Somatostatin receptor scintigraphy using [111In-DTPA0]RC-160 in humans: a comparison with [111In-DTPA0]octreotide

Somatostatin receptor-positive lesions can be visualized by scintigraphy using [¹¹¹In-DTPA⁰]octreotide. Recently, there have been reports of differences in receptor binding between somatostatin receptor subtypes and between somatostatin analogues, such as RC-160 and octreotide, as well as of differences in internalization between the somatostatin receptor subtypes. The possibility that certain somatostatin receptor-positive tissues and tumours which do not bind octreotide may bind and internalize RC-160 would open new scintigraphic or radiotherapeutic applications of radiolabelled RC-160. We investigated the metabolism and tissue distribution of [¹¹¹In-DTPA⁰]RC-160 in comparison with [¹¹¹In-DTPA⁰]octreotide in four patients after injection of 250 MBq (10 μg) of these radiopharmaceuticals. Patient 1 had a metastatic follicular thyroid carcinoma, patient 2 a metastatic medullary thyroid carcinoma, patient 3 tuberculosis and patient 4 an insulinoma. The plasma clearance of the [¹¹¹In-DTPA⁰]RC-160 was slower than that of [¹¹¹In-DTPA⁰]octreotide, with 5% and 2%, respectively, of the initial plasma radioactivity remaining at 10 h p.i. The urinary excretion of [¹¹¹In-DTPA⁰]RC-160 was initially also slower than that of [¹¹¹In-DTPA⁰]octreotide, but the cumulative excretion of radioactivity was not significantly different at 48 h p.i. Approximately 80% of injected radioactivity was cleared in the urine, while in one patient 20% of the injected dose was recovered in the faeces. The slower clearance of [¹¹¹In-DTPA⁰]RC-160 resulted in a higher background in all organs studied i.e. liver, spleen, kidneys and lungs, at 24 h p.i. Although the target to background ratio with [¹¹¹In-DTPA⁰]octreotide was higher, no differences were found between the two analogues with regard to their sensitivity in detecting lesions in these four patients. We conclude that although only four subjects were studied, [¹¹¹In-DTPA⁰]RC-160 does not appear to have additional value for scintigraphy and is associated with higher background activity. Received 20 October and in revised form 31 October 1997     

Tumour uptake of the radiolabelled somatostatin analogue [DOTA0,TYR3]octreotide is dependent on the peptide amount

Radiolabelled tumour receptor-binding peptides can be used for in vivo scintigraphic imaging. Recently, the somatostatin analogue [Tyr³]octreotide (d-Phe-c(Cys-Tyr-d-Trp-Lys-Thr-Cys)-Thr(ol)) was derivatized with the chelator DOTA (tetra-azacyclododecane-tetra-acetic acid), enabling stable radiolabelling with both the high-energy beta particle-emitter yttrium-90 and the Auger electron-emitter indium-111. The thus produced radiolabelled compounds are promising for peptide receptor radionuclide therapy. Our previous in vitro and in vivo (rat) experiments with these radiolabelled compounds showed favourable binding and biodistribution characteristics with high uptake and retention in the target organs. We also demonstrated receptor-specific, time- and temperature-dependent internalization of radiolabelled [DOTA⁰,Tyr³]octreotide in somatostatin receptor subtype 2 (sst₂)-positive rat pancreatic tumour cell lines. In this study we have investigated the effects of differences in the amount of injected peptide on tissue distribution of ¹¹¹In-labelled [DOTA⁰,Tyr³]octreotide in normal, i.e. non-tumour-bearing, and CA20948 tumour-bearing rats. This was done in order to find the amount of peptide at which the highest uptake in target tissues is achieved, and thereby to increase the potential of radionuclide therapy while simultaneously ensuring the lowest possible radiotoxicity in normal organs. Uptake of radiolabelled [DOTA⁰,Tyr³]octreotide in sst₂-positive organs showed different bell-shaped functions of the amount of injected peptide, being highest at 0.05 (adrenals), 0.05–0.1 (pituitary and stomach) and 0.25 (pancreas) μg. Uptake in the tumour was highest at 0.5 μg injected peptide. The highest uptake was found at peptide amounts that were lower than those reported for [¹¹¹In-DTPA⁰]octreotide ((d-Phe-c(Cys-Phe-d-Trp-Lys-Thr-Cys)-Thr(ol), DTPA = diethylene-triamine-penta-acetic acid), consistent with the higher receptor affinity of the first compound. Our observations of mass-dependent differences in uptake of radiolabelled [DOTA⁰, Tyr³]octreotide, being the resultant of a positive effect of increasing amounts of peptide on, for example, receptor clustering and a negative effect of receptor saturation, are of consequence for rat radionuclide therapy studies with radiolabelled peptides and may also be of consequence for human radionuclide therapy studies with this compound. Received 6 January and in revised form 16 February 1999     

Yttrium-90 and indium-111 labelling, receptor binding and biodistribution of [DOTA0,d-Phe1,Tyr3]octreotide, a promising somatostatin analogue for radionuclide therapy

