Renal uptake of radiolabeled octreotide in human subjects is efficiently inhibited by succinylated gelatin.

Peptide receptor-mediated radiotherapy of neuroendocrine and other somatostatin receptor-positive tumors with radiolabeled somatostatin analogs has been applied in several experimental settings. The kidneys are the organs responsible for dose-limiting toxicity attributable to the retention of radiolabeled octreotide in the renal cortex, leading to a relatively high radiation dose that may result in irreversible loss of kidney function. The administration of basic amino acids reduces renal uptake but does have significant side effects. We observed that gelatin-based plasma expanders induced tubular low-molecular-weight proteinuria in healthy volunteers, suggesting that components in these solutions can interfere with the tubular reabsorption of proteins and peptides. Here, we studied the effects of infusion of low doses of the plasma expander succinylated gelatin (GELO) on the renal uptake of 111In-labeled octreotide (111In-OCT). METHODS: Five healthy volunteers were given 111In-OCT, first in combination with normal saline and 2 wk later in combination with GELO. Scintigraphic images of the kidneys as well as blood and urine samples were analyzed. To exclude a nonspecific hemodynamic effect of the plasma expander, the procedure was repeated with 5 other volunteers who received the carbohydrate-based plasma expander hydroxyethyl starch (HES). RESULTS: Low doses of GELO were able to effectively reduce the kidney retention of 111In-OCT. The renal radiation dose was significantly reduced by 45% +/- 10% (mean +/- SD) (P = 0.006), whereas HES showed no significant effect (0% +/- 12%). The infusion of GELO did not cause any side effects. CONCLUSION: GELO effectively reduces the renal uptake of 111In-OCT. In contrast to currently used mixtures of amino acids, GELO does not cause any side effects.     

Oral versus intravenous administration of lysine: equal effectiveness in reduction of renal uptake of [111In-DTPA]octreotide.

During tumor therapy with radiolabeled somatostatin analogs, the kidneys are dose limiting. Renal uptake in patients can effectively be reduced by a 4- to 10-h intravenous infusion of a lysine/arginine solution, thereby increasing the maximum radiation dose to the tumor without renal side effects. Oral administration of amino acids could facilitate this labor-intensive procedure. Therefore, the effects of oral versus intravenous administration of D-lysine were compared in rats injected with [111In-diethylenetriaminepentaacetic acid (DTPA)]octreotide. METHODS: Rats were intravenously injected with 3 MBq/0.5 microg [111In-DTPA]octreotide and also received D-lysine intravenously or orally in various concentrations and following various time schedules. Twenty-four hours after injection, a biodistribution study and renal ex vivo autoradiography were performed. RESULTS: Renal uptake was reduced significantly-up to 40%-in all lysine-treated groups, without affecting the uptake in other organs. CONCLUSION: Renal uptake of this radiolabeled peptide can be reduced up to 40% both by oral and by intravenous administration of lysine in rats.     

177Lu-DOTAOTyr3]octreotate: comparison with [111In-DTPAo]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.     

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.     

Progressive nuclear translocation of somatostatin analogs.

Optimal cancer radiotherapy using Auger electron emitters requires selective localization of radionuclides in close proximity to tumor DNA. METHODS: Intracellular trafficking of (125)I-Tyr1-somatostatin-14 somatotropin-release inhibiting factor (SRIF) and 2 of its analogs, (125)I-WOC 4a and (111)In-pentetreotide, was studied in human neuroblastoma cells. RESULTS: After 24-h incubation, SRIF was degraded or recycled, whereas its protease-resistant analogs progressively accumulated in nuclear fractions. (111)In-pentetreotide binding to DNA increased over time in somatostatin receptor-positive cells but not in somatostatin receptor-negative cells. CONCLUSION: These in vitro studies show that prolonged exposure to radiolabeled SRIF analogs significantly increases their cellular internalization, nuclear translocation, and DNA binding. Clinically, infusion of radiolabeled somatostatin analogs may enhance tumor uptake and retention and provide more effective in situ radiotherapy.     

Uptake of [111In-DTPA0]octreotide in the rat kidney is inhibited by colchicine and not by fructose.

