Radiochemical studies of 99mTc complexes of modified cysteine ligands and bifunctional chelating agents

The synthesis of four novel ligands using the amino-acid cysteine and its ethyl carboxylate derivative is described. The synthetic method involves a two-step procedure, wherein the intermediate Schiff base formed by the condensation of the amino group of the cysteine substrate and salicylaldehyde is reduced to give the target ligands. The intermediates and the final products were characterized by high resolution nuclear magnetic resonance spectroscopy. Complexation studies of the ligands with ^99mTc were optimized using stannous tartrate as the reducing agent under varying reaction conditions. The complexes were characterized using standard quality control techniques such as thin layer chromatography, paper electrophoresis, and paper chromatography. Lipophilicities of the complexes were estimated by solvent extraction into chloroform. Substantial changes in net charge and lipophilicity of the ^99mTc complexes were observed on substituting the carboxylic acid functionality in ligands I and II with the ethyl carboxylate groups (ligands III and IV). All the ligands formed ^99mTc complexes in high yield. Whereas the complexes with ligands I and II were observed to be hydrophilic in nature and not extractable into CHCl₃, ligands III and IV resulted in neutral and lipophilic ^99mTc complexes. The ^99mTc complex with ligand III was not stable and on storage formed a hydrophilic and nonextractable species. The biodistribution of the complexes of ligands I and II showed that they cleared predominantly through the kidneys, whereas the complexes with ligands III and IV were excreted primarily through the hepatobiliary system. No significant brain uptake was observed with the ^99mTc complexes with ligands III and IV despite their favorable properties of neutrality, lipophilicity, and conversion into a hydrophilic species. These ligands offer potential for use as bifunctional chelating agents.     

Synthesis and evaluation of copper radiopharmaceuticals with mixed bis(thiosemicarbazone) ligands

Four “mixed” bis(thiosemicarbazone) derivatives of pyruvaldehyde were synthesized that incorporate two dissimilar thiosemicarbazone functions. The corresponding [⁶⁷Cu]copper(II) complexes were prepared and evaluated as possible copper radiopharmaceuticals. The pyruvaldehyde-based mixed bis(thiosemicarbazone) ligands, CH₃C[=NNHC(S)NHMe]CH[=NNHC(S)NHEt] (1), CH₃C[=NNHC(S)NHMe]CH[=NNHC(S)NEt₂] (2), CH₃C[=NNHC(S)NHMe]CH[=NNHC(S)-cyclo-N(CH₂)₅] (3), and CH₃C[=NNHC(S)NHMe]CH[=NNHC(S)-cyclo-N(CH₂)₆] (4), were obtained by reaction of the appropriate thiosemicarbazide derivative with pyruvaldehyde-2-N⁴-methylthiosemicarbazone (CH₃C[=NNHC(S)NHMe]CHO). The ⁶⁷Cu-labeled copper(II) complexes of ligands 1-4 were prepared and screened in a rat model to assess the potential of each chelate as a ⁶²Cu-radiopharmaceutical for imaging with positron emission tomography. The ⁶⁷Cu-complexes of ligands 1–4 exhibit significant uptake into the brain and heart 1 min following intravenous administration to rats. For the ⁶⁷Cu-complexes of ligands 2, 3, and 4, the cerebral and myocardial uptake of ⁶⁷Cu is two-to-threefold lower at 2 h than at 1 min postinjection, due to significant biological clearance of these ⁶⁷Cu-chelates. However, the ⁶⁷Cu-complex of 1 affords cerebral and myocardial uptake and retention comparable to that of [⁶⁷Cu]Cu-PTSM in this model. Although the kinetics of this new agent appear attractive, ultrafiltration studies using solutions of dog and human serum albumin reveal that the ⁶⁷Cu-complex of ligand 1, like Cu-PTSM, interacts more strongly with human albumin than dog albumin. Thus, this new agent would appear to offer no advantage over Cu-PTSM as a ⁶²Cu-labeled tracer for evaluation of regional tissue perfusion.     

