A class of asymmetrical nitrido 99mTc heterocomplexes as heart imaging agents with improved biological properties

Fundamentals of Rhenium-188 Radiopharmaceutical Chemistry

The β^- emitter, rhenium-188 (¹⁸⁸Re), has long been recognized as an attractive candidate for targeted cancer radionuclide therapy (TRNT). This transition metal shares chemical similarities with its congener element technetium, whose nuclear isomer technetium-99m (^99mTc) is the current workhorse of diagnostic nuclear medicine. The differences between these two elements have a significant impact on the radiolabelling methods and should always receive critical attention. This review aims to highlight what needs to be considered to design a successful radiopharmaceutical incorporating ¹¹⁸Re. Some of the most effective strategies for preparing therapeutic radiopharmaceuticals with ¹⁸⁸Re are illustrated and rationalized using the concept of the inorganic functional group (core) and a simple ligand field theoretical model combined with a qualitative definition of frontiers orbitals. Of special interest are the Re(V) oxo and Re(V) nitrido functional groups. Suitable ligands for binding to these cores are discussed, successful clinical applications are summarized, and a prediction of viable future applications is presented. Rhenium-188 decays through the emission of a high energy beta particle (2.12 MeV max energy) and a half-life of 16.9 h. An ideal biological target would therefore be a high-capacity target site (transporters, potential gradients, tumour microenvironment) with less emphasis on saturable targets such as overexpressed receptors on smaller metastases.

N-Centered Tripodal Phosphine Re(V) and Tc(V) Oxo Complexes: Revisiting a [3 + 2] Mixed-Ligand Approach

N-Triphos derivatives (NP₃^R, R = alkyl, aryl) and asymmetric variants (NP₂^RX^R′, R′ = alkyl, aryl, X = OH, NR₂, NRR′) are an underexplored class of tuneable, tripodal ligands in relation to the coordination chemistry of Re and Tc for biomedical applications. Mixed-ligand approaches are a flexible synthetic route to obtain Tc complexes of differing core structures and physicochemical properties. Reaction of the NP₃^Ph ligand with the Re(V) oxo precursor [ReOCl₃(PPh₃)₂] generated the bidentate complex [ReOCl₃(κ²-NP₂^PhOH^Ar)], which possesses an unusual AA’BB’XX’ spin system with a characteristic second-order NMR lineshape that is sensitive to the bi- or tridentate nature of the coordinating diphosphine unit. The use of the asymmetric NP₂^PhOH^Ar ligand resulted in the formation of both bidentate and tridentate products depending on the presence of base. The tridentate Re(V) complex [ReOCl₂(κ³-NP₂^PhO^Ar)] has provided the basis of a new reactive “metal-fragment” for further functionalization in [3 + 2] mixed-ligand complexes. The synthesis of [3 + 2] complexes with catechol-based π-donors could also be achieved under one-pot, single-step conditions from Re(V) oxo precursors. Analogous complexes can also be synthesized from suitable ⁹⁹Tc(V) precursors, and these complexes have been shown to exhibit highly similar structural properties through spectroscopic and chromatographic analysis. However, a tendency for the {M^VO}³⁺ core to undergo hydrolysis to the {M^VO₂}⁺ core has been observed both in the case of M = Re and markedly for M = ⁹⁹Tc complexes. It is likely that controlling this pathway will be critical to the generation of further stable Tc(V) derivatives.

An N-centered tripodal heterofunctionalized phosphine ligand was used to generate a reactive “metal-fragment” based on the {M^VO}³⁺ (M = Re, ⁹⁹Tc) core for the formation of mixed-ligand [3 + 2] complexes. Characteristic lineshapes arising from an AA’BB’XX’ spin system are diagnostic of bidentate vs tridentate coordination modes of the ligand.

Technetium-99m Radiopharmaceuticals for Ideal Myocardial Perfusion Imaging: Lost and Found Opportunities

The favorable nuclear properties in combination with the rich coordination chemistry make technetium-99m the radioisotope of choice for the development of myocardial perfusion tracers. In the early 1980s, [^99mTc]Tc-Sestamibi, [^99mTc]Tc-Tetrofosmin, and [^99mTc]Tc-Teboroxime were approved as commercial radiopharmaceuticals for myocardial perfusion imaging in nuclear cardiology. Despite its peculiar properties, the clinical use of [^99mTc]Tc-Teboroxime was quickly abandoned due to its rapid myocardial washout. Despite their widespread clinical applications, both [^99mTc]Tc-Sestamibi and [^99mTc]Tc-Tetrofosmin do not meet the requirements of an ideal perfusion imaging agent due to their relatively low first-pass extraction fraction and high liver absorption. An ideal radiotracer for myocardial perfusion imaging should have a high myocardial uptake; a high and stable target-to-background ratio with low uptake in the lungs, liver, stomach during the image acquisition period; a high first-pass myocardial extraction fraction and very rapid blood clearance; and a linear relationship between radiotracer myocardial uptake and coronary blood flow. Although it is difficult to reconcile all these properties in a single tracer, scientific research in the field has always channeled its efforts in the development of molecules that are able to meet the characteristics of ideality as much as possible. This short review summarizes the developments in ^99mTc myocardial perfusion tracers, which are able to fulfill hitherto unmet medical needs and serve a large population of patients with heart disease, and underlines their strengths and weaknesses, the lost and found opportunities thanks to the developments of the new ultrafast SPECT technologies.

PET and SPECT Tracers for Myocardial Perfusion Imaging

Coronary artery disease has been the leading cause of death since the 1960s, which has motivated the research and development of myocardial perfusion imaging (MPI) agents for early diagnosis and to guide treatment. MPI with SPECT has been the clinical workhorse for MPI, but over the past two decades PET MPI is experiencing growth due to enhanced image quality that results in superior diagnostic accuracy over SPECT. Furthermore, dynamic PET imaging of the tracer distribution process from time of tracer administration to tracer accumulation in the myocardium has enabled routine quantification of myocardial blood flow (MBF) and myocardial flow reserve (MFR) in absolute units. MBF and MFR incrementally improve diagnostic and prognostic accuracy over MPI alone. In some cases (eg, rubidium PET imaging with pharmacologic stress) MPI, MBF, and MFR can be acquired simultaneously without incremental cost, radiation exposure, or significant processing time. Nuclear cardiology clinics have been looking to incorporate MBF quantification into clinical routine, but traditional SPECT and MPI tracers are inadequate for this challenge. Cardiac dedicated SPECT scanners can also perform dynamic imaging and have stimulated research into MBF quantification using SPECT tracers. New perfusion tracers must be tailored for emerging clinical needs (including MBF quantification), technical capabilities of imaging instrumentation, market constraints, and supply chain feasibility. Because these conditions have been evolving, tracers previously considered inferior may be reconsidered for future applications and some recently developed tracers may be suboptimal. This article reviews current, clinically-available tracers and those under development showing greatest potential. It discusses for each tracer the rationale for development, physiological mechanism of uptake by the myocardium, published evaluation results and development state. Finally, it gauges the suitability of each tracer for clinical application. The article demonstrates an acceleration in the pace of perfusion radiotracer development due to better understanding of the relevant physiology, better chemistry tools and small animal imaging. Consequently, bad tracers may fail faster and with less wasted investment, and good tracers may translate more efficiently from bench to bedside.

