Tissue distribution of octreotide binding receptors in normal mice and strains prone to autoimmunity

Direct comparison of [18F]AlF-NOTA-JR11 and [18F]AlF-NOTA-octreotide for PET imaging of neuroendocrine tumors: Antagonist versus agonist

BACKGROUND

[¹⁸F]AlF-NOTA-octreotide is an ¹⁸F-labeled somatostatin analogue which is a good clinical alternative for ⁶⁸Ga-labeled somatostatin analogues. However, radiolabeled somatostatin receptor (SSTR) antagonists might outperform agonists regarding imaging sensitivity of neuroendocrine tumors (NETs). No direct comparison between the antagonist [¹⁸F]AlF-NOTA-JR11 and the agonist [¹⁸F]AlF-NOTA-octreotide as SSTR PET probes is available. Herein, we present the radiosynthesis of [¹⁸F]AlF-NOTA-JR11 and compare its NETs imaging properties directly with the established agonist radioligand [¹⁸F]AlF-NOTA-octreotide preclinically.

METHODS

[¹⁸F]AlF-NOTA-JR11 was synthesized in an automated synthesis module. The in vitro binding characteristics (IC₅₀) of [^natF]AlF-NOTA-JR11 and [^natF]AlF-NOTA-octreotide were evaluated and the in vitro stability of [¹⁸F]AlF-NOTA-JR11 was determined in human serum. In vitro cell binding and internalization was performed with [¹⁸F]AlF-NOTA-JR11 and [¹⁸F]AlF-NOTA-octreotide using SSTR2 expressing cells and the pharmacokinetics were evaluated using μPET/CT in mice bearing BON1.SSTR2 tumor xenografts.

RESULTS

Excellent binding affinity for SSTR2 was found for [^natF]AlF-NOTA-octreotide (IC₅₀ of 25.7 ± 7.9 nM). However, the IC₅₀ value for [^natF]AlF-NOTA-JR11 (290.6 ± 71 nM) was 11-fold higher compared to [^natF]AlF-NOTA-octreotide, indicating lower affinity for SSTR2. [¹⁸F]AlF-NOTA-JR11 was obtained in a good RCY (50 ± 6 %) but with moderate RCP of 94 ± 1 %. [¹⁸F]AlF-NOTA-JR11 demonstrated excellent stability in human serum (>95 % after 240 min). 2.7-fold higher cell binding was observed for [¹⁸F]AlF-NOTA-JR11 as compared to [¹⁸F]AlF-NOTA-octreotide after 60 min. μPET/CT images demonstrated comparable pharmacokinetics and tumor uptake between [¹⁸F]AlF-NOTA-JR11 (SUV_max: 3.7 ± 0.8) and [¹⁸F]AlF-NOTA-octreotide (SUV_max: 3.6 ± 0.4).

CONCLUSIONS

[¹⁸F]AlF-NOTA-JR11 was obtained in good RCY, albeit with a moderate RCP. The cell binding study showed significant higher binding of [¹⁸F]AlF-NOTA-JR11 compared to [¹⁸F]AlF-NOTA-octreotide, despite the higher IC₅₀ value of AlF-NOTA-JR11. However, pharmacokinetics and in vivo tumor uptake was comparable for both radiotracers. Novel Al¹⁸F-labeled derivatives of JR11 with higher SSTR2 affinity should be developed for increased tumor uptake and NET imaging sensitivity.

Semi-automated system for concentrating 68Ga-eluate to obtain high molar and volume concentration of 68Ga-Radiopharmaca for preclinical applications

INTRODUCTION

⁶⁸Ga-radiopharmaceuticals are common in the field of Nuclear Medicine to visualize receptor-mediated processes. In contrast to straightforward labeling procedures for clinical applications, preclinical in vitro and in vivo applications are hampered for reasons like e.g. volume restriction, activity concentration, molar activity and osmolality. Therefore, we developed a semi-automatic system specifically to overcome these problems. A difficulty appeared unexpectedly, as intrinsic trace metals derived from eluate (Zn, Fe and Cu) are concentrated as well in amounts that influence radiochemical yield and thus lower molar activity.

