Highly Efficient In Vivo Agonist-Induced Internalization of sst2 Receptors in Somatostatin Target Tissues

Altered biodistribution of [68Ga]Ga-DOTA-TOC during somatostatin analogue treatment

PURPOSE

The need for an interval between the administration of long-acting Somatostatin Receptor Analogues (SSA) and the [68Ga]Ga-DOTA-TATE PET has been questioned based on recent literature in the new EANM guidelines. Here an earlier studies showed that SSA injection immediately before SSTR PET had minimal effect on normal organ and tumor uptake (1). However, data are scarce and there are (small) differences between [68Ga]Ga-DOTA-TATE and [68Ga]Ga-DOTA-TOC binding affinity, and it remains unknown whether these findings can be directly translated to scans with [68Ga]Ga-DOTA-TOC as well. The purpose of this study was to assess the effect of SSA use on the biodistribution in a subsequent [68Ga]Ga-DOTA-TOC PET/CT and compare this intra-individually across several cycles of SSA treatments.

METHODS

Retrospectively, 35 patients with NENs were included. [68Ga]Ga-DOTA-TOC PET at staging and after the 1st and 2nd cycle of SSA were included. SUVmean and SUVmax of blood, visceral organs, primary tumor and two metastases were determined. Also, the interval between SSA therapy and the PET scan was registered.

RESULTS

Treatment with SSA resulted in a significantly higher bloodpool activity and lower visceral tracer uptake. This effect was maintained after a 2nd cycle of SSA therapy. Furthermore, there was an inverse relationship between bloodpool tracer availability and visceral tracer binding and a positive correlation between bloodpool tracer availability and primary tumor tracer uptake. With an interval of up to 5 days, there was a significantly higher bloodpool activity than at longer intervals.

CONCLUSION

Absolute comparison of the SUV on [68Ga]Ga-DOTA-TOC PET should be done with caution as the altered biodistribution of the tracer after SSA treatment should be taken into account. We recommend not to perform a scan within the first 5 days after the injection of lanreotide.

Comparison of 177Lu-octreotate and 177Lu-octreotide for treatment in human neuroblastoma-bearing mice

Background

Patients with high-risk neuroblastoma (NB) have a 5-year event-free survival of less than 50 %, and novel and improved treatment options are needed. Radiolabeled somatostatin analogs (SSTAs) could be a treatment option. The aims of this work were to compare the biodistribution and the therapeutic effects of 177Lu-octreotate and 177Lu-octreotide in mice bearing the human CLB-BAR NB cell line, and to evaluate their regulatory effects on apoptosis-related genes.

Methods

The biodistribution of 177Lu-octreotide in mice bearing CLB-BAR tumors was studied at 1, 24, and 168 h after administration, and the absorbed dose was estimated to tumor and normal tissues. Further, animals were administered different amounts of 177Lu-octreotate or 177Lu-octreotide. Tumor volume was measured over time and compared to a control group given saline. RNA was extracted from tumors, and the expression of 84 selected genes involved in apoptosis was quantified with qPCR.

Results

The activity concentration was generally lower in most tissues for 177Lu-octreotide compared to 177Lu-octreotate. Mean absorbed dose per administered activity to tumor after injection of 1.5 MBq and 15 MBq was 0.74 and 0.03 Gy/MBq for 177Lu-octreotide and 2.9 and 0.45 Gy/MBq for 177Lu-octreotate, respectively. 177Lu-octreotide treatment resulted in statistically significant differences compared to controls. Fractionated administration led to a higher survival fraction than after a single administration. The pro-apoptotic genes TNSFS8, TNSFS10, and TRADD were regulated after administration with 177Lu-octreotate. Treatment with 177Lu-octreotide yielded regulation of the pro-apoptotic genes CASP5 and TRADD, and of the anti-apoptotic gene IL10 as well as the apoptosis-related gene TNF.

Conclusion

177Lu-octreotide gave somewhat better anti-tumor effects than 177Lu-octreotate. The similar effect observed in the treated groups with 177Lu-octreotate suggests saturation of the somatostatin receptors. Pronounced anti-tumor effects following fractionated administration merited receptor saturation as an explanation. The gene expression analyses suggest apoptosis activation through the extrinsic pathway for both radiopharmaceuticals.

Intra-arterial peptide-receptor radionuclide therapy for neuro-endocrine tumour liver metastases: an in-patient randomised controlled trial (LUTIA)

PURPOSE

Peptide receptor radionuclide therapy (PRRT) using [177Lu]Lu-DOTATATE has been shown to effectively prolong progression free survival in grade 1-2 gastroenteropancreatic neuroendocrine tumours (GEP-NET), but is less efficacious in patients with extensive liver metastases. The aim was to investigate whether tumour uptake in liver metastases can be enhanced by intra-arterial administration of [177Lu]Lu-DOTATATE into the hepatic artery, in order to improve tumour response without increasing toxicity.

METHODS

Twenty-seven patients with grade 1-2 GEP-NET, and bi-lobar liver metastases were randomized to receive intra-arterial PRRT in the left or right liver lobe for four consecutive cycles. The contralateral liver lobe and extrahepatic disease were treated via a "second-pass" effect and the contralateral lobe was used as the control lobe. Up to three metastases (> 3 cm) per liver lobe were identified as target lesions at baseline on contrast-enhanced CT. The primary endpoint was the tumour-to-non-tumour (T/N) uptake ratio on the 24 h post-treatment [177Lu]Lu-SPECT/CT after the first cycle. This was calculated for each target lesion in both lobes using the mean uptake. T/N ratios in both lobes were compared using paired-samples t-test.

FINDINGS

After the first cycle, a non-significant difference in T/N uptake ratio was observed: T/NIA = 17·4 vs. T/Ncontrol = 16·2 (p = 0·299). The mean increase in T/N was 17% (1·17; 95% CI [1·00; 1·37]). Of all patients, 67% (18/27) showed any increase in T/N ratio after the first cycle.

CONCLUSION

Intra-arterial [177Lu]Lu-DOTATATE is safe, but does not lead to a clinically significant increase in tumour uptake.

Influence of the Molar Activity of 203/212Pb-PSC-PEG2-TOC on Somatostatin Receptor Type 2-Binding and Cell Uptake

(1) Background: In neuroendocrine tumors (NETs), somatostatin receptor subtype 2 is highly expressed, which can be targeted by a radioactive ligand such as [177Lu]Lu-1,4,7,10-tetraazacyclododecane-N,N',N″,N‴,-tetraacetic acid-[Tyr3,Thr8]-octreotide (177Lu-DOTA-TOC) and, more recently, by a lead specific chelator (PSC) containing 203/212Pb-PSC-PEG2-TOC (PSC-TOC). The molar activity (AM) can play a crucial role in tumor uptake, especially in receptor-mediated uptake, such as in NETs. Therefore, an investigation of the influence of different molar activities of 203/212Pb-PSC-TOC on cell uptake was investigated. (2) Methods: Optimized radiolabeling of 203/212Pb-PSC-TOC was performed with 50 µg of precursor in a NaAc/AcOH buffer at pH 5.3-5.5 within 15-45 min at 95° C. Cell uptake was studied in AR42 J, HEK293 sst2, and ZR75-1 cells. (3) Results: 203/212Pb-PSC-TOC was radiolabeled with high radiochemical purity >95% and high radiochemical yield >95%, with AM ranging from 0.2 to 61.6 MBq/nmol. The cell uptake of 203Pb-PSC-TOC (AM = 38 MBq/nmol) was highest in AR42 J (17.9%), moderate in HEK293 sstr (9.1%) and lowest in ZR75-1 (0.6%). Cell uptake increased with the level of AM. (4) Conclusions: A moderate AM of 15-40 MBq/nmol showed the highest cell uptake. No uptake limitation was found in the first 24-48 h. Further escalation experiments with even higher AM should be performed in the future. It was shown that AM plays an important role because of its direct dependence on the cellular uptake levels, possibly due to less receptor saturation with non-radioactive ligands at higher AM.

Comparative Saturation Binding Analysis of 64Cu-Labeled Somatostatin Analogues Using Cell Homogenates and Intact Cells

The development of novel ligands for G-protein-coupled receptors (GPCRs) typically entails the characterization of their binding affinity, which is often performed with radioligands in a competition or saturation binding assay format. Since GPCRs are transmembrane proteins, receptor samples for binding assays are prepared from tissue sections, cell membranes, cell homogenates, or intact cells. As part of our investigations on modulating the pharmacokinetics of radiolabeled peptides for improved theranostic targeting of neuroendocrine tumors with a high abundance of the somatostatin receptor sub-type 2 (SST₂), we characterized a series of ⁶⁴Cu-labeled [Tyr³]octreotate (TATE) derivatives in vitro in saturation binding assays. Herein, we report on the SST₂ binding parameters measured toward intact mouse pheochromocytoma cells and corresponding cell homogenates and discuss the observed differences taking the physiology of SST₂ and GPCRs in general into account. Furthermore, we point out method-specific advantages and limitations.