The high renal uptake of radiolabeled somatostatin analogs is dose limiting. Lowering this uptake permits higher radioactivity doses and, thus, tumor doses to be administered. We tested the effects of the microtubule drug colchicine on renal uptake of [(111)In-DTPA(0)]octreotide. Also, the effects of fructose were tested. METHODS: Organ radioactivity 24 h after injection of [(111)In-DTPA(0)]octreotide was determined in rats. RESULTS: Coinjection of 1 mg of colchicine per kilogram did not influence renal uptake of [(111)In-DTPA(0)]octreotide, whereas this dose administered 5 h before [(111)In-DTPA(0)]octreotide resulted in significant renal uptake reduction (63%). D-Lysine plus colchicine reduced the uptake by 76% (P < 0.01 vs. D-lysine alone). Liver and blood radioactivity levels were significantly elevated by colchicine. Fructose did not affect the biodistribution of [(111)In-DTPA(0)]octreotide. CONCLUSION: Renal uptake of [(111)In-DTPA(0)]octreotide is dependent on microtubule function in rats. The addition of colchicine to amino acid protocols may permit administration of higher doses, improving the therapeutic window of peptide receptor radionuclide therapy.     

Albumin-derived peptides efficiently reduce renal uptake of radiolabelled peptides

Purpose

In peptide-receptor radionuclide therapy (PRRT), the maximum activity dose that can safely be administered is limited by high renal uptake and retention of radiolabelled peptides. The kidney radiation dose can be reduced by coinfusion of agents that competitively inhibit the reabsorption of radiolabelled peptides, such as positively charged amino acids, Gelofusine, or trypsinised albumin. The aim of this study was to identify more specific and potent inhibitors of the kidney reabsorption of radiolabelled peptides, based on albumin.

Methods

Albumin was fragmented using cyanogen bromide and six albumin-derived peptides with different numbers of electric charges were selected and synthesised. The effect of albumin fragments (FRALB-C) and selected albumin-derived peptides on the internalisation of ¹¹¹In-albumin, ¹¹¹In-minigastrin, ¹¹¹In-exendin and ¹¹¹In-octreotide by megalin-expressing cells was assessed. In rats, the effect of Gelofusine and albumin-derived peptides on the renal uptake and biodistribution of ¹¹¹In-minigastrin, ¹¹¹In-exendin and ¹¹¹In-octreotide was determined.

Results

FRALB-C significantly reduced the uptake of all radiolabelled peptides in vitro. The albumin-derived peptides showed different potencies in reducing the uptake of ¹¹¹In-albumin, ¹¹¹In-exendin and ¹¹¹In-minigastrin in vitro. The most efficient albumin-derived peptide (peptide #6), was selected for in vivo testing. In rats, 5 mg of peptide #6 very efficiently inhibited the renal uptake of ¹¹¹In-minigastrin, by 88%. Uptake of ¹¹¹In-exendin and ¹¹¹In-octreotide was reduced by 26 and 33%, respectively.

Conclusions

The albumin-derived peptide #6 efficiently inhibited the renal reabsorption of ¹¹¹In-minigastrin, ¹¹¹In-exendin and ¹¹¹In-octreotide and is a promising candidate for kidney protection in PRRT.     

In vitro and in vivo evaluation of 64Cu-TETA-Tyr3-octreotate. A new somatostatin analog with improved target tissue uptake.

Radiolabeled somatostatin analogs have demonstrated potential as cancer therapeutic agents. Many of these agents are based on the analog octreotide (OC). Recently it has been shown that substitution of a tyrosine for phenylalanine in the 3-position and changing the C-terminus from an alcohol to an acid improves the targeting of somatostatin-rich tissues. The compound, 1,4,8,11-tetraazacyclotetradecane-N,N',N",N"'-tetraacetic acid-Tyr3-octreotate (TETA-Y3-TATE), was synthesized and radiolabeled with 64Cu. The receptor binding properties of 64Cu-TETA-Y3-TATE showed an estimated Kd value of 549 pM in somatostatin receptor-positive CA20948 tissue membrane. High tumor uptake was observed in two animal tumor models. Tumor uptakes of 2.37 %ID/g in CA20948 tumor-bearing rats and 21.60 %ID/g in AR42J tumor-bearing SCID mice were observed at 1 h, compared with 1.09 %ID/g and 11.24 %ID/g for 64Cu-TETA-OC. Higher uptake in other somatostatin-receptor rich tissues was also observed, compared with 64Cu-TETA-OC. Positron emission tomography (PET) imaging with 64Cu-TETA-Y3-TATE in a baboon showed significant uptake in the pituitary and adrenals, and clearance through the kidneys. 64Cu-TETA-Y3-TATE, a new OC analog for binding somatostatin receptors, demonstrated significantly greater uptake in somatostatin-rich tissues in two tumor-bearing animal models, and demonstrated great potential as a radiopharmaceutical for imaging and therapy of somatostatin receptor-positive tissues.     

