Direct labeling of proteins with 99mTc

The direct labeling of antibodies and antibody fragments to form a highly stable bond between technetium and the sulfide groups of proteins is now well established. To optimize this reaction, the antibody protein must have sufficient reactive sulfides available to accept that technetium metal ions that are formed by the reduction of pertechnetate in the presence of a weak complexing agent. The reactive sulfide groups are provided by first reducing a small fraction of the disulfide bridges in the antibody protein or by starting with Fab′ fragments, which already have reactive sulfide groups. When the antibody protein has been appropriately reduced, and the reactive sulfide groups protected by a metal ion with a lower binding affinity than technetium, such as tin or zinc, very high labeling yields of high-affinity-bonded ^99mTc can be achieved. This can be accomplished without loss of immunoreactivity, measured as either affinity or immunoreactive fraction.Side reactions can produce radiochemical impurities such as low-affinity, bound ^99mTc; ^99mTc colloids; ^99mTc peptides or antibody aggregates; or ^99mTc-complexes. Also, pertechnetate ions may be an impurity if the sodium pertechnetate solution added to the reduced antibodies is not completely reduced. The specifics of minimizing these side reactions have not been extensively discussed in the prior literature; however, it is clear that appropriate reduction of the protein prior to labeling and complete removal of the reducing agent, particularly if it contains reactive sulfide groups or is toxic, are critical.One- or two-step ^99mTc-labeling kits for preparing ^99mTc-labeled antibody or antibody fragments are rapidly being introduced for use in clinical nuclear medicine studies. These direct labeling methods employ a common sequence of chemical reactions, although the reducing agents for both the antibody and the [^99mTc]pertechnetate may vary. Different ^99mTc transfer agents may be used, but all transfer agents have the common feature of quickly forming weak to moderately strong complexes with reduced technetium. Most use Sn(II) to reduce the pertechnetate, although other reducing agents can be used.     

Development of 99mTc-labeled asymmetric urea derivatives that target prostate-specific membrane antigen for single-photon emission computed tomography imaging

Prostate-specific membrane antigen (PSMA) is expressed strongly in prostate cancers and is, therefore, an attractive diagnostic and radioimmunotherapeutic target. In contrast to previous reports of PMSA-targeting ^99mTc-tricarbonyl complexes that are cationic or lack a charge, no anionic ^99mTc-tricarbonyl complexes have been reported. Notably, the hydrophilicity conferred by both cationic and anionic charges leads to rapid hepatobiliary clearance, whereas an anionic charge might better enhance renal clearance relative to a cationic charge. Therefore, an improvement in rapid clearance would be expected with either cationic or anionic charges, particularly anionic charges. In this study, we designed and synthesized a novel anionic ^99mTc-tricarbonyl complex ([^99mTc]TMCE) and evaluated its use as a single-photon emission computed tomography (SPECT) imaging probe for PSMA detection. Direct synthesis of [^99mTc]TMCE from dimethyl iminodiacetate, which contains both the asymmetric urea and succinimidyl moiety important for PSMA binding, was performed using our microwave-assisted one-pot procedure. The chelate formation was successfully achieved even though the precursor included a complicated bioactive moiety. The radiochemical yield of [^99mTc]TMCE was 12–17%, with a radiochemical purity greater than 98% after HPLC purification. [^99mTc]TMCE showed high affinity in vitro, with high accumulation in LNCaP tumors and low hepatic retention in biodistribution and SPECT/CT studies. These findings warrant further evaluation of [^99mTc]TMCE as an imaging agent and support the benefit of this strategy for the design of other PSMA imaging probes.     

Synthesis and biological evaluation of a novel 99mTc cyclopentadienyl tricarbonyl complex ([(Cp-R)99mTc(CO)3]) for sigma-2 receptor tumor imaging

