Advances in technetium chemistry towards 99mTc receptor imaging agents

Transition Metal Chemistry -     

Chemical and biological evaluation of 99mTc (CO)3 and 99mTc complexes of some IDA derivatives

The confirmation that N-substituted imidodiacetic acids, as small and simple ligand systems containing amines and carboxylic acids, could be coordinated to the tricarbonyl core and form inert complexes with [^99mTc (CO)₃(H₂O)₃]⁺, is demonstrated. The HPLC quality control results of ^99mTc-carbonyl tagged IDA molecules, performed by gradient HPLC, have shown that HIDA, EHIDA and p-butyl-IDA form complexes with [^99mTc(CO)₃(H₂O)₃]⁺, with a labeling yield of ~90% for each of ^99mTc(CO)₃ IDA derivatives. However, the changes in the structure of labeled compounds, e.g., EHIDA, influence the changes in the biological behavior. In comparison with ^99mTc-EHIDA, the biliary excretion of ^99mTc(CO)₃ EHIDA was lower, but the urinary excretion higher. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

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 and comparison of 99mTc-labeled d-glucosamine derivatives with different 99mTc cores as potential tumor imaging agents

Isocyanide is a strong coordination ligand that can coordinate with [^99mTc(I)(CO)₃]⁺ core and [^99mTc(I)]⁺ core to produce stable ^99mTc complexes, therefore developing a ^99mTc-labeled isocyanide complex for single-photon emission computed tomography (SPECT) imaging is considered to be of great interest. In order to develop potential tumor imaging agents with satisfied tumor uptake and suitable pharmacokinetic properties in vivo, a novel d -glucosamine isocyanide derivative, 4-isocyano-N-(2,4,5-trihydroxy-6-(hydroxymethyl)tetrahydro-2H-pyran-3-yl)butanamide (CN3DG), was synthesized and radiolabeled with [^99mTc(I)]⁺ and [^99mTc(CO)₃]⁺ cores to obtain [^99mTc(CN3DG)₆]⁺ and [^99mTc(CO)₃(CN3DG)₃]⁺ in high radiolabeling yields (>95%). Both of the complexes showed good hydrophilicity and great stability in vitro. Cell uptake studies performed in S180 cells demonstrated they were transported into cells by glucose transporters. Biodistribution studies of the two complexes in mice bearing S180 tumor showed they had high tumor uptakes and rapid clearance from muscle and blood so that the tumor/blood and tumor/muscle ratios were high. By comparison, [^99mTc(CN3DG)₆]⁺ was superior to [^99mTc(CO)₃(CN3DG)₃]⁺ in regard to tumor uptake, tumor/blood and tumor/liver ratios. S180 tumors could be seen clearly from the SPECT/CT images with [^99mTc(CN3DG)₆]⁺. Considering its favorable properties, [^99mTc(CN3DG)₆]⁺ would be a promising tumor imaging agent and needs to be further studied.     

Radiosynthesis and evaluation of novel [99mTc(I)]+ and [99mTc(I)(CO)3]+ complexes with a 4-nitroimidazole isocyanide for imaging tumor hypoxia

[^99mTc(I)]⁺ and [^99mTc(I)(CO)₃]⁺ complexes with isocyanide exhibit high stability, which makes them suitable platforms to develop novel ^99mTc radiopharmaceuticals. To develop novel ^99mTc radiotracers for imaging hypoxia, in this study, a novel L ligand (4-nitroimidazole isocyanide derivative) was synthesized and labelled using [^99mTc(I)]⁺ core and [^99mTc(I)(CO)₃]⁺ core to produce [^99mTc(L)₆]⁺ and [^99mTc(CO)₃(L)₃]⁺ with high yields. To verify the structure of the ^99mTc complexes, corresponding rhenium analogues were synthesized and characterized. Both of the ^99mTc complexes were stable and hydrophilic. in vitro cellular uptake results showed they could exhibit good hypoxic selectivity. The evaluation of biodistribution in mice bearing S180 tumors indicated both of them could accumulate in tumor. Between them, [^99mTc(L)₆]⁺ exhibited higher tumor uptake and tumor/non-target ratio than [^99mTc(CO)₃(L)₃]⁺. Further, single photon emission computed tomography (SPECT) imaging studies of [^99mTc(L)₆]⁺ indicated an obvious accumulation in tumor and the value of the region-of-interest (ROI) ratio of the uptake for the tumor site to the corresponding non-tumor region was 5.64 ± 0.52. The above results suggested [^99mTc(L)₆]⁺ would be a potential tracer for imaging tumor hypoxia.     

