Hypoxia PET Tracer Research Articles

2‐Nitroimidazole Based PET Tracers: Development of a General Synthetic Approach and Fluorination Methodology

Abstract2‐Nitroimidazole based PET tracers are widely used for non‐invasive hypoxic tumor diagnosis. In this context, we report here a general method based on a central Mitsunobu reaction to easily access 2‐nitroimidazole precursors. Optimization of fluorination conditions is described and successfully applied to the radiofluorination of the 2‐nitroimidazole precursors. Moreover, the radiosynthesis of a new potential hypoxia PET tracer is also described. Finally, this article presents a SFC analytical method allowing both an easy monitoring of the fluorination reactions and the measurement of partition coefficients (logP).

Correlation of hypoxia PET tracer uptake with hypoxic radioresistance in cancer cells: PET biomarkers of resistance to stereotactic radiation therapy?

PurposeThe pO2 threshold of an ideal PET hypoxia tracer for radiotherapy planning in cancer would match those observed in clinically and biologically relevant processes such as radioresistance and HIF1α expression. To identify such tracers, we directly compared uptake in vitro of hypoxia PET tracers ([18F]FMISO, [64Cu]CuATSM, and analogues [64Cu]CuATS, [64Cu]CuATSE, [64Cu]CuCTS, [64Cu]CuDTS, [64Cu]CuDTSE, [64Cu]CuDTSM) with levels of radioresistance and HIF1α expression in cultured cancer cells under identical hypoxic conditions ranging from extreme hypoxia to normoxia. Pimonidazole uptake was also compared as a marker of hypoxia. MethodsA custom-built hypoxia apparatus enabled all experiments to be performed under identical hypoxic conditions with constant measurement of pO2 in media using an OxyLab pO2™ probe. HCT116 human colonic carcinoma and MCF-7 human Caucasian breast adenocarcinoma cells were irradiated using a cobalt teletherapy unit. Clonogenic assays were used to assess survival. HIF1α expression was determined by western blotting, tracer uptake by gamma counting and pimonidazole binding by flow cytometry. ResultsRadioresistance, pimonidazole binding and HIF1α expression increased gradually as pO2 decreased between 25 mmHg and 0 mmHg. In contrast, all the PET hypoxia tracers showed a sharp increase in uptake only when pO2 levels fell below 1 mmHg. Above this threshold, tracer uptake was not elevated above that in normoxic cells. ConclusionThis study highlights an important mismatch in pO2 thresholds between these PET tracers and other markers of hypoxia: tracer uptake only occurred at oxygen levels that were well below levels that induced radioresistance, pimonidazole uptake and HIF1α expression. Although their pO2 thresholds do not match the threshold for resistance to conventionally fractionated radiotherapy (pO2 2.5–10 mmHg), their specificity for extreme hypoxia (pO2 ≪ 1 mmHg) suggests these PET tracers may be of particular use to predict outcomes in stereotactic radiation therapy where these maximally resistant cells play a key role in determining the biological effect.

Synthesis, Radiosynthesis, and in vitro Studies on Novel Hypoxia PET Tracers Incorporating [18F]FDR

AbstractWe report the synthesis of five radiotracers incorporating different oxyamine spacers between the hypoxia‐reactive 2‐nitroimidazole moiety and the 5‐[18F]‐fluorodeoxyribose ([18F]FDR, 12) prosthetic group: three linear alkyl chains with 3, 5, 7 carbon atoms (15 a–c), a cyclopropyl ring (15 d) and a 1,4‐disubstituted‐1,2,3‐triazole (15 e). Experiments in hypoxic cells showed that 15 d displays superior uptake kinetics – and similar selectivity for hypoxic cells – relative to the gold standard hypoxia tracers [18F]fluoroazomycin arabinoside ([18F]FAZA) and [18F]fluoromisonidazole ([18F]FMISO). Lipophilicity and structural rigidity have strong influence on the selectivity of tracers 15 towards hypoxic cells: the lead tracer 15 d displays a logP=0.38 and the most rigid spacer. A sixth radiotracer (15 f), with a 2‐H‐imidazole replacing the 2‐nitroimidazole moiety of 15 d, was used to demonstrate that the cyclopropyl group does not play a meaningful role in the sensitivity towards hypoxia.

