Preparation and evaluation of a new 99mTc labeled bombesin derivative for tumor imaging

A variety of human tumors like prostate and breast cancer express bombesin receptors. Due to this a new bombesin analogue was labeled with ^99mTc via HYNIC and tricine as a coligand and investigated further. Peptide was synthesized on a solid phase using Fmoc strategy. Labeling with ^99mTc was performed at 100 °C for 10 min and radiochemical analysis involved ITLC and HPLC methods. The stability of radiopeptide was checked in the presence of human serum at 37 °C up to 24 h. Internalization was studied with the human GRP receptor cell line PC-3. The biodistribution was studied in mice. Labeling yield of >98% was obtained corresponding to a specific activity of ~80.9 GBq/μmol. Radiopeptide internalization into PC-3 cells was moderate and specific (10.7 ± 1.2% at 4 h). A high and specific GRP receptor expressing mouse tumor and pancreas uptake (1.12 ± 0.08 and 1.04 ± 0.11% ID/g after 1 h respectively) was also determined.     

Synthesis and evaluation of a new bombesin analog labeled with <Superscript>99m</Superscript>Tc as a GRP receptor imaging agent

Tumors such as prostate, small cell lung cancer, breast, gastric and colon cancer are known to overexpress receptors to bombesin (BBN). In this study, a new bombesin analogue was labeled with ^99mTc via HYNIC and tricine/EDDA as coligands and investigated further. HYNIC-GABA-Bombesin (7–14) NH₂ was synthesized using a standard Fmoc strategy. Labeling with ^99mTc was performed at 100 °C for 10 min and radiochemical analysis involved ITLC and HPLC methods. The stability of radiopeptide was checked in the presence of humane serum at 37 °C up to 24 h. The receptor bound internalization and externalization rates were studied in GRP receptor expressing PC-3 cells. Biodistribution of radiopeptide was studied in nude mice bearing PC-3 tumor. Labeling yield of >98% was obtained corresponding to a specific activity of ~2.6 MBq/nmol. Peptide conjugate showed good stability in the presence of human serum. The radioligand showed high and specific internalization into PC-3 cells (14.63 ± 0.41% at 4 h). In biodistribution studies, a receptor-specific uptake was observed in GRP-receptor-positive organs so that after 4 h the uptakes in mouse tumor and pancreas were 1.31 ± 0.18 and 1.2 ± 0.13% ID/g, respectively.     

Preparation and evaluation of a new neurotensin analog labeled with 99mTc for targeted imaging of neurotensin receptor positive tumors

Neurotensin receptors are overexpressed in several human tumors and can be targets for tumors diagnosis and therapy. In this study, a new neurotensin analogue was labeled with ^99mTc via HYNIC and tricine/EDDA as coligands and investigated further. [HYNIC⁰, Gly⁷, Lys⁹, d-Tyr¹¹]-Neurotensin (7–13) was synthesized using a standard Fmoc strategy. Labeling with ^99mTc was performed at 100 °C for 10 min and radiochemical analysis involved ITLC and HPLC methods. The stability of radiopeptide was checked in the presence of humane serum at 37 °C up to 24 h. The receptor bound internalization and externalization rates were studied in neurotensin receptor expressing HT-29 cells. Biodistribution of radiopeptide was studied in nude mice bearing HT-29 tumor. Labeling yield of 98.6 ± 0.54 % (n = 3) was obtained corresponding to a specific activity of 81 MBq/nmol. Peptide conjugate showed good stability in the presence of human serum. The radioligand showed specific internalization into HT-29 cells (12.43 ± 0.52 % at 4 h). In biodistribution studies, a receptor-specific uptake was observed in neurotensin receptor positive organs so that after 1 h the uptakes in mouse intestine and tumor were 0.87 ± 0.16 and 0.63 ± 0.12 % ID/g respectively.     

