18F-Fluoromisonidazole in tumor hypoxia imaging

Optimization and validation of low-field MP2RAGE T1 mapping on 0.35T MR-Linac: Toward adaptive dose painting with hypoxia biomarkers

BACKGROUND

Stereotactic MR-guided Adaptive Radiation Therapy (SMART) dose painting for hypoxia has potential to improve treatment outcomes, but clinical implementation on low-field MR-Linac faces substantial challenges due to dramatically lower signal-to-noise ratio (SNR) characteristics. While quantitative MRI and T1 mapping of hypoxia biomarkers show promise, T1-to-noise ratio (T1NR) optimization at low fields is paramount, particularly for the clinical implementation of oxygen-enhanced (OE)-MRI. The 3D Magnetization Prepared (2) Rapid Gradient Echo (MP2RAGE) sequence stands out for its ability to acquire homogeneous T1-weighted contrast images with simultaneous T1 mapping.

PURPOSE

To optimize MP2RAGE for low-field T1 mapping; conduct experimental validation in a ground-truth phantom; establish feasibility and reproducibility of low-field MP2RAGE acquisition and T1 mapping in healthy volunteers.

METHODS

The MP2RAGE optimization was performed to maximize the contrast-to-noise ratio (CNR) of T1 values in white matter (WM) and gray matter (GM) brain tissues at 0.35T. Low-field MP2RAGE images were acquired on a 0.35T MR-Linac (ViewRay MRIdian) using a multi-channel head coil. Validation of T1 mapping was performed with a ground-truth Eurospin phantom, containing inserts of known T1 values (400-850 ms), with one and two average (1A and 2A) MP2RAGE scans across four acquisition sessions, resulting in eight T1 maps. Mean (± SD) T1 relative error, T1NR, and intersession coefficient of variation (CV) were determined. Whole-brain MP2RAGE scans were acquired in 5 healthy volunteers across two sessions (A and B) and T1 maps were generated. Mean (± SD) T1 values for WM and GM were determined. Whole-brain T1 histogram analysis was performed, and reproducibility was determined with the CV between sessions. Voxel-by-voxel T1 difference maps were generated to evaluate 3D spatial variation.

RESULTS

Low-field MP2RAGE optimization resulted in parameters: MP2RAGETR of 3250 ms, inversion times (TI1/TI2) of 500/1200 ms, and flip angles (α1/α2) of 7/5°. Eurospin T1 maps exhibited a mean (± SD) relative error of 3.45% ± 1.30%, T1NR of 20.13 ± 5.31, and CV of 2.22% ± 0.67% across all inserts. Whole-brain MP2RAGE images showed high anatomical quality with clear tissue differentiation, resulting in mean (± SD) T1 values: 435.36 ± 10.01 ms for WM and 623.29 ± 14.64 ms for GM across subjects, showing excellent concordance with literature. Whole-brain T1 histograms showed high intrapatient and intersession reproducibility with characteristic intensity peaks consistent with voxel-level WM and GM T1 values. Reproducibility analysis revealed a CV of 0.46% ± 0.31% and 0.35% ± 0.18% for WM and GM, respectively. Voxel-by-voxel T1 difference maps show a normal 3D spatial distribution of noise in WM and GM.

CONCLUSIONS

Low-field MP2RAGE proved effective in generating accurate, reliable, and reproducible T1 maps with high T1NR in phantom studies and in vivo feasibility established in healthy volunteers. While current work is focused on refining the MP2RAGE protocol to enable clinically efficient OE-MRI, this study establishes a foundation for TOLD T1 mapping for hypoxia biomarkers. This advancement holds the potential to facilitate a paradigm shift toward MR-guided biological adaptation and dose painting by leveraging 3D hypoxic spatial distributions and improving outcomes in conventionally challenging-to-treat cancers.

