Synthesis, Solution-State and Solid-State Structural Characterization of Monocationic Nitrido Heterocomplexes [M(N)(DTC)(PNP)]+ (M =99Tc, Re; DTC = Dithiocarbamate; PNP = Heterodiphosphane)

Luminescence Color and Intensity Changes of Nitridorhenium(V) Complexes Induced by Protonation/Deprotonation on the Bidentate Azolylpyridine Ligands

Tricyanidonitridorhenium(V) complexes with azolylpyridines, namely, [ReN(CN)3(H-N2py)]- (1-H, H-N2py = 2-(3-pyrazolyl)pyridine) and [ReN(CN)3(L)]2- (2-a, L = 2-[1,2,3]-triazol-4-yl-pyridine anion (N3py-), and 3-a, that is, L = 2-(tetrazol-5-yl)-pyridine anion (N4py-)), were newly synthesized and characterized. The structures of the new complexes were determined by single-crystal X-ray analysis. The 1-H complex includes two geometrical isomers in which an isomer is the conformation with the pyridyl (py) and pyrazolyl (pyrz) moieties of H-N2py occupying the trans site to the nitrido (the ax site) and the trans site to the cyanido (the eq site), respectively, in a bidentate fashion; the other isomer is the py and pyrz moieties coordinated to the eq and ax sites. In 2-a and 3-a, the triazolyl (trz) and tetrazoly (tetrz) moieties in N3py- and N4py- occupy the eq site, and the py moieties in N3py- and N4py- coordinate to the ax site. The complex 1-H is deprotonated upon the addition of 1,8-diazabicyclo[5.4.0]undec-7-one or NaOH to produce [ReN(CN)3(N2py)]2- (1-a), and 2-a is protonated upon the addition of p-toluene sulfonic acid (TsOH) to give [ReN(CN)3(H-N3py)]- (2-H) in DMSO. The protonation reaction does not occur for 3-a with TsOH in DMSO. All the complexes show one-electron redox waves of the Re(VI)/Re(V) and azolylpyridine ligand-centered processes in 0.1 M (n-C4H9)4NPF6-DMSO. All the complexes exhibit photoluminescence in DMSO and in the crystalline phase at 296 K. The emissive excited states of the complexes in DMSO were assigned to MLCT with a spin triplet nature. The emission band shifts to shorter and longer wavelengths upon protonation and deprotonation of the coordinated azolylpyridines, respectively. The emission color and intensity changes of 2-H and 2-a in the presence of acidic and basic vapors were investigated.

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.

Synthesis, Structures, and Photoluminescent Properties of Tricyanidonitridorhenium(V) Complexes with Bipyridine-Type Ligands

Tricyanidonitridorhenium(V) complexes with 2,2'-bipyridine (bpy) derivatives in which the 4 and 4' positions were substituted by X, [ReN(CN)₃(X₂bpy)]^- (X = NMe₂, NH₂, OMe, Me, Cl, and Br), were newly synthesized and characterized. The structures of the new complexes were determined by single-crystal X-ray analysis. UV-vis spectra of the complexes in dimethyl sulfoxide (DMSO) showed that the peak maximum wavelengths of rhenium-to-π* bpy-type-ligand charge transfer were in the range of 474-542 nm. Cyclic voltammograms in n-(C₄H₉)₄NPF₆-DMSO showed one-electron oxidation and reduction waves corresponding to the Re(VI/V) and X₂bpy^0/- processes, respectively. The new complexes and [ReN(CN)₃bpy]^- showed photoluminescence in the crystalline phase at 295 and 80 K and in DMSO at 295 K. The origin of the emission in DMSO was attributed to the triplet nature of the rhenium-to-π* bpy-type-ligand charge-transfer transition. Density functional theory calculations showed that the highest occupied and lowest unoccupied molecular orbitals were primarily localized on the d_xy orbital of the rhenium and π* orbitals of the bpy-type ligand, respectively.

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.

High-valent nitridorhenium(v) complexes containing PNP ligands: implications of ligand flexibility

The synthesis of (PNP)Re(N)X (PNP = [2-P(CHMe₂)₂-4-MeC₆H₃]₂N, X = Cl and Me) complexes is described.

Synthesis and characterization of rhenium(iii) complexes with (Ph2PCH2CH2)2NR diphosphinoamine ligands

The synthesis and characterization of a new series of neutral, six-coordinated compounds [Re^IIIX₃(PNPR)], where X is Cl or Br and PNPR is a diphosphinoamine having the general formula (Ph₂PCH₂CH₂)₂NR (R = H, CH₃, CH₂CH₃, CH₂CH₂CH₃, CH₂CH₂CH₂CH₃ and CH₂CH₂OCH₃) are reported. Stable [Re^IIIX₃(PNPR)] complexes were synthesized, in variable yields, starting from precursors where the metal was in different oxidation states (iii and v), by ligand-exchange and/or redox-substitution reactions. The compounds were characterized by elemental analysis, proton NMR spectroscopy, cyclic voltammetry, UV/vis spectroscopy, positive-ion electrospray ionization mass spectrometry (ESI(+)-MS) and X-ray diffraction analysis. Although the formulation of the complexes allows either meridional or facial isomers, the latter arrangement was prevalent both in the solid and solution states. Only [ReCl₃(PNPH)] showed a meridional configuration both in solution and in the crystalline state. [ReBr₃(PNPme)] prefers the meridional configuration in the crystalline state and the facial one in solution. While ESI(+)-MS and voltammetric data seem to indicate some dependency from the nature of the alkyl substituent at the nitrogen, the available structural data of the complexes show only slight differences both for angles and bond lengths upon change of the alkyl chain tethered to the nitrogen.

