99mTc-centered one-pot synthesis for preparation of 99mTc radiotracers

Insights into the development of 99mTc-radioligands for serotonergic receptors imaging: Synthesis, labeling, In vitro, and In vivo studies

Serotonergic (5-hydroxytryptamine; 5-HT) receptors play critical roles in neurological and psychological disorders such as schizophrenia, anxiety, depression, and Alzheimer's diseases. Therefore, it is particularly important to develop novel radioligands or modify the existing ones to identify the serotonergic receptors involved in psychiatric disorders. Among the 16 subtypes of serotonergic systems, only technetium-99m based radiopharmaceuticals have been evaluated for serotonin-1A (5-HT1A), serotonin-2A (5-HT2A), 5-HT1A/7 heterodimers and serotonin receptor neurotransmitter (SERT). This review focuses on recent efforts in the design, synthesis and evaluation of 99mTc-radioligands used for single photon emission computerized tomography (SPECT) imaging of serotonergic (5-HT) receptors. Additionally, the discussion will cover aspects such as chemical structure, in vitro/vivo stability, affinity toward serotonin receptors, blood-brain barrier permeation (BBB), and biodistribution study.

Comparative Preclinical Evaluation of HYNIC-Modified Designed Ankyrin Repeat Proteins G3 for the 99mTc-Based Imaging of HER2-Expressing Malignant Tumors

HER2 status determination is a necessary step for the proper choice of therapy and selection of patients for the targeted treatment of cancer. Targeted radiotracers such as radiolabeled DARPins provide a noninvasive and effective way for the molecular imaging of HER2 expression. This study aimed to evaluate tumor-targeting properties of three 99mTc-labeled DARPin G3 variants containing Gly-Gly-Gly-Cys (G3C), (Gly-Gly-Gly-Ser)3-Cys ((G3S)3C), or Glu-Glu-Glu-Cys (E3C) amino acid linkers at the C-terminus and conjugated to the HYNIC chelating agent, as well as to compare them with the clinically evaluated DARPin G3 labeled with 99mTc(CO)3 using the (HE)3-tag at the N-terminus. The labeling of DARPin G3-HYNIC variants provided radiochemical yields in the range of 50-80%. Labeled variants bound specifically to human HER2-expressing cancer cell lines with affinities in the range of 0.5-3 nM. There was no substantial influence of the linker and HYNIC chelator on the binding of 99mTc-labeled DARPin G3 variants to HER2 in vitro; however, [99mTc]Tc-G3-(G3S)3C-HYNIC had the highest affinity. Comparative biodistribution of [99mTc]Tc-G3-G3C-HYNIC, [99mTc]Tc-G3-(G3S)3C-HYNIC, [99mTc]Tc-G3-E3C-HYNIC, and [99mTc]Tc-(HE)3-G3 in healthy CD1 mice showed that there was a strong influence of the linkers on uptake in normal tissues. [99mTc]Tc-G3-E3C-HYNIC had an increased retention of activity in the liver and the majority of other organs compared to the other conjugates. The tumor uptake of [99mTc]Tc-G3-(G3S)3C-HYNIC and [99mTc]Tc-(HE)3-G3 in Nu/j mice bearing SKOV-3 xenografts was similar. The specificity of tumor targeting in vivo was demonstrated for both tracers. [99mTc]Tc-G3-(G3S)3C-HYNIC provided comparable, although slightly lower tumor-to-lung, tumor-to spleen and tumor-to-liver ratios than [99mTc]Tc-(HE)3-G3. Radiolabeling of DARPin G3-HYNIC conjugates with 99mTc provided the advantage of a single-step radiolabeling procedure; however, the studied HYNIC conjugates did not improve imaging contrast compared to the 99mTc-tricarbonyl-labeled DARPin G3. At this stage, [99mTc]Tc-(HE)3-G3 remains the most promising candidate for the clinical imaging of HER2-overexpressing cancers.

Versatile Diphosphine Chelators for Radiolabeling Peptides with 99mTc and 64Cu

We have developed a diphosphine (DP) platform for radiolabeling peptides with 99mTc and 64Cu for molecular SPECT and PET imaging, respectively. Two diphosphines, 2,3-bis(diphenylphosphino)maleic anhydride (DPPh) and 2,3-bis(di-p-tolylphosphino)maleic anhydride (DPTol), were each reacted with a Prostate Specific Membrane Antigen-targeted dipeptide (PSMAt) to yield the bioconjugates DPPh-PSMAt and DPTol-PSMAt, as well as an integrin-targeted cyclic peptide, RGD, to yield the bioconjugates DPPh-RGD and DPTol-RGD. Each of these DP-PSMAt conjugates formed geometric cis/trans-[MO2(DPX-PSMAt)2]+ (M = 99mTc, 99gTc, natRe; X = Ph, Tol) complexes when reacted with [MO2]+ motifs. Furthermore, both DPPh-PSMAt and DPTol-PSMAt could be formulated into kits containing reducing agent and buffer components, enabling preparation of the new radiotracers cis/trans-[99mTcO2(DPPh-PSMAt)2]+ and cis/trans-[99mTcO2(DPTol-PSMAt)2]+ from aqueous 99mTcO4- in 81% and 88% radiochemical yield (RCY), respectively, in 5 min at 100 °C. The consistently higher RCYs observed for cis/trans-[99mTcO2(DPTol-PSMAt)2]+ are attributed to the increased reactivity of DPTol-PSMAt over DPPh-PSMAt. Both cis/trans-[99mTcO2(DPPh-PSMAt)2]+ and cis/trans-[99mTcO2(DPTol-PSMAt)2]+ exhibited high metabolic stability, and in vivo SPECT imaging in healthy mice revealed that both new radiotracers cleared rapidly from circulation, via a renal pathway. These new diphosphine bioconjugates also furnished [64Cu(DPX-PSMAt)2]+ (X = Ph, Tol) complexes rapidly, in a high RCY (>95%), under mild conditions. In summary, the new DP platform is versatile: it enables straightforward functionalization of targeting peptides with a diphosphine chelator, and the resulting bioconjugates can be simply radiolabeled with both the SPECT and PET radionuclides, 99mTc and 64Cu, in high RCYs. Furthermore, the DP platform is amenable to derivatization to either increase the chelator reactivity with metallic radioisotopes or, alternatively, modify the radiotracer hydrophilicity. Functionalized diphosphine chelators thus have the potential to provide access to new molecular radiotracers for receptor-targeted imaging.

