Detection of somatostatin receptor-positive tumours using the new 99mTc-tricine-HYNIC-D-Phe1-Tyr3-octreotide: first results in patients and comparison with 111In-DTPA-D-Phe1-octreotide

Head-to-Head Comparison between Peptide-Based Radiopharmaceutical for PET and SPECT in the Evaluation of Neuroendocrine Tumors: A Systematic Review

We compared head-to-head the most used radiolabeled peptides for single photon computed emission tomography (SPECT) and positron emission tomography (PET) imaging of neuroendocrine tumors (NETs). A comprehensive literature search was performed in PubMed, Web of Science, and Scopus databases. The following words, coupled two by two, were used: 68Ga-DOTATOC; 68Ga-DOTATATE; 68Ga-DOTANOC; 99mTc-EDDA/HYNIC-TOC; 64Cu-DOTATATE; and 111In-DTPA-octreotide. Moreover, a second-step search strategy was adopted by using the following combined terms: "Somatostatin receptor imaging,"; "Somatostatin receptor imaging" and "Functional,"; "Somatostatin receptor imaging" and "SPECT,"; and "Somatostatin receptor imaging" and "PET". Eligible criteria were: (1) original articles focusing on the clinical application of the radiopharmaceutical agents in NETs; (2) original articles in the English language; (3) comparative studies (head-to-head comparative or matched-paired studies). Editorials, letters to the editor, reviews, pictorial essays, clinical cases, or opinions were excluded. A total of 1077 articles were found in the three electronic databases. The full texts of 104 articles were assessed for eligibility. Nineteen articles were finally included. Most articles focused on the comparison between 111In-DTPA-Octreotide and 68Ga-DOTATOC/TATE. Few papers compared 64Cu-DOTATATE and 68Ga-DOTATOC/TATE, or SPECT tracers. The rates of true positivity were 63.7%, 58.5%, 78.4% and 82.4%, respectively, for 111In-DTPA-Octreotide, 99mTc-EDDA/HYNIC-TOC, 68Ga-DOTATATE/TOC and 64Cu-DOTATATE. In conclusion, as highly expected, PET tracers are more suitable for the in vivo identification of NETs. Indeed, in comparative studies, they demonstrated a higher true positive rate than SPECT agents.

A review of pediatric neuroendocrine tumors, their detection, and treatment by radioisotopes

Neuroendocrine tumors (NETs) are rare in childhood. Neuroblastoma is the most common pediatric extracranial solid tumor, occurring >90% in children younger than 5 years of age. Pheochromocytoma and paraganglioma are rare NETs, causing hypertension in 0.5-2% of hypertensive children. Gastroenteropancreatic NETs can occur in children and are classified into carcinoids and pancreatic tumors. Nuclear medicine procedures have an essential role both in the diagnosis and treatment of NETs. Metaiodobenzylguanidine (MIBG) labeled with radioiodine has a well-established role in diagnosis as well as therapeutic management of the neuroblastoma group of diseases. During recent decades, establishing the abundant expression of somatostatin receptors by NETs first led to scintigraphy with somatostatin analogs (i.e. Tc/In-octreotide) and, later, with the emergence of positron-emitting labeled agents (i.e. Ga-DOTATATE/DOTATOC/DOTANOC) PET scans with significantly higher detection efficiency became available. Therapy with somatostatin analogs labeled with beta emitters such as Lu-177 and Y-90, known as peptide receptor radionuclide therapy, is a promising new option in the management of patients with inoperable or metastasized NETs. In this article, pediatric NETs are briefly reviewed and the role of radioactive agents in the detection and treatment of these tumors is discussed.

99mTc Labelling Strategies for the Development of Potential Nitroimidazolic Hypoxia Imaging Agents

Technetium-99m has a rich coordination chemistry that offers many possibilities in terms of oxidation states and donor atom sets. Modifications in the structure of the technetium complexes could be very useful for fine tuning the physicochemical and biological properties of potential 99mTc radiopharmaceuticals. However, systematic study of the influence of the labelling strategy on the “in vitro” and “in vivo” behaviour is necessary for a rational design of radiopharmaceuticals. Herein we present a review of the influence of the Tc complexes’ molecular structure on the biodistribution and the interaction with the biological target of potential nitroimidazolic hypoxia imaging radiopharmaceuticals presented in the literature from 2010 to the present. Comparison with the gold standard [18F]Fluoromisonidazole (FMISO) is also presented.

A Picture of Modern Tc-99m Radiopharmaceuticals: Production, Chemistry, and Applications in Molecular Imaging

Even today, techentium-99m represents the radionuclide of choice for diagnostic radio-imaging applications. Its peculiar physical and chemical properties make it particularly suitable for medical imaging. By the use of molecular probes and perfusion radiotracers, it provides rapid and non-invasive evaluation of the function, physiology, and/or pathology of organs. The versatile chemistry of technetium-99m, due to its multi-oxidation states, and, consequently, the ability to produce a variety of complexes with particular desired characteristics, are the major advantages of this medical radionuclide. The advances in technetium coordination chemistry over the last 20 years, in combination with recent advances in detector technologies and reconstruction algorithms, make SPECT’s spatial resolution comparable to that of PET, allowing 99mTc radiopharmaceuticals to have an important role in nuclear medicine and to be particularly suitable for molecular imaging. In this review the most efficient chemical methods, based on the modern concept of the 99mTc-metal fragment approach, applied to the development of technetium-99m radiopharmaceuticals for molecular imaging, are described. A specific paragraph is dedicated to the development of new 99mTc-based radiopharmaceuticals for prostate cancer.