Synthesis and preliminary evaluation of 99mTc-spermine as a tumor imaging agent

Polyamines are essential for the growth and survival of all cells with biosynthesis and transportation of polyamines being very active in tumors. With the aim of developing a new tumor imaging agent, the endogenous polyamine, spermine was labeled with ^99mTc, and its characters were also evaluated via in vitro and in vivo studies. ^99mTc-labeled spermine probe (^99mTc-spermine) was synthesized by the direct pretinning procedure and the labeling procedure was optimized with regard to the pH, reaction time, amounts of spermine and SnCl₂. The stability of the ^99mTc-spermine and its capacity to accumulate into 4T1 tumor cells were also evaluated. Biodistribution of ^99mTc-spermine was studied in 4T1 tumor-bearing mice. In the optimal conditions, the whole radiosynthesis was accomplished within 10 min with a decay-corrected yield of 96.5 ± 1.3 % and radiochemical purity of >95 %.^99mTc-spermine was stable at both 37 and 4 °C for at least 6 h. In vitro tests revealed that the ability of ^99mTc-spermine to penetrate in 4T1 tumour cells and an excess of spermine blocked the accumulation of the compound in the models. Biodistribution studies showed a high tumor uptake peaked at 30 min post-injection with 1.82 ± 0.19 % ID%/g. The tumor to muscle uptake ratios of the probe were 3.60 ± 0.51, 4.48 ± 0.29, 4.82 ± 0.18, 5.64 ± 0.10, respectively at 30 min, 1, 2 and 4 h postinjection. Block studies indicated that ^99mTc-spermine had specific binding of tumor via polyamine transport systems. ^99mTc-spermine is a promising radiopharmaceutical in tumor imaging. Further studies are required to determine the usability of ^99mTc–spermine for diagnosis purposes.     

Evaluation of 99mTc-ceftazidime as bacterial infection imaging agent

Although our understanding of microorganisms has advanced significantly and antimicrobial therapy has become increasingly available, infection remains a major cause of patient morbidity and mortality. The use of radiopharmaceuticals for diagnosis of infection is increasing due to their ability to distinguish between septic and aseptic inflammation. A wide range of radiopharmaceuticals have been proposed to visualize infection and inflammation scintigraphically. Ceftazidime a cephalosporin antibiotic used to treat bacterial infections was investigated to label with ^99mTc. Labeling was performed using sodium dithionite as reducing agent at 100 °C for 10 min and radiochemical analysis involved ITLC and HPLC methods. The stability of labeled antibiotic was checked in the presence of human serum at 37 °C up to 24 h. The maximum radiolabeling yield was 95.4 ± 2.0 % corresponding to a specific activity of 178 GBq/mmol. Bacterial binding assay was performed with S. aureus and the in vivo distribution was studied in mice. Images showed minimal accumulation in nontarget tissues, with an average target/nontarget ratio of % 1.4 ± 0.2.     

Labeling bombesin-like peptide with <Superscript>99m</Superscript>Tc via hydrazinonicotinamide: description of optimized radiolabeling conditions

Bombesin (BNN)-like peptides have very high binding affinity for the gastrin-releasing peptide (GRP) receptor. The goal of the current study was to optimize the labeling conditions of a new ^99mTc-radiolabeled BNN-like peptide based on the bifunctional chelating ligand HYNIC using different co-ligands (EDDA and tricine). The radiolabeling conditions (pH, amount of co-ligand, amount of stannous chloride, temperature and reaction time) for newly-formed ^99mTc-tricine-HYNIC-Q-Litorin and ^99mTc-EDDA-HYNIC-Q-Litorin were optimized and evaluated by RHPLC and RTLC. Radiochemical yields for ^99mTc-tricine-HYNIC-Q-Litorin and ^99mTc-EDDA-HYNIC-Q-Litorin were 98.0 ± 1.7 and 97.5 ± 2.5%, respectively. When EDDA was used as co-ligand, the labeling of ^99mTc-EDDA-HYNIC-Q-Litorin was optimal in the following reaction mixture: HYNIC-peptide: EDDA: 10 μg/5 mg, pH 3, SnCl₂ concentration: 12 μg/0.1 mL, reaction temperature: 100 °C, reaction time: 15 min. Besides, the optimum conditions were HYNIC-peptide:tricine: 10 μg/50 mg, pH 5, SnCl₂ concentration: 12 μg/0.1 mL, reaction temperature: 100 °C, reaction time: 15 min for preparing ^99mTc-tricine-HYNIC-Q-Litorin. The manufactured ^99mTc-HYNIC-Q-Litorin conjugates may offer new possibilities for imaging cancer cells expressing bombesin receptors.     