In vitro octreotide receptor binding of [¹¹¹In-DOTA⁰,d-Phe¹,Tyr³]octreotide (¹¹¹In-DOTATOC) and the in vivo metabolism of⁹⁰Y or¹¹¹In-labelled DOTATOC were investigated in rats in comparison with [¹¹¹In-DTPA⁰]octreotide [¹¹¹In-DTPAOC).¹¹¹In-DOTATOC was found to have an affinity similar to octreotide itself for the octreotide receptor in rat cerebral cortex microsomes. Twenty-four hours after injection of⁹⁰Y or¹¹¹In-labelled DOTATOC, uptake of radioactivity in the octreotide receptor-expressing tissues pancreas, pituitary, adrenals and tumour was a factor of 2–6 that after injection of¹¹¹In-DTPAOC. Uptake of labelled DOTATOC in pituitary, pancreas, adrenals and tumour was almost completely blocked by pretreatment with 0.5 mg unlabelled octreotide, indicating specific binding to the octreotide receptors. These findings strongly indicate that⁹⁰Y-DOTATOC is a promising radiopharmaceutical for radiotherapy and that¹¹¹In-DOTATOC is of potential value for diagnosis of patients with octreotide receptor-positive lesions, such as most neuroendocrine tumours.     

Localisation and mechanism of renal retention of radiolabelled somatostatin analogues

Purpose Radiolabelled somatostatin analogues, such as octreotide and octreotate, are used for tumour scintigraphy and radionuclide therapy. The kidney is the most important critical organ during such therapy owing to the reabsorption and retention of radiolabelled peptides. The aim of this study was to investigate in a rat model both the localisation and the mechanism of renal uptake after intravenous injection of radiolabelled somatostatin analogues. The multi-ligand megalin/cubilin receptor complex, responsible for reabsorption of many peptides and proteins in the kidney, is an interesting candidate for renal endocytosis of these peptide analogues.Methods For localisation studies, ex vivo autoradiography and micro-autoradiography of rat kidneys were performed 1–24 h after injection of radiolabelled somatostatin analogues and compared with the renal anti-megalin immunohistochemical staining pattern. To confirm a role of megalin in the mechanism of renal retention of [¹¹¹In-DTPA]octreotide, the effects of three inhibitory substances were explored in rats. Results Renal ex vivo autoradiography showed high cortical radioactivity and lower radioactivity in the outer medulla. The distribution of cortical radioactivity was inhomogeneous. Micro-autoradiography indicated that radioactivity was only retained in the proximal tubules. The anti-megalin immunohistochemical staining pattern showed a strong similarity with the renal [¹¹¹In-DTPA]octreotide ex vivo autoradiograms. Biodistribution studies showed that co-injection of positively charged d-lysine reduced renal uptake to 60% of control. Sodium maleate reduced renal [¹¹¹In-DTPA]octreotide uptake to 15% of control. Finally, cisplatin pre-treatment of rats reduced kidney uptake to 70% of control.Conclusion Renal retention of [¹¹¹In-DTPA]octreotide is confined to proximal tubules in the rat kidney, in which megalin-mediated endocytosis may play an important part.     

Indium-111 octreotide scintigraphy in patients with bone tumours of the extremities

Radiolabelled somatostatin analogues are of potential value in the imaging of somatostatin receptor-positive tumours. Recently, somatostatin receptors have been demonstrated in the osteoblast precursor cells. In this preliminary study, we evaluated the uptake characteristics of indium-111 octreotide in two benign and two malignant bone tumours. Tracer accumulation was observed in all four cases, and overall lesion to background ratio (mean±SD) was 2.74±0.84 and 2.98±1.49 at 4 h and 24 h, respectively. There was no clear relationship between I¹¹¹In-octreotide accumulation and the benign or malignant nature of the tumour. In one patient, tracer uptake was inhibited by unlabelled octreotide administration. These results suggest that¹¹¹In-octreotide can be taken up by benign and malignant bone tumours. The inhibition of tumour uptake by treatment with cold octreotide supports the concept that specific uptake mechanisms are responsible for¹¹¹In-octreotide deposition by bone tumours.     

A new radiolabelled somatostatin analogue [111In-DTPA-D-Phe1]RC-160: preparation,biological activity,receptor scintigraphy in rats and comparison with [111In-DTPA-D-Phe1]octreotide