Comparison of somatostatin receptor expression in patients with neuroendocrine tumours with and without somatostatin analogue treatment imaged with [18F]SiTATE

Purpose

Somatostatin analogues (SSA) are frequently used in the treatment of neuroendocrine tumours. Recently, [18F]SiTATE entered the field of somatostatin receptor (SSR) positron emission tomography (PET)/computed tomography (CT) imaging. The purpose of this study was to compare the SSR-expression of differentiated gastroentero-pancreatic neuroendocrine tumours (GEP-NET) measured by [18F]SiTATE-PET/CT in patients with and without previous treatment with long-acting SSAs to evaluate if SSA treatment needs to be paused prior to [18F]SiTATE-PET/CT.

Methods

77 patients were examined with standardised [18F]SiTATE-PET/CT within clinical routine: 40 patients with long-acting SSAs up to 28 days prior to PET/CT examination and 37 patients without pre-treatment with SSAs. Maximum and mean standardized uptake values (SUVmax and SUVmean) of tumours and metastases (liver, lymphnode, mesenteric/peritoneal and bones) as well as representative background tissues (liver, spleen, adrenal gland, blood pool, small intestine, lung, bone) were measured, SUV ratios (SUVR) were calculated between tumours/metastases and liver, likewise between tumours/metastases and corresponding specific background, and compared between the two groups.

Results

SUVmean of liver (5.4 ± 1.5 vs. 6.8 ± 1.8) and spleen (17.5 ± 6.8 vs. 36.7 ± 10.3) were significantly lower (p < 0.001) and SUVmean of blood pool (1.7 ± 0.6 vs. 1.3 ± 0.3) was significantly higher (p < 0.001) in patients with SSA pre-treatment compared to patients without. No significant differences between tumour-to-liver and specific tumour-to-background SUVRs were observed between both groups (all p > 0.05).

Conclusion

In patients previously treated with SSAs, a significantly lower SSR expression ([18F]SiTATE uptake) in normal liver and spleen tissue was observed, as previously reported for 68Ga-labelled SSAs, without significant reduction of tumour-to-background contrast. Therefore, there is no evidence that SSA treatment needs to be paused prior to [18F]SiTATE-PET/CT.

Hyperfractionated Treatment with 177Lu-Octreotate Increases Tumor Response in Human Small-Intestine Neuroendocrine GOT1 Tumor Model

Simple Summary

Neuroendocrine tumors are slow growing and initially associated with vague symptoms and, therefore, often spread in the patient’s body at diagnosis, leading to a poor prognosis without means of curation through surgery. Although tumor-targeting treatments exist and are used in clinics, they are not fully optimized. The aim of this study was to test different dosages and time intervals of the radioactive pharmaceutical ¹⁷⁷Lu-octreotate. We found that dividing a dosage into several portions and administering it at short time intervals resulted in a stronger tumor reduction and/or prolonged time for regrowth in mice than if given as a single dose. The biggest differences were seen in the lower dosage levels of the study. The findings indicate that there is clear room for improvements in the treatment of neuroendocrine tumors with ¹⁷⁷Lu-octreotate.

Abstract

Radionuclide treatment of patients with neuroendocrine tumors has advanced in the last decades with favorable results using ¹⁷⁷Lu-octreotate. However, the gap between the high cure rate in animal studies vs. patient studies indicates a potential to increase the curation of patients. The aim of this study was to investigate the tumor response for different fractionation schemes with ¹⁷⁷Lu-octreotate. BALB/c mice bearing a human small-intestine neuroendocrine GOT1 tumor were either mock treated with saline or injected intravenously with a total of 30–120 MBq of ¹⁷⁷Lu-octreotate: 1 × 30, 2 × 15, 1 × 60, 2 × 30, 1 × 120, 2 × 60, or 3 × 40 MBq. The tumor volume was measured twice per week until the end of the experiment. The mean tumor volume for mice that received 2 × 15 = 30 and 1 × 30 MBq ¹⁷⁷Lu-octreotate was reduced by 61% and 52%, respectively. The mean tumor volume was reduced by 91% and 44% for mice that received 2 × 30 = 60 and 1 × 60 MBq ¹⁷⁷Lu-octreotate, respectively. After 120 MBq ¹⁷⁷Lu-octreotate, given as 1–3 fractions, the mean tumor volume was reduced by 91–97%. Multiple fractions resulted in delayed regrowth and prolonged overall survival by 20–25% for the 120 MBq groups and by 45% for lower total activities, relative to one fraction. The results indicate that fractionation and hyperfractionation of ¹⁷⁷Lu-octreotate are beneficial for tumor reduction and prolongs the time to regrowth.

A Physiologically Based Pharmacokinetic Model for In Vivo Alpha Particle Generators Targeting Neuroendocrine Tumors in Mice

In vivo alpha particle generators have great potential for the treatment of neuroendocrine tumors in alpha-emitter-based peptide receptor radionuclide therapy (α-PRRT). Quantitative pharmacokinetic analyses of the in vivo alpha particle generator and its radioactive decay products are required to address concerns about the efficacy and safety of α-PRRT. A murine whole-body physiologically based pharmacokinetic (PBPK) model was developed for ²¹²Pb-labeled somatostatin analogs (²¹²Pb-SSTA). The model describes pharmacokinetics of ²¹²Pb-SSTA and its decay products, including specific and non-specific glomerular and tubular uptake. Absorbed dose coefficients (ADC) were calculated for bound and unbound radiolabeled SSTA and its decay products. Kidneys received the highest ADC (134 Gy/MBq) among non-target tissues. The alpha-emitting ²¹²Po contributes more than 50% to absorbed doses in most tissues. Using this model, it is demonstrated that α-PRRT based on ²¹²Pb-SSTA results in lower absorbed doses in non-target tissue than α-PRRT based on ²¹²Bi-SSTA for a given kidneys absorbed dose. In both approaches, the energies released in the glomeruli and proximal tubules account for 54% and 46%, respectively, of the total energy absorbed in kidneys. The ²¹²Pb-SSTA-PBPK model accelerates the translation from bench to bedside by enabling better experimental design and by improving the understanding of the underlying mechanisms.

Distinct In Vitro Binding Profile of the Somatostatin Receptor Subtype 2 Antagonist [177Lu]Lu-OPS201 Compared to the Agonist [177Lu]Lu-DOTA-TATE

Treatment of neuroendocrine tumours with the radiolabelled somatostatin receptor subtype 2 (SST₂) peptide agonist [¹⁷⁷Lu]Lu-DOTA-TATE is effective and well-established. Recent studies suggest improved therapeutic efficacy using the SST₂ peptide antagonist [¹⁷⁷Lu]Lu-OPS201. However, little is known about the cellular mechanisms that lead to the observed differences. In the present in vitro study, we compared kinetic binding, saturation binding, competition binding, cellular uptake and release of [¹⁷⁷Lu]Lu-OPS201 versus [¹⁷⁷Lu]Lu-DOTA-TATE using HEK cells stably transfected with the human SST₂. While [¹⁷⁷Lu]Lu-OPS201 and [¹⁷⁷Lu]Lu-DOTA-TATE exhibited comparable affinity (K_D, 0.15 ± 0.003 and 0.08 ± 0.02 nM, respectively), [¹⁷⁷Lu]Lu-OPS201 recognized four times more binding sites than [¹⁷⁷Lu]Lu-DOTA-TATE. Competition assays demonstrated that a high concentration of the agonist displaced only 30% of [¹⁷⁷Lu]Lu-OPS201 bound to HEK-SST₂ cell membranes; an indication that the antagonist binds to additional sites that are not recognized by the agonist. [¹⁷⁷Lu]Lu-OPS201 showed faster association and slower dissociation than [¹⁷⁷Lu]Lu-DOTA-TATE. Whereas most of [¹⁷⁷Lu]Lu-OPS201 remained at the cell surface, [¹⁷⁷Lu]Lu-DOTA-TATE was almost completely internalised inside the cell. The present data identified distinct differences between [¹⁷⁷Lu]Lu-OPS201 and [¹⁷⁷Lu]Lu-DOTA-TATE regarding the recognition of receptor binding sites (higher for [¹⁷⁷Lu]Lu-OPS201) and their kinetics (faster association and slower dissociation of [¹⁷⁷Lu]Lu-OPS201) that explain, to a great extent, the improved therapeutic efficacy of [¹⁷⁷Lu]Lu-OPS201 compared to [¹⁷⁷Lu]Lu-DOTA-TATE.

Neuroblastoma xenograft models demonstrate the therapeutic potential of 177Lu-octreotate

BACKGROUND

Neuroblastoma (NB) is one of the most frequently diagnosed tumors in infants. NB is a neuroendocrine tumor type with various characteristics and features, and with diverse outcome. The most malignant NBs have a 5-year survival rate of only 40-50%, indicating the need for novel and improved treatment options. 177Lu-octreotate is routinely administered for treatment of neuroendocrine tumors overexpressing somatostatin receptors (SSTR). The aim of this study was to examine the biodistribution of 177Lu-octreotate in mice bearing aggressive human NB cell lines, in order to evaluate the potential usefulness of 177Lu-octreotate for treatment of NB.

METHODS

BALB/c nude mice bearing CLB-BAR, CLB-GE or IMR-32 tumor xenografts (n = 5-7/group) were i.v. injected with 0.15 MBq, 1.5 MBq or 15 MBq 177Lu-octreotate and sacrificed 1 h, 24 h, 48 h and 168 h after administration. The radioactivity concentration was determined for collected tissue samples, tumor-to-normal-tissue activity concentration ratios (T/N) and mean absorbed dose for each tissue were calculated. Immunohistochemical (IHC) staining for SSTR1-5, and Ki67 were carried out for tumor xenografts from the three cell lines.

