Effects of Chelator Modifications on 68Ga-Labeled [Tyr3]Octreotide Conjugates

Purpose

Somatostatin receptors (SSTR) have been reported as promising targets for imaging agents for cancer. Recently, ⁶⁸Ga-DOTATOC-based PET imaging has been used successfully for diagnosis and management of SSTR-expressing tumors. The purpose of this study was to evaluate the influence of chelator modifications and charge on ⁶⁸Ga-labeled peptide conjugates.

Procedures

We have synthesized a series of [Tyr³]octreotide conjugates that consisted of different NOTA-based chelators with two to five carboxylate moieties, and compared our results with ⁶⁸Ga-DOTATOC in both in vitro and in vivo studies.

Results

With the exception of ⁶⁸Ga-1 (three carboxylates), the increased number of carboxylates on the NOTA-based chelators resulted in a reduced binding affinity and internalization. Additionally, the tumor uptake for ⁶⁸Ga-2 (four carboxylates) and ⁶⁸Ga-3 (five carboxylates) was reduced compared to that of ⁶⁸Ga-DOTATOC (three carboxylates) and ⁶⁸Ga-NO2ATOC (two carboxylates) and ⁶⁸Ga-1 (three carboxylates) at 2 h p.i. suggesting the presence of an optimal charge for this compound.

Conclusions

Chelator modifications can lead to the altered pharmacokinetics. These results may impact further design considerations for peptide-based imaging agents.     

Imaging Characteristics and First Experience of [<Superscript>68</Superscript>Ga]THP-PSMA,a Novel Probe for Rapid Kit-Based Ga-68 Labeling and PET Imaging: Comparative Analysis with [<Superscript>68</Superscript>Ga]PSMA I&amp;T

Purpose

[⁶⁸Ga]Trishydroxypyridinone (THP)–prostate-specific membrane antigen (PSMA) is a novel tracer that can be labeled in one step by cold reconstitution of a kit with unprocessed generator eluate, targeting PSMA via the lysine-urea-glutamate (KuE) motif. The aim of this study was to evaluate the human imaging characteristics of [⁶⁸Ga]THP-PSMA.

Procedures

[⁶⁸Ga]THP-PSMA positron emission tomography (PET)/x-ray computed tomography (CT) was performed in 25 patients with biochemical recurrence after radical prostatectomy for prostate cancer. Urinary and biliary excretion and tumor lesion uptake were quantified using standardized uptake values (SUVs). Imaging characteristics were assessed in terms of non-target organ uptake, background activity, target-to-background ratios (TBRs) of tumor lesions, and frequency of bladder halo artifacts. Findings were compared to a matched cohort of 25 patients undergoing PET/CT with the established agent [⁶⁸Ga]PSMA I&T.

Results

Physiologic uptake of [⁶⁸Ga]THP-PSMA was significantly lower in salivary glands (P?<?0.0001), liver (P?<?0.0001), spleen (P?<?0.0001), and kidneys (P?<?0.0001) than with [⁶⁸Ga]PSMA I&T. While biliary tracer excretion of [⁶⁸Ga]THP-PSMA was negligible, urinary tracer excretion of [⁶⁸Ga]THP-PSMA was fast, and significantly higher than for [⁶⁸Ga]PSMA I&T, contributing to a higher frequency of bladder artifacts. Malignant lesion uptake of [⁶⁸Ga]THP-PSMA assessed as either SUV or TBR was significantly lower than with [⁶⁸Ga]PSMA I&T.

Conclusion

[⁶⁸Ga]THP-PSMA yields suitable in vivo uptake characteristics. The simplified synthesis method for [⁶⁸Ga]THP-PSMA may facilitate wider application and higher patient throughput with PSMA imaging. However, direct intraindividual comparison studies are needed to assess the relative performance of [⁶⁸Ga]THP-PSMA vs other PSMA ligands in terms of clinical detection rate and image quality.

     

Radiolabeling and Preliminary Evaluation of Ga-68 Labeled NODAGA-Ubiquicidin Fragments for Prospective Infection Imaging

Purpose

The present work was aimed at the development of prospective positron emission tomography (PET) agents for infection imaging. Towards this aim, ubiquicidin (UBI) fragments conjugated with the macrocyclic NODAGA chelator were radiolabeled with Ga-68 and evaluated.

