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

Preparation and Preliminary Evaluation of a Promising 99mTc-Labeled Isonitrile-Containing 6-Thia-Fatty Acid Derivative for Myocardial Metabolism Imaging

For over 40 years, none of the previous ^99mTc-labeled fatty acids for myocardial imaging has potential clinical use. ^99mTc-(C10-6-thia-CO₂H)(MIBI)₅ is the first ^99mTc-labeled fatty acid to exhibit good myocardial uptake (2.06 ± 0.06%ID/g) at 60 min post injection, high heart-to-liver ratio (6.43 ± 1.85 and 9.68 ± 0.76), high heart-to-lung ratio (9.48 ± 1.39 and 11.02 ± 0.89), and high heart-to-blood ratio (164.01 ± 43.51 and 197.36 ± 32.29) at 60 and 120 min in Sprague-Dawley (SD) rats, respectively. It also demonstrated excellent myocardial imaging quality. The above target-to-nontarget ratios exceeded those of [¹²³I]BMIPP and were higher than or close to those of ^99mTc-MIBI at 60 and 120 min. Most of ^99mTc-(C10-6-thia-CO₂H)(MIBI)₅ was partially β-oxidized to protein-bound metabolites in myocardium. Administration of trimetazidine dihydrochloride (TMZ, a fatty acid β-oxidation inhibitor) to rats caused 51% reduction in the myocardial uptake of ^99mTc-(C10-6-thia-CO₂H)(MIBI)₅ and 61% reduction in the distribution of ^99mTc-radioactivity in a residual tissue pellet at 60 min, indicating its considerable sensitivity to myocardial fatty acid β-oxidation.

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.

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.

Novel 99mTc(III)-azide complexes [99mTc(N3)(CDO)(CDOH)2B-R] (CDOH2=cyclohexanedione dioxime) as potential radiotracers for heart imaging

INTRODUCTION

In this study, novel ^99mTc(III)-azide complexes [^99mTc(N₃)(CDO)(CDOH)₂B-R] (^99mTc-ISboroxime-N₃: R=IS; ^99mTc-MPboroxime-N₃: R=MP; ^99mTc-PAboroxime-N₃: R=PA; ^99mTc-PYboroxime-N₃: R=PY; and ^99mTc-Uboroxime-N₃: R=5U) were evaluated as heart imaging agents.

METHODS

Complexes [^99mTc(N₃)(CDO)(CDOH)₂B-R] (R=IS, MP, PA, PY and 5U) were prepared by ligand exchange between NaN₃ and [^99mTcCl(CDO)(CDOH)₂B-R]. Biodistribution and imaging studies were carried out in Sprague-Dawley rats. Image quantification was performed to compare their initial heart uptake and myocardial retention.

RESULTS

^99mTc-ISboroxime-N₃, ^99mTc-PYboroxime-N₃ and ^99mTc-Uboroxime-N₃ were prepared with high RCP (93-98%) while the RCP of ^99mTc-MPboroxime-N₃ and ^99mTc-PAboroxime-N₃ was 80-85%. The myocardial retention curves of ^99mTc-ISboroxime-N₃, ^99mTc-PYboroxime-N₃ and ^99mTc-Uboroxime-N₃ were best fitted to the bi-exponential decay function. The half-time of the fast component was 1.6±0.4min for ^99mTc-ISboroxime-N₃, 0.7±0.1min for ^99mTc-PYboroxime-N₃ and 0.9±0.4min for ^99mTc-Uboroxime-N₃. The 2-min heart uptake from biodistribution studies followed the ranking order of ^99mTc-ISboroxime-N₃ (3.60±0.68%ID/g)>^99mTc-PYboroxime-N₃ (2.35±0.37%ID/g)≫^99mTc-Uboroxime-N₃ (1.29±0.06%ID/g). ^99mTc-ISboroxime-N₃ had the highest 2-min heart uptake among ^99mTc radiotracers revaluated in SD rats. High quality SPECT images were obtained with the right and left ventricular walls being clearly delineated. The best image acquisition window was 0-5min for ^99mTc-ISboroxime-N₃.

