Progestin radiopharmaceuticals labeled with technetium and rhenium: synthesis, binding affinity, and in vivo distribution of a new progestin N2S2-metal conjugate

Development of Norelgestromin Dissolving Bilayer Microarray Patches for Sustained Release of Hormonal Contraceptive

Microarray patches (MAPs) offer a noninvasive and patient-friendly drug delivery method, suitable for self-administration, which is especially promising for low- and middle-income country settings. This study focuses on the development of dissolving bilayer MAPs loaded with norelgestromin (NGMN) as a first step towards developing a future potential drug delivery system for sustained hormonal contraception. The fabricated MAPs were designed with the appropriate needle lengths to penetrate the stratum corneum, while remaining minimally stimulating to dermal nociceptors. Ex vivo assessments showed that the MAPs delivered an average of 176 ± 60.9 μg of NGMN per MAP into excised neonatal porcine skin, representing 15.3 ± 5.3% of the loaded drug. In vivo pharmacokinetic analysis in Sprague Dawley rats demonstrated a Tmax of 4 h and a Cmax of 67.4 ± 20.1 ng/mL for the MAP-treated group, compared to a Tmax of 1 h and a Cmax of 700 ± 138 ng/mL for the intramuscular (IM) injection group, with a relative bioavailability of approximately 10% for the MAPs. The MAP-treated rats maintained plasma levels sufficient for therapeutic effects for up to 7 days after a single application. These results indicate the potential of NGMN-loaded dissolving bilayer MAPs, with further development focused on extending the release duration and improving bioavailability for prolonged contraceptive effects.

Development and evaluation of a 99mTc(V)-nitrido complex derived from estradiol for breast cancer imaging

Estrogen receptors are overexpressed in 70% of breast cancer and identification of their presence is important to select the appropriate treatment. This work proposes the preparation and evaluation of an estradiol derived as potential ER imaging agent. Ethinylestradiol was derivatized to introduce a dithiocarbamate function for Tc coordination. Labeling was achieved through the formation of a symmetric Tc(V)-nitrido complex with a radiochemical purity (RCP) > 95%. Physicochemical evaluation, cell uptake, biodistribution in normal animals and in nude mice bearing induced ER + breast tumors showed promising results.

Chelators and metal complex stability for radiopharmaceutical applications

Diagnostic and therapeutic nuclear medicine relies heavily on radiometal nuclides. The most widely used and well-known radionuclide is technetium-99m (99mTc), which has dominated diagnostic nuclear medicine since the advent of the 99Mo/99mTc generator in the 1960s. Since that time, many more radiometals have been developed and incorporated into potential radiopharmaceuticals. One critical aspect of radiometal-containing radiopharmaceuticals is their stability under in vivo conditions. The chelator that is coordinated to the radiometal is a key factor in determining radiometal complex stability. The chelators that have shown the most promise and are under investigation in the development of diagnostic and therapeutic radiopharmaceuticals over the last 5 years are discussed in this review.

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".

Preparation and preliminary bioevaluation of 99mTc(CO)3-11β-progesterone derivative prepared via click chemistry route

INTRODUCTION

Progesterone receptors (PRs) overexpressed in breast cancers serve as potential targets for developing radiotracers for use in nuclear medicine. Hence, suitably derivatized progesterone can be envisaged as a potential vector for targeting overexpression of receptors in breast cancer. In the present article, we report the preparation of a (99m)Tc(CO)(3)-progesterone triazole using the Cu(I)-catalyzed novel click chemistry route. Preliminary evaluation of the radiolabeled derivative has been carried out in binding studies with MCF 7 cell lines.

METHODS

11-Hydroxyprogesterone has been synthetically derivatized to 11-azidoprogesterone. Subsequently, the cycloaddition reaction between progesterone azide and propargyl glycine was carried out to prepare 1,4-bifunctionalized progesterone triazole analogue. The clicked progesterone triazole derivative was radiolabeled with (99m)Tc and characterized by HPLC. The chemical characterization of (99m)Tc(CO)(3)-progesterone triazole has been carried out by preparing its corresponding rhenium complex using the [NEt(4)](2)[Re(CO)(3)Br(3)] precursor. While in vitro studies were carried out in MCF7 cell lines, in vivo distribution studies were performed in female Swiss mice.

