Potential Radiopharmaceuticals Research Articles

Chemical design of a radiolabeled gelatinase inhibitor peptide for the imaging of gelatinase activity in tumors

Since elevated levels of gelatinases [matrix metalloproteinase (MMP)-2 and MMP-9] are associated with a poor prognosis in cancer patients, these enzymes are potential targets for tumor imaging. In the present study, a cyclic decapeptide, cCTTHWGFTLC (CTT), was selected as a mother compound because of its selective inhibitory activity toward gelatinases. For imaging gelatinase activity in tumors, we designed a CTT-based radiopharmaceutical taking into consideration that (1) the HWGF motif of the peptide is important for the activity, (2) hydrophilic radiolabeled peptides show low-level accumulation in the liver and (3) an increase in the negative charge of radiolabeled peptides is effective in reducing renal accumulation. Thus, a highly hydrophilic and negatively charged radiolabel, indiun-111-diethylenetriaminepentaacetic acid ((111)In-DTPA), was attached to an N-terminal residue distant from the HWGF motif ((111)In-DTPA-CTT). In MMP-2 inhibition assays, In-DTPA-CTT significantly inhibited the proteolytic activity in a concentration-dependent fashion. When injected into normal mice, (111)In-DTPA-CTT showed low levels of radioactivity in the liver and kidney. A comparison of the pharmacokinetic characteristics of (111)In-DTPA-CTT with those of other CTT derivatives having different physicochemical properties revealed that the increase in hydrophilicity and negative charge caused by the conjugation of (111)In-DTPA reduced levels of radioactivity in the liver and kidney. In tumor-bearing mice, a significant correlation was observed between the accumulation in the tumor as well as tumor-to-blood ratio of (111)In-DTPA-CTT and gelatinase activity. These findings support the validity of the chemical design of (111)In-DTPA-CTT for reducing accumulation in nontarget tissues and maintaining the inhibitory activity of the mother compound. Furthermore, (111)In-DTPA-CTT derivatives would be potential radiopharmaceuticals for the imaging of gelatinase activity in metastatic tumors in vivo.