Hypoxia PET imaging beyond 18F-FMISO in patients with high-grade glioma: 18F-FAZA and other hypoxia radiotracers

High-grade gliomas are aggressive primitive brain tumors presenting aberrant vasculature, regional necrosis, and areas of hypoxia. Tumor hypoxia is associated with resistance to conventional treatment and worse prognosis. [18F]-fluoromisonidazole (18F-FMISO) is the most extensively investigated radiotracer for the evaluation of hypoxia. However, the use of 18F-FMISO in clinical practice has been hampered mainly due to the slow clearance of the unbound tracer from normoxic tissue and its low tumor-to-background ratio (TBR). The research community has therefore investigated other radiotracers to overcome the drawbacks of 18F-FMISO. This mini-review aims to present an update on the most relevant PET studies published in the last 15 years evaluating the utility of radiotracers for hypoxia imaging other than 18F-FMISO in high-grade glioma (HGG) patients. A comprehensive computer literature search of studies was carried out in PubMed/MEDLINE database to identify the most relevant studies published in the last 15 years which investigated the utility of hypoxia PET tracers other than 18F-FMISO in the assessment of tumor hypoxia in patients with HGG. 18F-flouroazomycin arabinoside (18F-FAZA) has been proposed as a valid alternative to 18F-FMISO for the assessment of hypoxia, due to its improved biodistribution and enhanced tumor-to-background ratio. Also 1-(2-[18F]fluoro-1[hydroxymethyl]ethoxy)methyl-2-nitroimidazole(18F-FRP170) seems a valuable hypoxia tracer in patients with brain tumor. The value of copper-diacetyl-bis(N4-methylthiosemicarbazone)(Cu-ATSM) seems controversial. Few evidences still exist regarding the utility of 18F-2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide (18F-EF5). Hypoxia PET imaging has the potential to provide useful information for the clinicians and to guide hypoxia tailored treatments. According to the present literature, the most promising hypoxic tracer seems to be 18F-FAZA, but well-designed and wide trials to validate hypoxia radiotracers and evaluate their clinical utility in daily practice are still lacking.

Effect of HNSCC Radiochemotherapy on Imaging Biomarker T2* MRI and its Relation to FMISO-PET Derived Hypoxia

Previous studies using hypoxia PET have shown significant reduction of tumor hypoxia (reoxygenation) during primary radiochemotherapy (RCT) of HNSCC and a significant correlation between reoxygenation on hypoxia PET and local control. Assessment of tumor hypoxia using MRI would offer independence from hypoxia PET tracer availability. The apparent MRI transverse relaxation time T2* has been proposed as an imaging biomarker and as a potential surrogate for hypoxia PET. The aim of this study was to assess the effect of primary RCT on T2* at an early (week 2) and late (week 5) time point during RCT and to analyse the relation between T2* and established hypoxia PET tracer 18F-misonidazole PET/CT (FMISO PET) and standard 18F-FDG PET/CT (FDG PET). In 10 T2-4N+ HNSCC patients, FDG PET was obtained at baseline and repeat FMISO PET and 3 Tesla MRI T2* in weeks 0, 2 and 5. MRI gross tumor volumes for tumor and lymph nodes (GTV-T, GTV-LN) were contoured and FMISO PET derived hypoxic tumor/lymph node subvolumes (HSV-T, HSV-LN) and complementary non-hypoxic subvolumes (nonHSV-T, nonHSV-LN) were generated using a threshold level of 1.4 times the mean SUV FMISO within muscle. Volumes were contoured for all time points (week 0, 2 and 5) individually and mean values for T2* and SUVmean FDG PET were obtained. Within GTVs (T, LN), r2 (FMISO to T2*) was calculated for [FMISO SUVmax GTV/SUVmean muscle] to [T2* mean GTV] and r2 (FMISO to FDG) was calculated for [FMISO SUVmax GTV/SUVmean muscle] to [FDG SUVmax GTV/SUVmean muscle]. From week 0 to 5 GTV-T and GTV-LN decreased by -56% and -63%, respectively and HSV-T and HSV-LN nearly completely resolved (n = 10). Mean T2* signal showed no significant change for GTV-T or GTV-LN. Within HSV-T mean T2* values were smaller compared to nonHSV-T: 15.0+/-4.6 vs. 18.3+/-2.9 (p = 0.051), whereas FDG SUVmean was significantly higher within hypoxic as compared to non-hypoxic regions: HSV-T 12.1+/-5.5 vs. nonHSV-T 6.1+/-2.6 and HSV-LN 10.2+/-3.9 vs. nonHSV-LN 4.7+/-1.9 (week 0, p≤0.026 and p≤0.011). Correlation between FMISO PET and FDG PET was higher than between FMSIO PET and T2*: r2 for GTV-T (FMISO/FDG)=0.81, r2 for GTV-T (FMISO/T2*)=0.32. Marked reduction of tumor hypoxia between week 0, 2 and 5 found on FMISO PET was not accompanied by a significant T2*change within GTVs over time. At baseline, smaller T2* values and significantly larger FDG SUVmean were found within HSV-T as compared to nonHSV-T. These results suggest a relation between tumor oxygenation status and T2* at baseline, however with the correlation coefficient r2 FMISO/T2* being lower than r2 FMISO/FDG. Overall, T2* quantitation was feasible and the findings on T2* imaging warrant further investigations but do not indicate a simple surrogate role for T2* as hypoxia imaging marker due to lack of correlation to established hypoxia marker FMISO PET. Our findings on reduction of FMISO uptake during RCT were in line with previous reports.