99mTc(L-L)3+ complexes containing ether analogs of DMPE

Novel ^99mTc(L-L)₃⁺ complexes have been investigated for potential use in myocardial perfusion imaging. Bidentate chelators have been prepared that are based on substituent analogs of 1,2-bis(dimethylphosphino)ethane, onto which alkyl ether groups have been incorporated. The new ligands are: (1) MMPE, 1,2-bis(methyl methoxyethyl phosphino)ethane, (2) MIBPE, 1,2-bis(methyl methoxyisobutyl phosphino)ethane, (3) FURPE, 1,2-bis(methyl tetrahydrofuran phosphino)ethane, and (4) PYRPE, 1,2-bis(methyl tetrahydropyran phosphino)ethane. These ligands have been reacted with ^99mTc and the resulting complexes evaluated for myocardial imaging properties. ^99mTc(MMPE)₃⁺ exhibited the most favorable myocardial imaging characteristics in animal models. Results indicate that pendent ether moieties can improve the myocardial imaging properties of cationic technetium complexes.     

Complexation and biodistribution study of 111In and 90Y complexes of bifunctional phosphinic acid analogs of H4dota

In this study, the complexation rates of two new phosphinate H₄dota (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) analogs, H₅do3ap^PrA and H₄do3ap^ABn, and H₄dota itself were compared under identical conditions (H₅do3ap^PrA=10-{[(2-carboxyethyl)hydroxyphosphoryl]methyl}-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid; H₄do3ap^ABn=10-{[(4-aminophenyl)hydroxyphosphoryl]methyl}-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid). The biodistribution of their ¹¹¹In and ⁹⁰Y complexes in healthy rats was also studied. Unlike the observation obtained under “chemical” conditions where differences between the ligands were observed no such differences in complexation rates were found under radiochemical conditions. The ligands bind the radiometals similarly to H₄dota. So, “chemical” formation kinetic data should be transferred into radiochemical conditions only with high care and “radiochemical” complexation experiments should be run as part of standard in vitro studies with new ligands considered as potential radiopharmaceuticals. Pharmacokinetic studies in rats showed similar distribution characteristics for both phosphinate H₄dota analogs radiolabelled with ¹¹¹In and ⁹⁰Y when compared with that of the ¹¹¹In–H₄dota complex. No specific uptake in any organ and tissue of rats was determined. The phosphinate complexes are not accumulated in calcified tissues.     

Electrolytic 99mTcO4− reduction: A different pathway to obtain 99mTc-labelled compounds

Electrolytic reduction of ^99mTcO₄⁻ at inert electrodes to obtain ^99mTc cationic complexes and in vitro stability of labelled compounds were studied. Amines were used as neutral N-donor ligands and a systematic analysis of various parameters involved in the reduction process was performed.Usefulness of electrolytic reduction was proved as an alternative ^99mTc-labelling method. Its most important advantages are: production of complexes with a high radiochemical purity, negligible presence of red-hyd-^99mTc, lack of foreign materials, simplicity of development and possibility of further applications.     

A new efficient method for the preparation of 99mTc-radiopharmaceuticals containing the TcN multiple bond

An improved method for the preparation of ^99mTc-radiopharmaceuticals containing the TcN multiple bond, in sterile and apyrogen conditions, is described. This method is based on the reaction of [^99mTc]pertechnetate with ligands derived from S-methyl dithiocarbazate [H₂NN(R)C(S)SCH₃ [R = H, CH₃], in the presence of HCl and tertiary phosphines. It was found that these derivatives can behave both as sources of nitride nitrogen ions (N³⁻) and as coordinating ligands. The reaction leads to the formation of intermediate technetium-nitrido complexes in high yield. These intermediate species can be used as suitable prereduced substrates for the preparation of technetium-nitrido radiopharmaceuticals through simple substitution reactions with appropriate exchanging ligands.     