We report the design, synthesis and biological evaluation of a novel ^99mTc 4-(4-cyclohexylpiperazine-1-yl)-butan-1-one-1-cyclopentadienyltricarbonyl technetium ([^99mTc]5) as a potential SPECT tracer for imaging of σ₂ receptors in tumors. [^99mTc]5 was prepared in 25 ± 5% isolated radiochemical yield with radiochemical purity of >99% via double-ligand transfer (DLT) reaction from the ferrocene precursor 2b (4-(4-cyclohexylpiperazine-1-yl)-1-ferrocenylbutan-1-one). The corresponding Re-complex 4 and the ferrocenyl complex 2b showed relatively high affinity towards σ₂ receptors in in vitro competition binding assay (K_i values of 4 and 2b were 64.4 ± 18.5 nM and 43.6 ± 21.3 nM, respectively) and moderate to high selectivity versus σ₁ receptors (K_iσ₁/K_iσ₂ ratios were 12.5 and 95.5, respectively). The log D value of [^99mTc]5 was determined to be 2.52 ± 0.33. Biodistribution studies in mice revealed comparably high initial brain uptake of [^99mTc]5 and slow washout. Administration of haloperidol 5 min prior to injection of [^99mTc]5 significantly reduced the radiotracer uptake in brain, heart, lung, and spleen by 40–50% at 2 h p.i.. Moreover, [^99mTc]5 showed high uptake in C6 glioma cell lines (8.6%) after incubation for 1 h. Blocking with haloperidol to compete with [^99mTc]5 significantly reduced the cell uptake. Preliminary blocking study in C6-brain-tumor bearing rats showed that [^99mTc]5 binds to σ receptors in the brain-tumor specifically. These results are encouraging for further exploration of ^99mTc-labeled probes for σ₂ receptor tumor imaging in vivo.     

Development of 99mTc radiolabeled A85380 derivatives targeting cerebral nicotinic acetylcholine receptor: Novel radiopharmaceutical ligand 99mTc-A-YN-IDA-C4

Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated ion channels that have been implicated in higher brain functions. To elucidate the functional mechanisms underlying nAChRs and contribute significantly to development of drugs targeting neurological and neuropsychiatric diseases, non-invasive nuclear medical imaging can be used for evaluation. In addition, technetium-99m (^99mTc) is a versatile radionuclide used clinically as a tracer in single-photon emission computed tomography. Because A85380 is known as a potent α4β2-nAChR agonist, we prepared A85380 derivatives labeled with ^99mTc using a bifunctional chelate system. A computational scientific approach was used to design the probe efficiently. We used non-radioactive rhenium (Re) for a ^99mTc analog and found that one of the derivatives, Re-A-YN-IDA-C4, exhibited high binding affinity at α4β2-nAChR in both the docking simulation (?19.3 kcal/mol) and binding assay (Ki = 0.4 ± 0.04 nM). Further, ^99mTc-A-YN-IDA-C4 was synthesized using microwaves, and its properties were examined. Consequently, we found that ^99mTc-A-YN-IDA-C4, with a structure optimized by using computational chemistry techniques, maintained affinity and selectivity for nAChR in vitro and possessed efficient characteristics as a nuclear medicine molecular imaging probe, demonstrated usefulness of computational scientific approach for molecular improvement strategy.     

The chemistry of rhenium and technetium as related to the use of isotopes of these elements in therapeutic and diagnostic nuclear medicine

The β emitting isotopes ¹⁸⁶Re and ¹⁸⁸Re are logical choices on which to base therapeutic radiopharmaceuticals that might be expected to be analogous to diagnostic radiopharmaceuticals based on ^99mTc. However, the chemistry of rhenium is sufficiently different from that of technetium so that the development of Re radiopharmaceuticals often cannot be predicated on the known chemistry and biological behavior of ^99mTc radiopharmaceuticals. The relevant chemical differences involve the greater stability of the higher oxidation states of Re (and thus the greater tendency of reduced Re radiopharmaceuticals to undergo re-oxidation to perrhenate), and the greater substitution inertness of reduced Re complexes. These differences are illustrated (1) in the preparation and use of ¹⁸⁶Re (Sn)-HEDP and ^99mTc(Sn)-HEDP diphosphonate radiopharmaceuticals designed, respectively, for palliative therapy and diagnosis of metastatic cancer to bone, and (2) in the preparation and biodistribution of tr-[¹⁸⁶Re(DMPE)₂Cl₂]⁺ and [¹⁸⁶Re(DMPE)₃]⁺, analogs to the potential myocardial perfusion imaging agents tr-[^99mTc(DMPE)₂Cl₂]⁺ and [^99mTc(DMPE)₃]⁺. [HEDP = (1-hydroxyethylidene)diphosphonate; DMPE = 1,2-bis(dimethylphosphino)ethane].     