Diphosphine Bioconjugates via Pt(0)-Catalyzed Hydrophosphination. A Versatile Chelator Platform for Technetium-99m and Rhenium-188 Radiolabeling of Biomolecules

The ability to append targeting biomolecules to chelators that efficiently coordinate to the diagnostic imaging radionuclide, 99mTc, and the therapeutic radionuclide, 188Re, can potentially enable receptor-targeted "theranostic" treatment of disease. Here we show that Pt(0)-catalyzed hydrophosphination reactions are well-suited to the derivatization of diphosphines with biomolecular moieties enabling the efficient synthesis of ligands of the type Ph2PCH2CH2P(CH2CH2-Glc)2 (L, where Glc = a glucose moiety) using the readily accessible Ph2PCH2CH2PH2 and acryl derivatives. It is shown that hydrophosphination of an acrylate derivative of a deprotected glucose can be carried out in aqueous media. Furthermore, the resulting glucose-chelator conjugates can be radiolabeled with either 99mTc(V) or 188Re(V) in high radiochemical yields (>95%), to furnish separable mixtures of cis- and trans-[M(O)2L2]+ (M = Tc, Re). Single photon emission computed tomography (SPECT) imaging and ex vivo biodistribution in healthy mice show that each isomer possesses favorable pharmacokinetic properties, with rapid clearance from blood circulation via a renal pathway. Both cis-[99mTc(O)2L2]+ and trans-[99mTc(O)2L2]+ exhibit high stability in serum. This new class of functionalized diphosphine chelators has the potential to provide access to receptor-targeted dual diagnostic/therapeutic pairs of radiopharmaceutical agents, for molecular 99mTc SPECT imaging and 188Re systemic radiotherapy.

Technetium Nitrido Complexes of Tetradentate Thiosemicarbazones: Kit-Based Radiolabeling, Characterization, and In Vivo Evaluation

Bis(thiosemicarbazone) and pyridylhydrazone-thiosemicarbazone chelators have demonstrated utility in nuclear medicine. In particular, the 64Cu2+ complexes have been extensively developed for hypoxia imaging and molecular imaging of peptide and protein markers of disease. However, the chemistry and application of bis(thiosemicarbazone) and pyridylhydrazone-thiosemicarbazone chelators in combination with 99mTc, the most widely used radionuclide in nuclear medicine, is underexplored. Herein, a series of bis(thiosemicarbazone) and pyridylhydrazone-thiosemicarbazone chelators were radiolabeled with nitrido-technetium-99m in an optimized one-pot synthesis from [99mTc]TcO4-. Optimization of the radiochemical syntheses allowed for production of the complexes in >90% radiochemical conversion with apparent molar activities of 3.3-5 GBq/μmol. Competition experiments demonstrated the excellent stability of the complexes. The nitrido-technetium-99 complexes were synthesized, and the chemical identities were investigated using mass spectrometry, spectroscopy, and density functional theory calculations. Complexation of nitrido-rhenium(V) was achieved with the N4-dialkylated bis(thiosemicarbazones). Planar imaging and ex vivo biodistribution studies of the five 99mTc complexes were conducted on healthy BALB/c mice to determine in vivo behavior. The lipophilic nature of the complexes resulted in uptake of 1.6-5.7% ID g-1 in the brain at 2 min postinjection and retention of 0.4-1.7% ID g-1 at 15 min postinjection. The stability of the complexes and the biodistribution data demonstrate that these chelators are ideal platforms for future production of radiopharmaceutical candidates.

Comparative Preclinical Evaluation of Peptide-Based Chelators for the Labeling of DARPin G3 with 99mTc for Radionuclide Imaging of HER2 Expression in Cancer

Non-invasive radionuclide imaging of human epidermal growth factor receptor type 2 (HER2) expression in breast, gastroesophageal, and ovarian cancers may stratify patients for treatment using HER2-targeted therapeutics. Designed ankyrin repeat proteins (DARPins) are a promising type of targeting probe for radionuclide imaging. In clinical studies, the DARPin [99mTc]Tc-(HE)3-G3 labeled using a peptide-based chelator His-Glu-His-Glu-His-Glu ((HE)3), provided clear imaging of HER2 expressing breast cancer 2-4 h after injection. The goal of this study was to evaluate if the use of cysteine-containing peptide-based chelators Glu-Glu-Glu-Cys (E3C), Gly-Gly-Gly-Cys (G3C), and Gly-Gly-Gly-Ser-Cys connected via a (Gly-Gly-Gly-Ser)3-linker (designated as G3-(G3S)3C) would further improve the contrast of imaging using 99mTc-labeled derivatives of G3. The labeling of the new variants of G3 provided a radiochemical yield of over 95%. Labeled G3 variants bound specifically to human HER2-expressing cancer cell lines with affinities in the range of 1.9-5 nM. Biodistribution of [99mTc]Tc-G3-G3C, [99mTc]Tc-G3-(G3S)3C, and [99mTc]Tc-G3-E3C in mice was compared with the biodistribution of [99mTc]Tc-(HE)3-G3. It was found that the novel variants provide specific accumulation in HER2-expressing human xenografts and enable discrimination between tumors with high and low HER2 expression. However, [99mTc]Tc-(HE)3-G3 provided better contrast between tumors and the most frequent metastatic sites of HER2-expressing cancers and is therefore more suitable for clinical applications.

Glycol-Chitosan-Based Technetium-99m-Loaded Multifunctional Nanomicelles: Synthesis, Evaluation, and In Vivo Biodistribution

We hereby propose the use of stable, biocompatible, and uniformly sized polymeric micelles as high-radiotracer-payload carriers at region-of-interest with negligible background activity due to no or low offsite radiolysis. We modified glycol chitosan (GC) polymer with varying levels of palmitoylation (P) and quaternization (Q). Quaternary ammonium palmitoyl glycol chitosan (GCPQ) with a Q:P ratio of 9:35 (Q9P35GC) offers >99% biocompatibility at 10 mg mL−1. Q9P35GC micelles exhibit >99% 99mTechnetium (99mTc) radiolabeling via the stannous chloride reduction method without heat. The 99mTc-Q9P35GC micelles (65 ± 3 nm) exhibit >98% 6 h serum stability at 37 °C and 7 day of radiochemical stability at 25 °C. HepG2 cells show a higher uptake of FITC-Q9P35GC than Q13P15GC and Q20P15GC. The in vivo 24 h organ cumulated activity (MBq h) order follows: liver (234.4) > kidneys (60.95) > GIT (0.73) > spleen (88.84). The liver to organ ratio remains higher than 2.4, rendering a better contrast in the liver. The radiotracer uptake decreases significantly in fibrotic vs. normal liver, whereas a blocking study with excess Q9P35GC significantly decreases the radiotracer uptake in a healthy vs. fibrotic liver. FITC-Q9P35GC shows in vivo hepato-specific uptake. Radiotracer liver uptake profile follows reversible binding kinetics with data fitting to two-tissue compartmental (2T), and graphical Ichise multilinear analysis (MA2) with lower AIC and higher R2 values, respectively. The study concludes that 99mTc-Q9P35GC can be a robust radiotracer for noninvasive hepatocyte function assessment and diagnosis of liver fibrosis. Furthermore, its multifunctional properties enable it to be a promising platform for nanotheranostic applications.