RESULTS

High 177Lu concentration levels and T/N values were observed in all NB tumors, with the highest for CLB-GE tumor xenografts (72%IA/g 24 h p.i.; 1.5 MBq 177Lu-octreotate). The mean absorbed dose to the tumor was 6.8 Gy, 54 Gy and 29 Gy for CLB-BAR, CLB-GE and IMR-32, respectively, p.i. of 15 MBq 177Lu-octreotate. Receptor saturation was clearly observed in CLB-BAR, resulting in higher concentration levels in the tumor when lower activity levels where administered. IHC staining demonstrated highest expression of SSTR2 in CLB-GE, followed by CLB-BAR and IMR-32.

CONCLUSION

T/N values for all three human NB tumor xenograft types investigated were high relative to previously investigated neuroendocrine tumor types. The results indicate a clear potential of 177Lu-octreotate as a therapeutic alternative for metastatic NB.

A physiologically based pharmacokinetic (PBPK) model to describe organ distribution of 68Ga-DOTATATE in patients without neuroendocrine tumors

Background

Physiologically based pharmacokinetic (PBPK) models combine drug-specific information with prior knowledge on the physiology and biology at the organism level. Whole-body PBPK models contain an explicit representation of the organs and tissue and are a tool to predict pharmacokinetic behavior of drugs. The aim of this study was to develop a PBPK model to describe organ distribution of ⁶⁸Ga-DOTATATE in a population of patients without detectable neuroendocrine tumors (NETs).

Methods

Clinical ⁶⁸Ga-DOTATATE PET/CT data from 41 patients without any detectable somatostatin receptor (SSTR) overexpressing tumors were included. Scans were performed at 45 min (range 30–60 min) after intravenous bolus injection of ⁶⁸Ga-DOTATATE. Organ (spleen, liver, thyroid) and blood activity levels were derived from PET scans, and corresponding DOTATATE concentrations were calculated. A whole-body PBPK model was developed, including an internalization reaction, receptor recycling, enzymatic reaction for intracellular degradation and renal clearance. SSTR2 expression was added for several organs. Input parameters were fixed or estimated using a built-in Monte Carlo algorithm for parameter identification.

Results

⁶⁸Ga-DOTATATE was administered with a median peptide amount of 12.3 µg (range 8.05–16.9 µg) labeled with 92.7 MBq (range 43.4–129.9 MBq). SSTR2 amounts for spleen, liver and thyroid were estimated at 4.40, 7.80 and 0.0108 nmol, respectively. Variability in observed organ concentrations was best described by variability in SSTR2 expression and differences in administered peptide amounts.

Conclusions

To conclude, biodistribution of ⁶⁸Ga-DOTATATE was described with a whole-body PBPK model, where tissue distribution was mainly determined by variability in SSTR2 organ expression and differences in administered peptide amounts.

A Rapid and Safe Infusion Protocol for 177Lu Peptide Receptor Radionuclide Therapy

Peptide receptor radionuclide therapy (PRRT) with ¹⁷⁷Lu-labeled somatostatin analogs in patients with somatostatin receptor-expressing tumors is often performed using administration protocols prescribing a 30-min infusion time. The most often used method of infusion is the gravity method, by which the complete dose is effectively administered exponentially. However, there is no evidence to explicitly support an infusion time of 30 min. This study aims to investigate the safety of an infusion time of less than 5 min. Methods: A cohort study was performed, examining the biochemical and clinical toxicity after PRRT when using a fast-infusion protocol with a maximum infusion time of 5 min. Data on patient characteristics, laboratory tests, follow-up visits, and pre- and posttreatment imaging using ⁶⁸Ga-DOTATOC PET/CT from patients treated with PRRT at the University Medical Center Utrecht (UMC Utrecht) were collected. All patients receiving PRRT using the fast-infusion protocol were included. If no laboratory or clinical follow-up was available, patients were excluded. In addition, a laboratory experiment was performed, simulating the standard-infusion protocol using the gravity method. Results: Thirty-one patients, treated using the fast-infusion protocol, were included. Clinical toxicity mainly consisted of grade 1/2 fatigue (87.1%) and grade 1 nausea or vomiting (67.7%) during follow-up. No acute or long-term clinical toxicity possibly related to the fast-infusion protocol was reported. Grade 3/4 hematologic toxicity occurred after PRRT in 1 patient (3.2%). No grade 3/4 renal toxicity occurred. The laboratory experiment showed that when using the gravity method for infusion, half of the activity is infused after 3.5 min, and 95% is infused within 15 min. Conclusion: A faster infusion of PRRT using an infusion time of less than 5 min is safe and feasible in clinical practice.

Synthesis, preclinical evaluation, and a pilot clinical imaging study of [18F]AlF-NOTA-JR11 for neuroendocrine neoplasms compared with [68Ga]Ga-DOTA-TATE

PURPOSE

A [¹⁸F]AlF-labeled somatostatin receptor (SSTR) antagonist was developed for imaging of neuroendocrine neoplasms (NENs), evaluated and compared with [⁶⁸Ga]Ga-DOTA-TATE.

METHOD

[¹⁸F]AlF-NOTA-JR11 was synthesized manually and qualified with high-performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS). The cellular uptake, internalization, and saturation binding were performed with HEK293-SSTR2 cells. Biodistribution and micro-PET imaging were carried out with HEK293-SSTR2 tumor-bearing mice. [¹⁸F]AlF-NOTA-JR11 PET/MR imaging and [⁶⁸Ga]Ga-DOTA-TATE PET/CT were performed with ten patients of NEN at 50~60 min post-injection (p.i.). Normal organ biodistribution and tumor detectability were evaluated.

RESULT

[¹⁸F]AlF-NOTA-JR11(24~36 GBq/μmol) was prepared within 30 min and 51.35 ± 3.30% (n > 10)of radiochemical yield. The radiochemical purity was 98.74 ± 1.24% (n > 10). Two stereoisomers were found and confirmed by LC-MS. The cellular uptake of [¹⁸F]AlF-NOTA-JR11 and [⁶⁸Ga]Ga-DOTA-TATE were 4.50 ± 0.31 and 4.50 ± 0.13 %AD/10⁵ cells at 30 min, and the internalization at 37 °C of [¹⁸F]AlF-NOTA-JR11 (5.47 ± 0.32% at 60 min) was significantly lower than [⁶⁸Ga]Ga-DOTA-TATE (66.89 ± 1.62% at 60 min). The affinity of [¹⁸F]AlF-NOTA-JR11 (K_d = 11.59 ± 1.31 nM) was slightly lower than [⁶⁸Ga]Ga-DOTA-TATE (K_d = 7.36 ± 1.02 nM); [¹⁸F]AlF-NOTA-JR11 showed high uptake in tumor (9.02 ± 0.92 %ID/g at 60 min p.i.) which can be blocked by 50 μg of NOTA-JR11 (3.40 ± 1.64 %ID/g at 60 min p.i.); the result was coincident with micro-PET imaging. Imaging study of NEN patients showed that more lesions were found only by [¹⁸F]AlF-NOTA-JR11 (n = 67 vs. 1 only by [⁶⁸Ga]Ga-DOTA-TATE), and the uptakes of [¹⁸F]AlF-NOTA-JR11 in majority normal organs were significantly lower than [⁶⁸Ga]Ga-DOTA-TATE. The target to nontarget of maximum of standard uptake value (SUVmax) of [¹⁸F]AlF-NOTA-JR11 in liver lesions were significantly higher than those of [⁶⁸Ga]Ga-DOTA-TATE.

CONCLUSION

Qualitied [¹⁸F]AlF-NOTA-JR11 is prepared conveniently with reasonable yield, and it can bind SSTR2 specifically with high affinity. Excellent imaging capability of [¹⁸F]AlF-NOTA-JR11 for NENs is superior to [⁶⁸Ga]Ga-DOTA-TATE, especially in digestive system. It has a great potential for imaging of NENs.

Direct labeling of a somatostatin receptor antagonist via peptide cyclization with Re, 99mTc and 186Re metal centers: Radiochemistry and in vitro evaluation

INTRODUCTION

With the long-term goal of developing a diagnostic (^99mTc) and therapeutic (¹⁸⁶Re) agent pair for targeting somatostatin receptor (SSTR)-positive neuroendocrine tumors (NETs), we developed novel metal-cyclized peptides through direct labeling of the potent SSTR2 antagonist Ac-4-NO₂-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)-DTyr-NH₂ (1) with Re (in Re-1), ^99mTc (in [^99mTc]Tc-1) and ¹⁸⁶Re (in [¹⁸⁶Re]Re-1).

METHODS

Re-1 was characterized by LC-ESI-MS and HR-ESI-MS and was tested for receptor affinity in SSTR-expressing cells (AR42J). Radiolabeling of the peptide was achieved via ligand exchange from ^99mTc-labeled glucoheptonate or [¹⁸⁶Re]ReOCl₃(PPh₃)₂, yielding [^99mTc]Tc-1 or [¹⁸⁶Re]Re-1, respectively. In vitro stability of [^99mTc]Tc-1/[¹⁸⁶Re]Re-1 in PBS (10 mM) at pH 7.4 and 37 °C was determined by HPLC analysis. Moreover, [^99mTc]Tc-1 stability was tested in cysteine (1 mM) and rat serum under the same conditions.