We have evaluated the potential usefulness of indium-111 labelled [DTPA-D-Phe¹]RC-160, derived from the octapeptide somatostatin analogue RC-160, as a radiopharmaceutical for the in vivo detection of somatostatin receptor-positive tumours. For this purpose ¹¹¹In-and ¹¹¹In-labelled [DTPA-D-Phe¹]RC-160 was tested for its biological activity, and applied for somatostatin receptor scintigraphy in vivo to rats bearing the transplantable rat pancreatic tumour CA20948, which expresses somatostatin receptors. We previously described the development of the ¹¹¹In-labelled somatostatin analogue [DTPA-D-Phe¹]octreotide and its use in the in vivo visualization of somatostatin receptor-positive tumours in rats and in humans. Like [¹¹¹In-DTPA-D-Phe¹]octreotide, [¹¹¹In-DTPA-D-Phe¹]RC-160 showed uptake in and specific binding in vivo to somatostatin receptor-positive organs and tumours, and the tumours were clearly visualized by gamma camera scintigraphy. However, as compared to [¹¹¹In-DTPA-D-Phe¹]octreotide, blood radioactivity (background) was higher, resulting in a lower tumour to blood (background) ratio. Using this animal model we therefore conclude that [¹¹¹In-DTPA-DPhe¹]RC-160 has no advantage over [¹¹¹In-DTPA-DPhe¹]octreotide as a radiopharmaceutical in the visualization of somatostatin receptors which bind both analogues. However, recent reports suggest the existence of different somatostatin receptor subtypes on some human cancers, which differentially bind RC-160 and not octreotide. These tumours include cancers of the breast, ovary, exocrine pancreas, prostate and colon. [¹¹¹In-DTPA-D-Phe¹]RC-160 might be of interest for future use in such cancer patients as a radiopharmaceutical for imaging somatostatin receptor-positive tumours, which do not bind octreotide.     

[177Lu-DOTA0,Tyr3]octreotate: comparison with [111In-DTPA0]octreotide in patients

The somatostatin analogue [DOTA⁰,Tyr³]octreotate has a nine-fold higher affinity for the somatostatin receptor subtype 2 as compared with [DOTA⁰,Tyr³]octreotide. Also, labelled with the beta- and gamma-emitting radionuclide lutetium-177, this compound has been shown to have a very favourable impact on tumour regression and animal survival in a rat model. Because of these reported advantages over the analogues currently used for somatostatin receptor-mediated radiotherapy, we decided to compare [¹⁷⁷Lu-DOTA⁰,Tyr³]octreotate (¹⁷⁷Lu-octreotate) with [¹¹¹In-DTPA⁰]octreotide (¹¹¹In-octreotide) in six patients with somatostatin receptor-positive tumours. Plasma radioactivity after ¹⁷⁷Lu-octreotate expressed as a percentage of the injected dose was comparable with that after ¹¹¹In-octreotide. Urinary excretion of radioactivity was significantly lower than after ¹¹¹In-octreotide, averaging 64% after 24 h. The uptake after 24 h, expressed as a percentage of the injected dose of ¹⁷⁷Lu-octreotate, was comparable to that after ¹¹¹In-octreotide for kidneys, spleen and liver, but was three- to fourfold higher for four of five tumours. The spleen and kidneys received the highest absorbed doses. The doses to the kidneys were reduced by a mean of 47% after co-infusion of amino acids. It is concluded that in comparison with the radionuclide-coupled somatostatin analogues that are currently available for somatostatin receptor-mediated radiotherapy, ¹⁷⁷Lu-octreotate potentially represents an important improvement. Higher absorbed doses can be achieved to most tumours, with about equal doses to potentially dose-limiting organs; furthermore, the lower tissue penetration range of ¹⁷⁷Lu as compared with ⁹⁰Y may be especially important for small tumours.     

Improved visualization of carcinoid liver metastases by indium-111 pentetreotide scintigraphy following treatment with cold somatostatin analogue

Five patients with hepatic metastases of midgut carcinoid underwent somatostatin receptor scintigraphy with indium-111 pentetreotide before and during treatment with octreotide. Octreotide treatment changed the biodistribution of ¹¹¹In-pentetreotide significantly. Whereas the radioactivity in liver, spleen and kidney decreased, hepatic metastases showed increased contrast. In one patient, liver metastases could only be detected during octreotide treatment. These data suggest that the diagnostic reliability of somatostatin receptor scintigraphy in carcinoid liver metastases is not necessarily compromised by octreotide therapy. Because of different biodistributions, the detection of liver metastases may even be improved during octreotide therapy. Correspondence to: U. Dörr     

Use of the rat pancreatic CA20948 cell line for the comparison of radiolabelled peptides for receptor-targeted scintigraphy and radionuclide therapy

