Evolution of Tc-99m in diagnostic radiopharmaceuticals

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

Technetium-99m radiopharmaceutical preparation by surface adsorbed stannous ions

A method of producing technetium-99m (99mTc) radiopharmaceuticals is described, where reduction of pertechnetate occurs through stannous ions adsorbed on the surface of an infusion catheter. This leads to radiopharmaceuticals containing microgram quantities of stannous ions and that, therefore, results in minimal blood-pool labeling and essentially the elimination of tin colloids. Other advantages of the method include a reduction in quantities of ligand required and the possibility to mass produce the "tinned" catheters as technetium "reduction" kits. A wide variety of 99mTc radiopharmaceuticals have been prepared in high yield. Excellent biodistribution in several of these is demonstrated.     

Detection of endotoxins in radiopharmaceutical preparations--III. Limulus test assessment using radiopharmaceutical preparations; correlation with the rabbit pyrogen test

Experiments using 17 radiopharmaceuticals containing known amounts of added endotoxin show that none of them inhibits the pyrogenic reaction of the rabbit. Gelation of the Limulus amoebocyte lysate (LAL) is inhibited by 4 of them: colloidal erbium 169Er citrate, colloidal rhenium 186Re sulfide, colloidal technetium 99mTc (Re) sulfide for liver scintigraphy and the colloidal technetium 99mTc (Re) sulfide for lymphography. This inhibition is cancelled, either by dilution or after neutral pH adjustment. Both controls were performed on 313 batches of various radiopharmaceuticals, 95% of results were identical (93% negative, 2% positive). The remaining 5% correspond to positive LAL tests vs negative rabbit tests on the same batches. No negative LAL test vs positive rabbit test was observed.     

Evaluation of formamidine sulfinic acid and other reducing agents for use in the preparation of Tc-99m labeled radiopharmaceuticals.

Various reducing agents have been evaluated for their potential usefulness in the preparation of 99mTc labeled radiopharmaceuticals for use in nuclear medicine. Adequate labeling of various radiopharmaceuticals was accomplished using formamidine sulfinic acid. Nitrogen-purging of solutions is not required, which is an advantage for in-house preparation. Tagging requires heating, however, so heat-labile material cannot be used. Various compounds that could not be labeled when stannous chloride was used, could be tagged with 99mTc when formanidine sulfinic acid was used as the reducing agent.     

Microautoradiographic study of technetium-99m colloid uptake by the rat liver

A new microautoradiographic technique was developed to study the distribution of 99mTc-labeled radiopharmaceuticals. Using a thick emulsion, it is possible to get microscopically visible tracks of internal conversion and Auger electrons. The liver uptake of microscopic particles has been thought to occur in Kupffer cells but no direct evidence has been provided for technetium colloids. Using this method, 99mTc-labeled colloids were clearly identified in Kupffer cells in the sinusoidal areas of liver. "Track" microautoradiography using a thick emulsion layer may be used on any frozen tissue sections and may provide an important tool to assess the biodistribution of 99mTc radiopharmaceuticals.     

Development of technetium-99m-based CNS receptor ligands: have there been any advances?

By virtue of its ideal nuclear physical characteristics for routine nuclear medicine diagnostics and its ready availability, technetium-99m is of outstanding interest in the development of novel radiopharmaceuticals. The potential for the development of 99mTc-based radioligands for the study the receptor function in the central nervous system (CNS) is also well recognised despite the difficulties to be overcome. A fundamental challenge is the pharmacologically acceptable integration of the transition metal technetium, with its specific coordination chemistry, into the molecular entity of CNS receptor ligands. Conceptually, the ligand molecule can be assembled by three building blocks: a small neutral chelate unit, an organic linker that may also serve as a pharmacological modifier and a receptor-binding region derived from selective receptor antagonists. The recent introduction of novel technetium chelate units, particularly mixed-ligand complexes and low-valency organometallic compounds of technetium, provides an impetus for the further development of CNS receptor ligands. Moreover, progress in receptor pharmacology and the experience gained with positron emission tomography radiotracers have facilitated the design of numerous 99mTc-based CNS receptor ligands. The formidable challenge of developing 99mTc probes as single-photon emission tomography imaging agents targeting CNS receptors can be viewed with optimism given the successful development of [99mTc]TRODAT-1 as a 99mTc complex for imaging dopamine transporters in the brain, although there are a number of receptor-specific imaging agents that have so far resisted all efforts to develop them. This review presents recent advances and discusses the remaining hurdles in the design of 99mTc-based CNS receptor imaging agents.     