Bridging the gap between in vitro and in vivo imaging: isostructural Re and 99mTc complexes for correlating fluorescence and radioimaging studies

A bifunctional ligand that is capable of forming Re and 99mTc complexes as complementary fluorescent and radioactive probes was developed. The tridentate bis(quinoline) amine ligand, which is referred to as the SAACQ system, was prepared in a single step from Fmoc protected lysine in high yield. Reaction of the SAACQ ligand with [Re(CO)3Br3]2- resulted in the formation of the SAACQ-(Re(CO)3)+complex which exhibits favorable fluorescence properties including a long lifetime and a large Stoke's shift. Because the SAACQ ligand is derived from an amino acid, it can readily be linked to or incorporated within peptides as a means of targeting the probe to specific receptors. To demonstrate this feature, the SAACQ ligand and the SAACQ-Re complex were incorporated into fMLFG, a peptide that binds to the formyl peptide receptor (FPR). Uptake of the fMLF[(SAACQ-Re(CO)3)+]G conjugate into human leukocytes in vitro was visualized by fluorescence microscopy, and the observed distribution of the peptide was similar to that of a well-established fluorescent FPR probe. The corresponding Tc complex, fMLF[(SAACQ-99mTc(CO)3)+]G, was prepared in excellent yield from [99mTc(CO)3(OH2)3]+, which affords the opportunity to correlate the results of the microscopy experiments with in vivo radioimaging studies because the probes are isostructural.     

99mTc—Sulphur colloid: A freeze-dried,single-step kit for liver imaging

The formulation of a freeze-dried sulphide colloid kit, to be directly labelled with technetium is based on the preparation of tin/II/ sulphide precolloid, stabilized with carboxy methyl cellulose. The formation kinetics of the labelled colloid assessed by GCS-method showing a fast reduction of^99mTc-pertechnetate, forming reduced^99mTc-colloid with subsequent build-up of the labelled sulphur colloid. This is assumed to be due to the formation of^99mTc/V/ to be bound to sulphide precolloid of the kit. The blood clearance data of the labelled colloid in rabbits showed two exponential disappearance phases of radioactivity from blood with biological half times of 3 and 67 min. The results of organ distribution in mice show that more than 85% of the administered activity is accumulated in the liver indicating a high colloidal yield of optimal particle size. The dynamic studies of the labelled colloid achieved in normal subjects using gamma camera indicate a rapid blood clearance and high liver uptake with low surrounding tissue background.This work was performed under the auspices of the Iraqi Atomic Energy Organization and presented in part at the Fifth International Symposium on Radiopharmaceutical Chemistry, Tokyo, August 1984.     

Rhenium fac tricarbonyl bisimine complexes: biologically useful fluorochromes for cell imaging applications

A series of lipophilic and hydrophilic fac tricarbonyl rhenium bisimine complexes have been prepared, their membrane-permeabilities explored in liposomes and their potential for application in fluorescence microscopy cell imaging demonstrated in the first application of MLCT-fluorescent rhenium complexes in cell imaging.     

99mTc(CO)3-labeled pamidronate and alendronate for bone imaging

Bone scintigraphy with (99m)Technetium-methylenediphosphonate ((99m)Tc-MDP) or (99m)Technetium-hydroxymethylenediphosphonate ((99m)Tc-HMDP) presents several limitations, namely low specificity, uncertainty in the radiopharmaceutical's molecular structure and long acquisition time after injection. Aiming to find bone-seeking radiotracers based on the core fac-[(99m)Tc(CO)(3)](+) with improved chemical and biological properties, we synthesized new conjugates (pz-PAM and pz-ALN), comprising a pyrazolyl-diamine chelating unit (pz: N,N,N donor atom set) for metal stabilization and a pendant pamidronate (PAM) or alendronate (ALN) moiety for bone targeting. The reaction of the conjugates with fac-[(99m)Tc(CO)(3)](+) yielded (> 95%) the stable complexes fac-[(99m)Tc(CO)(3)(pz-PAM)](-) (2a) and fac-[(99m)Tc(CO)(3)(pz-ALN)](-) (3a), which have been characterized by comparing their HPLC gamma-traces with the UV-vis traces of the Re surrogates 2 and 3, respectively. 2a and 3a bind strongly onto hydroxyapatite. The biodistribution studies in Balb-c mice have shown that 2a and 3a presented an high bone uptake (2a 18.3 ± 0.6% I.D./g, 3a 17.3 ± 6.1% I.D./g, at 1 h post injection), similar to (99m)Tc-MDP (17.1 ± 2.4% I.D./g, at 1 h post injection), with comparable clearance from most tissues and increased total excretion (2a 66% I.D., 3a 67% I.D. and (99m)Tc-MDP 49% I.D., at 1 h post injection). The bone-to-blood (2a 86.2, 3a 74.7) and the bone-to-muscle ratios (2a 77.7, 3a 79.0) are higher than the ones found for (99m)Tc-MDP (70.9, 47.9), at 4 h post injection. Planar whole-body gamma camera images of the rats injected with the (99m)Tc(CO)(3)-labeled pamidronate (2a) and alendronate (3a) confirmed the overall adequate biological profile of the new radiotracers for bone imaging.     