18F‐Labeling of Peptides by means of an Organosilicon‐Based Fluoride Acceptor

Fluorine-18 is among the most commonly used radionuclides for positron emission tomography (PET). This non-invasive imaging technique is capable of providing in vivo information about the distribution of radiolabeled biomolecules by 1808 coincidence detection of two simultaneously emitted photons from positron–electron annihilation. Although a number of different radiotracers have been successfully employed in PET, only a few, such as 2-[F]fluoro-2-deoxy-d-glucose (FDG) and [F]fluorodopa, have gained widespread application in nuclear medicine. The reason for this is that the regioselective introduction of F into tracer molecules is often non-specific and radiochemical yields (RCY) of the Flabelled product are low. The introduction of F into tracer molecules requires high temperatures and often leads to undesired by-products. The research over the last decade clearly indicates that the success of PET in nuclear medicine justifies the intense search for versatile labeling formulations for the syntheses of F-radiopharmaceuticals. Especially the development of rational F-labelling strategies for peptides, until now characterized by multistep procedures, is considered to be one of the most important tasks. As an alternative to conventional F-labelling chemistry, the use of [F]fluorosilanes as labeling synthons was first proposed by Rosenthal et al. who treated chlorotrimethylsilane with n.c.a. (no carrier added) F in aqueous acetonitrile isolating the corresponding [F]fluorosilane in 65% yield. A preliminary in vivo evaluation revealed fast hydrolysis of the compound accompanied by high radioactivity (F) uptake by the bone making it unsuitable as a labeling synthon. Another approach is based on the work by Pilcher et al. who fluorinated organosilanoles with nonradioactive HF in high yields. An analogous labeling strategy was proposed in a symposium abstract. However, so far no labeling experiments using aqueous F /[F]HF solutions have been reported. Most recently Ting et al. used organotriethoxysilanes as labeling precursors for the synthesis of [F]fluorosilanes but no practical application for the synthesis of potential radiopharmaceuticals has been demonstrated. Herein we report the syntheses of substituted [F]organofluorosilanes using organochlorosilanes as labeling precursors and their in vitro and in vivo stability. As an alternative labeling approach we also describe the F–F isotopic exchange using [F]di-tert-butylphenyl fluorosilane as a highly efficient silicon-based fluoride acceptor (SiFA compound). As proof of applicability we transferred the SiFA approach to the development of a simple and practical formulation for the synthesis of a F-labelled SiFA derivatized Tyr-octreotate, a peptide used in oncology for the visualization of neuro-endocrine tumors. We synthesized three [F]organofluorosilanes, namely [F]fluorotriphenylsilane (1), [F]fluoro-tert-butyldiphenylsilane (2), and [F]fluorodi-tert-butylphenylsilane (3), and evaluated their in vitro stability in human serum as well as their in vivo stability in rats, studied by animal-PET. These data are essential for finding the most suitable compound and for evaluating the labeling concept. The reaction in acetonitrile of the triorganochlorosilanes (5–11.8 mmolmL ) Ph3SiCl, tBuPh2SiCl, and tBu2PhSiCl, with the azeotropically dried complex F /Kryptofix2.2.2./ K at room temperature provided almost quantitatively the corresponding [F]triorganofluorosilanes 1–3 (Figure 1), as demonstrated by means of radio-HPLC. Their identities were confirmed by coelution of the radioactive probes spiked with the related nonradioactive F-analogues. The specific activity of 1, 2, and 3 was determined using UV-calibration curves and was in the range 1500–1700 GBqmmol . To investigate the applicability of the F-labelled compounds for the development of Si–F containing radiopharmaceuticals, their in vitro stability in human serum was investigated (Figure 1). In agreement with previously published data, the [F]triphenylfluorosilane 1 was found to be stable for 4 h in neutral water (data not shown) but unstable at pH 7.4–7.6 in human serum. In contrast, the tert[*] Dr. R. Schirrmacher, Dr. E. Schirrmacher, J. Tillmanns, Dr. T. Siessmeier, Dipl.-Ing. H. G. Buchholz, Prof. Dr. P. Bartenstein Klinik und Poliklinik f1r Nuklearmedizin Johannes Gutenberg Universit6t Mainz 55131 Mainz (Germany) Fax: (+49)6131-172386 E-mail: schirrmacher@klinik.nuklearmedizin.uni-mainz.de Dipl.-Chem. G. BradtmBller, Prof. Dr. K. Jurkschat Lehrstuhl f1r Anorganische Chemie Universit6t Dortmund 44221 Dortmund (Germany) Fax: (+49)231-755-5048 E-mail: klaus.jurkschat@uni-dortmund.de

Design and evaluation of “3 + 1” mixed ligand oxorhenium and oxotechnetium complexes bearing a nitroaromatic group with potential application in nuclear medicine oncology

The synthesis and evaluation of a series of oxotechnetium and oxorhenium complexes containing a nitroaromatic moiety as potential radiopharmaceuticals for targeting tumour hypoxia is presented. 99mTc labelling was performed in high yield (> 85%) and radiochemical purity (> 90%). Their structure was corroborated by means of the rhenium complexes. Reduction potentials were in the range for bioreducible compounds. 99mTc complexes III–VI were selected for “in vivo” experiments in view of the results of cytotoxicity studies. Biodistribution in normal animals was characterized by high initial blood, lung and liver uptake, fast blood and soft tissue depuration and preferential excretion via the hepatobiliary system. Initial tumour uptake was moderate but tumour/muscle ratios for complexes III and IV, were favourable at all time points. Although the results are encouraging further development is still necessary in order to achieve higher tumour uptake and lower gastrointestinal activity.