N-Centered Tripodal Phosphine Re(V) and Tc(V) Oxo Complexes: Revisiting a [3 + 2] Mixed-Ligand Approach

N-Triphos derivatives (NP₃^R, R = alkyl, aryl) and asymmetric variants (NP₂^RX^R′, R′ = alkyl, aryl, X = OH, NR₂, NRR′) are an underexplored class of tuneable, tripodal ligands in relation to the coordination chemistry of Re and Tc for biomedical applications. Mixed-ligand approaches are a flexible synthetic route to obtain Tc complexes of differing core structures and physicochemical properties. Reaction of the NP₃^Ph ligand with the Re(V) oxo precursor [ReOCl₃(PPh₃)₂] generated the bidentate complex [ReOCl₃(κ²-NP₂^PhOH^Ar)], which possesses an unusual AA’BB’XX’ spin system with a characteristic second-order NMR lineshape that is sensitive to the bi- or tridentate nature of the coordinating diphosphine unit. The use of the asymmetric NP₂^PhOH^Ar ligand resulted in the formation of both bidentate and tridentate products depending on the presence of base. The tridentate Re(V) complex [ReOCl₂(κ³-NP₂^PhO^Ar)] has provided the basis of a new reactive “metal-fragment” for further functionalization in [3 + 2] mixed-ligand complexes. The synthesis of [3 + 2] complexes with catechol-based π-donors could also be achieved under one-pot, single-step conditions from Re(V) oxo precursors. Analogous complexes can also be synthesized from suitable ⁹⁹Tc(V) precursors, and these complexes have been shown to exhibit highly similar structural properties through spectroscopic and chromatographic analysis. However, a tendency for the {M^VO}³⁺ core to undergo hydrolysis to the {M^VO₂}⁺ core has been observed both in the case of M = Re and markedly for M = ⁹⁹Tc complexes. It is likely that controlling this pathway will be critical to the generation of further stable Tc(V) derivatives.

An N-centered tripodal heterofunctionalized phosphine ligand was used to generate a reactive “metal-fragment” based on the {M^VO}³⁺ (M = Re, ⁹⁹Tc) core for the formation of mixed-ligand [3 + 2] complexes. Characteristic lineshapes arising from an AA’BB’XX’ spin system are diagnostic of bidentate vs tridentate coordination modes of the ligand.

A Review on the Current State and Future Perspectives of [99mTc]Tc-Housed PSMA-i in Prostate Cancer

Recently, prostate-specific membrane antigen (PSMA) has gained momentum in tumor nuclear molecular imaging as an excellent target for both the diagnosis and therapy of prostate cancer. Since 2008, after years of preclinical research efforts, a plentitude of radiolabeled compounds mainly based on low molecular weight PSMA inhibitors (PSMA-i) have been described for imaging and theranostic applications, and some of them have been transferred to the clinic. Most of these compounds include radiometals (e.g., ⁶⁸Ga, ⁶⁴Cu, ¹⁷⁷Lu) for positron emission tomography (PET) imaging or endoradiotherapy. Nowadays, although the development of new PET tracers has caused a significant drop in single-photon emission tomography (SPECT) research programs and the development of new technetium-99m (^99mTc) tracers is rare, this radionuclide remains the best atom for SPECT imaging owing to its ideal physical decay properties, convenient availability, and rich and versatile coordination chemistry. Indeed, ^99mTc still plays a relevant role in diagnostic nuclear medicine, as the number of clinical examinations based on ^99mTc outscores that of PET agents and ^99mTc-PSMA SPECT/CT may be a cost-effective alternative for ⁶⁸Ga-PSMA PET/CT. This review aims to give an overview of the specific features of the developed [^99mTc]Tc-tagged PSMA agents with particular attention to [^99mTc]Tc-PSMA-i. The chemical and pharmacological properties of the latter will be compared and discussed, highlighting the pros and cons with respect to [⁶⁸Ga]Ga-PSMA11.

One-step, kit-based radiopharmaceuticals for molecular SPECT imaging: a versatile diphosphine chelator for 99mTc radiolabelling of peptides

Radiotracers labelled with technetium-99m (99mTc) enable accessible diagnostic imaging of disease, provided that radiotracer preparation is simple. Whilst 99mTc radiopharmaceuticals for imaging perfusion are routinely prepared from kits, and regularly used in healthcare, there are no 99mTc-labelled receptor-targeted radiopharmaceuticals in widespread clinical use. This is in part due to the multistep radiosyntheses required for the latter. We demonstrate that the diphosphine, 2,3-bis(diphenylphosphino)maleic anhydride (BMA), is an excellent platform for preparation of kit-based, receptor-targeted 99mTc-labelled radiotracers: its conjugates are simple to prepare and can be easily labelled with 99mTc using one-step, kit-based protocols. Here, reaction of BMA with the αvβ3-integrin receptor targeted cyclic peptide, Arg-Gly-Asp-DPhe-Lys (RGD), provided the first diphosphine-peptide conjugate, DP-RGD. DP-RGD was incorporated into a "kit", and addition of a saline solution containing 99mTcO4- to this kit, followed by heating, furnished the radiotracer [99mTcO2(DP-RGD)2]+ in consistently high radiochemical yields (>90%). The analogous [ReO2(DP-RGD)2]+ compound was prepared and characterised, revealing that both [99mTcO2(DP-RGD)2]+ and [ReO2(DP-RGD)2]+ consist of a mixture of cis and trans geometric isomers. Finally, [99mTcO2(DP-RGD)2]+ exhibited high metabolic stability, and selectively targeted αvβ3-integrin receptors, enabling in vivo SPECT imaging of αvβ3-integrin receptor expression in mice.

