[99mTc]Demotate 2 in the detection of sst2-positive tumours: a preclinical comparison with [111In]DOTA-tate

Diagnostic Value of Radiolabelled Somatostatin Analogues for Neuroendocrine Tumour Diagnosis: The Benefits and Drawbacks of [64Cu]Cu-DOTA-TOC

Neuroendocrine tumours (NETs) arise from secondary epithelial cell lines in the gastrointestinal or respiratory system organs. The rate of development of these tumours varies from an indolent to an aggressive course, typically being initially asymptomatic. The identification of these tumours is difficult, particularly because the primary tumour is often small and undetectable by conventional anatomical imaging. Consequently, diagnosis of NETs is complicated and has been a significant challenge until recently. In the last 30 years, the advent of novel nuclear medicine diagnostic procedures has led to a substantial increase in NET detection. Great varieties of exclusive single photon emission computed tomography (SPECT) and positron emission tomography (PET) radiopharmaceuticals for detecting NETs are being applied successfully in clinical settings, including [111In]In-pentetreotide, [99mTc]Tc-HYNIC-TOC/TATE, [68Ga]Ga-DOTA-TATE, and [64Cu]Cu-DOTA-TOC/TATE. Among these tracers for functional imaging, PET radiopharmaceuticals are clearly and substantially superior to planar or SPECT imaging radiopharmaceuticals. The main advantages include higher resolution, better sensitivity and increased lesion-to-background uptake. An advantage of diagnosis with a radiopharmaceutical is the capacity of theranostics to provide concomitant diagnosis and treatment with particulate radionuclides, such as beta and alpha emitters including Lutetium-177 (177Lu) and Actinium-225 (225Ac). Due to these unique challenges involved with diagnosing NETs, various PET tracers have been developed. This review compares the clinical characteristics of radiolabelled somatostatin analogues for NET diagnosis, focusing on the most recently FDA-approved [64Cu]Cu-DOTA-TATE as a state-of-the art NET-PET/CT radiopharmaceutical.

Overview of Radiolabeled Somatostatin Analogs for Cancer Imaging and Therapy

Identified in 1973, somatostatin (SST) is a cyclic hormone peptide with a short biological half-life. Somatostatin receptors (SSTRs) are widely expressed in the whole body, with five subtypes described. The interaction between SST and its receptors leads to the internalization of the ligand-receptor complex and triggers different cellular signaling pathways. Interestingly, the expression of SSTRs is significantly enhanced in many solid tumors, especially gastro-entero-pancreatic neuroendocrine tumors (GEP-NET). Thus, somatostatin analogs (SSAs) have been developed to improve the stability of the endogenous ligand and so extend its half-life. Radiolabeled analogs have been developed with several radioelements such as indium-111, technetium-99 m, and recently gallium-68, fluorine-18, and copper-64, to visualize the distribution of receptor overexpression in tumors. Internal metabolic radiotherapy is also used as a therapeutic strategy (e.g., using yttrium-90, lutetium-177, and actinium-225). With some radiopharmaceuticals now used in clinical practice, somatostatin analogs developed for imaging and therapy are an example of the concept of personalized medicine with a theranostic approach. Here, we review the development of these analogs, from the well-established and authorized ones to the most recently developed radiotracers, which have better pharmacokinetic properties and demonstrate increased efficacy and safety, as well as the search for new clinical indications.

Imaging of inflammatory cellular protagonists in human atherosclerosis: a dual-isotope SPECT approach

Purpose

Atherosclerotic plaque development and progression signifies a complex inflammatory disease mediated by a multitude of proinflammatory leukocyte subsets. Using single photon emission computed tomography (SPECT) coupled with computed tomography (CT), this study tested a new dual-isotope acquisition protocol to assess each radiotracer’s capability to identify plaque phenotype and inflammation levels pertaining to leukocytes expressing leukocyte function-associated antigen-1 (LFA-1) and the leukocyte subset of proinflammatory macrophages expressing somatostatin receptor subtype-2 (SST₂). Individual radiotracer uptake was quantified and the presence of corresponding immunohistological cell markers was assessed.