Nitrido Technetium-99 m Core in Radiopharmaceutical Applications: Four Decades of Research

The knowledge on element 43 (Tc) of the periodic table, built over the years through the contributions given by the close relationship between chemistry and nuclear medicine, allowed the development of new and increasingly effective radiopharmaceuticals useful both as perfusion and target specific imaging agents for SPECT (single photon emission tomography). Among the manifold Tc-compounds, Tc(V) nitrido complexes played a relevant role in the search for new technetium-99m radiopharmaceuticals, providing efficient labeling procedures that can be conveniently exploited for the design and synthesis of agents, also incorporating small organic molecules or peptides having defined structural features. With this work, we present an overview of four decades of research on the chemistry and on the nuclear medicine applications of Tc(V) nitrido complexes.

New 99mTc Radiotracers for Myocardial Perfusion Imaging by SPECT

OBJECTIVE

Myocardial Perfusion Imaging (MPI) with radiotracers is an integral component in evaluation of the patients with known or suspected coronary artery diseases (CAD). 99mTc-Sestamibi and 99mTc-Tetrofosmin are commercial radiopharmaceuticals for MPI by single photon-emission computed tomography (SPECT). Despite their widespread clinical applications, they do not meet the requirements of an ideal perfusion imaging agent due to their inability to linearly track the regional myocardial blood flow rate at >2.5 mL/min/g. With tremendous development of CZT-based SPECT cameras over the past several years, the nuclear cardiology community has been calling for better perfusion radiotracers with improved extraction and biodistribution properties.

METHODS

This review will summarize recent research efforts on new cationic and neutral 99mTc radiotracers for SPECT MPI. The goal of these efforts is to develop a 99mTc radiotracer that can be used to detect perfusion defects at rest or under stress, determine the regional myocardial blood flow, and measure the perfusion and left ventricular function.

RESULTS

The advantage of cationic radiotracers (e.g. 99mTc-Sestamibi) is their long myocardial retention because of the positive molecular charge and fast liver clearance kinetics. 99mTc-Teboroxime derivatives have a high initial heart uptake (high first-pass extraction fraction) due to their neutrality. 99mTc- 3SPboroxime is the most promising radiotracer for future clinical translation considering its initial heart uptake, myocardial retention time, liver clearance kinetics, heart/liver ratios and SPECT image quality.

CONCLUSION

99mTc-3SPboroximine is an excellent example of perfusion radiotracers, the heart uptake of which is largely relies on the regional blood flow. It is possible to use 99mTc-3SPboroximine for detection of perfusion defect(s), accurate quantification and determination of regional blood flow rate. Development of such a 99mTc radiotracer is of great clinical benefit for accurate diagnosis of CAD and assessing the risk of future hard events (e.g. heart attack and sudden death) in cardiac patients.

New SPECT and PET radiopharmaceuticals for imaging cardiovascular disease

Nuclear cardiology has experienced exponential growth within the past four decades with converging capacity to diagnose and influence management of a variety of cardiovascular diseases. Single photon emission computed tomography (SPECT) myocardial perfusion imaging (MPI) with technetium-99m radiotracers or thallium-201 has dominated the field; however new hardware and software designs that optimize image quality with reduced radiation exposure are fuelling a resurgence of interest at the preclinical and clinical levels to expand beyond MPI. Other imaging modalities including positron emission tomography (PET) and magnetic resonance imaging (MRI) continue to emerge as powerful players with an expanded capacity to diagnose a variety of cardiac conditions. At the forefront of this resurgence is the development of novel target vectors based on an enhanced understanding of the underlying pathophysiological process in the subcellular domain. Molecular imaging with novel radiopharmaceuticals engineered to target a specific subcellular process has the capacity to improve diagnostic accuracy and deliver enhanced prognostic information to alter management. This paper, while not comprehensive, will review the recent advancements in radiotracer development for SPECT and PET MPI, autonomic dysfunction, apoptosis, atherosclerotic plaques, metabolism, and viability. The relevant radiochemistry and preclinical and clinical development in addition to molecular imaging with emerging modalities such as cardiac MRI and PET-MR will be discussed.

Comparison of ⁹⁹mTc-N-DBODC5 and ⁹⁹mTc-MIBI of myocardial perfusion imaging for diagnosis of coronary artery disease

Despite recent advances in therapeutic and diagnostic approaches, coronary artery disease (CAD) and its related cardiac disorders represent the most common cause of death in the United States. Nuclear myocardial perfusion imaging (MPI) technologies play a pivotal role in the diagnosis and treatment design for CAD. Recently, in order to develop improved MPI agents for diagnosis of CAD, (99m)Tc-[bis(dimethoxypropylphosphinoethyl)-ethoxyethyl-amine(PNP5)]-[bis(N-ethoxyethyl)dithiocarbamato(DBODC)]nitride(N-DBODC5)((99m)Tc-N-DBODC5) with a faster liver clearance than conventional single-photon emission computed tomography (SPECT) imaging agents (technetium 99m sestamibi ((99m)Tc-MIBI) or technetium 99m tetrofosmin) has been introduced. In preclinical and phase I studies, (99m)Tc-N-DBODC5 has shown characteristics of an essentially ideal MPI tracer. Importantly, however, there is no data to support the use of (99m)Tc-N-DBODC5 to evaluate myocardial ischemia in patients with suspected CAD. The present study was designed to assess the clinical value of this agent; the findings of stress and rest MPI after the administration of this agent were compared to those of stress and rest (99m)Tc-MIBI, as well as those of coronary angiography, with respect to the detection of CAD. Our findings indicated the usefulness of (99m)Tc-N-DBODC5 as a promising MPI agent.

Dithiocarbamate complexes as radiopharmaceuticals for medical imaging

Over the past 30 years dithiocarbamate ligands have found application in radiopharmaceutical metal-ligand complexes to image a range of disease states. The vast majority of research and applications, and the widest range of complex structures, have involved radionuclides of technetium and rhenium. Considering the extent of coordination chemistry of dithiocarbamate ligands described elsewhere in this issue, the extent of radiopharmaceutical application with metallic radionuclides is surprisingly narrow. Here we summarise the types of radiopharmaceutical complexes studied and the uses, and potential uses, to which they have been put in nuclear medicine.