Radiocharacterization of the 99mTc�Crufloxacin complex and biological evaluation in Staphylococcus aureus infected rat model

^99mTc?Crufloxacin (^99mTc?CRUN) complex was prepared by reaction of different amounts of reduced sodium pertechnetate with different amount of Rufloxacin (RUN) antibiotic for the in vivo scintigraphic localization of the Staphylococcus aureus (S. aureus) infectious foci in Male Wister Rats (MWR) model. The ^99mTc?CRUN complex was radiochemically and biologically characterized in terms of radiochemical stability in saline, serum, in vitro binding with S. aureus and biodistribution in artificially infected with S. aureus MWR. The ^99mTc?CRUN complex showed stability more than 90% up to 240 min in normal saline with a maximum stability value of 98.10 ± 0.18% at 30 min after reconstitution. At 37 °C the complex showed in vitro permanence in serum up to 16 h with 13.90% side products during incubation. The ^99mTc?CRUN complex showed saturated in vitro binding with S. aureus at different intervals with a maximum uptake value of 71.50%. Infected to normal muscle, infected to inflamed and inflamed to normal muscles ratios were approximately 6.04, 4.31 and 1.40. Based on the stability of the complex in saline, serum, in vitro binding with S. aureus and biodistribution results, the ^99mTc?CRUN complex is recommended for in vivo scintigraphic localization of the S. aureus in vivo infectious foci in human.     

Preparation,molecular modeling and biodistribution of 99mTc-phytochlorin complex

Phytochlorin [21H, 23H-Porphine-7-propanoicacid, 3-carboxy-5-(carboxymethyl)13-ethenyl-18-ethyl-7,8-dihydro-2,8,12,17-tetramethyl-,(7S,8S)] was labeled with ^99mTc and the factors affecting the labeling yield of ^99mTc-phytochlorin complex were studied in details. At pH 10, ^99mTc-phytochlorin complex was obtained with a high radiochemical yield of 98.4 ± 0.6 % by adding ^99mTc to 100 mg phytochlorin in the presence of 75 μg SnCl₂·2H₂O after 30 min reaction time. The molecular modeling study showed that the structure of ^99mTc-phytochlorin complex presents nearly linear HO–Tc–OH unit with an angle of 179.27° and a coplanar Tc(N1N2N3N4) unit. Biodistribution of ^99mTc-phytochlorin complex in tumor bearing mice showed high T/NT ratio (T/NT = 3.65 at 90 min post injection). This preclinical study showed that ^99mTc-phytochlorin complex is a potential selective radiotracer for solid tumor imaging and afford it as a new radiopharmaceutical suitable to proceed through the clinical trials for tumor imaging.     

Formulation and preclinical evaluation of 99mTc–gemcitabine as a novel radiopharmaceutical for solid tumor imaging

The aim of this study is the formulation of a new radiopharmaceutical for imaging solid tumor bearing. Gemcitabine is a nucleoside analogue used as chemotherapeutic agent. Gemcitabine was formulated and radiolabeled with one of the most important diagnostic radioactive isotopes (technetium-99m) to be investigated in solid tumor imaging. The labeling parameters such as gemcitabine amount, stannous chloride amount, pH of the reaction mixture, and reaction time were optimized. ^99mTc–gemcitabine was prepared at pH 9 with a maximum labeling yield of 96 ± 0.3 % without any notable decomposition at room temperature over a period of 8 h. The preclinical evaluation and biodistribution in solid tumor bearing mice showed that ^99mTc–gemcitabine had solid tumor selectivity, preclinical high biological accumulation in tumor cells and high retention. Tumor/normal muscle (T/NT) ratios increased with time showing high T/NT ratio (T/NT = 4.9 ± 0.27 at 120 min post injection) and high Tumor/Blood ratio (3.4 ± 0.06), suggesting ^99mTc–gemcitabine as a novel solid tumor imaging agent.     

Tetraamine‐Derived Bifunctional Chelators for Technetium‐99m Labelling: Synthesis,Bioconjugation and Evaluation as Targeted SPECT Imaging Probes for GRP‐Receptor‐Positive Tumours