We have evaluated the usefulness of the rat pancreatic CA20948 tumour as an in vitro cell culture model and as an in vivo model in Lewis rats comparing different radiolabelled peptides for receptor-targeted scintigraphy. In vitro the receptor-specific uptake and internalization of different radiolabelled analogues of somatostatin, bombesin, substance P and cholecystokinin were demonstrated. Analogues were selected based on high-affinity binding to their respective receptors. Their uptake and internalization in CA20948 cells were compared to these processes in AR42J cells, a well-known rat pancreatic tumour cell line used for peptide-receptor studies. Receptor-specific internalization, which was blocked by excess unlabelled peptide analogue, was found in both the CA20948 and AR42J cells for all the peptide analogues tested. This indicates specific receptor expression for all the different peptides, making these cells highly suitable for peptide studies. Internalization of the different peptides was as follows, in increasing order: [111In-DOTA0]CCK < [111In-DTPA0,Arg1]substance P < [111In-DTPA0]octreotide < [111In-DTPA0,Pro1,Tyr4]bombesin. Internalization appeared to be time and temperature dependent. In accordance with the in vitro experiments, receptor-specific uptake of all the peptide analogues was also found in vivo in the solid CA20948 tumour. The in vivo tumour uptake of [111n-DTPA0]octreotide was the highest amongst the peptides tested, the order of tumour uptake being [111In-DTPA0]octreotide >[111In-DTPA0,Pro1,Tyr4]bombesin >[111In-DTPA0,Arg1]substance P > [111In-DOTA0]CCK, which is different from the in vitro findings and points to either different receptor numbers on the tumour cells for the different peptide receptors in vitro and in vivo or to differences between the peptides with regard to metabolic stability. It can be concluded that the CA20948 tumour, both in cell culture and as a solid tumour in rats, is a very useful model for peptide receptor scintigraphy and radionuclide therapy studies.     

In vivo binding of [68Ga]-DOTATOC to somatostatin receptors in neuroendocrine tumours — impact of peptide mass

ObjectivesThe aim of this pilot study was to explore the impact of peptide mass on binding of [⁶⁸Ga]-DOTATOC to neuroendocrine tumour somatostatin receptors in vivo using a tracer of variable specific radioactivity (SRA) and to show the logistic feasibility of sequential PET scans in the same patient.Material and MethodsNine patients with gastroenteropancreatic neuroendocrine tumours were included. Six of them underwent three sequential PET-CT examinations with intravenous injections of [⁶⁸Ga]-DOTATOC proceeded by 0, 50 and 250 or 500 μg of octreotide, administered 10 min before the tracer. Three patients were examined by dynamic and static PET/CT for pharmacokinetic and dosimetric calculations. The [⁶⁸Ga]-DOTATOC synthesis included preconcentration and purification of the generator eluate and microwave heating in a semi-automated in-house procedure.Results[⁶⁸Ga]-DOTATOC synthesis and quality control were accomplished within 30 min and radiochemical purity was >95%. The tracer accumulation in the tumours varied and depended on the total amount of the administered peptide. In five of six patients, the highest tumour-to-normal tissue ratio was found when 50 μg of octreotide was preadministered. One patient showed a continuously increasing tumour uptake. Dosimetrically, a large variation in organ doses was found (kidney: 0.086–0.168 mSv/MBq; liver: 0.026–0.096 mSv/MBq; spleen: 0.046–0.226 mSv/MBq). The effective dose (0.015, 0.0067 and 0.0042 mSv/MBq) was correlated to the total amount of decays.DiscussionThree sequential PET-CT examinations using ⁶⁸Ga-based tracer was carried out in 1 day. The use of high SRA [⁶⁸Ga]-DOTATOC and unlabelled octreotide indicates an optimal mass leading to better image contrast. [⁶⁸Ga]-DOTATOC-PET-CT employing variable SRA may be utilised for accurate quantification of tumour uptake with subsequent dosimetry for personalized therapy management.     

[123I]Mtr-TOCA, a radioiodinated and carbohydrated analogue of octreotide: scintigraphic comparison with [111In]octreotide

Purpose Scintigraphy with maltotriose-[¹²³I]Tyr³-octreotate ([¹²³I]Mtr-TOCA) is compared with [¹¹¹In]DTPA-Phe¹-octreotide ([¹¹¹In]OC) to assess the differences in pharmacokinetics and imaging properties as well as to estimate the therapeutic potential of the corresponding [¹³¹I]Mtr-TOCA.Methods Six patients with somatostatin receptor (sstr)-positive tumours were assessed using a dual-head gamma camera. After injection of 137±28 MBq [¹²³I]Mtr-TOCA, dynamic data acquisition of the upper abdomen (30 min) was performed followed by whole-body scans at 0.5 h, 1 h, 3 h and 20 h as well as by SPECT imaging (tumour) at 2 h. [¹¹¹In]OC scintigraphy was performed by acquiring whole-body scans (4 h, 24 h) and SPECT (24 h). Using a region of interest (ROI) method, tissue and tumour bound activity was assessed and dosimetry performed.Results [¹²³I]Mtr-TOCA shows rapid tumour uptake. Up to 4 h, tumour/organ (tu/org) ratios are stable and generally higher than with [¹¹¹In]OC. From 3 h to 20 h, tu/org ratios increase for spleen, remain stable for liver and decrease significantly for all other tissues. In contrast, with [¹¹¹In]OC, tu/org ratios decrease slightly between 4 h and 24 h for liver, spleen and kidney and increase for all other tissues. On [¹²³I]Mtr-TOCA scintigraphy, a total of 27 lesions are detected, whereas 33 lesions are detected on [¹¹¹In]OC scintigraphy (p=0.50). Effective patient absorbed dose is 1.9±0.9 mSv per 100 MBq [¹²³I]Mtr-TOCA.Conclusion Compared with [¹¹¹In]OC, [¹²³I]Mtr-TOCA enables faster imaging of sstr-positive tumours with a lower radiation burden to the patient. [¹²³I]Mtr-TOCA and [¹¹¹In]OC allow for tumour imaging with almost identical contrast and diagnostic yield. As regards peptide receptor radionuclide therapy, radioiodinated Mtr-TOCA is hampered by limited intratumoural activity retention.     

