Neural Stem Cells as Carriers of Nucleoside-Conjugated Nanogels: A New Approach toward Cell-Mediated Delivery.

Neural stem cells (NSCs) present attractive natural drug delivery systems (DDSs). Their migratory potential enables crossing of the blood-brain barrier and efficient and selective accumulation near malignant cells. Here, we present the potential of NSCs as DDSs for nucleoside analogue-conjugated nanogels (NGs). Two different approaches were investigated: the intracellular loading and extracellular cell surface decoration with NGs. For both designs, the tumor-specific migratory potentials of NSCs remained unchanged; however, the intracellular loading showed a shorter NG retention. The cell surface decoration protocol yielded a high loading capacity of 100% after 1 h and a prolonged drug retention. A redox-sensitive linker between NGs and the nucleoside analogue 5-ethynyl-2'-deoxycytidine (EdC) allowed a tumor environment-specific drug release and its efficient and preferential incorporation into the DNA of the tumor cells. Interestingly, the tumor-trafficking potentials of NSCs were significantly potentiated by irradiation of tumor cells. In conclusion, this study indicates the potentials of cell surface-decorated NSCs as DDSs for tumor-specific release, cellular uptake, and incorporation of EdC into DNA.

Correction: Sankaranarayanan et al. Auger Emitter Conjugated PARP Inhibitor for Therapy in Triple Negative Breast Cancers: A Comparative In-Vitro Study. Cancers 2022, 14, 230.

The authors wish to replace the 'Author Contributions' statement and the affiliation for Jochen Maurer of this article [...].

Improved protocol for the radiosynthesis of [18 F]FTC-146: A potent and selective sigma-1 receptor radioligand.

[18 F]FTC-146 was introduced as a very potent and selective sigma-1 receptor radioligand, which has shown promising application as an imaging agent for neuropathic pain with positron emission tomography. In line with a multi-laboratory project on animal welfare, we chose this radioligand to investigate its potential for detecting neuropathic pain and tissue damage in tumor-bearing animals. However, the radiochemical yield (RCY) of around 4-7% was not satisfactory to us, and efforts were made to improve it. Herein, we describe an improved approach for the radiosynthesis of [18 F]FTC-146 resulting in a RCY, which is sevenfold higher than that previously reported. A tosylate precursor was synthesized and radio-fluorination experiments were performed via aliphatic nucleophilic substitution reactions using either K[18 F]F-Kryptofix®222 (K2.2.2 )-carbonate system or tetra-n-butylammonium [18 F]fluoride ([18 F]TBAF). Several parameters affecting the radiolabeling reaction such as solvent, 18 F-fluorination agent with the corresponding amount of base, labeling time, and temperature were investigated. Best labeling reaction conditions were found to be [18 F]TBAF and acetonitrile as solvent at 100°C. The new protocol was then translated to an automated procedure using a FX2 N synthesis module. Finally, the radiotracer reproducibly obtained with RCYs of 41.7 ± 4.4% in high radiochemical purity (>98%) and molar activities up to 171 GBq/µmol.

PARP targeted Auger emitter therapy with [125I]PARPi-01 for triple-negative breast cancer.

BACKGROUND: Triple-negative breast cancer (TNBC) lacks biomarkers for targeted therapy. Auger emitters display the best therapeutic effect, if delivered directly into the nucleus proximal to DNA. The nuclear protein Poly (ADP-ribose)-Polymerase 1 (PARP1) is a suitable target against which few inhibitors (PARPi) are clinically approved for treatment of breast cancer with germline BRCA mutation (BRCAmut). In this study, a theranostic approach was investigated in a TNBC xenografted mouse model by radiolabelling a close derivative of a PARPi Olaparib (termed PARPi-01) with the Auger emitters 123/125I. METHODS: TNBC cell line MDA-MB-231 was subcutaneously implanted in female NOD/SCID mice. At a tumour size of ~ 500mm3, [123I]PARPi-01 was administered intravenously, and SPECT/CT images were obtained at 4 h or 24 h post injection (p.i). A therapy study was performed with [125I]PARPi-01 in 4 doses (10 MBq/dose, 10 days apart). Tumour growth was monitored by CT scans longitudinally once per week. Upon reaching study endpoint, tissues were harvested and stained with TUNEL assay for detection of apoptosis induction. RESULTS: SPECT/CT images showed rapid hepatobiliary tracer clearance at 4 h post injection (p.i.). Retention in thyroid at 24 h p.i. suggested tracer deiodination in vivo. The tumour and liver uptake were 0.2%ID/g and 2.5%ID/g, respectively. The tumour: blood ratio was 1.3. Endogenous therapy induced a significant delay in tumour growth (doubling time increased from 8.3 to 14.2 days), but no significant survival advantage. Significantly higher apoptosis ratio was observed in [125I]PARPi-01 treated tumour tissues. No radiotoxicity was detected in the liver and thyroid. CONCLUSION: Considering the radio-cytotoxic effect in the tumour tissue and a delay on tumour doubling time, [125I]PARPi-01 presents a potential radiotherapeutics for treatment of TNBC. Improvements to overcome the suboptimal pharmacokinetics are necessary for its potential clinical application.

