Hypoxia Tracer Research Articles

A Review of Hypoxia Imaging Using 18F-Fluoromisonidazole Positron Emission Tomography.

Tumor hypoxia is an essential factor related to malignancy, prognosis, and resistance to treatment. Positron emission tomography (PET) is a modality that visualizes the distribution of radiopharmaceuticals administered into the body. PET imaging with [18F]fluoromisonidazole ([18F]FMISO) identifies hypoxic tissues. Unlike [18F]fluorodeoxyglucose ([18F]FDG)-PET, fasting is not necessary for [18F]FMISO-PET, but the waiting time from injection to image acquisition needs to be relatively long (e.g., 2-4h). [18F]FMISO-PET images can be displayed on an ordinary commercial viewer on a personal computer (PC). While visual assessment is fundamental, various quantitative indices such as tumor-to-muscle ratio have also been proposed. Several novel hypoxia tracers have been invented to compensate for the limitations of [18F]FMISO.

Abstract B020: Hypoxia as tumor-intrinsic regulator of Flt3L production by NK cells

Abstract Natural killer (NK) cell abundance in the tumor correlates with increased immunotherapy responses and better survival in cancer patients. As their name suggests, NK cells can control tumors through their direct cytotoxicity, but also play an important part in regulating immune responses through their immunomodulatory production of cytokines and chemokines. We recently discovered that NK cells control immunotherapy responses through their production of a cytokine called FMS-like tyrosine kinase 3 ligand (Flt3L) in the tumor, which regulates type 1 conventional dendritic cell (cDC1) abundance. However, the mechanisms regulating NK cell phenotype and function, including Flt3L production, in the tumor remain unknown. Here, we define tumor cell-intrinsic mechanisms that control these protective NK cell functions in the tumor. Across eight mouse melanoma tumor models, we found two genetically distinct murine ectopic tumors (referred to as Mel1 and Mel2) that are similarly infiltrated with total NK cells, but have distinct skewing of NK cell phenotypes and differential production of Flt3L, correlating with differences in cDC1 abundance. Pathway analysis of differentially expressed genes in tumor cells from these two models found a significant increase in hypoxia signaling in Mel1 tumors. Using an in vivo hypoxia tracer and in vitro hypoxia chamber studies, we found that NK cells in Mel1 tumors experience more severe hypoxia and that NK cells produce significantly less Flt3L under hypoxic conditions (1% O2) compared to NK cells in normoxic conditions (21% O2). Our findings suggest that hypoxia is a tumor-intrinsic regulator of Flt3L production by NK cells and highlight a previously unknown inhibitory pathway within the tumor that can shape the protective immunomodulatory effects of NK cells. Citation Format: Renske J.E. van den Bijgaart, Shayan C. Avanessian, Nayvin W. Chew, Thao T. Tang, Kevin C. Barry. Hypoxia as tumor-intrinsic regulator of Flt3L production by NK cells [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Tumor Immunology and Immunotherapy; 2023 Oct 1-4; Toronto, Ontario, Canada. Philadelphia (PA): AACR; Cancer Immunol Res 2023;11(12 Suppl):Abstract nr B020.

Predicting the Tumour Response to Radiation by Modelling the Five Rs of Radiotherapy Using PET Images

Despite the intensive use of radiotherapy in clinical practice, its effectiveness depends on several factors. Several studies showed that the tumour response to radiation differs from one patient to another. The non-uniform response of the tumour is mainly caused by multiple interactions between the tumour microenvironment and healthy cells. To understand these interactions, five major biologic concepts called the “5 Rs” have emerged. These concepts include reoxygenation, DNA damage repair, cell cycle redistribution, cellular radiosensitivity and cellular repopulation. In this study, we used a multi-scale model, which included the five Rs of radiotherapy, to predict the effects of radiation on tumour growth. In this model, the oxygen level was varied in both time and space. When radiotherapy was given, the sensitivity of cells depending on their location in the cell cycle was taken in account. This model also considered the repair of cells by giving a different probability of survival after radiation for tumour and normal cells. Here, we developed four fractionation protocol schemes. We used simulated and positron emission tomography (PET) imaging with the hypoxia tracer 18F-flortanidazole (18F-HX4) images as input data of our model. In addition, tumour control probability curves were simulated. The result showed the evolution of tumours and normal cells. The increase in the cell number after radiation was seen in both normal and malignant cells, which proves that repopulation was included in this model. The proposed model predicts the tumour response to radiation and forms the basis for a more patient-specific clinical tool where related biological data will be included.