Associations between the uptake of <Superscript>111</Superscript>In-DTPA-trastuzumab,HER2 density and response to trastuzumab (Herceptin) in athymic mice bearing subcutaneous human tumour xenografts

Purpose  The purpose of the study was to investigate the associations between uptake of ¹¹¹In-DTPA-trastuzumab, tumour HER2 density and response to trastuzumab (Herceptin) of human breast cancer (BC) xenografts in athymic mice. Materials and methods  The tumour uptake of ¹¹¹In-DTPA-trastuzumab in athymic mice bearing BC xenografts with increasing HER2 density (0 to 3+) was evaluated. Specific uptake ratios were established in biodistribution (SUR) and imaging studies (ROI-SUR) using ¹¹¹In-labeled mouse IgG (¹¹¹In-DTPA-mIgG). Further corrections were made for circulating radioactivity using tumour-to-blood ratios defined as a localization index (LI) and region-of-interest localization index (ROI-LI), respectively. Mice were treated with trastuzumab (Herceptin). A tumour growth inhibition index (TGI) was calculated and relative TGIs calculated by dividing the TGI of control by that of trastuzumab-treated mice. Results  Strong, nonlinear associations with HER2 density were obtained if the uptake of ¹¹¹In-DTPA-trastuzumab was corrected for nonspecific IgG localization (i.e., SUR; r ² = 0.99) and circulating radioactivity (i.e., LI; r ² = 0.87), but without these corrections, the association between HER2 density and tumour uptake was poor (r ² = 0.22). There was a strong association between ROI-SUR and ROI-LI values and HER2 expression (r ² = 0.90 and r ² = 0.95, respectively. All tumours were imaged. Relative TGI values were associated with increasing uncorrected tumour uptake of ¹¹¹In-DTPA-trastuzumab but not always with HER2 density (i.e., MCF-HER2-18 cells with trastuzumab-resistance). Conclusion  HER2 expression (0 to 3+) can be differentiated using ¹¹¹In-DTPA-trastuzumab, but requires correction of tumour uptake for nonspecific IgG localization and circulating radioactivity. The uncorrected uptake of ¹¹¹In-DTPA-trastuzumab was associated with tumour response to trastuzumab.     

The somatostatin receptor-targeted radiotherapeutic [90Y-DOTA-dPhe1,Tyr3]octreotide (90Y-SMT 487) eradicates experimental rat pancreatic CA 20948 tumours

Somatostatin receptor-expressing tumours are potential targets for therapy with radiolabelled somatostatin analogues. We have synthesized a number of such analogues in the past and identified [DOTA-dPhe¹, Tyr³]octreotide (SMT 487) as the most promising candidate molecule because of its advantageous properties in cellular and in vivo tumour models. In the current paper we describe the radiotherapeutic effect of yttrium-90 labelled SMT 487 in Lewis rats bearing the somatostatin receptor-positive rat pancreatic tumour CA 20948. SMT 487 binds with nanomolar affinity to both the human and the rat somatostatin receptor subtype 2 (sst₂) (human sst₂ IC₅₀=0.9 nM, rat sst₂ IC₅₀=0.5 nM). In vivo, ⁹⁰Y-SMT 487 distributed rapidly to the sst₂ expressing CA 20948 rat pancreatic tumour, with a tumour-to-blood ratio of 49.15 at 24 h post injection. A single intravenous administration of 10 mCi/kg ⁹⁰Y-SMT 487 resulted in a complete remission of the tumours in five out of seven CA 20948 tumour-bearing Lewis rats. No regrowth of the tumours occurred 8 months post injection. Control animals that were treated with 30 μg/kg of unlabelled SMT 487 had to be sacrificed 10 days post injection due to excessive growth or necrotic areas on the tumour surface. Upon re-inoculation of tumour cells into those rats that had shown complete remission, the tumours disappeared after 3–4 weeks of moderate growth without any further treatment. The present study shows for the first time the curative potential of ⁹⁰Y-SMT 487-based radiotherapy for somatostatin receptor-expressing tumours. Clinical phase I studies with yttrium-labelled SMT 487 have started in September 1997. Received 14 January and in revised form 16 March 1998     

DOTA-NOC,a high-affinity ligand of somatostatin receptor subtypes 2, 3 and 5 for labelling with various radiometals