Rhenium‐Dicarbonyl‐Nitrosyl‐Komplexe mit Imidazol

AbstractRhenium Dicarbonyl‐Nitrosyl Complexes with ImidazoleDifferent rhenium‐dicarbonyl‐nitrosyl complexes with imidazole (Im) as monodentate ligand have been synthesized and characterized, starting from [NEt4][ReCl3(CO)2(NO)] and [ReCl(μ−Cl)(CO)2(NO)]2. Whereas the complexes [ReCl2(Im)(CO)2(NO)] and [ReCl(Im)2(CO)2(NO)]+ were achieved in high yields, the complex [Re(Im)3(CO)2(NO)]2+ with three imidazole ligands could only be isolated after complete removal of all halide ions (with AgBF4) in low yield. The synthesis of a corresponding 99mTc‐dicarbonyl‐nitrosyl complex with imidazole opens a new perspective for such compounds as potential radiopharmaceuticals and alternatives to the already established 99mTc‐tricarbonyl complexes.

Double-stabilized neurotensin analogues as potential radiopharmaceuticals for NTR-positive tumors

Overexpression of neurotensin (NT) receptors in exocrine pancreatic cancer and other neuroendocrine cancers make them interesting targets for tumor imaging and therapy. Modifications at the cleavage bonds 8-9 and 11-12 led to the synthesis of NT-XII, NT-XIII and NT-XVIII, three new stabilized analogues. (NalphaHis)Ac was coupled to the N-terminus for labeling with [(99m)Tc]-tricarbonyl. Stability was tested in vitro in human plasma and HT-29 cells. Binding to NT1 receptors and internalization/efflux were analyzed in intact HT-29 cells. Biodistribution studies were performed in nude mice bearing HT-29 xenografts. All analogues were very stable in human plasma, with half-lives of 20-21 days. Degradation in HT-29 cells was more rapid (t(1/2) of 6.5, 5 and 2.5 h for NT-XII, NT-XIII and NT-XVIII, respectively). They also showed high affinity and specificity for NT1 receptors. Bound activity was rapidly internalized at 37 degrees C. The pattern of externalization was different. NT-XII was released more slowly than NT-XIII and NT-XVIII (half of the activity still inside the cells after 24 h). Bigger differences were found in the biodistribution studies. NT-XII showed the highest tumor uptake as well as the best tumor to nontumor ratios. The modifications introduced in NT(8-13) increased plasma stability, maintaining unaffected the in vitro binding properties. The best biodistribution corresponded to NT-XII, which shows to be a good candidate for NT1 receptors overexpressing tumors. First clinical trials are ongoing.

Novel 99mTc-Labeled Neurotensin Analogues with Optimized Biodistribution Properties

Two new 99mTc-labeled neurotensin(8-13) analogues containing the retro-N(alpha)-carboxymethyl-histidine ((N(alpha)His)Ac) chelator were synthesized as potential radiopharmaceuticals for visualization of pancreatic carcinoma. To improve the pharmacokinetic properties, (N(alpha)His)Ac-Arg-NMeArg-Pro-Tyr-Tle-Leu (NT-XII), which is metabolically stabilized at two positions, was further modified. Shikimic acid (3,4,5-trihydroxy-1-cyclohexene-1-carboxylic acid) was introduced to obtain a more hydrophilic peptide (NT-XVIII), or Tyr11 was replaced by 2,6-dimethyltyrosine (Dmt) resulting in a triple-stabilized NT(8-13) analogue (NT-XIX). The latter has the best biodistribution profile.

Synthesis of cyclopentadienyltricarbonylrhenium substituted benzhydryl species and oestrogen receptor binding properties

Abstract6‐(4‐Methoxyphenyl)fulvene has been efficiently engaged in a process implying nucleophilic attack of methyllithium followed by a transmetallation reaction in the presence of different organorhenium sources {XRe(CO)5: X = Cl, Br, OTf; BrRe(CH3CN)2(CO)3, [BrRe(CO)3THF]2} to afford the corresponding cyclopentadienyltricarbonylrhenium‐substituted compound. This study allowed us to determine the best complex for that transformation, and similar reaction conditions were used with success to provide new potential radiopharmaceuticals in the cold series. One of these compounds [the (4,4′‐dihydroxybenzhydryl)cyclopentadienyltricarbonylrhenium] displays a good recognition for the α form of the oestrogen receptor. Copyright © 2006 John Wiley & Sons, Ltd.