Synthesis of [18F]FMISO, a hypoxia-specific imaging probe for PET, an overview from a radiochemist’s perspective

Background[18F]fluoromisonidazole ([18F]FMISO, 1H-1-(3-[18F]fluoro-2-hydroxypropyl)-2-nitroimidazole) is a commonly used radiotracer for imaging hypoxic conditions in cells. Since hypoxia is prevalent in solid tumors, [18F]FMISO is in clinical application for decades to explore oxygen demand in cancer cells and the resulting impact on radiotherapy and chemotherapy.ResultsSince the introduction of [18F]FMISO as positron emission tomography imaging agent in 1986, a variety of radiosynthesis procedures for the production of this hypoxia tracer has been developed. This paper gives a brief overview on [18F]FMISO radiosyntheses published so far from its introduction until now. From a radiopharmaceutical chemist’s perspective, different precursors, radiolabeling approaches and purification methods are discussed as well as used automated radiosynthesizers, including cassette-based and microfluidic systems.ConclusionIn a GMP compliant radiosynthesis using original cassettes for FASTlab we produced [18F]FMISO in 49% radiochemical yield within 48 min with radiochemical purities > 99% and molar activities > 500 GBq/µmol. In addition, we report an easy and efficient radiosynthesis of [18F]FMISO, based on in-house prepared FASTlab cassettes, providing the radiotracer for research and preclinical purposes in good radiochemical yields (39%), high radiochemical purities (> 99%) and high molar activity (> 500 GBq/µmol) in a well-priced option.

Radiation Dosimetry of Theragnostic Pairs for Isotopes of Iodine in IAZA.

Theragnostic pairs of isotopes are used to infer radiation dosimetry for a therapeutic radiopharmaceutical from a diagnostic imaging study with the same tracer molecule labelled with an isotope better suited for the imaging task. We describe the transfer of radiation dosimetry from the diagnostic radioiodine isotope 123I, labelled for the hypoxia tracer molecule iodoazomycin arabinoside ([123I]IAZA), to isotopes 131I (therapeutic) and 124I (PET imaging). Uncertainties introduced by the dissimilar isotope half-lives are discussed in detail. Radioisotope dosimetries for [123I]IAZA were obtained previously. These data are used here to calculate residence times for 131I and 124I and their uncertainties. We distinguish two cases when extrapolating to infinity: purely physical decay (case A) and physical decay plus biological washout (case B). Organ doses were calculated using the MIRD schema with the OLIDNA/EXM code. Significant increases in some organ doses (in mSv per injected activity) were found for 131I and 124I. The most affected organs were the intestinal walls, thyroid, and urinary bladder wall. Uncertainty remained similar to 123I for case A but considerably greater for case B, especially for long biological half-lives (GI tract). Normal tissue dosimetries for IAZA must be considered carefully when substituting isotope species. A long biological half-life can significantly increase dosimetric uncertainties. These findings are relevant when considering PET imaging studies with [124I]IAZA or therapeutic administration of [131I]IAZA.

FMISO-Based Adaptive Radiotherapy in Head and Neck Cancer

Concurrent chemoradiotherapy represents one of the most used strategies in the curative treatment of patients with head and neck (HNC) cancer. Locoregional failure is the predominant recurrence pattern. Tumor hypoxia belongs to the main cause of treatment failure. Positron emission tomography (PET) using hypoxia radiotracers has been studied extensively and has proven its feasibility and reproducibility to detect tumor hypoxia. A number of studies confirmed that the uptake of FMISO in the recurrent region is significantly higher than that in the non-recurrent region. The escalation of dose to hypoxic tumors may improve outcomes. The technical feasibility of optimizing radiotherapeutic plans has been well documented. To define the hypoxic tumour volume, there are two main approaches: dose painting by contour (DPBC) or by number (DPBN) based on PET images. Despite amazing technological advances, precision in target coverage, and surrounding tissue sparring, radiation oncology is still not considered a targeted treatment if the “one dose fits all” approach is used. Using FMISO and other hypoxia tracers may be an important step for individualizing radiation treatment and together with future radiomic principles and a possible genome-based adjusting dose, will move radiation oncology into the precise and personalized era.

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.

Imaging of Tumor Hypoxia With 18F-EF5 PET/MRI in Cervical Cancer.