Value of Somatostatin Receptor Scintigraphy with 99mTc-HYNIC-TOC in Patients with Primary Sjögren Syndrome

Objectives: Primary Sjögren syndrome (SS) is diagnosed based on the American European Consensus Group (AECG) criteria, but lacks specificity, not only in the involvement of salivary glands, but also in extra-glandular involvement. Whole-body somatostatin receptor scintigraphy with ^99mTc-HYNIC-TOC scintigraphy could overcome these limitations. The aims of this study were to evaluate salivary gland uptake of ^99mTc-HYNIC-TOC in untreated patients with de-novo diagnosis of SS as compared to control subjects and as compared to conventional sialoscintigraphy with ^99mTcO₄⁻. We also aimed to evaluate the involvement of joints. Methods: ^99mTc-HYNIC-TOC was used with SS patients and uptake in joints and salivary glands was analyzed semi-quantitatively. Patients also underwent ^99mTcO₄ sialoscintigraphy. The control group that we analyzed consisted of 30 patients with neuroendocrine tumors. Results: Fifty-two females and 10 males fully met the AECG criteria for SS, and were included. A target background ratio (TBR) >1.18 in submandibular glands correctly classified 93% of the patients with SS in comparison to 27% for ^99mTcO₄ sialoscintigraphy. The area under the curve (ROC) analysis for TBR in submandibular glands was 0.95. In joints there was a huge variety in uptake. The median TBR was significantly higher in salivary glands in patients with SS compared to controls. Conclusions: ^99mTc-HYNIC-TOC scintigraphy identified active inflammatory processes not only in the salivary glands, but, unexpectedly, also in many joints in patients with primary SS, contrary to popular belief. This technique provides an objective parameter to evaluate the inflammation burden in salivary glands and joints and could be used to evaluate response to treatment.

International Union of Basic and Clinical Pharmacology. CV. Somatostatin Receptors: Structure, Function, Ligands, and New Nomenclature

Somatostatin, also known as somatotropin-release inhibitory factor, is a cyclopeptide that exerts potent inhibitory actions on hormone secretion and neuronal excitability. Its physiologic functions are mediated by five G protein-coupled receptors (GPCRs) called somatostatin receptor (SST)1-5. These five receptors share common structural features and signaling mechanisms but differ in their cellular and subcellular localization and mode of regulation. SST₂ and SST₅ receptors have evolved as primary targets for pharmacological treatment of pituitary adenomas and neuroendocrine tumors. In addition, SST₂ is a prototypical GPCR for the development of peptide-based radiopharmaceuticals for diagnostic and therapeutic interventions. This review article summarizes findings published in the last 25 years on the physiology, pharmacology, and clinical applications related to SSTs. We also discuss potential future developments and propose a new nomenclature.

Clinical application of 99mTc-HYNIC-TOC SPECT/CT in diagnosing and monitoring of pancreatic neuroendocrine neoplasms

OBJECTIVE

Our aim of this research was to determine the value of SPECT/CT with ^99mTc-HYNIC-TOC for evaluation of the pancreatic masses which were suspected as neuroendocrine neoplasms and follow-up of patients with pancreatic neuroendocrine neoplasms.

METHODS

We retrospectively analyzed 184 patients who performed ^99mTc-HYNIC-TOC SPECT/CT. All the patients were divided into two groups: one for assessment of diagnostic efficiency for pancreatic suspected masses (n = 140) and another for monitoring recurrence after surgery (n = 44). The image findings acquired at 2 h postinjection were compared to final diagnoses from pathological results and clinical follow-up. Then, the correlation between ratios of tumor-to-background (TBR) and tumor grade was analyzed.

RESULTS

In group 1, 95/140 (67.9%) patients were confirmed as neuroendocrine neoplasms including 85 neuroendocrine tumors and 10 neuroendocrine carcinomas. Patient-based analysis showed that the sensitivity, specificity and accuracy of diagnosing neuroendocrine neoplasms with SPECT/CT were 81.1, 84.4 and 82.1%. There was significant difference of TBRs among G1, G2 and G3 (F = 3.175, P = 0.048). In group 2, 22/44 (50.0%) patients occurred metastasis mainly in liver. The sensitivity, specificity and accuracy of monitoring recurrence were 87.0, 100 and 93.2%.

CONCLUSIONS

^99mTc-HYNIC-TOC SPECT/CT is a reliable method of diagnosing and monitoring of pancreatic neuroendocrine neoplasms, especially neuroendocrine tumors.

99mTc-HYNIC-(tricine/EDDA)-FROP peptide for MCF-7 breast tumor targeting and imaging

Background

Breast cancer is the most common malignancy among women in the world. Development of novel tumor-specific radiopharmaceuticals for early breast tumor diagnosis is highly desirable. In this study we developed ^99mTc-HYNIC-(tricine/EDDA)-Lys-FROP peptide with the ability of specific binding to MCF-7 breast tumor.

Methods

The FROP-1 peptide was conjugated with the bifunctional chelator hydrazinonicotinamide (HYNIC) and labeled with ^99mTc using tricine/EDDA co-ligand. The cellular specific binding of ^99mTc-HYNIC-FROP was evaluated on different cell lines as well as with blocking experiment on MCF-7 (human breast adenocarcinoma). The tumor targeting and imaging of this labeled peptide were performed on MCF-7 tumor bearing mice.

Results

Radiochemical purity for ^99mTc-HYNIC-(tricine/EDDA)-FROP was 99% which was determined with ITLC method. This radiolabeled peptide showed high stability in normal saline and serum about 98% which was monitored with HPLC method. In saturation binding experiments, the binding constant (K_d) to MCF-7 cells was determined to be 158 nM. Biodistribution results revealed that the ^99mTc-HYNIC-FROP was mainly exerted from urinary route. The maximum tumor uptake was found after 30 min post injection (p.i.); however maximum tumor/muscle ratio was seen at 15 min p.i. The tumor uptake of this labeled peptide was specific and blocked by co-injection of excess FROP. According to the planar gamma imaging result, tumor was clearly visible due to the tumor uptake of ^99mTc-HYNIC-(tricine/EDDA)-FROP in mouse after 15 min p.i.

Conclusions

The ^99mTc-HYNIC-(tricine/EDDA)-FROP is considered a promising probe with high specific binding to MCF-7 breast cancer cells.

Prospective of 68Ga Radionuclide Contribution to the Development of Imaging Agents for Infection and Inflammation

During the last decade, the utilization of ⁶⁸Ga for the development of imaging agents has increased considerably with the leading position in the oncology. The imaging of infection and inflammation is lagging despite strong unmet medical needs. This review presents the potential routes for the development of ⁶⁸Ga-based agents for the imaging and quantification of infection and inflammation in various diseases and connection of the diagnosis to the treatment for the individualized patient management.