Earlier studies have shown that modification of the octapeptide octreotide in positions 3 and 8 may result in compounds with increased somatostatin receptor affinity that, if radiolabelled, display improved uptake in somatostatin receptor-positive tumours. The aim of a recent research study in our laboratory was to employ the parallel peptide synthesis approach by further exchanging the amino acid in position 3 of octreotide and coupling the macrocyclic chelator DOTA(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) to these peptides for labelling with radiometals like gallium-67 or -68, indium-111, yttrium-90 and lutetium-177. The purpose was to find radiopeptides with an improved somatostatin receptor binding profile in order to extend the spectrum of targeted tumours. A first peptide, [¹¹¹In,⁹⁰Y-DOTA]-1-Nal³-octreotide (¹¹¹In,⁹⁰Y-DOTA-NOC), was isolated which showed an improved profile. In^III-DOTA-NOC exhibited the following IC₅₀ values (nM) when studied in competition with [¹²⁵I][Leu⁸, d-Trp²², Tyr²⁵]somatostatin-28 (values for Y^III-DOTA-NOC are shown in parentheses): sstr2, 2.9±0.1 (3.3±0.2); sstr3, 8±2 (26±1.9); sstr5, 11.2±3.5 (10.4±1.6). Affinity towards sstr1 and 4 was very low or absent. In^III-DOTA-NOC is superior to all somatostatin-based radiopeptides having this particular type of binding profile, including DOTA-lanreotide, and has three to four times higher binding affinity to sstr2 than In^III,Y^III-DOTA-Tyr³-octreotide (In^III,Y^III-DOTA-TOC). In addition, [¹¹¹In]DOTA-NOC showed a specific and high rate of internalization into AR4-2J rat pancreatic tumour cells which, after 4 h, was about two times higher than that of [¹¹¹In]DOTA-TOC and three times higher than that of [¹¹¹In]DOTA-octreotide ([¹¹¹In]DOTA-OC). The internalized radiopeptides were externalized intact upon 2 h of internalization followed by an acid wash. After 2–3 h of externalization a plateau is reached, indicating a steady-state situation explained by reactivation of the receptors followed by re-endocytosis. Biodistribution studies in CA 20948 tumour-bearing rats showed rapid clearance from all sstr-negative tissues except the kidneys. At 4 h the uptake of [¹¹¹In]DOTA-NOC in the tumour and sstr-positive tissues, such as adrenals, stomach and pancreas, was three to four times higher than that of [¹¹¹In]DOTA-TOC. Differential blocking studies indicate that this is at least partially due to the uptake mediated by sstr3 and sstr5. These very promising preclinical data justify the use of this new radiopeptide for imaging and potentially internal radiotherapy studies in patients.Abbreviations of the common amino acids are in accordance with the recommendations of IUPAC-IUB [IUPAC-IUB Commission of Biochemical Nomenclature (CBN), Symbols for amino-acid derivatives and peptides, recommendations 1971. Eur J Biochem 1972; 27:201–207].     

Radioiodinated somatostatin analogue RC-160: preparation,biological activity,in vivo application in rats and comparison with [123I-Tyr3]octreotide

We have evaluated the potential usefulness of the radioiodinated octapeptide RC-160, a somatostatin analogue, which might serve as a radiopharmaceutical for the in vivo detection of somatostatin receptor-positive tumours. For this purpose, iodine-123 and iodine-125 labelled RC-160 was tested for biological activity and applied in vivo in rats bearing the transplantable rat pancreatic tumour CA20948, which expresses somatostatin receptors. Our group has recently described the in vivo visualization of such tumours in rats and in humans with the radioiodinated somatostatin analogue [Tyr³]octreotide. Like [¹²³I-Tyr³]octreotide, ¹²³I-RC-160 showed uptake in and specific binding in vivo to somatostatin receptor-positive organs and tumours. However, blood radioactivity (background) was higher, resulting in a lower tumour to blood (background) ratio. We therefore conclude that in this animal model ¹²³I-RC-160 has no advantage over [¹²³I-Tyr³]octreotide as a radiopharmaceutical for the in vivo use as a somatostatin receptor imager, although, like [¹²³I-Tyr³]octreotide, ¹²³I-RC-160 shows specific binding to different somatostatin receptor-positive organs. Recently differences were reported in affinity between somatostatin and its analogues for somatostatin receptors expressed in different human cancers, like those of the breast, ovary, exocrine pancreas, prostate and colon. Therefore ¹²³I-RC-160 might be of interest for future use in humans as a radiopharmaceutical for imaging octreotide receptor-negative tumours. Correspondence to: W.A.P. Breeman, Department of Nuclear Medicine, University Hospital Dijkzigt, Dr. Molewaterplein 40, NL-3015 GD Rotterdam, The NetherlandsThe authors wish to thank Dr. Wil Kort, Ineke Hekking-Weyma, Reno Mekes, Marcello Harms and Ina Loeve for their expert assistance during the experiments.     