Preparation and characterization of technetium complexes with Schiff base and phosphine coordination. 1. Complexes of technetium-99g and -99m with substituted acac2en and trialkyl phosphines (where acac2en = N,N'-ethylenebis[acetylacetone iminato])

A series of 23 technetium(III) complexes of the type [TcL(PR3)2]+, where L represents a tetradentate Schiff base ligand in the equatorial plane and PR3 represents the axial phosphine ligands, are reported. Full ligand syntheses and characterizations are included. The technetium complexes were prepared with 99mTc to study the organ distribution in guinea pigs at 5 and 60 min postinjection. Four prototypical complexes of the series were also prepared with either 99gTc or 99gTc/99mTc (designated as carrier-added) to allow macroscopic characterization. Equivalence of the 99gTc and 99mTc complexes was demonstrated by dual detection high performance liquid chromatography (HPLC) techniques. The development of a one-step preparation from the standard two-step method is discussed for some complexes. Biodistribution data are related to structure and lipophilicity. None of the complexes in the series exhibited a tendency for in vivo reduction. Myocardial uptake was favorable for a number of complexes. The optimal agent from this series for further imaging development was chosen based on myocardial uptake, rapid blood and liver clearance, and ability to be formulated as a one-step kit.     

Synthesis and biological evaluation of silylated mixed-ligand 99mTc complexes with the [PNS/S] donor atom set

New oxotechnetium complexes of general formula [99mTc(O)(PNS)(S(CH2)nOSiR3)] (4-6) were synthesized by direct reduction of [99mTcO4]- with stannous chloride, in the presence of the tridentate heterofunctionalized phosphine H2PNS and of the monodentate silylated thiols [HS(CH2)nOSiR3] (n = 2, R = Ph (1); n = 3, R = Ph (2); n = 3, R = Et (3)). The mixed-ligand rhenium and technetium complexes of general formula [M(O)(PNS)(S(CH2)nOH)] (n = 2: M = 99mTc, (7), M = Re, (7a); n = 3: M = 99mTc, (8), M = Re, (8a)) were also prepared. All the 99mTc complexes were obtained with high radiochemical purity (> 95%), after purification by HPLC, and were characterized by comparison of their HPLC profiles with the ones obtained for the corresponding Re compounds. The silylated compounds 4-6 are stable in phosphate saline buffer (PBS) pH 7.4, rat plasma, human serum and whole blood, and do not bind to plasmatic proteins, and also do not challenge with glutathione. The biological behavior of [99mTc(O)(PNS)(S(CH2)nOH)] (7, 8) and [99mTc(O)(PNS)(S(CH2)nOSiR3)] (4-6) was studied. The effect of the pH on the cleavage of the O-Si bond in complexes 4-6 was also evaluated.     

New radiopharmaceuticals for oncologic diagnosis and therapy: developments in radioimmunoscintigraphy and radioimmunotherapy

131I labeled monoclonal antibodies are at a disadvantage for radioimmunoscintigraphy due to the in-vivo cleavage of radioiodine by deiodases. Alternative labeling with 99mTc was not very successful because of the lack of suitable ligands for stable technetium complexes. New developments in this field will be presented using stable 99mTc-N2S2 complexes. Some comments on radioimmunotherapy have also been added.     

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

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

Synthesis, characterization and biological evaluation of a novel "3 + 1" mixed ligand 99mTc complex having an aliphatic thiol as coligand.

A novel "3 + 1" mixed ligand 99mTc complex with N,N-bis(2-mercaptoethyl)-N'N'-diethyl-ethilenediamine as ligand and 1-octanethiol as coligand was prepared and evaluated as potential brain radiopharmaceutical. Preparation at tracer level was accomplished by substitution, using 99mTc-glucoheptonate as precursor and a coligand/ligand ratio of 5. Under these conditions the labeling yield was over 80% and a major product with radiochemical purity >80% was isolated by HPLC methods and used for biological evaluation. Chemical characterization at carrier level was developed using the corresponding rhenium and 99gTc complexes. Results were consistent with the expected "3 + 1" structure and X-ray diffraction study demonstrated that the complex adopted a distorted trigonal bipyramidal geometry. All sulphur atoms underwent ionization leading to the formation of a neutral compound. Biodistribution in mice demonstrated early brain uptake, fast blood clearance and excretion through hepatobiliary system. Although brain/blood ratio increased significantly with time, this novel 99mTc complex did not exhibit ideal properties as brain perfusion radiopharmaceutical since brain uptake was too low.     