Procedures

Conformations of custom synthesized NODAGA-UBI (29–41) and NODAGA-UBI (31–38) conjugates were compared with UBI (29–41) by circular dichroism (CD) spectroscopy. Optimization of labeling of NODAGA conjugates of UBI peptides with Ga-68 was performed and quality control analysis was carried out by chromatography techniques. In vitro uptake of [⁶⁸Ga] NODAGA-UBI (29–41) and [⁶⁸Ga]NODAGA-UBI (31–38) was studied in Staphylococcus aureus cells. In vivo distribution of [⁶⁸Ga]GaCl₃ and [⁶⁸Ga]NODAGA-UBI complexes was performed in normal Swiss mice.

Results

Conformations of NODAGA-UBI (29–41) and NODAGA-UBI (31–38) conjugates were found to be similar to UBI (29–41). NODAGA-UBI conjugates could be consistently labeled with Ga-68 in high radiochemical yields (>95 %) with high radiochemical purity (>95 %). [⁶⁸Ga]NODAGA-UBI (29–41) and [⁶⁸Ga]NODAGA-UBI (31–38) complexes showed retention time of 14 and 14.5 min, respectively, by HPLC radiochromatogram. Specific uptake of [⁶⁸Ga]NODAGA-UBI fragments was observed in S.aureus cells. Greater than 64 % of the injected dose was cleared via the renal route at 1 h post injection, and no significant uptake in vital organs of mice was observed with both the agents.

Conclusion

This is the first report on Ga-68 labeled NODAGA-UBI fragments for infection imaging and the agents hold tremendous prospect in PET imaging.

     

64Cu- and 68Ga-Labelled [Nle14,Lys40(Ahx-NODAGA)NH2]-Exendin-4 for Pancreatic Beta Cell Imaging in Rats

Purpose

Glucagon-like peptide-1 receptor (GLP-1R) is a molecular target for imaging of pancreatic beta cells. We compared the ability of [Nle¹⁴,Lys⁴⁰(Ahx-NODAGA-⁶⁴Cu)NH₂]-exendin-4 ([⁶⁴Cu]NODAGA-exendin-4) and [Nle¹⁴,Lys⁴⁰(Ahx-NODAGA-⁶⁸Ga)NH₂]-exendin-4 ([⁶⁸Ga]NODAGA-exendin-4) to detect native pancreatic islets in rodents.

Procedures

The stability, lipophilicity and affinity of the radiotracers to the GLP-1R were determined in vitro. The biodistribution of the tracers was assessed using autoradiography, ex vivo biodistribution and PET imaging. Estimates for human radiation dosimetry were calculated.

Results

We found GLP-1R-specific labelling of pancreatic islets. However, the pancreas could not be visualised in PET images. The highest uptake of the tracers was observed in the kidneys. Effective dose estimates for [⁶⁴Cu]NODAGA-exendin-4 and [⁶⁸Ga]NODAGA-exendin-4 were 0.144 and 0.012 mSv/MBq, respectively.

Conclusion

[⁶⁴Cu]NODAGA-exendin-4 might be more effective for labelling islets than [⁶⁸Ga]NODAGA-exendin-4. This is probably due to the lower specific radioactivity of [⁶⁸Ga]NODAGA-exendin-4 compared to [⁶⁴Cu]NODAGA-exendin-4. The radiation dose in the kidneys may limit the use of [⁶⁴Cu]NODAGA-exendin-4 as a clinical tracer.     

Development of Single Vial Kits for Preparation of 68Ga-Labelled Peptides for PET Imaging of Neuroendocrine Tumours

Purpose

The present work was aimed at the formulation and evaluation of freeze-dried kits of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-peptides for the preparation of ⁶⁸Ga-labelled peptides for PET imaging of neuroendocrine tumours. The ⁶⁸GaCl₃ was obtained from the locally produced nanoceria-PAN, composite-sorbent-based ⁶⁸Ge/⁶⁸Ga generator.

Procedures

Single vial kits of somatostatin analogues DOTA-[Tyr³]-octreotide (DOTA-TOC), DOTA-[NaI³]-octreotide (DOTA-NOC) and DOTA-Tyr³-Thre⁸-octreotide (DOTA-TATE) were formulated. Optimization of radiolabelling with ⁶⁸Ga from the in-house generator, characterization, long term evaluation of stability of kits and bioevaluation studies in animals was carried out.