Evaluation of potential heart affine compounds by use of the isolated perfused rat heart

^99mTc-organocation complexes were screened for heart affinity using the isolated perfused rat heart. The ^99mTc-I compounds are better extracted by the heart and retained in it than the ^99mTc-III compounds. ^99mTcCl₂(DMPE) ₂ ⁺ shows low uptake (15%) and fast washout, but no impairment of uptake after incubation with human serum albumin(HSA), with a modified preparation, initial uptake is not much better (18%) but washout is clearly diminished. ^99mTc(DMPE) ₃ ⁺ is taken up to a greater extent (33%) if administered solely, but when given in conjunction with HSA, uptake decreases to about 4% because of strong affinity of the complex to the protein, but this compound persists well in the tissue. Detergents like tween 80 and dioctylsulphosuccinate are able to cause higher uptake of ^99mTc(DMPE) ₃ ⁺ even in the presence of HSA, thus indicating a diminished binding of the compound to the protein. ^99mTc(CN-t-butyl) ₆ ⁺ shows very high uptake (80%) by the heart, which is only moderately diminished by addition of HSA (t0 38%), and persists excellently in the myocardium.Basic features of the compounds such as accumulation in and elimination from the myocardium as well as plasma protein binding and their mutual relationships are well reflected by the isolated heart model.     

Design of hypoxia-targeting radiopharmaceuticals: selective uptake of copper-64 complexes in hypoxic cells in vitro

The well-known perfusion tracer CuPTSM, labelled with ⁶²Cu or ⁶⁴Cu, is believed to be trapped in cells non-selectively by a bioreductive mechanism. It is proposed that by modifying the ligand to increase its electron donor strength (for example by adding alkyl functionality or replacing sulphur ligands with oxygen ligands), the copper complexes will become less easily reduced and tracers with selectivity for hypoxic tissues could thus be developed. The aim of this work was to prepare ⁶⁴Cu-labelled complexes of two series of ligands, based on the bis(thiosemicarbazone) (13 ligands) and bis(salicylaldimine) (3 ligands) skeletons, and to evaluate the hypoxia dependence of their uptake in cells. The complexes were incubated with Chinese hamster ovary cells under normoxic and hypoxic conditions, and the cells isolated by centrifugation to determine radioactivity uptake at various time points up to 90 min. Several members of both series demonstrated significant (P<0.05) or highly significant (P<0.01) hypoxia selectivity, indicating that both series of complexes offer a basis for development of hypoxia-targeting radiopharmaceuticals for positron emission tomography (⁶⁰Cu, ⁶¹Cu, ⁶²Cu, ⁶⁴Cu) and targeted radiotherapy (⁶⁴Cu, ⁶⁷Cu). Received 30 March 1998     

Synthesis and biodistribution of lipophilic and monocationic gallium radiopharmaceuticals derived from N,N′-bis(3-aminopropyl)-N,N′-dimethylethylenediamine: potential agents for PET myocardial imaging with 68Ga

IntroductionIn locations that lack nearby cyclotron facilities for radionuclide production, generator-based ⁶⁸Ga radiopharmaceuticals might have clinical utility for positron emission tomography (PET) studies of myocardial perfusion and other physiological processes.MethodsThe lipophilic and monocationic ⁶⁷Ga-labeled gallium chelates of five novel hexadentate bis(salicylaldimine) ligands the bis(salicylaldimine), bis(3-methoxysalicylaldimine), bis(4-methoxysalicylaldimine), bis(6-meth,oxysalicylaldimine), and bis(4,6-dimethoxysalicylaldimine) of N,N′-bis(3-aminopropyl)-N,N′-dimethylethylenediamine (BAPDMEN), were prepared. The structure of the unlabeled [Ga(4-MeOsal)₂BAPDMEN]⁺PF₆^? salt was determined by X-ray crystallography, and the biodistribution of each of the ⁶⁷Ga-labeled gallium chelates was determined in rats following intravenous administration and compared with the biodistribution of [⁸⁶Rb]rubidium chloride.ResultsThe [Ga(4-MeOsal)₂BAPDMEN]⁺PF₆^? complex exhibited the expected pseudo-octahedral N₄O₂^2? coordination sphere about the Ga³⁺ center with a trans disposition of the phenolate oxygen atoms. All five ⁶⁷Ga radiopharmaceuticals were found to afford the desired myocardial retention of the radiogallium. The [^67/68Ga][Ga(3-MeOsal)₂BAPDMEN]¹⁺ radiopharmaceutical appears to have the best properties for myocardial imaging, exhibiting 2% of the injected dose in the heart 1 min and 2 h postinjection and very high heart/nontarget ratios (heart/blood ratios of 7.6±1.0 and 54±10 at 1 and 120 min, respectively; heart/liver ratios of 1.8±0.4 and 39±3 at 1 and 120 min, respectively).ConclusionsMost of these new agents, particularly [^67/68Ga][Ga(3-MeOsal)₂BAPDMEN]¹⁺, would appear superior to previously reported bis(salicylaldimine) ligands of N,N′-bis(3-aminopropyl)ethylenediamine as candidates for PET imaging of the heart with ⁶⁸Ga.     