[68 Ga]Ga-FAPI uptake correlates with the state of chronic kidney disease.

PURPOSE: Kidney fibrosis leads to a progressive reduction in kidney function ultimately resulting in kidney failure. Diagnostic tools to detect kidney fibrosis are all invasive in nature requiring kidney biopsies with subsequent histological validation. In this retrospective study, the diagnostic value of three different radiotracers for the noninvasive prediction of kidney fibrosis was analyzed, taking into account the glomerular filtration rate (GFR) and the intra-renal parenchymal radiotracer uptake. METHODS: In 81 patients receiving either one of the following molecular imaging probes, [68 Ga]Ga-FAPI, [68 Ga]Ga-PSMA, or [68 Ga]Ga-DOTATOC, kidney function parameters were correlated with SUVmax and SUVmean of the renal parenchyma and background activity measured in lung parenchyma, myocardium, gluteal muscle, and the abdominal aorta. Patients were clustered according to their grade of chronic kidney disease (CKD), and a regression analysis and one-way ANOVA were conducted in this retrospective analysis. RESULTS: We found a negative correlation between GFR and [68 Ga]Ga-FAPI uptake for both SUVmax and SUVmean values, whereas background activity showed no correlation with GFR. [68 Ga]Ga-DOTATOC and [68 Ga]Ga-PSMA did not correlate between CKD stage and intra-renal parenchymal radiotracer uptake. Only [68 Ga]Ga-PSMA background activity exhibited a positive correlation with GFR suggesting an unspecific binding/retention potentially due to longer circulation times. CONCLUSION: There is a significant negative correlation between renal parenchymal [68 Ga]Ga-FAPI uptake and GFR, which was not the case for [68 Ga]Ga-DOTATOC and [68 Ga]Ga-PSMA. This correlation suggests a specific binding of FAPI rather than a potential unspecific retention in the renal parenchyma, underlining the potential value of [68 Ga]Ga-FAPI for the noninvasive quantitative evaluation of kidney fibrosis.

Auger Emitter Conjugated PARP Inhibitor for Therapy in Triple Negative Breast Cancers: A Comparative In-Vitro Study.

PARP1 inhibitors (PARPi) are currently approved for BRCAmut metastatic breast cancer, but they have shown limited response in triple negative breast cancer (TNBC) patients. Combination of an Auger emitter with PARPis enables PARP inhibition and DNA strand break induction simultaneously. This will enhance cytotoxicity and additionally allow a theranostic approach. This study presents the radiosynthesis of the Auger emitter [125I] coupled olaparib derivative: [125I]-PARPi-01, and its therapeutic evaluation in a panel of TNBC cell lines. Specificity was tested by a blocking assay. DNA strand break induction was analysed by γH2AX immunofluorescence staining. Cell cycle analysis and apoptosis assays were studied using flow cytometry in TNBC cell lines (BRCAwt/mut). Anchorage independent growth potential was evaluated using soft agar assay. [125I]-PARPi-01 showed PARP1-specificity and higher cytotoxicity than olaparib in TNBC cell lines irrespective of BRCA their status. Cell lines harbouring DNA repair deficiency showed response to [125I]-PARPi-01 monotherapy. Combined treatment with Dox-NP further enhanced therapeutic efficiency in metastatic resistant BRCAwt cell lines. The clonogenic survival was significantly reduced after treatment with [125I]-PARPi-01 in all TNBC lines investigated. Therapeutic efficacy was further enhanced after combined treatment with chemotherapeutics. [125I]-PARPi-01 is a promising radiotherapeutic agent for low radiation dosages, and mono/combined therapies of TNBC.

