18F-FSPG PET/CT Imaging of System xC- Transporter Activity in Patients with Primary and Metastatic Brain Tumors

Background The PET tracer (4S)-4-(3-[¹⁸F]fluoropropyl)-l-glutamate (¹⁸F-FSPG) targets the system x_C^- cotransporter, which is overexpressed in various tumors. Purpose To assess the role of ¹⁸F-FSPG PET/CT in intracranial malignancies. Materials and Methods Twenty-six patients (mean age, 54 years ± 12; 17 men; 48 total lesions) with primary brain tumors (n = 17) or brain metastases (n = 9) were enrolled in this prospective, single-center study (ClinicalTrials.gov identifier: NCT02370563) between November 2014 and March 2016. A 30-minute dynamic brain ¹⁸F-FSPG PET/CT scan and a static whole-body (WB) ¹⁸F-FSPG PET/CT scan at 60-75 minutes were acquired. Moreover, all participants underwent MRI, and four participants underwent fluorine 18 (¹⁸F) fluorodeoxyglucose (FDG) PET imaging. PET parameters and their relative changes were obtained for all lesions. Kinetic modeling was used to estimate the ¹⁸F-FSPG tumor rate constants using the dynamic and dynamic plus WB PET data. Imaging parameters were correlated to lesion outcomes, as determined with follow-up MRI and/or pathologic examination. The Mann-Whitney U test or Student t test was used for group mean comparisons. Receiver operating characteristic curve analysis was used for performance comparison of different decision measures. Results ¹⁸F-FSPG PET/CT helped identify all 48 brain lesions. The mean tumor-to-background ratio (TBR) on the whole-brain PET images at the WB time point was 26.6 ± 24.9 (range: 2.6-150.3). When ¹⁸F-FDG PET was performed, ¹⁸F-FSPG permitted visualization of non-¹⁸F-FDG-avid lesions or allowed better lesion differentiation from surrounding tissues. In participants with primary brain tumors, the predictive accuracy of the relative changes in influx rate constant K_i and maximum standardized uptake value to discriminate between poor and good lesion outcomes were 89% and 81%, respectively. There were significant differences in the ¹⁸F-FSPG uptake curves of lesions with good versus poor outcomes in the primary brain tumor group (P < .05) but not in the brain metastases group. Conclusion PET/CT imaging with (4S)-4-(3-[¹⁸F]fluoropropyl)-l-glutamate (¹⁸F-FSPG) helped detect primary brain tumors and brain metastases with a high tumor-to-background ratio. Relative changes in ¹⁸F-FSPG uptake with multi-time-point PET appear to be helpful in predicting lesion outcomes. Clinical trial registration no. NCT02370563 © RSNA, 2022 Online supplemental material is available for this article.

Tau PET imaging with 18F-PI-2620 in Patients with Alzheimer Disease and Healthy Controls: A First-in-Humans Study

¹⁸F-PI-2620 is a PET tracer with high binding affinity for aggregated tau, a key pathologic feature of Alzheimer disease (AD) and other neurodegenerative disorders. Preclinically, ¹⁸F-PI-2620 binds to both 3-repeat and 4-repeat tau isoforms. The purpose of this first-in-humans study was to evaluate the ability of ¹⁸F-PI-2620 to detect tau pathology in AD patients using PET imaging, as well as to assess the safety and tolerability of this new tau PET tracer. Methods: Participants with a clinical diagnosis of probable AD and healthy controls (HCs) underwent dynamic ¹⁸F-PI-2620 PET imaging for 180 min. ¹⁸F-PI-2620 binding was assessed visually and quantitatively using distribution volume ratios (DVR) estimated from noninvasive tracer kinetics and SUV ratio (SUVR) measured at different time points after injection, with the cerebellar cortex as the reference region. Time-activity curves and SUVR were assessed in AD and HC subjects, as well as DVR and SUVR correlations and effect size (Cohen's d) over time. Results: ¹⁸F-PI-2620 showed peak brain uptake around 5 min after injection and fast washout from nontarget regions. In AD subjects, focal asymmetric uptake was evident in temporal and parietal lobes, precuneus, and posterior cingulate cortex. DVR and SUVR in these regions were significantly higher in AD subjects than in HCs. Very low background signal was observed in HCs. ¹⁸F-PI-2620 administration was safe and well tolerated. SUVR time-activity curves in most regions and subjects achieved a secular equilibrium after 40 min after injection. A strong correlation (R ² > 0.93) was found between noninvasive DVR and SUVR for all imaging windows starting at more than 30 min after injection. Similar effect sizes between AD and HC groups were obtained across the different imaging windows. ¹⁸F-PI-2620 uptake in neocortical regions significantly correlated with the degree of cognitive impairment. Conclusion: Initial clinical data obtained in AD and HC subjects demonstrated a high image quality and excellent signal-to-noise ratio of ¹⁸F-PI-2620 PET for imaging tau deposition in AD subjects. Noninvasive quantification using DVR and SUVR for 30-min imaging windows between 30 and 90 min after injection-for example, 45-75 min-provides robust and significant discrimination between AD and HC subjects. ¹⁸F-PI-2620 uptake in expected regions correlates strongly with neurocognitive performance.

Evaluation of Dosimetry, Quantitative Methods, and Test-Retest Variability of 18F-PI-2620 PET for the Assessment of Tau Deposits in the Human Brain

¹⁸F-PI-2620 is a next-generation tau PET tracer that has demonstrated ability to image the spatial distribution of suspected tau pathology. The objective of this study was to assess the tracer biodistribution, dosimetry, and quantitative methods of ¹⁸F-PI-2620 in the human brain. Full kinetic modeling to quantify tau load was investigated. Noninvasive kinetic modeling and semiquantitative methods were evaluated against the full tracer kinetics. Finally, the reproducibility of PET measurements from test and retest scans was assessed. Methods: Three healthy controls (HCs) and 4 Alzheimer disease (AD) subjects underwent 2 dynamic PET scans, including arterial sampling. Distribution volume ratio (DVR) was estimated using full tracer kinetics (reversible 2-tissue-compartment [2TC] model and Logan graphical analysis [LGA]) and noninvasive kinetic models (noninvasive LGA [NI-LGA] and the multilinear reference tissue model [MRTM2]). SUV ratio (SUVR) was determined at different imaging windows after injection. The correlation between DVR and SUVR, effect size (Cohen's d), and test-retest variability (TRV) were evaluated. Additionally, 6 HCs received 1 tracer administration and underwent whole-body PET for dosimetry calculation. Organ doses and the whole-body effective dose were calculated using OLINDA 2.0. Results: A strong correlation was found across different kinetic models (R ² > 0.97) and between DVR(2TC) and SUVR between 30 and 90 min, with an R ² of more than 0.95. Secular equilibrium was reached at around 40 min after injection in most regions and subjects. TRV and effect size for SUVR across different regions were similar at 30-60 min (TRV, 3.8%; Cohen's d, 3.80), 45-75 min (TRV, 4.3%; Cohen's d, 3.77) and 60-90 min (TRV, 4.9%; Cohen's d, 3.73) and increased at later time points. Elimination was via the hepatobiliary and urinary systems. The whole-body effective dose was 33.3 ± 2.1 μSv/MBq for an adult female and 33.1 ± 1.4 μSv/MBq for an adult male, with a 1.5-h urinary bladder voiding interval. Conclusion: ¹⁸F-PI-2620 exhibits fast kinetics, suitable dosimetry, and low TRV. DVR measured using the 2TC model with arterial sampling correlated strongly with DVR measured by NI-LGA, MRTM2, and SUVR. SUVR can be used for ¹⁸F-PI-2620 PET quantification of tau deposits, avoiding arterial blood sampling. Static ¹⁸F-PI-2620 PET scans between 45 and 75 min after injection provide excellent quantification accuracy, a large effect size, and low TRV.

