Damian Wild

Radiolabeled somatostatin receptor antagonists are preferable to agonists for in vivo peptide receptor targeting of tumors

Targeting neuroendocrine tumors expressing somatostatin receptor subtypes (sst) with radiolabeled somatostatin agonists is an established diagnostic and therapeutic approach in oncology. While agonists readily internalize into tumor cells, permitting accumulation of radioactivity, radiolabeled antagonists do not, and they have not been considered for tumor targeting. The macrocyclic chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was coupled to two potent somatostatin receptor-selective peptide antagonists [NH2-CO-c(DCys-Phe-Tyr-DAgl8(Me,2-naphthoyl)-Lys-Thr-Phe-Cys)-OH (sst3-ODN-8) and a sst2-selective antagonist (sst2-ANT)], for labeling with 111/natIn. 111/natIn-DOTA-sst3-ODN-8 and 111/natIn-DOTA–[4-NO2-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)-DTyr-NH2] (111/natIn-DOTA-sst2-ANT) showed high sst3- and sst2-binding affinity, respectively. They did not trigger sst3 or sst2 internalization but prevented agonist-stimulated internalization. 111In-DOTA-sst3-ODN-8 and 111In-DOTA-sst2-ANT were injected intravenously into mice bearing sst3- and sst2-expressing tumors, and their biodistribution was monitored. In the sst3-expressing tumors, strong accumulation of 111In-DOTA-sst3-ODN-8 was observed, peaking at 1 h with 60% injected radioactivity per gram of tissue and remaining at a high level for >72 h. Excess of sst3-ODN-8 blocked uptake. As a control, the potent agonist 111In-DOTA–[1-Nal3]-octreotide, with strong sst3-binding and internalization properties showed a much lower and shorter-lasting uptake in sst3-expressing tumors. Similarly, 111In-DOTA-sst2-ANT was injected into mice bearing sst2-expressing tumors. Tumor uptake was considerably higher than with the highly potent sst2-selective agonist 111In-diethylenetriaminepentaacetic acid–[Tyr3,Thr8]-octreotide (111In-DTPA-TATE). Scatchard plots showed that antagonists labeled many more sites than agonists. Somatostatin antagonist radiotracers therefore are preferable over agonists for the in vivo targeting of sst3- or sst2-expressing tumors. Antagonist radioligands for other peptide receptors need to be evaluated in nuclear oncology as a result of this paradigm shift.

Glucagon-like peptide-1 receptor imaging for localization of insulinomas.

CONTEXT The surgical removal of insulinomas is hampered by difficulties to localize it using conventional radiological procedures. Recently these tumors were shown to exhibit a very high density of glucagon-like peptide-1 receptors (GLP-1R) in vitro that may be used as specific targets for in vivo receptor radiolabeling. OBJECTIVE The objective of the study was to test the 111In-labeled GLP-1R agonist 111In-DOTA-exendin-4 in localizing insulinomas using single photon emission computed tomography in combination with computed tomography images. DESIGN This was a prospective open-label investigation. SETTING The study was conducted at three tertiary referral centers in Switzerland. PATIENTS Patients included six consecutive patients with proven clinical and biochemical endogenous hyperinsulinemic hypoglycemia. INTERVENTION (111)In-DOTA-exendin-4 was administered iv at a dose of about 90 MBq (30 microg peptide) over 5 min. Whole-body planar images of the abdomen were performed at 20 min, 4 h, 23 h, 96 h, and up to 168 h after injection. After surgical removal of the insulinomas, GLP-1R expression was assessed in the tumor tissue in vitro by GLP-1R autoradiography. MAIN OUTCOME MEASURE The detection rate of insulinomas was measured. RESULTS In all six cases, the GLP-1R scans successfully detected the insulinomas identified using conventional methods in four cases. By using a gamma-probe intraoperatively, GLP-1R detection permitted a successful surgical removal of the tumors in all patients, diagnosed histopathologically as five pancreatic and one extrapancreatic insulinomas. In vitro GLP-1R autoradiography showed a high density of GLP-1R in all tested insulinomas. CONCLUSION In vivo GLP-1R imaging is an innovative, noninvasive diagnostic approach that successfully localizes small insulinomas pre- and intraoperatively and that may in the future affect the strategy of insulinoma localization.