Non-linear conversion of HX4 uptake for automatic segmentation of hypoxic volumes and dose prescription

Background: Tumour hypoxia is associated with increased radioresistance and poor response to radiotherapy. Pre-treatment assessment of tumour oxygenation could therefore give the possibility to tailor the treatment by calculating the required boost dose needed to overcome the increased radioresistance in hypoxic tumours. This study concerned the derivation of a non-linear conversion function between the uptake of the hypoxia-PET tracer 18F-HX4 and oxygen partial pressure (pO2).Material and methods: Building on previous experience with FMISO including experimental data on tracer uptake and pO2, tracer-specific model parameters were derived for converting the normalised HX4-uptake at the optimal imaging time point to pO2. The conversion function was implemented in a Python-based computational platform utilising the scripting and the registration modules of the treatment planning system RayStation. Subsequently, the conversion function was applied to determine the pO2 in eight non-small-cell lung cancer (NSCLC) patients imaged with HX4-PET before the start of radiotherapy. Automatic segmentation of hypoxic target volumes (HTVs) was then performed using thresholds around 10 mmHg. The HTVs were compared to sub-volumes segmented based on a tumour-to-blood ratio (TBR) of 1.4 using the aortic arch as the reference oxygenated region. The boost dose required to achieve 95% local control was then calculated based on the calibrated levels of hypoxia, assuming inter-fraction reoxygenation due to changes in acute hypoxia but no overall improvement of the oxygenation status.Results: Using the developed conversion tool, HTVs could be obtained using pO2 a threshold of 10 mmHg which were in agreement with the TBR segmentation. The dose levels required to the HTVs to achieve local control were feasible, being around 70–80 Gy in 24 fractions.Conclusions: Non-linear conversion of tracer uptake to pO2 in NSCLC imaged with HX4-PET allows a quantitative determination of the dose-boost needed to achieve a high probability of local control.

Hypoxia and positron emission tomography in patients with gliomas

The treatment of brain tumors poses a challenge for the medical community and is an interdisciplinary endeavor. Especially tumor cells of high-grade gliomas are able to develop chemo- and radioresistance. One of its key causes is known to be hypoxia. In the last two decades, positron emission tomography (PET) with tracers like [18F]fluoromisonidazole has become established to visualize hypoxic subvolumes, offering new treatment approaches for the radio-oncologist. This review is meant to summarize the association between the hypoxic subvolume in patients with glioma and clinical and histopathological parameters, while pointing out the challenge of using this knowledge to improve patient outcome from a radio-oncological perspective. We searched the PubMed database for the terms “brain tumor”, “hypoxia” and “PET”, as well as “brain tumor”, “hypoxia” and “radiotherapy”. In the end we found 13 studies dealing with hypoxia PET-tracers in humans with glioma. The hypoxic volume (HV) is associated with higher tumor grade and poor prognosis and it seems to overlap to some extent with the contrast-enhanced volume in magnetic resonance imaging. Hypoxia visualized by PET in patients with glioma indicates poor prognosis. Before HV can be specifically targeted by radiation therapy further studies are needed.