Nuclear Medicine Imaging Techniques in Glioblastomas

Glioblastomas are the most common primary malignant grade 4 tumors of the central nervous system (CNS). The treatment and management of such tumors requires a multidisciplinary approach and nuclear medicine techniques play an important role in this process. Glioblastoma, which recurs despite current treatments and becomes resistant to treatments, is among the tumors with the lowest survival rate, with a survival rate of approximately 8 months. Currently, the standard treatment of glioblastoma is adjuvant chemoradiotherapy after surgical resection. There have been many recent advances in the field of Nuclear Medicine in glioblastoma. PET scans are critical in determining tumor localization, pre-surgical planning, evaluation of post-treatment response and detection of recurrence. Advances in the treatment of glioblastoma and a better understanding of the biological characteristics of the disease have contributed to the development of nuclear medicine techniques. This review, in addition to other studies, is intended as a general imaging summary guide and includes some new expressions discovered in glioblastoma. This review discusses recent advances in nuclear medicine in glioblastoma.

Optimising hypoxia PET imaging and its applications in guiding targeted radiation therapy for non-small cell lung cancer: a scoping review

INTRODUCTION

Non-small cell lung cancer (NSCLC) is the leading cause of cancer-related death. Definitive treatment includes chemotherapy and radiation therapy. Tumour hypoxia impacts the efficacy of these treatment modalities. Novel positron-emission tomography (PET) imaging has been developed to non-invasively quantify hypoxic tumour subregions, and to guide personalised treatment strategies. This review evaluates the reliability of hypoxia imaging in NSCLC in relation to various tracers, its correlations to treatment-related outcomes, and to assess if this imaging modality can be meaningfully applied into radiation therapy workflows.

METHODS

A literature search was conducted on the Medline (Ovid) and Embase databases. Searches included terms related to 'hypoxia', 'positron-emission tomography', 'magnetic resonance imaging' and 'lung cancer'. Results were filtered to exclude studies prior to 2011, and animal studies were excluded. Only studies referring to a confirmed pathology of NSCLC were included, while disease staging was not a limiting factor. Full-text English language and translated literature examined included clinical trials, clinical cohort studies and feasibility studies.

RESULTS

Quantification of hypoxic volumes in a pre-treatment setting is of prognostic value, and indicative of treatment response. Dosimetric comparisons have highlighted potential to significantly dose escalate to hypoxic volumes without risk of additional toxicity. However, clinical data to support these strategies are lacking.

CONCLUSION

Heterogenous study design and non-standardised imaging parameters have led to a lack of clarity regarding the application of hypoxia PET imaging in NSCLC. PET imaging using nitroimidazole tracers is the most investigated method of non-invasively measuring tumour hypoxia and has potential to guide hypoxia-targeted radiation therapy. Further clinical research is required to elucidate the benefits versus risks of dose-escalation strategies.

PET radiomics in lung cancer: advances and translational challenges

Radiomics is an emerging field of medical imaging that aims at improving the accuracy of diagnosis, prognosis, treatment planning and monitoring non-invasively through the automated or semi-automated quantitative analysis of high-dimensional image features. Specifically in the field of nuclear medicine, radiomics utilizes imaging methods such as positron emission tomography (PET) and single photon emission computed tomography (SPECT) to evaluate biomarkers related to metabolism, blood flow, cellular activity and some biological pathways. Lung cancer ranks among the leading causes of cancer-related deaths globally, and radiomics analysis has shown great potential in guiding individualized therapy, assessing treatment response, and predicting clinical outcomes. In this review, we summarize the current state-of-the-art radiomics progress in lung cancer, highlighting the potential benefits and existing limitations of this approach. The radiomics workflow was introduced first including image acquisition, segmentation, feature extraction, and model building. Then the published literatures were described about radiomics-based prediction models for lung cancer diagnosis, differentiation, prognosis and efficacy evaluation. Finally, we discuss current challenges and provide insights into future directions and potential opportunities for integrating radiomics into routine clinical practice.