Dose-response effect of Gelofusine on renal uptake and retention of radiolabelled octreotate in rats with CA20948 tumours

Purpose  

Peptide receptor radionuclide therapy using β-emitting radiolabelled somatostatin analogues like DOTA,Tyr³-octreotate shows beneficial results in patients suffering from somatostatin receptor overexpressing tumours. However, after high-dose therapy partial renal reabsorption of radiopeptides may lead to nephrotoxicity. Co-infusion of lysine/arginine lowers renal retention of these radiopeptides without affecting tumour uptake. Recently co-administration of Gelofusine has been described to have a comparable kidney-protecting effect in rats. In the present study optimal dosing of Gelofusine co-administration was studied in tumour-bearing rats.     

Indication for different mechanisms of kidney uptake of radiolabeled peptides.

Nephrotoxicity due to renal reabsorption of radiolabeled peptides limits the tumor dose in peptide receptor radiotherapy (PRRT). Therefore, we evaluated the ability of several agents to inhibit the renal accumulation of different radiopeptides. METHODS: Male Wistar rats (4 per group) were injected intravenously with 1 MBq of (111)In-labeled octreotide (OCT), minigastrin (MG), bombesin (BOM), or exendin (EX), together with a potential inhibitor of renal uptake (lysine [Lys], poly-glutamic acid [PGA], and Gelofusine [GF], a gelatin-based plasma expander) or phosphate-buffered saline as a control. Organ uptake at 20 h after injection was determined as the percentage of injected activity per gram (%IA/g). Lys, PGA, and GF were also combined to determine whether an additive effect could be obtained. The localization of the peptides in the kidneys was investigated by autoradiography using a phosphor imager. RESULTS: OCT accumulation in the kidney was inhibited by Lys and GF (40.7%-45.1%), whereas PGA was ineffective. On the other hand, renal uptake of BOM, MG, and EX was inhibited by PGA and GF (15.4%-85.4%), whereas Lys was ineffective. The combination of GF and Lys showed additive effects in inhibiting OCT uptake, whereas PGA and GF had additive effects for the inhibition of EX uptake. The amount of kidney uptake correlated with the number of charged amino acids. All radiopeptides were localized in the renal cortex, as indicated by autoradiography. CONCLUSION: Inhibition of renal accumulation of the radiopeptides tested could be achieved by either Lys or PGA but not by both at the same time, suggesting 2 different uptake mechanisms. The differences in renal accumulation of radiopeptides may be related to the number of charges of a molecule. GF is the only compound that inhibited renal accumulation of all radiopeptides tested. Additional experiments are needed to further elucidate these findings and to optimize inhibition of renal accumulation of radiopeptides to reduce the kidney dose in PRRT.     

Renal uptake and retention of radiolabeled somatostatin, bombesin, neurotensin, minigastrin and CCK analogues: species and gender differences