Owing to its optimal nuclear properties, ready availability, low cost and favourable dosimetry, ^99mTc continues to be the ideal radioisotope for medical‐imaging applications. Bifunctional chelators based on a tetraamine framework exhibit facile complexation with Tc(V)O₂ to form monocationic species with high in vivo stability and significant hydrophilicity, which leads to favourable pharmacokinetics. The synthesis of a series of 1,4,8,11‐tetraazaundecane derivatives ( 01 – 06 ) containing different functional groups at the 6‐position for the conjugation of biomolecules and subsequent labelling with ^99mTc is described herein. The chelator 01 was used as a starting material for the facile synthesis of chelators functionalised with OH ( 02 ), N₃ ( 04 ) and O‐succinyl ester ( 05 ) groups. A straightforward and easy synthesis of carboxyl‐functionalised tetraamine‐based chelator 06 was achieved by using inexpensive and commercially available starting materials. Conjugation of 06 to a potent bombesin‐antagonist peptide and subsequent labelling with ^99mTc afforded the radiotracer ^99mTc‐N4‐BB‐ANT, with radiolabelling yields of >97 % at a specific activity of 37 GBq μmol^?1. An IC₅₀ value of (3.7±1.3) nM was obtained, which confirmed the high affinity of the conjugate to the gastrin‐releasing‐peptide receptor (GRPr). Immunofluorescence and calcium mobilisation assays confirmed the strong antagonist properties of the conjugate. In vivo pharmacokinetic studies of ^99mTc‐N4‐BB‐ANT showed high and specific uptake in PC3 xenografts and in other GRPr‐positive organs. The tumour uptake was (22.5±2.6) % injected activity per gram (% IA g^?1) at 1 h post injection (p.i.). and increased to (29.9±4.0) % IA g^?1 at 4 h p.i. The SPECT/computed tomography (CT) images showed high tumour uptake, clear background and negligible radioactivity in the abdomen. The promising preclinical results of ^99mTc‐N4‐BB‐ANT warrant its potential candidature for clinical translation.     

Radio labeling,quality control and biodistribution of <Superscript>99m</Superscript>Tc-cefotaxime as an infection imaging agent

Cefotaxime, a cephalosporin antibiotic, used to treat bacterial infections was investigated to label with ^99mTc. Labeling was performed using sodium dithionite as a reducing agent at 100 °C for 10 min and radiochemical analysis involved ITLC and HPLC methods. The stability of labeled antibiotic was checked in the presence of human serum at 37 °C up to 24 h. The maximum radiolabeling yield was 92 ± 2%. Bacterial binding assay was performed with S. aureus and the in vivo distribution was studied in mice. Images showed minimal accumulation in non-target tissues, with an average target/non-target ratio of 2.89 ± 0.58.     

99mTc-labeled bombesin analog for breast cancer identification

Bombesin is a tetradecapeptide that binds specifically to gastrin releasing peptide receptors in humans. Several forms of cancer, including lung, prostate, breast, and colon express receptors for bombesin-like peptides. Radiolabeled bombesin analogs with a high affinity for these receptors might therefore be used for scintigraphic imaging of these tumor types. A truncated bombesin derivative (HYNIC-βAla-Bombesin_(7–14)) was radiolabeled with technetium-99m using EDDA and tricine as coligands. In vitro stability was evaluated in presence of plasma and excess of cysteine. The receptor-binding affinity assays was evaluated in MDA-MB-231 cancer cell line. In addition, in vivo biodistribution was performed in nude mice bearing breast tumor. In vitro assay showed a good affinity for the MDA-MB-231 cell line, showing 20.0 % of internalization at 4 h post-administration. ^99mTc-HYNIC-βAla-Bombesin_(7–14) biodistribution revealed a rapid clearance and a significant renal excretion. In addition, tumor uptake was higher than non-excretory organs, such as the spleen, the liver, and muscles. Tumor-to-muscle and tumor-to-blood ratios for ^99mTc-HYNIC-βAla-Bombesin_(7–14) showed high values at 4 h post-injection (5.34 and 4.55, respectively). Furthermore, blocked studies using cold bombesin peptide were performed, which demonstrated an important decrease in tumor uptake, indicating a tumor specificity for ^99mTc-HYNIC-βAla-Bombesin_(7–14). The ^99mTc-HYNIC-βAla-Bombesin_(7–14) displayed suitable radiochemical characteristics, and adequate affinity to breast tumor cells (MDA-MB-231). Therefore, this analog can be considered as a candidate for the identification of bombesin-positive tumors.     