Methods

Human symptomatic carotid plaque segments were obtained from endarterectomy. Segments were incubated in dual-isotope radiotracers [¹¹¹In]In-DOTA-butylamino-NorBIRT ([¹¹¹In]In-Danbirt) and [^99mTc]Tc-[N^0–1₄,Asp⁰,Tyr³]-octreotate ([^99mTc]Tc-Demotate 2) before scanning with SPECT/CT. Plaque phenotype was classified as pathological intimal thickening, fibrous cap atheroma or fibrocalcific using histology sections based on distinct morphological characteristics. Plaque segments were subsequently immuno-stained with LFA-1 and SST₂ and quantified in terms of positive area fraction and compared against the corresponding SPECT images.

Results

Focal uptake of co-localising dual-radiotracers identified the heterogeneous distribution of inflamed regions in the plaques which co-localised with positive immuno-stained regions of LFA-1 and SST₂. [¹¹¹In]In-Danbirt and [^99mTc]Tc-Demotate 2 uptake demonstrated a significant positive correlation (r = 0.651; p = 0.001). Fibrous cap atheroma plaque phenotype correlated with the highest [¹¹¹In]In-Danbirt and [^99mTc]Tc-Demotate 2 uptake compared with fibrocalcific plaques and pathological intimal thickening phenotypes, in line with the immunohistological analyses.

Conclusion

A dual-isotope acquisition protocol permits the imaging of multiple leukocyte subsets and the pro-inflammatory macrophages simultaneously in atherosclerotic plaque tissue. [¹¹¹In]In-Danbirt may have added value for assessing the total inflammation levels in atherosclerotic plaques in addition to classifying plaque phenotype.

Paradigm shift in theranostics of neuroendocrine tumors: conceptual horizons of nanotechnology in nuclear medicine

We present a comprehensive review of Neuroendocrine Tumors (NET) and the current and developing imaging and therapeutic modalities for NET with emphasis on Nuclear Medicine modalities. Subsequently, nanotechnology and its emerging role in cancer management, especially NET, are discussed. The article is both educative and informative. The objective is to provide an insight into the developments made in nuclear medicine and nanotechnology towards management of NET, individually as well as combined together.

Radiolabeled new somatostatin analogs conjugated to DOMA chelator used as targeted tumor imaging agent: synthesis and radiobiological evaluation

Several receptor-specific radiopeptides have been developed and effective in the diagnosis of malignant diseases. Among them, somatostatin receptor (SSTR) scintigraphy with (111)In-DTPA-octreotide has become a tumor diagnostic radiopharmaceutical in nuclear medicine. However, it suffers some drawbacks concerning the imaging properties and elevated radiation burden of (111)In. Here, we report the synthesis of radiolabeled two new octapeptides with improved uptake in SSTR2-positive tumors in comparison with (99m)Tc-HYNIC-Tyr(3)-octreotide (HYNIC-TOC). Octapeptides were synthesized in high yield by Fmoc solid-phase synthesis and coupling the macrocyclic chelator DOMA(1,4,7-Tri-Boc-10-(carboxymethyl)-1,4,7,10-tetraazocyclododecane-1-yl-monoacetic acid) to these peptides for (99m)Tc labeling. New peptides DOMA-Asn(3)-octreotate(DOMA-AATE) and DOMA-Pro(3)-octreotate(DOMA-PATE) were purified, characterized by RP-HPLC, MALDI-mass, (1)H-NMR, (13)C-NMR. Labeling was performed by SnCl2 method to get products with excellent radiochemical purity (97 %). Radiopeptides were found to be substantially stable under physiological condition for 24 h. Internalization and receptor-binding studies 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 IC50 values of (99m)Tc-DOMA-AATE(1.10 ± 0.48 nM) and (99m)Tc-DOMA-PATE(1.76 ± 0.06 nM) showed high affinity binding for SSTR2 receptor and they internalized rapidly in C6 cells. Biodistribution and imaging studies were performed in C6 tumor-bearing rat under gamma camera showing significant uptake in kidney, urine and C6 tumor. Radiopeptides exhibited fast blood clearance and rapid elimination through the urinary systems. However, (99m)Tc-DOMA-AATE exhibited the highest tumor to muscle and tumor to blood uptake ratios among three. These favorable characteristics validate (99m)Tc-DOMA-AATE as a more promising (99m)Tc-radiotracer than (99m)Tc-DOMA-PATE, (99m)Tc-HYNIC-TOC for SSTR2-positive tumor scintigraphy.