New Bioconjugated Technetium and Rhenium Folates Synthesized by Transmetallation Reaction with Zinc Derivatives

The zinc dithiocarbamates functionalized with folic acid 2_Zn and 3_Zn were synthesized with a simple straightforward method, using an appropriated folic acid derivative and a functionalized zinc dithiocarbamate (1_Zn). Zinc complexes 2_Zn and 3_Zn show very low solubilities in water, making them useful for preparing Tc-99m radiopharmaceuticals with a potentially high molar activity. Thus, the transmetallation reaction in water medium between the zinc complexes 2_Zn or 3_Zn and the cation fac-[^99mTc(H₂O)₃(CO)₃]⁺, in the presence of the monodentate ligand TPPTS, leads to the formation of the 2 + 1 complexes fac-[^99mTc(CO)₃(SS)(P)] bioconjugated to folic acid (2_Tc and 3_Tc). In spite of the low solubility of 2_Zn and 3_Zn in water, the reaction yield is higher than 95%, and the excess zinc reagent is easily removed by centrifugation. The Tc-99m complexes were characterized by comparing their HPLC with those of the homologous rhenium complexes (2_Re and 3_Re) previously synthesized and characterized by standard methods. Preliminary in vivo studies with 2_Tc and 3_Tc indicate low specific binding to folate receptors. In summary, Tc-99m folates 2_Tc and 3_Tc were prepared in high yields, using a one-pot transmetallation reaction with low soluble zinc dithiocarbamates (>1 ppm), at moderate temperature, without needing a subsequent purification step.

Bioconjugated technetium carbonyls by transmetalation reaction with zinc derivatives

The transmetalation reaction between zinc dithiocarbamates functionalized with organic groups and the cation fac-[^99mTc(H₂O)₃(CO)₃]⁺ has been studied as a new strategy to bind biomolecules to this radionuclide for preparing radiopharmaceuticals with high molar activity. All complexes were obtained in high yields by heating at moderate temperatures and without subsequent purification. The chemical identity was ascertained by HPLC comparison with the homologous rhenium complexes. Stability studies in cysteine solution and serum have shown a good stability of the coordination set fac-[^99mTc(CO)₃(SS)(P)]. Preliminary biological studies of the radiocomplex functionalized with D-(+)-glucosamine with carcinoma cells have been performed.

Recent advances in the synthesis of (99mTechnetium) based radio-pharmaceuticals

Technetium radionuclide (99mTc) has excellent extent of disintegration properties and occupies a special place in the field of nuclear medicinal chemistry and other health disciplines. Current review describes recent approaches of synthesis in detailed ways for radio-pharmaceuticals of technetium which have been developed to treat and diagnose the biotic disorders. These technetium labeled radio-pharmaceuticals have been established to apply in the field of diagnostic nuclear medicine especially for imaging of different body parts such as brain, heart, kidney, bones and so on, through single photon emission computed tomography (SPECT) that is thought to be difficult to image such organs by using common X-ray and MRI (Magnetic Resonance Imaging) techniques. This review highlights and accounts an inclusive study on the various synthetic routes of technetium labeled radio-pharmaceuticals using ligands with various donor atoms such as carbon, nitrogen, sulphur, phosphorus etc. These compounds can be utilized as next generation radio-pharmaceuticals.

Manipulating Pharmacokinetics of Purification-Free 99mTc-Labeled Bivalent Probes for In Vivo Imaging of Saturable Targets

The accumulation of ^99mTc-labeled probes targeting saturable systems of the body is hindered by the presence of a large excess of unlabeled ligands needed to ensure high radiochemical yields in a short reaction time. To address the issue, we recently reported a novel concept of a metal-coordination-mediated synthesis of a bivalent ^99mTc-labeled probe from a monovalent ligand using d-penicillamine (Pen) as a chelating molecule and c(RGDfK) as a model targeting device. The Pen-conjugated c(RGDfK) via a hexanoate linkage (Pen-Hx-c(RGDfK)) provided a bivalent [^99mTc]Tc-[(Pen-Hx-c(RGDfK))₂ that possessed much higher integrin α_vβ₃ binding affinity than Pen-Hx-c(RGDfK) and visualized a murine tumor without purification. However, high radioactivity levels were observed in the abdominal regions, which necessitated improved pharmacokinetics of the probes for practical applications. In this study, a pharmacokinetic (PK) modifier was introduced to manipulate the pharmacokinetics of the ^99mTc-Pen₂-based bivalent probe. The Hx linkage in Pen-Hx-c(RGDfK) was replaced with acetyl-d-serine-d-serine-glycine (Ac-ssG) or hexanoyl-d-serine-d-serine-d-serine (Hx-sss) to prepare Pen-Ac-ssG-c(RGDfK) or Pen-Hx-sss-c(RGDfK). Pen-Ac-ssG-c(RGDfK) impaired the complexation ability of Pen-Hx-c(RGDfK), and a monovalent ^99mTc-labeled compound was generated at low ligand concentration. However, Pen-Hx-sss-c(RGDfK) provided the objective bivalent ^99mTc-labeled probe in high radiochemical yields at a concentration similar to that of Pen-Hx-c(RGDfK). [^99mTc]Tc-[Pen-Hx-sss-c(RGDfK)]₂ also possessed stability and integrin α_vβ₃ binding affinity similar to those of [^99mTc]Tc-[Pen-Hx-c(RGDfK)]₂. As a result, [^99mTc]Tc-[Pen-Hx-sss-c(RGDfK)]₂ exhibited tumor and abdominal radioactivity levels similar to and significantly lower than those of [^99mTc]Tc-[Pen-Hx-c(RGDfK)]₂. These findings indicate the incorporation of a tripeptide PK modifier to Pen-Hx-c(RGDfK) preserved the complexation ability and improved the pharmacokinetics of the resulting ^99mTc-labeled bivalent probe without impairing the targeting ability. Thus, the [Pen-Hx-(PK modifier)-(targeting device)] would constitute a basic formulation for preparing the ^99mTc-Pen₂-based bivalent probes for imaging saturable targets of the body.