Data Science Opportunities To Improve Radiotherapy Planning and Clinical Decision Making

Radiotherapy aims to achieve a high tumor control probability while minimizing damage to normal tissues. Personalizing radiotherapy treatments for individual patients, therefore, depends on integrating physical treatment planning with predictive models of tumor control and normal tissue complications. Predictive models could be improved using a wide range of rich data sources, including tumor and normal tissue genomics, radiomics, and dosiomics. Deep learning will drive improvements in classifying normal tissue tolerance, predicting intra-treatment tumor changes, tracking accumulated dose distributions, and quantifying the tumor response to radiotherapy based on imaging. Mechanistic patient-specific computer simulations ('digital twins') could also be used to guide adaptive radiotherapy. Overall, we are entering an era where improved modeling methods will allow the use of newly available data sources to better guide radiotherapy treatments.

18 F-Fluorination of Nitroimidazolyl-Containing Sultone: A Direct Access to a Highly Hydrophilic Radiotracer for High-Performance Positron Emission Tomography Imaging of Hypoxia

Hypoxia, characterized by nonphysiological levels of oxygen tension, is a key phenomenon common to the majority of malignant tumors with poor prognosis. Many efforts have been made to develop hypoxia imaging for diagnosis, staging, and monitoring of diseases, as well as for evaluating therapies. PET Imaging using 18F-fluoronitroimidazoles (i.e., [18F]FMISO as a lead radiotracer) has demonstrated potential for clinical investigations, but the poor contrast and prolonged acquisition times (>2.5 h) strongly limit its accuracy and routine developments. Here, we report an original [18F]fluoronitroimidazole bearing a sulfo group ([18F]FLUSONIM) that displays highly hydrophilic properties and rapid clearance, providing high-performance hypoxia specific PET imaging. We describe the synthesis and radiosynthesis of [18F]FLUSONIM, its in vivo preclinical evaluation by PET imaging in healthy rats and a rhabdomyosarcoma rat model, as well as its radiometabolization and histological studies. [18F]FLUSONIM was prepared in a single step by high yielding radiofluorination of a sultone precursor, highlighting the advantages of this new radiolabeling approach not yet explored for radiopharmaceutical development. PET imaging experiments were conducted by systematically comparing [18F]FLUSONIM to [18F]FMISO as a reference. The overall results unequivocally demonstrate that the developed radiopharmaceutical meets the criteria of an ideal candidate for hypoxia PET imaging-rapid and efficient radiosynthesis, total stability, exclusive urinary elimination, high specificity for hypoxic regions, unprecedented tumor/background ratios, short acquisition delays (<60 min), and promising potential for further preclinical and clinical applications.

Molecular imaging of tumor hypoxia: Evolution of nitroimidazole radiopharmaceuticals and insights for future development

Though growing evidence has been collected in support of the concept of dose escalation based on the molecular level images indicating hypoxic tumor sub-volumes that could be radio-resistant, validation of the concept is still a work in progress. Molecular imaging of tumor hypoxia using radiopharmaceuticals is expected to provide the required input to plan dose escalation through Image Guided Radiation Therapy (IGRT) to kill/control the radio-resistant hypoxic tumor cells. The success of the IGRT, therefore, is heavily dependent on the quality of images obtained using the radiopharmaceutical and the extent to which the image represents the true hypoxic status of the tumor in spite of the heterogeneous nature of tumor hypoxia. Available literature on radiopharmaceuticals for imaging hypoxia is highly skewed in favor of nitroimidazole as the pharmacophore given their ability to undergo oxygen dependent reduction in hypoxic cells. In this context, present review on nitroimidazole radiopharmaceuticals would be immensely helpful to the researchers to obtain a birds-eye view on what has been achieved so far and what can be tried differently to obtain a better hypoxia imaging agent. The review also covers various methods of radiolabeling that could be utilized for developing radiotracers for hypoxia targeting applications.