Synthesis and in vivo evaluation of [C]SA6298 as a PET sigma1 receptor ligand

The potential of a ¹¹C-labeled selective sigma₁ receptor ligand, 1-(3,4-dimethoxyphenethyl)-4-[3-(3,4-dichlorophenyl)propyl]piperazine ([¹¹C]SA6298), was evaluated as a positron emission tomography (PET) ligand for mapping sigma₁ receptors in the central nervous system and peripheral organs. [¹¹C]SA6298 was synthesized by methylation of the desmethyl SA6298 with [¹¹C]CH₃I, with the decay-corrected radiochemical yield of 39 ± 5% based on [¹¹C]CH₃I and with the specific activity of 53 ± 17 TBq/mmol within 20 min from end of bombardment (EOB). In mice, the uptake of [¹¹C]SA6298 was significantly decreased by carrier loading in the brain, liver, spleen, heart, lung, small intestine, and kidney in which sigma receptors are present as well as in the skeletal muscle. Pretreatment with SA6298 also blocked the uptake of [¹¹C]SA6298 by these organs except for the small intestine, but significant displacement of [¹¹C]SA6298 by posttreatment with SA6298 was observed only in the heart, lung, and muscle. In the blocking study with one of the eight sigma receptor ligands, including haloperidol, SA6298, NE-100, (+)-pentazocine, SA4503, (−)-pentazocine, (+)-3-PPP, and (+)-SKF 10,047 (in the order of the affinity for sigma₁ receptor subtype), only SA6298 and an analog SA4503 significantly reduced the brain uptake of [¹¹C]SA6298 to approximately 80% of the control, but the other six ligands did not. Peripherally, the uptake of [¹¹C]SA6298 by the organs described above was decreased predominantly by SA6298 or SA4503, but the blocking effects of the other five ligands except for NE-100 depended on their affinity for sigma₁ receptors. The saturable brain uptake of [¹¹C]SA6298, approximately 20%, was also observed by tissue dissection method in rats and by PET in a cat. Ex vivo autoradiography of the rat brain showed a high uptake in the cortex and thalamus. In the cat brain a relatively high uptake was found in the cortex, thalamus, striatum, and cerebellum. These results have indicated a receptor-mediated uptake of the tracer to some extent in the brain and peripheral organs. However, the tracer has a limited potential for the PET study of the brain receptors because of a relatively high nonspecific binding.     