The aim of this study was to evaluate the distribution of hypoxia using 18F-EF5 as a hypoxia tracer in cervical cancer patients with PET/MRI. We investigated the association between this 18F-EF5-PET tracer and the immunohistochemical expression of endogenous hypoxia markers: HIF1α, CAIX, and GLUT1. Nine patients with biopsy-proven primary squamous cell cervix carcinoma (FIGO 2018 radiological stages IB1-IIIC2r) were imaged with dual tracers 18F-EF5 and 18F-FDG using PET/MRI (Int J Gynaecol Obstet. 2019;145:129-135). 18F-EF5 images were analyzed by calculating the tumor-to-muscle ratio to determine the hypoxic tissue (T/M ratio >1.5) and further hypoxic subvolume (HSV) and percentage hypoxic area. These 18F-EF5 hypoxic parameters were correlated with the size and localization of tumors in 18F-FDG PET/MRI and the results of hypoxia immunohistochemistry. All primary tumors were clearly 18F-FDG and 18F-EF5 PET positive and heterogeneously hypoxic with multiple 18F-EF5-avid areas in locally advanced cancer and single areas in clinically stage I tumors. The location of hypoxia was detected mainly in the periphery of tumor. Hypoxia parameters 18F-EF5 max T/M ratio and HSV in primary tumors correlated independently with the advanced stage (P = 0.036 and P = 0.040, respectively), and HSV correlated with the tumor size (P = 0.027). The location of hypoxia in 18F-EF5 imaging was confirmed with a higher hypoxic marker expression HIF1α and CAIX in tumor fresh biopsies. The 18F-EF5 imaging has promising potential in detecting areas of tumor hypoxia in cervical cancer.

Chemoradiation With or Without Metformin in Locally Advanced Cervical Cancer: Phase II Randomized Trial

Tumor hypoxia is associated with poor response to radiation (RT) and chemotherapy, and worse treatment outcome. We previously discovered a novel mechanism of metformin in a xenograft model: enhancing tumor RT response by inhibiting tumor cell oxygen consumption. Our population-based study showed that cumulative dose of metformin after cervical cancer diagnosis was independently associated with a decreased risk of cervical cancer-specific mortality in diabetic women. Based on the pre-clinical and retrospective data, we hypothesized that metformin would decrease tumor hypoxia and improve tumor response to RT in locally advanced cervical cancer.A window-of-opportunity, phase II randomized trial was performed in women with stage IB-IVA cervical adenocarcinoma, squamous cell or adenosquamous carcinoma. Patients underwent screening positron emission tomography (PET) imaging with hypoxia tracer fluoroazomycin arabinoside (FAZA). Those with non-hypoxic tumor (no FAZA uptake) were excluded. Patients with FAZA uptake were randomized centrally in a 2:1 ratio (in favor of metformin) to receive either metformin in combination with standard chemoRT or standard chemoRT alone. Metformin was started at 850mg once daily x 3 days, followed by 850mg twice daily throughout the entire duration of external radiotherapy. A second FAZA-PET/CT scan was performed after 1 week of metformin or no intervention in control group, just before start of chemoRT. The hypoxic volume was defined as all voxels within a tumor with standardized uptake values (SUVs) greater than 3 standard deviations (SD) from the mean gluteus maximus muscle SUV value. The hypoxic fraction (HF) was defined as the ratio of the number of hypoxic voxels to the total number of tumor voxels. The primary endpoint was absolute mean change in HF between the two FAZA-PET scans, compared using the Wilcoxon sign rank test. Disease-free survival (DFS) was defined as the duration of time from randomization to the time of relapse or death and compared using the log-rank test. Target accrual was 48 patients; the study was closed early to accrual due to FAZA availability and the COVID-19 pandemic.Of the twenty patients who consented, 6 were excluded due to no FAZA uptake and 1 withdrew. The median age of the 13 enrolled patients was 52; 8 (62%) had squamous cell carcinoma and 8 had stage IIB disease. HF of the 10 patients in the metformin arm decreased by an average of 10.2% (from 44.4% to 34.2%) ± SD 16.9% after 1 week of metformin, compared to an average increase of 4.7% (from 29.1% to 33.8%) ± 11.5% for the 3 patients in the control arm (P = 0.027). With a median follow-up of 2.8 years, the 2-year DFS was 67% for the metformin arm vs 33% for control (P = 0.09).Metformin decreases cervical tumor hypoxia with a trend towards improved DFS in this trial. A larger confirmatory trial is warranted.