Synthesis and evaluation of Re/99mTc(I) complexes bearing a somatostatin receptor-targeting antagonist and labeled via a novel [N,S,O] clickable bifunctional chelating agent

The somatostatin receptor subtype 2 (SSTR2) is often highly expressed on neuroendocrine tumors (NETs), making it a popular in vivo target for diagnostic and therapeutic approaches aimed toward management of NETs. In this work, an antagonist peptide (sst₂-ANT) with high affinity for SSTR2 was modified at the N-terminus with a novel [N,S,O] bifunctional chelator (2) designed for tridentate chelation of rhenium(I) and technetium(I) tricarbonyl cores, [Re(CO)₃]⁺ and [^99mTc][Tc(CO)₃]⁺. The chelator-peptide conjugation was performed via a Cu(I)-assisted click reaction of the alkyne-bearing chelator (2) with an azide-functionalized sst₂-ANT peptide (3), to yield NSO-sst₂-ANT (4). Two synthetic methods were used to prepare Re-4 at the macroscopic scale, which differed based on the relative timing of the click conjugation to the [Re(CO)₃]⁺ complexation by 2. The resulting products demonstrated the expected molecular mass and nanomolar in vitro SSTR2 affinity (IC₅₀ values under 30?nM, AR42J cells, [¹²⁵I]iodo-Tyr¹¹-somatostatin-14 radioligand standard). However, a difference in their HPLC retention times suggested a difference in metal coordination modes, which was attributed to a competing N-triazole donor ligand formed during click conjugation. Surprisingly, the radiotracer scale reaction of [^99mTc][Tc(OH₂)₃(CO)₃]⁺ (^99mTc; t_½?=?6?h, 141?keV γ) with 4 formed a third product, distinct from the Re analogues, making this one of the unusual cases in which Re and Tc chemistries are not well matched. Nevertheless, the [^99mTc]Tc-4 product demonstrated excellent in vitro stability to challenges by cysteine and histidine (≥98% intact through 24?h), along with 75% stability in mouse serum through 4?h. In vivo biodistribution and microSPECT/CT imaging studies performed in AR42J tumor-bearing mice revealed improved clearance of this radiotracer in comparison to a similar [^99mTc][Tc(CO)₃]-labeled sst₂-ANT derivative previously studied. Yet despite having adequate tumor uptake at 1?h (4.9% ID/g), tumor uptake was not blocked by co-administration of a receptor-saturating dose of SS-14. Aimed toward realignment of the Re and Tc product structures, future efforts should include distancing the alkyne group from the intended donor atoms of the chelator, to reduce the coordination options available to the [M(CO)₃]⁺ core (M?=?Re, ^99mTc) by disfavoring involvement of the N-triazole.     

Preparation of classical Re/99mTc(CO)3+ and novel 99mTc(CO)2(NO)2+ cores complexed with flavonol derivatives and their binding characteristics for Aβ(1–40) aggregates

Classical ^99mTc(CO)₃⁺ and novel ^99mTc(CO)₂(NO)²⁺ cores complexed with flavonol derivatives were prepared. Autoradiography of postmortem AD transgenic mice (Tg C57, APP, PS1 12-month-old) brain section confirmed the binding property of [^99mTc(CO)₃⁺-3-OH-flavone]⁰ to Aβ_(1–40) aggregates, while the novel ^99mTc(CO)₂(NO)²⁺ labeled compounds showed no binding sites in AD transgenic mice sections. Intravenous administration of [^99mTc(CO)₃⁺-3-OH-flavone]⁰ resulted in moderate brain uptake (0.48 ± 0.05%ID/g) at 5 min post-injection and slow clearance from the brain issues in 2 h post-injection (120 min: 0.39 ± 0.08%ID/g). Then an Aβ_(1–40)-receptor-targeted Re(CO)₃⁺-3-OH-flavone, was prepared to identify the structure of the technetium complex. UV–vis absorption and fluorescence emission properties have been studied at room temperature in order to determine the natures of the lowest electronically excited states of Re(CO)₃⁺-3-OH-flavone and the ligand. The fluorescent rhenium complex Re(CO)₃⁺-3-OH-flavone showed high affinity for Aβ_(1–40) aggregates in vitro by fluorescence spectra (dissociation constant (K_d) = 11.16 nM). In conclusion, the results suggested that ^99mTc(CO)₃⁺-3-OH-flavone should be a suitable candidate as Aβ plaque SPECT imaging agent for AD.     