A Novel Ternary Ligand System Useful for Preparation of Cationic 99mTc-Diazenido Complexes and 99mTc-Labeling of Small Biomolecules

This report describes a novel ternary ligand system composed of a phenylhydrazine, a crown ether-containing dithiocarbamate (DTC), and a PNP-type bisphosphine (PNP). The combination of three different ligands with (99m)Tc results in cationic (99m)Tc-diazenido complexes, [(99m)Tc(NNAr)(DTC)(PNP)]+, with potential radiopharmaceuticals for heart imaging. Synthesis of cationic (99m)Tc-diazenido complexes can be accomplished in two steps. For example, the reaction of phenylhydrazine with (99m)TcO4- at 100 degrees C in the presence of excess stannous chloride and 1,2-diaminopropane-N,N,N',N'-tetraacetic acid (PDTA) results in the [(99m)Tc(NNPh)(PDTA)n] intermediate, which then reacts with sodium N-(dithiocarbamato)-2-aminomethyl-15-Crown-5 (L4) and N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]ethoxyethylamine (PNP6) at 100 degrees C for 15 min to give the complex, [(99m)Tc(NNPh)(L4)(PNP6)]+ in high yield (>90%). Cationic complexes [(99m)Tc(NNPh)(DTC)(PNP)]+ are stable for > or = 6 h. Their composition was determined to be 1:1:1:1 for Tc:NNPh:DTC:PNP using the mixed-ligand experiments on the tracer ((99m)Tc) level and was further confirmed by the ESI-MS spectral data of a model compound [Re(NNPh)(L4)(L6)]+. It was found that both DTCs and bisphosphines have a significant impact on the lipophilicity of their cationic (99m)Tc-diazenido complexes. Results from a (99m)Tc-labeling efficiency experiment showed that 4-hydrazinobenzoic acid (HYBA) might be useful as a bifunctional coupling agent for (99m)Tc-labeling of small biomolecules. However, the (99m)Tc-labeling efficiency of HYBA is much lower than that of 6-hydrazinonicotinic acid (HYNIC) with tricine and trisodium triphenylphosphine-3,3',3''-trisulfonate (TPPTS) as coligands.

Development of 5-HT1A receptor radioligands to determine receptor density and changes in endogenous 5-HT

[(18)F]FCWAY and [(18)F]FPWAY, analogues of the high affinity 5-HT(1A) receptor (5-HT(1A)R) antagonist WAY100635, were evaluated in rodents as potential radiopharmaceuticals for determining 5-HT(1A)R density and changes in receptor occupancy due to changes in endogenous serotonin (5-HT) levels. The in vivo hippocampus specific binding ratio [(hippocampus(uptake)/cerebellum(uptake))-1] of [(18)F]FPWAY was decreased to 32% of the ratio of [(18)F]FCWAY, indicating that [(18)F]FPWAY has lower affinity than [(18)F]FCWAY. The 5-HT(1A)R selectivity of [(18)F]FPWAY was confirmed using ex vivo autoradiography studies with 5-HT(1A)R knockout, heterozygous, and wildtype mice.Pre- or post-treatment of awake rodents in tissue dissection studies with paroxetine had no effect on hippocampal binding of [(18)F]FCWAY or [(18)F]FPWAY compared to controls, indicating neither tracer was sensitive to changes in endogenous 5-HT. In mouse ex vivo autoradiography studies in which awake mice were treated with fenfluramine following the [(18)F]FPWAY, a significant decrease was not observed in the hippocampus specific binding ratios. In rat dissection studies with fenfluramine administered following [(18)F]FPWAY or [(18)F]FBWAY ([(18)F]-MPPF) in awake or urethane-anesthetized rats, no significant differences in the specific binding ratios of the hippocampus were observed compared to their respective controls. [(18)F]FPWAY and [(18)F]FBWAY uptakes in all brain regions were increased variably in the anesthetized group (with the greatest increase in the hippocampus) vs. the awake group, but were decreased in the fenfluramine-treated anesthetized group vs. the anesthetized group. These data are best explained by changes in blood flow caused by urethane and fenfluramine, which varies from region to region in the brain.