Comparison of three 18F-labeled 2-nitroimidazoles for imaging hypoxia in breast cancer xenografts: [18F]FBNA, [18F]FAZA and [18F]FMISO

BACKGROUND

Tumour hypoxia is associated with increased metastasis, invasion, poor therapy response and prognosis. Most PET radiotracers developed and used for clinical hypoxia imaging belong to the 2-nitroimidazole family. Recently we have developed novel 2-nitroimidazole-derived PET radiotracer [18F]FBNA (N-(4-[18F]fluoro-benzyl)-2-(2-nitro-1H-imidazol-1-yl)-acet-amide), an 18F-labeled analogue of antiparasitic drug benznidazole. The present study aimed to analyze its radio-pharmacological properties and systematically compare its PET imaging profiles with [18F]FMISO and [18F]FAZA in preclinical triple-negative (MDA-MB231) and estrogen receptor-positive (MCF-7) breast cancer models.

METHODS

In vitro cellular uptake experiments were carried out in MDA-MB321 and MCF-7 cells under normoxic and hypoxic conditions. Metabolic stability in vivo was determined in BALB/c mice using radio-TLC analysis. Dynamic PET experiments over 3 h post-injection were performed in MDA-MB231 and MCF-7 tumour-bearing mice. Those PET data were used for kinetic modelling analysis utilizing the reversible two-tissue-compartment model. Autoradiography was carried out in tumour tissue slices and compared to HIF-1α immunohistochemistry. Detailed ex vivo biodistribution was accomplished in BALB/c mice, and this biodistribution data were used for dosimetry calculation.

RESULTS

Under hypoxic conditions in vitro cellular uptake was elevated in both cell lines, MCF-7 and MDA-MB231, for all three radiotracers. After intravenous injection, [18F]FBNA formed two radiometabolites, resulting in a final fraction of 65 ± 9 % intact [18F]FBNA after 60 min p.i. After 3 h p.i., [18F]FBNA tumour uptake reached SUV values of 0.78 ± 0.01 in MCF-7 and 0.61 ± 0.04 in MDA-MB231 tumours (both n = 3), representing tumour-to-muscle ratios of 2.19 ± 0.04 and 1.98 ± 0.15, respectively. [18F]FMISO resulted in higher tumour uptakes (SUV 1.36 ± 0.04 in MCF-7 and 1.23 ± 0.08 in MDA-MB231 (both n = 4; p < 0.05) than [18F]FAZA (0.66 ± 0.11 in MCF-7 and 0.63 ± 0.14 in MDA-MB231 (both n = 4; n.s.)), representing tumour-to-muscle ratios of 3.24 ± 0.30 and 3.32 ± 0.50 for [18F]FMISO, and 2.92 ± 0.74 and 3.00 ± 0.42 for [18F]FAZA, respectively. While the fraction per time of radiotracer entering the second compartment (k3) was similar within uncertainties for all three radiotracers in MDA-MB231 tumours, it was different in MCF-7 tumours. The ratios k3/(k3 + k2) and K1*k3/(k3 + k2) in MCF-7 tumours were also significantly different, indicating dissimilar fractions of radiotracer bound and trapped intracellularly: K1*k3/(k2 + k3) [18F]FMISO (0.0088 ± 0.001)/min, n = 4; p < 0.001) > [18F]FAZA (0.0052 ± 0.002)/min, n = 4; p < 0.01) > [18F]FBNA (0.003 ± 0.001)/min, n = 3). In contrast, in MDA-MB231 tumours, only K1 was significantly elevated for [18F]FMISO. However, this did not result in significant differences for K1*k3/(k2 + k3) for all three 2-nitroimidazoles in MDA-MB231 tumours.

CONCLUSION

Novel 2-nitroimidazole PET radiotracer [18F]FBNA showed uptake into hypoxic breast cancer cells and tumour tissue presumably associated with elevated HIF1-α expression. Systematic comparison of PET imaging performance with [18F]FMISO and [18F]FAZA in different types of preclinical breast cancer models revealed a similar tumour uptake profile for [18F]FBNA with [18F]FAZA and, despite its higher lipophilicity, still a slightly higher muscle tissue clearance compared to [18F]FMISO.