Results

DOTA-TOC, DOTA-NOC and DOTA-TATE kits could be successfully formulated. Consistently high radiochemical yields (>95 %) were obtained on radiolabelling with ⁶⁸Ga. The radiolabelled peptides exhibited excellent in vitro stability. Biodistribution studies in normal non-tumour bearing Swiss mice revealed fast clearance of activity via renal route as reported for the respective peptides.

Conclusion

Availability of ready to use DOTA-peptide kits in conjunction with ⁶⁸Ge/⁶⁸Ga generators would pave way for the establishment of ⁶⁸Ga radiopharmacy, a long-felt need of the nuclear medicine community.     

In Vitro and In Vivo Structure–Property Relationship of 68Ga-Labeled Schiff Base Derivatives for Functional Myocardial PET Imaging

Purpose

SPECT (e.g., with ^99mTc-sestamibi) is routinely used for imaging myocardial damage, even though PET could offer a higher spatial resolution. Using the generator-gained isotope ⁶⁸Ga would allow a rapid supply of the tracer in the diagnostic unit. For this reason, the aim of the study was to develop ⁶⁸Ga-labeled PET tracers based on different Schiff base amines and to evaluate the cardiomyocyte uptake in vitro as well as the biodistribution of the tracers in vivo.

Procedures

Fifteen different Schiff bases (basing on 3 different backbones) were synthesized and labeled with ⁶⁸Ga. Lipophilicity varied between 0.87?±?0.24 and 2.72?±?0.14 (logD value). All tracers were positively charged and stable in plasma and apo-transferrin solution. In vitro uptake into cardiomyocytes was assessed in HL-1 cells in the absence and presence of the ionophor valinomycin. In vivo accumulation in the heart and in various organs was assessed by small animal PET imaging as well as by ex vivo biodistribution. The results were compared with ^99mTc-sestamibi and ¹⁸F-flurpiridaz.

Results

All cationic Schiff bases were taken up into cardiomyocytes but the amount varied by a factor of 10. When destroying the membrane potential, the cellular uptake was markedly reduced in most of the tracers, indicating the applicability of these tracers for identifying ischemic myocardium. PET imaging revealed that the in vivo myocardial uptake reached a constant value approximately 10 min after injection but the intracardial amount of the tracer varied profoundly (SUV 0.46 to 3.35). The most suitable tracers showed a myocardial uptake which was comparable to that of ^99mTc-sestamibi.

Conclusions

⁶⁸Ga-based Schiff bases appear suitable for myocardial PET images with uptake comparable to ^99mTc-sestamibi but offering higher spatial resolution. By systematical variation of the backbone and the side chains, tracers with optimal properties can be identified for further clinical evaluation.     

<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.

     

Suitability of [18F]Altanserin and PET to Determine 5-HT2A Receptor Availability in the Rat Brain: In Vivo and In Vitro Validation of Invasive and Non-Invasive Kinetic Models

Purpose

While the selective 5-hydroxytryptamine type 2a receptor (5-HT_2AR) radiotracer [¹⁸F]altanserin is well established in humans, the present study evaluated its suitability for quantifying cerebral 5-HT_2ARs with positron emission tomography (PET) in albino rats.

Procedures

Ten Sprague Dawley rats underwent 180 min PET scans with arterial blood sampling. Reference tissue methods were evaluated on the basis of invasive kinetic models with metabolite-corrected arterial input functions. In vivo 5-HT_2AR quantification with PET was validated by in vitro autoradiographic saturation experiments in the same animals.

Result

Overall brain uptake of [¹⁸F]altanserin was reliably quantified by invasive and non-invasive models with the cerebellum as reference region shown by linear correlation of outcome parameters. Unlike in humans, no lipophilic metabolites occurred so that brain activity derived solely from parent compound. PET data correlated very well with in vitro autoradiographic data of the same animals.

Conclusion

[¹⁸F]Altanserin PET is a reliable tool for in vivo quantification of 5-HT_2AR availability in albino rats. Models based on both blood input and reference tissue describe radiotracer kinetics adequately. Low cerebral tracer uptake might, however, cause restrictions in experimental usage.     