RESULTS

The radiolabeled complex could be prepared in >95% radiochemical yield as determined by HPLC. In vitro studies of (99m)Tc(CO)(3)-progesterone complex in MCF7 cell lines overexpressing receptors for breast cancer showed binding up to 30%. In vivo distribution studies in female Swiss mice have shown uterine uptake of 0.41 (0.06) % ID/g at 3 h postinjection (pi) and retention therein till 24 h pi.

CONCLUSION

The present study demonstrates a novel and facile route for preparation of (99m)Tc-labeled progesterone complex using click chemistry. This strategy can be further extended towards preparation of radiolabeled complexes of other steroidal derivatives.

Bifunctional coupling agents for radiolabeling of biomolecules and target-specific delivery of metallic radionuclides

Receptor-based radiopharmaceuticals are of great current interest in molecular imaging and radiotherapy of cancers, and provide a unique tool for target-specific delivery of radionuclides to the diseased tissues. In general, a target-specific radiopharmaceutical can be divided into four parts: targeting biomolecule (BM), pharmacokinetic modifying (PKM) linker, bifunctional coupling or chelating agent (BFC), and radionuclide. The targeting biomolecule serves as a "carrier" for specific delivery of the radionuclide. PKM linkers are used to modify radiotracer excretion kinetics. BFC is needed for radiolabeling of biomolecules with a metallic radionuclide. Different radiometals have significant difference in their coordination chemistry, and require BFCs with different donor atoms and chelator frameworks. Since the radiometal chelate can have a significant impact on physical and biological properties of the target-specific radiopharmaceutical, its excretion kinetics can be altered by modifying the coordination environment with various chelators or coligand, if needed. This review will focus on the design of BFCs and their coordination chemistry with technetium, copper, gallium, indium, yttrium and lanthanide radiometals.

[Development of bifunctional radiopharmaceuticals for targeted imaging and therapy]

In vivo radiopharmaceuticals have two different uses - for nuclear diagnostic imaging and for internal radiation therapy. For nuclear diagnostic imaging, it is necessary to make the difference of radioactivity levels between in the target regions and in the other regions at an early time after administration. For internal radiation therapy, a more selective accumulation of the radioactivity to the target regions is required to minimize an adverse effect. In order to achieve the highly selective accumulation of in vivo radiopharmaceuticals, it is necessary to find an appropriate target molecule in the first place and design a compound which can recognize the target molecule and stably label it with radionuclide. There are several proposed approaches to chemical design for this purpose. However, even with the specific recognition and stable radiolabel, targeted imaging and therapy are not necessarily achieved. We have been developing in vivo radiopharmaceuticals based on a chemical design called "bifunctional radiopharmaceutical." Bifunctional radiopharmaceuticals have the recognition site of the target molecule and binding site for the radionuclide independently in one molecule. This review summarizes our examples of chemical design of in vivo radiopharmaceuticals to achieve the targeted imaging and therapy.

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.

Aryl cyclopentadienyl tricarbonyl rhenium complexes: novel ligands for the estrogen receptor with potential use as estrogen radiopharmaceuticals