18 F-FMISO PET of an Intracranial Tuberculoma : A Great Mimicker

ABSTRACT

Solitary intracranial tuberculomas are rare and frequently misdiagnosed as brain tumors. We report a case of intracranial tuberculous granuloma mimicking a high-grade glioma with avid uptake on 18 F-fluoromisonidazole PET/CT. It has been believed that hypoxia exists within the tuberculosis granuloma, and that this hypoxic environment causes Mycobacterium tuberculosis to lie dormant and asymptomatic infection to occur. This hypoxic and necrotic condition inside tuberculous granuloma may lead to high accumulation of 18 F-fluoromisonidazole in this case.

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.

RESULTS

Since 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.

CONCLUSION

In 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.

Application of Metabolic Reprogramming to Cancer Imaging and Diagnosis

Cellular metabolism governs the signaling that supports physiological mechanisms and homeostasis in an individual, including neuronal transmission, wound healing, and circadian clock manipulation. Various factors have been linked to abnormal metabolic reprogramming, including gene mutations, epigenetic modifications, altered protein epitopes, and their involvement in the development of disease, including cancer. The presence of multiple distinct hallmarks and the resulting cellular reprogramming process have gradually revealed that these metabolism-related molecules may be able to be used to track or prevent the progression of cancer. Consequently, translational medicines have been developed using metabolic substrates, precursors, and other products depending on their biochemical mechanism of action. It is important to note that these metabolic analogs can also be used for imaging and therapeutic purposes in addition to competing for metabolic functions. In particular, due to their isotopic labeling, these compounds may also be used to localize and visualize tumor cells after uptake. In this review, the current development status, applicability, and limitations of compounds targeting metabolic reprogramming are described, as well as the imaging platforms that are most suitable for each compound and the types of cancer to which they are most appropriate.

Potential novel imaging targets of inflammation in cardiac sarcoidosis

Cardiac sarcoidosis (CS) is an inflammatory disease with high morbidity and mortality, with a pathognomonic feature of non-caseating granulomatous inflammation. While 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) is a well-established modality to image inflammation and diagnose CS, there are limitations to its specificity and reproducibility. Imaging focused on the molecular processes of inflammation including the receptors and cellular microenvironments present in sarcoid granulomas provides opportunities to improve upon FDG-PET imaging for CS. This review will highlight the current limitations of FDG-PET imaging for CS while discussing emerging new nuclear imaging molecular targets for the imaging of cardiac sarcoidosis.

The Role of Imaging Biomarkers to Guide Pharmacological Interventions Targeting Tumor Hypoxia

Hypoxia is a common feature of solid tumors that contributes to angiogenesis, invasiveness, metastasis, altered metabolism and genomic instability. As hypoxia is a major actor in tumor progression and resistance to radiotherapy, chemotherapy and immunotherapy, multiple approaches have emerged to target tumor hypoxia. It includes among others pharmacological interventions designed to alleviate tumor hypoxia at the time of radiation therapy, prodrugs that are selectively activated in hypoxic cells or inhibitors of molecular targets involved in hypoxic cell survival (i.e., hypoxia inducible factors HIFs, PI3K/AKT/mTOR pathway, unfolded protein response). While numerous strategies were successful in pre-clinical models, their translation in the clinical practice has been disappointing so far. This therapeutic failure often results from the absence of appropriate stratification of patients that could benefit from targeted interventions. Companion diagnostics may help at different levels of the research and development, and in matching a patient to a specific intervention targeting hypoxia. In this review, we discuss the relative merits of the existing hypoxia biomarkers, their current status and the challenges for their future validation as companion diagnostics adapted to the nature of the intervention.