CONCLUSION

Both azide coligand and boronate caps had significant impact on the heart uptake and myocardial retention of complexes [^99mTc(N₃)(CDO)(CDOH)₂B-R]. Among the radiotracers evaluated in SD rats, ^99mTc-ISboroxime-N₃ has the highest initial heart uptake with the heart retention comparable to that of ^99mTc-Teboroxime. ^99mTc-ISboroxime-N₃ is a promising alternative to ^99mTc-Teboroxime for SPECT MPI.

Effect of co-ligands on chemical and biological properties of (99m)Tc(III) complexes [(99m)Tc(L)(CDO)(CDOH)2BMe] (L=Cl, F, SCN and N3; CDOH2=cyclohexanedione dioxime)

INTRODUCTION

(99m)Tc-Teboroxime ([(99m)TcCl(CDO)(CDOH)2BMe]) is a member of the BATO (boronic acid adducts of technetium dioximes) class of (99m)Tc(III) complexes. This study sought to explore the impact of co-ligands on solution stability, heart uptake and myocardial retention of [(99m)Tc(L)(CDO)(CDOH)2BMe] ((99m)Tc-Teboroxime: L=Cl; (99m)Tc-Teboroxime(F): L=F; (99m)Tc-Teboroxime(SCN): L=SCN; and (99m)Tc-Teboroxime(N3): L=N3).

METHODS

Radiotracers (99m)Tc-Teboroxime(L) (L=F, SCN and N3) were prepared by reacting (99m)Tc-Teboroxime with NaF, NaSCN and NaN3, respectively. Biodistribution and imaging studies were carried out in Sprague-Dawley rats. Image quantification was performed to compare their heart retention and liver clearance kinetics.

RESULTS

Complexes (99m)Tc-Teboroxime(L) (L=F, SCN and N3) were prepared in high yield with high radiochemical purity. All new radiotracers were stable for >6h in the kit matrix. In its HPLC chromatogram, (99m)Tc-Teboroxime showed one peak at ~15.5 min, which was shorter than that of (99m)Tc-Teboroxime(F) (~16.4 min). There were two peaks for (99m)Tc-Teboroxime(SCN) at 16.5 and 18.3 min. (99m)Tc-Teboroxime(N3) appeared as a single peak at 18.4 min. Their heart retention and liver clearance curves were best fitted to the bi-exponential decay function. The half-times of fast/slow components were 1.6±0.4/60.7±8.9 min for (99m)Tc-Teboroxime, 0.8±0.2/101.7±20.7 min for (99m)Tc-Teboroxime(F), 1.2±0.3/84.8±16.6 min for (99m)Tc-Teboroxime(SCN), and 2.9±0.9/51.6±5.0 min for (99m)Tc-Teboroxime(N3). The 2-min heart uptake followed the order of (99m)Tc-Teboroxime (3.00±0.37%ID/g)>(99m)Tc-Teboroxime(N3) (2.66±0.01 %ID/g)≈(99m)Tc-Sestamibi (2.55±0.46 %ID/g)>(99m)TcN-MPO (2.38±0.15 %ID/g). (99m)Tc-Teboroxime remains the best in first-pass extraction. The best image acquisition window is 0-5 min for (99m)Tc-Teboroximine and 0-15 min for (99m)Tc-Teboroximine(N3).

CONCLUSION

Co-ligands had significant impact on the heart uptake and myocardial retention of complexes [(99m)Tc(L)(CDO)(CDOH)2BMe] (L=Cl, F, SCN and N3). Future studies should be directed towards minimizing the liver uptake and radioactivity accumulation in the blood vessels while maintaining their high heart uptake.

Development of kit formulations for (99m) TcN-MPO: a cationic radiotracer for myocardial perfusion imaging

