Strain Differences Determine the Suitability of Animal Models for Noninvasive <Emphasis Type="Italic">In Vivo</Emphasis> Beta Cell Mass Determination with Radiolabeled Exendin

Purpose

Noninvasive beta cell mass (BCM) quantification is a crucial tool to understand diabetes development and progression. [¹¹¹In]exendin is a promising agent for in vivo beta cell imaging, but tracer testing has been hampered by the lack of well-defined rodent models.

Procedures

Biodistribution and pancreatic uptake of [¹¹¹In]exendin were compared in rats and mice. In selected models, the amount of [¹¹¹In]exendin accumulation in the pancreas and other organs was determined using a model of alloxan-induced beta cell loss. GLP-1R expression levels were analyzed by RT-PCR and immunohistochemistry.

Results

Namely Brown Norway rats showed beta-cell-specific tracer accumulation and favorable pancreas-to-background ratios for noninvasive BCM determination. Mice displayed receptor-mediated [¹¹¹In]exendin uptake in endocrine and exocrine pancreas, in spite of very low GLP-1R expression in exocrine tissue.

Conclusions

Rats display better characteristics for in vivo BCM determination than mice and are suggested as a more adequate model for humans.

     

Preclinical Evaluation of an Anti-Nectin-4 ImmunoPET Reagent in Tumor-Bearing Mice and Biodistribution Studies in Cynomolgus Monkeys

Purpose

Nectin-4 is selectively overexpressed in a variety of cancers and is currently under clinical investigation as a therapeutic target. A monoclonal antibody against nectin-4 (AGS-22M6) was evaluated as an Immuno-positron emission tomography (ImmunoPET) reagent. Its ability to assay nectin-4 expression as well as detect nectin-4 positive tumors in the liver and bone was evaluated using mouse models.

Procedures

The biodistribution of [⁸⁹Zr]AGS-22M6 was evaluated in mice bearing tumors with varying levels of nectin-4 expression. An isogenic breast cancer tumor line was used to model metastatic liver and bone disease in mice. The biodistribution of [¹⁸F]AGS-22M6 in cynomolgus monkeys was evaluated.

Results

A positive correlation was demonstrated between tumor nectin-4 expression and [⁸⁹Zr]AGS-22M6 uptake. Tumors in the liver and bone were detected and differentiated based on nectin-4 expression. [¹⁸F]AGS-22M6 showed limited uptake in cynomolgus monkey tissues.

Conclusions

[⁸⁹Zr]AGS-22M6 is a promising ImmunoPET reagent that can assay nectin-4 expression in both primary and metastatic lesions.

     

Preclinical Evaluation of 4-[<Superscript>18</Superscript>F]Fluoroglutamine PET to Assess ASCT2 Expression in Lung Cancer

Purpose

Alanine-serine-cysteine transporter 2 (ASCT2) expression has been demonstrated as a promising lung cancer biomarker. (2S,4R)-4-[¹⁸F]Fluoroglutamine (4-[¹⁸F]fluoro-Gln) positron emission tomography (PET) was evaluated in preclinical models of non-small cell lung cancer as a quantitative, non-invasive measure of ASCT2 expression.

Procedures

In vivo microPET studies of 4-[¹⁸F]fluoro-Gln uptake were undertaken in human cell line xenograft tumor-bearing mice of varying ASCT2 levels, followed by a genetically engineered mouse model of epidermal growth factor receptor (EGFR)-mutant lung cancer. The relationship between a tracer accumulation and ASCT2 levels in tumors was evaluated by IHC and immunoblotting.

Result

4-[¹⁸F]Fluoro-Gln uptake, but not 2-deoxy-2-[¹⁸F]fluoro-D-glucose, correlated with relative ASCT2 levels in xenograft tumors. In genetically engineered mice, 4-[¹⁸F]fluoro-Gln accumulation was significantly elevated in lung tumors, relative to normal lung and cardiac tissues.

Conclusions

4-[¹⁸F]Fluoro-Gln PET appears to provide a non-invasive measure of ASCT2 expression. Given the potential of ASCT2 as a lung cancer biomarker, this and other tracers reflecting ASCT2 levels could emerge as precision imaging diagnostics in this setting.