The need for imaging agents for estrogen receptor positive (ER+) tumors that are both cost effective and widely available, as well as the need for novel radiotherapeutic agents for the treatment of breast cancer, has prompted us to investigate cyclopentadienyl tricarbonyl metal [CpMet(CO)(3), Met=Re, Tc-99m] complexes that bind well to the ER. Thus, we have prepared a series of p-hydroxyphenyl-substituted CpRe(CO)(3) complexes and evaluated them (and, in some cases, their cyclopentadiene precursors) for binding to ER. These compounds constitute a new class of structurally integrated organometallic ligands for ER in which the CpMet(CO)(3 )organometallic unit forms the very structural core of these molecules and thus is necessarily intimately involved in their interaction with the receptor. The CpRe(CO)(3) compounds were prepared by reaction of the lithium salt of the arene-substituted cyclopentadiene with a suitable Re(CO)(3)(+) precursor, followed by deprotection of the methyl ether. The X-ray crystal structure of one of these analogues shows that it has the classical 'piano stool'-like geometry, with the alkyl groups directed upward, away from the tripodyl metal carbonyl base. The aryl-substituted CpRe(CO)(3) complexes that we have prepared all bind to the ER, some with affinity as great as 20% that of the native ligand, estradiol. In general, at least two p-hydroxyphenyl substituents and one to two alkyl groups attached to the organometallic cyclopentadienyl core are needed for high ER affinity. Where we have been able to make comparisons, the metal complexes bind to ER with an affinity greater than their cyclopentadiene precursors. The high affinity of some of these complexes indicates that the bulky Re(CO)(3) unit is able to exploit the considerable volume in the center of the ER ligand binding pocket that is not occupied by most ligands, a consideration that is supported by molecular modeling. The preparation of the best of these agents in technetium-99m labeled form is currently being investigated.

Recent advances in 99mTc radiopharmaceuticals

99mTc radiopharmaceuticals play an important role in widespread applications of nuclear medicine. When 99mTc radiopharmaceuticals first came into use, major efforts were directed toward the development of 99mTc radiopharmaceuticals for bone imaging and for the excretory functions of the liver and kidneys. In the past 20 years, a significant advance has been made in technetium chemistry, which provided 99mTc radiopharmaceuticals for assessment of regional cerebral and myocardial blood flow. Recent efforts have been directed toward the design of 99mTc-labeled compounds for estimating receptor or transporter functions. A number of bifunctional chelating agents that provide 99mTc labeled proteins and peptides of high in vivo stability with high radiochemical yields have also been developed. More recently, organometallic technetium and rhenium compounds have been introduced as another class of 99mTc radiopharmaceutical design. In this manuscript, recent progress in 99mTc radiopharmaceuticals is reviewed with the major emphasis laid on key innovations in this field to provide the 99mTc radiopharmaceuticals available today.

Evolution of Tc-99m in diagnostic radiopharmaceuticals

The progress in diagnostic nuclear medicine over the years since the discovery of 99mTc is indeed phenomenal. Over 80% of the radiopharmaceuticals currently being used make use of this short-lived, metastable radionuclide, which has reigned as the workhorse of diagnostic nuclear medicine. The preeminence of 99mTc is attributable to its optimal nuclear properties of a short half-life and a gamma photon emission of 140 keV, which is suitable for high-efficiency detection and which results in low radiation exposure to the patient. 99mTcO4-, which is readily available as a column eluate from a 99Mo/99mTc generator, is reduced in the presence of chelating agents. The versatile chemistry of technetium emerging from the 8 possible oxidation states, along with a proper understanding of the structure-biologic activity relationship, has been exploited to yield a plethora of products meant for morphologic and functional imaging of different organs. This article reviews the evolution of 99mTc dating back to its discovery, the development of 99Mo/99mTc generators, and the efforts to exploit the diverse chemistry of the element to explore a spectrum of compounds for diagnostic imaging, planar, and single photon emission computed tomography. A brief outline of the 99mTc radiopharmaceuticals currently being used has been categorically presented according to the organs being imaged. Newer methods of labeling involving bifunctional chelating agents (which encompass the "3 + 1" ligand system, Tc(CO)3(+1)-containing chelates, hydrazinonicotinamide, water-soluble phosphines, and other Tc-carrying moieties) have added a new dimension for the preparation of novel technetium compounds. These developments in technetium chemistry have opened new avenues in the field of diagnostic imaging. These include fundamental aspects in the design and development of target-specific agents, including antibodies, peptides, steroids, and other small molecules that have specific receptor affinity.

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.