Preliminary PET/CT Imaging with Somatostatin Analogs [<Superscript>68</Superscript>Ga]DOTAGA-TATE and [<Superscript>68</Superscript>Ga]DOTAGA-TOC

Purpose

Somatostatin receptor positron emission tomography/X-ray computed tomography (SSTR-PET/CT) is a well-established technique for staging and detection of neuroendocrine tumors (NETs). Ga-68-labeled DOTA-conjugated octreotide analogs are the privileged radiotracers for diagnosis and therapeutic monitoring of NETs. Hence, we were interested in assessing the influence of promising, newer variant DOTAGA on the hydrophilicity, pharmacokinetics, and lesion pick-up of somatostatin analogs. Herein, the potential of ([⁶⁸Ga]DOTAGA, Tyr³, Thr⁸) octreotide ([⁶⁸Ga]DOTAGA-TATE) and ([⁶⁸Ga]DOTAGA, Tyr³) octreotide ([⁶⁸Ga]DOTAGA-TOC) as NET imaging agents has been investigated.

Procedures

Amenability of [⁶⁸Ga]DOTAGA-(TATE/TOC) to kit-type formulation has been demonstrated. Biodistribution studies were carried out in normal rats at 1 h post-injection (p.i.). [⁶⁸Ga]DOTAGA-(TATE/TOC) PET/CT scans were carried out in patients (70–170 MBq, 1 h p.i.) with histologically confirmed well-differentiated NETs.

Results

[⁶⁸Ga]DOTAGA-TATE exhibited hydrophilicity similar to [⁶⁸Ga]DOTA-TATE (log P = ?3.51 vs ?3.69) whereas [⁶⁸Ga]DOTAGA-TOC was more hydrophilic than [⁶⁸Ga]DOTA-TOC (log P = ?3.27 vs ?2.93). [⁶⁸Ga]DOTAGA-TATE and [⁶⁸Ga]DOTA-TATE showed almost identical blood and kidney uptake in normal rats whereas significantly fast clearance (p < 0.05) of [⁶⁸Ga]DOTAGA-TATE was observed from other non-specific organs (liver, lungs, spleen, intestine). [⁶⁸Ga]DOTAGA-TOC also demonstrated rapid clearance from blood and kidneys (p < 0.05) in comparison to [⁶⁸Ga]DOTA-TOC. The metastatic lesions in NET patients were well identified by [⁶⁸Ga]DOTAGA-TATE and [⁶⁸Ga]DOTAGA-TOC.

Conclusion

The phenomenal analogy was observed between [⁶⁸Ga]DOTAGA-TATE and [⁶⁸Ga]DOTA-TATE as well as between [⁶⁸Ga]DOTAGA-TOC and [⁶⁸Ga]DOTA-TOC in biodistribution studies in rats. The good lesion detection ability of the two radiotracers indicates their potential as NET imaging radiotracers.

     

Synthesis and Evaluation of 13N-Labelled Azo Compounds for β-Amyloid Imaging in Mice

Purpose

The aim of the present study was to develop short half-lived tools for in vitro and in vivo β-amyloid imaging in mice, for which no suitable PET tracers are available.

Procedures

Five ¹³N-labelled azo compounds (1–5) were synthesized using a three-step process using cyclotron-produced [¹³N]NO₃ ^?. Biodistribution studies were performed using positron emission tomography–computed tomography (PET–CT) on 20-month-old healthy, wild-type (WT) mice. In vivo and in vitro binding assays were performed using PET-CT and autoradiography, respectively, on 20-month-old healthy (WT) mice and transgenic (Tg2576) Alzheimer's disease model mice.

Results

¹³N-labelled azo compounds were prepared with decay corrected radiochemical yields in the range 27?±?4 % to 39?±?4 %. Biodistribution studies showed good blood–brain barrier penetration for compounds 1 and 3–5; good clearance data were also obtained for compounds 1–3 and 5. Compounds 2, 3 and 5 (but not 1) showed a significant uptake in β-amyloid-rich structures when assayed in in vitro autoradiographic studies. PET studies showed significant uptake of compounds 2 and 3 in the cortex of transgenic animals that exhibit β-amyloid deposits.

Conclusions

The results underscore the potential of compounds 2 and 3 as in vitro and in vivo markers for β-amyloid in animal models of Alzheimer's disease.     