IntroductionDuring therapy with radiolabeled peptides, the kidney is most often the critical organ. Newly developed peptides are evaluated preclinically in different animal models before their application in humans. In this study, the renal retention of several radiolabeled peptides was compared in male and female rats and mice.MethodsAfter intravenous injection of radiolabeled peptides [somatostatin, cholecystokinin (CCK), minigastrin, bombesin and neurotensin analogues], renal uptake was determined in both male and female Lewis rats and C57Bl mice. In addition, ex vivo autoradiography of renal sections was performed to localize accumulated radioactivity.ResultsAn equal distribution pattern of renal radioactivity was found for all peptides: high accumulation in the cortex, lower accumulation in the outer medulla and no radioactivity in the inner medulla of the kidneys.In both male rats and mice, an increasing renal uptake was found: [¹¹¹In-DTPA]CCK8<[¹¹¹In-DTPA-Pro¹,Tyr⁴]bombesin≈[¹¹¹In-DTPA]neurotensin<[¹¹¹In-DTPA]octreotide≪[¹¹¹In-DTPA]MG0. Renal uptake of [¹¹¹In-DTPA]octreotide in rats showed no gender difference, and renal radioactivity was about constant over time. In mice, however, renal uptake in females was significantly higher than that in males and decreased rapidly over time in both genders. Moreover, renal radioactivity in female mice injected with [¹¹¹In-DTPA]octreotide showed a different localization pattern.ConclusionsRegarding the renal uptake of different radiolabeled peptides, both species showed the same ranking order. Similar to findings in patients, rats showed comparable and constant renal retention of radioactivity in both genders, in contrast to mice. Therefore, rats appear to be the more favorable species for the study of the renal retention of radioactivity.     

Megalin is essential for renal proximal tubule reabsorption of (111)In-DTPA-octreotide.

Radiolabeled somatostatin analogs have been shown to be important radiopharmaceuticals for tumor diagnosis and radionuclide therapy. The kidney has appeared to be the critical organ during radionuclide therapy because of peptide reabsorption and retention in the proximal tubules after glomerular filtration. The molecular mechanism of renal reabsorption of these analogs has not been clarified. A possible receptor candidate is megalin, a multiligand scavenger receptor in the renal proximal tubules. The objective of this study was to investigate the role of megalin in tubular reabsorption of radiolabeled somatostatin analogs by using kidney-specific megalin-deficient mice versus mice with normal renal megalin expression. [(111)In-Diethylenetriaminepentaacetic acid (DTPA)]octreotide was used as a practical model of peptide. METHODS: Renal uptake of [(111)In-DTPA]octreotide was determined by animal SPECT scintigraphy at different time points after injection of the tracer and by measurement of radioactivity after isolation of the organs. Furthermore, ex vivo autoradiography of renal sections revealed the zonal distribution of radioactivity in the megalin-deficient and megalin-expressing kidneys. RESULTS: SPECT scintigraphy of [(111)In-DTPA]octreotide at 3 and 24 h after injection clearly showed lower renal radioactivity in megalin-deficient kidneys than in megalin-expressing kidneys, both in male and in female mice, in accordance with counts obtained after isolation of the organ (70%-85% reduction of uptake in the megalin-deficient kidneys, P < 0.001). Renal uptake of [(111)In-DTPA]octreotide was significantly higher in female than in male kidneys (P < 0.001). Ex vivo autoradiograms clearly showed that renal radioactivity was not homogeneously distributed in the megalin-expressing kidneys but localized in the renal cortex. Quantification of the autoradiogram data confirmed the reduced radioactivity in the renal cortex of megalin-deficient kidneys. CONCLUSION: This study revealed the molecular mechanism of [(111)In-DTPA]octreotide uptake in renal proximal tubules involving the receptor megalin. Identification of megalin may be crucial for further research into strategies to reduce renal uptake.     

Increased cell death after therapy with an Arg-Gly-Asp-linked somatostatin analog.