Single-photon emission computed tomography tracers in the diagnostics of neuroendocrine tumors

Different imaging strategies have been developed targeting the peculiar features of neuroendocrine tumors (NETs). Metabolic characteristics and receptor expression on the tumor surface have been studied, and expertise and knowledge are increasing as a result of the implementation of fusion imaging and the development of more detailed positron emission tomography tracers. Scintigraphic study of NETs is the most diffused and convenient technique for evaluating patients suspected to have NETs.

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.

Radiopeptide internalisation and externalization assays: cell viability and radioligand integrity

Various aspects of radiopeptide receptor-mediated cell internalisation and externalization assays were assessed, including the integrity of externalized peptides and the effect of varying the pH and incubation time of the acid wash step (to remove surface receptor-bound ligand) on efficacy and cell viability. The observed intact proportion of externalized peptide was 5-10%, and acid wash buffers with pH 2.8 or below were found to be detrimental to cell viability and integrity, particularly following prolonged incubation times.

Tumor imaging and therapy using radiolabeled somatostatin analogues

Molecular imaging plays an essential role in balancing the clinical benefits and risks of radionuclide-based cancer therapy. To effectively treat individual patients, careful assessment of biodistribution, dosimetry, and toxicity is essential. In this Account, we describe advances that combine features of molecular imaging and radionuclide therapy to provide new avenues toward individualized cancer treatment. Selective receptor-targeting radiopeptides have emerged as an important class of radiopharmaceuticals for molecular imaging and therapy of tumors that overexpress peptide receptors on the cell membrane. After such peptides labeled with gamma-emitting radionuclides bind to their receptors, they allow clinicians to visualize receptor-expressing tumors non-invasively. Peptides labeled with beta-particle emitters could also eradicate receptor-expressing tumors. The somatostatin receptors, which are overexpressed in a majority of neuroendocrine tumors, represent the first and best example of targets for radiopeptide-based imaging and radionuclide therapy. The somatostatin analogue (111)In-octreotide permits the localization and staging of neuroendocrine tumors that express the appropriate somatostatin receptors. Newer modified somatostatin analogues, including Tyr(3)-octreotide and Tyr(3)-octreotate, are successfully being used for tumor imaging and radionuclide therapy. Because there are few effective therapies for patients with inoperable or metastasized neuroendocrine tumors, this therapy is a promising novel treatment option for these patients. Peptide receptor imaging and radionuclide therapy can be combined in a single probe, called a "theranostic". To select patients who are likely to benefit from this type of intervention, we first use a peptide analogue labeled with a diagnostic radionuclide to obtain a scan. Selected patients will be treated using the same or a similar peptide analogue labeled with a therapeutic radionuclide. The development of such theranostics could greatly advance the development of personalized treatments. Apart from patient selection for radionuclide therapy, other imaging applications of targeted radiopeptides include localization of primary tumors, detection of metastatic disease (staging/restaging), dosimetry (prediction of response and radiotoxicity), monitoring effects of surgery, radio(nuclide)therapy or chemotherapy, and detection of progression of disease or relapse (follow up). For further evaluation of tumor receptor expression and to increase the value of cancer targeting using radiopeptides, researchers have introduced and evaluated different radiolabeled analogues of other peptide families, such as cholecystokinin (CCK), gastrin, bombesin, substance P, vasoactive intestinal peptide (VIP), and neuropeptide (NP)-Y analogues. We expect improvements in the development of new peptide analogues: such advances could reduce side effects and allow for the use of combination therapy (for example, combining radiopeptide analogues with chemotherapeutics).

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.