Real-Time Assessment of Ultrasound-Mediated Drug Delivery Using Fibered Confocal Fluorescence Microscopy

Purpose

Transport across the plasma membrane is a critical step of drug delivery for weakly permeable compounds with intracellular mode of action. The purpose of this study is to demonstrate real-time monitoring of ultrasound (US)-mediated cell-impermeable model drug uptake with fibered confocal fluorescence microscopy (FCFM).

Procedures

An in vitro setup was designed to combine a mono-element US transducer, a cell chamber with a monolayer of tumor cells together with SonoVue microbubbles, and a FCFM system. The cell-impermeable intercalating dye, SYTOX Green, was used to monitor US-mediated uptake.

Results

The majority of the cell population showed fluorescence signal enhancement 10 s after US onset. The mean rate constant k of signal enhancement was calculated to be 0.23?±?0.04 min^?1.

Conclusions

Feasibility of real-time monitoring of US-mediated intracellular delivery by FCFM has been demonstrated. The method allowed quantitative assessment of model drug uptake, holding great promise for further local drug delivery studies.     

Metabolism of Radiolabeled Methionine in Hepatocellular Carcinoma

Purpose

Radiolabeled methionine (Met) promises to be useful in the positron emission tomography (PET) imaging of hepatocellular carcinoma (HCC). However, its metabolic routes in HCC have not yet been fully understood. In this study, the metabolic pathway(s) of radiolabeled Met in HCC were investigated.

Procedures

To simulate the rapid blood clearance of radiolabeled Met, pulse–chase experiments were conducted. l-[methyl-³H]-Met or l-[1-¹⁴C]-Met was pulsed over control or cycloheximide-treated WCH17 cells and rat hepatocytes for 5 min and chased with cold media. The water-soluble, lipid-soluble, DNA, RNA, and protein phases were subsequently extracted and measured from the acid-precipitable and acid-soluble fractions of whole cells. The radioactive metabolites Met, S-adenosylmethionine (SAM), S-adenosylhomocysteine, Met sulfoxide, and Met sulfone were further separated by radio thin layer chromatography.

Results

(1) The uptake of l-[methyl-³H]-Met in both cell types was higher than that of l-[1-¹⁴C]-Met. In rat hepatocytes, the uptake of l-[methyl-³H]-Met was significantly higher than that of l-[1-¹⁴C]-Met, which may contribute to its physiologic accumulation in surrounding hepatic tissues seen in PET imaging of HCC using l-[methyl-¹¹C]-Met. Compared to rat hepatocytes, WCH17 cells had significantly higher uptake of both radiotracers. (2) For l-[methyl-³H]-Met, the major intracellular uptake was found mostly in the protein phase and, to a lesser degree, in the phosphatidylethanolamine (PE) methylation pathway, which is fairly stabilized within the 55-min chase period (the main metabolites were SAM, Met, Met sulfoxide, and Met sulfone). In contrast, the uptake of Met in rat hepatocytes mainly points to phosphatidylcholine (PC) synthesis through the PE methylation pathway (the main metabolite was PC). (3) Both cell types incorporated l-[1-¹⁴C]-Met predominantly into protein synthesis. (4) Finally, when the protein synthesis pathway was inhibited, the incorporation of SAM derived from l-[methyl-³H]-Met to lipid class (PC was the main metabolite) occurred at a reduced rate in WCH17 cells, suggesting that the route may be impaired in HCC.

Conclusions

This study demonstrated that different metabolic pathways of radiolabeled Met exist between HCC and surrounding hepatic tissue and contribute to the patterns of increased uptake of radiolabeled Met in HCC.     

In Vitro and In Vivo Characterization of Two C-11-Labeled PET Tracers for Vesicular Acetylcholine Transporter

Purpose

The vesicular acetylcholine transporter (VAChT) is a specific biomarker for imaging presynaptic cholinergic neurons. Herein, two potent and selective ¹¹C-labeled VAChT inhibitors were evaluated in rodents and nonhuman primates for imaging VAChT in vivo.

Procedures

For both (?)-[¹¹C]2 and (?)-[¹¹C]6, biodistribution, autoradiography, and metabolism studies were performed in male Sprague Dawley rats. Positron emission tomography (PET) brain studies with (?)-[¹¹C]2 were performed in adult male cynomolgus macaques; 2 h dynamic data was acquired, and the regions of interest were drawn by co-registration of the PET images with the MRI.