Commentary to 18F-GP1, a Novel PET Tracer Designed for High-Sensitivity, Low-Background Detection of Thrombi: Imaging Activated Platelets in Clots-Are We Getting There?

Thrombus formation can lead to heart attacks, stroke and pulmonary embolism, which are major causes of mortality. Current standard diagnostic imaging methods detect anatomic abnormalities such as vascular flow impairment but have limitations. By using a targeted molecular imaging approach critical components of a pathology can be selectively visualized and exploited for an improved diagnosis and patient management. The GPIIb/IIIa receptor is abundantly and specifically exposed on activated platelets and is the key receptor in thrombus formation. This commentary describes the current status of GPIIb/IIIa-based PET imaging approaches with a focus on the recently published preclinical data of the small-molecule PET tracer ¹⁸F-GP1. Areas of future research and potential clinical applications are discussed that may lead to an improved detection of critical thromboembolic events and an optimization of available antithrombotic therapies by tracking activated platelets.

Exploratory Clinical Investigation of (4S)-4-(3-18F-Fluoropropyl)-L-Glutamate PET of Inflammatory and Infectious Lesions

We explored system [Formula: see text] transporter activity and the detection of inflammatory or infectious lesions using (4S)-4-(3-(18)F-fluoropropyl)-l-glutamate ((18)F-FSPG) PET.

METHODS

In 10 patients with various inflammatory or infectious diseases, as many as 5 of the largest lesions were selected as reference lesions. (18)F-FSPG images were assessed visually and quantitatively. Expression levels of xCT, CD44, and surface markers of inflammatory cells were evaluated by immunohistochemistry.

RESULTS

(18)F-FSPG PET detected all reference lesions. (18)F-FSPG uptake in sarcoidosis was significantly higher than that in nonsarcoidosis. The lesion-to-blood-pool SUV ratio for (18)F-FSPG was comparable to that for (18)F-FDG in sarcoidosis. In nonsarcoidosis, however, it was significantly lower. In 5 patients with available tissue samples, the SUVmax for (18)F-FSPG and CD163 were negatively correlated (ρ = -0.872, P = 0.054).

CONCLUSION

(18)F-FSPG PET may detect inflammatory lesions when activated macrophages or monocytes are present, such as in sarcoidosis.

Targeted ED-B fibronectin SPECT in vivo imaging in experimental atherosclerosis

AIM

The extracellular matrix protein ED-B fibronectin (ED-B) is upregulated in inflammatory atherosclerotic lesions. However, functional in vivo imaging of ED-B-containing plaques has not been explored. This study evaluated whether [(99m)Tc]-conjugated AP39 ([(99m)Tc]-AP39), a single-chain antibody specific to ED-B, can be used for in vivo detection of atherosclerotic plaques in Western diet (WD)-fed, apolipoprotein E-deficient (apoE-/-) mice as compared to wildtype (WT) control mice.

METHODS

Using SPECT, 12-month-old WD-fed apoE-/- and WT mice were studied 4 hours after injecting [(99m)Tc]-AP39 (148 MBq). Subsequently, mice were sacrificed, thoracic aortas measured in a g-counter, and plaques analyzed using histology, immuno-histochemistry, autoradiography, and morphometry.

RESULTS

In vivo [(99m)Tc]-AP39-SPECT imaging of apoE-/- mice demonstrated a significant signal activity in the plaque-ridden thoracic aorta (52.236 ± 40.646 cpm/cm³) that co-localized with the aortic arch and the supra-aortic arteries in MRI scans. Low signal activity (9.468 ± 4.976 cpm/cm³) was observed in WT mice. In apoE-/- mice, the strongest signals were detected in the aortic root, aortic arch and along the abdominal aorta. Autoradiography analysis of aortas from apoE-/- mice confirmed the in vivo observation by demonstrating signal localization in atherosclerotic plaques. The size of autoradiography-positive plaque areas correlated significantly with the size of ED-B-positive (r=0.645, P=0.044) or macrophage-infiltrated (r=0.84, P<0.002) plaques. A significant correlation was found between the sizes of ED-B-positive and macrophage-infiltrated plaque areas (r=0.93, P<0.01).

CONCLUSION

[(99m)Tc]-AP39-SPECT in vivo imaging detects inflammatory plaque lesions in WD-fed apoE-/- mice.

Aβ imaging with 18F-florbetaben in prodromal Alzheimer's disease: a prospective outcome study

BACKGROUND

We assessed the clinical utility of β-amyloid (Aβ) imaging with (18)F-florbetaben (FBB) in mild cognitive impairment (MCI) by evaluating its prognostic accuracy for progression to Alzheimer's disease (AD), comparing semiquantitative with visual scan assessment, and exploring the relationships among Aβ, hippocampal volume (HV) and memory over time.

METHODS

45 MCI underwent FBB positron emission tomography, MRI and neuropsychological assessment at baseline and 2 years and clinical follow-up at 4 years. Positive FBB (FBB+), defined by a cortical to cerebellar cortex standardised uptake value ratio (SUVR) ≥ 1.45, was compared with visual assessment by five readers. Amnestic MCI (aMCI) was defined by a composite episodic memory (EM) Z-score of <-1.5.

RESULTS

At baseline, 24 (53%) MCI were FBB+. Majority reads agreed with SUVR classification (κ 0.96). In 2 years, 18 (75%) FBB+ progressed to AD compared with 2 (9.5%) FBB-, yielding a predictive accuracy of 83% (95% CI 61% to 94%). Four FBB- developed non-AD dementia. Predictive accuracies of HV (58% (95% CI 42% to 73%)) and aMCI status (73% (95% CI 58% to 81%)) were lower. Combinations did not improve accuracy. By 4 years, 21 (87.5%) FBB+ had AD whereas 5 (24%) FBB- had non-AD dementia yielding a predictive accuracy of 94% (95% CI 74% to 99%). While the strong baseline association between FBB SUVR and EM declined over 2 years, the association between EM and HV became stronger. FBB SUVR increased 2.2%/year in FBB+ with no change in FBB-.

CONCLUSIONS

(18)F-florbetaben Aβ imaging facilitates accurate detection of prodromal AD. As neurodegeneration progresses, and in contrast with the early stages of the disease, hippocampal atrophy and not Aβ, seems to drive memory decline.

TRIAL REGISTRATION NUMBER

NCT01138111.

Gene expression levels of matrix metalloproteinases in human atherosclerotic plaques and evaluation of radiolabeled inhibitors as imaging agents for plaque vulnerability

INTRODUCTION

Atherosclerotic plaque rupture is the primary cause for myocardial infarction and stroke. During plaque progression macrophages and mast cells secrete matrix-degrading proteolytic enzymes, such as matrix metalloproteinases (MMPs). We studied levels of MMPs and tissue inhibitor of metalloproteinases-3 (TIMP-3) in relation to the characteristics of carotid plaques. We evaluated in vitro two radiolabeled probes targeting active MMPs towards non-invasive imaging of rupture-prone plaques.

METHODS

Human carotid plaques obtained from endarterectomy were classified into stable and vulnerable by visual and histological analysis. MMP-1, MMP-2, MMP-8, MMP-9, MMP-10, MMP-12, MMP-14, TIMP-3, and CD68 levels were investigated by quantitative polymerase chain reaction. Immunohistochemistry was used to localize MMP-2 and MMP-9 with respect to CD68-expressing macrophages. Western blotting was applied to detect their active forms. A fluorine-18-labeled MMP-2/MMP-9 inhibitor and a tritiated selective MMP-9 inhibitor were evaluated by in vitro autoradiography as potential lead structures for non-invasive imaging.