Repeatability of hypoxia dose painting by numbers based on EF5-PET in head and neck cancer

Background Hypoxia dose painting is a radiotherapy technique to increase the dose to hypoxic regions of the tumour. Still, the clinical effect relies on the reproducibility of the hypoxic region shown in the medical image. 18F-EF5 is a hypoxia tracer for positron emission tomography (PET), and this study investigated the repeatability of 18F-EF5-based dose painting by numbers (DPBN) in head and neck cancer (HNC). Materials and methods Eight HNC patients undergoing two 18F-EF5-PET/CT sessions (A and B) before radiotherapy were included. A linear conversion of PET signal intensity to radiotherapy dose prescription was employed and DPBN treatment plans were created using the image basis acquired at each PET/CT session. Also, plan A was recalculated on the image basis for session B. Voxel-by-voxel Pearson’s correlation and quality factor were calculated to assess the DPBN plan quality and repeatability. Results The mean (SD) correlation coefficient between DPBN prescription and plan was 0.92 (0.02) and 0.93 (0.02) for sessions A and B, respectively, with corresponding quality factors of 0.02 (0.002) and 0.02 (0.003), respectively. The mean correlation between dose prescriptions at day A and B was 0.72 (0.13), and 0.77 (0.12) for the corresponding plans. A mean correlation of 0.80 (0.08) was found between plan A, recalculated on image basis B, and plan B. Conclusion Hypoxia DPBN planning based on 18F-EF5-PET/CT showed high repeatability. This illustrates that 18F-EF5-PET provides a robust target for dose painting.

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.

Synthesis and Preliminary Evaluation of a Novel 18F-Labeled 2-Nitroimidazole Derivative for Hypoxia Imaging.

ObjectiveHypoxia is prevalent in tumors and plays a pivotal role in resistance to chemoradiotherapy. 18F-MISO (18F-labeled fluoromisonidazole) is currently the preferred choice of PET hypoxia tracers in clinical practice, but has severe disadvantages involving complex labeling methods and low efficient imaging due to lipophilicity. We aimed to design a novel nitroimidazole derivative labeled by 18F via a chelation technique to detect hypoxic regions and provide a basis for planning radiotherapy.Materials and MethodsFirst, we synthesized a 2-nitroimidazole precursor, 2-[4-(carboxymethyl)-7-[2-(2-(2-nitro-1H-imidazol-1-yl)acetamido)ethyl]-1,4,7-triazanonan-1-yl]acetic acid (NOTA-NI). For 18F-labeling, a 18F solution was reacted with a mixture of AlCl3 and NOTA-NI at pH 3.5 and 100°C for 20 min, and the radiochemical purity and stability were evaluated. Biological behaviors of Al18F-NOTA-NI were analyzed by an uptake study in ECA109 normoxic and hypoxic cells, and a biodistribution study and microPET imaging in ECA109 xenografted mice.ResultsAl18F-NOTA-NI required a straightforward and efficient labeling procedure compared with 18F-MISO. The uptake values were distinctly higher in hypoxic tumor cells. Animal studies revealed that the imaging agent was principally excreted via the kidneys. Due to hydrophilicity, the radioactivities in blood and muscle were decreased, and we could clearly distinguish xenografted tumors from para-carcinoma tissue by PET imaging.ConclusionsThe nitroimidazole tracer Al18F-NOTA-NI steadily accumulated in hypoxic areas in tumors and was rapidly eliminated from normal tissue. It appears to be a promising candidate for hypoxia imaging with high sensitivity and resolution.

18F]-FDG-PET/CT and [18F]-FAZA-PET/CT Hypoxia Imaging of Metastatic Thyroid Cancer: Association with Short-Term Progression After Radioiodine Therapy.