Incremental value of 99mTc-HYNICTOC SPECT/CT over whole-body planar scintigraphy and SPECT in patients with neuroendocrine tumours

Aim: The aim of this study was to evaluate the additional value of 99mTc-HYNIC-TOC SPECT/CT over planar whole-body (WB) scintigraphy and SPECT alone in the detection and accurate localisation of neuroendocrine tumour (NET) lesions. Methods: This study included 65 patients with a definitive histological diagnosis of NET prior to scintigraphy. Planar WB scintigraphy, SPECT, and SPECT/CT images were acquired at 4 h postadministration of 670 MBq 99mTc-HYNIC- TOC. Additional SPECT images at 10 min after tracer administration were also acquired. Clinical and imaging follow-up findings were considered as the reference standards (minimum follow-up period, 15 months). Patient and lesion-based analyses of the efficacies of the imaging modalities were performed. Results: While 38 patients exhibited metastasis of NETs, 27 presented no evidence of metastasis. Upon patient- based analysis, the sensitivity and specificity of SPECT/CT were found to be 88.9 and 79.3 %, respectively. The diagnostic accuracies of WB scintigraphy, 4h-SPECT, and SPECT/CT were 72.3, 73.8, and 84.6 %o, respectively. The area under curve (AUC) value for SPECT/CT (0.84) was the highest, followed by those for 4h-SPECT (0.75) and WB scintigraphy (0.74). The accuracy and AUC values of SPECT/CT were significantly better compared to those of WB scintigraphy (p < 0.001), 10 min-SPECT (p < 0.001), and 4 h-SPECT (p = 0.001). The findings of SPECT/CT led to the change in treatment plan of 11 patients (16.9 %o). Conclusion: The sensitivity and diagnostic accuracy of SPECT/CT in the evaluation of NET lesions outperforms planar WB imaging or SPECT alone.

99mTc-Hynic-TOC imaging in the diagnostic of neuroendocrine tumors

The aim of this study was to assess the potential of ^99mTc-Hynic-TOC imaging in the primary diagnosis and follow-up of midgut neuroendocrine tumors (NETs). In comparison to ¹¹¹In-octreotide, ^99mTc-Hynic-TOC has a higher imaging quality and leads to a lower radiation absorption in patients. ^99mTc-Hynic-TOC was used for assessing primary diagnosis (n = 14) and during follow-up (n = 17) in patients with NETs. The scintigraphic findings were compared with computed tomography scans and follow-up. In 31 patients, 34 somatostatin receptor scans using 99mTc-Hynic-TOC were performed. The primary diagnoses were midgut NET. The scintigraphy was true positive in 17 patients, true negative in 9, false negative in 4, and false positive in 1. From these data, a sensitivity of 81%, specificity of 90%, positive predictive value of 94%, and negative predictive value of 69% were calculated. In summary, ^99mTc-TOC represents a useful radiotracer in imaging SSTR-expressing tumor lesions with slightly higher sensitivity, higher imaging quality, and lower radiation exposure for patients compared to ¹¹¹In-octreotide. A 1-day double-acquisition protocol should be used to reduce false-positive findings of the gut.

Physiological expression of pancreatic somatostatin receptors in 99mTc-HYNIC-TOC scintigraphy

PURPOSE

To describe the frequency of head and/or pancreas uncinate process uptake of 99mTc-HYNIC-TOC, to study its nature, and analyze its diagnostic value.

MATERIALS AND METHODS

Retrospective evaluation of 47 consecutive 99mTc-HYNIC-TOC examinations was conducted. Head and/or pancreas uncinate process uptake was considered to be physiological in patients with normal CT at the same episode and in follow-up. It was analyzed if age or diabetes mellitus was justifying the existence or not of uptake.

RESULTS

32.5% patients showed uptake; 73% of them were mild. 84.6% patients with uptake have no pathology and 4% had neuroendocrine pancreatic disease at CT. Neither the age nor the diabetes mellitus established differences in patients without lesion.

CONCLUSIONS

Near one-third of patients show physiological uptake by head and/or pancreas uncinate process at 99mTc-HYNIC-TOC scintigraphy. It seems that neither the diabetes nor the ages are factors that determine this physiological uptake.

Diagnostic utility of 99mTc-EDDA-tricine-HYNIC-Tyr3-octreotate SPECT for differentiation of active from inactive pulmonary tuberculosis

OBJECTIVE

The current study was performed to evaluate the impact of Tc-EDDA-tricine-HYNIC-Tyr-octreotate in the differentiation of active from inactive pulmonary tuberculosis lesions.

PATIENTS AND METHODS

Ten consecutive patients (six male and four female, age range 24-83 years) with proven pulmonary tuberculosis (with a positive smear or culture) were enrolled in the study. At 120 min after injection of 740 MBq of Tc-EDDA-tricine-HYNIC-Tyr-octreotate, planar and single-photon emission computed tomography (SPECT) images of the thorax were taken. A semiquantitative evaluation of lesion and nonlesion areas was performed. The scan was repeated following the same protocol after standard treatment for tuberculosis after a negative sputum culture.

RESULTS

Semiquantitative evaluation of the lesions showed a statistically significant higher uptake before treatment in both planar and SPECT images (P=0.005 and 0.007, respectively). Lesion-to-nonlesion ratios were also higher in the pretreatment sets on both planar and SPECT images (1.4±0.2 vs. 1.19±0.15, P=0.001, for planar images and 2.32±0.55 vs. 1.32±0.32, P=0.0001, for SPECT images).

CONCLUSION

Tc-EDDA-tricine-HYNIC-Tyr-octreotate scintigraphy may help to differentiate between active and inactive pulmonary tuberculosis. SPECT imaging and semiquantitative evaluation are indispensable for increasing the diagnostic yield of this method. Larger studies are needed to corroborate our results.

Syntheses and biological evaluation of99mTc-HYNIC-fatty acid complexes for myocardial imaging

The aim of the present study is to identify a^99mTc-labeled fatty acid tracer which could be a possible substitute of the widely used¹²³I-labeled fatty acids in studying myocardial metabolism and in detection of myocardial abnormalities.

68Ga-DOTATATE PET/CT, 99mTc-HYNIC-octreotide SPECT/CT, and whole-body MR imaging in detection of neuroendocrine tumors: a prospective trial

There are different metabolic imaging methods, various tracers, and emerging anatomic modalities to stage neuroendocrine tumor (NET). We aimed to compare NET lesion detectability among (99m)Tc-hydrazinonicotinamide (HYNIC)-octreotide (somatostatin receptor scintigraphy [SSRS]) SPECT/CT, (68)Ga-DOTATATE PET/CT, and whole-body diffusion-weighted MR imaging (WB DWI).