Receptor-targeted scintigraphy and radionuclide therapy with radiolabeled somatostatin analogs are successfully applied for somatostatin receptor-positive tumors. The synergistic effects of an apoptosis-inducing factor, for example, the Arg-Gly-Asp (RGD) motif, can increase the radiotherapeutic efficacy of these peptides. Hence, the tumoricidal effects of the hybrid peptide RGD-diethylaminetriaminepentaacetic acid (DTPA)-Tyr3-octreotate (cyclic[c](Arg-Gly-Asp-D-Tyr-Asp)-Lys(DTPA)-D-Phe-c(Cys-Tyr-D-Trp-Lys-Thr-Cys)-Thr), hereafter referred to as RGD-DTPA-octreotate, were evaluated in comparison with those of RGD (c(Arg-Gly-Asp-D-Tyr-Asp)) and Tyr3-octreotate (D-Phe-c(Cys-Tyr-D-Trp-Lys-Thr-Cys)-Thr). METHODS: The therapeutic effects of RGD-111In-DTPA-octreotate, 111In-DTPA-RGD, and 111In-DTPA-Tyr3-octreotate were investigated with various cell lines by use of a colony-forming assay, and caspase-3 activity was also determined. RESULTS: Tumoricidal effects were found with 111In-DTPA-RGD, 111In-DTPA-Tyr3-octreotate, and RGD-111In-DTPA-octreotate, in order from least effective to most effective. Also, the largest increase in caspase-3 levels was found with RGD-111In-DTPA-octreotate. CONCLUSION: RGD-111In-DTPA-octreotate has more pronounced tumoricidal effects than 111In-DTPA-RGD and 111In-DTPA-Tyr3-octreotate, because of increased apoptosis, as indicated by increased caspase-3 activity.     

[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.     

Pretargeted radioimmunotherapy with a single-chain antibody/streptavidin construct and radiolabeled DOTA-biotin: strategies for reduction of the renal dose.

Multistep immune targeting holds great promise for radioimmunodiagnosis and therapy of cancer. Pretargeting of the tetrameric single-chain, variable-fragment streptavidin construct of the tetrameric monoclonal antibody CC49 with subsequent administration of radiolabeled 1,4,7,10-tetraazacyclododecane-N,N',N',N'-tetraacetic acid (DOTA)-biotin has yielded promising results in TAG-72-expressing tumor xenograft models. A potential limitation of this approach, however, has been high and prolonged renal uptake of radioactivity. The objective of the current study, therefore, was to evaluate the reduction of kidney uptake of radiolabeled DOTA-biotin achieved by each of 4 different methods. METHODS: A human pancreatic adenocarcinoma xenograft model (HPAC) in nude mice was used. The animals were intravenously injected with the antibody-streptavidin construct and a synthetic clearing agent (biotinylated N-acetyl-galactosamine) 24 and 4 h, respectively, before the administration of (67)Ga-DOTA-biotin. For reduction of the renal uptake, different groups of mice were treated with streptavidin saturated with biotin, with several administrations of lysine or colchicine or with a succinylated antibody-streptavidin construct (resulting in a decreased electrical charge). All animals were sacrificed 24 h after injection of the (67)Ga-DOTA-biotin for biodistribution and quantitative autoradiography (QAR) studies and selected animals underwent microSPECT/microCT studies. RESULTS: There was marked targeting of the radiolabeled DOTA-biotin to tumor in all groups except in negative-control animals. Only succinylation of the scFv-CC49-streptavidin fusion protein significantly reduced ( approximately 30%) kidney uptake without affecting tumor activity. QAR corroborated these results and demonstrated that radiolabeled DOTA-biotin localized selectively in the renal cortex. Among the other experimental groups, there was no change in kidney uptake of the radiolabeled biotin. CONCLUSION: In contrast to directly labeled antibodies and antibody fragments, administration of the negatively charged amino acid lysine was largely ineffective in pretargeting strategies with a single-chain-immuno-streptavidin fusion protein. Succinylation of the scFv-CC49-streptavidin construct, on the other hand, reduces kidney uptake of subsequently administered radiolabeled biotin, presumably by inhibiting reuptake of the fusion protein in the proximal renal tubules, and, therefore, could significantly reduce renal doses and improve therapeutic indices associated with multistep immune targeting approaches to radioimmunotherapy.     