A Prospective Investigation of Tumor Hypoxia Imaging with 68Ga-Nitroimidazole PET/CT in Patients with Carcinoma of the Cervix Uteri and Comparison with 18F-FDG PET/CT: Correlation with Immunohistochemistry

Hypoxia in cervical cancer has been associated with a poor prognosis. Over the years ⁶⁸Ga labelled nitroimidazoles have been studied and have shown improved kinetics. We present our initial experience of hypoxia Positron Emission Tomography (PET) imaging in cervical cancer with ⁶⁸Ga-Nitroimidazole derivative and the correlation with ¹⁸F-FDG PET/CT and immunohistochemistry. Twenty women with cervical cancer underwent both ¹⁸F-FDG and ⁶⁸Ga-Nitroimidazole PET/CT imaging. Dual-point imaging was performed for ⁶⁸Ga-Nitroimidazole PET. Immunohistochemical analysis was performed with hypoxia inducible factor-1α (HIF-1α). We documented SUVmax, SUVmean of the primary lesions as well as tumor to muscle ratio (TMR), tumor to blood (TBR), metabolic tumor volume (MTV) and hypoxic tumor volume (HTV). There was no significant difference in the uptake of ⁶⁸Ga-Nitroimidazole between early and delayed imaging. Twelve patients had uptake on ⁶⁸Ga-Nitroimidazole PET. Ten patients demonstrated varying intensities of HIF-1α expression and six of these also had uptake on ⁶⁸Ga-Nitroimidazole PET. We found a strong negative correlation between HTV and immunohistochemical staining (r = −0.660; p = 0.019). There was no correlation between uptake on PET imaging and immunohistochemical analysis with HIF-1α. Two-thirds of the patients demonstrated hypoxia on ⁶⁸Ga-Nitroimidazole PET imaging.

Hypoxia Imaging As a Guide for Hypoxia-Modulated and Hypoxia-Activated Therapy

Significance: Oxygen imaging techniques, which can probe the spatiotemporal heterogeneity of tumor oxygenation, could be of significant clinical utility in radiation treatment planning and in evaluating the effectiveness of hypoxia-activated prodrugs. To fulfill these goals, oxygen imaging techniques should be noninvasive, quantitative, and capable of serial imaging, as well as having sufficient temporal resolution to detect the dynamics of tumor oxygenation to distinguish regions of chronic and acute hypoxia. Recent Advances: No current technique meets all these requirements, although all have strengths in certain areas. The current status of positron emission tomography (PET)-based hypoxia imaging, oxygen-enhanced magnetic resonance imaging (MRI), 19F MRI, and electron paramagnetic resonance (EPR) oximetry are reviewed along with their strengths and weaknesses for planning hypoxia-guided, intensity-modulated radiation therapy and detecting treatment response for hypoxia-targeted prodrugs. Critical Issues: Spatial and temporal resolution emerges as a major concern for these areas along with specificity and quantitative response. Although multiple oxygen imaging techniques have reached the investigative stage, clinical trials to test the therapeutic effectiveness of hypoxia imaging have been limited. Future Directions: Imaging elements of the redox environment besides oxygen by EPR and hyperpolarized MRI may have a significant impact on our understanding of the basic biology of the reactive oxygen species response and may extend treatment possibilities.

PET Use in Cancer Diagnosis, Treatment, and Prognosis

Tumorigenesis is a multistep process marked by variations in numerous metabolic pathways that affect cellular architectures and functions. Cancer cells reprogram their energy metabolism to enable several basic molecular functions, including membrane biosynthesis, receptor regulations, bioenergetics, and redox stress. In recent years, cancer diagnosis and treatment strategies have targeted these specific metabolic changes and the tumor's interactions with its microenvironment. Positron emission tomography (PET) captures all molecular alterations leading to abnormal function and cancer progression. As a result, the development of PET radiotracers increasingly focuses on irregular biological pathways or cells that overexpress receptors that have the potential to function as biomarkers for early diagnosis and treatment measurements as well as research. This chapter reviews both established and evolving PET radiotracers used to image tumor biology. We have also included a few advantages and disadvantages of the routinely used PET radiotracers in cancer imaging.