Preliminary studies of (99m)Tc-memantine derivatives for NMDA receptor imaging

IntroductionNovel technetium-labeled ligands, ^99mTc-NCAM and ^99mTc-NHAM were developed from the N-methyl-d-aspartate (NMDA) receptor agonist memantine as a lead compound by coupling with N₂S₂. This study evaluated the binding affinity and specificity of the ligands for the NMDA receptor.MethodsLigand biodistribution and uptake specificity in the brain were investigated in mice. Binding affinity and specificity were determined by radioligand receptor binding assay. Three antagonists were used for competitive binding analysis. In addition, uptake of the complexes into SH-SY5Y nerve cells was evaluated.ResultsThe radiochemical purity of ^99mTc-labeled ligands was more than 95%. Analysis of brain regional uptake showed higher concentration in the frontal lobe and specific uptake in the hippocampus. ^99mTc-NCAM reached a higher target to nontarget ratio than ^99mTc-NHAM. The results indicated that ^99mTc-NCAM bound to a single site on the NMDA receptor with a K_d of 701.21 nmol/l and a B_max of 62.47 nmol/mg. Specific inhibitors of the NMDA receptor, ketamine and dizocilpine, but not the dopamine D₂ and 5HT_1A receptor partial agonist aripiprazole, inhibited specific binding of ^99mTc-NCAM to the NMDA receptor. Cell physiology experiments showed that NCAM can increase the viability of SH-SY5Y cells after glutamate-induced injury.ConclusionsThe new radioligand ^99mTc-NCAM has good affinity for and specific binding to the NMDA receptor, and easily crosses the blood–brain barrier; suggesting that it might be a potentially useful tracer for NMDA receptor expression.     

Synthesis, in vitro and in vivo characterization of novel 99mTc-'4+1'-labeled 5-nitroimidazole derivatives as potential agents for imaging hypoxia

The evaluation of oxygenation status of solid tumors is an important field of radiopharmaceutical research. With the aim to develop new potential ^99mTc-radiopharmaceuticals for imaging hypoxia, we have synthesized two novel isocyanide derivatives of metronidazole, which has demonstrated high affinity for hypoxic tumors in vitro and in vivo.MethodsMetronidazole derivatives 4-isocyano-N-[2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethyl]butanamide (M1) and 1-(4-isocyanobutanoyl)-4-[2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethyl]piperazine (M2) were synthesized, and labeling was performed through preparation of their corresponding ^99mTc-(4+1) complexes, ^99mTc-NS₃M1 and ^99mTc-NS₃M2. The structure of the technetium complexes was corroborated by preparation and characterization of the corresponding rhenium complexes. We have studied the main physicochemical properties (stability, lipophilicity and plasma protein binding). Biological behavior in HCT-15 cells both in oxia and in hypoxia was assessed. Biodistribution in normal mice and in animals bearing induced 3LL Lewis murine lung carcinoma was also studied.ResultsMetronidazole derivatives were successfully synthesized. Labeling with high radiochemical purity was achieved for both ligands. ^99mTc complexes were stable in labeling milieu and human plasma. However, presence of the piperazine linker in M2 resulted in higher lipophilicity and protein binding. Although cell uptake in hypoxic conditions was observed for both radiotracers, ^99mTc-NS₃M2 biodistribution was considered unsuitable for a potential radiopharmaceutical due to high liver uptake and poor blood clearance. However, ^99mTc-NS₃M1 demonstrated a very favorable in vivo profile both in normal mice and in mice bearing induced tumors.ConclusionSelective uptake and retention in tumor together with favorable tumor/muscle ratio make ^99mTc-NS₃M1 a promising candidate for further evaluation as potential hypoxia imaging agent in tumors.     

The effect of formulation conditions on the distribution of 99mTc(NaBH4)-HEDP components separated by anion exchange HPLC

Reduction of ^99mTcO₄⁻ by NaBH₄ in the presence of HEDP leads to a mixture of ^99mTc-containing complexes which can be separated by anion exchange high performance liquid chromatography (HPLC). The distribution of complexes within this mixture can be varied by controlling pH, the concentration of HEDP, the concentration of technetium, and the presence of air. Suitable control of these formulation conditions can yield mixtures which consist of essentially (85%) one component. Arguments are presented to support the view that the components of ^99mTc(NaBH₄)-HEDP and ^99mTc(NaBH₄)-MDP mixtures are in fact oligomeric or polymeric complexes that can contain technetium centers in at least two different oxidation states.     