Radiotherapeutic efficacy of (153)Sm-CMDTPA-Tyr(3)-octreotate in tumor-bearing rats

A number of radiolabeled somatostatin analogs have been evaluated in animal tumor models for radiotherapeutic efficacy. The majority of the agents tested have used either high-energy beta-emitters, such as Y-90 or Re-188, or the Auger electron-emitting radionuclide, In-111. Because a medium-energy beta-emitter might have equivalent efficacy compared to high-energy emitters, and lower toxicity to non-target tissues, we have evaluated the therapeutic potential of the beta-emitting nuclide, Sm-153, chelated to the somatostatin analog, CMDTPA-Tyr(3)-octreotate. Using an in vitro binding assay, this octreotate derivative was shown to have high affinity for the somatostatin subtype-2 receptor (IC(50) = 2.7 nM). Biodistribution studies in CA20948 tumor-bearing Lewis rats demonstrate that the Sm-153 labeled compound has high uptake and retention in tumor tissue (1.7% injected dose/g tissue, 4 hrs post injection) and has rapid overall clearance properties from non-target tissue. Radiotherapy studies were carried out using (153)Sm-CMDTPA-Tyr(3)-octreotate and CA20948 tumor bearing Lewis rats at 7 days post implant. Dose regimens consisting of single and multiple i.v. injections of 5.0 mCi/rat (185 MBq) were employed over a time span of 7 days. Suppression of tumor growth rate was observed in all treated animals compared to untreated controls. Greater inhibition of tumor growth was observed in animals that received multiple doses. These studies indicate that medium-energy beta-emitting isotopes have considerable potential for the treatment of somatostatin receptor-positive tumors.     

Peptide receptor imaging and therapy.

This article reviews the results of somatostatin receptor imaging (SRI) in patients with somatostatin receptor-positive neuroendocrine tumors, such as pituitary tumors, endocrine pancreatic tumors, carcinoids, gastrinomas, and paragangliomas, or other diseases in which somatostatin receptors may also be expressed, like sarcoidosis and autoimmune diseases. [(111)In-DTPA0]octreotide is a radiopharmaceutical that has great potential for helping visualize whether somatostatin receptor-positive tumors have recurred. The overall sensitivity of SRI to localize neuroendocrine tumors is high. In several neuroendocrine tumor types, inclusion of SRI in the localization or staging procedure may be very rewarding in terms of cost effectiveness, patient management, or quality of life. The value of SRI in patients with other tumors, such as breast cancer or malignant lymphomas, or in patients with granulomatous diseases has to be established. The application of radiolabeled peptides may be clinically useful in another way: after the injection of [(111)In-DTPA0]octreotide, surgeons can detect tumor localizations by a probe that is used during the operation. This may be of particular value if small tumors with a high receptor density are present (e.g., gastrinomas). As the success of peptide receptor scintigraphy for tumor visualization became clear, the next logical step was to try to label these peptides with radionuclides emitting alpha or beta particles, or Auger or conversion electrons, and to perform radiotherapy with these radiolabeled peptides. The results of the described studies with 90Y- and (111)In-labeled octreotide show that peptide receptor radionuclide therapy using radionuclides with appropriate particle ranges may become a new treatment modality. One might consider the use of radiolabeled somatostatin analogs first in an adjuvant setting after surgery of somatostatin receptor-positive tumors to eradicate occult metastases and second for cancer treatment at a later stage.     

Somatostatin receptor-targeted radionuclide therapy of tumors: preclinical and clinical findings