METHODS

Nineteen consecutive patients (34-77 y old; mean, 54.3 ± 10.4 y old; 10 men and 9 women) underwent SSRS SPECT/CT, (68)Ga-DOTATATE PET/CT, and WB DWI. Images were acquired with a maximum interval of 3 mo between them and were analyzed with masking by separate teams. Planar whole-body imaging and SPECT/CT were performed from thorax to pelvis using a double-head 16-slice SPECT/CT scanner 4 h after injection of 111-185 MBq of (99m)Tc-HYNIC-octreotide. (68)Ga-DOTATATE PET/CT was performed from head to feet using a 16-slice PET/CT scanner 45 min after injection of 185 MBq of tracer. WB DWI was performed in the coronal plane using a 1.5-T scanner and a body coil. The standard method of reference for evaluation of image performance was undertaken: consensus among investigators at the end of the study, clinical and imaging follow-up, and biopsy of suggestive lesions.

RESULTS

McNemar testing was applied to evaluate the detectability of lesions using (68)Ga-DOTATATE PET/CT in comparison to SSRS SPECT/CT and WB DWI: a significant difference in detectability was noted for pancreas (P = 0.0455 and P = 0.0455, respectively), gastrointestinal tract (P = 0.0455 and P = 0.0455), and bones (P = 0.0082 and P = 0.0082). Two unknown primary lesions were identified solely by (68)Ga-DOTATATE PET/CT. (68)Ga-DOTATATE PET/CT, SSRS SPECT/CT, and WB DWI demonstrated, respectively, sensitivities of 0.96, 0.60, and 0.72; specificities of 0.97, 0.99, and 1.00; positive predictive values of 0.94, 0.96, and 1.00; negative predictive values of 0.98, 0.83, and 0.88; and accuracies of 0.97, 0.86, and 0.91.

CONCLUSION

(68)Ga PET/CT seems to be more sensitive for detection of well-differentiated NET lesions, especially for bone and unknown primary lesions. NET can be staged with (68)Ga-DOTATATE PET/CT. WB DWI is an efficient new method with high accuracy and without ionizing radiation exposure. SSRS SPECT/CT should be used only when (68)Ga-DOTATATE PET/CT and WB DWI are not available.

[Radioguided surgery in neuroendocrine tumors. A review of the literature]

Radioguided surgery can be a useful technique in the localization of neuroendocrine tumors. It detects more and smaller lesions compared to pre-surgical imaging and intraoperative digital palpation by the surgeon. It detects residual lesions and also indicates the shortest access route to the lesion. Nevertheless, its use has not become widespread because of technical difficulties. There is a limited number of published series, a lack of standardized protocol because of the great variability regarding type of radiopharmaceutical, dose of radiotracer, timing between injection and surgery. In this paper, we review these issues, describing the experience of different authors in diverse tumors.

PET and PET/CT with 68gallium-labeled somatostatin analogues in Non GEP-NETs Tumors

Somatostatin (SST) is a 28-amino-acid cyclic neuropeptide mainly secreted by neurons and endocrine cells. A major interest for SST receptors (SSTR) as target for in vivo diagnostic and therapeutic purposes was born since a series of stable synthetic SST-analouges PET became available, being the native somatostatin non feasible for clinical use due to the very low metabolic stability. The rationale for the employment of SST-analogues to image cancer is both based on the expression of SSTR by tumor and on the high affinity of these compounds for SSTR. The primary indication of SST-analogues imaging is for neuroendocrine tumors (NETs), which usually express a high density of SSTR, so they can be effectively targeted and visualized with radiolabeled SST-analogues in vivo. Particularly, SST-analogues imaging has been widely employed in gastroenteropancreatic (GEP) NETs. Nevertheless, a variety of tumors other than NETs expresses SSTR thus SST-analogues imaging can also be used in these tumors, particularly if treatment with radiolabeled therapeutic SST-analouges PET is being considered. The aim of this paper is to provide a concise overview of the role of positron emission tomography/computed tomography (PET/CT) with (68)Ga-radiolabeled SST-analouges PET in tumors other than GEP-NETs.

Evaluation of K(HYNIC)(2) as a bifunctional chelator for (99m)Tc-labeling of small biomolecules

This study sought to evaluate K(HYNIC)(2) (K = lysine and HYNIC = 6-hydrazinonicotinyl) as a bifunctional chelator for (99m)Tc-labeling of biomolecule. In this study, four K(HYNIC)(2)-conjugated cyclic RGD peptides, K(HYNIC)(2)-RGD(2) (RGD(2) = E[c(RGDfK)](2)), K(HYNIC)(2)-3G-RGD(2) (3G-RGD(2) = Gly-Gly-Gly-E[Gly-Gly-Gly-c(RGDfK)](2)), K(HYNIC)(2)-2P-RGD(2) (2P-RGD(2) = E[PEG4-c(RGDfK)](2), and PEG(4) = 15-amino-4,7,10,13-tetraoxapentadecanoic acid), and K(HYNIC)(2)-3P-RGD(2) (3P-RGD(2) = PEG4-E[PEG4-c(RGDfK)]2) were prepared, and evaluated for their integrin αvβ3 binding affinity. IC(50) values were determined to be 47 ± 2, 35 ± 2, 37 ± 2, 85 ± 2, and 422 ± 15 nM for K(HYNIC)(2)-2P-RGD(2), K(HYNIC)(2)-3P-RGD(2), K(HYNIC)(2)-3G-RGD(2), K(HYNIC)(2)-RGD(2), and c(RGDyK), respectively, against (125)I-echistatin bound to U87MG cells. Macrocyclic complexes [(99m)Tc(K(HYNIC)(2)-RGD(2))(tricine)] (1), [(99m)Tc(K(HYNIC)(2)-3G-RGD(2))(tricine)] (2), [(99m)Tc(K(HYNIC)(2)-2P-RGD(2))(tricine)] (3), and [(99m)Tc(K(HYNIC)(2)-3P-RGD(2))(tricine)] (4) were prepared, and evaluated in athymic nude mice bearing U87MG glioma xenografts for their tumor targeting capability and biodistribution. It was found that 1-4 all had high solution stability and more than two isomers, as evidenced by the presence of multiple radiometric peaks in their radio-HPLC chromatograms. The tumor uptake of 1-4 was 3.78 ± 0.81, 7.46 ± 1.68, 9.74 ± 1.65, and 8.59 ± 1.52%ID/g, respectively, which was completely consistent with trend of integrin α(v)β(3) binding affinity for cyclic RGD peptides. Replacing [(99m)Tc(HYNIC)(tricine)(TPPTS)] (TPPTS = trisodium triphenylphosphine-3,3',3"-trisulfonate) with [(99m)Tc(K(HYNIC)(2))(tricine)] had little impact on radiotracer tumor uptake; but it had significant effect on the uptake of radiotracer in kidneys, lungs, and spleen. The tumor was clearly visualized by SPECT/CT with excellent contrast in a glioma-bearing mouse administered with 4. K(HYNIC)(2) would be particularly useful for (99m)Tc-labeling of small biomolecules with one or more disulfide linkages.