Labelling via [Al18F]2+ Using Precomplexed Al-NODA Moieties.

Over the past 20 years, 68Ga-labelled radiopharmaceuticals have become an important part in clinical routine. However, the worldwide supply with 68Ge/68Ga generators is limited as well as the number of patient doses per batch of 68Ga radiopharmaceutical. In the recent years, a new technique appeared, making use of the ease of aqueous labelling via chelators as with 68Ga but using 18F instead. This technique takes advantage of the strong coordinative bond between aluminium and fluoride, realized in the aqueous cation [Al18F]2+. Most applications to date make use of one-pot syntheses with free Al(III) ions in the system. In contrast, we investigated the labelling approach split into two steps: generating the Al-bearing precursor in pure form and using this Al compound as a precursor in the labelling step with aqueous [18F]fluoride. Hence, no free Al3+ ions are present in the labelling step. We investigated the impact of parameters: temperature, pH, addition of organic solvent, and reaction time using the model chelator NH2-MPAA-NODA. With optimized parameters we could stably achieve a 80% radiochemical yield exerting a 30-min reaction time at 100 °C. This technique has the potential to become an important approach in radiopharmaceutical syntheses.

Sodium [18F]Fluoride PET Can Efficiently Monitor In Vivo Atherosclerotic Plaque Calcification Progression and Treatment.

Given the high sensitivity and specificity of sodium [18F]Fluoride (Na[18F]F) for vascular calcifications and positive emerging data of vitamin K on vascular health, the aim of this study is to assess the ability of Na[18F]F to monitor therapy and disease progression in a unitary atherosclerotic mouse model. ApoE-/- mice were placed on a Western-type diet for 12-weeks and then split into four groups. The early stage atherosclerosis group received a chow diet for an additional 12-weeks, while the advanced atherosclerosis group continued the Western-type diet. The Menaquinone-7 (MK-7) and Warfarin groups received MK-7 or Warfarin supplementation during the additional 12-weeks, respectively. Control wild type mice were fed a chow diet for 24-weeks. All of the mice were scanned with Na[18F]F using a small animal positron emission tomography (PET)/computed tomography (CT). The Warfarin group presented spotty calcifications on the CT in the proximal aorta. All of the spots corresponded to dense mineralisations on the von Kossa staining. After the control, the MK-7 group had the lowest Na[18F]F uptake. The advanced and Warfarin groups presented the highest uptake in the aortic arch and left ventricle. The advanced stage group did not develop spotty calcifications, however Na[18F]F uptake was still observed, suggesting the presence of micro-calcifications. In a newly applied mouse model, developing spotty calcifications on CT exclusively in the proximal aorta, Na[18F]F seems to efficiently monitor plaque progression and the beneficial effects of vitamin K on cardiovascular disease.

HER2-directed antibodies, affibodies and nanobodies as drug-delivery vehicles in breast cancer with a specific focus on radioimmunotherapy and radioimmunoimaging.

PURPOSE: The aim of the present paper is to review the role of HER2 antibodies, affibodies and nanobodies as vehicles for imaging and therapy approaches in breast cancer, including a detailed look at recent clinical data from antibody drug conjugates and nanobodies as well as affibodies that are currently under development. RESULTS: Clinical and preclinical studies have shown that the use of monoclonal antibodies in molecular imaging is impaired by slow blood clearance, associated with slow and low tumor uptake and with limited tumor penetration potential. Antibody fragments, such as nanobodies, on the other hand, can be radiolabelled with short-lived radioisotopes and provide high-contrast images within a few hours after injection, allowing early diagnosis and reduced radiation exposure of patients. Even in therapy, the small radioactively labeled nanobodies prove to be superior to radioactively labeled monoclonal antibodies due to their higher specificity and their ability to penetrate the tumor. CONCLUSION: While monoclonal antibodies are well established drug delivery vehicles, the current literature on molecular imaging supports the notion that antibody fragments, such as affibodies or nanobodies, might be superior in this approach.

An international multi-center investigation on the accuracy of radionuclide calibrators in nuclear medicine theragnostics.