     

Evaluation of PET Imaging Performance of the TSPO Radioligand [<Superscript>18</Superscript>F]DPA-714 in Mouse and Rat Models of Cancer and Inflammation

Purpose

Many radioligands have been explored for imaging the 18-kDa translocator protein (TSPO), a diagnostic and therapeutic target for inflammation and cancer. Here, we investigated the TSPO radioligand [¹⁸F]DPA-714 for positron emission tomography (PET) imaging of cancer and inflammation.

Procedures

[¹⁸F]DPA-714 PET imaging was performed in 8 mouse and rat models of breast and brain cancer and 4 mouse and rat models of muscular and bowel inflammation.

Results

[¹⁸F]DPA-714 showed different uptake levels in healthy organs and malignant tissues of mice and rats. Although high and displaceable [¹⁸F]DPA-714 binding is observed ex vivo, TSPO-positive PET imaging of peripheral lesions of cancer and inflammation in mice did not show significant lesion-to-background signal ratios. Slower [¹⁸F]DPA-714 metabolism and muscle clearance in mice compared to rats may explain the elevated background signal in peripheral organs in this species.

Conclusion

Although TSPO is an evolutionary conserved protein, inter- and intra-species differences call for further exploration of the pharmacological parameters of TSPO radioligands.

     

Al[<Superscript>18</Superscript>F]NOTA-T140 Peptide for Noninvasive Visualization of CXCR4 Expression

Purpose

Chemokine receptor CXCR4 plays an important role in tumor aggressiveness, invasiveness, and metastasis formation. Quantification of CXCR4 expression by tumors may have an impact on prediction and evaluation of tumor response to therapies. In this study, we developed a robust and straightforward F-18 labeling route of T140, a CXCR4 peptide-based antagonist.

Procedures

T140 derivative was conjugated to 1,4,7-triazacyclononane-triacetic acid (NOTA) and labeled with Al[¹⁸F]. Al[¹⁸F]NOTA-T140 was evaluated in vitro in cell-based assay and stability in mouse serum and in vivo using CXCR4 positive and negative tumor xenograft models.

Results

Labeling of Al[¹⁸F]NOTA-T140 was completed within 30 min with a radiochemical yield of 58?±?5.3 % at the end of synthesis, based on fluoride-18 activity. Al[¹⁸F]NOTA-T140 accumulated in CHO-CXCR4 positive but not negative tumors. Al[¹⁸F]NOTA-T140 uptake in the tumors correlated with CXCR4 protein expression. Moreover, Al[¹⁸F]NOTA-T140 had high accumulation in CXCR4-positive metastatic tumors.

Conclusions

The simplicity of Al[¹⁸F]NOTA-T140 labeling along with its properties to specifically image CXCR4 expression by tumors warrant further clinical application for the diagnosis of CXCR4 clinically.

     

Pre-clinical Evaluation of a Cyanine-Based SPECT Probe for Multimodal Tumor Necrosis Imaging

Purpose

Recently we showed that a number of carboxylated near-infrared fluorescent (NIRF) cyanine dyes possess strong necrosis avid properties in vitro as well as in different mouse models of spontaneous and therapy-induced tumor necrosis, indicating their potential use for cancer diagnostic- and prognostic purposes. In the previous study, the detection of the cyanines was achieved by whole body optical imaging, a technique that, due to the limited penetration of near-infrared light, is not suitable for investigations deeper than 1 cm within the human body. Therefore, in order to facilitate clinical translation, the purpose of the present study was to generate a necrosis avid cyanine-based NIRF probe that could also be used for single photon emission computed tomography (SPECT). For this, the necrosis avid NIRF cyanine HQ4 was radiolabeled with ¹¹¹indium, via the chelate diethylene triamine pentaacetic acid (DTPA).

Procedures

The necrosis avid properties of the radiotracer [¹¹¹In]DTPA-HQ4 were examined in vitro and in vivo in different breast tumor models in mice using SPECT and optical imaging. Moreover, biodistribution studies were performed to examine the pharmacokinetics of the probe in vivo.

Results

Using optical imaging and radioactivity measurements, in vitro, we showed selective accumulation of [¹¹¹In]DTPA-HQ4 in dead cells. Using SPECT and in biodistribution studies, the necrosis avidity of the radiotracer was confirmed in a 4T1 mouse breast cancer model of spontaneous tumor necrosis and in a MCF-7 human breast cancer model of chemotherapy-induced tumor necrosis.