Preparation and biological behaviour of some neutral 99mTc-carbonyl dithiocarbamates showing rapid hepatobiliary excretion

A simple procedure for the preparation of ^99mTc—carbonyl complexes of dithiocarbamates in high yield and radiochemical purity has been developed and used for the preparation of ^99mTc—carbonyl complexes of bis(2-hydroxyethyl)dithiocarbamate and bis(2-hydroxypropyl)dithiocarbamate. These complexes were found to be extremely stable and their biological behaviour was studied in mice and compared to that of the ^99mTcN- and the ^99mTc-complexes [prepared by dithionite (dit) reduction] of the same ligands. The carbonyl complexes were found to be efficient hepatobiliary agents and cleared more rapidly than the corresponding ^99mTcN- and ^99mTc(dit)-complexes.     

Synthesis and binding affinities of Re(I) and Tc(I)-containing 16α-substituted estradiol complexes: Models for potential breast cancer imaging agents

To develop technetium and rhenium-labeled imaging agents for estrogen receptor (ER) positive breast tumors, we have synthesized tridentate metal tricarbonyl chelates substituted at the 16α-position of estradiol. Their structures were characterized by IR, ¹H NMR, ¹³C NMR, HRMS or elemental analysis. The rhenium complex 7b showed the highest ER binding affinity (RBA = 25.7) among these compounds, so ligand 6b was selected to be labeled by the precursor [^99mTc(H₂O)₃(CO)₃]⁺ to yield technetium(I)-99m complex 7b′ with good radiochemical yields. The lipophilicity of corresponding technetium(I)-99m complex 7b′ was appropriately reduced, which might be favorable to target tissue selectivity in vivo. The stability of complex 7b′ is excellent in 1 mM histidine, 1 mM cysteine, PBS and bovine serum within 6 h in vitro.     

Synthesis of Tc-99m labeled glucosamino-Asp-cyclic(Arg-Gly-Asp-d-Phe-Lys) as a potential angiogenesis imaging agent

Angiogenesis imaging agents for single photon emission computed tomography (SPECT) play a role in diagnosing tumor-induced angiogenesis as well as tumor metastasis. We synthesized and evaluated radiolabeled RGD glycopeptides by incorporation of the [^99mTc(CO)₃(H₂O)₃]⁺. ^99mTc labeled glucosamino-D-c(RGDfK) ([^99mTc]2) was prepared in 90–93% radiochemical yields (decay corrected). In vitro cell binding assays demonstrated selective binding [^99mTc]2 to human umbilical vein endothelial (HUVE) cells, with inhibition of binding to 37.3% of control levels by 10 μM of cold authentic compounds. In addition, [^99mTc]2 was shown to have high binding affinity to purified α_vβ₃ integrin (IC₅₀ = 1.5 nM). These results suggest that these radiolabeled RGD glycopeptides may have value for non-invasive assessment of angiogenesis.     

Analysis of elimination mechanisms of some 99mTc-complexes

The biological behaviour of complexes of ^99mTc with aminopolycarboxylic and aminocarbohydroxamic ligands EDTA (ethylenediaminetetraacetic acid), DTPA (diethylenetriaminepentaacetic acid), EDTAH (ethylenediaminetetraacetohydroxamic acid) and HIDAmH (N-2-hydroxyethyl-N-carboxymethyl-aminoacetohydroxamic acid) was studied in rabbits. The pharmacokinetic parameters determined in intact rabbits were compared with the results obtained in the study of renal and hepatic clearance of the complexes under study. Hepatobiliary excretion, which in [^99mTc]EDTA forms 20–30% of the total excreted amount, is of negligible magnitude in the other ^99mTc-complexes studied (<2%). Their renal clearance is not influenced by the inhibition of tubular secretion with probenecid. Binding to plasma proteins increases in the order [^99mTc]DTPA < [^99mTc]EDTA <[^99mTc]HIDAmH <[^99mTc]EDTAH and the elimination half-life increases in the same order. The value of renal clearance of the complexes studied related to inulin clearance correlates well with the fraction of the free drug in the plasma. In rabbits the complexes under study are excreted mainly by the mechanism of glomerular filtration in the kidney.     