The objective of this study was to develop a kit formulation for [(99m) TcN(mpo)(PNP5)](+) (MPO = 2-mercaptopyridine oxide), ((99m) TcN-MPO) to support its clinical evaluations as a SPECT radiotracer. Radiolabeling studies were performed using three different formulations (two-vial formulation and single-vial formulations with/without SnCl2 ) to explore the factors influencing radiochemical purity (RCP) of (99m) TcN-MPO. We found that the most important factor affecting the RCP of (99m) TcN-MPO was the purity of PNP5. (99m) TcN-MPO was prepared >98% RCP (n = 20) using the two-vial formulation. For single-vial formulations with/without SnCl2 , β-cyclodextrin (β-CD) is particularly useful as a stabilizer for PNP5. The RCP of (99m) TcN-MPO was 95-98% using β-CD, but its RCP was only 90-93% with γ-cyclodextrin (γ-CD). It seems that PNP5 fits better into the inner cavity of β-CD, which forms more stable inclusion complex than γ-CD in the single-vial formulations. The results from biodistribution and imaging studies in Sprague-Dawley rats clearly demonstrated biological equivalence of three different formulations. Single photon-emission computed tomography data suggested that high quality images could be obtained at 0-30-min post-injection without significant interference from the liver radioactivity. Considering the ease for (99m) Tc-labeling and high RCP of (99m) TcN-MPO, the non-SnCl2 single-vial formulation is an attractive choice for future clinical studies.

(99m)Tc(N)-DBODC(5), a potential radiolabeled probe for SPECT of multidrug resistance: in vitro study

[(99m)Tc(N)(DBODC)(PNP5)](+) [DBODC is bis(N-ethoxyethyl)dithiocarbamato; PNP5 is bis(dimethoxypropylphosphinoethyl)ethoxyethylamine], abbreviated as (99m)Tc(N)-DBODC(5), is a lipophilic cationic mixed compound investigated as a myocardial imaging agent. The findings that this tracer accumulates in mitochondrial structures through a mechanism mediated by the negative mitochondrial membrane potential and that the rapid efflux of (99m)Tc(N)-DBODC(5) from nontarget tissues seems to be associated with the multidrug resistance (MDR) P-glycoprotein (P-gp) transport function open up the possibility to extend its clinical applications to tumor imaging and noninvasive MDR studies. The rate of uptake at 4 and 37 °C of (99m)Tc(N)-DBODC(5) was evaluated in vitro in selected human cancer cell lines and in the corresponding sublines before and after P-gp and/or MDR-associated protein (MRP) modulator/inhibitor treatment using (99m)Tc-sestamibi as a reference. The results indicated that (1) the uptake of both (99m)Tc(N)-DBODC(5) and (99m)Tc-sestamibi is correlated to metabolic activity of the cells and (2) the cellular accumulation is connected to the level of P-gp/MRP expression; in fact, an enhancement of uptake in resistant cells was observed after treatment with opportune MDR inhibitor/modulator, indicating that the selective blockade of P-gp/MRP prevented efflux of the tracers. This study provides a preliminary indication of the applicability of (99m)Tc(N)-DBODC(5) in tumor imaging and in detecting P-gp/MRP-mediated drug resistance in human cancer. In addition, the possibility to control the hydrophobicity and pharmacological activity of this heterocomplex through the variation of the substituents on the ligands backbone without affecting the P2S2 coordinating sphere makes (99m)Tc(N)-DBODC(5) a suitable scaffold for the preparation of a molecular probe for single photon emission computed tomography of MDR.

On the structural modification of 2-nitroimidazole-(99m)Tc(CO)(3) complex, a hypoxia marker, for improving in vivo pharmacokinetics

INTRODUCTION

A 2-nitroimidazole-(99m)Tc(CO)(3) complex reported earlier showed promise with respect to its uptake and retention in hypoxic tumor. However, significant uptake and slow clearance from liver imposed severe limitations towards advocating its possible practical utility. In an attempt to improving its liver clearance, an ether linkage, which is known to help in liver clearance, was introduced in the molecule.

METHODS

The 2-nitroimidazole iminodiacetic acid (IDA) derivative containing an ether linkage was synthesized in a five step procedure from 2-nitroimidazole. This derivative was radiolabeled using [(99m)Tc(CO)(3)(H(2)O)(3)](+) precursor complex. The corresponding Re(CO)(3) analogue was also synthesized in the macroscopic level for structural characterization. The (99m)Tc(CO)(3) complex was evaluated in an animal model bearing fibrosarcoma tumor.