Results

The resolved enantiomers (?)-2 and (?)-6 were very potent and selective for VAChT in vitro (K _i ?35-fold selectivity for VAChT vs. σ receptors); both radioligands, (?)-[¹¹C]2 and (?)-[¹¹C]6, demonstrated high accumulation in the VAChT-enriched striatum of rats. (?)-[¹¹C]2 had a higher striatum to cerebellum ratio of 2.4-fold at 60 min; at 30 min, striatal uptake reached 0.550?±?0.086 %ID/g. Uptake was also specific and selective; following pretreatment with (±)-2, striatal uptake of (?)-[¹¹C]2 in rats at 30 min decreased by 50 %, while pretreatment with a potent sigma ligand had no significant effect on striatal uptake in rats. In addition, (?)-[¹¹C]2 displayed favorable in vivo stability in rat blood and brain. PET studies of (?)-[¹¹C]2 in nonhuman primates indicate that it readily crosses the blood-brain barrier (BBB) and provides clear visualization of the striatum; striatal uptake reaches the maximum at 60 min, at which time the target to nontarget ratio reached ~2-fold.

Conclusions

The radioligand (?)-[¹¹C]2 has high potential to be a suitable PET radioligand for imaging VAChT in the brain of living subjects.     

Diagnostic performance of somatostatin receptor PET/CT using 68Ga-DOTANOC in gastrinoma patients with negative or equivocal CT findings

Purpose

Contrast-enhanced CT (CECT) is a standard investigative procedure in the localization of gastrinomas. Small tumors are often missed and metastatic lesions may remain occult on CT. The purpose of present study was to prospectively evaluate the diagnostic performance of ⁶⁸Ga-labeled [1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid]-1-NaI³-Octreotide (⁶⁸Ga-DOTANOC) positron emission tomography/computed tomography (PET/CT) in gastrinoma patients with negative or equivocal CT findings.

Methods

Twenty-five patients (age 46.6 ± 13.3 years; male 60%) with clinical/biochemical diagnosis of gastrinoma and negative or equivocal findings on CECT were prospectively evaluated. All of them underwent ⁶⁸Ga-DOTANOC PET/CT which was evaluated by two nuclear medicine physicians in consensus. Combination of histopathology, serum gastrin, endoscopy, and follow-up imaging were taken as reference standard.

Results

⁶⁸Ga-DOTANOC PET/CT was positive in 17 patients and negative in 8 patients, yielding an overall detection rate of 68%. It was positive 13/20 patients who underwent baseline evaluation and in 4/5 post-treatment patients. Of the 11 patients who had a negative CT result, ⁶⁸Ga-DOTANOC PET/CT was positive in four cases (detection rate 36.4%), while it was abnormal in 13/14 patients who had equivocal CT findings (detection rate 92.8%). Diagnostic performance of ⁶⁸Ga-DOTANOC PET/CT was superior in patients with equivocal CECT findings than that in patients with negative CECT (P = 0.010).

Conclusion

⁶⁸Ga-DOTANOC PET/CT appears to be useful in patients with gastrinoma with negative or equivocal results on CECT, especially the latter group.     

[<Superscript>68</Superscript>Ga]PSMA-HBED-CC Uptake in Osteolytic,Osteoblastic, and Bone Marrow Metastases of Prostate Cancer Patients

Purpose

The aim of this study was to evaluate potential differences in “Glu-NH-CO-NH-Lys” radio-labeled with [⁶⁸Ga]gallium N,N-bis[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine-N,N-diacetic acid ([⁶⁸Ga]PSMA-HBED-CC) uptake in osteolytic, osteoblastic, mixed, and bone marrow metastases in prostate cancer (PC) patients.

Procedures

This retrospective study was approved by the local ethics committee. Patients who received [⁶⁸Ga]PSMA-HBED-CC positron emission tomography/computed tomography ([⁶⁸Ga]PSMA-PET/CT) with at least one positive bone metastasis were included in this study. Only patients who have not received systemic therapy for their PC were included. Bone metastases had to be confirmed by at least one other imaging modality or follow-up investigation. The maximum standardized uptake value (SUV_max) and mean Hounsfield units (HU_mean) of each metastasis were measured. Based on CT, each metastasis was classified as osteolytic (OL), osteoblastic (OB), bone marrow (BM), or mixed (M).