RESULTS

Gene expression levels of all MMPs and CD68 were elevated in plaques. MMP-1, MMP-9, MMP-12 and MMP-14 were significantly higher in vulnerable than stable plaques. TIMP-3 expression was highest in stable and low in vulnerable plaques. Immunohistochemistry revealed intensive staining of MMP-9 in vulnerable plaques. Western blotting confirmed presence of the active form in plaque lysates. In vitro autoradiography showed binding of both inhibitors to stable and vulnerable plaques.

CONCLUSIONS

MMPs differed in their expression patterns among plaque phenotypes, providing possible imaging targets. The two tested MMP-2/MMP-9 and MMP-9 inhibitors may be useful to detect atherosclerotic plaques, but not the vulnerable lesions selectively.

Studies toward the development of new silicon-containing building blocks for the direct (18)F-labeling of peptides

Silicon-containing prosthetic groups have been conjugated to peptides to allow for a single-step labeling with (18)F radioisotope. The fairly lipophilic di-tert-butylphenylsilane building block contributes unfavorably to the pharmacokinetic profile of bombesin conjugates. In this article, theoretical and experimental studies toward the development of more hydrophilic silicon-based building blocks are presented. Density functional theory calculations were used to predict the hydrolytic stability of di-tert-butylfluorosilanes 2-23 with the aim to improve the in vivo properties of (18)F-labeled silicon-containing biomolecules. As a further step toward improving the pharmacokinetic profile, hydrophilic linkers were introduced between the lipophilic di-tert-butylphenylsilane building block and the bombesin congeners. Increased tumor uptake was shown with two of these peptides in xenograft-bearing mice using positron emission tomography and biodistribution studies. The introduction of a hydrophilic linker is thus a viable approach to improve the tumor uptake of (18)F-labeled silicon-bombesin conjugates.

(4S)-4-(3-18F-fluoropropyl)-L-glutamate for imaging of xC transporter activity in hepatocellular carcinoma using PET: preclinical and exploratory clinical studies

(4S)-4-(3-(18)F-fluoropropyl)-l-glutamate ((18)F-FSPG, or BAY 94-9392) is a new tracer to assess system x(C)(¯) transporter activity with PET. The aim of this study was to explore the tumor detection rate of (18)F-FSPG, compared with that of (18)F-FDG, in patients with hepatocellular carcinoma (HCC).

METHODS

Preclinically, in vivo HCC models of orthotopically implanted Huh7 and MH3924a cancer cells were studied with (18)F-FSPG in Naval Medical Research Institute nude mice (n = 3) and August-Copenhagen Irish rats (n = 4), respectively. Clinically, 5 patients with HCC who had hyper- or isometabolic lesions on (18)F-FDG PET were enrolled for evaluation of the tracer. Dynamic whole-body PET images with (18)F-FSPG were acquired for up to 120 min after injection of approximately 300 MBq of (18)F-FSPG. Immunohistochemical expression levels of the xCT subunit of the system x(C)(¯) and CD44 of HCC were studied in 4 patients with HCC.

RESULTS

Strong tumor uptake and low background from nontarget tissue allowed excellent tumor visualization in animal models with orthotopically implanted liver tumors. (18)F-FSPG PET procedures were well tolerated in all patients. (18)F-FSPG PET and (18)F-FDG detected lesions in 5 of 5 and 3 of 5 patients, respectively. The maximal standardized uptake values (SUV) were comparable ((18)F-FSPG, 4.7 ± 3.2; (18)F-FDG, 6.1 ± 2.9). The ratios of maximal SUV of the tumor to mean SUV of normal liver were also comparable ((18)F-FSPG, 3.6 ± 2.2; (18)F-FDG, 2.7 ± 1.3), but the mean SUV of normal liver of (18)F-FSPG was significantly lower than that of (18)F-FDG (P < 0.05). Two patients with HCC who showed both xCT and CD44 expression had moderate or intense accumulation of (18)F-FSPG, but the remaining 2 patients with negative CD44 expression showed mild uptake.

CONCLUSION

(18)F-FSPG was successfully translated from preclinical evaluation into patients with HCC. (18)F-FSPG may be a promising tumor PET agent with a high cancer detection rate in patients with HCC.

Investigations into the synthesis, radiofluorination and conjugation of a new [¹⁸F]fluorocyclobutyl prosthetic group and its in vitro stability using a tyrosine model system

The [(18)F]fluorocyclobutyl group has the potential to be a metabolically stable prosthetic group for PET tracers. The synthesis of the radiolabeling precursor cis-cyclobutane-1,3-diyl bis(toluene-4-sulfonate) 8 was obtained from epibromohydrin in 7 steps (2% overall yield). The radiolabeling of this precursor 8 and its conjugation to L-tyrosine as a model system was successfully achieved to give the new non-natural amino acid 3-[(18)F]fluorocyclobutyl-L-tyrosine (L-3-[(18)F]FCBT) [(18)F]17 in 8% decay-corrected yield from the non-carrier-added [(18)F]fluoride. L-3-[(18)F]FCBT was investigated in vitro in different cancer cell lines to determine the uptake and stability. The tracer [(18)F]17 showed a time dependent uptake into different tumor cell lines (A549, NCI-H460, DU145) with the best uptake of 5.8% injected dose per 5×10(5) cells after 30min in human lung carcinoma cells A549. The stability of L-3-[(18)F]FCBT in human and rat plasma and the stability of the non-radioactive L-3-FCBT in rat hepatocytes were both found to be excellent. These results show that the non-natural amino acid L-3-[(18)F]FCBT is a promising metabolically stable radiotracer for positron emission tomography.

Exploratory clinical trial of (4S)-4-(3-[18F]fluoropropyl)-L-glutamate for imaging xC- transporter using positron emission tomography in patients with non-small cell lung or breast cancer

PURPOSE

(4S)-4-(3-[(18)F]fluoropropyl)-l-glutamate (BAY 94-9392, alias [(18)F]FSPG) is a new tracer to image x(C)(-) transporter activity with positron emission tomography (PET). We aimed to explore the tumor detection rate of [(18)F]FSPG in patients relative to 2-[(18)F]fluoro-2-deoxyglucose ([(18)F]FDG). The correlation of [(18)F]FSPG uptake with immunohistochemical expression of x(C)(-) transporter and CD44, which stabilizes the xCT subunit of system x(C)(-), was also analyzed.

EXPERIMENTAL DESIGN

Patients with non-small cell lung cancer (NSCLC, n = 10) or breast cancer (n = 5) who had a positive [(18)F]FDG uptake were included in this exploratory study. PET images were acquired following injection of approximately 300 MBq [(18)F]FSPG. Immunohistochemistry was done using xCT- and CD44-specific antibody.

RESULTS

[(18)F]FSPG PET showed high uptake in the kidney and pancreas with rapid blood clearance. [(18)F]FSPG identified all 10 NSCLC and three of the five breast cancer lesions that were confirmed by pathology. [(18)F]FSPG detected 59 of 67 (88%) [(18)F]FDG lesions in NSCLC, and 30 of 73 (41%) in breast cancer. Seven lesions were additionally detected only on [(18)F]FSPG in NSCLC. The tumor-to-blood pool standardized uptake value (SUV) ratio was not significantly different from that of [(18)F]FDG in NSCLC; however, in breast cancer, it was significantly lower (P < 0.05). The maximum SUV of [(18)F]FSPG correlated significantly with the intensity of immunohistochemical staining of x(C)(-) transporter and CD44 (P < 0.01).

CONCLUSIONS

[(18)F]FSPG seems to be a promising tracer with a relatively high cancer detection rate in patients with NSCLC. [(18)F]FSPG PET may assess x(C)(-) transporter activity in patients with cancer.