Radiosynthesis and Evaluation of 11C-Labeled Diaryl-Substituted Imidazole and Indole Derivatives for Mapping Cyclooxygenase-2

11C-labeled analogs of 4-chloro-5-(3-fluoro-4-methoxyphenyl)-1-(4-methylsulfonylphenyl)imidazole ([11C]1), 4-[4-chloro-5-(3-fluoro-4-methoxyphenyl)imidazol-1-yl]benzenesulfonamide ([11C]2) and 2-(4-aminosulfonylphenyl)-3-(4-methoxyphenyl)indole ([11C]3), which have been shown to have excellent potency and high selectivity for cyclooxygenase isoform 2 (COX-2) inhibiting activity, were prepared and evaluated in rats as potential radiopharmaceuticals for imaging the COX-2 enzyme by positron emission tomography. These 11C-labeled COX-2 inhibitors were synthesized in high radiochemical yields by O-[11C]methylation of phenolic precursors with [11C]methyl triflate in acetone containing NaOH as a base. In vivo evaluation in rats bearing AH109A hepatoma showed no specific binding of any tracer to COX-2 in any tissue such as the brain, heart, lung, kidney, and AH109A hepatoma. In ex vivo autoradiography, [11C]1 showed regionally different distribution in the brain, while [11C]2 and [11C]3 were not substantially taken up by the brain. In in vitro monolayer efflux assays, compound 3 was found to be a substrate for the P-glycoprotein (P-gp) efflux pump, but pretreatment of rats with the potent P-gp inhibitor, cyclosporine A, did not have any significant influence on the cerebral uptake of [11C]3. These results indicate that all three tracers were not suitable for in vivo imaging of COX-2. There seem to be some obstacles to finding a useful candidate for in vivo imaging application of COX-2 selective inhibitors only by standard consideration of in vitro affinity and selectivity, and the lipophilicity of the compound.

On the99mTc-labeling of isoniazid with different99mTc cores

Recent advances in the chemistry of radiolabeling with 99mTc such as use of the 99mTc tricarbonyl and 99mTc–HYNIC cores have revived interest in 99mTc-labeling of small biomolecules and further investigation as potential radiopharmaceuticals. Isoniazid, a drug commonly used for treatment of tuberculosis, has been chosen for the present study. Three distinct strategies were utilized to radiolabel isoniazid with 99mTc. In the direct labeling protocol, the hydrazino amide functional group of isoniazid was used for 99mTc-labeling in the HYNIC sense using tricine and EDDA as co-ligands. The other strategies of 99mTc-labeling involved the derivatization of isoniazid to its N, N-diacetic acid derivative, which in turn was either used as a tridentate ligand for labeling with the [99mTc(CO)3(H2O)3]+ synthon or directly labeled by the conventional route wherein 99mTc is in the +3 oxidation state. The complexes prepared in >95% yields were characterized by paper chromatography, thin layer chromatography and HPLC. Comparison of the three approaches showed that maximum specific activity and stability was obtained in the 99mTc–isoniazid derivative synthesized via the tricarbonyl method. However, in vitro cell-binding studies indicated that none of the 99mTc–isoniazid complexes prepared had any appreciable uptake in Mycobacterium tuberculosis. Copyright © 2005 John Wiley & Sons, Ltd.

Carbohydrate conjugates for molecular imaging and radiotherapy: 99mTc(I) and 186Re(I) tricarbonyl complexes of N-(2'-Hydroxybenzyl)-2-amino-2-deoxy-D-glucose.