RESULTS

Re-1 consisted of two isomers, confirmed by LC-ESI-MS, with good SSTR2 affinity (IC₅₀ = 43 ± 6 nM). Optimization of the ^99mTc labeling through varying reaction parameters such as pH, reaction time, and Sn²⁺ and ligand concentrations resulted in high radiochemical yield (RCY ≥92%). Similarly, [¹⁸⁶Re]Re-1 was prepared in reasonable RCY (≥50%). Both ^99mTc/¹⁸⁶Re-tracers consisted of two product isomers as identified by HPLC co-injection with Re-1. While [^99mTc]Tc-1 was sufficiently stable in vitro (≥71% intact through 4 h in PBS, cysteine and rat serum), [¹⁸⁶Re]Re-1 exhibited more moderate in vitro stability (58% intact after 1 h in PBS).

CONCLUSIONS

Novel ^99mTc/¹⁸⁶Re-cyclized SSTR2 antagonist peptides were synthesized and characterized using the Re-cyclized analogue as a reference. Due to the nanomolar SSTR2 affinity of Re-1 and good in vitro stability of [^99mTc]Tc-1, the latter shows early promise for development as a radiodiagnostic agent for SSTR-expressing NETs.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

The ^99mTc-cyclized complex showed promising in vitro properties, and future in vivo studies will determine the potential for translating such a design into the human clinic.

Overview of Radiolabeled Somatostatin Analogs for Cancer Imaging and Therapy

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

The theranostic promise for Neuroendocrine Tumors in the late 2010s - Where do we stand, where do we go?

More than 25 years after the first peptide receptor radionuclide therapy (PRRT), the concept of somatostatin receptor (SSTR)-directed imaging and therapy for neuroendocrine tumors (NET) is seeing rapidly increasing use. To maximize the full potential of its theranostic promise, efforts in recent years have expanded recommendations in current guidelines and included the evaluation of novel theranostic radiotracers for imaging and treatment of NET. Moreover, the introduction of standardized reporting framework systems may harmonize PET reading, address pitfalls in interpreting SSTR-PET/CT scans and guide the treating physician in selecting PRRT candidates. Notably, the concept of PRRT has also been applied beyond oncology, e.g. for treatment of inflammatory conditions like sarcoidosis. Future perspectives may include the efficacy evaluation of PRRT compared to other common treatment options for NET, novel strategies for closer monitoring of potential side effects, the introduction of novel radiotracers with beneficial pharmacodynamic and kinetic properties or the use of supervised machine learning approaches for outcome prediction. This article reviews how the SSTR-directed theranostic concept is currently applied and also reflects on recent developments that hold promise for the future of theranostics in this context.

Somatostatin receptor 2A in gliomas: Association with oligodendrogliomas and favourable outcome

Somatostatin receptor subtype 2A (SSTR2A) is a potential therapeutic target in gliomas. Data on SSTR2A expression in different glioma entities, however, is particularly conflicting. Our objective was to characterize SSTR2A status and explore its impact on survival in gliomas classified according to the specific molecular signatures of the updated WHO classification. In total, 184 glioma samples were retrospectively analyzed for SSTR2A expression using immunohistochemistry with monoclonal antibody UMB-1. Double staining with CD68 was used to exclude microglia and macrophages from analyses. SSTR2A staining intensity and its localization in tumor cells was evaluated and correlated with glioma entities and survival. Diagnoses included 101 glioblastomas (93 isocitrate dehydrogenase (IDH) -wildtype, 3 IDH-mutant, 5 not otherwise specified (NOS)), 60 astrocytomas (22 IDH-wildtype, 37 IDH-mutant, 1 NOS), and 23 oligodendrogliomas (19 IDH-mutant and 1p/19q-codeleted, 4 NOS). SSTR2A expression significantly associated with oligodendrogliomas (79% SSTR2A positive) compared to IDH-mutant or IDH-wildtype astrocytomas (27% and 23% SSTR2A positive, respectively), and especially glioblastomas of which only 13% were SSTR2A positive (p < 0.001, Fisher's exact test). The staining pattern in glioblastomas was patchy whereas more homogeneous membranous and cytoplasmic staining was detected in oligodendrogliomas. Positive SSTR2A was related to longer overall survival in grade II and III gliomas (HR 2.7, CI 1.2-5.8, p = 0.013). In conclusion, SSTR2A expression is infrequent in astrocytomas and negative in the majority of glioblastomas where it is of no prognostic significance. In contrast, oligodendrogliomas show intense membranous and cytoplasmic SSTR2A expression, which carries potential diagnostic, prognostic, and therapeutic value.

Adjuvant celecoxib and lanreotide following transarterial chemoembolisation for unresectable hepatocellular carcinoma: a randomized pilot study

Recurrence of hepatocellular carcinoma (HCC) after transarterial chemoembolisation (TACE) is common due to neoangiogenesis. Cyclooxygenase-2 inhibitors and somatostatin analogues were reported to inhibit tumour angiogenesis. The pilot randomized controlled trial was aimed to prospectively evaluate the protocol of TACE combined with celecoxib and lanreotide (TACE+C+L) in patients with unresectable and advanced HCC. A total of 71 patients with HCC were enrolled and randomly assigned to either TACE (n=35) or TACE+C+L (n=36) group. Overall survival, disease control rate (DCR), and adverse events were assessed during a 3-year follow-up period. The median overall survival of the TACE+C+L group (15.0 months) was doubled compared to that of TACE group (7.5 months), p = 0.012. DCR of the TACE+C+L group was significantly higher than that of the TACE group either at 6 months (72.2% vs 42.9%, p = 0.012) or at 12 months (61.1% vs 28.6%, p = 0.006). The median overall survivals (13 months vs 4.5 months, p = 0.013) and DCR at 12 months (50% vs 13.6%, p = 0.008) of patients with advanced HCC in TACE+C+L groups were significantly higher than those in TACE group. No significant difference of adverse events was observed between the two groups. The occurrence of post-embolisation syndrome in TACE+C+L group was significantly lower than that in TACE group (16.7% vs 60.0%, p = 0.001). In conclusion, the regimen of TACE+C+L prolonged overall survival, enhanced tumour response, reduced post-embolisation syndrome and was well-tolerable in the patients with unresectable HCC. It may be more beneficial for advanced HCC.

The somatostatin receptor 2 antagonist 64Cu-NODAGA-JR11 outperforms 64Cu-DOTA-TATE in a mouse xenograft model

Copper-64 is an attractive radionuclide for PET imaging and is frequently used in clinical applications. The aim of this study was to perform a side-by-side comparison of the in vitro and in vivo performance of 64Cu-NODAGA-JR11 (NODAGA = 1,4,7-triazacyclononane,1-glutaric acid,4,7-acetic acid, JR11 = p-Cl-Phe-cyclo(D-Cys-Aph(Hor)-D-Aph(cbm)-Lys-Thr-Cys)D-Tyr-NH2), a somatostatin receptor 2 antagonist, with the clinically used sst2 agonist 64Cu-DOTA-TATE ((TATE = D-Phe-cyclo(Cys-Tyr-D-Trp-Lys-Thr-Cys)Thr). In vitro studies demonstrated Kd values of 5.7±0.95 nM (Bmax = 4.1±0.18 nM) for the antagonist 64/natCu-NODAGA-JR11 and 20.1±4.4. nM (Bmax = 0.48±0.18 nM) for the agonist 64/natCu-DOTA-TATE. Cell uptake studies showed the expected differences between agonists and antagonists. Whereas 64Cu-DOTA-TATE (the agonist) showed very effective internalization in the cell culture assay (with 50% internalized at 4 hours post-peptide addition under the given experimental conditions), 64Cu-NODAGA-JR11 (the antagonist) showed little internalization but strong receptor-mediated uptake at the cell membrane. Biodistribution studies of 64Cu-NODAGA-JR11 showed rapid blood clearance and tumor uptake with increasing tumor-to-relevant organ ratios within the first 4 hours and in some cases, 24 hours, respectively. The tumor washout was slow or non-existent in the first 4 hours, whereas the kidney washout was very efficient, leading to high and increasing tumor-to-kidney ratios over time. Specificity of tumor uptake was proven by co-injection of high excess of non-radiolabeled peptide, which led to >80% tumor blocking. 64Cu-DOTA-TATE showed less favorable pharmacokinetics, with the exception of lower kidney uptake. Blood clearance was distinctly slower and persistent higher blood values were found at 24 hours. Uptake in the liver and lung was relatively high and also persistent. The tumor uptake was specific and similar to that of 64Cu-NODAGA-JR11 at 1 h, but release from the tumor was very fast, particularly between 4 and 24 hours. Tumor-to-normal organ ratios were distinctly lower after 1 hour. This is indicative of insufficient in vivo stability. PET studies of 64Cu-NODAGA-JR11 reflected the biodistribution data with nicely delineated tumor and low background. 64Cu-NODAGA-JR11 shows promising pharmacokinetic properties for further translation into the clinic.