Conclusions

The radiotracer [¹¹¹In]DTPA-HQ4 possessed strong and selective necrosis avidity in vitro and in various mouse models of tumor necrosis in vivo, indicating its potential to be clinically applied for diagnostic purposes and to monitor anti-cancer treatment efficacy.

     

pH-Sensitive,Long-Circulating Liposomes as an Alternative Tool to Deliver Doxorubicin into Tumors: a Feasibility Animal Study

Purpose

Therapeutic agents used in chemotherapy have low specificity leading to undesired severe side effects. Hence, the development of drug delivery systems that improve drug specificity, such as liposome moieties, is an alternative to overcome chemotherapy limitations and increase antitumor efficacy. In this study, the biodistribution profile evaluation of pH-sensitive long-circulating liposomes (SpHL) containing [^99mTc]DOX in 4T1 tumor-bearing BALB/c mice is described.

Procedures

[^99mTc]DOX was radiolabeled by direct method. Liposomes were prepared and characterized. [^99mTc]DOX was encapsulated into liposomes by freezing and thawing. Circulation time for SpHL-[^99mTc]DOX was determined by measuring the blood activity from healthy animals. Biodistribution studies were carried out in tumor-bearing mice at 1, 4, and 24 h after injection.

Results

Blood levels of the SpHL-[^99mTc]DOX declined in a biphasic manner, with an α half-life of 14.1 min and β half-life of 129.0 min. High uptake was achieved in the liver and spleen, due to the macrophages captured. Moreover, tumor uptake was higher than control tissue, resulting in high tumor-to-muscle ratios, indicating higher specificity for the tumor area.

Conclusion

[^99mTc]DOX was successfully encapsulated in liposomes. Biodistribution indicated high tumor-to-muscle ratios in breast tumor-bearing BALB/c mice. In summary, these results showed the higher accumulation of SpHL-[^99mTc]DOX in the tumor area, suggesting selective delivery of doxorubicin into tumor.

     

P-Glycoprotein,not BCRP,Limits the Brain Uptake of [<Superscript>18</Superscript>F]Mefway in Rodent Brain

Purpose

The aim of this study was to determine whether the brain uptake of [¹⁸F]Mefway is influenced by the action of P-glycoprotein (P-gp) and breast cancer resistance protein (Bcrp) in rodents.

Procedures

[¹⁸F]Mefway was applied to rats pharmacologically inhibited with tariquidar (TQD) and to genetically disrupted mice.

Results

Pretreatment of TQD results in 160 % higher hippocampal uptake compared with control rats. In genetically disrupted mice, a maximal brain uptake value of 3.2 SUV in the triple knockout mice (tKO, Mdr1a/b^(?/?)Bcrp1^(?/?)) was comparable to that of the double knockout mice (dKO, Mdr1a/b^(?/?)) and 2-fold those of the wild-type and Bcrp1^(?/?) knockout mice. The differences of binding values were statistically insignificant between control and experimental groups. The brain-to-plasma ratios for tKO mice were also two to five times higher than those for other groups.

Conclusions

[¹⁸F]Mefway is modulated by P-gp, and not by Bcrp in rodents.

     

<Emphasis Type="Italic">In Vitro</Emphasis> and <Emphasis Type="Italic">In Vivo</Emphasis> Comparison of Selected Ga-68 and Zr-89 Labelled Siderophores

Purpose

Some [⁶⁸Ga]siderophores show promise in specific and sensitive imaging of infection. Here, we compare the in vitro and in vivo behaviour of selected Ga-68 and Zr-89 labelled siderophores.

Procedures

Radiolabelling was performed in HEPES or sodium acetate buffer systems. Radiochemical purity of labelled siderophores was determined using chromatography. Partition coefficients, in vitro stability and protein binding affinities were determined. Ex vivo biodistribution and animal imaging was studied in mice.

Results

Certain differences among studied siderophores were observed in labelling efficiency. Protein binding and stability tests showed highest stabilities and lowest protein binding affinities for Ga-68 and [⁸⁹Zr]triacetylfusarinine C (TAFC). All studied Ga-68 and [⁸⁹Zr]siderophores exhibited a similar biodistribution and pharmacokinetics in mice with the exception of [⁸⁹Zr]ferrioxamine E (FOXE).

Conclusions

Zr-89 and [⁶⁸Ga]siderophores showed analogous in vitro and in vivo behaviour. Tested [⁸⁹Zr]siderophores could be applied for longitudinal positron emission tomography (PET) studies of fungal infections and especially TAFC for the development of novel bioconjugates.