An approach to a new class of potential radiopharmaceuticals is demonstrated by the labeling of a glucosamine derivative with the tricarbonyls of 99mTc and 186Re. The proligand HL2 (N-(2'-hydroxybenzyl)-2-amino-2-deoxy-D-glucose) was produced by hydrogenation of the corresponding Schiff base and reacted with [NEt4]2[Re(CO)3Br3] to form the neutral complex [(L2)Re(CO)3] in 40% yield. 1H and 13C NMR spectra indicate that the [Re(CO)3] core is bound in a tridentate fashion via the amino N, phenolato O, and C-3 hydroxyl O atoms of the ligand. At the tracer-level, labeling of HL2 with [99mTc(CO)3(H2O)3]+ and [186Re(CO)3(H2O)3]+ was achieved in aqueous conditions in 95 +/- 2% and 94 +/- 3% average radiochemical yields, respectively.

Application of n.c.a. 4‐[18F]fluorophenol in diaryl ether syntheses of 2‐(4‐[18F]fluorophenoxy)‐benzylamines

AbstractThe availability of no‐carrier‐added (n.c.a.) 4‐[18F]fluorophenol offers the possibility of introducing the 4‐[18F]fluorophenoxy moiety into potential radiopharmaceuticals. Besides alkyl–aryl ether synthesis using n.c.a. 4‐[18F]fluorophenol the diaryl ether coupling is an attractive synthetic method to enlarge the spectrum of interesting labelling procedures. As examples the syntheses of n.c.a. 2‐(4‐[18F]fluorophenoxy)‐N,N‐dimethylbenzylamine and n.c.a. 2‐(4‐[18F]fluorophenoxy)‐N‐methylbenzylamine were realized by an Ullmann ether synthesis of corresponding 2‐bromobenzoic acid amides using tetrakis(acetonitrile)copper(I) hexafluorophosphate as catalyst and a subsequent reduction of the amides formed. The radiochemical yield of the coupling varied between 5 and 65% based on labelled 4‐[18F]fluorophenol. Both compounds are structural analogues of recently published radiotracers for imaging the serotonin reuptake transporter sites (SERT). However, in vitro binding assays of both molecules showed only a low affinity towards monoamine transporters. Copyright © 2004 John Wiley & Sons, Ltd.

Direct Synthesis of Tricarbonyl(cyclopentadienyl)rhenium and Tricarbonyl(cyclopentadienyl)technetium Units from Ferrocenyl Moieties − Preparation of 17α‐Ethynylestradiol Derivatives Bearing a Tricarbonyl(cyclopentadienyl)technetium Group

AbstractThe preparation of new tricarbonyl(cyclopentadienyl)rhenium and tricarbonyl(cyclopentadienyl)technetium derivatives is described. The approach used was applied to a cold target model, 17α‐ethynylestradiol substituted at the 17α position by a ketotricarbonyl(cyclopentadienyl)rhenium group. The novel organometallic reaction applied here, when extended to potential radiopharmaceuticals, consists of a cyclopentadienyl‐ligand transfer between ketoferrocenes and fac‐[99mTc(H2O)3(CO)3]+ in a water/DMSO 1:1 mixture. This reaction lead to the synthesis of a new estradiol derivative, labeled with 99mTc. The synthesis of another product was performed by using [99TcCl3(CO)3]2− as the Tc precursor and its X‐ray crystal structure was determined. In each case high yields were obtained. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Contribution of electrospray mass spectrometry for the characterization, design, and development of nitrido technetium and rhenium heterocomplexes as potential radiopharmaceuticals.