To examine the relationships between 2-deoxy-2-[18F]fluoro-D-glucose ([18F]-FDG) and hypoxia tracer [18F]fluoro-azomycinarabinofuranoside ([18F]-FAZA) and between 131I and [18F]-FAZA uptake in patients with metastatic thyroid cancer and to evaluate imaging features associated with short-term progression after 131I therapy. The study population was 20 patients (17 women and 3 men; mean age, 67years) with metastatic thyroid cancer who underwent both [18F]-FDG- and [18F]-FAZA-positron emission tomography (PET)/X-ray computed tomography (CT) examinations before 131I therapy. Short-term response to radioiodine was assessed (mean follow-up, 19months ± 9). PET parameters including [18F]-FDG-SUVmax, [18F]-FAZA-SUVmax, and [18F]-FAZA-tumor-to-muscle [T/M] were obtained. Mann-Whitney U, Wilcoxon signed-rank, or χ2 tests were used to assess differences between two quantitative variables or compare categorical data. Predictive factors for short-term progression were investigated with logistic regression analysis. Eleven lymph node metastatic lesions were identified in 9 patients and 46 distant metastatic lesions (lung, 19; bone, 17; and liver, 10) in 14 patients. A total of 24 131I-positive and 33 131I-negative lesions were detected. SUVmax was significantly lower with [18F]-FAZA-PET/CT (1.3 ± 0.6) than with [18F]-FDG-PET/CT (6.4 ± 5.9, p < 0.001). No significant correlation was observed between [18F]-FAZA-PET/CT and 131I imaging concerning visibility (p = 0.36). After 131I therapy, 31 of 57 metastatic lesions displayed short-term progression. Multivariate logistic regression revealed that [18F]-FDG-SUVmax (p = 0.022) and [18F]-FAZA-T/M (p = 0.002) showed significant associations with short-term progression. Although [18F]-FAZA uptake was low in metastatic thyroid cancers, not only glucose metabolism but also hypoxic conditions may be associated with progression after 131I therapy in patients with metastatic thyroid cancer.

The cellular basis of increased PET hypoxia tracer uptake in focal cerebral ischemia with comparison between [18F]FMISO and [64Cu]CuATSM

PET hypoxia imaging can assess tissue viability in acute ischemic stroke (AIS). [18F]FMISO is an established tracer but requires substantial accumulation time, limiting its use in hyperacute AIS. [64Cu]CuATSM requires less accumulation time and has shown promise as a hypoxia tracer. We compared these tracers in a M2-occlusion model (M2CAO) with preserved collateral blood flow. Rats underwent M2CAO and [18F]FMISO (n = 12) or [64Cu]CuATSM (n = 6) examinations. [64Cu]CuATSM animals were also examined with MRI. Pimonidazole was used as a surrogate for [18F]FMISO in an immunofluorescence analysis employed to profile levels of hypoxia in neurons (NeuN) and astrocytes (GFAP). There was increased [18F]FMISO uptake in the M2CAO cortex. No increase in [64Cu]CuATSM activity was found. The pimonidazole intensity of neurons and astrocytes was increased in hypoxic regions. The pimonidazole intensity ratio was higher in neurons than in astrocytes. In the majority of animals, immunofluorescence revealed a loss of astrocytes within the core of regions with increased pimonidazole uptake. We conclude that [18F]FMISO is superior to [64Cu]CuATSM in detecting hypoxia in AIS, consistent with an earlier study. [18F]FMISO may provide efficient diagnostic imaging beyond the hyperacute phase. Results do not provide encouragement for the use of [64Cu]CuATSM in experimental AIS.

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.

Correlation of hypoxia as measured by fluorine-18 fluoroerythronitroimidazole (18F-FETNIM) PET/CT and overall survival in glioma patients.

Hypoxia is important in the biology of glioma in humans. Positron emission tomography/computed tomography (PET/CT) with a hypoxia tracer offers a noninvasive method to differentiate individual tumor biology and potentially modify treatment for patients with malignancies. The purpose of this study was to determine whether hypoxia, as measured by fluorine-18 fluoroerythronitroimidazole (18F-FETNIM) PET/CT, was associated with tumor grade, overall survival (OS), and immunohistochemical features related to hypoxia, proliferation, angiogenesis, and the invasion of gliomas. Twenty-five patients with gliomas in whom gross maximal resection could be safely attempted were analyzed. All patients underwent 18F-FETNIM PET/CT studies before surgery. The maximum standardized uptake value (SUVmax) was obtained from the PET images of tumor tissues. Tumor specimens were stereotactically obtained for the immunohistochemical staining of hypoxia-inducible factor-1 alpha (HIF-1α), Ki-67, vascular endothelial growth factor (VEGF), and matrix metalloproteinase 9 (MMP-9). A correlation between the SUVmax and glioma grade was found (r = 0.881, P < 0.001). The SUVmax was significantly correlated with the expression of HIF-1α, Ki-67, VEGF, and MMP-9 (r = 0.820, 0.747, 0.606, and 0.727; all P < 0.001). Patients with a high SUVmax had significantly worse 3-year OS than those with a low SUVmax (24.4% vs. 82.1%, P = 0.003). 18F-FETNIM PET/CT provides an excellent noninvasive assessment of hypoxia in glioma. It can be used to understand the mechanisms by which hypoxia affects the OS of glioma patients.