Synthesis of novel progestin-rhenium conjugates as potential ligands for the progesterone receptor

To assist in the development of technetium-based radiopharmaceuticals that are useful for the diagnostic imaging of steroid receptor-positive breast tumors, we have synthesized a series of small-sized metal chelates according to 'n + 1' mixed-ligand, thioether-carbonyl and organometallic designs. In these preliminary investigations, rhenium was used as a model for the radioactive technetium. The metal chelates contain the rhenium metal in several oxidation states, being + 5, + 3, and + 1, and they were attached to 21-substituted progesterone derivatives. A competitive receptor-binding assay (rat uterine cytosol, 0 degrees C) was used to determine the binding affinity of these conjugates for the progesterone receptor. The highest affinity of 9% (RU5020 = 100%) was obtained with a '3 + 1' mixed-ligand complex, containing a NMe group as the central donor atom in the tridentate ligand part. This value reflects a relative binding affinity of 75% compared with the parent steroid progesterone.

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.

Synthesis and binding affinities of new 17 alpha-substituted estradiol-rhenium "n + 1" mixed-ligand and thioether-carbonyl complexes

The development of technetium and rhenium-based radiotracers for the steroid receptor system requires the use of suitable donor groups on the steroid to provide stable binding sites for the metal. Previous approaches have mainly exploited methods involving various N- and S-coordinating chelate systems or organometallic complexes. In this work, we have prepared several novel chelate systems attached to a series of 17 alpha-substituted estradiol derivatives and examined their binding to the estrogen receptor (ER). The neutral "n + 1" mixed-ligand and dithioether-carbonyl complexes that we prepared contain the metal in three oxidation states, +5, +3 or +1, attached to a 17 alpha-substituted estradiol derivative through a thiol group, an isocyanide group, or a dithioether unit, respectively. In our preliminary investigations, we used rhenium as a nonradioactive analog of the radionuclide technetium. All complexes synthesized were evaluated in a competitive radiometric receptor binding assay at 0 degree C and 25 degrees C to determine their relative binding affinities (RBA) to the ER (relative to 3,17 beta-estradiol, RBA = 100%). The complexes show binding affinities up to 23.4% at 0 degree C and 14.1% at 25 degrees C.

Specificity of diastereomers of [99mTc]TRODAT-1 as dopamine transporter imaging agents

Recently, we reported the first human study of [99mTc]TRODAT-1, technetium, 2-[[2-[[[3-(4-chlorophenyl)-8-methyl-8-azabicyclo[3.2.1]oct-2- yl]methyl](2-mercaptoethyl)amino]ethyl]amino]-ethanethiolato(3-)-o xo- [1R-(exo-exo)]-, as an imaging agent of central nervous system (CNS) dopamine transporters. Due to the existence of several chiral centers on this molecule, upon the formation of [99mTc]TRODAT-1 complex (2) several diastereomers could be created. Two major diastereomers of [99mTc]TRODAT-1 (2), designated as peak A (2A) and peak B (2B), were separated by HPLC. Biodistribution of the purified diastereomers 2A,B was evaluated in rats. It appears that 2A displayed a higher lipophilicity than 2B (PC = 305 and 229, respectively), and a similar trend was observed for the initial brain uptake at 2 min postinjection (0.50% and 0.28% dose/organ for 2A,B, respectively). At 60 min post-iv-injection, the specific uptakes, as measured by [striatum - cerebellum]/cerebellum ([ST-CB]/CB) ratio, were 1.72 and 2.79 for 2A,B, respectively. The higher [ST-CB]/CB ratio observed for 2B was corroborated by the results of an in vitro binding assay. Higher binding affinity for dopamine transporters was observed for 3B (Ki = 13.87 and 8.42 nM for the analogous rhenium complexes 3A,B, respectively). The structure of the [99mTc]TRODAT-1 complexes was deduced using nonradioactive rhenium as a surrogate for radioactive technetium complex. Reacting free TRODAT-1 ligand with [Bu4N][ReOCl4] yielded two major complexes: Re-TRODAT-1A (3A) and Re-TRODAT-1B (3B) (corresponding with peaks A and B of [99mTc]TRODAT-1, respectively), whose structures were determined by X-ray analysis. The X-ray structures show that both complexes have a pseudo-square-pyramidal structure of [RevO]3+N2S2 core with oxygen occupying the apical position and the N-alkyl substitution in syn-configuration to the oxo-rhenium bond. In conclusion, TRODAT-1 formed at least two diastereomers after complexing with a metal(V)-oxo (M = 99mTc, Re) center core. The two isomers display different binding affinities toward dopamine transporters and distinct properties of localization in the striatum area of the brain where the transporters are located.