     

Investigation of Image Reconstruction Parameters of the Mediso nanoScan PC Small-Animal PET/CT Scanner for Two Different Positron Emitters Under NEMA NU 4-2008 Standards

Purpose

The Tera-Tomo 3D image reconstruction algorithm (a version of OSEM), provided with the Mediso nanoScan® PC (PET8/2) small-animal positron emission tomograph (PET)/x-ray computed tomography (CT) scanner, has various parameter options such as total level of regularization, subsets, and iterations. Also, the acquisition time in PET plays an important role. This study aims to assess the performance of this new small-animal PET/CT scanner for different acquisition times and reconstruction parameters, for 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG) and Ga-68, under the NEMA NU 4-2008 standards.

Procedures

Various image quality metrics were calculated for different realizations of [¹⁸F]FDG and Ga-68 filled image quality (IQ) phantoms.

Results

[¹⁸F]FDG imaging produced improved images over Ga-68. The best compromise for the optimization of all image quality factors is achieved for at least 30 min acquisition and image reconstruction with 52 iteration updates combined with a high regularization level.

Conclusion

A high regularization level at 52 iteration updates and 30 min acquisition time were found to optimize most of the figures of merit investigated.

     

Terbutaline lessens protein fluxes across the alveolo-capillary barrier during high-volume ventilation

Objective

To evaluate whether a β₂-adrenergic agonist may reduce acute alveolo-capillary barrier alterations during high-volume ventilation.

Design

Experimental study.

Setting

Animal research laboratory.

Subjects

A total of 48 male Wistar rats.

Interventions

A zone of alveolar flooding was produced by liquid instillation in a distal airway. Proteins in the instilled solution were traced with ^99mTc-albumin. ¹¹¹In, which binds to transferrin, was injected into the systemic circulation. Terbutaline was administered in the instilled solution or intra-peritoneally. Conventional ventilation was applied for 30?min followed by different ventilation strategies for 90?min: conventional ventilation, high-volume ventilation with or without 6?cmH₂O PEEP.

Measurements and main results

Protein fluxes across the alveolar and microvascular barriers were evaluated by scintigraphy. High-volume ventilation resulted in immediate leakage of ^99mTc-albumin from alveolar spaces and increased pulmonary uptake of systemic ¹¹¹In-transferrin. Terbutaline in the instilled solution and PEEP lessened alveolar ^99mTc-albumin leakage and pulmonary ¹¹¹In-transferrin uptake due to high-volume ventilation, whereas terbutaline given intra-peritoneally only lessened ¹¹¹In-transferrin uptake. Terbutaline in the instilled solution also lessened the increase in lung wet-to-dry weight ratio due to high-volume ventilation.

Conclusions

Terbutaline reduces protein fluxes across the alveolar epithelial and pulmonary microvascular barriers during high-volume ventilation in vivo. The route of administration may be important.

     

CD80 Is Upregulated in a Mouse Model with Shear Stress-Induced Atherosclerosis and Allows for Evaluating CD80-Targeting PET Tracers

Purpose

A shear stress-induced atherosclerosis mouse model was characterized for its expression of inflammation markers with focus on CD80. With this model, we evaluated two positron emission tomography (PET) radiotracers targeting CD80 as well as 2-deoxy-2-[¹⁸F]fluoro-d-mannose ([¹⁸F]FDM) in comparison with 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG).

Procedure

A flow constrictive cuff implanted around the common carotid artery in apolipoprotein E knockout mice resulted in plaque formation. CD80 expression levels and plaque histopathology were evaluated. Serial PET/X-ray computed tomography scans were performed to follow inflammation.

Results

Plaque formation with increased levels of CD80 was observed. Histologically, plaques presented macrophage-rich and large necrotic areas covered by a thin fibrous cap. Of the CD80-specific tracers, one displayed an increased uptake in plaques by PET. Both [¹⁸F]FDG and [¹⁸F]FDM accumulated in atherosclerotic plaques.

Conclusion

This mouse model presented, similar to humans, an increased expression of CD80 which renders it suitable for non-invasively targeting CD80-positive immune cells and evaluating CD80-specific radiotracers.