Assessment of the best N(3-) donors in preparation of [M(N)(PNP)]-based (M=(99m)Tc-; (188)Re) target-specific radiopharmaceuticals: Comparison among succinic dihydrazide (SDH), N-methyl-S-methyl dithiocarbazate (HDTCZ) and PEGylated N-methyl-S-methyl dithiocarbazate (HO2C-PEG600-DTCZ)

Succinic dihydrazide (SDH), N-methyl-S-methyl dithiocarbazate (HDTCZ) and PEGylated N-methyl-S-methyl dithiocarbazate (HO2C-PEG600-DTCZ) are nitrido nitrogen atom donors employed for the preparation of nitride [M(N)]-complexes (M=(99m)Tc and (188)Re). This study aims to compare the capability and the efficiency of these three N(3-) group donors, in the preparation of [M(N)PNP]-based target-specific compounds (M=(99m)Tc, (188)Re; PNP=aminodiphosphine). For this purpose, three different kit formulations (SDH kit; HO2C-PEG600-DTCZ kit; HDTCZ kit) were assembled and used in the preparation of [M(N)(cys~)(PNP3)](0/+) complexes (cys~=cysteine derivate ligands). For each formulation, the radiochemical yield (RCY) of the [M(N)(~cys)(PNP3)] compounds, was determined by HPLC. The deviation of the percentage of RCY, due to changes in concentration of the N(3-) donors and of the exchanging ligand, was determined. For (99m)Tc, data clearly show that HDTCZ is the most efficient donor of N(3-); however, SDH is the most suitable nitrido nitrogen atom donor for the preparation of [(99m)Tc(N)(PNP)]-based target-specific agents with high specific activity. When HO2C-PEG600-DTCZ or HDTCZ are used in N(3-) donation, high amounts of the exchanging ligand (10(-4)M) were required for the formation of the final complex in acceptable yield. The possibility to use microgram amounts of HDTCZ also in [(188)Re(N)] preparation (0.050mg) reduces its ability to compete in ligand exchange reactions, minimizing the quantity of chelators required to obtain the final complex in high yield. This finding can be exploit for increasing the radiolabeling efficiency in [(188)Re(N)]-radiopharmaceutical preparations compared to the previously reported HDTCZ-based procedure, notwithstanding a purification process could be necessary to improve the specific activity of the complexes.

(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.

99mTc-centered one-pot synthesis for preparation of 99mTc radiotracers

Nuclear medicine relies on two main imaging modalities: single photon emission computed tomography (SPECT) and positron emission tomography (PET). Radiopharmaceuticals (or radiotracers) are the blood stream of nuclear medicine for the diagnosis or therapy of diseases. Diagnostic radiotracers that are small molecules labelled with a gamma-emitter for SPECT or positron-emitter for PET provide a non-invasive method to assess the disease or disease states and monitor the therapeutic efficacy of a specific treatment regime. Over the past four decades, radiopharmaceutical research has been practising one-pot synthesis at the tracer level (10(-7)-10(-6) M). Many (99m)Tc radiotracers currently used in nuclear medicine are routinely prepared by following the basic principles of one-pot synthesis. Unlike traditional organic one-pot synthesis, which often involves the formation of multiple C-C and C-heteroatom bonds in a single step, the (99m)Tc-centered one-pot synthesis requires the formation of multiple coordination bonds between Tc and various donor atoms, such as N, O, S and P. This review will illustrate how the (99m)Tc-centered one-pot synthesis is utilized for routine preparations of different (99m)Tc radiotracers.

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.

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

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

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.

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.

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.

The [Tc(N)(PNP)]2+ metal fragment labeled cholecystokinin-8 (CCK8) peptide for CCK-2 receptors imaging:in vitro andin vivo studies

The radiolabeling of the natural octapeptide CCK8, derivatized with a cysteine residue (Cys-Gly-CCK8), by using the metal fragment [99mTc(N)(PNP3)]2+ (PNP3 = N,N-bis(dimethoxypropylphosphinoethyl)methoxyethylamine) is reported. The [99mTc(N)(NS-Cys-Gly-CCK8)(PNP3)]+ complex was obtained according to two methods (one-step or two-step procedure) that gave the desired compound in high yield. The complex is stable in aqueous solution and in phosphate buffer. In vitro challenge experiments with an excess of cysteine and glutathione indicate that no transchelation reactions occur, confirming the high thermodynamic stability and kinetic inertness of this compound. Stability studies carried out in human and mouse serum, as well as in mouse liver homogenates, show that the radiolabeled compound remains intact for prolonged incubation at 37 degrees C. Binding properties give Kd (19.0 +/- 4.6 nmol/l) and Bmax (approximately 10(6) sites/cell) values in A431 cells overexpressing the CCK2-R. In vivo evaluation of the compound shows rapid and specific targeting to CCK2-R, a fourfold higher accumulation compared to nonreceptor expressing tumors.

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.