New technetium 99m-labeled brain perfusion imaging agents

In developing new brain perfusion imaging agents, three types of neutral and lipid-soluble technetium 99m complexes have been reported: 99mTc-ethylene cysteinate dimer, 99mTc-hexamethyl propylene amine oxime, and 99mTc-boric acid adduct of technetium oxime. Structure-activity relationships of these complexes on brain uptake and retention have been investigated. The new 99mTc-brain perfusion imaging agents are useful in detecting various cerebral vascular abnormalities. The exact trapping mechanisms and their relationship with potential clinical applications still remain to be elucidated. However, initial clinical studies with 99mTc agents do show parallel clinical efficacy as those reported for the iodine 123-labeled brain perfusion agents, isopropyliodoamphetamine and N,N,N'-trimethyl-N'-[2-hydroxy-3-methyl-5-iodobenzyl]-1,3-propanediamine .     

A simple kit for the preparation of 99mTc-labeled red blood cells.

A simple kit for the preparation of 99mTc-labeled red blood cells in a closed sterile system is described. Whole blood (3-6 ml) is automatically drawn from the patient into a 10-ml Vacutainer tube, containing a freeze-dried stannous citrate formulation, with heparin as an anticoagulant. The kit provides consistent 97% yields in 20 min with small blood samples. The radionuclide is added at the end of the labeling sequence to minimize operator exposure. Freeze-dried kits have performed well after more than 12 months. Some technetium solutions produced low labeling yields. Experimental evidence suggests that the poor yields were due to carrier 99Tc. This effect may explain low labeling yields obtained with some other radiopharmaceuticals.     

Preincubation with Sn-complexes causes intensive intracellular retention of 99mTc in thyroid cells in vitro

Technetium radiopharmaceuticals are well established in nuclear medicine. Besides its well-known gamma radiation, 99mTc emits an average of five Auger and internal conversion electrons per decay. The biological toxicity of these low-energy, high-LET (linear energy transfer) emissions is a controversial subject. One aim of this study was to estimate in a cell model how much 99mTc can be present in exposed cells and which radiobiological effects could be estimated in 99mTc-overloaded cells. Methods: Sodium iodine symporter (NIS)-positive thyroid cells were used. 99mTc-uptake studies were performed after preincubation with a non-radioactive (cold) stannous pyrophosphate kit solution or as a standard 99mTc pyrophosphate kit preparation or with pure pertechnetate solution. Survival curves were analyzed from colony-forming assays. Results: Preincubation with stannous complexes causes irreversible intracellular radioactivity retention of 99mTc and is followed by further pertechnetate influx to an unexpectedly high 99mTc level. The uptake of 99mTc pertechnetate in NIS-positive cells can be modified using stannous pyrophosphate from 3-5% to >80%. The maximum possible cellular uptake of 99mTc was 90Bq/cell. Compared with nearly pure extracellular irradiation from routine 99mTc complexes, cell survival was reduced by 3-4 orders of magnitude after preincubation with stannous pyrophosphate. Conclusions: Intracellular 99mTc retention is related to reduced survival, which is most likely mediated by the emission of low-energy electrons. Our findings show that the described experiments constitute a simple and useful in vitro model for radiobiological investigations in a cell model.     

Preparation and evaluation of 99mTc-labeled cyclic arginine–glycine–aspartate (RGD) peptide for integrin targeting

Technetium coordination chemistry has been a subject of interest in the development of radiopharmaceuticals, especially imaging radiotracers. Due to the extensive work done on developing chelates for ^99mTc, various chelators have been investigated and applied to radiopharmceuticals. Previous studies on the coordination chemistry of the [^99mTc=O] core have established peptide-derived sequences as effective chelating ligands. These observations led to the design of tetradentate ligands derived from amino acid sequences. Such amino acid sequences provide a tetradentate coordination site for chelation to the radionuclide and an effective functional group for conjugation to biomolecules using conventional solid-phase synthetic routes. A derivative of a novel tripeptide chelating sequence, Pro–Gly–Cys (PGC) has been developed where it is possible to form stable technetium complexes with the [^99mTc=O] via N₃S₁ tetradentate coordination core that serves this function and can be readily incorporated into biomolecules using solid-phase synthesis techniques. As a model system, the RGD peptide was selected which has been well known to target the integrin receptor for angiogenesis and tumor imaging agents. The results of in vivo studies with these novel radiolabeled compounds in tumor xenografts demonstrated a distribution in tumor targeting and other organs, such as kidney, liver and intestines.     