Technetium-99m-labeled receptor-specific small-molecule radiopharmaceuticals: recent developments and encouraging results

The development of technetium-99m-labeled small-molecule radiopharmaceuticals directed at specific high-affinity binding sites, as are found in receptors for hormones and neurotransmitters, transport systems, and certain enzymes, is a natural outgrowth from the successful development of technetium radiopharmaceuticals for imaging flow and metabolism. Although many receptor-specific radiopharmaceuticals labeled with PET and other SPECT isotopes already exist, the low cost and widespread availability of technetium-99m would make their 99mTc-labeled counterparts much more accessible to the medical community. This review has four goals: (a) To survey and analyze critically the results of a flurry of research activity in this area in recent years, which has led to the preparation of a number of novel technetium-labeled radiopharmaceuticals targeted at high-affinity sites, a few of which appear to be very promising; (b) to provide a conceptual analysis of how these agents are being designed; (c) to provide a context in terms of binding and uptake behavior by which these agents should be judged; and (d) to highlight emerging knowledge on the structure of receptors and related high-affinity binding biomolecules and their distribution, which may serve as reference points for understanding the results that have been obtained so far, and may be useful guides for future design.

A technetium-99m SPECT imaging agent which targets the dopamine transporter in primate brain

The dopamine transporter (DAT), located presynaptically on dopamine neurons, provides a marker for certain neurological diseases. In particular, the DAT is depleted in Parkinson's disease, and the extent of depletion correlates with the loss of dopamine. Herein we describe the design, synthesis, and biological evaluation of technepine, the first 99mTc-labeled SPECT imaging agent which targets the dopamine transporter in striatum. We have demonstrated that the DAT can accommodate a chelating unit attached to the 8-amine function of a tropane skeleton. Further, we have demonstrated for the first time that a molecule can be designed to carry the radionuclide 99mTc across the blood-brain barrier in sufficient quantity to obtain in vivo images of the striatum in monkeys. This advance will undoubtedly lead to the design of new receptor and transporter-mediated 99mTc agents which can label specific transporter and receptor targets in the central nervous system.

Synthesis and characterization of technetium-99m-labeled tropanes as dopamine transporter-imaging agents

In the development of novel Tc-99m-labeled tropane derivatives as dopamine transporter (reuptake site)-imaging agents, a series of neutral and lipophilic complexes containing bis-(aminoethanethiol) as a neutral complexing moiety for a [99mTc]TcO3+ center core was successfully prepared. Biological evaluation of the Tc-99m-labeled complexes 13-16 as central nervous system (CNS) dopamine transporter-imaging agents was reported. Synthesis of the tropane derivatives was achieved by stepwise reactions adding two aminoethanethiol units. The final free thiol ligands were obtained by deblocking the 4-methoxybenzyl protecting group with Hg(OAc)2 to obtain trifluoroacetate salts. All of the Tc-99m complexes, with the exception of 16, displayed good initial brain uptake and selective uptake in the striatal area, where dopamine transporters are concentrated. One of the compounds, [2-[[2-[[[3-(4-chlorophenyl)-8-methyl-8-azabicyclo [3.2.1]oct-2-yl]methyl](2-mercaptoethyl)amino]ethy] amino]ethanethiolato-(3-)-N2,N2',S2,S2'] oxo-[1R-(exo-exo)]- [99mTc]technetium,[99mTc]TRODAT-1 (13), displayed the highest initial uptake in rat brain (0.4% at 2 min post iv injection); the striatal/cerebellar (ST/ CB) ratio reached 2.8 at 60 min after an iv injection. The specific uptake in rat brain can be blocked by pretreating rats with a competing dopamine transporter binding agent, beta-CIT (RTI-55, 2 beta-carbomethoxy-3 beta-(4-iodophenyl)tropane; iv, 1 mg/kg), which reduced the regional brain uptake ratio (ST/CB) to 1.2. In contrast, the specific striatal uptake was not affected by pretreating rats with a noncompeting ligand, haldol (iv, 1 mg/kg). After an iv injection of 9 mCi of [99mTc]TRODAT-1 (13), in vivo images of baboon brain using single-photon emission-computed tomography exhibited excellent localization in striatum (basal ganglia), where dopamine neurons are known to be concentrated. This series of compounds may provide a convenient source of short-lived imaging agents for routine diagnosis of CNS diseases (i.e., Parkinson's disease) in which changes in the dopamine transporter concentration are implicated.