Diagnostic nuclear medicine (NM) is among the imaging procedures (together with X-ray, computerized tomography, magnetic resonance, and echography) the clinicians can routinely adopt to image organs or tissues and related disorders. (99m)Tc-based agents are the radiopharmaceuticals of election in diagnostic NM because of the ideal physical properties of the 99mTc nuclide (t1/2 6.01 hr; Egamma 142 keV), low cost, and easy availability through the commercial 99Mo/99mTc generator, and chemical versatility of the element. In the last two decades the synergistic work of clinics, pharmacologists, and coordination chemists has had a tremendous impact in the development of new 99mTc-based radiopharmaceuticals through the recognition of the structure at the molecular level of the agent utilized. This has been achieved by studying the physico-chemical properties of the long-lived 99gTc (t1/2 2.11 x 10(5) year; Ebeta 292 keV) and third-row congener Re isostructural compounds. Electrospray ionization mass spectrometry (ESI-MS) and collision experiments (MS/MS) represent valuable analytical techniques suitable for the characterization of both technetium and rhenium complexes relevant to NM. Unequivocal structural identification of these bioinorganic compounds, either simple coordination complexes ("essential radiopharmaceuticals") or more sophisticated structures carrying bioactive fragments ("receptor-specific" radiopharmaceuticals), can be realized in combination with multinuclear NMR spectroscopy. MS/MS experiments provide useful information on the different metal-ligand bond strength, and comparison of the fragmentation profiles of isostructural technetium and rhenium compounds give additional details on the role played by the metal in determining preferred decomposition channels. The analysis of these data contribute to design novel synthetic strategies for the obtainment of technetium and rhenium compounds relevant to NM. The chemistry underlying the production of a new class of potential radiopharmaceuticals including a terminal nitrogen bond and a mixed coordination sphere comprising heterodiphosphines and/or dithiocarbamates (DTC) is presented in detail together with the ESI-MS and MS/MS investigations.

A novel route to radioiodinated [ 123I]- N-succinimidyl-3-iodobenzoate, a reagent for radioiodination of bioactive peptides

Radiolabeled peptides continue to emerge as potential radiopharmaceuticals for targeting several diseases such as cancer, infection and inflammation and even tissue and organ rejection. The classical method for labeling these molecules has been the electrophilic route. Evidence suggests that most molecules labeled via this route perturb their biological activity. Moreover, this method is not applicable to peptides lacking a tyrosine moiety in their structure. Hence, there is the need to develop alternate methods such as the prosthetic approach. We have optimized a solid-state radioiodination by exchange to produce [ 123I]-metaiodobenzylguanidine ([ 123I]-mIBG). The mIBG served as a precursor to obtain an activated N-succinimidyl ester for efficient coupling to amine functions in peptides, preferably the lysine group(s). The method was used to label a model chemotactic peptide and evaluated in vivo.

Current use and future potential of organometallic radiopharmaceuticals.

Contrary to common belief, organometallic compounds exhibit remarkable stability in aerobic and even diluted aqueous solutions. Technetium-sestamibi (Cardiolite) is one of the most prominent examples of this class of compounds routinely used in nuclear medicine. This review summarises the recent progress in labelling of biomolecules with organometallic complexes for diagnostic and therapeutic application in radiopharmacy and exemplifies in detail developments focussing on organometallic technetium- and rhenium-tricarbonyl technologies. The value of such technologies has been recognised and they have become a valuable alternative to common labelling methodologies. An increasing number of groups have started to employ an organometallic precursor for the purpose of radioactive labelling of various classes of biomolecules, and the advantages and limitations of this new technique are compared with those of other labelling methods. The synthetic access to appropriate precursors via double-ligand exchange or aqueous carbonyl kit preparation for routine application is described. Strategies and examples for the design of appropriate bifunctional chelating agents for the Tc/Re-tricarbonyl core are given. The functionalization of biomolecules such as tracers for the central nervous system (dopaminergic and serotonergic), tumour affine peptides (somatostatin receptors, neuroreceptors) and tumour binding single-chain antibody fragments is summarised. Where possible and appropriate, the in vitro and in vivo results in respect of these examples are compared with those obtained with classical (99m)Tc/(188)Re(V)- and (111)In-labelled analogues. The preclinical results show the in many ways superior characteristics of organometallic labelling techniques.