Development of a Radiolabeled Peptide-Based Probe Targeting MT1-MMP for Breast Cancer Detection

Breast cancer is one of the most frequent and aggressive primary tumors among women of all races. Matrix metalloproteinase (MMPs), a family of zinc- and calcium-dependent secreted or membrane anchored endopeptidases, is overexpressed in varieties of diseases including breast cancer. Therefore, noninvasive visualization and quantification of MMP in vivo are of great interest in basic research and clinical application for breast cancer early diagnosis. Herein, we developed a ^99mTc labeled membrane type I matrix metalloproteinase (MT1-MMP) specific binding peptide, [^99mTc]-(HYNIC-AF7p)(tricine)(TPPTS), for in vivo detection of MDA-MB-231 breast tumor by single photon emission computed tomography (SPECT). [^99mTc]-(HYNIC-AF7p)(tricine)(TPPTS) demonstrated nice biostability and high MT1-MMP binding affinity in vitro and in vivo. Tumor-to-muscle ratio was found to reach to the highest (4.17±0.49) at 2 hour after intravenously administration of [^99mTc]-(HYNIC-AF7P)(tricine)(TPPTS) into MDA-MB-231 tumor bearing mice. Overall, [^99mTc]-(HYNIC-AF7P)(tricine)(TPPTS) demonstrated great potential for MT1-MMP targeted detection in vivo and it would be a promising molecular imaging probe that are probably beneficial to breast cancer early diagnoses.     

First example of well-characterized Re and Tc tricarbonyl complexes of ciprofloxacin and norfloxacin in the development of infection-specific imaging agents

Aiming at the development of ^99mTc-based infection-specific imaging agents, the synthesis and characterization of rhenium and technetium-99m tricarbonyl complexes with derivatized ciprofloxacin and norfloxacin is hereby reported. The ligands were prepared by coupling the tridentate chelator picolylamino-N,N-diacetic acid (PADA) with the piperazinyl (NH) nitrogen of ciprofloxacin or norfloxacin, through the employment of the PADA anhydride. The corresponding rhenium complexes were synthesized using the fac-[NEt₄]₂[ReBr₃(CO)₃] precursor and were fully characterized by elemental analysis and NMR spectroscopy. X-ray crystallography of the ciprofloxacin complex showed that the geometry about rhenium is distorted octahedral defined by the NNO donor atom set of the tridentate chelator and the three carbonyl groups. The analogous technetium-99m complexes were prepared quantitatively through the use of the fac-[^99mTc(H₂O)₃(CO)₃]⁺ precursor and their structure was established by comparative HPLC studies using the well-characterized rhenium complexes as reference. Preliminary studies with the technetium-99m complexes showed high bacterial uptake in vitro.     

Functionalized thymidine derivatives as carriers for the γ-emitter technetium tricarbonyl moiety

Nuclear medicine imaging of cell proliferation has gained broad interest in clinical oncology. A good tracer to image cell proliferation, possibly associated to tumour progression, should rapidly and specifically be incorporated into growing DNA. Following this idea, we have singled out thymidine as a potential biological carrier for delivery of the organometallic fragment [M(CO)₃]⁺ (M = “cold” Re, γ-emitter ^99mTc) to DNA. In the present paper we report the two-step synthesis of thymidine N-3 derivatives and their coordination complexes. Thymidine could be alkylated without protecting the sugar alcoholic functions. Methyl-2,2′-diaminodiethylamine thymidine derivative was quickly reacted with rhenium and technetium carbonyls in almost quantitative yield. The resulting ^99mTc complex represents a convenient radiopharmaceutical for single photon emission tomography (SPECT).     