     

The Synergistic Effect of Selumetinib/Docetaxel Combination Therapy Monitored by [<Superscript>18</Superscript> F]FDG/[<Superscript>18</Superscript> F]FLT PET and Diffusion-Weighted Magnetic Resonance Imaging in a Colorectal Tumor Xenograft Model

Purpose

Positron emission tomography (PET) and diffusion-weighted MRI (DW-MRI) were used to characterize the treatment effects of the MEK1/2 inhibitor selumetinib (AZD6244), docetaxel, and their combination in HCT116 tumor-bearing mice on the molecular level.

Procedures

Mice were treated with vehicle, selumetinib (25 mg/kg), docetaxel (15 mg/kg), or a combination of both drugs for 7 days and imaged at four time points with 2-deoxy-2-[¹⁸?F]fluoro-D-glucose ([¹⁸?F]FDG) or 3′-deoxy-3′-[¹⁸?F]fluorothymidine ([¹⁸?F]FLT) followed by DW-MRI to calculate the apparent diffusion coefficient (ADC). Data was cross-validated using the Pearson correlation coefficient (PCC) and compared to histology (IHC).

Results

Each drug led to tumor growth inhibition but their combination resulted in regression. Separate analysis of PET or ADC could not provide significant differences between groups. Only PCC combined with IHC analysis revealed the highest therapeutic impact for combination therapy.

Conclusion

Combination treatment of selumetinib/docetaxel was superior to the respective mono-therapies shown by PCC of PET and ADC in conjunction with histology.

     

[<Superscript>89</Superscript>Zr]Trastuzumab: Evaluation of Radiation Dosimetry,Safety, and Optimal Imaging Parameters in Women with HER2-Positive Breast Cancer

Purpose

The purpose of the present study is to evaluate safety, human radiation dosimetry, and optimal imaging time of [⁸⁹Zr]trastuzumab in patients with HER2-positive breast cancer.

Procedures

Twelve women with HER2-positive breast cancer underwent [⁸⁹Zr]trastuzumab positron emission tomography (PET)/X-ray computed tomography (CT) twice within 7 days post-injection. Biodistribution data from whole-torso PET/CT images and organ time-activity curves were created using data from all patients. Human dosimetry was calculated using OLINDA with the adult female model.

Results

High-quality images and the greatest tumor-to-nontumor contrast were achieved with images performed 5?±?1 day post-injection. Increased [⁸⁹Zr]trastuzumab uptake was seen in at least one known lesion in ten patients. The liver was the dose-limiting organ (retention of ～12 % of the injected dose and average dose of 1.54 mSv/MBq). The effective dose was 0.47 mSv/MBq. No adverse effects of [⁸⁹Zr]trastuzumab were encountered.

Conclusion

[⁸⁹Zr]trastuzumab was safe and optimally imaged at least 4 days post-injection. The liver was the dose-limiting organ.

     

Comparison of Somatostatin Receptor 2-Targeting PET Tracers in the Detection of Mouse Atherosclerotic Plaques

Purpose

Rupture-prone atherosclerotic plaques are characterized by accumulation of macrophages, which have shown to express somatostatin type 2 receptors. We aimed to investigate whether somatostatin receptor-targeting positron emission tomography (PET) tracers, [⁶⁸Ga]DOTANOC, [¹⁸F]FDR-NOC, and [⁶⁸Ga]DOTATATE, can detect inflamed atherosclerotic plaques.

Procedures

Atherosclerotic IGF-II/LDLR^?/?ApoB^100/100 mice were studied in vivo and ex vivo for tracer uptake into atherosclerotic plaques. Furthermore, [⁶⁸Ga]DOTANOC and [⁶⁸Ga]DOTATATE were compared in a head-to-head setting for in vivo PET/X-ray computed tomography (CT) imaging characteristics.

Results

Ex vivo uptake of [⁶⁸Ga]DOTANOC and [⁶⁸Ga]DOTATATE in the aorta was higher in atherosclerotic mice compared to control C57Bl/6N mice, while the aortic uptake of [¹⁸F]FDR-NOC showed no genotype difference. Unlike [¹⁸F]FDR-NOC, [⁶⁸Ga]DOTANOC and [⁶⁸Ga]DOTATATE showed preferential binding to atherosclerotic plaques with plaque-to-wall ratio of 1.7?±?0.3 and 2.1?±?0.5, respectively. However, the aortic uptake and aorta-to-blood ratio of [⁶⁸Ga]DOTANOC were higher compared to [⁶⁸Ga]DOTATATE in in vivo PET/CT imaging.