APPLICATION OF TECHNETIUM AND RHENIUM IN NUCLEAR MEDICINE

Technetium and Rhenium are the two lower elements in the manganese triad. Whereas rhenium is known as an important part of high resistance alloys, technetium is mostly known as a cumbersome product of nuclear fission. It is less known that its metastable isotope 99mTc is of utmost importance in nuclear medicine diagnosis. The technical application of elemental rhenium is currently complemented by investigations of its isotope 188Re , which could play a central role in the future for internal, targeted radiotherapy. This article will briefly describe the basic principles behind diagnostic methods with radionuclides for molecular imaging, review the 99mTc -based radiopharmaceuticals currently in clinical routine and focus on the chemical challenges and current developments towards improved, radiolabeled compounds for diagnosis and therapy in nuclear medicine.

Evaluation of (99) (m)TcN-MPO as a new myocardial perfusion imaging agent in normal dogs and in an acute myocardial infarction canine model: comparison with (99) (m)Tc-sestamibi

PURPOSE

(99)  (m)TcN-MPO ([(99)  (m)TcN(mpo)(PNP5)](+): mpo = 2-mercaptopyridine oxide and PNP5 = N-ethoxyethyl-N,N-bis[2-(bis(3-methoxypropyl)phosphino)ethyl]amine) is a cationic (99)  (m)Tc-nitrido complex, which has favorable biodistribution and myocardial uptake with rapid liver clearance in Sprague Dawley rats. The objective of this study was to compare the biodistribution and pharmacokinetics of (99)  (m)TcN-MPO and (99)  (m)Tc-Sestamibi in normal dogs, and to evaluate the potential of (99)  (m)TcN-MPO as a myocardial perfusion agent in canines with acute myocardial infarction.

METHODS

Five normal mongrel dogs were injected intravenously with (99)  (m)TcN-MPO. Venous blood samples were collected via a femoral vein catheter at 0.5, 1, 2, 3, 4, 5, 10, 20, 30, 40, 60, and 90 min post-injection (p.i.). Anterior-posterior planar images were acquired by γ-camera at 10, 20, 30, 60, 90, and 120 min p.i. Regions of interest (ROIs) were drawn around the heart, liver, and lungs. The heart/liver and heart/lung ratios were calculated by dividing the mean counts in heart ROI by the mean counts in the liver and lung ROI, respectively. For comparison, (99)  (m)Tc-sestamibi was also evaluated in the same five dogs. The interval period between the two examinations was 1 week to eliminate possible interference between these two radiotracers. In addition, single positron emission computed tomography (SPECT) images in the canine infarct model were collected 24 h after myocardial infarction at 30 and 60 min after the administration of (99)  (m)TcN-MPO (n = 4) or (99)  (m)Tc-Sestamibi (n = 4).

RESULTS

It was found that (99)  (m)TcN-MPO and (99)  (m)Tc-Sestamibi displayed very similar blood clearance characteristics during the first 90 min p.i. Both (99)  (m)TcN-MPO and (99)  (m)Tc-Sestamibi had a rapid blood clearance with less than 50% of initial radioactivity remaining at 1 min and less than 5% at 30 min p.i. (99)  (m)TcN-MPO and (99)  (m)Tc-Sestamibi both showed good heart/lung contrast. The heart/liver ratio of (99)  (m)TcN-MPO increased with time (0.53 ± 0.06 at 10 min, 0.90 ± 0.062 at 30 min, and 1.22 ± 0.06 at 60 min p.i.), whereas the heart/liver ratio of (99)  (m)Tc-Sestamibi remained low at all time points (0.50 ± 0.03 at 10 min, 0.64 ± 0.03 at 30 min, and 0.60 ± 0.02 at 60 min p.i.). SPECT imaging studies in canines with acute myocardial infarction indicated that good visualization of the left ventricular wall and perfusion defects could be achieved at 30 min after administration of (99)  (m)TcN-MPO but not after (99)  (m)Tc-Sestamibi.

CONCLUSION

The combination of reasonable heart uptake with rapid hepatobiliary excretion makes (99)  (m)TcN-MPO a promising new radiotracer for myocardial perfusion imaging.

Kinetic characterization of a novel cationic (99m)Tc(I)-tricarbonyl complex, (99m)Tc-15C5-PNP, for myocardial perfusion imaging

BACKGROUND

Intense liver uptake of (99m)Tc-sestamibi (MIBI) often interferes with visualization of myocardial perfusion in the inferior wall of the left ventricle. To develop improved myocardial perfusion agents, crown ether-containing dithiocarbamates and bisphosphines have been introduced in recent years. This study was designed to investigate the myocardial imaging properties and in vivo kinetics of a cationic (99m)Tc(I)-tricarbonyl complex, (99m)Tc-15C5-PNP, in comparison with MIBI.

METHODS

Dynamic cardiac images were acquired for 60 minutes after intravenous tracer injection using a small-animal SPECT system in healthy control rats and rats with myocardial infarcts. Myocardial and liver time-activity curves were generated for radiopharmaceutical kinetic analysis.

RESULTS

Good visualization of the left ventricular wall and perfusion defects could be achieved 20 minutes after (99m)Tc-15C5-PNP administration. (99m)Tc-15C5-PNP images in all hearts with infarcts showed perfusion defects, which were comparable to MIBI images. The kinetic curves plotted from 1 to 60 minutes demonstrated that (99m)Tc-15C5-PNP has a shorter washout half-life (6.4 ± 3.2 vs 124 ± 30.5 minutes, P < .01) in the liver, lower residual liver activity (14.5 ± 10.2% vs 36.5 ± 28.9%, P < .01), and higher heart/liver ratio than MIBI.

CONCLUSIONS

(99m)Tc-15C5-PNP has potential for rapid myocardial perfusion imaging with low liver uptake.