Abstract 1875: [18F]HX4 shows potential as a predictive biomarker for the radiosensitizing capacities of metformin in a NSCLC xenograft model

Abstract Introduction Metformin (MET) may improve tumor oxygenation and thus radiotherapy (RT) response. However, appropriate imaging biomarkers for patient stratification, critically needed to be able to advance to clinical trials with MET as a radiosensitizer, are still lacking. In this proof-of-concept (POC) study, we first assessed the effect of acute MET administration on NSCLC xenograft tumor hypoxia with PET using the novel hypoxia PET tracer [18F]HX4. Second, we verified the influence of a single dose of MET prior to RT on treatment outcome. Experimental procedures A549 tumor-bearing mice (n=9; animals were inoculated in both hind legs) underwent a [18F]HX4 PET/CT scan to determine baseline tumor hypoxia. The next day, mice were administered 100 mg/kg MET IV. [18F]HX4 was administered IV 30 min. later, whereupon a second PET/CT scan was performed to assess changes in tumor hypoxia. Two days after this second scan, mice were randomized into three groups (n=3/group): a control group (1), a RT group (2), and a MET+RT group (3) with comparable tumor volumes (259±103; 218±81 and 247±121 mm3, resp.). Animals were administered 0.9% saline (groups 1-2) or 100 mg/kg MET (group 3) IV, followed by a single dose of 10 Gy 30 min. after administration (groups 2-3). Control animals of group 1 underwent sham RT. During the entire study, tumor growth was monitored triweekly by caliper measurements. Calculation of the relative tumor volumes (RTV) was initiated when tumors reached a mean baseline volume of 100 mm3 with the formula RTV=Vtime x/Vbaseline. Tumor growth inhibition (TGI) was calculated for each tumor in both treatment groups with the formula TGI=1-(RTVtreated/RTVcontrol). Results MET significantly altered A549 tumor hypoxia, as the mean [18F]HX4 tumor-to-blood ratio (TBR) was reduced from 3.03±0.27 to 2.82±0.25 (p=0.040) after MET administration. The tempering influence of MET on tumor hypoxia improved RT response, as fifteen days after irradiation TGI was 63±7% in the MET+RT group as compared to only 36±9% in the RT group, however significance was not reached. To date, tumors are still being measured triweekly and this will be continued until a volume of 1500 mm3 is reached. Then, animals will be sacrificed for histological validations on tumor tissue to assess differences in a.o. proliferation and hypoxia. Conclusions Using non-invasive imaging, we showed in A549 xenograft tumors that MET acts as a radiosensitizer possibly by decreasing tumor hypoxia. Our results imply that [18F]HX4 PET imaging holds potential as a predictive biomarker for the beneficial effect of MET in the treatment of NSCLC; however, the small group sizes in this POC study did not allow us to draw sound conclusions. These promising findings will be validated in a larger follow-up study in order to validate [18F]HX4 PET as a predictive biomarker for MET and RT response. Citation Format: Sven De Bruycker, Christel Vangestel, Tim Van den Wyngaert, Steven Deleye, Dominique Vanderghinste, Steven Staelens, Sigrid Stroobants. [18F]HX4 shows potential as a predictive biomarker for the radiosensitizing capacities of metformin in a NSCLC xenograft model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1875. doi:10.1158/1538-7445.AM2017-1875

Automated PET Radiotracer Manufacture on the BG75 System and Imaging Validation Studies of [18F]fluoromisonidazole ([18F]FMISO).

The hypoxia PET tracer, 1-[18F]fluoro-3-(2-nitro-1Himidazol- 1-yl)-propan-2-ol ([18F]FMISO) is the first radiotracer developed for hypoxia PET imaging and has shown promising for cancer diagnosis and prognosis. However, access to [18F]FMISO radiotracer is limited due to the needed cyclotron and radiochemistry expertise. The study aimed to develop the automated production method on the [18F]FMISO radiotracer with the novel fully automated platform of the BG75 system and validate its usage on animal tumor models. [18F]FMISO was produced with the dose synthesis cartridge automatically on the BG75 system. Validation of [18F]FMISO hypoxia imaging functionality was conducted on two tumor mouse models (FaDu/U87 tumor). The distribution of [18F]FMISO within tumor was further validated by the standard hypoxia marker EF5. The average radiochemical purity was (99±1) % and the average pH was 5.5±0.2 with other quality attributes passing standard criteria (n=12). Overall biodistribution for [18F]FMISO in both tumor models was consistent with reported studies where bladder and large intestines presented highest activity at 90 min post injection. High spatial correlation was found between [18F]FMISO autoradiography and EF5 hypoxia staining, indicating high hypoxia specificity of [18MF]FMISO. This study shows that qualified [18F]FMISO can be efficiently produced on the BG75 system in an automated "dose-on-demand" mode using single dose disposable cards. The possibilities of having a low-cost, automated system manufacturing ([18F]Fluoride production + synthesis + QC) different radiotracers will greatly enhance the potential for PET technology to reach new geographical areas and underserved patient populations.