Conclusion

Our results demonstrate superior applicability for [⁶⁸Ga]DOTANOC and [⁶⁸Ga]DOTATATE in the detection of atherosclerotic plaques compared to [¹⁸F]FDR-NOC.

     

Preclinical Characterization of the Phosphodiesterase 10A PET Tracer [<Superscript>11</Superscript>C]MK-8193

Purpose

A positron emission tomography (PET) tracer for the enzyme phosphodiesterase 10A (PDE10A) is desirable to guide the discovery and development of PDE10A inhibitors as potential therapeutics. The preclinical characterization of the PDE10A PET tracer [¹¹C]MK-8193 is described.

Procedures

In vitro binding studies with [³H]MK-8193 were conducted in rat, monkey, and human brain tissue. PET studies with [¹¹C]MK-8193 were conducted in rats and rhesus monkeys at baseline and following administration of a PDE10A inhibitor.

Results

[³H]MK-8193 is a high-affinity, selective PDE10A radioligand in rat, monkey, and human brain tissue. In vivo, [¹¹C]MK-8193 displays rapid kinetics, low test-retest variability, and a large specific signal that is displaced by a structurally diverse PDE10A inhibitor, enabling the determination of pharmacokinetic/enzyme occupancy relationships.

Conclusions

[¹¹C]MK-8193 is a useful PET tracer for the preclinical characterization of PDE10A therapeutic candidates in rat and monkey. Further evaluation of [¹¹C]MK-8193 in humans is warranted.

     

Evaluation of Quantitative Imaging Biomarkers in the DSS Colitis Model

Purpose

In humans, colonoscopy is the gold standard for the diagnosis of inflammatory changes of the colon wall. Aim of this study was the identification of less invasive imaging biomarkers in the dextran sodium sulfate (DSS) colitis model to provide additional information on transmural changes of the colon wall.

Procedures

Colitis was induced in C57BL/6 mice by administration of 2, 3, and 4 % DSS over a period of 5 days. Colon wall thickness was measured using magnetic resonance imaging (MRI), ultrasound (US), and x-ray computed tomography (CT), gut inflammation by positron emission tomography/CT, and mucosal changes of the colon wall by colonoscopy. Colon samples were examined histologically.

Results

MRI, CT, US, and histological data revealed increased colon wall thickness in DSS-treated mice compared to healthy controls. Elevated 2-deoxy-2[¹⁸F]fluoro-d-glucose uptake and colonoscopy confirmed high inflammatory load in the guts of colitis mice.

Conclusions

The established quantitative imaging readouts offer promising perspectives to develop new compounds and to translate these methods into the clinical setting.

     

Repeatability of [<Superscript>68</Superscript>Ga]DKFZ11-PSMA PET Scans for Detecting Prostate-specific Membrane Antigen-positive Prostate Cancer

Purpose

We studied the effect of varying specific activity of [⁶⁸Ga]DKFZ-PSMA11 ([⁶⁸Ga]DP11) on repeated imaging of prostate-specific membrane antigen-positive (PSMA+) xenograft tumors.

Procedures

Athymic nude mice bearing PC3-PIP (PSMA+) and PC3 (PSMA?) bilateral flank tumors were assessed to study intra- and inter-day repeatability of [⁶⁸Ga]DP11 imaging in mice administered [⁶⁸Ga]DP11 or [⁶⁷Ga]DP11 (as a dilution tracer) using imaging and biodistribution studies.

Results

Region of interest (ROI) analysis of the [⁶⁸Ga]DP11 imaging study indicated that the uptake was constant on the same day or consecutive days. Prior imaging with [⁶⁸Ga]DP11 did not significantly influence the subsequent uptake of [⁶⁸Ga]DP11. Uptake of [⁶⁸Ga]DP11 (60 min) and [⁶⁷Ga]DP11 (24 h) in PC3-PIP tumors was 12.37 ± 4.19 %ID/g and 12.49 ± 6.88 %ID/g, respectively; [⁶⁸Ga]DP11 was 13.83 ± 3.77 and 17.76 ± 1.84 on same-day and 15.98 ± 5.82 %ID/g on second-day imaging.

Conclusions

This study demonstrates that [⁶⁸Ga]DP11, in a given PSMA+ lesion, is constant under several same-day or serial-day imaging conditions.