Minimizing liver uptake of cationic Tc radiotracers with ether and crown ether functional groups

Ischemia-related diseases, particularly coronary artery disease (CAD), account for the majority of deaths worldwide. Myocardial ischemia is a serious condition and the delay in reperfusion of ischemic tissues can be life-threatening. This is particular true in the aged population. Rapid and accurate early detection of myocardial ischemia is highly desirable so that various therapeutic regiments can be given before irreversible myocardial damage occurs. Myocardial perfusion imaging with radiotracers is an integral component in evaluations of patients with known or suspected CAD. (99m)Tc-Sestamibi and (99m)Tc-Tetrofosmin are commercial radiopharmaceuticals currently available for myocardial perfusion imaging. Despite their widespread clinical applications, both (99m)Tc-Sestamibi and (99m)Tc-Tetrofosmin do not meet the requirements of an ideal perfusion imaging agent, largely due to their high liver uptake. The intense liver uptake makes it difficult to interpret the heart activity in the inferior and left ventricular wall. Photon scattering from the high liver radioactivity accumulation remains a significant challenge for diagnosis of heart diseases. This review will summarize the most recent research efforts to minimize the liver uptake of cationic (99m)Tc radiotracers by using ether and crown ether-containing chelators. Fast liver clearance will shorten the duration of imaging protocols (< 30 min post-injection), and allow for early acquisition of heart images with high quality. Improvement of heart/liver ratio may permit better detection of the presence and extent of coronary artery disease. Identification of such a new radiotracer that allows for the improved noninvasive assessment of myocardial perfusion would be of considerable benefit in treatment of patients with suspected CAD.

Technetium and rhenium in five-coordinate symmetrical and dissymmetrical nitrido complexes with alkyl phosphino-thiol ligands. Synthesis and structural characterization

The reactivity of bulky alkylphosphino-thiol ligands (PSH) toward nitride-M(V, VI) (M = Tc/Re) precursors was investigated. Neutral five-coordinate monosubstituted complexes of the type [M(N)(PS)Cl(PPh(3))] (Tc1-4, Re1-2) were prepared in moderate to high yields. It was found that these [M(N)(PS)Cl(PPh(3))] species underwent ligand-exchange reactions under mild conditions when reacted with bidentate mononegative ligands having soft donor atoms such as dithiocarbamates (NaL(n)) to afford stable dissymmetrical mixed-substituted complexes of the type [M(N)(PS)(L(n))] (Tc5,8-10, Re5-9) containing two different bidentate chelating ligands bound to the [M[triple bond]N](2+) moiety. In these reactions, the dithiocarbamate replaced the two labile monodentate ligands (Cl and PPh(3)) leaving the [M(N)(PS)](+) building block intact. In the above reactions, technetium and rhenium were found to behave in a similar way. Instead, under more drastic conditions, reactions of PSH with [M(N)Cl(2)(PPh(3))(2)] gave a mixture of monosubstituted [M(N)(PS)Cl(PPh(3))] and bis-substituted species [M(N)(PS)(2)] (Tc11-14) in the case of technetium, whereas only monosubstituted [M(N)(PS)Cl(PPh(3))] complexes were recovered for rhenium. All isolated products were characterized by elemental analysis, IR and multinuclear ((1)H, (13)C, and (31)P) NMR spectroscopies, ESI MS spectrometry, and X-ray crystal structure determination of the representative monosubstituted [Tc(N)(PStbu)Cl(PPh(3))] (Tc4) and mixed-substituted [Re(N)(PScy)(L(3))] (Re7) and [Re(N)(PSiso)(L(4))] (Re9) complexes. The latter rhenium complexes represent the first example of a square-pyramidal nitrido Re species with the basal plane defined by a PS(3) donor set. Monosubstituted [M(N)(PS)Cl(PPh(3))] species bearing the substitution-inert [M(N)(PS)](+) moieties act as suitable building blocks proposed for the construction of new classes of dissymmetrical nitrido compounds with potential application in the development of essential and target specific (99m)Tc and (188)Re radiopharmaceuticals for imaging and therapy, respectively.

Synthesis and biodistribution of a novel [99mTcN(PNP5)(DMCHDTC)]+ complex as a potential myocardial perfusion imaging agent

The [(99m)TcN(PNP5)(DMCHDTC)](+)(DMCHDTC: 2,3-dimethyl cyclohexyl dithiocarbamate, PNP5:bis(dimethoxypropylphosphinoethyl)ethoxyethylamine) complex was synthesized through a ligand-exchange reaction. The two-step procedure involved the initial reaction of (99m)TcO(4)(-) with succinic dihydrazide (SDH) as a donor of nitride nitrogen atom (N(3-)) in the presence of stannous chloride dihydrate as reducing agent and propylenediamine tetraacetic acid (PDTA) as complexant, followed by the addition of the PNP5 ligand and the DMCHDTC ligand. The radiochemical purity (RCP) of the product was over 90% as measured by thin layer chromatography (TLC). No decomposition of the complex at room temperature was observed over a period of 6 h. Its partition coefficient indicated that it was a lipophilic complex. The electrophoresis results showed the complex was cationic. The biodistribution results in mice indicated that [(99m)TcN(PNP5)(DMCHDTC)](+) was significantly retained into the heart. The heart uptake (ID%/g) was 14.47, 12.23 and 8.76 at 5, 30 and 60 min post-injection, respectively. The heart/liver, heart/lung and heart/blood ratios of the complex were 1.24, 3.62 and 23.05 at 60 min post-injection, suggesting it will be a potential myocardial imaging agent.

Evaluation of 99mTcN-15C5 as a new myocardial perfusion imaging agent in normal dogs and canines with coronary stenosis

OBJECTIVES

This study was designed to evaluate the biodistribution and blood clearance characteristics of 99mTcN-15C5 and its potential as a myocardial perfusion radiotracer.

METHODS

Five normal mongrel dogs were injected with 99mTcN-15C5 intravenously. Blood samples were collected at 0.5, 1, 2, 3, 4, 5, 10, 15, and 30 min postinjection (p.i.). Anterior planar images were acquired at 5, 15, 30, 60, 90, and 120 min p.i. Regions of interest (ROIs) were drawn around heart, liver, and lungs. The raw activity in each ROI was expressed as counts/pixel/min. Heart/liver and heart/lung ratios were calculated by dividing the mean counts in heart ROI by the mean counts in liver and lung ROI, respectively. For comparison, 99mTc-sestamibi was also evaluated in the same five dogs. In dogs with coronary stenoses, single photon emission computed tomography images were acquired at 30, 60, and 120 min after administration of 99mTcN-15C5 with/without adenosine.

RESULTS

99mTcN-15C5 and 99mTc-sestamibi had very similar blood clearance characteristics during the first 30 min p.i. The heart/liver ratio of 99mTcN-15C5 increased from 0.48+/-0.05 at 5 min p.i. to 1.85+/-0.11 at 120 min p.i., whereas the heart/liver ratio of 99mTc-sestamibi was improved only slightly from 0.45+/-0.04 at 5 min p.i. to 0.74+/-0.15 at 120 min p.i. SPECT imaging studies in canines with coronary stenoses indicated that good visualization of the perfusion defect could be achieved at 30 min after administration of 99mTcN-15C5 with the adenosine stress.

CONCLUSION

The combination of high heart uptake and rapid liver clearance makes 99mTcN-15C5 a promising new radiotracer for myocardial perfusion imaging.