Evaluation of astatine-211-labeled octreotide as a potential radiotherapeutic agent for NSCLC treatment

Octreotide is a somatostatin (SST) analogue currently used in the treatment of neuroendocrine tumors (NETs) with high binding affinity for the somatostatin receptor-2 (SSTR2) that is also overexpressed in non-small cell lung cancer cell (NSCLC). Alpha-particle-emitting astatine-211 (²¹¹At) is a promising radionuclide with appropriate physical and chemical properties for use in targeted anticancer therapies. To obtain an additional pharmacological agent for the treatment of NSCLC, we present the first investigation of the possible use of ²¹¹At-labeled octreotide as a potential alpha-radionuclide therapeutic agent for NSCLC treatment. ²¹¹At-SPC-octreotide exhibited observable higher uptake in lung, spleen, stomach and intestines than in other tissues. Through histological examination, ²¹¹At-SPC-octreotide demonstrated much more lethal effect than control groups (PBS, octreotide and free ²¹¹At). These promising preclinical results suggested that ²¹¹At labeled octreotide deserved to be further developed as a new anticancer agent for NSCLC.

Changes in biodistribution on 68Ga-DOTA-Octreotate PET/CT after long acting somatostatin analogue therapy in neuroendocrine tumour patients may result in pseudoprogression

Background

To evaluate the effects of long-acting somatostatin analogue (SSA) therapy on ⁶⁸Ga-DOTA-octreotate (GaTate) uptake at physiological and metastatic sites in neuroendocrine tumour (NET) patients.

Methods

Twenty-one patients who underwent GaTate PET/CT before and after commencement of SSA therapy were reviewed. Maximum standardized uptake values (SUVmax) were measured in normal organs. Changes in uptake of 49 metastatic lesions in 12 patients with stable disease were also compared. Serum chromogranin-A (CgA) levels were available for correlation between scans in 17/21 patients.

Results

Mean thyroid, spleen and liver SUVmax decreased significantly following SSA therapy from a baseline of 5.9 to 3.5, 30.3 to 23.1 and 10.3 to 8.0, respectively (p = < 0.0001 for all). Pituitary SUVmax increased from 10.2 to 11.0 (p = 0.004) whereas adrenal and salivary gland SUVmax did not change. Tumour SUVmax increased in 7 of 12 patients with stable disease; CgA was stable or decreasing in 5 of these patients. 30/49 (61%) metastatic lesions had an increase in SUVmax and lesion-to-liver uptake ratio increased in 40/49 (82%) following SSA therapy.

Conclusion

Long-acting SSA therapy decreases GaTate uptake in the thyroid, spleen and liver but in most cases increases intensity of uptake within metastases. This has significant implications for interpretation of GaTate PET/CT following commencement of therapy as increased intensity alone may not represent true progression. Our findings also suggest pre-dosing with SSA prior to PRRT may enable higher doses to be delivered to tumour whilst decreasing dose to normal tissues.

Electronic supplementary material

The online version of this article (10.1186/s40644-018-0136-x) contains supplementary material, which is available to authorized users.

In Vitro Evaluation of Molecular Tumor Targets in Nuclear Medicine: Immunohistochemistry Is One Option, but Under Which Conditions?

The identification of new molecular targets for diagnostic and therapeutic applications using in vitro methods is an important challenge in nuclear medicine. One such method is immunohistochemistry, increasingly popular because it is easy to perform. This review presents the case for conducting receptor immunohistochemistry to evaluate potential molecular targets in human tumor tissue sections. The focus is on the immunohistochemistry of G-protein-coupled receptors, one of the largest families of cell surface proteins, representing a major class of drug targets and thus playing an important role in nuclear medicine. This review identifies common pitfalls and challenges and provides guidelines on performing such immunohistochemical studies. An appropriate validation of the target is a prerequisite for developing robust and informative new molecular probes.

Long-Acting Somatostatin Analog Therapy Differentially Alters 68Ga-DOTATATE Uptake in Normal Tissues Compared with Primary Tumors and Metastatic Lesions

Synthetic somatostatin analogs have been posed as a potential source of error in somatostatin receptor imaging through interference with tumor detection; however, experimental models and clinical studies have shown a complex mechanism of the effect of octreotide on tumors. The aim of this study was to assess whether ⁶⁸Ga-DOTATATE uptake before treatment with long-acting somatostatin analogs differs from that after treatment. Methods: Thirty patients (15 men; age [mean ± SD], 64.6 ± 13.4 y) who had intermediately differentiated to well-differentiated neuroendocrine tumors and who underwent ⁶⁸Ga-DOTATATE PET/CT scanning before and after receiving long-acting repeatable octreotide (Sandostatin LAR) were included in the study. The SUV_max and SUV_mean of healthy target organs, residual primary tumor, and up to 5 lesions with the highest SUV_max in each organ were compared before and after octreotide treatment. Results: The mean time interval between the 2 ⁶⁸Ga-DOTATATE studies was 9.6 ± 7.2 mo, and the mean time gap between the last Sandostatin LAR injection and the second ⁶⁸Ga-DOTATATE study was 25.1 ± 14.8 d. The pretreatment mean SUV_max and SUV_mean were both significantly higher in the thyroid, liver, and spleen (P < 0.05) than the values measured after the administration of Sandostatin LAR. No significant differences were found among the uptake indices for residual primary tumor or any metastatic lesions in the liver, bone, lung, or lymph nodes before and after Sandostatin LAR administration (P > 0.05). Conclusion: Long-acting octreotide treatment diminished ⁶⁸Ga-DOTATATE uptake in the liver, spleen, and thyroid but did not compromise tracer uptake in residual primary tumor and metastatic lesions. These findings have a direct impact on the interpretation of ⁶⁸Ga-DOTATATE PET/CT scans.

Neodymium-140 DOTA-LM3: Evaluation of an In Vivo Generator for PET with a Non-Internalizing Vector

¹⁴⁰Nd (t_1/2 = 3.4 days), owing to its short-lived positron emitting daughter ¹⁴⁰Pr (t_1/2 = 3.4 min), has promise as an in vivo generator for positron emission tomography (PET). However, the electron capture decay of ¹⁴⁰Nd is chemically disruptive to macrocycle-based radiolabeling, meaning that an in vivo redistribution of the daughter ¹⁴⁰Pr is expected before positron emission. The purpose of this study was to determine how the delayed positron from the de-labeled ¹⁴⁰Pr affects preclinical imaging with ¹⁴⁰Nd. To explore the effect, ¹⁴⁰Nd was produced at CERN-ISOLDE, reacted with the somatostatin analogue, DOTA-LM3 (1,4,7,10- tetraazacyclododecane, 1,4,7- tri acetic acid, 10- acetamide N - p-Cl-Phecyclo(d-Cys-Tyr-d-4-amino-Phe(carbamoyl)-Lys-Thr-Cys)d-Tyr-NH2) and injected into H727 xenograft bearing mice. Comparative pre- and post-mortem PET imaging at 16 h postinjection was used to quantify the in vivo redistribution of ¹⁴⁰Pr following ¹⁴⁰Nd decay. The somatostatin receptor-positive pancreas exhibited the highest tissue accumulation of ¹⁴⁰Nd-DOTA-LM3 (13% ID/g at 16 h) coupled with the largest observed redistribution rate, where 56 ± 7% (n = 4, mean ± SD) of the in situ produced ¹⁴⁰Pr washed out of the pancreas before decay. Contrastingly, the liver, spleen, and lungs acted as strong sink organs for free ¹⁴⁰Pr³⁺. Based upon these results, we conclude that ¹⁴⁰Nd imaging with a non-internalizing vector convolutes the biodistribution of the tracer with the accumulation pattern of free ¹⁴⁰Pr. This redistribution phenomenon may show promise as a probe of the cellular interaction with the vector, such as in determining tissue dependent internalization behavior.

Time-integrated activity coefficient estimation for radionuclide therapy using PET and a pharmacokinetic model: A simulation study on the effect of sampling schedule and noise

PURPOSE

The aim of this study was to investigate the accuracy of PET-based treatment planning for predicting the time-integrated activity coefficients (TIACs).

METHODS

The parameters of a physiologically based pharmacokinetic (PBPK) model were fitted to the biokinetic data of 15 patients to derive assumed true parameters and were used to construct true mathematical patient phantoms (MPPs). Biokinetics of 150 MBq (68)Ga-DOTATATE-PET was simulated with different noise levels [fractional standard deviation (FSD) 10%, 1%, 0.1%, and 0.01%], and seven combinations of measurements at 30 min, 1 h, and 4 h p.i. PBPK model parameters were fitted to the simulated noisy PET data using population-based Bayesian parameters to construct predicted MPPs. Therapy simulations were performed as 30 min infusion of (90)Y-DOTATATE of 3.3 GBq in both true and predicted MPPs. Prediction accuracy was then calculated as relative variability vorgan between TIACs from both MPPs.

RESULTS

Large variability values of one time-point protocols [e.g., FSD = 1%, 240 min p.i., vkidneys = (9 ± 6)%, and vtumor = (27 ± 26)%] show inaccurate prediction. Accurate TIAC prediction of the kidneys was obtained for the case of two measurements (1 and 4 h p.i.), e.g., FSD = 1%, vkidneys = (7 ± 3)%, and vtumor = (22 ± 10)%, or three measurements, e.g., FSD = 1%, vkidneys = (7 ± 3)%, and vtumor = (22 ± 9)%.

CONCLUSIONS

(68)Ga-DOTATATE-PET measurements could possibly be used to predict the TIACs of (90)Y-DOTATATE when using a PBPK model and population-based Bayesian parameters. The two time-point measurement at 1 and 4 h p.i. with a noise up to FSD = 1% allows an accurate prediction of the TIACs in kidneys.