Amifostine protects rat kidneys during peptide receptor radionuclide therapy with [<Superscript>177</Superscript>Lu-DOTA<Superscript>0</Superscript>,Tyr<Superscript>3</Superscript>]octreotate

Purpose In peptide receptor radionuclide therapy (PRRT) using radiolabelled somatostatin analogues, the kidneys are the major dose-limiting organs, because of tubular reabsorption and retention of radioactivity. Preventing renal uptake or toxicity will allow for higher tumour radiation doses. We tested the cytoprotective drug amifostine, which selectively protects healthy tissue during chemo- and radiotherapy, for its renoprotective capacities after PRRT with high-dose [¹⁷⁷Lu-DOTA⁰,Tyr³]octreotate. Methods Male Lewis rats were injected with 278 or 555 MBq [¹⁷⁷Lu-DOTA⁰,Tyr³]octreotate to create renal damage and were followed up for 130 days. For renoprotection, rats received either amifostine or co-injection with lysine. Kidneys, blood and urine were collected for toxicity measurements. At 130 days after PRRT, a single-photon emission computed tomography (SPECT) scan was performed to quantify tubular uptake of ^99mTc-dimercaptosuccinic acid (DMSA), a measure of tubular function. Results Treatment with 555 MBq [¹⁷⁷Lu-DOTA⁰,Tyr³]octreotate resulted in body weight loss, elevated creatinine and proteinuria. Amifostine and lysine treatment significantly prevented this rise in creatinine and the level of proteinuria, but did not improve the histological damage. In contrast, after 278 MBq [¹⁷⁷Lu-DOTA⁰,Tyr³]octreotate, creatinine values were slightly, but not significantly, elevated compared with the control rats. Proteinuria and histological damage were different from controls and were significantly improved by amifostine treatment. Quantification of ^99mTc-DMSA SPECT scintigrams at 130 days after [¹⁷⁷Lu-DOTA⁰,Tyr³]octreotate therapy correlated well with 1/creatinine (r ² = 0.772, p < 0.001). Conclusion Amifostine and lysine effectively decreased functional renal damage caused by high-dose [¹⁷⁷Lu-DOTA⁰,Tyr³]octreotate. Besides lysine, amifostine might be used in clinical PRRT as well as to maximise anti-tumour efficacy.     

[1-<Superscript>11</Superscript>C]Acetate uptake is not increased in renal cell carcinoma

Purpose The purpose of this study was to investigate the potential of [1-¹¹C]acetate (AC) as a metabolic tracer for renal cell cancer in human subjects. Methods Twenty-one patients with suspected kidney tumours were investigated with AC and dynamic PET. AC uptake was scored on a five-step scale. Tumour localisation was known from CT/MRI. Histology was available in 18/21 patients. The results in these 18 patients are reported. Results AC uptake by the tumour was less than (n = 11), equal to (n = 5) or higher than (n = 2) uptake in the surrounding renal parenchyma. Histological tumour types showed a typical distribution, with a predominance of clear cell carcinomas (n = 14) and only a small number of papillary cell carcinomas (n = 2) and oncocytomas (n = 2). Only the benign oncocytomas were highly positive with AC. Conclusion In most kidney tumours the AC accumulation was not higher than in normal kidney parenchyma. Therefore, AC PET cannot be recommend for the characterisation of a renal mass.     

111In-labelled somatostatin analogues in a rat tumour model: somatostatin receptor status and effects of peptide receptor radionuclide therapy

Purpose Peptide receptor scintigraphy with the radioactive somatostatin analogue ¹¹¹In-DTPA-octreotide is a sensitive and specific technique to show in vivo the presence of somatostatin receptors on various tumours. Since ¹¹¹In emits not only gamma rays but also therapeutic Auger and internal conversion electrons with a medium to short tissue penetration (0.02–10 m and 200–550 m, respectively), ¹¹¹In-DTPA-octreotide is also being used for peptide receptor radionuclide therapy (PRRT). In this study we investigated the therapeutic effects of ¹¹¹In-DTPA-octreotide in tumours of various sizes. Regrowth of a tumour despite PRRT with ¹¹¹In-DTPA-octreotide may be due to the lack of crossfire from ¹¹¹In, whereby any possible receptor-negative tumour cell can multiply. We therefore also investigated the somatostatin receptor status of the tumour before and after PRRT.Methods The radiotherapeutic effects of different doses of ¹¹¹In-DTPA-octreotide in vivo were investigated in Lewis rats bearing small (1 cm²) or large (8 cm²) somatostatin receptor-positive rat pancreatic CA20948 tumours expressing the somatostatin receptor subtype 2 (sst₂). In addition, the somatostatin receptor density on the tumour after injection of a therapeutic labelled somatostatin analogue was investigated when the tumour was either declining in size or regrowing after an initial reduction in size. To initiate a partial response of the tumour (so that regrowth would follow) and not a complete response, a relatively low dose was administered.Results Impressive radiotherapeutic effects of ¹¹¹In-labelled octreotide were observed in this rat tumour model. Complete responses (up to 50%) were found in the animals bearing small (1 cm²) tumours after at least three injections of 111 MBq or a single injection of 370 MBq ¹¹¹In-DTPA-octreotide, leading to a dose of 6.3–7.8 mGy/MBq (1–10 g tumour). In the rats bearing the larger (8 cm²) tumours, the effects were much less pronounced and only partial responses were achieved in these groups. Clear sst₂ expression was found in the control as well as in the treated tumours. A significantly higher tumour receptor density (p<0.001) was found when the tumours regrew after an initial decline in size after low-dose PRRT in comparison with the untreated tumours.Conclusion Therapy with ¹¹¹In-labelled somatostatin analogues is feasible but should preferably start as early as possible during tumour development. One might also consider the use of radiolabelled somatostatin analogues in an adjuvant setting after surgery of somatostatin receptor-positive tumours in order to eradicate occult metastases. We showed that PRRT led to an increase in the density of somatostatin receptors when the tumours regrew after an initial decline in size because of PRRT. Upregulation of the somatostatin receptor may lead to higher uptake of radiolabelled peptides in therapeutic applications, which would probably make repeated injections of radiolabelled peptides more effective.     