Synthesis of Tc-99m labeled 1,2,3-triazole-4-yl c-met binding peptide as a potential c-met receptor kinase positive tumor imaging agent

The mesenchymal-epithelial transition factor (c-Met), which is related to tumor cell growth, angiogenesis and metastases, is known to be overexpressed in several tumor types. In this study, we synthesized technetium-99m labeled 1,2,3-triazole-4-yl c-Met binding peptide (cMBP) derivatives, prepared by solid phase peptide synthesis and the ‘click-to-chelate’ protocol for the introduction of tricarbonyl technetium-99m, as a potential c-Met receptor kinase positive tumor imaging agent, and evaluated their in vitro c-Met binding affinity, cellular uptake, and stability. The ^99mTc labeled cMBP derivatives ([^99mTc(CO)₃]12, [^99mTc(CO)₃]13, and [^99mTc(CO)₃]14) were prepared in 85-90% radiochemical yields. The cold surrogate cMBP derivatives, [Re(CO)₃]12, [Re(CO)₃]13, and [Re(CO)₃]14, were shown to have high binding affinities (0.13 μM, 0.06 μM, and 0.16 μM, respectively) to a purified cMet/Fc chimeric recombinant protein. In addition, the in vitro cellular uptake and inhibition studies demonstrated the high specific binding of these ^99mTc labeled cMBP derivatives ([^99mTc(CO)₃]12–14) to c-Met receptor positive U87MG cells.     

Direct 99mTc labeling of monoclonal antibodies: Radiolabeling and in vitro stability

Direct labeling involves ^99mTc binding to different donor groups on the protein, giving multiple binding sites of various affinities resulting in an in vivo instability. The stability has been considerably improved by activating the antibody using a controlled reduction reaction (using 2-aminoethanethiol). This reaction generates sulfhydryl groups, which are known to strongly bind ^99mTc. The direct ^99mTc antibody labeling method was explored using whole antibodies and fragments. Analytical methods were developed for routine evaluation of radiolabeling yield and in vitro stability.Stable direct antibody labeling with ^99mTc requires the generation of sulfhydryl groups, which show high affinity binding sites for ^99mTc. Such groups are obtained with 2-aminoethanethiol (AET), which induces the reduction of the intrachain or interchain disulfide bond, with no structural deterioration or any loss of immunobiological activity of the antibody. The development of fast, reliable analytical methods has made possible the qualitative and quantitative assessment of technetium species generated by the radiolabeling process. Labeling stability is determined by competition of the ^99mTc-antibody bond with three ligands, Chelex 100 (a metal chelate-type resin), free DTPA solution and 1% HSA solution.Very good ^99mTc-antibody stability is obtained with activated IgG (IgGa) and Fab′ fragment, which makes these substances possible candidates for immunoscintigraphy use.     

Synthesis and SAR of 99mTc/Re-labeled small molecule prostate specific membrane antigen inhibitors with novel polar chelates

Prostate specific membrane antigen (PSMA) is recognized as an attractive molecular target for the development of radiopharmaceuticals to image and potentially treat metastatic prostate cancer. A series of novel ^99mTc/Re-tricarbonyl radiolabeled PSMA inhibitors were therefore synthesized by the attachment of glutamate-urea-lysine (Glu-urea-Lys) and glutamate-urea-glutamate (Glu-urea-Glu) pharmacophore to single amino acid chelate (SAAC) where the SAAC ligand was either bis(pyridin-2-ylmethyl)amino (DPA), bis((1-methyl-1H-imidazol-2-yl)methyl)amino (NMI), bis((1-(carboxymethyl)-1H-imidazol-2-yl)methyl)amino (CIM) or bis((1-(2-(bis(carboxymethyl)amino)-2-oxoethyl)-1H-imidazol-2-yl)methyl)amino (TIM). The in vitro binding affinity of the rhenium complexes was evaluated using PSMA-expressing human prostate cancer LNCaP cells. IC₅₀ values ranged from 3.8 ± 2 to >2000 nM. A linker between the SAAC chelate and pharmacophore was required for high affinity binding. However, extending the length of the linker did not substantially improve binding. PSMA binding was also influenced by the nature of the SAAC chelate. One of the most potent compounds, 23b (IC₅₀ = 4.8 ± 2.7 nM), was radiolabeled with technetium tricarbonyl ({^99mTc(CO)₃}⁺) to afford the {^99mTc(CO)₃}⁺ complex in excellent yield and high purity. This effort has led to the identification of a diverse series of promising high affinity {^99mTc(CO)₃}⁺ radiolabeled PSMA inhibitors.     