Phage-Enabled Nanomedicine: From Probes to Therapeutics in Precision Medicine

Both lytic and temperate bacteriophages (phages) can be applied in nanomedicine, in particular, as nanoprobes for precise disease diagnosis and nanotherapeutics for targeted disease treatment. Since phages are bacteria-specific viruses, they do not naturally infect eukaryotic cells and are not toxic to them. They can be genetically engineered to target nanoparticles, cells, tissues, and organs, and can also be modified with functional abiotic nanomaterials for disease diagnosis and treatment. This Review will summarize the current use of phage structures in many aspects of precision nanomedicine, including ultrasensitive biomarker detection, enhanced bioimaging for disease diagnosis, targeted drug and gene delivery, directed stem cell differentiation, accelerated tissue formation, effective vaccination, and nanotherapeutics for targeted disease treatment. We will also propose future directions in the area of phage-based nanomedicines, and discuss the state of phage-based clinical trials.

[Use of somatostatin analogues in the pancreonecrosis therapy (kliniko-experimental research)]

The article is devoted to the study and optimization of therapeutic strategy in pancreatic necrosis. We studied the effectiveness of clinical application of human somatostatin analog - octreotide in different dosages. Submitted investigationconsists of experimental and clinical parts.

MATERIAL AND METHODS

Experimentalmodel of acute destructive pancreatitis onrats of Wistar line with weight of 300±30 gwas reproduced.There were 5 animal groups (one control group and four experimental groups). Experimental animals received oktreotidin different doses. In 2 days there were estimated the blood biochemistryand histopathologic features of liver and pancreas of operated animals. In clinical part there were formed two groups of patients out of selected case histories by a principle of similarity of complex therapy differing in oktreotid doses, namely: 1) pancreatonecrosis patients estimated of 3 to 8 points by Ranson scale in whose treatment regimen oktreotid 300 mkg/day (n=70) dose was included; 2) pancreatonecrosispatients estimated of 3 to 8 points by Ransonin whose treatment regimenoktreoid1200 mkg/day (n=38) was included. There were compared indices of lethal outcomes, average bed-days, and dynamics of laboratory findings, number and volume of surgical procedures.

RESULTS AND CONCLUSION

Dose-related effect of a synthetic analogue of somatostatin, acetateoktreotid, was proved. Introduction in complex therapy of pancreatonecrosisthe highest possible authorized doses of oktreotid (1200 mkg/day) is conducive to more favorable course of illness, dynamic reduction of amilazemia, decrease of lethal outcomes and total bed-day, and also, reduction of initial and repeated operative procedures.

Priming increases the anti-tumor effect and therapeutic window of 177Lu-octreotate in nude mice bearing human small intestine neuroendocrine tumor GOT1

Background

¹⁷⁷Lu-[DOTA⁰, Tyr³]-octreotate (¹⁷⁷Lu-octreotate) is used for treatment of patients with somatostatin receptor (SSTR) expressing neuroendocrine tumors. However, complete tumor remission is rarely seen, and optimization of treatment protocols is needed. In vitro studies have shown that irradiation can up-regulate the expression of SSTR1, 2 and 5, and increase ¹⁷⁷Lu-octreotate uptake.

The aim of the present study was to examine the anti-tumor effect of a ¹⁷⁷Lu-octreotate priming dose followed 24 h later by a second injection of ¹⁷⁷Lu-octreotate compared to a single administration of ¹⁷⁷Lu-octreotate, performed on the human small intestine neuroendocrine tumor cell line, GOT1, transplanted to nude mice.

Results

Priming resulted in a 1.9 times higher mean absorbed dose to the tumor tissue per administered activity, together with a reduced mean absorbed dose for kidneys. Priming gave the best overall anti-tumor effects. Magnetic resonance imaging showed no statistically significant difference in tumor response between treatment with and without priming. Gene expression analysis demonstrated effects on cell cycle regulation. Biological processes associated with apoptotic cell death were highly affected in the biodistribution and dosimetry study, via differential regulation of, e.g., APOE, BAX, CDKN1A, and GADD45A.

Conclusions

Priming had the best overall anti-tumor effects and also resulted in an increased therapeutic window. Results indicate that potential biomarkers for tumor regrowth may be found in the p53 or JNK signaling pathways. Priming administration is an interesting optimization strategy for ¹⁷⁷Lu-octreotate therapy of neuroendocrine tumors, and further studies should be performed to determine the mechanisms responsible for the reported effects.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-016-0247-y) contains supplementary material, which is available to authorized users.

Radiation Dosimetry for 177Lu-PSMA I&T in Metastatic Castration-Resistant Prostate Cancer: Absorbed Dose in Normal Organs and Tumor Lesions

Prostate-specific membrane antigen (PSMA)-targeted radioligand therapy is increasingly used in metastatic castration-resistant prostate cancer. We aimed to estimate the absorbed doses for normal organs and tumor lesions using ¹⁷⁷Lu-PSMA I&T (I&T is imaging and therapy) in patients undergoing up to 4 cycles of radioligand therapy. Results were compared with pretherapeutic Glu-NH-CO-NH-Lys-(Ahx)-[⁶⁸Ga(HBEDCC)] (⁶⁸Ga-PSMA-HBED-CC) PET. Methods: A total of 34 cycles in 18 patients were analyzed retrospectively. In 15 patients the first, in 9 the second, in 5 the third, and in 5 the fourth cycle was analyzed, respectively. Whole-body scintigraphy was performed at least between 30-120 min, 24 h, and 6-8 d after administration. Regions of interest covering the whole body, organs, and up to 4 tumor lesions were drawn. Organ and tumor masses were derived from pretherapeutic ⁶⁸Ga-PSMA-HBED-CC PET/CT. Absorbed doses for individual cycles were calculated using OLINDA/EXM. SUVs from pretherapeutic PET were compared with absorbed doses and with change of SUV. Results: The mean whole-body effective dose for all cycles was 0.06 ± 0.03 Sv/GBq. The mean absorbed organ doses were 0.72 ± 0.21 Gy/GBq for the kidneys; 0.12 ± 0.06 Gy/GBq for the liver; and 0.55 ± 0.14 Gy/GBq for the parotid, 0.64 ± 0.40 Gy/GBq for the submandibular, and 3.8 ± 1.4 Gy/GBq for the lacrimal glands. Absorbed organ doses were relatively constant among the 4 different cycles. Tumor lesions received a mean absorbed dose per cycle of 3.2 ± 2.6 Gy/GBq (range, 0.22-12 Gy/GBq). Doses to tumor lesions gradually decreased, with 3.5 ± 2.9 Gy/GBq for the first, 3.3 ± 2.5 Gy/GBq for the second, 2.7 ± 2.3 Gy/GBq for the third, and 2.4 ± 2.2 Gy/GBq for the fourth cycle. SUVs of pretherapeutic PET moderately correlated with absorbed dose (r = 0.44, P < 0.001 for SUV_max; r = 0.43, P < 0.001 for SUV_mean) and moderately correlated with the change of SUV (r = 0.478, P < 0.001 for SUV_max, and r = 0.50, P < 0.001 for SUV_mean). Conclusion: Organ- and tumor-absorbed doses for ¹⁷⁷Lu-PSMA I&T are comparable to recent reports and complement these with information on an excellent correlation between the 4 therapy cycles. With the kidneys representing the critical organ, a cumulative activity of 40 GBq of ¹⁷⁷Lu-PSMA I&T appears to be safe and justifiable. The correlation between pretherapeutic SUV and absorbed tumor dose emphasizes the need for PSMA-ligand PET imaging for patient selection.

Specific biology of neuroendocrine tumors: Peptide receptors as molecular targets

Neuroendocrine tumors (NET) are characterized by a high over-expression of many different peptide hormone receptors. These receptors represent important molecular targets for imaging and therapy, using either radiolabeled or cold peptide analogs. The clinically best established example is somatostatin receptor targeting. A relatively new application is glucagon-like peptide 1 (GLP-1) receptor-targeted imaging of insulinomas, which is highly sensitive. A potential future candidate for peptide receptor targeting is the gastric inhibitory peptide (GIP) receptor. It was recently found to exhibit a very wide expression in NET and may be a particularly suitable target in somatostatin and GLP-1 receptor negative tumors. With increasing use of peptide receptor targeting, reliable morphologic in vitro tools to assess peptide receptors in tissues are mandatory, such as in vitro receptor autoradiography or thoroughly established immunohistochemical procedures.

Rapid kit-based (68)Ga-labelling and PET imaging with THP-Tyr(3)-octreotate: a preliminary comparison with DOTA-Tyr(3)-octreotate

BACKGROUND

Ge/(68)Ga generators provide an inexpensive source of a PET isotope to hospitals without cyclotron facilities. The development of new (68)Ga-based molecular imaging agents and subsequent clinical translation would be greatly facilitated by simplification of radiochemical syntheses. We report the properties of a tris(hydroxypyridinone) conjugate of the SSTR2-targeted peptide, Tyr(3)-octreotate (TATE), and compare the (68)Ga-labelling and biodistribution of [(68)Ga(THP-TATE)] with the clinical radiopharmaceutical [(68)Ga(DOTATATE)].

