarget areas. The application of such models will be assisted by the development of estrogen receptor binding ligands that are not converted to circulating metabolites.

Radiobrominated spiroperidol for the study of dopamine D2 receptors

Some neuropsychiatric disorders have been suggested to be related to the CNS dopaminergic system. In order to probe this neurotransmitter system, radiobrominated spiroperidol, a potent dopamine (DA) D2-receptor antagonist, was developed. This review deals with the routine synthesis of [75, 76, or 77Br]-p-bromospiroperidol (BrSP), its validation as a radiopharmaceutical directed to DA D2 receptors, and the imaging studies which have been done using this radioligand with either single photon tomography (SPECT) or positron tomography (PET). [*Br]BrSP will be compared to other D2 ligands currently available. The present state and future direction of DA receptor research, in particular, the quantitative aspects, will be discussed.

NCA 16 alpha-[18F]fluoroestradiol-17 beta: the effect of reaction vessel on fluorine-18 resolubilization, product yield, and effective specific activity

Although the reported synthesis of the title compound resulted in a high radiochemical yield (43% based on resolubilized 18F), the effective specific activity at EOS was low (166 Ci/mmol). Reduction in the amount of carrier fluoride in the target water improved the effective specific activity of the product, but with a concommitant decrease in the resolubilized yield of the fluoroestradiol (12.7%). A re-examination of the labeling parameters was performed to determine the conditions that would increase the yield of the fluoroestrogen and maintain a high effective activity for the product. Since the amount of resolubilized 18F in THF is important in obtaining high specific activity compounds in this type of synthesis, several types of vessels were investigated to determine their effect on the evaporation of the [18O]H2O target water and subsequent resolubilization of 18F into THF. Of the vessels (Pt crucible, borosilicate glass, siliconized borosilicate glass, Vacutainer), the Vacutainer afforded the highest resolubilization of 18F into THF (90%), resulting in an improved resolubilized yield for the fluoroestradiol (28%) and an increased effective specific activity at EOS for the product (1600-3939 Ci/mmol).