Efficient preparation of 2-nitroimidazole nucleosides as precursors for hypoxia PET tracers

2-Deoxy-D-ribose was converted to α/β-mixtures of methyl 3-O-acetyl- and methyl 3-O-benzoyl-2-deoxy-5-(p-toluenesulfonyl)-D-ribofuranosides. These were reacted with boron trichloride to generate ribofuranosyl chlorides, which afforded precursors for tracers to image tumor hypoxia on substitution with salts of 2-nitroimidazole. The anomeric ratio of the nucleosides was delicately influenced by the reaction conditions.Graphical abstract

Pharmacokinetic modeling of a novel hypoxia PET tracer [18F]HX4 in patients with non-small cell lung cancer.

Background[18F]HX4 is a promising new PET tracer developed to identify hypoxic areas in tumor tissue. This study analyzes [18F]HX4 kinetics and assesses the performance of simplified methods for quantification of [18F]HX4 uptake.To this end, eight patients with non-small cell lung cancer received dynamic PET scans at three different time points (0, 120, and 240 min) after injection of 426 ± 72 MBq [18F]HX4, each lasting 30 min. Several compartment models were fitted to time activity curves (TAC) derived from various areas within tumor tissue using image-derived input functions.ResultsBest fits were obtained using the reversible two-tissue compartment model with blood volume parameter (2T4k+VB). Simplified measures correlated well with VT estimates (tumor-to-blood ratio (TBr) R 2 = 0.96, tumor-to-muscle ratio R 2 = 0.94, standardized uptake value R 2 = 0.89).Conclusions[18F]HX4 shows reversible kinetics in tumor tissue: 2T4k+VB. TBr based on static imaging at 2 or 4 h can be used for quantification of [18F]HX4 uptake.Electronic supplementary materialThe online version of this article (doi:10.1186/s40658-016-0167-y) contains supplementary material, which is available to authorized users.

64Cu-ATSM Reflects pO2 Levels in Human Head and Neck Cancer Xenografts but Not in Colorectal Cancer Xenografts: Comparison with 64CuCl2.

The hypoxia PET tracer (64)Cu-diacetyl-bis(N(4)-methylthiosemicarbazonate) ((64)Cu-ATSM) has shown promising results in clinical studies. However, concerns have been raised with regard to the possible effect of copper metabolism and free copper on tumor uptake and thereby the robustness of (64)Cu-ATSM as a hypoxia marker. In this study, accumulation and distribution of (64)Cu-ATSM and (64)CuCl2 in tumor tissue were compared with partial pressure of oxygen (pO2) probe measurements. One-hour dynamic PET scans were performed on nude mice bearing subcutaneous human head and neck tumors (FaDu) and human colorectal tumors (HT29) after administration of either (64)Cu-ATSM or (64)CuCl2. Subsequently, tracks were generated and track markers were positioned in tumors to allow for registration of their exact location on the high-resolution CT scan. After completion of the CT scan, pO2 probe measurements were performed along each track. PET and CT images were coregistered and ROIs drawn on the basis of the location of track markers and pO2 probe measurement depth. A linear mixed model for repeated measures was applied for the comparison of PET tracer uptake to corresponding pO2 values. Comparable uptake of (64)Cu-ATSM and (64)CuCl2 was found in the kidney, muscle, and liver of all animals, but (64)CuCl2 showed a higher uptake 10-60 min after injection in both tumor models. Significant differences were also found for both tumor-to-muscle and tumor-to-liver ratios. The intratumoral distribution of (64)Cu-ATSM, but not (64)CuCl2, showed a significant negative relationship with pO2 measurements in FaDu tumors. However, this relationship was not found in HT29 tumors. (64)Cu-ATSM and (64)CuCl2 displayed different uptake in tumors. In human head and neck xenografts, (64)Cu-ATSM but not (64)CuCl2 reflected pO2 measurements, indicating that (64)Cu-ATSM is a hypoxia-specific marker in this tumor type. However, data from colorectal cancer xenografts indicated that (64)Cu-ATSM may not be a hypoxia marker in all tumor types.