METHODS

A tris(hydroxypyridinone) with a pendant isothiocyanate group was conjugated to the primary amine terminus of H2N-PEG2-Lys(iv-Dde)(5)-TATE, and the resulting conjugate was deprotected to provide THP-TATE. THP-TATE was radiolabelled with (68)Ga(3+) from a (68)Ge/(68)Ga generator. In vitro uptake was assessed in SSTR2-positive 427-7 cells and SSTR2-negative 427 (parental) cells. Biodistribution of [(68)Ga(THP-TATE)] was compared with that of [(68)Ga(DOTATATE)] in Balb/c nude mice bearing SSTR2-positive AR42J tumours. PET scans were obtained 1 h post-injection, after which animals were euthanised and tissues/organs harvested and counted.

RESULTS

[(68)Ga(THP-TATE)] was radiolabelled and formulated rapidly in <2 min, in ≥95 % radiochemical yield at pH 5-6.5 and specific activities of 60-80 MBq nmol(-1) at ambient temperature. [(68)Ga(THP-TATE)] was rapidly internalised into SSTR2-positive cells, but not SSTR2-negative cells, and receptor binding and internalisation were specific. Animals administered [(68)Ga(THP-TATE)] demonstrated comparable SSTR2-positive tumour activity (11.5 ± 0.6 %ID g(-1)) compared to animals administered [(68)Ga(DOTATATE)] (14.4 ± 0.8 %ID g(-1)). Co-administration of unconjugated Tyr(3)-octreotate effectively blocked tumour accumulation of [(68)Ga(THP-TATE)] (2.7 ± 0.6 %ID g(-1)). Blood clearance of [(68)Ga(THP-TATE)] was rapid and excretion was predominantly renal, although compared to [(68)Ga(DOTATATE)], [(68)Ga(THP-TATE)] exhibited comparatively longer kidney retention.

CONCLUSIONS

Radiochemical synthesis of [(68)Ga(THP-TATE)] is significantly faster, proceeds under milder conditions, and requires less manipulation than that of [(68)Ga(DOTATATE)]. A (68)Ga-labelled tris(hydroxypyridinone) conjugate of Tyr(3)-octreotate demonstrates specificity and targeting affinity for SSTR2 receptors, with comparable in vivo targeting affinity to the clinical PET tracer, [(68)Ga(DOTATATE)]. Thus, peptide conjugates based on tris(hydroxypyridinones) are conducive to translation to kit-based preparation of PET tracers, enabling the expansion and adoption of (68)Ga PET in hospitals and imaging centres without the need for costly automated synthesis modules.

SPECT- and PET-based patient-tailored treatment in neuroendocrine tumors: a comprehensive multidisciplinary team approach

The overexpression of somatostatin receptors on the tumor cell surface of neuroendocrine tumors (NETs) detected by multimodal functional imaging modalities such as SPECT and PET tracers constitutes a therapeutic option using targeting radiolabeled compounds. We will introduce the theranostic concept in general, explain in more detail its development in NETs, and discuss available SPECT and PET tracers regarding their potential for diagnostic imaging, visualization of target expression, and treatment tailoring. Moreover, we will discuss the currently available peptide receptor radionuclide therapy principles and compare them to previously published studies. Finally, we will discuss which new concepts will most likely influence the theranostic treatment approach in NETs in the future.

PET tracers for somatostatin receptor imaging of neuroendocrine tumors: current status and review of the literature

Neuroendocrine tumors have shown rising incidence mainly due to higher clinical awareness and better diagnostic tools over the last 30 years. Functional imaging of neuroendocrine tumors with PET tracers is an evolving field that is continuously refining the affinity of new tracers in the search for the perfect neuroendocrine tumor imaging tracer. (68)Ga-labeled tracers coupled to synthetic somatostatin analogs with differences in affinity for the five somatostatin receptor subtypes are now widely applied in Europe. Comparison of sensitivity between the most used tracers - (68)Ga-DOTA-Tyr3-octreotide, (68)Ga-DOTA-Tyr3-octreotate and (68)Ga-DOTA-l-Nal3-octreotide - shows little difference and expertise on the specific tracer used, and knowledge regarding physiological uptake might be more important than in vitro-proven differences in affinity. Using isotopes such as (18)F or (64)Cu might improve these PET tracers further.

Pharmacokinetic digital phantoms for accuracy assessment of image-based dosimetry in (177)Lu-DOTATATE peptide receptor radionuclide therapy

Patient-specific image-based dosimetry is considered to be a useful tool to limit toxicity associated with peptide receptor radionuclide therapy (PRRT). To facilitate the establishment and reliability of absorbed-dose response relationships, it is essential to assess the accuracy of dosimetry in clinically realistic scenarios. To this end, we developed pharmacokinetic digital phantoms corresponding to patients treated with (177)Lu-DOTATATE. Three individual voxel phantoms from the XCAT population were generated and assigned a dynamic activity distribution based on a compartment model for (177)Lu-DOTATATE, designed specifically for this purpose. The compartment model was fitted to time-activity data from 10 patients, primarily acquired using quantitative scintillation camera imaging. S values for all phantom source-target combinations were calculated based on Monte-Carlo simulations. Combining the S values and time-activity curves, reference values of the absorbed dose to the phantom kidneys, liver, spleen, tumours and whole-body were calculated. The phantoms were used in a virtual dosimetry study, using Monte-Carlo simulated gamma-camera images and conventional methods for absorbed-dose calculations. The characteristics of the SPECT and WB planar images were found to well represent those of real patient images, capturing the difficulties present in image-based dosimetry. The phantoms are expected to be useful for further studies and optimisation of clinical dosimetry in (177)Lu PRRT.

Pretherapeutic estimation of kidney function in patients treated with peptide receptor radionuclide therapy: can renal scintigraphy be safely omitted?

OBJECTIVE

A pretherapeutic assessment of kidney function before peptide receptor radionuclide therapy (PRRT) is considered essential because of the potential renal toxicity associated with PRRT. The aim of this study was to investigate the diagnostic performance of laboratory testing and Tc-mercaptoacetyltriglycine (MAG3) renal scintigraphy with a focus on patients treated with PRRT.

MATERIALS AND METHODS

From January to December 2013 the kidney function of 152 patients was assessed using laboratory tests [creatinine, blood urea nitrogen (BUN), and glomerular filtration rate (GFR)] and Tc-MAG3 clearance. In 27 patients, kidney function was assessed before PRRT. Results of blood tests and Tc-MAG3 renal scintigraphy, considered the reference standard, were compared in the entire patient cohort (n=152) and in both subgroups (PRRT and non-PRRT) using Student's t-test. The cutoff values for the laboratory tests for the prediction of abnormal Tc-MAG3 clearance were determined by means of receiver operating characteristic analysis. In a further mathematical approach using discriminant analysis, a formula was derived for the prediction of kidney function that included all of the serum parameters.

RESULTS

In the PRRT subgroup, laboratory test-derived kidney function correlated significantly with Tc-MAG3 clearance (creatinine: r=-0.429, P=0.037; BUN: r=-0.45, P=0.027; GFR: r=0.44, P=0.022). The correlation was confirmed in the non-PRRT subgroup. The receiver operating characteristic analysis for prediction of abnormal Tc-MAG3 clearance resulted in area under the curves of 0.779 for creatinine alone (sensitivity 74.3%, specificity 71.1%; cutoff ≥0.995 mg/dl) and for the combination of creatinine, BUN, and GFR (sensitivity was 74.3% and specificity was 69.3%).

CONCLUSION

Laboratory tests of kidney function correlate significantly with Tc-MAG3 clearance. Because of the moderate accuracy for laboratory tests, Tc-MAG3 clearance is recommended as a standard test to assess kidney function before PRRT.

Peptide receptor radionuclide therapy: an overview

Peptide receptor radionuclide therapy (PRRT) is a site-directed targeted therapeutic strategy that specifically uses radiolabeled peptides as biological targeting vectors designed to deliver cytotoxic levels of radiation dose to cancer cells, which overexpress specific receptors. Interest in PRRT has steadily grown because of the advantages of targeting cellular receptors in vivo with high sensitivity as well as specificity and treatment at the molecular level. Recent advances in molecular biology have not only stimulated advances in PRRT in a sustainable manner but have also pushed the field significantly forward to several unexplored possibilities. Recent decades have witnessed unprecedented endeavors for developing radiolabeled receptor-binding somatostatin analogs for the treatment of neuroendocrine tumors, which have played an important role in the evolution of PRRT and paved the way for the development of other receptor-targeting peptides. Several peptides targeting a variety of receptors have been identified, demonstrating their potential to catalyze breakthroughs in PRRT. In this review, the authors discuss several of these peptides and their analogs with regard to their applications and potential in radionuclide therapy. The advancement in the availability of combinatorial peptide libraries for peptide designing and screening provides the capability of regulating immunogenicity and chemical manipulability. Moreover, the availability of a wide range of bifunctional chelating agents opens up the scope of convenient radiolabeling. For these reasons, it would be possible to envision a future where the scope of PRRT can be tailored for patient-specific application. While PRRT lies at the interface between many disciplines, this technology is inextricably linked to the availability of the therapeutic radionuclides of required quality and activity levels and hence their production is also reviewed.