Synthesis, characterization, conformational analysis of a cyclic conjugated octreotate peptide and biological evaluation of (99m)Tc-HYNIC-His(3)-Octreotate as novel tracer for the imaging of somatostatin receptor-positive tumors

Peptides are attracting increasing interest in nuclear oncology for targeted tumor diagnosis and therapy. We therefore synthesized new cyclic octapeptides conjugated with HYNIC by Fmoc solid-phase peptide synthesis. These were purified and analyzed by RP-HPLC, MALDI mass, (1)H NMR, (13)C NMR, HSQC, HMBC, COSY and IR spectroscopy. Conformational analysis of the peptides was performed by circular dichroism spectroscopy, in pure water and trifluoroethanol-water (1:1), revealed the presence of strong secondary structural features like β-sheet and random coils. Labeling was performed with (99m)Tc using Tricine and EDDA as coligands by SnCl(2) method to get products with excellent radiochemical purity >99.5 %. Metabolic stability analysis did not show any evidence of breaking of the labeled compounds and formation of free (99m)Tc. Internalization studies were done and IC(50) values were determined in somatostatin receptor-expressing C6 glioma cell line and rat brain cortex membrane, and the results compared with HYNIC-TOC as standard. The IC(50) values of (99m)Tc-HYNIC-His(3)-Octreotate (21 ± 0.93 nM) and (99m)Tc-HYNIC-TOC (2.87 ± 0.41 nM) proved to be comparable. Biodistribution and image study on normal rat under gamma camera showed very high uptake in kidney and urine, indicating kidney as primary organ for metabolism and route of excretion. Biodistribution and image study on rats bearing C6 glioma tumor found high uptake in tumor (1.27 ± 0.15) and pancreas (1.71 ± 0.03). Using these findings, new derivatives can be prepared to develop (99m)Tc radiopharmaceuticals for imaging somatostatin receptor-positive tumors.

Radiopharmaceutical development of radiolabelled peptides

Receptor targeting with radiolabelled peptides has become very important in nuclear medicine and oncology in the past few years. The overexpression of many peptide receptors in numerous cancers, compared to their relatively low density in physiological organs, represents the molecular basis for in vivo imaging and targeted radionuclide therapy with radiolabelled peptide-based probes. The prototypes are analogs of somatostatin which are routinely used in the clinic. More recent developments include somatostatin analogs with a broader receptor subtype profile or with antagonistic properties. Many other peptide families such as bombesin, cholecystokinin/gastrin, glucagon-like peptide-1 (GLP-1)/exendin, arginine-glycine-aspartic acid (RGD) etc. have been explored during the last few years and quite a number of potential radiolabelled probes have been derived from them. On the other hand, a variety of strategies and optimized protocols for efficient labelling of peptides with clinically relevant radionuclides such as (99m)Tc, M(3+) radiometals ((111)In, (86/90)Y, (177)Lu, (67/68)Ga), (64/67)Cu, (18)F or radioisotopes of iodine have been developed. The labelling approaches include direct labelling, the use of bifunctional chelators or prosthetic groups. The choice of the labelling approach is driven by the nature and the chemical properties of the radionuclide. Additionally, chemical strategies, including modification of the amino acid sequence and introduction of linkers/spacers with different characteristics, have been explored for the improvement of the overall performance of the radiopeptides, e.g. metabolic stability and pharmacokinetics. Herein, we discuss the development of peptides as radiopharmaceuticals starting from the choice of the labelling method and the conditions to the design and optimization of the peptide probe, as well as some recent developments, focusing on a selected list of peptide families, including somatostatin, bombesin, cholecystokinin/gastrin, GLP-1/exendin and RGD.

Specificity and sensitivity of ⁹⁹mTc-EDDA/HYNIC-Tyr³-octreotide (⁹⁹mTc-TOC) for imaging neuroendocrine tumors

OBJECTIVES

Gastroenteropancreatic neuroendocrine tumors (NETs) are cancers originating from neuroendocrine organs such as the pancreas, pituitary, thyroid, and adrenal glands and tumors arising from the diffuse neuroendocrine cells that are widely distributed throughout the body. NETs express somatostatin (SS) and contain a high density of SS receptors; therefore, they can be specifically targeted with SS-based radiopharmaceuticals. The aim of this research was to determine the validity in terms of specificity, sensitivity, and the agreement beyond chance with the biopsy (gold standard) of the ⁹⁹mTc-EDDA-HYNIC-Tyr³octreotide (⁹⁹mTc-TOC) to image and localize NETs and their metastases.

MATERIALS AND METHODS

Freeze-dried kits containing 0.0125 mg HYNIC-octreotide and co-ligands were easily labeled and quality controlled within the hospital radiopharmacy. Fifty-six consecutive Mexican patients with a previous presumptive diagnosis of NETs underwent several clinical and laboratory studies and were referred to the Nuclear Medicine Department for a routine scan with ⁹⁹mTc-TOC. The patients were injected with 500-600 MBq ⁹⁹mTc-TOC, and whole-body images were obtained 2 h later with a SPECT or a SPECT/CT camera. Two nuclear medicine physicians observed the images and classified them as 17 negative and 39 positive. After correlating the image of each patient with our 'gold standard' (biopsy, clinical history, morphological images, and tumor marker assays), the ⁹⁹mTc-TOC images were classified by the same two physicians as 12 true negatives, five false negatives, 38 true positives and one false positive.

RESULTS

The validity of ⁹⁹mTc-TOC in terms of relative frequencies with corresponding 95% confidence intervals were as follows: 92.3% (64-100%) specificity; 88.4% (78-97%) sensitivity; and the agreement beyond chance was 73% (60-84%). The positive predictive value was 97.4% (87-100%); the negative predicted value was 70.6% (48-93%); the accuracy was 89.3% (89-97%); and the prevalence was 76.8% (64-87%).