Robustness of quantitative hypoxia PET image analysis for predicting local tumor control

ABSTRACTBackground. Previous studies suggested the maximum tumor to background ratio (TBRmax) in FMISO PET images as a potentially predictive parameter for local control after radio-chemotherapy (CRT) in head and neck squamous cell carcinomas (HNSCC). However, different TBRmax thresholds for stratification were reported, implying that a common threshold cannot readily be used among different institutions without the risk of reducing prediction accuracy. Therefore, this study investigated the robustness of using a common pre-defined TBRmax, simulating a multicenter clinical trial.Material and methods. FMISO PET/CT was performed four hours post-injection in 22 patients with advanced HNSCC in a phase II FMISO dose escalation study. PET background regions of interest (ROIs) were manually defined in deep neck muscles. TBRmax was calculated as the mean of the highest-valued voxels within the high risk RT planning target volume. Its predictive power with respect to local control was tested, classifying patients using median TBRmax as threshold. The influence of systematically varying quantification between institutions was studied in silico by applying offsets of ± 10% and ± 20% to the TBRmax of all patients, while the threshold remained constant. The effect was analyzed using a receiver operating characteristic (ROC). True positive and false positive rates (TPR/FPR) as well as positive and negative predictive values (PPV/NPV) were evaluated.Results. For the reference condition without an offset the median TBRmax was 2.0 (1.4–3.5). Patients were classified using this threshold and TPR = 0.7, FPR = 0.4, PPV = 0.5 and NPV = 0.8 were observed. Accuracy declined with increasing offsets. Negative offsets of -10% and -20% resulted in TPR = 0.43 and 0.14, FPR = 0.20 and 0.13, PPV = 0.50 and 0.33 and NPV = 0.75 and 0.68, respectively. Positive offsets of + 10% and + 20% resulted in TPR = 1.00 and 1.00, FPR = 0.53 and 0.67, PPV = 0.47 and 0.41 and NPV = 1.00 and 1.00, respectively.Conclusions. Using a common pre-defined TBRmax threshold in multicenter trials requires careful standardization and harmonization of all steps from patient preparation to image analysis. Our results indicate that TBRmax should deviate less than 10% from reference conditions (absolute value in this dataset ± 0.2). This conclusion likely applies to all low contrast nitroimidazole hypoxia PET tracers.

Preliminary research on 1-(4-bromo-2-nitroimidazol-1-yl)-3-[18F]fluoropropan-2-ol as a novel brain hypoxia PET tracer in a rodent model of stroke

The synthesis of the new radiotracer precursor 4-Br-NITTP and the radiolabeling of the new tracer 1-(4-bromo-2-nitroimidazol-1-yl)-3-[18F]fluoropropan-2-ol (4-Br-[18F]FMISO) is reported. The cyclic voltammetry behaviour, neuronal cell toxicity, transport through the brain endothelial cell monolayer, in vivo PET imaging and preliminary calculations of the tracer uptake for a rodent model of stroke were studied for the new compound and the results were compared to those obtained with [18F]FMISO, the current gold standard PET hypoxia tracer. The new PET brain hypoxia tracer is more easily reduced, has higher CLogP than [18F]FMISO and it diffuses more rapidly through brain endothelial cells. The new compound is non-toxic to neuronal cells and it allows the in vivo mapping of stroke in mice with higher sensitivity. 4-Br-[18F]FMISO is a good candidate for further development in ischemic stroke.

Hypoxia PET Tracers in EBRT Dose Planning in Head and Neck Cancer.

Over the years, external beam radiotherapy (EBRT) has been used in the treatment management of many malignancies, including head and neck squamous cell carcinomas (HNSCC). Hypoxia is a common feature in HNSCC. Hypoxic segments in HNSCC have proven to be more resistant to radiotherapy. The ability to identify hypoxia using SPECT and PET tracers has been investigated since the late 1970s. Nitroimidazole-based compounds labelled with positron emitters have been developed to more specifically image hypoxia. This article reviews the current data from research publications on(18)F-FMISO, (18)F-FAZA, (18)F-EF5 and (64)Cu-ATSM.