BACKGROUND: Personalized molecular radiotherapy based on theragnostics requires accurate quantification of the amount of radiopharmaceutical activity administered to patients both in diagnostic and therapeutic applications. This international multi-center study aims to investigate the clinical measurement accuracy of radionuclide calibrators for 7 radionuclides used in theragnostics: 99mTc, 111In, 123I, 124I, 131I, 177Lu, and 90Y. METHODS: In total, 32 radionuclide calibrators from 8 hospitals located in the Netherlands, Belgium, and Germany were tested. For each radionuclide, a set of four samples comprising two clinical containers (10-mL glass vial and 3-mL syringe) with two filling volumes were measured. The reference value of each sample was determined by two certified radioactivity calibration centers (SCK CEN and JRC) using two secondary standard ionization chambers. The deviation in measured activity with respect to the reference value was determined for each radionuclide and each measurement geometry. In addition, the combined systematic deviation of activity measurements in a theragnostic setting was evaluated for 5 clinically relevant theragnostic pairs: 131I/123I, 131I/124I, 177Lu/111In, 90Y/99mTc, and 90Y/111In. RESULTS: For 99mTc, 131I, and 177Lu, a small minority of measurements were not within ± 5% range from the reference activity (percentage of measurements not within range: 99mTc, 6%; 131I, 14%; 177Lu, 24%) and almost none were outside ± 10% range. However, for 111In, 123I, 124I, and 90Y, more than half of all measurements were not accurate within ± 5% range (111In, 51%; 123I, 83%; 124I, 63%; 90Y, 61%) and not all were within ± 10% margin (111In, 22%; 123I, 35%; 124I, 15%; 90Y, 25%). A large variability in measurement accuracy was observed between radionuclide calibrator systems, type of sample container (vial vs syringe), and source-geometry calibration/correction settings used. Consequently, we observed large combined deviations (percentage deviation > ± 10%) for the investigated theragnostic pairs, in particular for 90Y/111In, 131I/123I, and 90Y/99mTc. CONCLUSIONS: Our study shows that substantial over- or underestimation of therapeutic patient doses is likely to occur in a theragnostic setting due to errors in the assessment of radioactivity with radionuclide calibrators. These findings underline the importance of thorough validation of radionuclide calibrator systems for each clinically relevant radionuclide and sample geometry.

Radionuclide tumor necrosis factor-alpha activity in herniated lumbar disc correlates with severe leg pain.

BACKGROUND: Lumbar disc herniation is often associated with an inflammatory process. In this context, inflammation has been considered a key factor in the modulation of pain. Here, we present a case of inflammatory activity directly documented in a patient with a lumbar disc herniation. CASE DESCRIPTION: A 49-year-old male presented with progressive low back pain and left-sided S1 radiculopathy, without a focal neurological deficit. The lumbar MR revealed a prominent herniated disc at the L5-S1 level, with compression of the left S1 root. The patient underwent a L5-S1 discectomy using a standard interlaminar approach. Although initially he was pain free, he required three additional operations to address recurrent pain complaints. As research indicates that local inflammation contributes to neuropathic pain, we had the patient undergoes single-photon emission computed tomography (SPECT) imaging using technetium-99m-labeled-infliximab (an anti-tumor necrosis factor [TNF]-alpha monoclonal antibody) before a proposed fourth operation. The SPECT study documented a strong signal at the site of the herniated disc, thus confirming the diagnosis of a pro-inflammatory process involving the S1 nerve root. Nine months after the fourth operation, the patient was pain free. Of interest, the second SPECT study in the now asymptomatic patient demonstrated no detectable/ residual signal at the operative/disc site. CONCLUSION: Absence of a SPECT TNF-alpha signal in a pain-free patient following a lumbar discectomy correlates with the reduction/resolution of the local preoperative inflammatory response.

Targeting of prostate-specific membrane antigen for radio-ligand therapy of triple-negative breast cancer.