Subcellular distribution and metabolism studies of the potential myocardial imaging agent [99mTc(N)(DBODC)(PNP5)]+

99mTc(N)-DBODC5 is the lead compound of a new series of monocationic 99mTc(N)-based potential myocardial imaging agents that exhibit original biodistribution properties. This study was addressed to elucidate the mechanisms of distribution, retention, and elimination of this promising 99mTc(N)-agent.

METHODS

The sex-related in vitro and in vivo stability and the subcellular distribution of 99mTc(N)-DBODC5 were investigated. Studies were performed by considering binding to the serum proteins; stability in rat serum, human serum, and rat liver homogenates; and the chemical integrity of the complex after extraction from rat tissues such as heart, liver, and kidney, as well as from intestinal fluids and urine. The effect of cyclosporin A on the in vivo pharmacokinetic properties of 99mTc(N)-DBODC5 was also evaluated. Subcellular distribution of 99mTc(N)-DBODC5 in ex vivo rat heart was determined by standard differential centrifugation techniques.

RESULTS

No significant in vitro serum protein binding and no notable biotransformation of the native compound into different species by the in vitro action of the serum and liver enzymes was evidenced. In vivo experiments showed that sex affects the pharmacokinetic profile of the 99mTc(N)-complexes including metabolism and excretion. Chromatographic profiles of 99mTc(N)-radioactivity extracted from tissues and fluids of female rats were always coincident with the control. Conversely, a small percentage of metabolized species was detected by high-performance liquid chromatography in liver extracts of male rats. Furthermore, administration of cyclosporin A caused a significant reduction of lung, liver, and kidney washout along with a considerable variation in activity distribution in the intestinal tract in both male and female rats, thus indicating a possible implication of Pgp transporters in determining the biologic behavior of 99mTc(N)-DBODC5. However, this phenomenon was more pronounced in females. Subcellular distribution studies showed that 86.3% +/- 7.4% of 99mTc(N)-DBODC5 was localized into mitochondrial fraction as a result of the interaction with the negative membrane potential.

CONCLUSION

Evidence showing that the new 99mTc(N)-myocardial tracers behave as multidrug resistance-associated protein P-glycoprotein substrates, combined with their selective mitochondrial accumulation, strongly supports the possibility that diagnostic application of 99mTc(N)-DBODC5 can be extended to tumor imaging and noninvasive multidrug resistance studies.

Whole-body biodistribution and radiation dosimetry of the new cardiac tracer 99mTc-N-DBODC

Our purpose was to evaluate the safety profile and biodistribution behavior in healthy human volunteers of the new myocardial perfusion tracer bis[(dimethoxypropylphosphanyl)ethyl]ethoxyethylamine N,N'-bis(ethoxyethyl)dithiocarbamato nitrido technetium(V) (99mTc-N-DBODC).

METHODS

Ten healthy male volunteers were injected with 99mTc-N-DBODC under both stress and rest conditions. Anterior and posterior planar gamma-camera images were collected at 5, 30, 60, 240, and 1,440 min after injection, with organ uptake quantified by region-of-interest analysis. Tracer kinetics in body fluids were determined by collecting blood and urine samples at different time points.

RESULTS

After injection, 99mTc-N-DBODC showed significant accumulation in the myocardium and prolonged retention. Under rest conditions, uptake in the heart, lungs, and liver at 5 min after injection was 1.67% +/- 0.13%, 1.16% +/- 0.07%, and 10.85% +/- 1.72%, respectively, of administered activity. Under stress conditions, heart uptake was significantly higher (2.07% +/- 0.22%). Radioactivity in the liver decreased to 3.64% +/- 0.98% and 2.37% +/- 0.48% at 60 and 240 min, respectively, after injection. This rapid liver clearance led to favorable heart-to-liver ratios, reaching values of 0.74 +/- 0.13 at rest and 1.26 +/- 0.28 during exercise 60 min after tracer administration. Radiation dose estimates were comparable to those obtained with other myocardial perfusion cationic compounds.

CONCLUSION

The high uptake in the myocardium and the fast liver washout of 99mTc-N-DBODC will allow SPECT images of the left ventricle to be acquired early and with excellent quality.

Tc-99m-N-MPO: novel cationic Tc-99m radiotracer for myocardial perfusion imaging

BACKGROUND

Technetium 99m-N-MPO ([Tc-99m-N(mpo)(PNP5)](+)) is a cationic Tc-99m nitrido complex. The objective of this study is to evaluate its potential as a new radiotracer for myocardial perfusion imaging.

METHODS AND RESULTS

Biodistribution studies were performed in Sprague-Dawley rats and guinea pigs to compare the myocardial uptake and excretion kinetics of Tc-99m-N-MPO from noncardiac organs, such as the liver and lungs, with those of the known cationic Tc-99m radiotracers: Tc-99m-N-DBODC5 and Tc-99m-sestamibi. Planar imaging was performed in Sprague-Dawley rats to evaluate the utility of Tc-99m-N-MPO as a myocardial perfusion imaging agent. Metabolism studies were carried out by use of both Sprague-Dawley rats and guinea pigs. In general, the heart uptake of Tc-99m-N-MPO was between that of Tc-99m-sestamibi and Tc-99m-N-DBODC5 over the 2-hour study period. However, the heart-liver ratio of Tc-99m-N-MPO (12.75 +/- 3.34) at 30 minutes after injection was more than twice that of Tc-99m-N-DBODC5 (6.01 +/- 1.45) and approximately 4 times higher than that of Tc-99m-sestamibi (2.90 +/- 0.22). The heart uptake and heart-liver ratio of Tc-99m-N-MPO and Tc-99m-sestamibi in guinea pigs were significantly lower than those obtained in Sprague-Dawley rats. The metabolism studies demonstrated no detectable Tc-99m-N-MPO metabolites in the urine and feces samples of the Sprague-Dawley rats at 120 minutes after injection. In guinea pigs no Tc-99m-N-MPO metabolites were detected in the urine at 120 minutes, but only approximately 60% of Tc-99m-N-MPO remained intact in the feces samples. In contrast, there was no intact Tc-99m-sestamibi detected in urine samples, and less than 15% of Tc-99m-sestamibi remained intact in the feces samples. Planar imaging studies indicated that clinically useful images of the heart may be obtained as early as 15 minutes after injection of Tc-99m-N-MPO.

CONCLUSION

The combination of favorable organ biodistribution and myocardial uptake with rapid liver clearance makes Tc-99m-N-MPO a very promising myocardial perfusion radiotracer worthy of further evaluation in various preclinical animal models.