A Comparison of the Imaging Characteristics and Microregional Distribution of 4 Hypoxia PET Tracers

We compared the imaging characteristics and hypoxia selectivity of 4 hypoxia PET radiotracers ((18)F-fluoromisonidazole [(18)F-FMISO], (18)F-flortanidazole [(18)F-HX4], (18)F-fluoroazomycin arabinoside [(18)F-FAZA], and (64)Cu-diacetyl-bis(N4-methylsemicarbazone) [(64)Cu-ATSM]) in a single murine xenograft tumor model condition using small-animal PET imaging and combined ex vivo autoradiography and fluorescence immunohistochemistry. Nude mice bearing SQ20b xenograft tumors were administered 1 of 4 hypoxia PET tracers and images acquired 80-90 min after injection. Frozen sections from excised tumors were then evaluated for tracer distribution using digital autoradiography and compared with histologic markers of tumor hypoxia (pimonidazole, carbonic anydrase 9 [CA9]) and vascular perfusion (Hoechst 33342). The highest tumor uptake was observed with (64)Cu-ATSM (maximum standardized uptake values [SUV(max)], 1.26 ± 0.13) and the lowest with (18)F-FAZA (SUVmax, 0.41 ± 0.24). (18)F-FMISO and (18)F-HX4 had similar intermediate tumor uptake (SUV(max), 0.76 ± 0.38 and 0.65 ± 0.19, respectively). Digital autoradiographs of hypoxia tracer distribution were compared pixel by pixel with images of immunohistochemistry stains. The fluorinated nitroimidazoles all showed radiotracer uptake increasing with pimonidazole and CA9 staining. (64)Cu-ATSM showed the opposite pattern, with highest radiotracer uptake observed in regions with the lowest pimonidazole and CA9 staining. The fluorinated nitroimidazoles showed similar tumor distributions when compared with immunohistochemistry markers of hypoxia. Variations in tumor standardized uptake value and normal tissue distribution may determine the most appropriate clinical setting for each tracer. (64)Cu-ATSM showed the highest tumor accumulation and little renal clearance. However, the lack of correlation between (64)Cu-ATSM distribution and immunohistochemistry hypoxia markers casts some doubt on the hypoxia selectivity of (64)Cu-ATSM.

Abstract B144: Hypoxia imaging with [18F]HX4 PET compared with [18F]FMISO and [18F]FAZA: From preclinical to clinical studies.

Abstract Introduction: Tumor hypoxia is an important prognostic factor in overall survival for chemotherapy and radiotherapy. Quantifying and localizing hypoxia non-invasively is therefore important. At present, hypoxia PET tracers lack high tumor-to-background (TBR) ratios. The nitroimidazole [18F]HX4, has shown promise in pre-clinical (Dubois et al. PNAS 2011) and phase I human setting (van Loon et al. EJNMMI 2010). However, until now no comparative research has been conducted between the latest promising hypoxia tracers in order to define the best marker. In a pre-clinical study [18F]HX4 was compared to the hypoxia tracers [18F]FMISO and [18F]FAZA on 3 endpoints; optimal imaging time point, spatial reproducibility and oxygen dependency. Clinical studies investigated the optimal time point and spatial-temporal stability for [18F]HX4 imaging. Methods: In pre-clinical experiments, WAG/Rij rats bearing rhabdomyosarcoma R1 tumors were injected with 20 ± 4 MBq of [18F]HX4, [18F]FMISO or [18F]FAZA, followed by PET/CT imaging up to 6h post-injection (h p.i.). For each tracer TBR was calculated and reproducibility was investigated by voxel-to-voxel analysis or a DICE similarity coefficient. Oxygen sensitivity was studied by altering the tumoral hypoxia status either by carbogen/nicotinamide treatment or 7% oxygen breathing. In clinical trials, NSCLC (N=15) [NCT01024829, NCT01210378] and HNSCC (N=10) [NCT01347281] patients were injected with 403 ± 53 MBq [18F]HX4 and PET/CT imaging was performed at several time points up to 4h p.i.. Tumor uptake (TBR>1.4), image contrast changes over time, hypoxic fraction (HF) and spatial-temporal stability were evaluated. Results: In pre-clinical setting, [18F]HX4 uptake reached a stabilized and maximal TBR at 3h p.i. (TBR=7.2) which is significantly higher compared to the plateau phase of [18F]FAZA reached at 2h p.i. (TBR=3.9, P<0.05). [18F]FMISO did not reach a plateau. Voxel-to-voxel comparisons and DICE similarity coefficient calculations showed a high short- and long-term spatial reproducibility except for [18F]FAZA, which performed poor on a 48 hour interval. Increasing the hypoxic fraction by 7% oxygen breathing resulted in enhanced mean standardized uptake values for both [18F]HX4 (P<0.01) and [18F]FAZA (P<0.05). Only [18F]FMISO uptake was found to be reversible upon reoxygenation using nicotinamide and carbogen. In patients, a TBR>1.4 was observed in 72% of NSCLC and in 74% of HNSCC target lesions, with average HF of 14 ± 17% and 25 ± 22%, respectively. In the hypoxic lesions, TBR increased up to 4h p.i., providing a better contrast at the latest time-points. [18F]HX4 uptake was stable in time and space for the heterogeneous pattern in NSCLC lesions (2h vs. 4h p.i.: r = 0.77 ± 0.10). Conclusions: Both pre-clinical and clinical studies support [18F]HX4 as a robust hypoxia tracer, with the highest image contrast from 3h p.i. onwards, sensitivity to hypoxia and high reproducibility. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):B144. Citation Format: Sarah Peeters, Catharina Zegers, Natasja Lieuwes, Wouter van Elmpt, Frank Hoebers, Hoda Sharifi, Bart Reymen, Michel Öllers, Felix Mottaghy, Jonas Eriksson, Albert Windhorst, Guus van Dongen, Dirk de Ruysscher, Ludwig Dubois, Philippe Lambin. Hypoxia imaging with [18F]HX4 PET compared with [18F]FMISO and [18F]FAZA: From preclinical to clinical studies. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr B144.