Tumour angiogenesis pathways: related clinical issues and implications for nuclear medicine imaging.

Tumour angiogenesis is essential for growth, invasion and metastasis. Retrospective studies suggest that it is an independent prognostic factor that merits prospective validation. Furthermore, as tumour blood vessels show many differences from normal vessels and are not genetically unstable, they form a key area for therapy development. However, as anti-angiogenic therapy is primarily cytostatic and not cytotoxic, novel tailor-made specific end-points for treatment monitoring are required. In this regard, suitable molecular parameters for imaging tumour angiogenesis by means of nuclear medicine are being explored. Here we review current knowledge on the multiple pathways controlling tumour angiogenesis and try to assess which are the most clinically relevant for nuclear medicine imaging. Parameters that may influence the imaging potential of radiopharmaceuticals for angiogenesis imaging such as molecular weight and structure, their targeted location within the tumour and their usefulness in terms of specificity and constancy of the targeted molecular pathway are discussed.

Synthesis of novel 1,3,5- cis, cis-triaminocyclohexane ligand based Cu(II) complexes as potential radiopharmaceuticals and correlation of structure and serum stability

The present study describes the synthesis of Cu(II) and radioactive 67,64Cu(II) complexes with N, N′, N″-tris(2-pyridylmethyl)-1,3,5- cis, cis-triaminocyclohexane (tachpyr) and a new pyridyl ring alkylated derivative, N, N′, N″-tris(6-methyl-2-pyridylmethyl)-1,3,5- cis, cis-triaminocyclohexane (tach-6-Me-pyr) and the investigation of their structural and solution properties as well as stability in human serum. Facile formation of both the ‘cold’ and 67,64Cu-radiolabeled complexes, [Cu(tachpyr)] 2+ and [Cu(tach-6-Me-pyr)] 2+, occurs in aqueous (0.15 M NH 4OAc buffer, pH 6.5) or methanolic solvent. X-ray crystallography reveals that Cu(II) is hexacoordinate in [Cu(tachpyr)] 2+ while methylation at the 6-position of the pyridyl donors in tachpyr produces considerable steric hindrance. Thus one pyridyl arm of tach-6-Mepyr is rotated entirely away from copper in [Cu(tach-6-Mepyr)] 2+, resulting in a highly distorted trigonal-bipyramidal coordination geometry. The solid-state structures of the complexes correlate well with their stabilities in human serum at 37 °C. Thus, [ 67Cu(tachpyr)] 2+ possesses excellent stability, being unchanged after a period of 7 days, while [ 67Cu(tach-6-Mepyr)] 2+ decomposes within 1 h. Tachpyr may be an excellent candidate for developing new Cu(II) radiopharmaceuticals for diagnosis and therapy.

Radiopharmaceuticals to image infection and inflammation

Scintigraphic imaging of infection and inflammation is a powerful diagnostic tool in the management of patients with infectious or inflammatory diseases. Most infectious and inflammatory foci can be visualized accurately with radiolabeled autologous leukocytes. However, the preparation of this radiopharmaceutical is laborious and requires the handling of potentially contaminated blood. A few radiopharmaceuticals are available that could be used instead of radiolabeled leukocytes to scintigraphically visualize infectious and inflammatory foci, such as 67Ga-citrate and 99mTc-labeled antigranulocyte antibody preparations. Various agents labeled with 99mTc are currently developed for this application. Most of these newly developed agents are ligands that bind receptors on white blood cell subpopulations, ie, monoclonal antibodies, chemotactic peptides, and cytokines. Furthermore, agents are developed that potentially could distinguish between infection and nonmicrobial inflammation. In addition, 18F-fluorodeoxyglucose positron emission tomography imaging was proposed to visualize inflammatory foci when a high spatial resolution is required. In this article, the characteristics and diagnostic potential of established and experimental radiopharmaceuticals for infection and inflammation imaging are reviewed.