Glucagon-like peptide 1-receptor scans to localize occult insulinomas

The precise localization of some insulinomas with the use of conventional imaging techniques is a challenging clinical problem. These findings indicate that GLP-1–receptor scanning may offer a new ...

68Ga-DOTANOC: a first compound for PET imaging with high affinity for somatostatin receptor subtypes 2 and 5

Existing somatostatin-based radiotracers (e.g. In-DOTA TOC) have sole affinity for somatostatin receptor subtype 2 (sst2). This represents a drawback, given that sst1–5 have been shown to be over-expressed in different tumours, alone or concomitantly [1]. Our goal, therefore, was to develop radiopeptides with broader receptor subtype profiles. Ga-DOTANOC is a first compound for PET imaging with high affinity for sst2 and sst5 [2]. Its affinity profile (IC50 nM) for human sst1–5 is, respectively, >10,000, 1.9± 0.4, 40±5.8, 260±74 and 7.2±1.6. For comparison, the values for the standard compound, In-DOTATOC, are >10,000, 4.6±0.2, 120±26, 230±82 and 130±17. Here we present the 60 min p.i. Ga-DOTANOC PET images and the 21 h p.i. In-DOTATOC planar images of a 52-year-old patient with an advanced neuroendocrine tumour. The two examinations were performed within 4 weeks. During this time interval the patient received bisphosphonates. Preparation and application of Ga-DOTANOC PET and Ga-DOTATOC PET are comparable [3]. In the reported case study, the Ga-DOTANOC PET scan shows high radioligand uptake in the liver and bone metastases. Although many bone metastases appeared visually similar in the two scans, the right sixth rib and left occipital bone metastases (arrows) are much more visible on the Ga-DOTANOC PET scan. This selective difference cannot be explained simply by the advantages of the PET technique. The possible predominance of sst5 in these two bone metastases and the high sst5 affinity of GaDOTANOC are in fact the probable reasons for the high Ga-DOTANOC and low In-DOTATOC uptake. The enlarged liver and somatostatin receptor-positive organs such as the spleen (high uptake) and pituitary gland and thyroid (moderate uptake) are also visible. These normal organs, known to express more sst than just sst2, are better visualised with Ga-DOTANOC (see in particular the spleen). We conclude that Ga-DOTANOC is an excellent candidate for primary diagnostic and follow-up investigations in patients with suspected or proven somatostatin receptorpositive tumours. Furthermore, in this case, predictive imaging indicates that Yor Lu-DOTANOC has greater potential for treatment of this patient than Yor LuDOTATOC.

Exendin-4–Based Radiopharmaceuticals for Glucagonlike Peptide-1 Receptor PET/CT and SPECT/CT