Direct labeling of doxorubicin with technetium-99m: its optimization, characterization and quality control

Doxorubicin (DOX) is an anthracycline antineoplastic and one of the most potent and widely used drugs in clinical oncology. It is used in the treatment of a wide variety of cancers. The aim of this study was the direct labeling of DOX with ^99mTc; its optimization, characterization and quality control of the radiolabeled DOX. Labeling efficiency was determined by paper chromatography. More than 92% labeling was obtained at pH 6?C7, 10?C12???g stannous chloride and 200???g of DOX. The stability of ^99mTc?CDOX was studied up to 5?h. All the experiments were performed at room temperature (25±?2?°C). The characterization of the labeling compound was performed by HPLC and electrophoresis. Electrophoresis indicates that labeled DOX has no charge and HPLC shows single specie of labeled compound.     

Preparation of radioiodinated ritodrine as a potential agent for lung imaging

Ritodrine (a beta-2 adrenergic receptor agonist) was successfully labeled with ¹²⁵I via direct electrophilic substitution reaction at ambient temperature. ¹²⁵I-ritodrine was obtained with a maximum labeling yield of 97 ± 0.163 % and in vitro stability up to 24 h. Biodistribution studies showed that maximum in vivo uptake of ¹²⁵I-ritodrine in lungs was 20.4 ± 0.22 % injected activity/g tissue at 1 h post-injection, whereas the clearance from mice appeared to proceed mainly via the renal pathway. ¹²⁵I-ritodrine is not a blood product and so it is more safe than the currently available ^99mTc-MAA, and its lung uptake is higher than that of the recently discovered ^99mTc(CO)₅I and ^99mTc-DHPM. As a conclusion, radioiodinated ritodrine could be used as a novel radiopharmaceutical for lung perfusion scan safer than the currently available ^99mTc-MAA and more potential than the recently discovered ^99mTc(CO)₅I and ^99mTc-DHPM.     

99mTc-labeling and in vitro characterization of N4- and N3S-RGDS-derivative peptides

The aim of this work was to characterize the in vitro behavior of N4- and N3S-RGDS-derivative peptides labeled with ^99mTc. Peptides AGGG-Abu-GRGDSPK-NH₂ (F22) and C(acm)-GGG-Abu-GRGDSPK-NH₂ (SMA1) were synthesized by solid phase. The stability of ^99mTc-labeled peptides was assessed in a 30-fold molar excess of cysteine and in plasma. The affinity for plasma proteins was also evaluated. Labeling yield was >95% for both peptides. ^99mTc-F22 was not stable in presence of cysteine, but 63% of ^99mTc remained chelated to SMA1 up to 24 hours. Both peptides showed low affinity to plasma proteins. N3S-RGDS-derivative peptide (SMA1) showed more stable coordination binding with ^99mTc and a higher stability in plasma with regard to N4-RGDS-derivative peptide (F22).     

Synthesis of 99mTc-oxybutynin for M3-receptor-mediated imaging of urinary bladder

Radiolabeling of oxybutynin, a muscarinic acetylcholine (mACh) receptor antagonist agent with ^99mTc is of considerable interest for imaging of urinary bladder. This study is aimed to optimize radiolabeling yield of oxybutynin with ^99mTc using SnCl₂·2H₂O as a reducing agent with respect to factors that affect the reaction conditions such as oxybutynin amount, stannous chloride amount, reaction time and pH of the reaction mixture. In vitro stability of the radiolabeled complex was checked and it was found to be stable for up to 8 h. ^99mTc-oxybutynin was injected via subcutaneous and intravenous administration routes into normal Sprague?CDawley rats. Biodistribution studies have revealed that ^99mTc-oxybutynin exhibits high affinity and specificity for the muscarinic M₃ subtype located on the smooth muscle of urinary bladder relative to the M₁ and M₂ subtypes of the G protein coupled receptor (GPCR) superfamily. In vivo uptake of subcutaneous ^99mTc-oxybutynin in urinary bladder was 19.6 ± 0.42% ID at 0.5 h, whereas in intravenous administration route the accumulation in the urinary bladder was found to be 9.4 ± 0.31% ID at 0.5 h post injection. Administration of cold oxybutynin effectively blocked urinary bladder uptake and further confirms the high specificity of this complex for the M₃ receptor.     