Preparation of 99mTc-nitrido asymmetrical heterocomplex with 4-(cyclohexylpiperazin-1-yl)-dithioformate and its biological evaluation as a potential myocardial imaging agent

Potassium 4-(cyclohexylpiperazin-1-yl)-dithioformate (HPDTF) was synthesized from 1-cyclohexylpiperazine. The corresponding (99m)Tc-nitrido asymmetrical heterocomplex [(99m)TcN(PNP5)(HPDTF)](+) [PNP5=bis-(dimethoxypropylphosphinoethyl)ethoxyethylamine] was prepared via the [(99m)TcN](int)(2+) precursor in high yields (>95%) and characterized by HPLC and paper electrophoresis. It was found to be lipophilic and cationic, with high stability in vitro. Studies of its biodistribution in mice showed high heart uptake and good myocardial retention ((11.12+/-1.41)% ID/g at 5 min and (10.88+/-1.45)% ID/g at 120 min), as well as rapid clearance from liver, blood and soft tissues. At 60 min post-injection, the heart-to-liver, heart-to-lung and heart-to-blood ratios were 1.30, 3.89 and 27.56, respectively, which suggested that the complex might be suitable for myocardial imaging.

Rhenium and technetium tricarbonyl complexes anchored by pyrazole-based tripods: novel lead structures for the design of myocardial imaging agents

This report describes the synthesis and biological evaluation of cationic (99m)Tc-tricarbonyl complexes anchored by ether-containing tris(pyrazolyl)methane or bis(pyrazolyl)ethanamine ligands to be applied in the design of radiopharmaceuticals for myocardial imaging: fac-[(99m)Tc(CO)(3){RC(pz)(3)}](+) (R = H (1a), MeOCH(2) (2a), EtOCH(2) (3a), (n)PrOCH(2) (4a)) and fac-[(99m)Tc(CO)(3){RNHCH(2)CH(pz)(2)}](+) (R = H (5a), MeO(CH(2))(2) (6a)) (pz = pyrazolyl). At the no carrier added level, complexes 1a-6a were obtained in high radiochemical yield (> 98%) by reaction of fac-[(99m)Tc(CO)(3)(H(2)O)(3)](+) with the corresponding tripod chelator in aqueous medium. All these complexes display a high in vitro and in vivo stability, except 6a which metabolizes in vivo yielding fac-[(99m)Tc(CO)(3){HO(CH(2))(2)NHCH(2)CH(pz)(2)}](+) (7a). Biological studies in mice have shown that among the radiotracers evaluated in this work, 3a, anchored by a tris(pyrazolyl)methane chelator bearing an ethyl methyl ether substituent, has the highest heart uptake (3.6 +/- 0.5%ID g(-1) at 60 min p.i.). Complex 3a presents also the best heart: blood, heart: liver and heart: lung ratios, appearing as the most promising as a potential myocardial imaging agent. The chemical identity of 1a-7a was ascertained by HPLC comparison with the previously reported fac-[Re(CO)(3){HC(pz)(3)}]Br (1) and with the novel fac-[Re(CO)(3){RC(pz)(3)}]Br (R = MeOCH(2) (2), EtOCH(2) (3), (n)PrOCH(2)(4)) and fac-[Re(CO)(3){RNHCH(2)CH(pz)(2)}]Br (R = H (5), MeO(CH(2))(2) (6) HO(CH(2))(2) (7)). The novel Re(I) tricarbonyl complexes, 2-7, were characterized by the common analytical techniques, including single crystal X-ray diffraction analysis. The solid state structure confirmed the presence of facial and tridentate (kappa(3)-N(3)) anchor ligands. Solution NMR studies have also shown that this kappa(3)-N(3) coordination mode is retained in solution for all complexes (2-7).

Evaluation of novel cationic 99mTc(I)-tricarbonyl complexes as potential radiotracers for myocardial perfusion imaging

This report describes the evaluation of three cationic (99m)Tc(I)-tricarbonyl complexes--[(99m)Tc(CO)(3)(L)](+) (L=N-methoxyethyl-N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]amine (ME-PNP), N-[15-crown-5)-2-yl]-N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]amine (15C5-PNP) and N-[18-crown-6)-2-yl]-N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]amine (18C6-PNP))--as potential radiotracers for myocardial perfusion imaging. Biodistribution, imaging and metabolism studies were performed using Sprague-Dawley rats. It was found that bisphosphine ligands have a significant impact on the biodistribution characteristics and clearance kinetics of their cationic (99m)Tc(I)-tricarbonyl complexes. Among the three radiotracers evaluated in this study, [(99m)Tc(CO)(3)(15C5-PNP)](+) has a very high initial heart uptake and is retained in the rat myocardium for >2 h. It also shows rapid clearance from the liver and lungs. The heart/liver ratio of [(99m)Tc(CO)(3)(15C5-PNP)](+) is approximately 2.5 times better than that of (99m)Tc-sestamibi at 30 min postinjection. [(99m)Tc(CO)(3)(15C5-PNP)](+) is almost identical to (99m)TcN-DBODC5 with respect to heart uptake, heart/lung ratio and heart/liver ratio. Results from metabolism studies show that there is no significant metabolism for [(99m)Tc(CO)(3)(15C5-PNP)](+) in the urine, but it does show a small metabolite peak (<10%) in the radio high-performance liquid chromatography chromatogram of the feces sample at 120 min postinjection. Results planar imaging studies demonstrate that [(99m)Tc(CO)(3)(15C5-PNP)](+) has a much better liver clearance profile than (99m)Tc-sestamibi and might give clinically useful images of the heart as early as 30 min postinjection. [(99m)Tc(CO)(3)(15C5-PNP)](+) is a very promising candidate for more preclinical evaluations in various animal models.

Impact of bidentate chelators on lipophilicity, stability, and biodistribution characteristics of cationic 99mTc-nitrido complexes