Rhenium analogues of promising renal imaging agents with a [99mTc(CO)3]+ core bound to cysteine-derived dipeptides, including lanthionine

The coordination chemistry of lanthionine (LANH2) and cystathionine (CSTH2) dipeptides, which respectively consist of two cysteines and one cysteine and one homocysteine linked by a thioether bridge, is almost unstudied. Recently for fac-[99mTc(CO)3(LAN)]- isomers, the first small 99mTc(CO)3 agents evaluated in humans were found to give excellent renal images and to have a high specificity for renal excretion. Herein we report the synthesis and characterization of Re complexes useful for interpreting the nature of tracer 99mTc radiopharmaceuticals. Treatment of [Re(CO)3(H2O)3]OTf with commercially available LANH2 (a mixture of meso (d,l) and chiral (dd,ll) isomers) gave three HPLC peaks, 1A, 1B, and 1C, but treatment with CSTH2 (l,l isomer) gave one major product, Re(CO)3(CSTH) (2). Crystalline Re(CO)3(LANH) products were best obtained with synthetic LANH2, richer in meso or chiral isomers. X-ray crystallography showed that these dipeptides coordinate as tridentate N2S-bound ligands with two dangling carboxyls. The LANH ligand is meso in 1A and 1C and chiral in 1B. While 1A (kinetically favored) is stable at ambient temperature for days, it converted into 1C (thermodynamically favored) at 100 degrees C; after 6 h, equilibrium was reached at a 1A:1C ratio of 1:2 at pH 8. The structures provide a rationale for this behavior and for the fact that 1A and 1C have simple NMR spectra. This simplicity results from fluxional interchange between an enantiomer with both chelate rings having the same delta pucker and an enantiomer with both chelate rings having the same lambda pucker. Agents with the [99mTc(CO)3]+ core and N2S ligands show promise of becoming an important class of 99mTc radiopharmaceuticals. The chemistry of Re analogues with these ligands, such as the LAN2- complexes reported here, provides a useful background for designing new small agents and also tagged large agents because two uncoordinated carboxyl groups are available for conjugation with biological molecules such as proteins.     

Preparation of two 99mTc(CO)3 labelled complexes with a 4-nitroimidazole isocyanide at different temperatures for molecular imaging of tumor hypoxia

Journal of Radioanalytical and Nuclear Chemistry - A 4-nitroimidazole isocyanide derivative (6) was synthesized and radiolabelled with 99mTc(CO)3 core in high yield. It was interesting to note that...     

Preparation and evaluation of a new neurotensin analog labeled with 99mTc for targeted imaging of neurotensin receptor positive tumors

Neurotensin receptors are overexpressed in several human tumors and can be targets for tumors diagnosis and therapy. In this study, a new neurotensin analogue was labeled with ^99mTc via HYNIC and tricine/EDDA as coligands and investigated further. [HYNIC⁰, Gly⁷, Lys⁹, d-Tyr¹¹]-Neurotensin (7–13) was synthesized using a standard Fmoc strategy. Labeling with ^99mTc was performed at 100 °C for 10 min and radiochemical analysis involved ITLC and HPLC methods. The stability of radiopeptide was checked in the presence of humane serum at 37 °C up to 24 h. The receptor bound internalization and externalization rates were studied in neurotensin receptor expressing HT-29 cells. Biodistribution of radiopeptide was studied in nude mice bearing HT-29 tumor. Labeling yield of 98.6 ± 0.54 % (n = 3) was obtained corresponding to a specific activity of 81 MBq/nmol. Peptide conjugate showed good stability in the presence of human serum. The radioligand showed specific internalization into HT-29 cells (12.43 ± 0.52 % at 4 h). In biodistribution studies, a receptor-specific uptake was observed in neurotensin receptor positive organs so that after 1 h the uptakes in mouse intestine and tumor were 0.87 ± 0.16 and 0.63 ± 0.12 % ID/g respectively.     