In preclinical studies in rats we evaluated biodistribution and therapeutic effects of different somatostatin analogs, [(111)In-DTPA]octreotide, [(90)Y-DOTA,Tyr(3)]octreotide and [(177)Lu-DOTA,Tyr(3)]octreotate, currently also being applied in clinical radionuclide therapy studies. [Tyr(3)]octreotide and [Tyr(3)]octreotate, chelated with DTPA or DOTA, both showed high affinity binding to somatostatin receptor subtype 2 (sst(2)) in vitro. The radiolabelled compounds all showed high tumor uptake in sst(2)-positive tumors in vivo in rats, the highest uptake being reached with [(177)Lu-DOTA,Tyr(3)]octreotate. In preclinical therapy studies in vivo in rats, excellent, dose dependent, tumor size responses were found, responses appeared to be dependent on tumor size at therapy start. These preclinical data showed the great promise of radionuclide therapy with radiolabelled somatostatin analogues. They emphasised the concept that especially the combination of somatostatin analogs radiolabeled with different radionuclides, like (90)Y and (177)Lu, is most promising to reach a wider tumor size region of high curability. Furthermore, different phase I clinical studies, using [(111)In-DTPA]octreotide, [(90)Y-DOTA,Tyr(3)]octreotide or [(177)Lu-DOTA, Tyr(3)]octreotate are described. Fifty patients with somatostatin receptor-positive tumors were treated with multiple doses of [(111)In-DTPA(0)]octreotide. Forty patients were evaluable after cumulative doses of at least 20 GBq up to 160 GBq. Therapeutic effects were seen in 21 patients: partial remission in 1 patient, minor remissions in 6 patients, and stabilization of previously progressive tumors in 14 patients. The toxicity was generally mild bone marrow toxicity, but 3 of the 6 patients who received more than 100 GBq developed a myelodysplastic syndrome or leukemia. Radionuclide therapy with [(90)Y-DOTA,Tyr(3)]octreotide started in 3 different phase I trials. Overall, antimitotic effects have been observed: about 20% partial response and 60% stable disease (N = 92) along with complete symptomatic cure of several malignant insulinoma and gastrinoma patients. Maximum cumulative [(90)Y-DOTA,Tyr(3)]octreotide dose was about 26 GBq, without reaching the maximum tolerable dose. New is the use of [(177)Lu-DOTA,Tyr(3)]octreotate, which shows the highest tumor uptake of all tested octreotide analogs so far, with excellent tumor-to-kidney ratios. Radionuclide therapy with this analog in a phase 1 trial started recently in our center in 63 patients (238 administrations), Interim analysis of 18 patients with neuroendocrine tumors was performed very recently. According to the WHO, toxicity criteria no dose limiting toxicity was observed. Minor CT-assessed tumor shrinkage (25% - 50% reduction) was noticed in 6% of 18 patients and partial remission (50% - 100% reduction, SWOG criteria) in 39%. Eleven percent of patients had tumor progression and in 44% no changes were seen. These data show that radionuclide therapy with radiolabelled somatostatin analogs, like [DOTA, Tyr(3)]octreotide and [DOTA, Tyr(3)octreotate is a most promising new treatment modality for patients who have sst(2)-positive tumors.     

New trends in peptide receptor radioligands.

The high level expression of somatostatin receptors (SSTR) on various tumor cells has provided the molecular basis for successful use of radiolabeled octreotide/lanreotide analogs as tumor tracers in nuclear medicine. The vast majority of human tumors seem to overexpress the one or the other of five distinct hSSTR sub-type receptors. Whereas neuroendocrine tumors frequently overexpress hSSTR2, intestinal adenocarcinomas seem to over-express more often hSSTR3 or hSSTR4, or both of these hSSTR. In contrast to 111In-DTPA-DPhe1-octreotide (OCTREOSCAN) which binds to hSSTR2 and 5 with high affinity (Kd 0.1-5 nM), to hSSTR3 with moderate affinity (Kd 10-100 nM) and does not bind to hSSTR1 and hSSTR4, 111In/90Y-DOTA-lanreotide was found to bind to hSSTR2, 3, 4, and 5 with high affinity, and to hSSTR1 with lower affinity (Kd 200 nM). Based on its unique hSSTR binding profile, 111In-DOTA-lanreotide was suggested to be a potential radioligand for tumor diagnosis, and 90Y-DOTA-lanreotide suitable for receptor-mediated radionuclide therapy. As opposed to 111In-DTPA-DPhe1-octreotide and 111In-DOTA-DPhe1-Tyr3-octreotide, discrepancies in the scintigraphic results were seen in about one third of (neuroendocrine) tumor patients concerning both the tumor uptake as well as detection of tumor lesions. On a molecular level, these discrepancies seem to be based on a "higher" high-affinity binding of 111In-DOTA-DPhe1-Tyr3-octreotide to hSSTR2. Other somatostatin analogs with divergent affinity to the five known hSSTR subtype receptors have also found their way into the clinics, including 99mTc-HYNIC-octreotide or 99mTc-depreotide (NEOSPECT; NEOTECT). Most of the imaging results are reported for neuroendocrine tumors (octreotide analogs) or non-small cell lung cancer (99mTc-depreotide), indicating high diagnostic capability of this type of receptor tracers. Consequently to their use as receptor imaging agents, hSSTR recognizing radioligands have also been implemented for experimental receptor-targeted radionuclide therapy. The study "MAURITIUS" (MulticenterAnalysis of a Universal Receptor Imaging and Treatment Initiative, a eUropean Study), a Phase IIa study, showed in patients with a calculated tumor dose >10 Gy/GBq 90Y-DOTA-lanreotide, the proof-of-principle for treating tumor patients with receptor imaging agents. Overall treatment results in >60 patients indicated stable tumor disease in roughly 35% of patients and regressive disease in 15% of tumor patients with different tumor entities. No acute or chronic severe hematological toxicity, change in renal or liver function parameters due to 90Y-DOTA-lanreotide, was reported. 90In-DOTA-DPhe1-Tyr3-octreotide may show a higher tumor uptake in neuroendocrine tumor lesions and may therefore provide even better treatment results in tumor patients, but there is only limited excess to long-term and survival data at present. Besides newer approaches and recent developments of 188Re-labeled radioligands no clinical results on the treatment response is available yet. In conclusion, several radioligands have been implemented on the basis of peptide receptor recognition throughout the last decade. A plentitude of preclinical data and clinical studies confirm "proof-of-principle" for their use in diagnosis as well as therapy of cancer patients. However, an optimal radiopeptide formulation does not yet exist for receptor-targeted radionuclide therapy.     