This report describes synthesis and evaluation of novel cationic 99mTc-nitrido complexes, [99mTcN(L)(PNP)](+) (L = ma, ema, tma, etma and mpo; PNP = PNP5, PNP6, and L6), as potential radiotracers for heart imaging. Cationic complexes [99mTcN(L)(PNP)](+) were prepared in two steps. For example, reaction of succinic dihydrazide with 99mTcO4(-) in the presence of excess stannous chloride and PDTA resulted in the [99mTcN(PDTA)n] intermediate, which then reacted Hmpo and PNP6 at 100 degrees C for 10-15 min to give [99mTcN(mpo)(PNP6)](+) in >90% yield. It was found that bidentate chelators have a significant impact on lipophilicity, solution stability, biodistribution, and metabolic stability of cationic 99mTc-nitrido complexes. The fact that [99mTcN(ema)(PNP6)](+) decomposes rapidly in the presence of cysteine (1 mg/mL) while [99mTcN(etma)(PNP6)](+) and [99mTcN(mpo)(PNP6)](+) remain stable for >6 h under the same conditions strongly suggests that thione-S donors in bidentate chelators increase the solution stability of their cationic 99mTc-nitrido complexes. Biodistribution studies were performed on four cationic 99mTc-nitrido complexes in Sprague-Dawley rats. [99mTcN(etma)(PNP5)](+) is of particular interest due to its high initial heart uptake (1.81 +/- 0.35 %ID/g at 5 min postinjection), and long myocardial retention (1.99 +/- 0.47 %ID/g at 120 min postinjection). The heart/liver ratio of [99mTcN(etma)(PNP5)](+) (6.06 +/- 1.48) at 30 min postinjection is almost identical that of 99mTcN-DBODC5 (6.01 +/- 1.45), and is >2 times better than that of 99mTc-sestamibi (2.90 +/- 0.22). Results from metabolism studies show that [99mTcN(etma)(PNP5)](+) has no significant metabolism in the urine, but it does show significant metabolism in feces samples at 120 min postinjection. Planar imaging studies suggest that [99mTcN(etma)(PNP5)](+) might be able to give clinically useful images of the heart as early as 30 min postinjection. [99mTcN(etma)(PNP5)](+) is a very promising candidate for more preclinical evaluations in various animal models.

Organ biodistribution and myocardial uptake, washout, and redistribution kinetics of Tc-99m N-DBODC5 when injected during vasodilator stress in canine models of coronary stenoses

BACKGROUND

Technetium 99m N-DBODC5 is a new myocardial perfusion tracer shown to exhibit high heart uptake and rapid liver clearance in normal rats. The objectives of this canine study were (1) to compare the organ biodistribution and myocardial uptake, washout, and redistribution kinetics of Tc-99m N-DBODC5 with Tc-99m sestamibi over a period of 3 hours in a more clinically relevant large animal species and (2) to compare the myocardial uptake of Tc-99m N-DBODC5 with thallium 201 when co-injected during vasodilator stress in dogs with coronary stenoses.

METHODS AND RESULTS

At peak adenosine-induced hyperemia, 10 dogs with critical left anterior descending artery stenoses received either Tc-99m N-DBODC5 (n = 6) or Tc-99m sestamibi (n = 4) and microspheres, followed by serial imaging and blood sampling over a period of 3 hours. Another 14 dogs with either critical (n = 7) or mild (n = 7) left anterior descending artery stenoses underwent simultaneous injection of Tc-99m N-DBODC5, Tl-201, and microspheres during peak vasodilator stress. Like sestamibi, Tc-99m N-DBODC5 showed good myocardial uptake with slow washout and minimal redistribution over a period of 3 hours (P = not significant); however, Tc-99m N-DBODC5 cleared more rapidly from the liver (heart-lung ratio at 30 minutes, 0.92+/-0.11 versus 0.51 +/- 0.05; P < .05). When injected during hyperemic flow, the myocardial extraction plateau for Tc-99m N-DBODC5 was lower than that for Tl-201 and was intermediate between Tc-99m sestamibi and Tc-99m tetrofosmin.

CONCLUSIONS

Excellent organ biodistribution and myocardial uptake and clearance kinetic properties, combined with rapid liver clearance and a favorable flow-extraction relationship, make Tc-99m N-DBODC5 a very promising new myocardial perfusion imaging agent.

Ether and crown ether-containing cationic 99mTc complexes useful as radiopharmaceuticals for heart imaging

While radiopharmaceutical research has been focused on the development of target-specific radiotracers for early detection and radiotherapy of cancers in the last decade, there is a limited effort on new cationic 99mTc radiotracers for heart imaging. This review will summarize some of the most recent developments in ether- and crown ether-containing cationic 99mTc radiotracers that have a fast liver clearance with a heart/liver ratio substantially better than that of 99mTc-Sestamibi and 99mTc-Tetrofosmin, the two commercial 99mTc radiopharmaceuticals currently available for myocardial perfusion imaging. Fast liver clearance might shorten the duration of imaging protocols (<30 min post-injection), and allow for early acquisition of heart images of high quality. Improvement of heart/liver ratio may permit better detection of the presence and extent of coronary artery disease. Identification of such a new radiotracer that allows for the improved non-invasive delineation of myocardial perfusion would be of considerable benefit in treatment of patients with suspected coronary artery disease.

Evaluation of novel cationic (99m)Tc-nitrido complexes as radiopharmaceuticals for heart imaging: improving liver clearance with crown ether groups

This report describes the evaluation of a series of novel cationic (99m)Tc-nitrido complexes, [(99m)TcN(DTC)(PNP)]+ (DTC = crown ether-containing dithiocarbamates; PNP = bisphosphine), as potential radiotracers for myocardial perfusion imaging. Synthesis of cationic (99m)Tc-nitrido complexes was accomplished in two steps according to literature methods. Biodistribution studies were performed in rats. Planar images of Sprague-Dawley rats administered with 15+/-2 MBq of cationic (99m)Tc radiotracer were obtained using a PhoGama large field-of-view Anger camera. Samples from both urine and feces were analyzed by a reversed-phase radio-HPLC method. Results from biodistribution studies showed that most of the cationic (99m)Tc-nitrido complexes have a high initial heart uptake with a long myocardial retention. They also show a rapid clearance from the liver and lungs. Cationic complexes [(99m)TcN(L2)(L6)]+ and [(99m)TcN(L4)(L6)]+ show heart/liver ratios four to five times better than that of (99m)Tc-sestamibi due to their much faster liver clearance. Their heart uptake and heart/liver ratio are comparable to that of (99m)TcN-DBODC5 within the experimental error. These findings have been confirmed by the results from imaging studies. Radio-HPLC analysis of urine and feces samples indicated that there was very little metabolism of cationic (99m)Tc-nitrido complexes in rats under anesthesia. The key finding of this study is that lipophilicity remains the most important factor affecting both heart uptake and target-to-background (T/B) ratios. Crown ethers are very useful functional groups to improve the liver clearance of cationic (99m)Tc-nitrido complexes. It is the combination of the appropriate DTCs and bisphosphines that results in cationic (99m)Tc-nitrido complexes with high heart uptake and fast clearance from the liver at the same time. The fast liver clearance of [(99m)TcN(L2)(L6)]+ and [(99m)TcN(L4)(L6)]+ suggests that they might be used to obtain clinically useful images as early as 30 min postinjection. [(99m)TcN(L2)(L6)]+ and [(99m)TcN(L4)(L6)]+ are very promising candidates for further evaluation in more extensive preclinical animal models.