Are radiogallium-labelled DOTA-conjugated somatostatin analogues superior to those labelled with other radiometals?

Purpose Gallium-68 is a metallic positron emitter with a half-life of 68 min that is ideal for the in vivo use of small molecules, such as [⁶⁸Ga-DOTA,Tyr³]octreotide, in the diagnostic imaging of somatostatin receptor-positive tumours. In preclinical studies it has shown a striking superiority over its ¹¹¹In-labelled congener. The purpose of this study was to evaluate whether third-generation somatostatin-based, radiogallium-labelled peptides show the same superiority. Methods Peptides were synthesised on solid phase. The receptor affinity was determined by in vitro receptor autoradiography. The internalisation rate was studied in AR4-2J and hsst-HEK-transfected cell lines. The pharmacokinetics was studied in a rat xenograft tumour model, AR4-2J. Results All peptides showed high affinities on hsst2, with the highest affinity for the Ga^III-complexed peptides. On hsst3 the situation was reversed, with a trend towards lower affinity of the Ga^III peptides. A significantly increased internalisation rate was found in sst2-expressing cells for all ⁶⁷Ga-labelled peptides. Internalisation into HEK-sst3 was usually faster for the ¹¹¹In-labelled peptides. No internalisation was found into sst5. Biodistribution studies employing [⁶⁷Ga-DOTA,1-Nal³]octreotide in comparison to [¹¹¹In-DOTA,1-Nal³]octreotide and [⁶⁷Ga-DOTA,Tyr³]octreotide showed a significantly higher and receptor-mediated uptake of the two ⁶⁷Ga-labelled peptides in the tumour and somatostatin receptor-positive tissues. A patient study illustrated the potential advantage of a broad receptor subtype profile radiopeptide over a high-affinity sst2-selective radiopeptide. Conclusion This study demonstrates that ^67/68Ga-DOTA-octapeptides show distinctly better preclinical, pharmacological performances than the ¹¹¹In-labelled peptides, especially on sst2-expressing cells and the corresponding animal models. They may be excellent candidates for further development for clinical studies.     

Renal accumulation of [<Superscript>111</Superscript>In]DOTATOC in rats: influence of inhibitors of the organic ion transport and diuretics

Aim Radiation exposure to the kidney limits therapy with radiometal labelled DOTATOC. This study evaluates the organic anion and cation transport (inhibitors: probenecid and cimetidine/dexamethason) as well as diuresis (furosemide and mannitol) regarding renal uptake of [¹¹¹In]DOTATOC. Methods One hundred eight male Fisher rats were injected with [¹¹¹In]DOTATOC via the tail vein. Prior to activity injection a total of 84 rats underwent injection with probenecid vs. sodium chloride 0.9% (48 rats), cimetidine vs. dexamethasone vs. sodium chloride 0.9% (18 rats), and furosemide vs. mannitol vs. sodium chloride 0.9% (18 rats). Rats were sacrificed at predetermined time points up to 48 h after activity injection. Kidneys, adrenal glands, pancreas, spleen, blood, liver, and muscle were harvested and injected activity per gram tissue was determined. Autoradiographic images of the kidneys were acquired in a total of 24 rats. Results Probenecid led to a reduction in renal uptake by up to 30% while not significantly changing the activity accumulation in the other organs investigated. This reduction was attributable to the renal cortex (ratio cortex/medulla 1.72 vs. 1.99; p = 0.006). Cimetidine and dexamethasone had no effect in any of the organs. Furosemide led to a 44% increase in renal activity accumulation attributable to enhanced renal medullary uptake (ratio cortex/medulla 1.44 versus 1.69; p = 0.006). Mannitol had no effect on renal activity uptake. Conclusion Inhibition of the organic anion transport by probenecid may help reduce renal uptake regarding therapy with radiometal labelled DOTATOC. The enhancing effect of furosemide may be unfavourable for therapy. The results must be confirmed by human studies.