CONCLUSION

Because of these high values, we strongly recommend scintigraphy with the Mexican-produced ⁹⁹mTc-TOC for the localization of NETs and their metastases, and we conclude that it is a good tool for detecting neuroendocrine disease in a Mexican population.

Nuclear medicine techniques for the imaging and treatment of neuroendocrine tumours

Nuclear medicine plays a pivotal role in the imaging and treatment of neuroendocrine tumours (NETs). Somatostatin receptor scintigraphy (SRS) with [(111)In-DTPA(0)]octreotide has proven its role in the diagnosis and staging of gastroenteropancreatic NETs (GEP-NETs). New techniques in somatostatin receptor imaging include the use of different radiolabelled somatostatin analogues with higher affinity and different affinity profiles to the somatostatin receptor subtypes. Most of these analogues can also be labelled with positron-emitting radionuclides that are being used in positron emission tomography imaging. The latter imaging modality, especially in the combination with computed tomography, is of interest because of encouraging results in terms of improved imaging quality and detection capabilities. Considerable advances have been made in the imaging of NETs, but to find the ideal imaging method with increased sensitivity and better topographic localisation of the primary and metastatic disease remains the ultimate goal of research. This review provides an overview of the currently used imaging modalities and ongoing developments in the imaging of NETs, with the emphasis on nuclear medicine and puts them in perspective of clinical practice. The advantage of SRS over other imaging modalities in GEP-NETs is that it can be used to select patients with sufficient uptake for treatment with radiolabelled somatostatin analogues. Peptide receptor radionuclide therapy (PRRT) is a promising new tool in the management of patients with inoperable or metastasised NETs as it can induce symptomatic improvement with all Indium-111, Yttrium-90 or Lutetium-177-labelled somatostatin analogues. The results that were obtained with [(90)Y-DOTA(0),Tyr(3)]octreotide and [(177)Lu-DOTA(0),Tyr(3)]octreotate are even more encouraging in terms of objective tumour responses with tumour regression and documented prolonged time to progression. In the largest group of patients receiving PRRT, treated with [(177)Lu-DOTA(0),Tyr(3)]octreotate, a survival benefit of several years compared with historical controls has been reported.

Comparative evaluation of glutamate-sensitive radiopharmaceuticals: Technetium-99m-glutamic acid and technetium-99m-diethylenetriaminepentaacetic acid-bis(glutamate) conjugate for tumor imaging

Single-photon emission computed tomography has become a significant imaging modality with huge potential to visualize and provide information of anatomic dysfunctions that are predictive of future diseases. This imaging tool is complimented by radiopharmaceuticals/radiosubstrates that help in imaging specific physiological aspects of the human body. The present study was undertaken to explore the utility of technetium-99m (⁹⁹(m)Tc)-labeled glutamate conjugates for tumor scintigraphy. As part of our efforts to further utilize the application of chelating agents, glutamic acid was conjugated with a multidentate ligand, diethylenetriaminepentaacetic acid (DTPA). The DTPA-glutamate conjugate [DTPA-bis(Glu)] was well characterized by IR, NMR, and mass spectroscopy. The biological activity of glutamic acid was compared with its DTPA conjugate by radiocomplexation with ⁹⁹(m)Tc (labeling efficiency ≥98%). In vivo studies of both the radiolabeled complexes ⁹⁹(m)Tc-Glu and ⁹⁹(m)Tc-DTPA-bis(Glu) were then carried out, followed by gamma scintigraphy in New Zealand albino rabbits. Improved serum stability of ⁹⁹(m)Tc-labeled DTPA conjugate indicated that ⁹⁹(m)Tc remained bound to the conjugate up to 24 hours. Blood clearance showed a relatively slow washout of the DTPA conjugate when compared with the labeled glutamate. Biodistribution characteristics of the conjugate in Balb/c mice revealed that DTPA conjugation of glutamic acid favors less accumulation in the liver and bone and rapid renal clearance. Tumor scintigraphy in mice showed increasing tumor accumulation, stable up to 4 hours. These preliminary studies show that ⁹⁹(m)Tc-DTPA-bis(Glu) can be a useful radiopharmaceutical for diagnostic applications in single-photon emission computed tomography imaging.

Bifunctional coupling agents for radiolabeling of biomolecules and target-specific delivery of metallic radionuclides

Receptor-based radiopharmaceuticals are of great current interest in molecular imaging and radiotherapy of cancers, and provide a unique tool for target-specific delivery of radionuclides to the diseased tissues. In general, a target-specific radiopharmaceutical can be divided into four parts: targeting biomolecule (BM), pharmacokinetic modifying (PKM) linker, bifunctional coupling or chelating agent (BFC), and radionuclide. The targeting biomolecule serves as a "carrier" for specific delivery of the radionuclide. PKM linkers are used to modify radiotracer excretion kinetics. BFC is needed for radiolabeling of biomolecules with a metallic radionuclide. Different radiometals have significant difference in their coordination chemistry, and require BFCs with different donor atoms and chelator frameworks. Since the radiometal chelate can have a significant impact on physical and biological properties of the target-specific radiopharmaceutical, its excretion kinetics can be altered by modifying the coordination environment with various chelators or coligand, if needed. This review will focus on the design of BFCs and their coordination chemistry with technetium, copper, gallium, indium, yttrium and lanthanide radiometals.

Factors influencing radioguided surgery of neuroendocrine tumours using 99mTc-ethylenediamine N,N'-diacetic acid/6-Hydrazinopyridine-3-carboxylic acid-D-Phe1-Tyr3-octreotide

OBJECTIVE

To determine factors influencing radioguided surgery of neuroendocrine tumours using Tc-ethylenediamine N,N'-diacetic acid/6-Hydrazinopyridine-3-carboxylic acid-D-Phe-Tyr-octreotide and gamma-probe.

METHODS

Patients with 17 neuroendocrine tumours were operated on. Three to 6 h before surgery, 550-650 MBq of Tc-labelled octreotide were injected into the patients. Scintigraphy was performed immediately before surgery to confirm uptake of the radiopharmaceutical by the tumour. The uptake was graded on a scale from 0 to 4. Local measurements of radioactivity were performed during surgery using a gamma-probe and small lead shields.