A novel ternary ligand system useful for preparation of cationic (99m)Tc-diazenido complexes and (99m)Tc-labeling of small biomolecules

This report describes a novel ternary ligand system composed of a phenylhydrazine, a crown ether-containing dithiocarbamate (DTC), and a PNP-type bisphosphine (PNP). The combination of three different ligands with (99m)Tc results in cationic (99m)Tc-diazenido complexes, [(99m)Tc(NNAr)(DTC)(PNP)]+, with potential radiopharmaceuticals for heart imaging. Synthesis of cationic (99m)Tc-diazenido complexes can be accomplished in two steps. For example, the reaction of phenylhydrazine with (99m)TcO4- at 100 degrees C in the presence of excess stannous chloride and 1,2-diaminopropane-N,N,N',N'-tetraacetic acid (PDTA) results in the [(99m)Tc(NNPh)(PDTA)n] intermediate, which then reacts with sodium N-(dithiocarbamato)-2-aminomethyl-15-Crown-5 (L4) and N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]ethoxyethylamine (PNP6) at 100 degrees C for 15 min to give the complex, [(99m)Tc(NNPh)(L4)(PNP6)]+ in high yield (>90%). Cationic complexes [(99m)Tc(NNPh)(DTC)(PNP)]+ are stable for > or = 6 h. Their composition was determined to be 1:1:1:1 for Tc:NNPh:DTC:PNP using the mixed-ligand experiments on the tracer ((99m)Tc) level and was further confirmed by the ESI-MS spectral data of a model compound [Re(NNPh)(L4)(L6)]+. It was found that both DTCs and bisphosphines have a significant impact on the lipophilicity of their cationic (99m)Tc-diazenido complexes. Results from a (99m)Tc-labeling efficiency experiment showed that 4-hydrazinobenzoic acid (HYBA) might be useful as a bifunctional coupling agent for (99m)Tc-labeling of small biomolecules. However, the (99m)Tc-labeling efficiency of HYBA is much lower than that of 6-hydrazinonicotinic acid (HYNIC) with tricine and trisodium triphenylphosphine-3,3',3''-trisulfonate (TPPTS) as coligands.

The crucial role of the diphosphine heteroatom X in the stereochemistry and stabilization of the substitution-inert [M(N)(PXP)]2+ metal fragments (M = Tc, Re; PXP = diphosphine ligand)

The nature of the heteroatom X incorporated in the five-membered PXP-diphosphine bridging chain was found to play a primary unit role both in the overall stability and in the stereochemical arrangement of nitrido-containing [M(N)(PXP)](2+) metal fragments (M = Tc, Re). Thus, by mixing PXP ligands with labile [Re(N)Cl(4)](-) and Tc(N)Cl(2)(PPh(3))(2) nitrido precursors in CH(2)Cl(2)/MeOH mixtures, a series of neutral M(N)Cl(2)(PXP) complexes (M = Tc, 1-5; M = Re, 8, 9) was collected. In the resulting distorted octahedrons, PXP adopted facial or meridional coordination, and combination with halide co-ligands produced three different stereochemical arrangements, that is, fac,cis, mer,cis, and mer,trans, depending primarily on the nature of the diphosphine heteroatom X. When X = NH, mer,cis-Tc(N)Cl(2)(PNP1), 1, was the only isomer formed. Alternatively, when a tertiary amine nitrogen (X = NR; R = CH(3), CH(2)CH(2)OCH(3)) was introduced in the bridging chain, fac,cis-M(N)Cl(2)(PN(R)P) complexes (M = Tc, 2, 3; M = Re, 8f) were obtained. Isomerization into the mer,cis-Re(N)Cl(2)(PN(R)P), 8m, species was observed only in the case of rhenium when the tertiary amine group carried the less encumbering methyl substituent. fac,cis-Tc(N)Cl(2)(PSP), 4f, was isolated in the solid state when X = S, but a mixture of fac,cis-Tc(N)Cl(2)(PSP) and mer,trans-Tc(N)Cl(2)(PSP), 4m, isomers was found in equilibrium in the solution state. A similar equilibrium between fac,cis-M(N)Cl(2)(POP) (M = Tc, 5f; M = Re, 9f) and mer,trans-M(N)Cl(2)(POP) (M = Tc, 5m; M = Re, 9m) species was detected in POP-containing complexes. The molecular structure of all of these complexes was assessed by means of conventional physicochemical techniques including multinuclear NMR spectroscopy and X-ray diffraction analysis of representative mer,cis-Tc(N)Cl(2)(PN(H)P), 1, fac,cis-Tc(N)Cl(2)(PSP), 4f, and mer,cis-Re(N)Cl(2)(PN(Me)P), 8m, compounds.

Contribution of electrospray mass spectrometry for the characterization, design, and development of nitrido technetium and rhenium heterocomplexes as potential radiopharmaceuticals

Diagnostic nuclear medicine (NM) is among the imaging procedures (together with X-ray, computerized tomography, magnetic resonance, and echography) the clinicians can routinely adopt to image organs or tissues and related disorders. (99m)Tc-based agents are the radiopharmaceuticals of election in diagnostic NM because of the ideal physical properties of the 99mTc nuclide (t1/2 6.01 hr; Egamma 142 keV), low cost, and easy availability through the commercial 99Mo/99mTc generator, and chemical versatility of the element. In the last two decades the synergistic work of clinics, pharmacologists, and coordination chemists has had a tremendous impact in the development of new 99mTc-based radiopharmaceuticals through the recognition of the structure at the molecular level of the agent utilized. This has been achieved by studying the physico-chemical properties of the long-lived 99gTc (t1/2 2.11 x 10(5) year; Ebeta 292 keV) and third-row congener Re isostructural compounds. Electrospray ionization mass spectrometry (ESI-MS) and collision experiments (MS/MS) represent valuable analytical techniques suitable for the characterization of both technetium and rhenium complexes relevant to NM. Unequivocal structural identification of these bioinorganic compounds, either simple coordination complexes ("essential radiopharmaceuticals") or more sophisticated structures carrying bioactive fragments ("receptor-specific" radiopharmaceuticals), can be realized in combination with multinuclear NMR spectroscopy. MS/MS experiments provide useful information on the different metal-ligand bond strength, and comparison of the fragmentation profiles of isostructural technetium and rhenium compounds give additional details on the role played by the metal in determining preferred decomposition channels. The analysis of these data contribute to design novel synthetic strategies for the obtainment of technetium and rhenium compounds relevant to NM. The chemistry underlying the production of a new class of potential radiopharmaceuticals including a terminal nitrogen bond and a mixed coordination sphere comprising heterodiphosphines and/or dithiocarbamates (DTC) is presented in detail together with the ESI-MS and MS/MS investigations.