Dual-tracer PET of viable tumor volume and hypoxia for identification of necrosis-containing radio-resistant Sub-volumes

Introduction: Positron emission tomography (PET) using hypoxia-selective tracers like FAZA may guide radiation dose-escalation approaches. However, poor resolution combined with slow tracer retention in relatively inaccessible target cells and slow clearance of unbound tracer results in low-contrast images, and areas where viable hypoxic tracer retaining cells and necrosis (no tracer) are intermixed may pass unnoticed during image thresholding. Here we hypothesized that a clinical feasible one-day dual tracer approach that combines a short-lived (e.g., 11C labeled) metabolic tracer that provides voxel-wise information on viable tissue volume (preferably independently of tumor microenvironment) and a hypoxia marker, may limit threshold-based errors.Material and methods: 11C-acetate and 11C-methionine uptake was quantified in tumor cell lines under tumor microenvironment-mimicking conditions of high/low O2 (21%/0%) and pH (7.4/6.7). Next, tumor-bearing mice were administered FAZA and sacrificed 1 h (mimics a clinical low-contrast image scenario) or 4 h (high contrast) later. In addition, all mice were administered pimonidazole (hypoxia) and 14C-methionine 1 h prior to sacrifice. Tumor tissue sections were analyzed using dual-tracer autoradiography. Finally, FAZA, or FAZA normalized to 14C-methionine retention (to adjust for differences in viable tissue volume) was compared to hypoxic fraction (deduced from immune-histological analysis of pimonidazole; ground truth) in PET-mimicking macroscopic pixels with variable extent of necrosis/hypoxia.Results/conclusions: Low pH stimulated 11C-acetate retention in many cell lines, and uptake was further modified by anoxia, compromising its usefulness as a universal marker of viable tumor volume. In contrast, 11C-methionine was largely unaffected by the in vitro microenvironment and was further tested in mice. Necrosis increased the risk of missing hypoxia-containing pixels during thresholding and hypoxic fraction and FAZA signal correlated poorly in the low contrast-scenario. Voxel-based normalization to 14C-methionine increased the likelihood of detecting voxels harboring hypoxic cells profoundly, but did not consistently improve the correlation between the density of hypoxic cells and tracer signal.

Biodistribution and radiation dosimetry of the novel hypoxia PET probe [18F]DiFA and comparison with [18F]FMISO

BackgroundTo facilitate hypoxia imaging in a clinical setting, we developed 1-(2,2-dihydroxymethyl-3-[18F]-fluoropropyl)-2-nitroimidazole ([18F]DiFA) as a new tracer that targets tumor hypoxia with its lower lipophilicity and efficient radiosynthesis. Here, we evaluated the radiation dosage, biodistribution, human safety, tolerability, and early elimination after the injection of [18F]DiFA in healthy subjects, and we performed a preliminary clinical study of patients with malignant tumors in a comparison with [18F]fluoromisonidazole ([18F]FMISO).ResultsThe single administration of [18F]DiFA in 8 healthy male adults caused neither adverse events nor abnormal clinical findings. Dynamic and sequential whole-body scans showed that [18F]DiFA was rapidly cleared from all of the organs via the hepatobiliary and urinary systems. The whole-body mean effective dose of [18F]DiFA estimated by using the medical internal radiation dose (MIRD) schema with organ level internal dose assessment/exponential modeling (OLINDA/EXM) computer software 1.1 was 14.4 ± 0.7 μSv/MBq. Among the organs, the urinary bladder received the largest absorbed dose (94.7 ± 13.6 μSv/MBq). The mean absorbed doses of the other organs were equal to or less than those from other hypoxia tracers. The excretion of radioactivity via the urinary system was very rapid, reaching 86.4 ± 7.1% of the administered dose. For the preliminary clinical study, seven patients were subjected to [18F]FMISO and [18F]DiFA positron emission tomography (PET) at 48-h intervals to compare the two tracers’ diagnostic ability for tumor hypoxia. The results of the tumor hypoxia evaluation by [18F]DiFA PET at 1 h and 2 h were not significantly different from those obtained with [18F]FMISO PET at 4 h ([18F]DiFA at 1 h, p = 0.32; [18F]DiFA at 2 h, p = 0.08). Moreover, [18F]DiFA PET at both 1 h (k = 0.68) and 2 h (k = 1.00) showed better inter-observer reproducibility than [18F]FMISO PET at 4 h (k = 0.59).Conclusion[18F]DiFA is well tolerated, and its radiation dose is comparable to those of other hypoxia tracers. [18F]DiFA is very rapidly cleared via the urinary system. [18F]DiFA PET generated comparable images to [18F]FMISO PET in hypoxia imaging with shorter waiting time, demonstrating the promising potential of [18F]DiFA PET for hypoxia imaging and for a multicenter trial.