Strong overexpression of glucagonlike peptide-1 (GLP-1) receptors in human insulinoma provides an attractive target for imaging. The first clinical trials demonstrated that GLP-1 receptor SPECT/CT using [Lys40(Ahx [6-aminohexanoic acid]-DOTA-111In)NH2]-exendin-4 can localize hardly detectable insulinomas. However, [Lys40(Ahx-DOTA-111In)NH2]-exendin-4 imaging has drawbacks related to the use of 111In in that it is costly and carries a relatively high radiation burden for the patient. The aim of this study was the preclinical evaluation of [Lys40(Ahx-DOTA-68Ga)NH2]-exendin-4 for PET/CT and [Lys40(Ahx-hydrazinonicotinamide [HYNIC]-99mTc)NH2]-exendin-4 for SPECT/CT. Methods: Internalization, biodistribution, dosimetry, and imaging studies were performed in the Rip1Tag2 mouse model of pancreatic β-cell carcinogenesis and compared with our gold standard [Lys40(Ahx-DOTA-111In)NH2]-exendin-4. Poly-glutamic acid and Gelofusine, a gelatin-based plasma expander, were used for renal uptake reduction studies. Results: The tumor uptake of [Lys40(Ahx-DOTA-68Ga)NH2]-exendin-4 was 205 ± 59 percentage injected activity per gram of tissue at 4 h. Other GLP-1 receptor–positive organs showed more than 4.8 times lower radioactivity uptake. [Lys40(Ahx-HYNIC-99mTc/ethylenediaminediacetic acid [EDDA])NH2]-exendin-4, compared with its 111In- and 68Ga-labeled sister compounds, showed significantly less tumor and organ uptake. The significantly lower tumor and organ uptake of [Lys40(Ahx-HYNIC-99mTc/EDDA)NH2]-exendin-4 did not result in inferior tumor-to-organ ratios or reduced image quality. All radiopeptides tested showed a high tumor-to-background ratio, resulting in the visualization of small tumors (maximum diameter between 1.0 and 3.2 mm) by SPECT and PET. The only exception was the kidneys, which also showed high uptake. This uptake could be reduced by 49%−78% using poly-glutamic acid, Gelofusine, or a combination of the 2. The estimated effective radiation dose was 3.7 μSv/MBq for [Lys40(Ahx-HYNIC-99mTc/EDDA)NH2]-exendin-4, which was 8 times less than that for [Lys40(Ahx-DOTA-68Ga)NH2]-exendin-4 and 43 times less than that for [Lys40(Ahx-DOTA-111In)NH2]-exendin-4. Conclusion: These promising pharmacokinetic and imaging data show that [Lys40(Ahx-DOTA-68Ga)NH2]-exendin-4 and [Lys40(Ahx-HYNIC-99mTc/EDDA)NH2]-exendin-4 are suitable candidates for clinical GLP-1 receptor imaging studies.

Glucagon-Like Peptide-1 Versus Somatostatin Receptor Targeting Reveals 2 Distinct Forms of Malignant Insulinomas

Glucagon-like peptide-1 (GLP-1) receptor imaging is superior to somatostatin receptor subtype 2 (sst2) imaging in localizing benign insulinomas. Here, the role of GLP-1 and sst2 receptor imaging in the management of malignant insulinoma patients was investigated. Methods: Eleven patients with malignant insulinoma were prospectively included. 111In-[Lys40(Ahx-diethylenetriaminepentaacetic acid [DTPA])NH2]-exendin-4 SPECT/CT, 68Ga- DOTATATE PET/CT, and in vitro receptor autoradiography were performed to assess the receptor status and to evaluate the detection rate. Results: GLP-1 receptor targeting was positive in 4 of 11 patients, and sst2 receptor expression was positive in 8 of 11. In only 1 patient were both receptors expressed. In 1 patient, GLP-1 receptor imaging was the only method that successfully localized the primary tumor in the pancreas. In 3 patients with sst2-expressing tumors, DOTATATE radiotherapy was effectively applied. Conclusion: As opposed to benign insulinomas, malignant insulinomas often lack GLP-1 receptors. Conversely, malignant insulinomas often express sst2, which can be targeted therapeutically.

Glucagon-like peptide-1 receptor imaging for the localisation of insulinomas: a prospective multicentre imaging study