Abstract LB-227: Pilot clinical trials of FSPG (BAY 94-9392): An 18F-labeled glutamate derivative for PET imaging of system xC- activity in tumors

Visualizing adaptations of tumor metabolism by employing positron emission tomography (PET) with a specific imaging probe has the potential to better characterize tumors with higher sensitivity and specificity. The glutamate-cystine exchanger (system xC-, a SLC7A11/SLC3A2 heterodimer) is an important transporter in tumor cells and a promising new target for imaging. This exchanger transports the amino acids L-cystine and L-glutamate, thus providing a rate-limiting precursor for glutathione biosynthesis. This contributes to redox maintenance, which is especially important in tumor cells. Recently, it was reported that a splice variant of the stem cell marker CD44 stabilizes the xCT subunit of system xC- at the cell membrane and promotes its proper function. FSPG (BAY 94-9392) is an 18F-labeled glutamate derivative that provides specific visualization of system xC- activity with PET/CT. Through this multicenter pilot trial, FSPG has successfully been translated into cancer patients and shows favorable pharmacokinetics and an absence of adverse events. A favorable biodistribution pattern previously seen in mice was also confirmed in these patients with different tumor types. A high lesion detection rate (80-100%) was found in lung cancer (n=10), hepatocellular cancer (n=5), head and neck cancer (n=5), and colorectal cancer (n=5) patients who also had positive FDG-PET/CT scans. Importantly, very low background uptake in healthy brain parenchyma enabled the detection of high-grade brain cancers as well as small brain metastases and skull base lesions (n=4) not well seen with FDG-PET. Primary breast tumors (n=4) and a low-grade brain tumor, however, showed minimal uptake of FSPG. Immunohistochemical (IHC) analyses on these subjects’ pathology samples showed significant correlation between FSPG uptake and protein expression of both the xCT subunit and CD44. In breast tumor samples specifically, IHC showed that absence of CD44 results in low signal from FSPG-PET even if xCT is present, indicating the importance of CD44 co-expression for system xC- function. Future clinical studies will further explore the potential of this compound in additional tumor types and expand upon the initial findings in a larger number of subjects. Here we present the first clinical examples demonstrating the ability of the tracer to measure system xC- activity in patients. FSPG-PET/CT may provide improved characterization and sensitivity of tumor detection in patients and further improve the understanding of the role of system xC- in cancer biology. It could be a valuable tool for improved tumor classification in patients, linking uptake of the tracer with malignancy and cancer stem cell properties of tumors. Furthermore, the ability to noninvasively probe this important pathway in humans may allow for more rational treatment planning, patient stratification and better development of new drugs.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-227. doi:1538-7445.AM2012-LB-227

Abstract 2800: BAY 94-9392 – A new derivative of L-glutamate for tumor specific PET imaging

There is a high demand for a tumor specific imaging, staging and therapy monitoring by PET. Currently, FDG (2-[F-18]fluoro-2-deoxyglucose) is the commonly used agent for tumor detection and staging. However, this method cannot differentiate between tumor and inflammatory lesions. In addition, FDG does not accumulate in tumors with low glycolytic activities, such as prostate cancer. It is not optimal for specific detection of brain tumors and metastases due to a high background signal resulting from healthy brain metabolism. The amino acids L-glutamine and L-glutamate as well as their corresponding transporters play a key role in the adapted intermediary metabolism of tumors. We synthesized and evaluated a new PET imaging agent BAY 94-9392, a derivative of L-glutamate. This agent was successfully used for tumor detection with high sensitivity and specificity as demonstrated in various preclinical models.

Methods: The radiosynthesis of (4S)-4-(3-[F-18]fluoropropyl)-L-glutamate (BAY 94-9392) was established on a commercial synthesis module. BAY 94-9392 was investigated in a variety of tumor cell lines as well as tumor models for uptake, biodistribution, mode of action, and its potential to differentiate between tumor versus inflammation.

Results: BAY 94-9392 is specifically transported by the cystine/glutamate exchanger (xCT, SLC7A11) and it is not metabolized. Compared to FDG, the agent displayed a high accumulation rate in a panel of tumor cells. PET imaging with excellent tumor visualization and high tumor to background ratios was achieved in a variety of preclinical tumor models. BAY 94-9392 did not accumulate in inflammatory lesions, whereas FDG shows a strong uptake in this turpentine oil induced and histopathologically confirmed rat model.

Conclusions: BAY 94-9392 is a new, promising tumor specific PET tracer. Both preclinical and clinical studies are in progress for further characterization.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2800. doi:10.1158/1538-7445.AM2011-2800

4-[18F]fluoroglutamic acid (BAY 85-8050), a new amino acid radiotracer for PET imaging of tumors: synthesis and in vitro characterization

There is a high demand for tumor specific PET tracers in oncology imaging. Besides glucose, certain amino acids also serve as energy sources and anabolic precursors for tumors. Therefore, (18)F-labeled amino acids are interesting probes for tumor specific PET imaging. As glutamine and glutamate play a key role in the adapted intermediary metabolism of tumors, the radiosynthesis of 4-[(18)F]fluoro l-glutamic acid (BAY 85-8050) as a new specific PET tracer was established. Cell-uptake studies revealed specific tumor cell accumulation.

Targeting of HER2-expressing tumors with a site-specifically 99mTc-labeled recombinant affibody molecule, ZHER2:2395, with C-terminally engineered cysteine

The detection of human epidermal growth factor receptor type 2 (HER2) expression in malignant tumors provides important information influencing patient management. Radionuclide in vivo imaging of HER2 may permit the detection of HER2 in both primary tumors and metastases by a single noninvasive procedure. Small (7 kDa) high-affinity anti-HER2 Affibody molecules may be suitable tracers for SPECT visualization of HER2-expressing tumors. The use of generator-produced (99m)Tc as a label would facilitate the prompt translation of anti-HER2 Affibody molecules into use in clinics.

METHODS

A C-terminal cysteine was introduced into the Affibody molecule Z(HER2:342) to enable site-specific labeling with (99m)Tc. Two recombinant variants, His(6)-Z(HER2:342)-Cys (dissociation constant [K(D)], 29 pM) and Z(HER2:2395)-Cys, lacking a His tag (K(D), 27 pM), were labeled with (99m)Tc in yields exceeding 90%. The binding specificity and the cellular processing of Affibody molecules were studied in vitro. Biodistribution and gamma-camera imaging studies were performed in mice bearing HER2-expressing xenografts.

RESULTS

(99m)Tc-His(6)-Z(HER2:342)-Cys was capable of targeting HER2-expressing SKOV-3 xenografts in SCID mice, but the liver radioactivity uptake was high. A series of comparative biodistribution experiments indicated that the presence of the His tag caused elevated accumulation in the liver. (99m)Tc-Z(HER2:2395)-Cys, not containing a His tag, showed low uptake in the liver and high and specific uptake in HER2-expressing xenografts. Four hours after injection, the radioactivity uptake values (percentage of injected activity per gram of tissue [%IA/g]) were 6.9 +/- 2.5 (mean +/- SD) %IA/g in LS174T xenografts (moderate level of HER2 expression) and 15 +/- 3 %IA/g in SKOV-3 xenografts (high level of HER2 expression). The corresponding tumor-to-blood ratios were 88 +/- 24 and 121 +/- 24, respectively. Both LS174T and SKOV-3 xenografts were clearly visualized with a clinical gamma-camera 1 h after injection of (99m)Tc-Z(HER2:2395)-Cys.

CONCLUSION

The Affibody molecule (99m)Tc-Z(HER2:2395)-Cys is a promising tracer for SPECT visualization of HER2-expressing tumors.