Hypoxia Imaging and Adaptive Radiotherapy: A State-of-the-Art Approach in the Management of Glioma.

Severe hypoxia [oxygen partial pressure (pO2) below 5–10 mmHg] is more frequent in glioblastoma multiforme (GBM) compared to lower-grade gliomas. Seminal studies in the 1950s demonstrated that hypoxia was associated with increased resistance to low–linear energy transfer (LET) ionizing radiation. In experimental conditions, the total radiation dose has to be multiplied by a factor of 3 to achieve the same cell lethality in anoxic situations. The presence of hypoxia in human tumors is assumed to contribute to treatment failures after radiotherapy (RT) in cancer patients. Therefore, a logical way to overcome hypoxia-induced radioresistance would be to deliver substantially higher doses of RT in hypoxic volumes delineated on pre-treatment imaging as biological target volumes (BTVs). Such an approach faces various fundamental, technical, and clinical challenges. The present review addresses several technical points related to the delineation of hypoxic zones, which include: spatial accuracy, quantitative vs. relative threshold, variations of hypoxia levels during RT, and availability of hypoxia tracers. The feasibility of hypoxia imaging as an assessment tool for early tumor response to RT and for predicting long-term outcomes is discussed. Hypoxia imaging for RT dose painting is likewise examined. As for the radiation oncologist's point of view, hypoxia maps should be converted into dose-distribution objectives for RT planning. Taking into account the physics and the radiobiology of various irradiation beams, preliminary in silico studies are required to investigate the feasibility of dose escalation in terms of normal tissue tolerance before clinical trials are undertaken.

Impact of SBRT fractionation in hypoxia dose painting - Accounting for heterogeneous and dynamic tumor oxygenation.

Tumor hypoxia, often found in nonsmall cell lung cancer (NSCLC), implies an increased resistance to radiotherapy. Pretreatment assessment of tumor oxygenation is, therefore, warranted in these patients, as functional imaging of hypoxia could be used as a basis for dose painting. This study aimed at investigating the feasibility of using a method for calculating the dose required in hypoxic subvolumes segmented on 18 F-HX4 positron emission tomography (PET) imaging of NSCLC. Positron emission tomography imaging data based on the hypoxia tracer 18 F-HX4 of 19 NSCLC patients were included in the study. Normalized tracer uptake was converted to oxygen partial pressure (pO2 ) and hypoxic target volumes (HTVs) were segmented using a threshold of 10mmHg. Uniform doses required to overcome the hypoxic resistance in the target volumes were calculated based on a previously proposed method taking into account the effect of interfraction reoxygenation, for fractionation schedules ranging from extremely hypofractionated stereotactic body radiotherapy (SBRT) to conventionally fractionated radiotherapy. Gross target volumes ranged between 6.2 and 859.6cm3 , and the hypoxic fraction<10mmHg between 1.2% and 72.4%. The calculated doses for overcoming the resistance of cells in the HTVs were comparable to those currently prescribed in clinical practice as well as those previously tested in feasibility studies on dose escalation in NSCLC. Depending on the size of the HTV and the distribution of pO2 , HTV doses were calculated as 43.6-48.4Gy for a three-fraction schedule, 51.7-57.6Gy for five fractions, and 59.5-66.4Gy for eight fractions. For patients in whom the HTV pO2 distribution was more favorable, a lower dose was required despite a bigger volume. Tumor control probability was lower for single-fraction schedules, while higher levels of tumor control probability were found for schedules employing several fractions. The method to account for heterogeneous and dynamic hypoxia in target volume segmentation and dose prescription based on 18 F-HX4-PET imaging appears feasible in NSCLC patients. The distribution of oxygen partial pressure within HTV could impact the required prescribed dose more than the size of the volume.