Radioguided sentinel node detection in breast cancer patients: comparison of 99mTc phytate and 99mTc rhenium colloid efficacy

BACKGROUND: Radioguided sentinel node biopsy (SNB) of breast cancer patients has become a standard method for detecting early stage breast cancer. However, no standard radiopharmaceutical exists. METHODS: 99mTc rhenium colloid or 99mTc phytate SNB was used to aid detection in breast cancer patients. For each radiopharmaceutical, 100 patients were examined. The following points were compared: (1) scintigraphic detection rate of axillary sentinel nodes (detectability and number when detectable) and internal mammary sentinel nodes; (2) the number of nodes detected scintigraphically and the number detected during surgery; (3) sensitivity, specificity, accuracy, negative predictive value, and positive predictive value for axillary sentinel nodes. RESULTS: Axillary sentinel nodes of patients were biopsied using either 99mTc rhenium or 99mTc phytate. The number of axillary nodes surgically removed from patients given 99mTc rhenium was 2.28+/-1.08 (mean+/-SD), and the number of axillary nodes surgically removed from patients given 99mTc phytate was 1.68+/-0.82. Some patients given 99mTc rhenium showed a spill-over of radioactivity from sentinel nodes. Concordance of scintigraphically detected nodes and surgical removed nodes was superior for 99mTc phytate compared to that with 99mTc rhenium, with a statistically significant difference. The sensitivity and negative predictive value was superior with 99mTc phytate compared to that with 99mTc rhenium, even though no statistical difference was detectable. However, visualization of internal mammary nodes was superior with 99mTc rhenium. CONCLUSION: In breast cancer patients, 99mTc phytate is a better choice for the detection of axillary SNB than 99mTc rhenium colloid. However, 99mTc rhenium colloid is a better choice for the detection of internal mammary nodes.     

Synthesis and preliminary evaluation of Tc-99m-labeled somatostatin analog (RC-160) using "3+1" mixed ligand approach

The success of (111)In-pentetreotide as a cancer-imaging agent has given impetus to the search for other peptide-based radiopharmaceuticals. The labeling with Tc-99m has become even more attractive because of the ready availability and near ideal physical properties. Additionally, the kinetics of the peptide-receptor interactions favors the radiolabeling with technetium-99m. A somatostatin analog RC-160 has been labeled with Tc-99m using the "3+1" mixed ligand approach utilizing the NNS/S coordination sites. The ternary complex was formed in greater than 95% within 30 min by simultaneous reduction and complexation of technetium-99m pertechnetate. The Tc-99m and the surrogate rhenium complexes showed similar chromatographic behavior. The complex was evaluated by in vitro receptor binding studies carried out on HTB-121 breast cancer cell line and biodistribution studies performed in normal mice. Our findings suggest that RC-160 can be labeled by the mixed ligand approach with the complex retaining its biological activity and warrants further studies.     

Synthesis and structural characterization of a Re(V) complex with a 2N1S donor and the radiochemical behavior of its Tc analog.

A rhenium(V) complex with 2-(2-pyridylmethylthio)-aniline has been synthesized and characterized by UV-VIS absorption, IR, MS and X-ray crystallography. The complex contains a ReO3+ moiety and the amine compound, acting as a monobasic tridentate(NSN) ligand. The chemical behavior of its technetium analog has been studied radioanalytically by solvent extraction, TLC, electrophoresis and HPLC using 99mTc tracer.     

Cationic complexes of technetium for myocardial imaging

Over the past 15 years a major goal of research in cardiovascular nuclear medicine has been the development of 99mTc complexes that could replace 201Tl and thus enhance the utility of myocardial perfusion imaging. This paper presents an overview of the current state-of-art of the development of cationic 99mTc complexes for this purpose. Cationic 99mTc complexes that have been evaluated as myocardial perfusion imaging agents in human volunteers and/or animals are discussed and classified on the basis of the oxidation state of the technetium center.