Results

One hundred fifty-four bone metastases in 30 patients were evaluated. Eighty out of 154 (51.9%) metastases were classified as OB, 21/154 (13.6%) as OL, 23/154 (14.9%) as M, and 30/154 (19.5%) as BM. The SUV_max for the different types of metastases were 10.6 ± 7.07 (OB), 24.0 ± 19.3 (OL), 16.0 ± 21.0 (M), and 14.7 ± 9.9 (BM). The SUV_max of OB vs. OL and OB vs. BM metastases differed significantly (p ≤ 0.025). A significant negative correlation between HU_mean and SUV_max (r = ?0.23, p < 0.05) was measured.

Conclusions

[⁶⁸Ga]PSMA-HBED-CC uptake is higher in osteolytic and bone marrow metastases compared to osteoblastic metastases. Information derived from [⁶⁸Ga]PSMA-PET and CT complement each other for the reliable diagnosis of the different types of bone metastases in PC patients.

     

Biodistribution of [<Superscript>68</Superscript>Ga]PSMA-HBED-CC in Patients with Prostate Cancer: Characterization of Uptake in Normal Organs and Tumour Lesions

Purpose

The aim of this study was to determine the physiological and pathophysiological biodistribution of [⁶⁸Ga]PSMA-HBED-CC (PSMA-11) ([⁶⁸Ga]PSMA) in patients with prostate cancer (PCA) to establish the range of normal uptake in relevant organs and primary prostate tumours, locally recurrent PCA, lymph and bone metastases and other metastatic lesions. Additionally, we aimed to determine a cut-off uptake value for differentiation of primary tumours from normal prostate tissue.

Procedures

Overall, [⁶⁸Ga]PSMA positron emission tomography/x-ray computed tomography (PET/CT) of 101 patients (mean age 69.1 years) with PCA was analysed retrospectively. For assessment of tracer biodistribution, maximum standardized uptake values (SUVmax) were calculated for various normal organs, as well as for primary tumours (PT) and/or metastases. Results are presented as median, interquartile range (IQR; 25th quantil–75th quantil) and range (minimum–maximum).

Results

[⁶⁸Ga]PSMA PET/CT was performed 50 min (range 30–126) after injection of 109 MBq (range 84–158). Regarding biodistribution, highest uptake (median/IQR/range) of the tracer was found in the kidneys (49.6/40.7–57.6/2.7–97.0) followed by the submandibular glands (17.3/13.7–21.2/7.5–30.4), parotid glands (16.1/12.2–19.8/5.5–30.9) and duodenum (13.8/10.5–17.2/5.8–26.9). The best cut-off value for differentiating physiological uptake in the primary tumour from that in the prostate was found to be an SUVmax of 3.2. The median SUVmax in the PT (n?=?35), locally recurrent PCA (n?=?8), lymph node (n?=?166), bone (n?=?157) and other metastases (n?=?3) were 10.2, 5.9, 6.2, 7.4 and 3.8, respectively. The best cut-off values for differentiating non-pathological uptake in lymph nodes and bones from tumour uptake were found to be SUVmax of 3.2 and 1.9, respectively. Patients with PSA <2 had significantly lower SUVmax in bone metastases as compared to patients with PSA ≥2 (p?<?0.01).

Conclusions

This biodistribution study provided a broad range of uptake data of [⁶⁸Ga]PSMA-11 for normal organs/tissues, primary prostate tumours and metastatic lesions based on a large patient cohort. Both PT and small metastatic lesions were detectable due to their high tracer uptake. Four-times-higher median uptake in PT in comparison to normal prostate stroma resulted in a high diagnostic accuracy that could potentially be used for multimodal image-guided biopsy with dedicated reconstruction software.

     

An Evaluation of 2-deoxy-2-[18F]Fluoro-D-Glucose and 3′-deoxy-3′-[18F]-Fluorothymidine Uptake in Human Tumor Xenograft Models

Purpose

The aim of this study is to assess the variability of 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]-FDG) and 3??-deoxy-3??-[¹⁸F]-fluorothymidine ([¹⁸F]-FLT) uptake in pre-clinical tumor models and examine the relationship between imaging data and related histological biomarkers.

Procedures

[¹⁸F]-FDG and [¹⁸F]-FLT studies were carried out in nine human tumor xenograft models in mice. A selection of the models underwent histological analysis for endpoints relevant to radiotracer uptake. Comparisons were made between in vitro uptake, in vivo imaging, and ex vivo histopathology data using quantitative and semi-quantitative analysis.