A brief overview of metal complexes as nuclear imaging agents

Metallic radionuclides have been instrumental in the field of nuclear imaging for over half a century. While recent years have played witness to a dramatic rise in the use of radiometals as labels for chelator-bearing biomolecules, imaging agents based solely on coordination compounds of radiometals have long played a critical role in the discipline as well. In this work, we seek to provide a brief overview of metal complex-based radiopharmaceuticals for positron emission tomography (PET) and single photon emission computed tomography (SPECT). More specifically, we have focused on imaging agents in which the metal complex itself rather than a pendant biomolecule or targeting moiety is responsible for the in vivo behavior of the tracer. This family of compounds contains metal complexes based on an array of different nuclides as well as probes that have been used for the imaging of a variety of pathologies, including infection, inflammation, cancer, and heart disease. Indeed, two of the defining traits of transition metal complexes-modularity and redox chemistry-have both been creatively leveraged in the development of imaging agents. In light of our audience, particular attention is paid to structure and mechanism, though clinical data is addressed as well. Ultimately, it is our hope that this review will not only educate readers about some of the seminal work performed in this space over the last 30 years but also spur renewed interest in the creation of radiopharmaceuticals based on small metal complexes.

Towards 99mTc- and Re-Based Multifunctional Silica Platforms for Theranostic Applications

Taking advantage of the radiation properties of 99mTc and 186/188Re and the photophysical characteristics of the {M(CO)3}+ moiety (M = Re), we developed a multifunctional silica platform with the theranostic pair 99mTc/Re with high potential for (nano)medical applications. Starting with a general screening to evaluate the most suitable mesoporous silica construct and the development of appropriate chelate systems, multifunctional mesoporous silica microparticles (SBA-15) were synthesized. These particles act as a model towards the synthesis of the corresponding nanoconstructs. The particles can be modified at the external surface with a targeting function and labeled with the {M(CO)3}+ moiety (M = 99mTc, Re) at the pore surface. Thus, a silica platform is realized, whose bioprofile is not altered by the loaded modalities. The described synthetic procedures can be applied to establish a target-specific theranostic nanoplatform, which enables the combination of fluorescence and radio imaging, with the possibility of radio- and chemotherapy.

An improved kit formulation for one-pot synthesis of [99m Tc]Tc-HYNIC-E[c(RGDfK)]2 for routine clinical use in cancer imaging

Radiolabeled Arg-Gly-Asp (RGD) peptide derivatives have immense potential for non-invasive monitoring of malignancies overexpressing integrin α_v β₃ receptors. Easy availability of suitable radiotracers would augment the utility of this class of molecular imaging agents. Towards this, the present article describes the development of an improved lyophilized kit for the routine clinical formulation of [^99m Tc]Tc complex of HYNIC-conjugated dimeric cyclic RGD peptide derivative E-[c(RGDfK)]₂ (E = glutamic acid, f = phenyl alanine, K = lysine) without using Sn²⁺ and systematic evaluation of its efficacy. Five batches of the kits were prepared, and [^99m Tc]Tc-HYNIC-E[c(RGDfK)]₂ radiotracer was synthesized with high radiochemical purity (98.6 ± 0.5%) and specific activity (124.8 GBq/μmol) using the kits. Biodistribution studies in C57BL/6 mice bearing melanoma tumor exhibited significant accumulation of the radiotracer in tumor (5.32 ± 0.56 %ID/g at 60 min p.i.), and this uptake was also found to be receptor-specific by blocking studies. Preliminary human clinical investigations carried out in 10 breast cancer patients revealed high radiotracer uptake in the tumor along with good tumor-to-background contrast. The developed kit formulation showed an exceptionally high shelf-life of at least 18 months. These results demonstrated promising attributes of the developed kit formulation and warrant more extensive clinical investigations.

Preparation and evaluation of 99mTc-labeled porphyrin complexes prepared using PNP and HYNIC cores: studying the effects of core selection on pharmacokinetics and tumor uptake in a mouse model

Porphyrins are tetrapyrrolic macrocyclic ligands known for their affinity towards neoplastic tissues and once radiolabeled with a suitable diagnostic radioisotope could potentially be used for the imaging of tumorous lesions. In the present study, an unsymmetrically substituted porphyrin derivative namely 5-(p-amino-propyloxyphenyl)-10,15,20-tris(carboxymethyleneoxyphenyl)-porphyrin was synthesized and modified further to enable radiolabeling with ^99mTc using two different ^99mTc-cores viz. ^99mTc-HYNIC (hydrazino nicotinic acid) and ^99mTc(N)PNP2 (PNP2 = bis-[(2-dimethylphosphino)ethyl]-methoxy-ethylamine) in order to study the effect of employing different ^99mTc-cores on tumor affinity and pharmacokinetic behavior of the resultant ^99mTc-labeled porphyrin complexes. ^99mTc-Porphyrin complexes were characterized by reversed phase HPLC studies and could be prepared with >95% radiochemical purity under optimized radiolabeling conditions. Both ^99mTc-complexes were found to be adequately stable in human blood serum till 3 h post-preparation. Bio-distribution studies, carried out in Swiss mice bearing fibrosarcoma tumors, revealed relatively higher tumor uptake for the ^99mTc-HYNIC-porphyrin complex (3.95 ± 1.42 and 3.28 ± 0.27% IA per g) compared to that exhibited by the ^99mTc(N)PNP-DTC-porphyrin complex (1.52 ± 0.53 and 1.56 ± 0.10% IA per g) at 1.5 and 3 h post-administration, although the former complex exhibited comparatively lower lipophilicity in the octanol-water system. Higher uptake and longer retention in the blood were observed for the ^99mTc-HYNIC-porphyrin complex (6.63 ± 0.75 and 4.36 ± 0.25% IA per g) compared to that exhibited by the ^99mTc(N)PNP-DTC-porphyrin complex (2.41 ± 0.54 and 2.30 ± 0.16% IA per g) at both 1.5 and 3 h post-administration. However, relatively lower liver uptake was observed for the former complex (19.26 ± 3.48 and 18.45 ± 1.05% IA per g) than that exhibited by the latter one (39.37 ± 3.88 and 34.15 ± 8.25% IA per g) at both 1.5 and 3 h post-administration. This study indicates that the in vivo behavior exhibited by the ^99mTc-labeled porphyrins not only depends on their lipophilicity/hydrophilicity but is also governed by the Tc-cores employed for radiolabeling.