Chemical Properties of Water-soluble Porphyrins. 5. Reactions of Some Manganese (III) Porphyrins with the Superoxide and Other Reducing Radicals

Summary

Solution properties of three manganese porphyrins, in monomeric form, were investigated. These were the ‘picket-fence-like’ porphyrin Mn(III)-α,α,α,β-tetra-ortho(N-methylisonicotinamidophenyl)porphyrin (Mn(III)PFP) and two ‘planar unhindered’ porphyrins, the Mn(III)TMPyP (tetrakis (4-N-methylpyridyl)porphyrin) and Mn(III)TAP (tetra(4-NNN-trimethyl-anilinium)porphyrin). The porphyrin properties studied were: the absorption spectra in their manganic and manganous forms; acid/base properties of the aquo complexes; the effect of potential axial ligands (up to a concentration of 0·1 mol dm^?3) and their one electron reduction potentials. Knowing these properties, the reaction of the Mn(III) porphyrins with the superoxide radical and other reducing radicals were studied using the pulse radiolysis technique. The second-order reaction rate constant of O^?₂ with the Mn(III) porphyrins, which governs the catalytic efficiency of the metalloporphyrins upon the disproportionation of the superoxide radical, was 5·1 × 10⁷ to 4·0 × 10⁵ dm³ mol^?1s^?1, depending on the pH and the nature of the metalloporphyrin. These values are at least one order of magnitude lower than found for Fe(III)TMPyP. One electron reduction of the three Mn(III) porphyrins by e^?_aq, CO^?₂, CH₂OH and (CH₃)₂COH had similar second-order rate constants (10⁹-10¹⁰ dm³ mol^?1 s^?1). That for (CH₃)₂(CH₂)COH was about 10⁵ dm³ mol^?1 s^?1. Reduction in all cases produced the corresponding Mn(II) porphyrin and no intermediate was found. The oxidation reaction of the Mn(II) porphyrins by O^?₂ was approximately two orders of magnitude faster when compared to the reduction of Mn(III) porphyrins with the same radical. Since the reactivities of O^?₂ towards the three manganese (III) compounds follow their reduction potentials, it is suggested that these reactions are governed by an outer-sphere mechanism. This suggestion is corroborated by the finding that water molecules acting as axial ligands, in these aqueous solution systems, are not replaced by another potential ligand when the latter is in the concentration range of 100 mm or less.     

A study on nitroimidazole-99mTc(CO)3 complexes as hypoxia marker: Some observations towards possible improvement in in vivo efficacy

IntroductionHypoxia plays a negative role in the clinical management of cancer. Detection of hypoxic status of a cancer is important for selecting patients for hypoxia directed therapy. Though [¹⁸ F]fluoromisonidazole ([¹⁸ F]FMISO), a PET radiopharmaceutical, is presently being used in the clinic for the detection of hypoxia, considering the logistical advantages of ^99mTc and wider availability of SPECT scanners, a radiopharmaceutical based on this isotope may find wider applicability.MethodsNine nitroimidazole (2-, 4- and 5-nitroimidazole) ligands were synthesized and radiolabeled using [^99mTc(CO)₃(H₂O)₃]⁺ precursor to obtain a group of complexes possessing different single electron reduction potential (SERP), overall charge and lipophilicity, the three attributes which decide the efficacy of the complex to detect hypoxic cells in vivo. The nitroimidazole-^99mTc(CO)₃ complexes as well as [¹⁸ F]FMISO were evaluated in fibrosarcoma tumor bearing mice.ResultsThe ^99mTc(CO)₃ complexes of nitroimidazole iminodiacetic acid (IDA) showed better tumor uptake and retention than nitroimidazole diethylenetriamine (DETA) and nitroimidazole aminoethylglycine (AEG) complexes. Tumor uptake observed with [¹⁸ F]FMISO was higher than any of the nitroimidazole-IDA- ^99mTc(CO)₃ complexes. However, [¹⁸ F]FMISO clearance from tumor was found to be faster compared to 2-nitroimidazole-IDA-^99mTc(CO)₃ complex. Observed tumor uptake and retention of the radiotracers evaluated could be correlated to its blood clearance pattern and SERP.ConclusionsResults of the present study indicated that uptake of the radiotracer in tumor is closely associated with its rate of clearance from blood. The study also indicated that along with SERP, clearance of radiotracer from blood (net effect of charge and lipophilicity) is a critical factor which decides the in vivo efficacy of the hypoxia detecting radiopharmaceutical.     