Results

In vitro data revealed that [1-¹⁴C]-2-deoxy-d-glucose ([¹⁴C]-2DG) uptake in the tumor cell lines was variable. In vivo, [¹⁸F]-FDG and [¹⁸F]-FLT uptake was highly variable across tumor types and uptake of one tracer was not predictive for the other. [¹⁴C]-2DG uptake in vitro did not predict for [¹⁸F]-FDG uptake in vivo. [¹⁸F]-FDG SUV was inversely proportional to Ki67 and necrosis levels and positively correlated with HKI. [¹⁸F]-FLT uptake positively correlated with Ki67 and TK1.

Conclusion

When evaluating imaging biomarkers in response to therapy, the choice of tumor model should take into account in vivo baseline radiotracer uptake, which can vary significantly between models.     

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.

     

[68Ga]-Albumin-PET in the Monitoring of Left Ventricular Function in Murine Models of Ischemic and Dilated Cardiomyopathy: Comparison with Cardiac MRI

Purpose

The purpose of this study is to evaluate left ventricular functional parameters in healthy mice and in different murine models of cardiomyopathy with the novel blood pool (BP) positron emission tomography (PET) tracer [⁶⁸Ga]-albumin.

Procedures

ECG-gated microPET examinations were obtained in healthy mice, and mice with dilative (DCM) and ischemic cardiomyopathy (ICM) using the novel BP tracer [⁶⁸Ga]-albumin (Alb_BP), as well as [¹⁸F]-FDG microPET. Cine-magnetic resonance imaging (MRI) examination performed on a clinical 1.5-T MRI provided the reference standard measurements.

Results

When considering the combined group of healthy controls, DCM and ICM Alb_BP-PET significantly overestimated the magnitudes of EDV (Alb_BP, 181?±?86 μl; cine-MRI, 125?±?80 μl; P?<?0.001) and ESV (Alb_BP, 136?±?92 μl; cine-MRI, 96?±?77 μl; P?<?0.001), whereas the EF (Alb_BP, 31?±?16 %; cine-MRI, 33?±?21 %; P?=?0.910) matched closely to cine-MRI results, as did findings with [¹⁸F]-FDG. High correlations were found between the measured cardiac parameters (EDV: R?=?0.978, ESV: R?=?0.989, and LVEF: R?=?0.992).

Conclusions

Measuring left ventricular function in mice with [⁶⁸Ga]-albumin BP PET is feasible and showed a high correlation compared to cine-MRI, which was used as a reference standard.     

Tumor Cell Uptake of 99mTc-Labeled 1-Thio-β-d-Glucose and 5-Thio-d-Glucose in Comparison with 2-Deoxy-2-[18 F]Fluoro-d-Glucose In Vitro: Kinetics,Dependencies, Blockage and Cell Compartment of Accumulation

Purpose

This study was conducted to investigate the capacity of ^99mTc-labeled 1-thio-β-d-glucose (^99mTc-1-TG) and 5-thio-d-glucose (^99mTc-5-TG) to act as a marker for glucose metabolism in tumor cells in vitro.

Procedures

We investigated the cellular uptake of ^99mTc-1-TG, ^99mTc-5-TG, and 2-deoxy-2-[¹⁸?F]fluoro-d-glucose(¹⁸?F-FDG) in a human colorectal carcinoma and human lung adenocarcinoma cell line (HCT-116, A549) at different time points and varying glucose/insulin concentrations and under transporter blockage by cytochalasin-B and phloretin. Cell compartment analysis was performed.

Results

A significant uptake and time dependency thereof, a significant uptake dependency on glucose and insulin and a significant uptake inhibition by cytochalasin-B for ^99mTc-1-TG and ^99mTc-5-TG, was shown. Albeit substantial, the uptake was less pronounced in ^99mTc-1-TG and ^99mTc-5-TG compared with ¹⁸?F-FDG. ^99mTc-1-TG and ^99mTc-5-TG showed a higher accumulation in the cell membranes compared with ¹⁸?F-FDG.

Conclusion

Tc-1-TG and ^99mTc-5-TG showed an uptake in vitro with glucose analog characteristics but with membranous accumulation. Tumor imaging should be investigated in an animal model.