New 99mTc Radiotracers for Myocardial Perfusion Imaging by SPECT

OBJECTIVE

Myocardial Perfusion Imaging (MPI) with radiotracers is an integral component in evaluation of the patients with known or suspected coronary artery diseases (CAD). 99mTc-Sestamibi and 99mTc-Tetrofosmin are commercial radiopharmaceuticals for MPI by single photon-emission computed tomography (SPECT). Despite their widespread clinical applications, they do not meet the requirements of an ideal perfusion imaging agent due to their inability to linearly track the regional myocardial blood flow rate at >2.5 mL/min/g. With tremendous development of CZT-based SPECT cameras over the past several years, the nuclear cardiology community has been calling for better perfusion radiotracers with improved extraction and biodistribution properties.

METHODS

This review will summarize recent research efforts on new cationic and neutral 99mTc radiotracers for SPECT MPI. The goal of these efforts is to develop a 99mTc radiotracer that can be used to detect perfusion defects at rest or under stress, determine the regional myocardial blood flow, and measure the perfusion and left ventricular function.

RESULTS

The advantage of cationic radiotracers (e.g. 99mTc-Sestamibi) is their long myocardial retention because of the positive molecular charge and fast liver clearance kinetics. 99mTc-Teboroxime derivatives have a high initial heart uptake (high first-pass extraction fraction) due to their neutrality. 99mTc- 3SPboroxime is the most promising radiotracer for future clinical translation considering its initial heart uptake, myocardial retention time, liver clearance kinetics, heart/liver ratios and SPECT image quality.

CONCLUSION

99mTc-3SPboroximine is an excellent example of perfusion radiotracers, the heart uptake of which is largely relies on the regional blood flow. It is possible to use 99mTc-3SPboroximine for detection of perfusion defect(s), accurate quantification and determination of regional blood flow rate. Development of such a 99mTc radiotracer is of great clinical benefit for accurate diagnosis of CAD and assessing the risk of future hard events (e.g. heart attack and sudden death) in cardiac patients.

The synthesis of a 99mTc-labeled tetravalent targeting probe upon isonitrile coordination to 99mTcI for enhanced target uptake in saturable systems

The difference in 2-proton's acidity between L_β and L_G led to dramatically different results of their reactions with [^99mTc][Tc(CO)₃(OH₂)₃]⁺.

A luminescent dicyanidonitridotechnetium(v) core with tridentate ligand coordination sites

A novel, luminescent technetium complex, [TcN(CN)2bpa] (bpa = bis-(2-pyridylmethyl)amine), with tridentate ligand coordination sites was synthesized and characterized. Photoemission with a maximum wavelength at 666 nm was observed in the solid-state at 296 K.

Multifunctional Cyclopentadienes as a Scaffold for Combinatorial Bioorganometallics in [(η5 -C5 H2 R1 R2 R3 )M(CO)3 ] (M=Re, 99m Tc) Piano-Stool Complexes

Multifunctional cyclopentadiene (Cp) ligands and their rhenium and ^99m Tc complexes were prepared by a versatile synthetic route. The properties of these Cp ligands can be tuned on demand, either during their synthesis (variation of R₁ ) or through post-synthetic functionalization with two equal or different vectors (V₁ and V₂ ). Variation of these groups enables a combinatorial approach in the synthesis of bioorganometallic complexes. This is demonstrated by the preparation of Cp ligands containing both electron-donating and electron-withdrawing groups at the R₁ position and their subsequent homo- or heterofunctionalization with biovector models (benzylamine and phenylalanine) under standard amide bond-formation conditions. All ligands can be coordinated to the fac-[Re(CO)₃ ]⁺ and fac-[^99m Tc(CO)₃ ]⁺ cores to give tetrafunctional complexes in straightforward and functional-group-tolerant procedures. The ^99m Tc complexes were prepared in one step, in 30 min, and under aqueous conditions from generator-eluted [^99m TcO₄ ]^- .

Novel Approach for 99mTc-Labeling of Red Blood Cells: Evaluation of 99mTc-4SAboroxime as a Blood Pool Imaging Agent

Angiography with radiolabeled red blood cells (RBCs) plays an important role in diagnosis and prognosis in vascular diseases. Both in vitro and in vivo methods have been developed for ^99mTc-labeling of RBCs. However, these methods are complicated and lack reproducibility. Therefore, it is highly desirable to develop an alternative method for routine ^99mTc-labeling of RBCs. In this report, we present a novel approach for ^99mTc-labeling of RBCs. We prepared a new ^99mTc(III) radiotracer [^99mTcCl(CDO)(CDOH)₂B-4AS] (^99mTc-4ASboroxime: 4AS-B(OH)₂ = 4-aminosulfonylphenyl)boronic acid, and CDOH₂ = cyclohexanedione dioxime) in >95% radiochemical purity. Imaging and biodistribution studies were performed in Sprague-Dawley (SD) rats. It was found that the blood radioactivity was ∼6.0%ID/g (∼90% injected dose for 200-225 g SD rats) for ^99mTc-4ASboroxime with low uptake in the myocardium, kidneys, liver, lungs, and muscle, most likely due to lack of leakage of ^99mTc-labeled RBCs from the intravascular space. The blood radioactivity was almost unchanged over the 2 h period, suggesting that the binding of ^99mTc-4ASboroxime to blood components (cells, proteins, and plasma) is stable. The results from γ-counting of the isolated blood components showed that ^99mTc-4ASboroxime had >95% of blood radioactivity binding to RBCs, ∼1% to albumin, and ∼3% remaining free in blood plasma, demonstrating its RBC-specificity. The results from imaging studies in SD rats indicated that ^99mTc-4ASboroxime is predominantly distributed in the blood pool. Main blood vessels were well delineated in the head/neck and abdominal regions. This statement was further substantiated by the results from imaging studies in pigs. ^99mTc-4ASboroxime is an excellent blood pool agent with the potential for diagnosis and prognosis of vascular diseases.

99m Tc-labeled rHuEpo for imaging of the erythropoietin receptor in tumors

To analyze erythropoietin receptor (EpoR) status in tumors, recombinant human erythropoietin (rHuEpo) was labeled with ^99m Tc by ^99m Tc-centered 1-pot synthesis, resulting in high radiochemical purity, stability, and biological activity. Both in vitro cell culture experiments and biodistribution studies of normal rats demonstrated successful EpoR targeting. The biodistribution of labeled rHuEpo in a NCI-H1975 xenograft model showed tumor accumulation (tumor-to-muscle ratio, 4.27 ± 1.77), confirming the expression of active EpoR in tumors. Thus, as a novel single positron emission computerized tomography tracer for the imaging of EpoR expression in vivo, ^99m Tc-rHuEpo is effective for exploring the role of EpoR in cancer growth, metastasis and angiogenesis.