(124)I-L19-SIP for immuno-PET imaging of tumour vasculature and guidance of (131)I-L19-SIP radioimmunotherapy

Purpose

The human monoclonal antibody (MAb) fragment L19-SIP is directed against extra domain B (ED-B) of fibronectin, a marker of tumour angiogenesis. A clinical radioimmunotherapy (RIT) trial with ¹³¹I-L19-SIP was recently started. In the present study, after GMP production of ¹²⁴I and efficient production of ¹²⁴I-L19-SIP, we aimed to demonstrate the suitability of ¹²⁴I-L19-SIP immuno-PET for imaging of angiogenesis at early-stage tumour development and as a scouting procedure prior to clinical ¹³¹I-L19-SIP RIT.

Methods

¹²⁴I was produced in a GMP compliant way via ¹²⁴Te(p,n)¹²⁴I reaction and using a TERIMO™ module for radioiodine separation. L19-SIP was radioiodinated by using a modified version of the IODO-GEN method. The biodistribution of coinjected ¹²⁴I- and ¹³¹I-L19-SIP was compared in FaDu xenograft-bearing nude mice, while ¹²⁴I PET images were obtained from mice with tumours of <50 to ∼700 mm³.

Results

¹²⁴I was produced highly pure with an average yield of 15.4 ± 0.5 MBq/μAh, while separation yield was ∼90% efficient with <0.5% loss of TeO₂. Overall labelling efficiency, radiochemical purity and immunoreactive fraction were for ¹²⁴I-L19-SIP: ∼80 , 99.9 and >90%, respectively. Tumour uptake was 7.3 ± 2.1, 10.8 ± 1.5, 7.8 ± 1.4, 5.3 ± 0.6 and 3.1 ± 0.4%ID/g at 3, 6, 24, 48 and 72 h p.i., resulting in increased tumour to blood ratios ranging from 6.0 at 24 h to 45.9 at 72 h p.i.. Fully concordant labelling and biodistribution results were obtained with ¹²⁴I- and ¹³¹I-L19-SIP. Immuno-PET with ¹²⁴I-L19-SIP using a high-resolution research tomograph PET scanner revealed clear delineation of the tumours as small as 50 mm³ and no adverse uptake in other organs.

Conclusions

¹²⁴I-MAb conjugates for clinical immuno-PET can be efficiently produced. Immuno-PET with ¹²⁴I-L19-SIP appeared qualified for sensitive imaging of tumour neovasculature and for predicting ¹³¹I-L19-SIP biodistribution.

Small-animal PET of tumor angiogenesis using a (76)Br-labeled human recombinant antibody fragment to the ED-B domain of fibronectin

The aim of this study was to image the extra domain B (ED-B) of fibronectin, an angiogenesis-related target, in solid tumors using small-animal PET. Toward this aim, an ED-B fibronectin-binding human antibody derivative (L19-SIP) was labeled with (76)Br via an enzymatic approach. Biodistribution and imaging studies were performed in human teratoma-bearing mice for up to 48 h after injection.

METHODS

L19-SIP was labeled with (76)Br using bromoperoxidase/H(2)O(2). The stability of the labeled antibody was tested both in vitro and in vivo. Biodistribution and small-animal imaging studies (PET and CT) were performed in F9-bearing 129/sv mice (n = 3 or 4).

RESULTS

The enzymatic radiobromination approach afforded the labeled antibody in high yield (>55%) under mild reaction conditions. (76)Br-L19-SIP stability in mouse serum proved to be similar to that of the (125)I-labeled analog (>80% of intact material at 48 h after injection). Fast and specific in vivo targeting was obtained in tumors and other organs expressing ED-B fibronectin (i.e., ovaries and uterus). However, slow renal clearance and persistent activity predominately in blood and stomach suggests partial (76)Br-L19-SIP debromination in vivo. This debromination was confirmed in a metabolism study in normal mice. The F9 tumors were clearly imaged by small-animal PET at each considered time point, starting at 5 h up to 48 h after injection.

CONCLUSION

(76)Br-L19-SIP specifically accumulated at the target site, enabling detailed small-animal PET of tumor neovasculature. Therefore, targeting the angiogenesis-associated expression of ED-B fibronectin can be a valuable tool for tumor detection using molecular imaging with PET.

Somatostatin Receptor-Binding Peptides Suitable for Tumor Radiotherapy with Re-188 or Re-186. Chemistry and Initial Biological Studies

Somatostatin derivative peptides previously designed for radiodiagnostic purposes (99mTc P829 or 99mTc depreotide) were reoptimized for radiotherapy of tumors with rhenium radioisotopes. An optimized pharmacophore peptide P1839 was derived by in vitro binding affinity assay to AR42J rat pancreatic tumor cell membranes. Peptides with chelating domains and their oxorhenium(V) complexes were tested in vitro for binding to NCI H69 human SCLC tumor membranes. Further optimization entailed radiolabeling with 99mTc and biodistribution in an AR42J xenograft mouse model. Kidney uptake was decreased substantially by removing positively charged residues. Neutral N3S diamide amine thiol chelators with no adjacent positive charges had the best overall properties. Substituting an aromatic amino acid into the chelator approximately doubled the tumor uptake. The final optimized peptide P2045 (39) radiolabeled with 99mTc exhibited increased tumor uptake ( approximately 25 %ID/g at 1.5 h), lower kidney uptake ( approximately 4.8 %ID/g at 1.5 h), and extensive urinary excretion (59 %ID at 1.5 h). Finally, comparison biodistribution studies between 99mTc and 188Re (39) showed a good correlation between the two metal complexes and demonstrated prolonged tumor retention (> or =24 h).

Bispecific antibody pretargeting of tumor neovasculature for improved systemic radiotherapy of solid tumors

PURPOSE

Extra domain B (ED-B) fibronectin is a specific tumor matrix marker for targeting angiogenesis in solid tumors. In this study, the radiotherapeutic potential of the directly radioiodinated divalent anti-ED-B antibody fragment, L19 small immunoprotein (L19-SIP; 75,000 Da), was compared with a pretargeting approach using the bispecific antibody AP39xm679 (bsMAb; 75,000 Da).

EXPERIMENTAL DESIGN

The bsMAb was prepared by coupling an anti-ED-B single-chain Fv (AP39) to the Fab' of the murine antibody m679, which binds to the small peptidic hapten histamine-succinyl-glycine (HSG). As an effector molecule for the pretargeting approach, the 111In-labeled HSG-DOTA complex was injected 25 or 41 hours after the bsMAb. The kinetics of both the iodinated bsMAb and the pretargeted 111In-labeled HSG hapten were investigated in mice bearing human glioblastoma xenografts (U251) and compared with the kinetics and tumor accumulation of radioiodinated L19-SIP. 111In and 125I were used as surrogate marker for the therapeutic radioisotopes 90Y/177Lu and 131I, respectively.

RESULTS

Tumor uptake of the pretargeted 111In-labeled peptide was significantly higher than 125I-L19-SIP over 7 days. At the calculated maximally tolerated dose for each agent (with the kidney being the dose-limiting organ for pretargeting and the bone marrow for direct targeting), a mouse tumor dose of 146 Gy could be given by pretargeting versus 45 Gy delivered by the direct approach.

CONCLUSIONS

These data suggest that pretargeting of ED-B with AP39xm679 and subsequent injection of the 90Y-hapten-peptide would improve the therapeutic efficacy in solid tumors by >3-fold compared with directly radiolabeled 131I-L19-SIP.

Imaging of tumor angiogenesis using 99mTc-labeled human recombinant anti-ED-B fibronectin antibody fragments

The aim of this study was to target the angiogenesis-associated extracellular matrix protein ED-B fibronectin for molecular imaging of solid tumors. Recombinant and chemically modified derivatives of the single-chain antibody fragment (scFv) L19, capable of being labeled with 99mTc, were synthesized and radiolabeled. The resulting compounds 99mTc-AP39, 99mTc-L19-His, and 99mTc-L19-Hi20 were assessed for their imaging properties in vivo.