18F-EF5 PET Imaging as an Early Response Biomarker for the Hypoxia-Activated Prodrug SN30000 Combined with Radiation Treatment in a Non–Small Cell Lung Cancer Xenograft Model

Hypoxia is a significant therapeutic problem for solid tumors because hypoxic cells are treatment-resistant and more aggressive. Hypoxia-activated prodrugs such as SN30000 use a mechanism of activation in hypoxic cells similar to that of 2-nitroimidazole hypoxia PET tracers. Therefore, we have evaluated the usefulness of 2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-(18)F-pentafluoropropyl)-acetamide ((18)F-EF5) PET to monitor and predict tumor response to SN30000 plus radiation treatment (RT). Human non-small cell lung cancer xenografts (H460) in athymic rats were imaged with (18)F-EF5 PET before and after treatment with SN30000 (90 mg/kg), with or without 15-Gy RT. The feasibility of imaging early changes in hypoxia in response to SN30000 was examined 24 h after treatment, followed by ex vivo γ-counting and immunohistochemical examination to study drug-induced apoptosis. Subsequently, the therapeutic effects of SN30000 with or without RT were evaluated in tumor growth delay studies and compared with early treatment-induced changes observed by (18)F-EF5 PET. Changes in tumor hemoglobin oxygen saturation as a function of time after treatment measured by optical spectroscopy were compared with PET data. The uptake of (18)F-EF5 was significantly lower in SN30000-treated tumors than in saline controls 24 h after treatment (mean standardized uptake value, 0.44 ± 0.08 vs. 0.56 ± 0.08 for control group; P < 0.05). Apoptosis was significantly higher in SN30000-treated tumors than in controls. Early treatment-induced changes in (18)F-EF5 uptake were indicative of tumor response in growth delay studies at the group level. SN30000 plus RT significantly decreased (18)F-EF5 uptake relative to baseline and resulted in complete tumor remission in 5 of 7 animals. SN30000 alone decreased (18)F-EF5 uptake, generally in tumors with high initial standardized uptake values, and showed a minor tumor growth delay effect. The changes induced by SN30000 with or without RT in (18)F-EF5 uptake correlated with baseline hypoxia levels. RT caused significant increases in tumor oxygen concentration and hemoglobin oxygen saturation. A hypoxia PET imaging agent can measure changes in tumor hypoxic fraction in response to SN30000. These results suggest the utility of (18)F-EF5 PET for monitoring early response to tumor treatment with SN30000 plus RT in the clinical development of this novel hypoxia-activated prodrug.

PET of Hypoxia: Current and Future Perspectives

In the past 25 y, a large amount of clinical experience with hypoxia PET tracers has accumulated. This article discusses recent improvements in image acquisition protocols and tracer pharmacology that have resulted in improved understanding of the underlying physiologic processes. The widespread clinical adoption of hypoxia PET tracers will depend largely on their utility in treatment prescription and in outcome monitoring. The establishment and validation of hypoxia-directed treatment protocols are still under development, and it is envisaged that the design and use of future hypoxia PET tracers will develop as part of this process.

9 oral: Simulation of Hypoxia Pet Tracer Dynamics Based on Tumour Tissue Microsections

9 oral: Simulation of Hypoxia Pet Tracer Dynamics Based on Tumour Tissue Microsections