Labelled bleomycin as a tumour-localizing agent II. The effect of dose loading of the different forms of 57Cobalt-bleomycin A2, B2, and pepleomycin on the tissue distribution and tumour uptake in tumour-bearing rats

The different forms of ⁵⁷cobalt-bleomycin (⁵⁷Co-blm) A₂ and B₂ as well as ⁵⁷Co-pepleomycin (⁵⁷Co-pep), were investigated in tumour-bearing rats in relation to the dose given. It appeared that the tumour-localizing properties of both forms of ⁵⁷Co-blm are identical if ⁵⁷Co-blm is injected as a bleomycin solution without carrier cobalt.Differences between the biological behaviour of the various cobalt complexes (which differ in ligand arrangement) were found if these complexes were injected together with inactive cobalt bleomycin complexes of the same form. In this case Co-blm B₂ form I and Co-pep form I localize better than Co-blm B₂ form II and Co-pep form II respectively. Such a decrease in uptake by the tumour, compared with form I, was not observed for Co-blm A₂-II.For part I, see Vos et al. 1979     

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.     

Radiochemical studies of technetium complexes of tetradentate amine-phenols

We have synthesized amine-phenol ligands of the general structure bis-(2-hydroxybenzylamino)X, where X is a hydrocarbon backbone. Radiochemical studies of ⁹⁹Tc and ^99mTc complexes were done at total technetium concentrations of 0.1–100 μM. Complex yields were measured by ascending solvent paper chromatography, TLC, solvent extraction and reversed phase HPLC. Complexes were readily formed and were neutral and lipophilic. Lipophilicity was enhanced by the addition of methyl groups to the carbon backbone. Similar complexes formed from the corresponding Schiff base ligands were less lipophilic and were formed in lower yields.     

Imaging and assessment of regional clearances of indium-111 labelled circulating immune complexes in humans

The in vivo function of the reticulo-endothelial system in humans is currently studied by the blood clearance of ⁵¹Cr labeled RBC sensitized with IgG anti-Rh. By replacing ⁵¹Cr by ¹¹¹In we developed a method which allows imaging and assessment of clearances of circulating immune complexes (CIC) by individual organs. Nine normal subjects were studied successively with ⁵¹Cr labelled CIC and ¹¹¹In-labelled CIC by blood sampling and sequential imaging. Time activity curves were obtained over spleen, liver, heart lungs and kidneys. The blood clearance values were similar with ¹¹¹In (T _1/2±SD=23.4 min±6.4 min) and ⁵¹Cr (T _1/2=22.9 min±5.8 min). The blood clearance derived from the time activity curve obtained over the cardiac area (T _1/2=26.6 min±8.4 min) was not significantly different from that obtained by blood sampling. Most of the CIC were taken up by the spleen with a T _1/2 of 20.3 min±4.1 min. Although hepatic uptake was observed in all subjects, it was lower than that of the spleen and CIC were subsequently released from the liver in five of the nine patients. The spleen/liver ratio increased with time, reaching a plateau averaging 24.0±5.8 within 80 min in all subjects. CIC imaging allows, by a non-invasive procedure, the computation of CIC blood clearance and the estimation of the respective role of spleen and liver in phagocytic function.Abbreviations CIC circulating immune complexes - RBC Red blood cells - ROI Region of interest     

Gel chromatographic analysis of the bone seeking radiopharmaceutical 99mTc(Sn)-EHDP: The influence of pH and EHDP-concentration on the size of its constituents

^99mTc(Sn)-EHDP complexes have been produced by reduction of TcO₄⁻ with Sn(II) in the presence of EHDP at varying pH and EHDP concentration. The mixture was separated by means of gel-chromatography with an eluent of the same composition and pH as the reaction mixture.It appears that at neutral pH larger complexes are formed than under acidic or basic reaction conditions. Larger complexes are also formed at higher EHDP concentrations.