BACKGROUND Small benign insulinomas are hard to localise, leading to difficulties in planning of surgical interventions. We aimed to prospectively assess the insulinoma detection rate of single-photon emission CT in combination with CT (SPECT/CT) with a glucagon-like peptide-1 receptor avid radiotracer, and compare detection rates with conventional CT/MRI techniques. METHODS In our prospective imaging study, we enrolled adults aged 25-81 years at centres in Germany, Switzerland, and the UK. Eligible patients had proven clinical and biochemical endogenous hyperinsulinaemic hypoglycaemia and no evidence for metastatic disease on conventional imaging. CT/MRI imaging was done at referring centres according to standard protocols. At three tertiary nuclear medicine centres, we used whole body planar images and SPECT/CT of the abdomen up to 168 h after injection of (111)In-[Lys40(Ahx-DTPA-(111)In)NH2]-exendin-4 ((111)In-DTPA-exendin-4) to identify insulinomas. Consenting patients underwent surgery and imaging findings were confirmed histologically. FINDINGS Between Oct 1, 2008, and Dec 31, 2011, we recruited 30 patients. All patients underwent (111)In-DTPA-exendin-4 imaging, 25 patients underwent surgery (with histological analysis), and 27 patients were assessed with CT/MRI. (111)In-DTPA-exendin-4 SPECT/CT correctly detected 19 insulinomas and four additional positive lesions (two islet-cell hyperplasia and two uncharacterised lesions) resulting in a positive predictive value of 83% (95% CI 62-94). One true negative (islet-cell hyperplasia) and one false negative (malignant insulinoma) result was identified in separate patients by (111)In-DTPA-exendin-4 SPECT/CT. Seven patients (23%) were referred to surgery on the basis of (111)In-DTPA-exendin-4 imaging alone. For 23 assessable patients, (111)In-DTPA-exendin-4 SPECT/CT had a higher sensitivity (95% [95% CI 74-100]) than did CT/MRI (47% [27-68]; p=0.011). INTERPRETATION (111)In-DTPA-exendin-4 SPECT/CT could provide a good second-line imaging strategy for patients with negative results on initial imaging with CT/MRI. FUNDING Oncosuisse, the Swiss National Science Foundation, and UK Department of Health.

Comparison of 68Ga-DOTANOC and 68Ga-DOTATATE PET/CT Within Patients with Gastroenteropancreatic Neuroendocrine Tumors

Somatostatin receptor PET tracers such as [68Ga-DOTA,1-Nal3]-octreotide (68Ga-DOTANOC) and [68Ga-DOTA,Tyr3]-octreotate (68Ga-DOTATATE) have shown promising results in patients with neuroendocrine tumors, with a higher lesion detection rate than is achieved with 18F-fluorodihydroxyphenyl-l-alanine PET, somatostatin receptor SPECT, CT, or MR imaging. 68Ga-DOTANOC has high affinity for somatostatin receptor subtypes 2, 3, and 5 (sst2,3,5). It has a wider receptor binding profile than 68Ga-DOTATATE, which is sst2-selective. The wider receptor binding profile might be advantageous for imaging because neuroendocrine tumors express different subtypes of somatostatin receptors. The goal of this study was to prospectively compare 68Ga-DOTANOC and 68Ga-DOTATATE PET/CT in the same patients with gastroenteropancreatic neuroendocrine tumors (GEP-NETs) and to evaluate the clinical impact of 68Ga-DOTANOC PET/CT. Methods: Eighteen patients with biopsy-proven GEP-NETs were evaluated with 68Ga-DOTANOC and 68Ga-DOTATATE using a randomized crossover design. Labeling of DOTANOC and DOTATATE with 68Ga was standardized using a fully automated synthesis device. PET/CT findings were compared with 3-phase CT scans and in some patients with MR imaging, 18F-FDG PET/CT, and histology. Uptake in organs and tumor lesions was quantified and compared by calculation of maximum standardized uptake values (SUVmax) using volume computer-assisted reading. Results: Histology revealed low-grade GEP-NETs (G1) in 4 patients, intermediate grade (G2) in 7, and high grade (G3) in 7. 68Ga-DOTANOC and 68Ga-DOTATATE were false-negative in only 1 of 18 patients. In total, 248 lesions were confirmed by cross-sectional and PET imaging. The lesion-based sensitivity of 68Ga-DOTANOC PET was 93.5%, compared with 85.5% for 68Ga-DOTATATE PET (P = 0.005). The better performance of 68Ga-DOTANOC PET is attributed mainly to the significantly higher detection rate of liver metastases rather than tumor differentiation grade. Multivariate analysis revealed significantly higher SUVmax in G1 tumors than in G3 tumors (P = 0.009). This finding was less pronounced with 68Ga-DOTANOC (P > 0.001). Altogether, 68Ga-DOTANOC changed treatment in 3 of 18 patients (17%). Conclusion: The sst2,3,5-specific radiotracer 68Ga-DOTANOC detected significantly more lesions than the sst2-specific radiotracer 68Ga-DOTATATE in our patients with GEP-NETs. The clinical relevance of this finding has to be proven in larger studies.