Thiourea derivatives as chelating agents for bioconjugation of rhenium and technetium

A^99mTc complex with a tetradentate thiocarbamoylbenzamidine group was used for the conjugation of angiotensin-II. The resulting bioconjugate is stablein vivoandin vitro.

Simultaneous SPECT imaging of multi-targets to assist in identifying hepatic lesions

Molecular imaging technique is an attractive tool to detect liver disease at early stage. This study aims to develop a simultaneous dual-isotope single photon emission computed tomography (SPECT)/CT imaging method to assist diagnosis of hepatic tumor and liver fibrosis. Animal models of liver fibrosis and orthotopic human hepatocellular carcinoma (HCC) were established. The tracers of ¹³¹I-NGA and ^99mTc-3P-RGD₂ were selected to target asialoglycoprotein receptor (ASGPR) on the hepatocytes and integrin α_vβ₃ receptor in tumor or fibrotic liver, respectively. SPECT imaging and biodistribution study were carried out to verify the feasibility and superiority. As expected, ^99mTc-3P-RGD₂ had the ability to evaluate liver fibrosis and detect tumor lesions. ¹³¹I-NGA showed that it was effective in assessing the anatomy and function of the liver. In synchronized dual-isotope SPECT/CT imaging, clear fusion images can be got within 30 minutes for diagnosing liver fibrosis and liver cancer. This new developed imaging approach enables the acquisition of different physiological information for diagnosing liver fibrosis, liver cancer and evaluating residual functional liver volume simultaneously. So synchronized dual-isotope SPECT/CT imaging with ^99mTc-3P-RGD₂ and ¹³¹I-NGA is an effective approach to detect liver disease, especially liver fibrosis and liver cancer.

Radiolabeled cyclic RGD peptides as radiotracers for tumor imaging

The integrin family comprises 24 transmembrane receptors, each a heterodimeric combination of one of 18α and one of 8β subunits. Their main function is to integrate the cell adhesion and interaction with the extracellular microenvironment with the intracellular signaling and cytoskeletal rearrangement through transmitting signals across the cell membrane upon ligand binding. Integrin α_vβ₃ is a receptor for the extracellular matrix proteins containing arginine–glycine–aspartic (RGD) tripeptide sequence. The α_vβ₃ is generally expressed in low levels on the epithelial cells and mature endothelial cells, but it is highly expressed in many solid tumors. The α_vβ₃ levels correlate well with the potential for tumor metastasis and aggressiveness, which make it an important biological target for development of antiangiogenic drugs, and molecular imaging probes for early tumor diagnosis. Over the last decade, many radiolabeled cyclic RGD peptides have been evaluated as radiotracers for imaging tumors by SPECT or PET. Even though they are called “α_vβ₃-targeted” radiotracers, the radiolabeled cyclic RGD peptides are also able to bind α_vβ₅, α₅β₁, α₆β₄, α₄β₁, and α_vβ₆ integrins, which may help enhance their tumor uptake due to the “increased receptor population.” This article will use the multimeric cyclic RGD peptides as examples to illustrate basic principles for development of integrin-targeted radiotracers and focus on different approaches to maximize their tumor uptake and T/B ratios. It will also discuss important assays for pre-clinical evaluations of the integrin-targeted radiotracers, and their potential applications as molecular imaging tools for noninvasive monitoring of tumor metastasis and early detection of the tumor response to antiangiogenic therapy.

Purification-Free Method for Preparing Technetium-99m-Labeled Multivalent Probes for Enhanced in Vivo Imaging of Saturable Systems

Metallic radionuclides provide target-specific radiolabeled probes for molecular imaging in radiochemical yields sufficient for administration to subjects without purification. However, unlabeled ligands in the injectate can compete for targeted molecules with radiolabeled probes, which eventually necessitates postlabeling purification. Herein we describe a "1 to 3" design to circumvent the issue by taking advantage of inherent coordination properties of technetium-99m ((99m)Tc). A monovalent RGD ligand possessing an isonitrile as a coordinating moiety (CN-RGD) was reacted with [(99m)Tc(CO)3(OH2)3](+) to prepare [(99m)Tc(CO)3(CN-RGD)3](+) in over 95% radiochemical yields. This complex exhibited higher integrin αvβ3 binding affinity than its unlabeled monovalent ligand, primarily due to its multivalency. This compound visualized a murine tumor without removing unlabeled ligands, while a (99m)Tc-labeled monovalent probe derived from a monovalent ligand could not. The metal coordination-mediated synthesis of radiolabeled multivalent probes thereby can be a useful approach for preparing ready-to-use target-specific probes labeled with (99m)Tc and other metallic radionuclides of interest.

Radiolabeled Cyclic RGD Peptide Bioconjugates as Radiotracers Targeting Multiple Integrins

Angiogenesis is a requirement for tumor growth and metastasis. The angiogenic process depends on vascular endothelial cell migration and invasion, and is regulated by various cell adhesion receptors. Integrins are such a family of receptors that facilitate the cellular adhesion to and migration on extracellular matrix proteins in the intercellular spaces and basement membranes. Among 24 members of the integrin family, αvβ3 is studied most extensively for its role in tumor angiogenesis and metastasis. The αvβ3 is expressed at relatively low levels on epithelial cells and mature endothelial cells, but it is highly expressed on the activated endothelial cells of tumor neovasculature and some tumor cells. This restricted expression makes αvβ3 an excellent target to develop antiangiogenic drugs and diagnostic molecular imaging probes. Since αvβ3 is a receptor for extracellular matrix proteins with one or more RGD tripeptide sequence, many radiolabeled cyclic RGD peptides have been evaluated as "αvβ3-targeted" radiotracers for tumor imaging over the past decade. This article will use the dimeric and tetrameric cyclic RGD peptides developed in our laboratories as examples to illustrate basic principles for development of αvβ3-targeted radiotracers. It will focus on different approaches to maximize the radiotracer tumor uptake and tumor/background ratios. This article will also discuss some important assays for preclinical evaluations of integrin-targeted radiotracers. In general, multimerization of cyclic RGD peptides increases their integrin binding affinity and the tumor uptake and retention times of their radiotracers. Regardless of their multiplicity, the capability of cyclic RGD peptides to bind other integrins (namely, αvβ5, α5β1, α6β4, α4β1, and αvβ6) is expected to enhance the radiotracer tumor uptake due to the increased integrin population. The results from preclinical and clinical studies clearly show that radiolabeled cyclic RGD peptides (such as (99m)Tc-3P-RGD2, (18)F-Alfatide-I, and (18)F-Alfatide-II) are useful as the molecular imaging probes for early cancer detection and noninvasive monitoring of the tumor response to antiangiogenic therapy.