RESULTS

Intraoperatively 12 tumours in 11 patients were successfully localized using a gamma-probe: three tumours with uptake grade 4, seven with grade 3 and two with grade 2. Seven tumours were localized in the region of the pancreas, two next to the aortic bifurcation, one in the mesentery and two in the ileocecal region. Tumours were not found in five patients. They were all positive on scintigraphy with tracer uptake grade 4 in one patient, grade 3 in one, and grade 2 in three patients. Localization was confirmed by endoscopic ultrasound, computed tomography and/or MRI. All five tumours were located in the pancreatic area. In four of these patients, tumours were seen in the same localization as preoperatively on follow-up scintigraphy 6-12 months later.

CONCLUSION

Factors influencing the success of surgery were high octreotide uptake (grades 3 and 4) and localization of the tumour away from kidneys and spleen. Positive localization on endoscopic ultrasound, computed tomography or MRI, size of tumour, pathohistology and reoperations did not influence surgical success.

99mTc-labeling of HYNIC-conjugated cyclic RGDfK dimer and tetramer using EDDA as coligand

In this study, EDDA (ethylenediamine- N, N'-diacetic acid) was used as the coligand for 99mTc-labeling of cyclic RGDfK conjugates: HYNIC-dimer (HYNIC = 6-hydrazinonicotinamide; dimer = E[c(RGDfK)]2) and HYNIC-tetramer (tetramer = E{E[c(RGDfK)]2}2). First, HYNIC-dimer was allowed to react with 99mTcO4 (-) in the presence of excess tricine and stannous chloride to form the intermediate complex [99mTc(HYNIC-dimer)(tricine)2], which was then allowed to react with EDDA to afford [99mTc(HYNIC-dimer)(EDDA)] with high yield (>90%) and high specific activity ( approximately 8.0 Ci/micromol). Under the same radiolabeling conditions, the yield for [99mTc(HYNIC-tetramer)(EDDA)] was always <65%. The results from a mixed-ligand experiment show that there is only one EDDA bonding to the 99mTc-HYNIC core in [99mTc(HYNIC-dimer)(EDDA)]. The athymic nude mice bearing subcutaneous U87MG human glioma xenografts were used to evaluate the impact of EDDA coligand on the biodistribution characteristics and excretion kinetics of the 99mTc-labeled HYNIC-dimer and HYNIC-tetramer. Surprisingly, [99mTc(HYNIC-dimer)(EDDA)] and [99mTc(HYNIC-tetramer)(EDDA)] had almost identical tumor uptake over the 2 h period. The use of EDDA as coligand to replace tricine/TPPTS (TPPTS = trisodium triphenylphosphine-3,3',3''-trisulfonate) did not significantly change the uptake of the 99mTc-labeled HYNIC-dimer in noncancerous organs, such as the liver, kidneys, and lungs; but it did result in a significantly lower kidney uptake for the 99mTc-labeled HYNIC-tetramer due to faster renal excretion. It was also found that the radiotracer tumor uptake decreases in a linear fashion as the tumor size increases. The smaller the tumors are, the higher the tumor uptake is regardless of the identity of radiotracer.

Somatostatin receptor scintigraphy using 99mTc-EDDA/HYNIC-TOC in patients with medullary thyroid carcinoma

PURPOSE

Several new somatostatin analogues have been developed for the diagnosis and therapy of different tumours. Since somatostatin receptors are often over-expressed in medullary thyroid carcinoma (MTC), the aim of our study was to evaluate the utility of scintigraphy with the somatostatin analogue (99m)Tc-EDDA/HYNIC-TOC in MTC in comparison with other diagnostic techniques.

METHODS

Forty-five patients with MTC, aged 14-83 years, were investigated. Scintigraphy using (99m)Tc-EDDA/HYNIC-TOC (Tektrotyd) was performed 2 and 4 h post injection of 740 MBq (20 mCi) of the tracer. Other imaging techniques were also applied and analysed in individual cases (ultrasonography, computed tomography, (99m)Tc(V)-DMSA, (131)I-MIBG, (99m)Tc-MDP, (111)In-DTPA-octreotide and (18)F-FDG-PET) and compared with (99m)Tc-EDDA/HYNIC-TOC.

RESULTS

In group 1 (eight patients before thyroidectomy), uptake of the tracer was found in the primary tumours. In group 2 (six patients with remission), a false positive result was found in one patient; in the remaining five patients, no pathological foci were visualised. In group 3 (31 patients with post-surgical hypercalcitoninaemia), scintigraphy was true positive in 23 patients (74.2%): uptake in the thyroid bed was found in five patients, in the lymph nodes in 18 and in bone metastases in four. Using (99m)Tc-EDDA/HYNIC-TOC scintigraphy, the overall sensitivity was 79.5%, specificity 83.3%, accuracy 80.0%, positive predictive value 96.9% and negative predictive value 38.5%.

CONCLUSION

(99m)Tc-EDDA/HYNIC-TOC is clinically useful for scintigraphy in the follow-up of patients with MTC. It can be used in clinical practice for preoperative evaluation, for localisation of local recurrence or distant metastases and particularly for therapy decision making.

99mTc-labeled cyclic RGDfK dimer: initial evaluation for SPECT imaging of glioma integrin alphavbeta3 expression

This report describes the evaluation of biodistribution properties of three radiotracers, [(99m)Tc(SQ168)(EDDA)], [(99m)Tc(SQ168)(tricine)(PDA)], and [(99m)Tc(SQ168)(tricine)(TPPTS)] (SQ168 = [2-[[[5-[carboonyl]-2-pyridinyl]hydrazono]methyl]benzenesulfonic acid]-Glu(cyclo{Lys-Arg-Gly-Asp-d-Phe})-cyclo{Lys-Arg-Gly-Asp-d-Phe}; EDDA = ethylenediamine-N,N'-diacetic acid; PDA = 2,5-pyridinedicarboxylic acid; TPPTS = trisodium triphenylphosphine-3,3',3' '-trisulfonate), and their potential to image the glioma integrin alpha(v)beta(3) expression in BALB/c nude mice bearing the U87MG human glioma xenografts. It was found that all three radiotracers were able to localize in glioma tumors with a relatively high tumor uptake and long tumor retention time by binding to the integrin alpha(v)beta(3) expressed on both tumor cells and endothelial cells of tumor neovasculature. It seems that the coligand has minimal effect on integrin alpha(v)beta(3) targeting capability of the (99m)Tc-labeled RGDfK dimer, but it has a significant impact on their biodistribution properties. For example, the complex [(99m)Tc(SQ168)(tricine)(TPPTS)] has the lowest liver uptake and the highest metabolic stability in normal BALB/c nude mice. Results from SPECT imaging studies show that the glioma tumors can be clearly visualized with all three radiotracers at 4 h postinjection. Among the three radiotracers evaluated in this study, [(99m)Tc(SQ168)(tricine)(TPPTS)] has the best imaging quality and is a promising candidate for more preclinical evaluations in the future.