Somatostatin receptor imaging in intracranial tumours

The somatostatin analogue [¹¹¹In-DTPA-d-Phe¹]-octreotide (¹¹¹In-octreotide) allows scintigraphic visualization of somatostatin receptor-expressing tissue. While it is well known that a large variety of tissues express somatostatin receptors and ¹¹¹In-octreotide scintigraphy has a clearly defined role in various neuroendocrine diseases, the clinical value of ¹¹¹In-octreotide scintigraphy in brain tumours is still under clinical investigation. In 124 patients with 141 brain lesions (63 meningiomas, 24 pituitary adenomas, 10 gliomas WHO class I and II, 12 gliomas WHO class III and IV, 11 neurinomas and 2 neurofibromas, 7 metastases and 12 other varieties: three non-Hodgkin B-cell lymphomas, two epidermoids, one abscess, one angioleiomyoma, one chordoma, one haemangiopericytoma, one osteosarcoma, one plasmacytoma and one pseudocyst), ¹¹¹In-octreotide scintigraphy was performed 4–6 and 24 h after i.v. injection of 110–220 MBq ¹¹¹In-octreotide. Planar images of the head in four views with a 128×128 matrix and single-photon emission tomographic images (64×64 matrix) were acquired, and lesions were graded according to qualitative tracer uptake. Fifty-nine of the 63 meningiomas showed moderate to intense tracer uptake. Nine of 24 pituitary adenomas were visible; the remaining 15 did not show any tracer uptake. None of the class I and II gliomas with an intact blood-brain barrier were detected whereas 11/12 class III and IV gliomas showed ¹¹¹In-octreotide uptake. None of the neurinomas or neurofibromas were positive. Five of seven metastases were classified as positive, as were the osteosarcoma, two of three non-Hodgkin B-cell lymphomas, one abscess, one angioleiomyoma, one chordoma and one haemangiopericytoma. The other varieties (one non-Hodgkin B-cell lymphoma, two epidermoids, one plasmacytoma and one pseudocyst) did not show ¹¹¹In-octreotide uptake. The results demonstrate that a large variety of intracranial lesions express somatostatin receptors and therefore can be visualized by [¹¹¹In-DTPA-d-Phe¹]-octreotide scintigraphy. This technique can be valuable in the differentiation between meningiomas and pituitary adenomas, based on qualitative tracer uptake. [¹¹¹In-DTPA-d-Phe¹]-octreotide scintigraphy allows differentiation between meningiomas and neurinomas or neurofibromas and therefore provides complementary information to computed tomography or magnetic resonance imaging. Furthermore, this technique allows differentiation between scar tissue and recurrent meningiomas postoperatively and can help in non-invasive tumour differentiation of multiple intracranial lesions, which can be of value in defining the most adequate therapeutic strategy. Received 1 December 1997 and in revised form 17 March 1998