[Lys40(Ahx-DTPA-111In)NH2]-Exendin-4 is a highly efficient radiotherapeutic for glucagon-like peptide-1 receptor-targeted therapy for insulinoma.

Purpose: Although metabolic changes make diagnosis of insulinoma relatively easy, surgical removal is hampered by difficulties in locating it, and there is no efficient treatment for malignant insulinoma. We have previously shown that the high density of glucagon-like peptide-1 receptors (GLP-1R) in human insulinoma cells provides an attractive target for molecular imaging and internal radiotherapy. In this study, we investigated the therapeutic potential of [Lys40(Ahx-DTPA-111In)NH2]-Exendin-4, an 111In-labeled agonist of GLP-1, in a transgenic mouse model of human insulinoma. Experimental Design: [Lys40(Ahx-DTPA-111In)NH2]-Exendin-4 was assessed in the Rip1Tag2 mouse model of pancreatic β-cell carcinogenesis, which exhibits a GLP-1R expression comparable with human insulinoma. Mice were injected with 1.1, 5.6, or 28 MBq of the radiopeptide and sacrificed 7 days after injection. Tumor uptake and response, the mechanism of action of the radiopeptide, and therapy toxicity were investigated. Results: Tumor uptake was >200% injected activity per gram, with a dose deposition of 3 Gy/MBq at 40 pmol [Lys40(Ahx-DTPA-111In)NH2]-Exendin-4. Other GLP-1R–positive organs showed ≥30 times lower dose deposition. A single injection of [Lys40(Ahx-DTPA-111In)NH2]-Exendin-4 resulted in a reduction of the tumor volume by up to 94% in a dose-dependent manner without significant acute organ toxicity. The therapeutic effect was due to increased tumor cell apoptosis and necrosis and decreased proliferation. Conclusions: The results suggest that [Lys40(Ahx-DTPA-111In)NH2]-Exendin-4 is a promising radiopeptide capable of selectively targeting insulinoma. Furthermore, Auger-emitting radiopharmaceuticals such as 111In are able to produce a marked therapeutic effect if a high tumor uptake is achieved.

First Clinical Evidence That Imaging with Somatostatin Receptor Antagonists Is Feasible

Preclinical studies have indicated that somatostatin receptor (sst)–expressing tumors demonstrate higher uptake of radiolabeled sst antagonists than of sst agonists. In this study, we evaluated whether imaging with sst antagonists was feasible in patients. Methods: Biodistribution and tumor uptake of the sst antagonist 111In-DOTA-pNO2-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)DTyrNH2 (111In-DOTA-BASS) were studied in 5 patients with metastatic thyroid carcinoma or neuroendocrine tumors. Findings were compared with 111In-pentetreotid (111In-DTPA-octreotide) scan. Results: No adverse effects of 111In-DOTA-BASS (20 μg) were observed. 111In-DOTA-BASS detected 25 of 28 lesions, whereas 111In-DTPA-octreotide detected only 17 of 28 lesions. In the same patient, 111In-DOTA-BASS showed higher tumor and lower renal uptake than 111In-DTPA-octreotide (3.5 ± 2.8 percentage injected activity [%IA] vs. 1.0 ± 0.99%IA and 1.5 ± 0.3 %IA vs. 2.3 ± 0.7 %IA) at 4 h after injection. Conclusion: Imaging of neuroendocrine tumors with sst antagonists is clinically feasible. The favorable human biodistribution data suggest that sst antagonists could significantly affect peptide receptor–mediated imaging and therapy.