Synthesis and evaluation of a ligand targeting the somatostatin-2 receptor for drug delivery to neuroendocrine cancers

Over-expression of the somatostatin-2 (SST2) receptor on plasma membranes of neuroendocrine cancer cells renders it attractive for use in targeting both imaging and therapeutic agents to neuroendocrine tumors. Peptide analogs of somatostatin have dominated this approach to date, however, many peptide analogs are either unstable in vivo or exhibit unwanted non-specific uptake in the liver and kidneys. The purpose of this Letter is to describe the preparation and evaluation of a non-peptide SST2 agonist for use in targeting drugs to neuroendocrine cancers. A non-peptide ligand for the SST2 receptor was identified from the literature as a candidate for development of targeted pharmaceuticals for neuroendocrine tumors, based on its SST2 binding affinity and selectivity for SST2 over other somatostatin receptors. It also offered a multiplicity of possible conjugation sites. Rhodamine conjugates in two positions were used for optical imaging and two compounds were internalized in an SST2 receptor transduced cell line (C6-SST2) via SST2 receptor-mediated endocytosis. Radionuclide conjugates were prepared for in vivo imaging and biodistribution studies in mice. The in vitro binding affinity of (99m)Tc conjugates ranged from a Kd of 37-494. Of these, one (99m)Tc conjugate was selected and dosed by IV injection into mice bearing C6-SST2 tumor xenografts. The highest uptake was into tumor, intestine and skin four hours after IV injection. Competition studies with octreotide, a synthetic peptide and SST2 agonist, confirmed that uptake was SST2 receptor mediated. While relatively high uptake in intestine, liver, kidneys and skin discouraged further development of the conjugate for delivery of chemotherapeutic agents, the conjugate may still be worthy of further development for neuroendocrine tumor imaging.

Prediction of clinically relevant hyperkalemia in patients treated with peptide receptor radionuclide therapy

Background

Peptide receptor radionuclide therapy (PRRT) is applied in patients with advanced neuroendocrine tumors. Co-infused amino acids (AA) should prevent nephrotoxicity. The aims of this study were to correlate the incidence of AA-induced hyperkalemia (HK) (≥5.0 mmol/l) and to identify predictors of AA-induced severe HK (>6.0).

Methods

In 38 patients, standard activity of ¹⁷⁷Lu-labelled somatostatin analogs was administered. Pre-therapeutic kidney function was assessed by renal scintigraphy and laboratory tests. For kidney protection, AA was co-infused. Biochemical parameters (potassium, glomerular filtration rate, creatinine, blood urea nitrogen (BUN), sodium, phosphate, chloride, and lactate dehydrogenase (LDH)) were obtained prior to 4 and 24 h after the AA infusion. Incidence of HK (≥5.0) was correlated with pre-therapeutic kidney function and serum parameters. Formulas for the prediction of severe hyperkalemia (>6.0) were computed and prospectively validated.

Results

At 4 h, HK (≥5.0) was present in 94.7% with severe HK (>6.0) in 36.1%. Values normalized after 24 h in 84.2%. Pre-therapeutic kidney function did not correlate with the incidence of severe HK.

Increases in K⁺ were significantly correlated with decreases in phosphate (r = −0.444, p < 0.005) and increases in BUN (r = 0.313, p = 0.056). A baseline BUN of >28 mg/dl had a sensitivity of 84.6% and a specificity of 60.0% (AUC = 0.75) in predicting severe HK of >6.0 (phosphate, AUC = 0.37).

Computing of five standard serum parameters (potassium, BUN, sodium, phosphate, LDH) resulted in a sensitivity of 88.9% and a specificity of 79.3% for the prediction of severe HK >6.0 (accuracy = 81.6%).

Conclusions

A combination of serum parameters predicted prospectively the occurrence of relevant HK with an accuracy of 81.6% underlining its potential utility for identifying ‘high-risk’ patients prone to PRRT.

The somatostatin analog 188Re-P2045 inhibits the growth of AR42J pancreatic tumor xenografts

P2045 is a peptide analog of somatostatin with picomolar affinity for the somatostatin receptor subtype 2 (SSTR2) upregulated in some pancreatic tumors. Studies were conducted in rat AR42J pancreatic tumor xenograft mice to determine whether (188)Re-P2045 could inhibit the growth of pancreatic cancer in an animal model.

METHODS

(188)Re-P2045 was intravenously administered every 3 d for 16 d to nude mice with AR42J tumor xenografts that were approximately 20 mm(3) at study initiation. Tumor volumes were recorded throughout the dosing period. At necropsy, all tissues were assessed for levels of radioactivity and evaluated for histologic abnormalities. Clinical chemistry and hematology parameters were determined from terminal blood samples. The affinity of nonradioactive (185/187)Re-P2045 for somatostatin receptors was compared in human NCI-H69 and rat AR42J tumor cell membranes expressing predominantly SSTR2.

RESULTS

In the 1.85- and 5.55-MBq groups, tumor growth was inhibited in a dose-dependent fashion. In the 11.1-MBq group, tumor growth was completely inhibited throughout the dosing period and for 12 d after the last administered dose. The radioactivity level in tumors 4 h after injection was 10 percentage injected dose per gram, which was 2-fold higher than in the kidneys. (188)Re-P2045 was well tolerated in all dose groups, with no adverse clinical, histologic, or hematologic findings. The nonradioactive (185/187)Re-P2045 bound more avidly (0.2 nM) to SSTR2 in human than rat tumor membranes, suggesting that these studies are relevant to human studies.

CONCLUSION

(188)Re-P2045 is a promising therapeutic candidate for patients with somatostatin receptor-positive cancer.

Influence of the amount of co-infused amino acids on post-therapeutic potassium levels in peptide receptor radionuclide therapy

Background

Peptide receptor radionuclide therapy (PRRT) is routinely used for advanced or metastasized neuroendocrine tumours (NET). To prevent nephrotoxicity, positively charged amino acids (AA) are co-infused. The aim of this study was to correlate the risk for therapy-related hyperkalaemia with the total amount of AA infused.

Methods

Twenty-two patients undergoing PRRT with standard activities of ¹⁷⁷Lu-DOTATATE/-TOC were monitored during two following treatment cycles with co-infusion of 75 and 50 g of AA (l-arginine and l-lysine), respectively. Mean serum levels of potassium and other parameters (glomerular filtration rate [GFR], creatinine, blood urea nitrogen [BUN], phosphate, chloride, lactate dehydrogenase) prior to, 4 h and 24 h after AA infusion were compared.

Results

Self-limiting hyperkalaemia (>5.0 mmol/l) resolving after 24 h occurred in 91% (20/22) of patients in both protocols. Potassium levels, BUN, creatinine, GFR, phosphate, chloride and LDH showed a similar range at 4 h after co-infusion of 75 or 50 g of AA, respectively (p?>?0.05). Only GFR and creatinine levels at 24 h varied significantly between the two co-infusion protocols (p?<?0.05).

Conclusions

Hyperkalaemia is a frequent side effect of AA infusion in PRRT. Varying the dose of co-infused amino acids did not impact on the incidence and severity of hyperkalaemia.

Feasibility of experimental BT4C glioma models for somatostatin receptor 2-targeted therapies

Somatostatin receptor subtype 2 (sstr2) is regarded as a potential target in malignant gliomas for new therapeutic approaches. Therefore, visualizing and quantifying tumor sstr2 expression in vivo would be highly relevant for the future development of sstr2-targeted therapies. The purpose of this study was to evaluate sstr2 status in experimental BT4C malignant gliomas.

METHODS

Rat BT4C malignant glioma cells were injected into BDIX rat brain or subcutaneously into nude mice. Tumor uptake of [(68)Ga]DOTA-(Tyr(3))-Octreotide ([(68)Ga]DOTATOC), a somatostatin analog binding to sstr2, was studied by positron emission tomography/computed tomography (PET/CT). Additionally, subcutaneous tumor-bearing mice underwent PET imaging with 5-deoxy-5-[(18)F]fluororibose-NOC ([(18)F]FDR-NOC), a novel glycosylated peptide tracer also targeting sstr2. Ex vivo tissue radioactivity measurements, autoradiography and immunohistochemistry were performed to study sstr2 expression.

RESULTS

Increased tumor uptake of [(68)Ga]DOTATOC was detected at autoradiography with mean tumor-to-brain ratio of 68 ± 30 and tumor-to-muscle ratio of 9.2 ± 3.8 for rat glioma. High tumor-to-muscle ratios were also observed in subcutaneous tumor-bearing mice after injection with [(68)Ga]DOTATOC and [(18)F]FDR-NOC with both autoradiography (6.7 ± 1.5 and 4.3 ± 0.8, respectively) and tissue radioactivity measurements (6.5 ± 0.8 and 4.8 ± 0.6, respectively). Furthermore, sstr2 immunohistochemistry showed positive staining in both tumor models. However, surprisingly low tumor signal compromised PET imaging. Mean SUVmax for rat gliomas was 0.64 ± 0.28 from 30 to 60 min after [(68)Ga]DOTATOC injection. The majority of subcutaneous tumors were not visualized by [(68)Ga]DOTATOC or [(18)F]FDR-NOC PET.

CONCLUSIONS

Experimental BT4C gliomas show high expression of sstr2. Weak signal in PET imaging, however, suggests only limited benefit of [(68)Ga]DOTATOC or [(18)F]FDR-NOC PET/CT in this tumor model for in vivo imaging of sstr2 status.