METHODS

L19 was genetically modified by inserting either the (Gly)3-Cys-Ala (AP39) or a (His)6 tag (L19-His) sequence at the C-terminal end. Chemical modifications were performed by conjugating the bifunctional chelator Hi20 (L19-Hi20) at epsilon-Lys-NH2 residues of the molecule to allow for a direct chelator-based labeling with 99mTc. Tumor-targeting, pharmacokinetic, and scintigraphic imaging properties of the radiolabeled scFvs were evaluated in nude mice bearing murine F9 teratocarcinoma.

RESULTS

99mTc labeling of the L19 derivatives yielded radiochemically pure proteins maintaining high immunoreactivity to ED-B fibronectin, as measured by affinity chromatography. Size-exclusion high-performance liquid chromatographic analysis of labeled L19 derivatives demonstrated either dimeric species (L19-His) or a mixture of predominantly associative dimeric and monomeric species (AP39, L19-Hi20). 99mTc-AP39 showed the most favorable biodistribution and imaging properties with high and fast tumor uptake (8.3 percentage injected dose per gram at 3 h after injection), rapid blood clearance and renal excretion, leading to high signal-to-noise ratios (tumor-to-blood ratio of 6.4 at 3 h after injection), and excellent planar scintigraphy in vivo.

CONCLUSION

ED-B fibronectin can be efficiently targeted by 99mTc-AP39 and scintigraphically visualized in tumor-bearing mice, providing a potentially useful clinical tool for imaging of angiogenesis-associated ED-B fibronectin-expressing human tumors.

Radioimmunotherapy of head and neck cancer xenografts using 131I-labeled antibody L19-SIP for selective targeting of tumor vasculature

The extra domain B of fibronectin (ED-B) is a marker of tumor angiogenesis. The human monoclonal antibody (mAb) L19-SIP (approximately 80 kDa; SIP is "small immunoprotein") has been selected for targeting of ED-B. The aim of this study was to evaluate the potential of radioimmunotherapy (RIT) with L19-SIP, either alone or in combination with cetuximab, for treatment of head and neck squamous cell carcinoma (HNSCC). Combination with cetuximab was considered because this anti-EGFR (epidermal growth factor receptor) mAb has proven value for the treatment of HNSCC.

METHODS

HNSCC xenograft lines FaDu and HNX-OE were evaluated for ED-B and EGFR expression. L19-SIP was radiolabeled with 2 candidate radionuclides for RIT, 177Lu and 131I (or 125I as substitute). The biodistribution of coinjected 177Lu-L19-SIP and 125I-L19-SIP was assessed in FaDu-bearing nude mice, whereas 131I-L19-SIP was evaluated in both xenograft lines. After labeling with high-dose 131I (623-789 MBq/mg), the maximum tolerated dose (MTD) was assessed. The efficacy of RIT with injected 131I-L19-SIP, either alone or in combination with unlabeled cetuximab (1 mg 2 times a week intraperitoneally for 4 wk), was evaluated in both xenograft lines.

RESULTS

Xenograft lines expressed both antigens, with similar EGFR expression and the highest ED-B expression in FaDu. Radioiodinated L19-SIP performed better than 177Lu-L19-SIP and was further exploited. The biodistribution of 131I-L19-SIP was most favorable in FaDu-bearing mice, with tumor uptake values at 24, 48, and 72 h after injection of 8.6 +/- 1.6, 5.8 +/- 0.4, and 3.4 +/- 0.2 %ID/g (%ID/g is percentage injected dose per gram of tissue), respectively, and ratios of tumor to normal tissues that gradually increased in time, such as for blood from 4.4 +/- 1.8 at 24 h to 21.4 +/- 1.7 at 72 h, after injection. RIT at the MTD level of 74 MBq caused significant tumor growth delay and improved survival in both lines. Although FaDu was most sensitive for RIT, with size reduction of all tumors, HNX-OE was most sensitive for treatment with cetuximab. The best survival and cure rates were obtained, however, when RIT and cetuximab were combined.

CONCLUSION

RIT with 131I-L19-SIP appeared efficacious in HNSCC xenografts. The efficacy of RIT was enhanced by combination with cetuximab, without increase of toxicity.

Novel Approach in Radionuclide Tumor Therapy: Dose Enhancement by High Z-Element Contrast Agents

PURPOSE

The aim of this study was to first calculate the dose-enhancement effect from internalized radiation by the presence of exogenous contrast media using Monte Carlo simulations, and then provide in vitro proof-of-concept for this novel method of radiation-dose enhancement.

MATERIALS AND METHODS

The Monte Carlo program EGSnrc (Electron Gamma Shower) was used to simulate the interaction of internalizing radiation with iodine (I) or gadolinium (Gd) containing contrast media. Experimentally, the doseenhancement effect induced by I or Gd was evaluated in cell culture assays using internalizing peptides chelated with beta- emitting radionuclides and clinically available contrast media.

RESULTS

Monte Carlo simulations predicted significant enhancement (approximately 70-340%) of radiation dose in the presence of high Zelement contrast media. This enhancement is radiation and Z-element dependent. Calculations showed that in the presence of contrast media, low-energy radionuclides favor localization of secondary particles, whereas higher energy beta- emitters localize radiation by reducing the pathway of the primary beta-particle. The dose enhancement was verified in vitro in two cell lines.

CONCLUSIONS

Monte Carlo simulations in parallel with in vitro studies provide proof-of-principle for dose enhancement that occurs when utilizing an internalized source of radiation followed by the addition of exogenous contrast media. This dose enhancement is both radiation and Z-element dependent.

Radioimmunotherapy of solid tumors by targeting extra domain B fibronectin: identification of the best-suited radioimmunoconjugate

PURPOSE

The expression of extra domain B (ED-B) fibronectin is always associated with angiogenic processes and can be exclusively observed in tissues undergoing growth and/or extensive remodeling. Due to this selective expression, ED-B fibronectin is an interesting target for radioimmunotherapy of malignant diseases. The aim of this study was to identify the most appropriate ED-B-targeting radioimmunoconjugate for the therapy of solid tumors.

EXPERIMENTAL DESIGN

Three ED-B fibronectin-binding human antibody formats of L19 were investigated: dimeric single-chain Fv (approximately 50 kDa), "small immunoprotein" (SIP, approximately 80 kDa), and immunoglobulin G1 (IgG1, approximately 150 kDa). These L19 derivatives were either labeled with I-125 or with In-111 (using MX-diethylenetriaminepentaacetic acid, MX-DTPA). Pharmacokinetics and tumor accumulation of the radiolabeled immunoconjugates were investigated in F9 (murine teratocarcinoma) tumor-bearing mice. Subsequently, dosimetry for the corresponding therapeutic isotopes I-13-1 and Y-90 was done. After testing the myelotoxicity of I-131-L19-SIP and I-131-L19-IgG1 in non-tumor-bearing mice, the therapeutic efficacy of these iodinated antibody formats was finally investigated in F9 tumor-bearing mice.

RESULTS

The most favorable therapeutic index was found for I-131-L19-SIP followed by I-131-L19-IgG1. The therapeutic index of all In-111-labeled derivatives was significantly inferior. Considering the bone marrow as the dose-limiting organ, it was calculated that activities of 74 MBq I-131-L19-SIP and 25 MBq I-131-L19-IgG1 could be injected per mouse without causing severe myelotoxicity. The best therapeutic efficacy was observed using I-131-L19-SIP, resulting in significant tumor growth delay and prolonged survival after a single injection.