METHODS

To purify Gallium-68 and to reduce the high elution volume of a ⁶⁸Ga-generator, a NaCl-based method using a column containing PS-H⁺ was implemented in a low volume PEEK system. Influence on reducing osmolality, acidity and the amount of PS-H⁺ resin (15-50 mg) was investigated. [⁶⁸Ga]Ga was desorbed from the PS-H⁺ resin with acidified 2-5 M NaCl (containing 0.05 M of HCl) and ⁶⁸Ga-activity was collected. DOTA-TATE was used as a peptide model. All buffers and additives used for labeling were mixed with Chelex 100 (~1 g/50 mL) for >144 h and eventually filtered using a 0.22 μm filter (Millipore). Quantification of metals was performed after labeling by HPLC (UV).

RESULTS

Gallium-68 activity could be desorbed from PS-H⁺ cation column with 3 M NaCl, and >60% (120-180 MBq) of [⁶⁸Ga]Ga was collected in <0.3 mL. Taking into account the used amount of ⁶⁸Ga-eluate, buffer and other excipients, the overall amount of trace metal per labeling was <1.5 nmol. DOTA-TATE could be labeled with [⁶⁸Ga]Ga with high radiochemical yield, >99% (ITLC), and a radiochemical purity of >95% (HPLC).

CONCLUSION

With the here described concentration system and metal purification technique, a low activity containing ⁶⁸Ga-generator can be used to label DOTA-peptide in preclinical applicable amounts >60 MBq/nmol (40-60 MBq/0.1 mL) and within 20 min.

New SPECT and PET Radiopharmaceuticals for Imaging Inflammatory Diseases: A Meta-analysis of the Last 10 Years

Modern molecular nuclear medicine is rapidly developing in the field of imaging of chronic inflammatory diseases, and many new radiopharmaceuticals have been recently described and tested in animals and man. These can detect early pathophysiological changes before the development of anatomical changes and, often, before clinical onset of symptoms. This field includes new radiopharmaceuticals for SPECT and PET use to define new strategies for imaging immune cells as well as tissue modifications induced by the inflammatory process. In this review, we present the results of a meta-analysis based on radiopharmaceuticals (for SPECT or PET) that are not commercially available and that have been used, at least once, in humans in the last 10 years.

Improved safety and efficacy of 213Bi-DOTATATE-targeted alpha therapy of somatostatin receptor-expressing neuroendocrine tumors in mice pre-treated with L-lysine

Background

Targeted alpha therapy (TAT) offers advantages over current β-emitting conjugates for peptide receptor radionuclide therapy (PRRT) of neuroendocrine tumors. PRRT with ¹⁷⁷Lu-DOTATATE or ⁹⁰Y-DOTATOC has shown dose-limiting nephrotoxicity due to radiopeptide retention in the proximal tubules. Pharmacological protection can reduce renal uptake of radiopeptides, e.g., positively charged amino acids, to saturate in the proximal tubules, thereby enabling higher radioactivity to be safely administered. The aim of this preclinical study was to evaluate the therapeutic effect of ²¹³Bi-DOTATATE with and without renal protection using L-lysine in mice. Tumor uptake and kinetics as a function of injected mass of peptide (range 0.03–3 nmol) were investigated using ¹¹¹In-DOTATATE. These results allowed estimation of the mean radiation absorbed tumor dose for ²¹³Bi-DOTATATE. Pharmacokinetics and dosimetry of ²¹³Bi-DOTATATE was determined in mice, in combination with renal protection. A dose escalation study with ²¹³Bi-DOTATATE was performed to determine the maximum tolerated dose (MTD) with and without pre-administration of l-lysine as for renal protection. Neutrophil gelatinase-associated lipocalin (NGAL) served as renal biomarker to determine kidney injury.

Results

The maximum mean radiation absorbed tumor dose occurred at 0.03 nmol and the minimum at 3 nmol. Similar mean radiation absorbed tumor doses were determined for 0.1 and 0.3 nmol with a mean radiation absorbed dose of approximately 0.5 Gy/MBq ²¹³Bi-DOTATATE. The optimal mass of injected peptide was found to be 0.3 nmol. Tumor uptake was similar for ¹¹¹In-DOTATATE and ²¹³Bi-DOTATATE at 0.3 nmol peptide. Lysine reduced the renal uptake of ²¹³Bi-DOTATATE by 50% with no effect on the tumor uptake. The MTD was <13.0 ± 1.6 MBq in absence of l-lysine and 21.7 ± 1.9 MBq with l-lysine renal protection, both imparting an LD₅₀ mean renal radiation absorbed dose of 20 Gy. A correlation was found between the amount of injected radioactivity and NGAL levels.