Biodistribution of model 105Rh-labeled tetradentate thiamacrocycles in rats

105Rh(III)Cl2 complexes with a limited series of [14]ane- and [16]ane- thia macrocycles were prepared and their biodistributions in Sprague-Dawley rats studied. These studies demonstrate that modifications in the structure and composition of the 105Rh-thia macrocycle complexes produce significant differences in their uptake and retention in both the liver and kidneys. The results indicate that the cis-Rh(III)Cl2-[14]ane thiamacrocycles exhibit less kidney retention than the corresponding trans-Rh(III)Cl2-[16]ane thiamacrocycles. In addition, the presence of a side chain containing a carboxylate group will produce decreased retention of activity in the kidneys. HPLC analysis of urine from these animals indicates no observable in vivo metabolism or dissociation of these chelates in the blood stream.

Heterodimeric Bis(amino thiol) Complexes of Oxorhenium(V) That Mimic the Structure of Steroid Hormones. Synthesis and Stereochemical Issues

We have prepared a series of bis-bidentate complexes of rhenium that mimic the size, shape, and peripheral functionality of steroidal androgens. In a model system, we used 2D NMR and X-ray crystallographic analysis to show that adjacent N-methyl and oxo substitutents adopt an anti configuration during the coordination reaction. We have synthesized a bis-bidentate oxorhenium(V) complex whose structure and peripheral functionality mimic 5alpha-dihydrotestosterone. 2D-NMR analysis indicates that the N-methyl and oxo substituents are driven into the steroidal anti configuration (beta-N-methyl, alpha-oxo) by the beta-orientation of the methyl group equivalent to C-18. Thus, this metal complex provides a remarkable structural and stereochemical mimic of a steroid. Its in vivo stability, however, appears to be limited.

Cloning and sequencing of V genes from anti-osteosarcoma monoclonal antibodies TP-1 and TP-3: location of lysine residues and implications for radiolabeling

Monoclonal antibodies TP-1 and TP-3 are of potential utility for the radioimmunodiagnosis of osteosarcoma in both human and canine patients. The V genes of these antibodies were cloned and sequenced and to facilitate radiolabeling of these proteins, the location of the lysine residues within these sequences have been determined. The V-domains of TP-1 contain a total of 12 lysines, 10 in the framework region and 2 in the CDR region, while the V-domains of TP-3 contain a total of 14 lysines, 11 in the framework region and 3 in the CDR regions. Using space-filling models, the availability of each lysine residue for radiolabeling, and potential interference with antigen binding was predicted.

Radiolabeling with technetium-99m to study high-capacity and low-capacity biochemical systems

After a brief review of the history of the development of technetium-99m-labeled radiopharmaceuticals, the use of technetium chelates in high-capacity systems is discussed. The latter are used in the study of five organ systems, the kidneys, liver, bone, brain, and heart. The chemical characterization of 99mTc complexes is also reviewed, followed by discussion of the various approaches to the labeling of proteins with direct labeling, the preformed chelate approach, and the antibody chelator conjugate approach. Thereafter, the labeling of biochemicals with 99mTc for use with easily saturated sites, e.g., receptors and enzymes, is considered. Finally, attention is given to factors that affect the preparation of high specific activity, high affinity 99mTc-labeled biochemicals.