BACKGROUND: Triple-negative breast cancer has extremely high risk of relapse due to the lack of targeted therapies, intra- and inter-tumoral heterogeneity, and the inherent and acquired resistance to therapies. In this study, we evaluate the potential of prostate-specific membrane antigen (PSMA) as target for radio-ligand therapy (RLT). METHODS: Tube formation was investigated after incubation of endothelial HUVEC cells in tumor-conditioned media and monitored after staining using microscopy. A binding study with 68Ga-labeled PSMA-addressing ligand was used to indicate targeting potential of PSMA on tumor-conditioned HUVEC cells. For mimicking of the therapeutic application, tube formation potential and vitality of tumor-conditioned HUVEC cells were assessed following an incubation with radiolabeled PSMA-addressing ligand [177Lu]-PSMA-617. For in vivo experiments, NUDE mice were xenografted with triple-negative breast cancer cells MDA-MB231 or estrogen receptor expressing breast cancer cells MCF-7. Biodistribution and binding behavior of [68Ga]-PSMA-11 was investigated in both tumor models at 30 min post injection using µPET. PSMA- and CD31-specific staining was conducted to visualize PSMA expression and neovascularization in tumor tissue ex vivo. RESULTS: The triple-negative breast cancer cells MDA-MB231 showed a high pro-angiogenetic potential on tube formation of endothelial HUVEC cells. The induced endothelial expression of PSMA was efficiently addressed by radiolabeled PSMA-specific ligands. 177Lu-labeled PSMA-617 strongly impaired the vitality and angiogenic potential of HUVEC cells. In vivo, as visualized by µPET, radiolabeled PSMA-ligand accumulated specifically in the triple-negative breast cancer xenograft MDA-MB231 (T/B ratio of 43.3 ± 0.9), while no [68Ga]-PSMA-11 was detected in the estrogen-sensitive MCF-7 xenograft (T/B ratio of 1.1 ± 0.1). An ex vivo immunofluorescence analysis confirmed the localization of PSMA on MDA-MB231 xenograft-associated endothelial cells and also on TNBC cells. CONCLUSIONS: Here we demonstrate PSMA as promising target for two-compartment endogenous radio-ligand therapy of triple-negative breast cancer.

Optimization of the reduction of 74As(V) to 74As(III) and of the labelling of dithiol dihydrolipoic acid.

The radiochemical separation of n.c.a. arsenic on its own or for radio-labelling purposes usually involves the issue of reducing arsenic(V). Numerous approaches for reducing pentavalent arsenic have been examined. A novel HPLC method has also been presented for accessing the efficiency of the reduction in terms of *As(III)/*As(V). Labelling with trivalent radioarsenic seems to be a promising research field to access new radiopharmaceuticals, for example, using arsenic as a surrogate for phosphorus. Moreover, as a model system, the labelling reaction of *As(III) with dihydrolipoic acid has been systematically optimized.

Relevant tumor sink effect in prostate cancer patients receiving 177Lu-PSMA-617 radioligand therapy.

AIM: In metastatic prostate cancer patients PSMA targeting radioligands have gained significant impact as theranostic probes. In this study a correlation between total tumor volume (TTV) and measured kidney dose as well as salivary glands (SG) uptake in 177Lu-PSMA-617 therapy was evaluated. METHODS: Eleven consecutive prostate cancer patients receiving a first cylcle of 177Lu-PSMA-617 (administered activity of approximately 6GBq) were included. The 68Ga-PSMA-11 PET/CT scan previous to therapy was used to determine TTV and SG uptake (glandulae submandibularis) employing PMOD version 3.403 with different 68Ga-PSMA-11 thresholds based on the standardized uptake value (SUV).The kidney dose was estimated with the software ULMDOS using planar whole-body scintigrams. RESULTS: Kidney dose and SG uptake was inversely correlated to TTV, indicating high kidney dose and high SG uptake in case of low tumor load and low kidney dose and low SG uptake in case of high tumor load. CONCLUSION: Our data support the hypothesis that in 177Lu-PSMA-617 therapy an individualized treatment activity based on total tumor volume could be beneficiary.

Modulation of glutathione promotes apoptosis in triple-negative breast cancer cells.