Synthesis and biodistribution of a novel 99mTc nitrido radiopharmaceutical with proline dithiocarbamate as a potential tumor imaging agent

In the present study, proline dithiocarbamate (PRODTC) ligand was radiolabeled with the [^99mTc≡N]²⁺ core successfully to obtain the ^99mTcN-PRODTC complex with high radiochemical purity. No decomposition of the complex at room temperature was observed over a period of 6 h. Its partition coefficient indicated that it was a hydrophilic complex. The electrophoresis results showed that the complex was negative. The biodistribution of ^99mTcN-PRODTC in mice bearing S 180 tumor showed that the complex accumulated in the tumor with a certain uptake. The tumor/blood and tumor/muscle ratios reached 2.19 and 4.54 at 2 h post-injection, suggesting it would be a promising candidate for tumor imaging.     

Synthesis and biological evaluation of novel 99mTc-oxo and 99mTc-nitrido complexes with phenylalanine dithiocarbamate for tumor imaging

In this study, phenylalanine dithiocarbamate (PHEDTC) ligand was successfully synthesized and then radiolabeled with [^99mTcO]³⁺ core and [^99mTc≡N]²⁺ core to produce ^99mTcO–PHEDTC and ^99mTcN–PHEDTC, respectively. Both complexes were prepared with high radiochemical purity and had good stability. The partition coefficient results showed they were hydrophilic, while ^99mTcN–PHEDTC was more hydrophilic than ^99mTcO–PHEDTC. The biodistribution study in mice bearing S 180 tumor showed that ^99mTcO–PHEDTC and ^99mTcN–PHEDTC had high tumor uptake at 2 h post-injection, 1.91 and 1.21, respectively. The good uptake and retention in tumor together with favorable tumor-to-muscle ratios make them promising candidates for further evaluation as potential tumor imaging agents.     

Preparation and characterization of [99TcO] apcitide: a technetium labeled peptide

[99mTcO] apcitide (99mTcO(P246)), the technetium complex of the 13 amino acid, apcitide, cyclo-(D-Tyr-Apc-Gly-Asp-Cys)-Gly-Gly-Cys(Acm)-Gly-Cys(Acm)-Gly-Gly-Cys-NH2, where Apc is L-[S-(3-aminopropyl)]cysteine (an arginine mimetic) and Acm is the acetamidomethyl protecting group, has high affinity and selectivity for the GPIIb/IIIa receptor that is expressed on the membrane surface of activated platelets and plays an integral role in platelet aggregation and thrombus formation. Bibapcitide, a 26 amino acid, bis-succinimidomethyl ether-linked dimer of the peptide apcitide has been formulated as a single-vial, lyophilized kit having the trade name AcuTect. When sterile, nonpyrogenic sodium pertechnetate (99mTcO4-) in 0.9% sodium chloride is added to the AcuTect radiopharmaceutical kit and the resulting kit is heated, [99mTcO] apcitide forms. This is the first radiopharmaceutical to target acute deep vein thrombosis (DVT) in the lower extremities. We report here the preparation, purification, and isolation of the 99Tc complex of apcitide and its characterization to determine the mode of binding of Tc to apcitide. [99TcO] apcitide was prepared, on the macroscopic level, by reaction of [99TcOCl4]- with apcitide, purified by preparative HPLC and isolated as a trifluoroacetate salt. [99TcO] apcitide can also be formed from the reaction of bibapcitide and 99TcO4- in the presence of Sn(II) and glucoheptonate at 80 degrees C, conditions that mimic the radiopharmaceutical kit preparation. FTIR data show a Tc=O stretch at 961.2 cm(-1), in the range observed for anionic [TcVO]3+ amide thiolate complexes. The mass spectral data is in agreement with the formula, [C51H73O20N17S5Tc]-, consistent with retention of Acm groups and the Tc binding in the Gly11-Gly12-Cys13 region of the peptide. Despite significant spectral overlap due to numerous similar amino acids, all protons of apcitide and [99TcO] apcitide were unambiguously assigned. The observation of two nonequivalent Acm groups and the observation of only 10 NH-CH cross-peaks in the TOCSY and COSY spectra of [99TcO] apcitide (NH-CH cross-peaks were absent for Gly11-Gly12-Cys13), compared to all 13 cross-peaks found in apcitide, provided compelling evidence to support the 99Tc binding to the terminal Gly11-Gly12-Cys13 region of apcitide.