Nuclear medicine imaging and therapy of neuroendocrine tumours

Radiolabelled peptides are used for specific targeting of receptors (over-)expressed by tumour cells. Dependent on the kind of labelling and the radionuclide used, these compounds may be utilised for imaging or for therapy. A concise overview is provided on basic principles of designing and developing radiopeptides for these applications. Furthermore, clinical application of these compounds for imaging and therapy is described. Advantages of the method compared to other techniques (such as the use of radiolabelled antibodies or antibody fragments) are discussed as well as pitfalls and limitations.

A novel ternary ligand system useful for preparation of cationic (99m)Tc-diazenido complexes and (99m)Tc-labeling of small biomolecules

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

99mTc-EDDA/HYNIC-octreotate scintigraphy, an efficient method for the detection and staging of carcinoid tumours: results of 3 years’ experience

PURPOSE

At all stages of the disease, serious difficulties are encountered in the imaging diagnosis of carcinoids. Somatostatin receptor scintigraphy (SRS) holds great promise for detecting primary tumours and metastases. 99mTc-EDDA/HYNIC-octreotate should significantly improve the diagnosis of carcinoids in comparison with 111In-Octreoscan owing to the better affinity for SSR2 and the higher count rate. The aim of this study was to assess the diagnostic efficiency of 99mTc-EDDA/HYNIC-octreotate scintigraphy in the detection and staging of carcinoid tumours.

METHODS

The study population comprised 75 patients (age 48.5+/-15.5 years): 46 with histological confirmation of carcinoid and 29 with suspected disease. 99mTc-EDDA/HYNIC-octreotate (740 MBq) SRS and CT were performed in all patients. Fifteen patients were examined with 111In-Octreoscan.

RESULTS

High-quality 99mTc-EDDA/HYNIC-octreotate images were obtained in all cases, with maximum tumour tracer accumulation 4 h p.i. The mean target/non-target ratios for whole body (WB) and SPECT scans were, respectively, as follows: primary lesions: 4.5 and 10.2; metastases: liver, 3.1 and 12.3; abdominal focal lesions, 2.7 and 5.8; lung, 2.7 and 8.3; mediastinum, 3.4 and 7.6; bones, 6.8 and 19.0. 99mTc-EDDA/HYNIC-octreotate WB scans revealed more metastases than 111In-Octreoscan, with better individual separation. 99mTc-EDDA/HYNIC-octreotate SRS revealed new metastatic lesions in seven patients with confirmed carcinoid, and in four with dissemination the primary focus was found. Five patients qualified for radioguided surgery and 11 were referred to 90Y-DOTA-TATE therapy. The sensitivity of SRS in comparison with CT was higher for primary lesions and liver and abdominal lymph node metastases. In the subgroup of patients with suspected neuroendocrine tumours, two duodenal carcinoids, one thymic carcinoid and one ileal carcinoid were found.

CONCLUSION

99mTc-EDDA/HYNIC-octreotate, with high imaging quality, is an excellent alternative to 111In-Octreoscan for staging of carcinoids, and it seems to be the method of choice for detection of the primary focus in patients with metastases from an unknown primary tumour.

Effect of coligands on biodistribution characteristics of ternary ligand 99mTc complexes of a HYNIC-conjugated cyclic RGDfK dimer

This report describes biodistribution characteristics of three ternary ligand complexes [(99m)Tc(SQ168)(tricine)(L)] (SQ168 = [2-[[[5-[carboonyl]-2-pyridinyl]hydrazono]methyl]-benzenesulfonic acid]-Glu(cyclo{Lys-Arg-Gly-Asp-d-Phe})-cyclo{Lys-Arg-Gly-Asp-d-Phe}; L = TPPTS (trisodium triphenylphosphine-3,3',3' '-trisulfonate), ISONIC (isonicotinic acid) and PDA (2,5-pyridinedicarboxylic acid)) in athymic nude mice bearing MDA-MB-435 human breast cancer xenografts. Ternary ligand complexes [(99m)Tc(SQ168)(tricine)(L)] (L = TPPTS, ISONIC and PDA) were prepared and were analyzed by a reversed HPLC method. Surprisingly, coligands have little impact on log P values of their ternary ligand (99m)Tc complexes even though HPLC retention times suggest that [(99m)Tc(SQ168)(tricine)(PDA)] and [(99m)Tc(SQ168)(tricine)(ISONIC)] are more hydrophilic than [(99m)Tc(SQ168)(tricine)(TPPTS)]. The results from biodistribution studies indicated that excretion kinetics of the (99m)Tc-labeled cyclic RGDfK dimer can be modified by the choice of coligand. The fact that all three radiotracers show high tumor uptake during the 2 h study period suggests that the coligand has minimal effect on the tumor targeting capability of the (99m)Tc-labeled cyclic RGDfK dimer. Results from the blocking experiment suggest that the tumor localization of the (99m)Tc-labeled cyclic RGDfK dimer is integrin alpha(v)beta(3)-mediated. On the basis of their liver uptake and tumor/liver ratios, we believe that PDA has the advantage over TPPTS and ISONIC for the (99m)Tc-labeling of HYNIC-biomolecule conjugates.

New directions in the coordination chemistry of 99mTc: a reflection on technetium core structures and a strategy for new chelate design

Bifunctional chelates offer a general approach for the linking of radioactive metal cations to macromolecules. In the specific case of 99mTc, a variety of technologies have been developed for assembling a metal-chelate-biomolecule complex. An evaluation of these methodologies requires an appreciation of the coordination characteristics and preferences of the technetium core structures and oxidation states, which serve as platforms for the development of the imaging agent. Three technologies, namely, the MAG3-based bifunctional chelates, the N-oxysuccinimidylhydrazino-nicotinamide system and the recently described single amino acid chelates for the {Tc(CO)3}1+ core, are discussed in terms of the fundamental coordination chemistry of the technetium core structures. In assessing the advantages and disadvantages of these technologies, we conclude that the single amino acid analogue chelates (SAAC), which are readily conjugated to small peptides by solid-phase synthesis methods and which form robust complexes with the {Tc(CO)3}1+ core, offer an effective alternative to the previously described methods.