Conclusions

The therapeutic potential of ²¹³Bi-DOTATATE was illustrated by significantly decreased tumor burden and improved overall survival. Renal protection with l-lysine immediately prior to TAT with ²¹³Bi-DOTATATE prolonged survival providing substantial evidence for pharmacological nephron blockade to mitigate nephrotoxicity.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-016-0240-5) contains supplementary material, which is available to authorized users.

Targeted entry via somatostatin receptors using a novel modified retrovirus glycoprotein that delivers genes at levels comparable to those of wild-type viral glycoproteins

Here we report a novel viral glycoprotein created by replacing a natural receptor-binding sequence of the ecotropic Moloney murine leukemia virus envelope glycoprotein with the peptide ligand somatostatin. This new chimeric glycoprotein, which has been named the Sst receptor binding site (Sst-RBS), gives targeted transduction based on three criteria: (i) a gain of the use of a new entry receptor not used by any known virus; (ii) targeted entry at levels comparable to gene delivery by wild-type ecotropic Moloney murine leukemia virus and vesicular stomatitis virus (VSV) G glycoproteins; and (iii) a loss of the use of the natural ecotropic virus receptor. Retroviral vectors coated with Sst-RBS gained the ability to bind and transduce human 293 cells expressing somatostatin receptors. Their infection was specific to target somatostatin receptors, since a synthetic somatostatin peptide inhibited infection in a dose-dependent manner and the ability to transduce mouse cells bearing the natural ecotropic receptor was effectively lost. Importantly, vectors coated with the Sst-RBS glycoprotein gave targeted entry of up to 1 × 10(6) transducing U/ml, a level comparable to that seen with infection of vectors coated with the parental wild-type ecotropic Moloney murine leukemia virus glycoprotein through the ecotropic receptor and approaching that of infection of VSV G-coated vectors through the VSV receptor. To our knowledge, this is the first example of a glycoprotein that gives targeted entry of retroviral vectors at levels comparable to the natural capacity of viral envelope glycoproteins.

The role of cortistatin in the human immune system

Cortistatin (CST) is a recently described neuropeptide that shares high homology with somatostatin (somatotropin release-inhibiting factor, SRIF) and binds with high affinity to all somatostatin (sst) receptor subtypes. CST is currently known to have a widespread distribution in many human organs including the immune system. The activities specific to CST may be partially attributable to its binding to the growth hormone secretagogue (GHS)-receptor (GHS-R) and the orphan G-protein-coupled receptor MrgX2. Human immune cells produce CST, whereas macrophage lineage and activated endothelium express sst2, and human lymphocytes express sst3. The human thymus expresses sst1, 2, 3, MrgX2 and almost all immune cells express GHS-R. Moreover, at this very moment promising research with CST in experimental animal models is being performed. On the basis of these promising results, studies aiming to further evaluate the possibilities of CST as a therapeutic agent in human immune-mediated inflammatory diseases are warranted.

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

INTRODUCTION

During 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.

METHODS

After 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.

RESULTS

An 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: [(111)In-DTPA]CCK8<[(111)In-DTPA-Pro(1),Tyr(4)]bombesin approximately [(111)In-DTPA]neurotensin<[(111)In-DTPA]octreotide<<[(111)In-DTPA]MG0. Renal uptake of [(111)In-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 [(111)In-DTPA]octreotide showed a different localization pattern.

CONCLUSIONS

Regarding 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.

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.

Induction of experimental allergic encephalomyelitis in a low-susceptible Albino Oxford rat strain by somatostatin analogue SMS 201-995

OBJECTIVE

The effect of the somatostatin analogue SMS 201-995 (octreotide; OCT) on the course of experimental allergic encephalomyelitis (EAE) in the relatively resistant Albino Oxford (AO) strain of rats was studied.

METHODS

Animals were actively immunized with bovine brain homogenate in complete Freund's adjuvant. OCT was given subcutaneously in the hind legs on days 7, 8 and 9 after immunization, at a dose of 3 x 5 microg/kg/day. Rats in control groups were treated with saline or were left untreated. EAE was scored clinically and immunophenotypically, estimating by flow cytometry the changes in the popliteal lymph nodes (PLN) and spleen and monitoring immunohistologically the brain sections of rats recovered from disease.