     

<Emphasis Type="Italic">In Vivo</Emphasis> Stabilized SB3, an Attractive GRPR Antagonist,for Pre- and Intra-Operative Imaging for Prostate Cancer

Purpose

The gastrin-releasing peptide receptor (GRPR), overexpressed on various tumor types, is an attractive target for receptor-mediated imaging and therapy. Another interesting approach would be the use of GRPR radioligands for pre-operative imaging and subsequent radio-guided surgery, with the goal to improve surgical outcome. GRPR radioligands were successfully implemented in clinical studies, especially Sarabesin 3 (SB3) is an appealing GRPR antagonist with high receptor affinity. Gallium-68 labeled SB3 has good in vivo stability, after labeling with Indium-111; however, the molecule shows poor in vivo stability, which negatively impacts tumor-targeting capacity. A novel approach to increase in vivo stability of radiopeptides is by co-administration of the neutral endopeptidase (NEP) inhibitor, phosphoramidon (PA). We studied in vivo stability and biodistribution of [¹¹¹In]SB3 without/with (?/+) PA in mice. Furthermore, SPECT/MRI on a novel, state-of-the-art platform was performed.

Procedures

GRPR affinity of SB3 was determined on PC295 xenograft sections using [¹²⁵I]Tyr⁴-bombesin with tracer only or with increasing concentrations of SB3. For in vivo stability, mice were injected with 200/2000 pmol [¹¹¹In]SB3 ?/+ 300 μg PA. Blood was collected and analyzed. Biodistribution and SPECT/MRI studies were performed at 1, 4, and 24 h postinjection (p.i.) of 2.5 MBq/200 pmol or 25 MBq/200 pmol [¹¹¹In]SB3 ?/+ 300 μg PA in PC-3-xenografted mice.

Results

SB3 showed high affinity for GRPR (IC₅₀ 3.5 nM). Co-administration of PA resulted in twice higher intact peptide in vivo vs [¹¹¹In]SB3 alone. Biodistribution studies at 1, 4, and 24 h p.i. show higher tumor uptake values with PA co-administration (19.7?±?3.5 vs 10.2?±?1.5, 17.6?±?5.1 vs 8.3?±?1.1, 6.5?±?3.3 vs 3.1?±?1.9 % ID/g tissue (P?<?0.0001)). Tumor imaging with SPECT/MRI clearly improved after co-injection of PA.

Conclusions

Co-administration of PA increased in vivo tumor targeting capacity of [¹¹¹In]SB3, making this an attractive combination for GRPR-targeted tumor imaging.

     

<Emphasis Type="Italic">In Vivo</Emphasis> Evaluation of a Bombesin Analogue Labeled with Ga-68 and Co-55/57

Purpose

The purpose of this study was to apply an analogue of bombesin, NOTA-AMBA, labeled with Co-55 or Ga-68, for preclinical imaging of prostate cancer.

Procedures

The peptide NOTA-AMBA was labeled with Ga-68 or Co-55 by microwave irradiation. Biodistribution in xenograft mice (PC3) was performed at 1, 4, and 24 h (only cobalt at 24 h) using a fixed amount of peptide. Four weeks post-inoculation, xenograft mice were positron emission tomography/X-ray computed tomography scanned after tail vein injection of [⁶⁸Ga]NOTA-AMBA or [⁵⁵Co]NOTA-AMBA.

Results

Labeling with Ga-68 and Co-55/57 was achieved in yields greater than 90 %. A radiochemical purity (RCP) of 95 and 90 % were obtained for Ga-68 and Co-55, respectively. Both radiopeptides showed high uptake in the intestines, stomach, pancreas, and in the tumor ([⁶⁸Ga]NOTA-AMBA, 10.3 %ID/g at 1 h to 6.4 %ID/g at 4 h; [⁵⁷Co]NOTA-AMBA, 8.2 %ID/g at 1 h to 5.3%ID/g at 24 h). Normal tissue cleared over time improving tumor-to-background ratios.

Conclusions

NOTA-AMBA was labeled in high yields and RCP with Ga-68 and Co-55/57. High tumor uptake in a subcutaneous mouse prostate cancer model was observed. At 24 h, [^55/57Co]NOTA-AMBA showed better tumor-to-organ ratios than [⁶⁸Ga]NOTA-AMBA at both 1 and 4 h post-injection. Hence, for imaging, [⁵⁵Co]NOTA-AMBA was found to be superior compared to [⁶⁸Ga]NOTA-AMBA.