CONCLUSION

Compared with other L19-based radioimmunoconjugates, I-131-L19-SIP is characterized by superior antitumor efficacy and toxicity profile in the F9 teratocarcinoma animal model. These results indicate that ED-B fibronectin-targeted radioimmunotherapy using I-131-L19-SIP has potential to be applied to treatment of solid cancers.

Prevention of carotid artery restenosis after sirolimus-coated stent implantation in pigs

BACKGROUND AND PURPOSE

To test the feasibility of self-expanding drug-coated nitinol stents for prevention of restenosis in an animal model. Stent implantation in the carotid artery (CA) has been shown to be feasible for treatment of CA stenosis. Even though the restenosis rate in CA is reported to be lower than in the coronary and peripheral arteries, problems may arise with increasing numbers of treated patients and lengthier follow-up.

METHODS

After predilatation with 8-mm balloons, 8 Goettinger minipigs were randomly selected to receive a sirolimus-eluting self-expanding nitinol stent (7 mm/80 mm) as well as the same stent without sirolimus/polymer coating in the right or left CA. Aspirin was given starting 3 days before the intervention and administered for an additional 4 weeks. Clopidogrel was administered for 10 days.

RESULTS

After 6 weeks, 2 subacute occlusions were observed in both groups. In the remaining vessels, the neointima was significantly reduced by sirolimus/polymer-coated stents (5.9+/-2.5 versus 0.7+/-1.0 mm2).

CONCLUSIONS

Sirolimus self-expanding nitinol stents may be an effective tool in reducing neointimal formation in CA.

Reduction of Intimal Hyperplasia withRe-188-labeled Stents in a Rabbit Model at 7and 26 Weeks: An Experimental Study

The aim of this study was to analyze the feasibility of (188)Re-labeled stents to reduce neointimal formation in a rabbit atherosclerosis model and to test the long-term effects at 7 and 26 weeks. Fifty-nine male New Zealand White rabbits were fed a 0.5% cholesterol diet for 4 weeks before balloon angioplasty and insertion of Palmaz stents in the infrarenal aorta. The animals were sacrificed 7 and 26 weeks after stent implantation. Control stents were compared with (188)Re stents: (dose 1) 11.3 +/- 1.8 MBq; (dose 2) 37.3 +/- 4.2 MBq, and (dose 3) 80.1 +/- 7.8 MBq. Each activity group consisted of a short-term (7 weeks) and a long-term group (26 weeks), resulting in a total of eight study groups. No thrombotic occlusion was observed. The neointimal formation in the control group was 2.11 [95% confidence interval (CI): 0.68--6.52] mm(2) at 7 weeks and 2.10 (0.62--7.11) at 26 weeks. In the treatment groups, neointima reduction was detectable at 7 weeks [dose 1: 0.33 (0.09--1.22) mm(2); dose 2: 0.17 (0.05--0.57) mm(2); dose 3: 0.03 (0.01--0.13) mm(2)]. After 26 weeks, a catch-up of neointimal formation in the radioactive groups was most obvious in the low-dose group [dose 1: 0.80 (0.28--2.29) mm(2); dose 2: 0.18([0.06--0.52) mm(2); dose 3: 0.50 (0.17--1.42) mm(2)]. Compared to the long-term control group, neointimal reduction was still >60%. No induction of neointimal formation was observed at the edges of the stents. Radiation resulted in delayed re-endothelialization. (188)Re stents were capable to reduce intimal hyperplasia and did not cause thrombosis. The edge effect, which was the major limitation of (32)P stents, was not observed in (188)Re stents.

Molecular targeting of angiogenesis for imaging and therapy

Angiogenesis, i.e. the proliferation of new blood vessels from pre-existing ones, is an underlying process in many human diseases, including cancer, blinding ocular disorders and rheumatoid arthritis. The ability to selectively target and interfere with neovascularisation would potentially be useful in the diagnosis and treatment of angiogenesis-related diseases. This review presents the authors' views on some of the most relevant markers of angiogenesis described to date, as well as on specific ligands which have been characterised in pre-clinical animal models and/or clinical studies. Furthermore, we present an overview on technologies which are likely to have an impact on the way molecular targeting of angiogenesis is performed in the future.

Targeting of endothelin receptors for molecular imaging of atherosclerosis in rabbits

We wanted to determine whether a previously described in vivo accumulation of a (99m)Tc-labeled endothelin derivative in atherosclerotic lesions is mediated by binding to endothelin receptors. Furthermore, the expression of endothelin receptors in atherosclerotic lesions of 2 different rabbit animal models for atherosclerosis was to be evaluated to determine whether endothelin receptors generally are a suitable target for atherosclerosis imaging.

METHODS

Normal vessels from untreated New Zealand White rabbits (NZW), balloon-denuded aortas from cholesterol-fed NZW, and atherosclerotic aortas from Watanabe Heritable Hyperlipidemic rabbits (WHHL) were used either as cross sections (cryosections) for receptor binding studies or for superfusion with a medium containing (125)I-labeled endothelin-1 or the (99m)Tc-labeled endothelin derivative.

RESULTS

Cross sections of aortas from untreated NZW contained 45 +/- 11.10(6) endothelin A receptors per square millimeter and 55 +/-11.10(6).endothelin B receptors per square millimeter, cross sections of balloon-denuded aortas from cholesterol-fed NZW contained 106 +/- 16.10(6) endothelin A receptors per square millimeter and 27 +/- 16.10(6) endothelin B receptors per square millimeter, and cross sections of atherosclerotic aortas from WHHL contained 40 +/- 13.10(6) endothelin A receptors per square millimeter and 5 +/- 13.10(6) endothelin B receptors per square millimeter. Balloon-denuded aortas from cholesterol-fed NZW (366 +/- 132 amol.mm(-2), P < 0.001) and atherosclerotic aortas from WHHL (338 +/- 175 amol.mm(-2), P < 0.002) accumulated significantly more of the (99m)Tc-labeled endothelin derivative than did vessels from control animals (137 +/- 26 amol.mm(-2)). On the contrary, (125)I-labeled endothelin-1--bound receptor mediated to superfused aortas from untreated NZW (12 +/- 9 amol.mm(-2)) and to balloon-denuded aortas from cholesterol-fed NZW (19 +/- 5 amol.mm(-2)) but not to aortas from WHHL. This lack of receptor-specific accumulation of (125)I-endothelin-1 in atherosclerotic areas of WHHL aortas, and this receptor-specific accumulation in atherosclerotic balloon-denuded NZW aortas that does not significantly increase in comparison with normal aortas of untreated NZW, cause failure of endothelins to detect atherosclerotic lesions.

CONCLUSION

Although the density and the ratio of endothelin receptor subtypes change because of the development of atherosclerotic lesions in rabbit aortas, endothelin receptor targeting for imaging of atherosclerosis is not suitable.

Tc-99m-labeled endothelin derivative for imaging of experimentally induced atherosclerosis

OBJECTIVE

to characterize the potential of an endothelin derivative labeled with technetium-99m (Tc-99m) for the imaging of experimentally induced atherosclerosis.

METHODS

neointima of different cellularity and severity of stenosis was induced in 32 rabbits by balloon denudation followed by distinct dietary regimens and drug application. Angiograms and scintigrams after injection of the Tc-99m-labeled endothelin derivative were obtained. The aorta was dissected for autoradiography, sudan-III-staining, morphometry, and immunohistology.

RESULTS

the lesions induced could be detected in vivo (whole body scintigram) in all the animals 15 min after the injection of the Tc-99m endothelin derivative. Autoradiography revealed a strong relationship between tracer accumulation and sudan-III-staining of lesions. Accumulation of the endothelin derivative correlated with the number of neointimal smooth muscle cells (SMC), but not with the number of medial SMC, neointimal macrophages, and neointimal area.