Activation of [(99(m))TcO4](-) by phosphonium cations

Phosphonium salts in solution or bound to solid phase supports interact with the robust [(99(m))TcO4](-) anion in neutral water. This activated form of [(99(m))TcO4](-) represents a source for complexes with the fac-{(99m)Tc(VII)O3}(+) core, which can be synthesized in high yields and purity.

Metal complexes containing natural and and artificial radioactive elements and their applications

Recent advances (during the 2007–2014 period) in the coordination and organometallic chemistry of compounds containing natural and artificially prepared radionuclides (actinides and technetium), are reviewed. Radioactive isotopes of naturally stable elements are not included for discussion in this work. Actinide and technetium complexes with O-, N-, N,O, N,S-, P-containing ligands, as well π-organometallics are discussed from the view point of their synthesis, properties, and main applications. On the basis of their properties, several mono-, bi-, tri-, tetra- or polydentate ligands have been designed for specific recognition of some particular radionuclides, and can be used in the processes of nuclear waste remediation, i.e., recycling of nuclear fuel and the separation of actinides and fission products from waste solutions or for analytical determination of actinides in solutions; actinide metal complexes are also usefulas catalysts forcoupling gaseous carbon monoxide, as well as antimicrobial and anti-fungi agents due to their biological activity. Radioactive labeling based on the short-lived metastable nuclide technetium-99m (^99mTc) for biomedical use as heart, lung, kidney, bone, brain, liver or cancer imaging agents is also discussed. Finally, the promising applications of technetium labeling of nanomaterials, with potential applications as drug transport and delivery vehicles, radiotherapeutic agents or radiotracers for monitoring metabolic pathways, are also described.

Future prospects for SPECT imaging using the radiolanthanide terbium-155 - production and preclinical evaluation in tumor-bearing mice

INTRODUCTION

We assessed the suitability of the radiolanthanide (155)Tb (t1/2=5.32 days, Eγ=87 keV (32%), 105keV (25%)) in combination with variable tumor targeted biomolecules using preclinical SPECT imaging.

METHODS

(155)Tb was produced at ISOLDE (CERN, Geneva, Switzerland) by high-energy (~1.4 GeV) proton irradiation of a tantalum target followed by ionization and on-line mass separation. (155)Tb was separated from isobar and pseudo-isobar impurities by cation exchange chromatography. Four tumor targeting molecules - a somatostatin analog (DOTATATE), a minigastrin analog (MD), a folate derivative (cm09) and an anti-L1-CAM antibody (chCE7) - were radiolabeled with (155)Tb. Imaging studies were performed in nude mice bearing AR42J, cholecystokinin-2 receptor expressing A431, KB, IGROV-1 and SKOV-3ip tumor xenografts using a dedicated small-animal SPECT/CT scanner.

RESULTS

The total yield of the two-step separation process of (155)Tb was 86%. (155)Tb was obtained in a physiological l-lactate solution suitable for direct labeling processes. The (155)Tb-labeled tumor targeted biomolecules were obtained at a reasonable specific activity and high purity (>95%). (155)Tb gave high quality, high resolution tomographic images. SPECT/CT experiments allowed excellent visualization of AR42J and CCK-2 receptor-expressing A431 tumors xenografts in mice after injection of (155)Tb-DOTATATE and (155)Tb-MD, respectively. The relatively long physical half-life of (155)Tb matched in particular the biological half-lives of (155)Tb-cm09 and (155)Tb-DTPA-chCE7 allowing SPECT imaging of KB tumors, IGROV-1 and SKOV-3ip tumors even several days after administration.

CONCLUSIONS

The radiolanthanide (155)Tb may be of particular interest for low-dose SPECT prior to therapy with a therapeutic match such as the β(-)-emitting radiolanthanides (177)Lu, (161)Tb, (166)Ho, and the pseudo-radiolanthanide (90)Y.

Tumour targeting with radiometals for diagnosis and therapy

Use of radiometals in nuclear oncology is a rapidly growing field and encompasses a broad spectrum of radiotracers for imaging via PET (positron emission tomography) or SPECT (single-photon emission computed tomography) and therapy via α, β(-), or Auger electron emission. This feature article opens with a brief introduction to the imaging and therapy modalities exploited in nuclear medicine, followed by a discussion of the multi-component strategy used in radiopharmaceutical development, known as the bifunctional chelate (BFC) method. The modular assembly is dissected into its individual components and each is discussed separately. The concepts and knowledge unique to metal-based designs are outlined, giving insight into how these radiopharmaceuticals are evaluated for use in vivo. Imaging nuclides (64)Cu, (68)Ga, (86)Y, (89)Zr, and (111)In, and therapeutic nuclides (90)Y, (177)Lu, (225)Ac, (213)Bi, (188)Re, and (212)Pb will be the focus herein. Finally, key examples have been extracted from the literature to give the reader a sense of breadth of the field.

Preparation and biological evaluation of (99m) TcN-labeled pteroyl-lys derivative as a potential folate receptor imaging agent

In order to develop a novel (99m) Tc-labeled folate receptor (FR) imaging agent, a dithiocarbamate derivative, pteroyl-lys-DTC, was synthesized and radiolabeled with (99m) Tc through the [(99m) TcN](2+) intermediate. The radiochemical purity of the corresponding (99m) Tc-complex, (99m) TcN-pteroyl-lys-DTC, was over 95% as measured by reversed-phase HPLC. The (99m) TcN complex was stable under physiological conditions. (99m) TcN-pteroyl-lys-DTC exhibited specific FR binding in FR-positive KB cells in vitro. The biodistribution in tumor-bearing mice showed that the (99m) TcN-labeled radiotracer had good uptake (3.56 ± 0.09%ID/g at 2 h postinjection) in FR-positive KB tumors, as well as in the kidneys (30.34 ± 3.53%ID/g at 2 h postinjection). After coinjection with excess folic acid, the uptake in tumor and kidneys was significantly blocked. The results indicated that (99m) TcN-pteroyl-lys-DTC was able to target the FR-positive tumor cells and tissues specifically both in vitro and in vivo.