Synthesis and biological evaluation of 99mTc(CO)3(His–CB) as a tumor imaging agent

The chlorambucil l-histidine conjugate was synthesized and radiolabeled with [^99mTc(CO)₃]⁺ core to form the ^99mTc(CO)₃(His–CB) complex. The radiochemical purity of the complex was over 90%. It had good hydrophilicity and was stable at room temperature. The high initial tumor uptake with certain retention, fast clearance from background, good tumor/non-tumor ratios and satisfactory scintigraphic images highlighted the potential of ^99mTc(CO)₃(His–CB) as a tumor imaging agent.     

Complexes with the fac-{M(CO)3} (M = Tc, Re) moiety and long alkyl chain ligands as Lipiodol surrogates

Accumulation of radiopharmaceuticals in the liver is frequently observed and represents in general a limiting factor when developing novel labeled compounds for any purpose in nuclear medicine. Aiming at the treatment of liver cancer with radiopharmaceuticals, such accumulation is desired but the compounds have to remain in the liver over an extended time period rather than being washed out or redistributed over time in the whole body. Lipiodol is known to remain in the liver and we present here a study for the preparation of ¹⁸⁶Re and ^99mTc labeled Lipiodol surrogates expected to behave similarly. We have synthesized two bidentate and two tridentate ligands conjugated to a pendant C18 chain as well as their corresponding fac-[Re(CO)₃]⁺ and fac-[Tc(CO)₃]⁺ complexes. Three of the rhenium complexes have been structurally characterized. Labelling with [¹⁸⁶Re(OH₂)₃(CO)₃]⁺ and [^99mTc(OH₂)₃(CO)₃]⁺, respectively, gave yields in the range of 90%. The complexes could be extracted into Lipiodol due to their high lipophilicity and close structural relationship with the major components of Lipiodol. The complexes are stable in water and in Lipiodol for more than 24 h. These Lipiodol surrogates present new low-valent technetium and rhenium complexes for applications in liver cancer imaging and therapy.     

A study of the concept of non-radioactive unit-dosed reagent kits [cold unit doses (CUDs)] as an efficient and cost-saving method for 99mTc radiopharmaceutical preparation

Traditionally, when preparing ^99mTc-labeled radiopharmaceuticals, [^99mTc]pertechnetate is added to the entire contents of a vial of reagent kit, and patient doses are subsequently withdrawn from the vial. This technique of compounding can be potentially wasteful for two reasons: (1) once reconstituted with ^99mTc, most reagent kits have a relatively short shelf-life, and thus the entire contents may not be used before expiration and (2) due to a need to conserve radioactivity in many hospitals, enough [^99mTc]pertechnetate is added to the reagent kit in order to retrieve only 1–2 patient doses, even though adequate chemicals (ligand, reducing agent, etc.) are present in the reagent kit to supply as many as 5–10 doses. Hence, a method for optimizing the efficient use of reagent kits would be desirable. The purpose of this study was to determine the feasibility of unit-dosing non-radioactive reagent kits and storing these cold unit doses (CUDs) for eventual labeling with ^99mTc. To evaluate this concept, unit doses were prepared from reagent kits of medronate (MDP) and pentetate (DTPA). The specific variables studied in this research were the effects of storage time, storage temperature and reconstitution volume (dilution) on the unit doses. These effects were monitored by measuring the radiochemical and biodistribution properties of the unit doses following their final reconstitution with [^99mTc]pertechnetate. The labeling efficiency was determined using instant thin layer chromatography (ITLC), and the biodistribution patterns of these radiolabeled CUDs were studied in mice. The results showed that MDP- and DTPA-CUDs stored at −18 °C retained the properties which resulted in acceptable radiochemical purity and biodistribution in mice for as long as 30 days. On the other hand, the radiochemical purity of MDP and DTPA unit doses stored at 25 °C deteriorated rapidly. Mean radiochemical purities as low as 0.58–19.4% were observed on day 30. Altered biodistributions were observed in a manner consistent with the decreased labeling efficiencies. The CUDs of lower dilution (3 mL) appeared to be more stable than the CUDs of higher dilution (10 mL). However, the effect of reconstitution volume was much less significant than the temperature effect on the CUDs. In conclusion, the concept of unit-dosing non-radioactive reagent kits appears to provide an efficient and cost-saving method for preparing infrequent and emergency radiopharmaceutical doses. The study also showed that the storage temperature of these unit doses is critical to the success of the procedure. The volume of reconstitution has a minimal impact on the stability of CUDs if stored at the appropriate temperature.