CONCLUSIONS

the results indicate that in vivo imaging of atherosclerosis with an endothelin derivative is a feasible method of detecting and characterizing atherosclerotic arterial wall lesions at early stages.

Prophylaxis of restenosis with (186)re-labeled stents in a rabbit model

BACKGROUND

Intraluminal beta-irradiation has been shown to decrease neointimal proliferation after angioplasty in experimental models. The purpose of this study was to test the technical feasibility and biological effects of (186)Re-labeled stents.

METHODS AND RESULTS

Thirty-four New Zealand White rabbits were fed a 0.5% cholesterol diet before balloon angioplasty and insertion of Palmaz stents in the infrarenal aorta. The animals were killed 7 weeks after stent implantation. Two of 34 animals died prematurely (aortic leak, pneumonia). Control stents (n=7) were compared with (186)Re stents (2.6 MBq [n=6], 8.1 MBq [n=5], 16.0 MBq [n=6], and 25.3 MBq [n=8]). Stent application was successful in all cases. No thrombus occlusion was observed. After 7 weeks, neointima formation was 2.2+/-0.2 mm(2) in the control group. In the treatment groups, a dose-dependent neointima reduction was detectable (0.5+/-0.5 mm(2) [2.6 MBq], 0.4+/-0.4 mm(2) [8.1 MBq], and 0 mm(2) [16.0 MBq, 25.3 MBq]). No induction of neointimal formation was observed at the edges of the stents. Radiation resulted in delayed reendothelialization.

CONCLUSIONS

(186)Re stents were capable of reducing neointima formation in a dose-dependent fashion. (186)Re stents did not cause late thrombosis or neointimal induction at the stent margins in the observation period of 7 weeks.

Local intra-arterial drug delivery for prevention of restenosis: comparison of the efficiency of delivery of different radiopharmaceuticals through a porous catheter

RATIONALE AND OBJECTIVES

Several radiopharmaceuticals were administered through a porous balloon catheter to compare the absolute amount deposited and the retention in the vessel wall. The reported efficiency of local drug delivery ranges from 0.001% to 0.1%, with poor retention after 24 hours.

METHODS

An endothelin derivative (n = 6), pertechnetate (n = 6), hexamethylpropylene amineoxime (HMPAO) (n = 5), ethyl cysteinate dimer (ECD) (n = 5), and tin colloid (n = 5) were labeled with 185 MBq/mL 99m-technetium. After balloon denudation of the infrarenal aorta in 27 New Zealand White rabbits, 100 microL of each agent was administered through a porous balloon at a pressure of 4 bar. Dynamic and static whole-body scintigrams were obtained for 24 hours. The infrarenal aorta was excised and the activity calculated in a gamma counter.

RESULTS

Apart from their retention in the region of local administration, the radiopharmaceuticals showed different distribution patterns. The highest regional tracer retention was observed with HMPAO. After administration of HMPAO, a significant difference between regional (vessel wall plus surrounding tissue: 14.5% of injected dose [ID]/24 hours) and local (vessel wall: 1.8% ID/24 hours) delivery was found. In contrast, ECD was eliminated quickly (local retention after 24 hours = 0% ID). The retention efficiencies were HMPAO > endothelin derivative > tin colloid > pertechnetate > ECD.

CONCLUSIONS

The different physicochemical and pharmacokinetic properties of radiopharmaceuticals resulted in different delivery efficiencies after local application.

Molecular imaging of atherosclerosis using a technetium-99m-labeled endothelin derivative

Endothelins have been implicated in the pathogenesis of atherosclerosis and restenosis. The aim of this study was to characterize the potential of an endothelin derivative labeled with 99mTc for imaging experimental atherosclerosis in vivo.

METHODS

Atherosclerosis was induced by balloon denudation of the infrarenal aorta in eight New Zealand white rabbits followed by a 6-wk period of a standard or 0.5% cholesterol diet in four animals, respectively. Another four rabbits served as controls, without balloon denudation and cholesterol feeding. Digital subtraction angiograms and planar whole-body scintigrams were obtained after intravenous injection of 74 MBq of the 99mTc-labeled endothelin derivative. The aorta was dissected for autoradiography, sudan-III staining, morphometry and immunohistology (anti-alpha-actin, RAM 11) 5 hr after injection.

RESULTS

The lesions induced in the infrarenal aorta could be detected in vivo (whole-body scintigrams) in all treated animals only 15 min after injection of 99mTc-endothelin derivative. Autoradiography of the excised aorta revealed good correlation of tracer accumulation and sudan-III-stained lesions. The ratio of accumulation between the induced lesions and untreated vessel wall was 6.8 +/- 1.4 in the cholesterol-fed animals and 6.3 +/- 1.8 in the animals without cholesterol feeding. Accumulation of the endothelin derivative correlated with the number of smooth muscle cells (r = 0.924) but not with the amount of macrophages, the area or the maximum thickness of the plaques.

CONCLUSION

Scintigraphic visualization of experimentally induced atherosclerosis in vivo is feasible using an endothelin derivative labeled with 99mTc.

Transport and metabolism of L-glutamate during oxygenation, anoxia, and reoxygenation of rat cardiac myocytes

The intracellular glutamate concentration of oxygenated, isolated adult rat heart cells incubated with 0.15 mM glutamate amounts to 2.89 +/- 0.6 mM. Under these conditions the velocity of glutamate transport was 24.3 +/- 1.6 pmol.min-1.mg protein-1 and occurs via a high-affinity carrier characterized by an apparent affinity (K(m)) value of 0.18 +/- 0.03 mM. At high glutamate concentrations ( > 1mM) this high-affinity transport system is superimposed by additional uptake processes of a low affinity but a high capacity for glutamate. The 1.6-fold increased uptake of glutamate observed during 30 min of anoxic incubation of cardiomyocytes does not prevent an intracellular decrease in this amino acid to a concentration of 0.49 mM. After 15 min reoxygenation of cardiomyocytes the intracellular glutamate content increases to the control values of oxygenated cells. Only 2.4% of the glutamate increase after reoxygenation is due to the transport o glutamate from the incubation medium. The competitive inhibitor of transaminases, aminooxyacetate, prevents both the observed intracellular decrease in glutamate during anoxia and the increase in intracellular glutamate after reoxygenation of cardiomyocytes. Half of the amino groups needed for the synthesis of glutamate originate from intracellular alanine, which increases during anoxia and is metabolized during reoxygenation of cardiomyocytes. The velocity of the glutamate uptake of cardiomyocytes incubated in a medium containing 10 mM L-glutamate amounted to 728 +/- 140 pmol.min-1.mg protein-1. During anoxic incubation of cardiomyocytes at this high extracellular glutamate concentration, the intracellular glutamate breakdown may be compensated by a simultaneous uptake of this amino acid via the transport processes characterized by a high capacity

Characterization and pH dependence of L-glutamate transport in sarcolemmal vesicles from rat hearts

Transport of L-glutamate in purified sarcolemmal vesicles from rat heart was Na+ dependent and characterized by an apparent affinity value of 0.149 +/- 0.04 mM (n = 6). Na+ binding to the carrier was not very specific, since an inwardly directed K+ gradient could also increase vesicular uptake of L-glutamate. Uptake of L-glutamate into sarcolemmal vesicles was inhibited by L-aspartate and L-cysteinate. These characteristics of glutamate uptake are typical of transport via system XAG-. Acidification of either the intravesicular or the extravesicular space led to a significant increase in L-glutamate transport. This increase was more pronounced when the intravesicular space had been acidified. Thus intracellular acidosis of hypoxic or ischemic myocardium is directly linked to an increased uptake of glutamate.