Triple-negative breast cancer has an extremely high rate of relapse. This is particularly due to the existence and survival of cancer stem cells (CSCs) characterized by increased amounts of glutathione (GSH). In this study, we evaluated the potential of pharmacological GSH depletion to sensitize CSCs to ionizing radiotherapy with an I-125-labeled nucleoside analog, 5-iodo-4'-thio-2'-deoxyuridine (ITdU). CSCs were isolated using CD24-- and CD44+-specific microbeads. GSH and reactive oxygen species (ROS) were evaluated by fluorescence-activated cell sorting. GSH synthesis was inhibited with buthionine sulfoximine (BSO). Apoptotic cells were identified with propidium iodide and double-strand DNA breaks were detected by γ-H2AX staining. For therapy study, BSO treated and untreated mice xenografted with breast CSCs received weekly I-125-ITdU. Therapy efficiency was monitored by fluorodeoxyglucose-18-µ-positron emission tomography. We showed that GSH modulation sensitizes CD24- and CD44+ breast cancer cells to endogenous nanoradiotherapy. BSO synergistically affects ROS generation induced by I-125-ITdU. In an in vivo study, we demonstrated a complete tumor regression as a consequence of preconditioning with a GSH-synthesis inhibitor prior to treatment with I-125-ITdU. GSH modulation in combination with an oxidative stress-generating treatment such as endogenous radiotherapy using an Auger emitter offers an extraordinary opportunity for selective and efficient eradication of drug-resistant CSCs.-Miran, T., Vogg, A. T. J., Drude, N., Mottaghy, F. M., Morgenroth, A. Modulation of glutathione promotes apoptosis in triple-negative breast cancer cells.

Dual addressing of thymidine synthesis pathways for effective targeting of proliferating melanoma.

Here, we examined the potential of blocking the thymidine de novo synthesis pathways for sensitizing melanoma cells to the nucleoside salvage pathway targeting endogenous DNA irradiation. Expression of key nucleotide synthesis and proliferation enzymes thymidylate synthase (TS) and thymidine kinase 1 (TK1) was evaluated in differentiated (MITFhigh [microphthalmia-associated transcription factor] IGR1) and invasive (MITFmedium IGR37) melanoma cells. For inhibition of de novo pathways cells were incubated either with an irreversible TS inhibitor 5-fluoro-2'-deoxyuridine (FdUrd) or with a competitive dihydrofolate-reductase (DHFR) inhibitor methotrexate (MTX). Salvage pathway was addressed by irradiation-emitting thymidine analog [123/125 I]-5-iodo-4'-thio-2'-deoxyuridine (123/125 I-ITdU). The in vivo targeting efficiency was visualized by single-photon emission computed tomography. Pretreatment with FdUrd strongly increased the cellular uptake and the DNA incorporation of 125 I-ITdU into the mitotically active IGR37 cells. This effect was less pronounced in the differentiated IGR1 cells. In vivo, inhibition of TS led to a high and preferential accumulation of 123 I-ITdU in tumor tissue. This preclinical study presents profound rationale for development of therapeutic approach by highly efficient and selective radioactive targeting one of the crucial salvage pathways in melanomas.

Radioiodinated indomethacin amide for molecular imaging of cyclooxygenase-2 expressing tumors.

Cyclooxygenase-2 (COX-2) is an important biomarker in several tumors. Available imaging probes display relatively low tumor to background ratios (smaller than 2:1). We evaluated newly developed indomethacin (Ind) derivatives for in vivo molecular imaging of COX-2 expressing carcinoma. Radioiodinated Ind derivatives Ind-NH-(CH2)4-NH-3-[I-125]I-Bz ([I-125]5), Ind-NH-(CH2)4-NH-5-[I-124/125]I-Nic ([I-124/125]6) and Ind-NH-(CH2)4-NH-5-[I-125]I-Iphth ([I-125]7) were prepared from the respective SnBu3-precursors (45-80% radiochemical yield; > 95% radiochemical purity). The cellular uptake of [I-125]5 and [I-125]6 correlated with COX-2 expression determined by SDS page/Western blot analysis. [I-125]5 was predominantly localized in the cell membrane while [I-125]6 was internalized and displayed a diffuse and favorable cytoplasmic distribution. In contrast, [I-125]7 showed only low uptake in COX-2 positive cells. Co-incubation with the COX-2 inhibitor Celecoxib led to an almost complete suppression of cellular uptake of [I-125]5 and [I-125]6. In vivo molecular imaging using positron emission tomography (PET) in SCID mice xenografted with COX-2+ (HT29) and COX-2- (HCT116) human colorectal carcinoma cells was performed for [I-124]6. HT29 xenografts displayed a significantly higher uptake than HCT-116 xenografts (5.6 ± 1.5 vs. 0.5 ± 0.1 kBq/g, P < 0.05) with an extraordinary high tumor to muscle ratio (50.3 ± 1.5). Immunohistological staining correlated with the imaging data. In conclusion, the novel radioiodinated indomethacin derivative ([I-124/125]6) could become a valuable tool for development of molecular imaging probes for visualization of COX-2 expressing tumors.