RESULTS

The in vivo evaluation of the complex indicated that, as envisaged, introduction of the ether linkage has improved clearance from the liver. The complex also showed higher retention in tumor compared to the 2-nitroimidazole-IDA-(99m)Tc(CO)(3) complex reported earlier. Though the tumor to muscle ratio improved with time, the tumor to blood ratio did not show any significant improvement. Despite improved liver clearance, there was significant liver activity present even at 3h p.i. attributable to gradual accumulation of activity cleared from muscle and blood.

CONCLUSIONS

Though the introduction of ether linkage improved liver clearance of the modified 2-nitroimidazole complex, it was found that a single ether linkage was not sufficient to achieve the desirable level of clearance. Probably, a linker with multiple ether groups, such as a di- or tri-ethylene glycol spacer, may be a possible solution to this issue.

Studies of the myocardial uptake and excretion mechanisms of a novel 99mTc heart perfusion agent

INTRODUCTION

(99m)Tc-TMEOP is a novel heart perfusion radiotracer exhibiting high initial and persistent heart uptake associated with rapid blood and liver clearance. This study aimed at determining the mechanisms of myocardial localization and fast liver clearance of (99m)Tc-TMEOP.

METHODS

Subcellular distribution of (99m)Tc-TMEOP was determined in excised rat heart tissue by differential centrifugation. The effect of cyclosporin A on the pharmacokinetic behaviour of (99m)Tc-TMEOP was evaluated by both ex vivo biodistribution and in vivo planar imaging studies.

RESULTS

Subcellular distribution studies showed that more than 73% of (99m)Tc-TMEOP was associated with the mitochondrial fraction. Comparison with subcellular distribution of (99m)Tc-sestamibi showed no significant difference in the mitochondrial accumulation between the two tracers. Biodistribution studies in the presence of cyclosporin A revealed an increase in kidneys and liver uptake of (99m)Tc-TMEOP, suggesting the involvement of multidrug resistance transporters in determining its pharmacokinetic profile.

CONCLUSIONS

The heart uptake mechanism of (99m)Tc-TMEOP is similar to that of the other reported monocationic (99m)Tc cardiac agents and is associated with its accumulation in the mitochondria. Cyclosporin A studies indicate that the fast liver and kidney clearance kinetics is mediated by P-glycoprotein (Pgp), supporting the potential interest of this radiotracer for imaging Pgp function associated with multidrug-resistant tumours.

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.

Mechanism for myocardial localization and rapid liver clearance of Tc-99m-N-MPO: a new perfusion radiotracer for heart imaging

BACKGROUND

[Tc-99m-N(mpo)(PNP5)](+) (Tc-99m-N-MPO: Hmpo = 2-mercaptopyridine N-oxide and PNP5 = N-ethoxyethyl-N,N-bis[2-(bis(3-methoxypropyl)phosphino)ethyl]amine) is a new Tc-99m radiotracer useful for myocardial perfusion imaging. The main objective of this study is to elucidate the mechanism for myocardial localization and fast liver clearance of Tc-99m-N-MPO in comparison with Tc-99m-sestamibi ([Tc-99m-(MIBI)(6)](+): MIBI = 2-methoxy-2-methylpropylisonitrile).

METHODS AND RESULTS

Subcellular distribution of Tc-99m-N-MPO and Tc-99m-sestamibi was examined in the excised Sprague-Dawley (SD) rat myocardium. Biodistribution and planar imaging studies were performed using SD rats in the absence/presence of Cyclosporin-A. Due to negative plasma and mitochondrial potentials, 84.5% +/- 3.2% of Tc-99m-N-MPO was found in the mitochondrial fraction as compared to 88.0% +/- 1.5% of Tc-99m-sestamibi. There was no significant difference in their mitochondrial accumulation. Tc-99m-N-MPO was also able to retain its chemical integrity in rat myocardium. Pre-treatment of SD rats with Cys-A result in significant increase in the kidney and liver uptake of Tc-99m-N-MPO.

CONCLUSION

Tc-99m-N-MPO and Tc-99m-sestamibi share almost identical subcellular distribution and localization mechanism. The MDR transport function of hepatocytes and renal cells is responsible for the fast clearance kinetics of Tc-99m-N-MPO from liver and kidneys, respectively. Tc-99m-N-MPO is a very promising myocardial perfusion radiotracer with favorable biodistribution properties and rapid liver clearance.

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.

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.

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.