RESULTS

In control AO rats, EAE was induced in only 2 of 22 rats (9%). In OCT-treated rats, however, EAE developed in 11 of 20 rats (55%), in comparison with 3 of 17 saline-treated animals (17%) (p <0.05). In PLN of OCT-treated rats during the clinical course of EAE, a decreased proportion of OX8+ cells was seen, followed by increases in OX39+ and W3/25+ cells on days 17 and 26. In spleen, OCT decreased the proportion of OX1+, OX39+ and OX8+ cells (on days 12 and/or 17), and increased the proportion of OX39+ cells on days 26 and 31. In the brain sections of saline-treated rats recovered from EAE, numerous Mac-1+, Mac-3+ and OX8+ cells were found. These cells were, however, absent in OCT-treated rats; instead, several W3/25+ cells were noticed.

CONCLUSIONS

These data imply that OCT increases the susceptibility of AO rats to EAE, interfering with specific and/or nonspecific defense mechanisms operating in both the initial and recovery phase of EAE.

First (18)F-labeled tracer suitable for routine clinical imaging of sst receptor-expressing tumors using positron emission tomography

PURPOSE

Despite excellent radionuclide characteristics, no (18)F-labeled peptides are available for quantitative peptide receptor mapping using positron emission tomography (PET) so far, mainly due to time-consuming multistep radiosyntheses with limited overall yields. A newly developed two-step chemoselective conjugation method allows rapid and high-yield [(18)F]fluorination of peptides via oxime formation and was applied for the synthesis of new (18)F-labeled carbohydrated Tyr(3)-octreotate (TOCA) analogs with optimized pharmacokinetics suitable for clinical routine somatostatin-receptor (sst) imaging.

EXPERIMENTAL DESIGN

(18)F-labeled glucose (Gluc-S-) and cellobiose (Cel-S-) derivatives of aminooxy-functionalized TOCA were synthesized via oxime formation with 4-[(18)F]fluorobenzaldehyde ([(18)F]FBOA-peptides). Both the in vitro internalization profile of Gluc-S-Dpr([(18)F]FBOA)TOCA and Cel-S-Dpr([(18)F]FBOA)TOCA in hsst(2)-expressing Chinese hamster ovary cells (dual tracer protocol) and their biodistribution in AR42J tumor-bearing mice were investigated and compared with two [(18)F]fluoropropionylated ([(18)F]FP) analogs, Gluc-Lys([(18)F]FP)TOCA and Gluc-S-Dpr([(18)F]FP)TOCA.

RESULTS

In contrast to [(18)F]FP-labeling (3 h), chemo-selective [(18)F]FBOA-formation (50 min) afforded the respective radiopeptides in high yields (65-85%). In vitro, Gluc-S-Dpr([(18)F]FBOA)TOCA and Cel-S-Dpr([(18)F]FBOA)-TOCA showed high internalization (139 +/- 2 and 163 +/- 8 of the reference [(125)I]Tyr(3)-octreotide, respectively), which was reflected by high tumor accumulation in vivo [21.8 +/- 1.4 and 24.0 +/- 2.5% of injected dose/g (1 h), respectively]. How-ever, only Cel-S-Dpr([(18)F]FBOA)TOCA and Gluc-S-Dpr([(18)F]FP)TOCA (tumor: 15.1 +/- 1.5% of injected dose/g) with its very low accumulation in all of the nontarget organs showed improved tumor:organ ratios compared with Gluc-Lys([(18)F]FP)TOCA. For Cel-S-Dpr([(18)F]FBOA)TOCA,tumor:organ ratios (1 h) were 42:1, 27:1, 15:1, 3:1, and 208:1 for blood, liver, intestine, kidney, and muscle, respectively.

CONCLUSION

Due to the fast and high-yield chemoselective radiofluorination strategy and to its excellent pharmacokinetics, Cel-S-Dpr([(18)F]FBOA)TOCA represents the first tracer suitable for routine clinical application in PET somatostatin receptor imaging.

Receptor-Mediated Tumor Targeting with Radiopeptides. Part 1. General Concepts and Methods: Applications to Somatostatin Receptor-Expressing Tumors

Radiolabeled peptides have become important tools in nuclear oncology, both as diagnostics and more recently also as therapeutics. They represent a distinct sector of the molecular targeting approach, which in many areas of therapy will implement the old "magic bullet" concept by specifically directing the therapeutic agent to the site of action. In this three-part review, we present a comprehensive overview of the literature on receptor-mediated tumor targeting with the different radiopeptides currently studied. Part I summarizes the general concepts and methods of targeting, the selection of radioisotopes, chelators, and the criteria of peptide ligand development. Then, the >400 studies on the application to somatostatin/somatostatin-release inhibiting factor receptor-mediated tumor localization and treatment will be reviewed, demonstrating that peptide radiopharmaceuticals have gained an important position in clinical medicine.