Synthesis,bioevaluation and gamma scintigraphy of <Superscript>99m</Superscript>Tc-N-2-(furylmethyl iminodiacetic acid) complex as a new renal radiopharmaceutical

A ligand of N-2-(furylmethyl iminodiacetic acid) (FMIDA) has been easily labeled by a tetradentate chelating agent of [^99mTc]. Factors like a stannous chloride solution as a reducing agent (100 μg), substrate amount (100 μg), pH (7), in vitro stability (8 h) and temperature (37 °C) have been systematically studied to optimize high radiochemical yield (98.0%). The radiochemical conversion was calculated on thin-layer chromatography, paper electrophoresis, and high performance liquid chromatography. Biodistribution study showed that this complex was removed from the kidneys and bladder path way during 1 h post injection. Therefore, [^99mTc]FMIDA may be used as renal function radiotracer.     

Synthesis and biological distribution of 99mTc–norfloxacin complex, a novel agent for detecting sites of infection

The optimization of the radiolabeling yield of ciprofloxacin analogous, norfloxacin, with technetium-99m (^99mTc) was described. Dependence of the labeling yield of ^99mTc–norfloxacin complex on the concentration of norfloxacin, SnCl₂·2H₂O content, pH of the reaction mixture and reaction time was studied. Norfloxacin was labeled with ^99mTc at pH 3 with a labeling yield of 95.4% by using 5 mg norfloxacin, 50 μg SnCl₂·2H₂O and 30 min reaction time. The formed ^99mTc–norfloxacin complex was stable for a time up to 3 h. Biological distribution of ^99mTc–norfloxacin complex was investigated in experimentally induced inflammation rats using Staphylococcus aureus (bacterial infection model) and heat killed Staphylococcus aureus and turpentine oil (sterile inflammation model). In case of bacterial infection, the T/NT value for ^99mTc–norfloxacin complex was found to be 6.9 ± 0.4 which was higher than that of the commercially available ^99mTc–ciprofloxacin under the same experimental condition.     

Preparation and SPECT imaging of the novel Anxa 1-targeted probe 99mTc-p-SCN-Bn-DTPA-GGGRDN-IF7

Annexin 1 (Anxa1) is a highly specific surface marker of tumor vasculature in several solid tumors. The IF7 peptide was modified with a hydrophic linkers,GGGRDN, and introduced into a new bifunctional chelating agent p-SCN-Bn-DTPA. The resulting peptides (p-SCN-Bn-DTPA-GGGRDN-IF7) was successfuly labeled with ^99mTc. The targeting characteristics to Anxa1 of the radiolabeled peptide were evaluated by in vitro and in vivo study is on U87 tumor models. SPECT imaging revealed that the U87 tumors were clearly visualized (3.64 ± 0.44%ID/g at 2 h p.i.). ^99mTc-p-SCN-Bn-DTPA-GGGRDN-IF7 (Tc-RIF7) might be a potential target probe for detecting Anxa1 levels in cancers due to the favorable characterizations such as convenient synthesis, specific Anxa1 targeting and moderate tumor uptakes.

     

Novel 99mTc labeled σ receptor ligand as a potential tumor imaging agent

A novel ^99mTc labeled complex, [N-[2-((2-oxo-2-(4-(3-phenylpropyl)piperazin-1-yl)ethyl) (2-mercaptoethyl)amino)acetyl]-2-aminoethanethiolato]Technetium(V) oxide (PPPE-MAMA’-^99mTcO) ([ ^99m Tc]-2) has been designed and prepared based on the integrated approach. The corresponding rhenium complex (PPPE-MAMA’-ReO)(Re-2) has been prepared and characterized. In vitro competition binding assays show moderate affinity of Re-2 towards σ₁ and σ₂ receptors with K _i values of 8.67 ± 0.07 and 5.71 ± 1.88 μmol, respectively. Planar images obtained at 0.5 h, 4 h, 20 h after i.v. injection indicate the accumulation of [ ^99m Tc]-2 in MCF-7 human breast tumor bearing mice at 20 h. Furthermore, the accumulation of [ ^99m Tc]-2 has been inhibited at 20 h after co-injection of [ ^99m Tc]-2 plus haloperidol (1 mg/kg). Biodistribution studies of [ ^99m Tc]-2 display an in vivo tumor uptake of 0.14% ± 0.01% ID/g at 24 h post i.v. injection with a tumor/muscle ratio of 6.02 ± 0.87. The above results suggest that [ ^99m Tc]-2, derived from a previously published lead compound, retains certain tumor uptake and affinity for σ receptors. [ ^99m Tc]-2 may be used as a basis for further structural modifications to develop tumor imaging agents with high affinity for σ receptors.