Paired 18F-Fluorodeoxyglucose (18F-FDG), and 64Cu-Copper(II)-diacetyl-bis(N(4)-methylthiosemicarbazone) (64Cu-ATSM) PET Scans in Dogs with Spontaneous Tumors and Evaluation for Hypoxia-Directed Therapy

Hypoxia is associated with neoplastic tissue, protecting cancer cells from death by irradiation and chemotherapy. Identification of hypoxic volume of tumors could optimize patient selection for hypoxia-directed medical, immunological, and radiation therapies. Clostridium novyi-NT (CNV-NT) is an oncolytic bacterium derived from attenuated wild-type Clostridium novyi spores, which germinates exclusively in the anaerobic core of tumors with low-oxygen content. The hypothesis was that 64Cu-ATSM would localize to regions of hypoxia, and that greater hypoxic volume would result in greater germination of Clostridium novyi-NT (CNV-NT). Tumor-bearing companion dogs were recruited to a veterinary clinical trial. Dogs received a CT scan, 18F-FDG PET scan (74 MBq) and 64Cu-ATSM PET scan (74 MBq). Scan regions of interest were defined as the highest 20% of counts/voxel for each PET scan, and regions with voxels overlapping between the two scans. Maximum standardized uptake value (MaxSUV) and threshold volume were calculated. Direct oximetry was performed in select tumors. Tumor types evaluated included nerve sheath tumor (10), apocrine carcinoma (1), melanoma (3) and oral sarcoma (6). MaxSUVATSM ranged from 0.3-6.6. Measured oxygen tension ranged from 0.05-89.9 mmHg. Inverse of MaxSUVATSM had a linear relationship with oxygen tension (R2 = 0.53, P = 0.0048). Hypoxia <8 mmHg was associated with an SUVATSM > 1.0. Hypoxic volume ranged from 0 to 100% of gross tumor volume (GTV) and MaxSUVATSM was positively correlated with hypoxic volume (R = 0.674; P = 0.0001), but not GTV (P = 0.182). Tumor hypoxic volume was heterogeneous in location and distribution. 64Cu-ATSM-avid regions were associated with differential CT attenuation. Hypoxic volume did not predict CNV-NT germination. 64Cu-ATSM PET scanning predicts hypoxia patterns within spontaneously occurring tumors of dogs as measured by direct oxymetry. Total tumor volume does not accurately predict degree or proportion of tumor hypoxia.

Evaluation of a Refined Implantable Resonator for Deep-Tissue EPR Oximetry in the Clinic

OBJECTIVES

(1) Summarize revisions made to the implantable resonator (IR) design and results of testing to characterize biocompatibility;(2) Demonstrate safety of implantation and feasibility of deep tissue oxygenation measurement using electron paramagnetic resonance (EPR) oximetry.

STUDY DESIGN

In vitro testing of the revised IR and in vivo implantation in rabbit brain and leg tissues.

METHODS

Revised IRs were fabricated with 1-4 OxyChips with a thin wire encapsulated with two biocompatible coatings. Biocompatibility and chemical characterization tests were performed. Rabbits were implanted with either an IR with 2 oxygen sensors or a biocompatible-control sample in both the brain and hind leg. The rabbits were implanted with IRs using a catheter-based, minimally invasive surgical procedure. EPR oximetry was performed for rabbits with IRs. Cohorts of rabbits were euthanized and tissues were obtained at 1 week, 3 months, and 9 months after implantation and examined for tissue reaction.

RESULTS

Biocompatibility and toxicity testing of the revised IRs demonstrated no abnormal reactions. EPR oximetry from brain and leg tissues were successfully executed. Blood work and histopathological evaluations showed no significant difference between the IR and control groups.

CONCLUSIONS

IRs were functional for up to 9 months after implantation and provided deep tissue oxygen measurements using EPR oximetry. Tissues surrounding the IRs showed no more tissue reaction than tissues surrounding the control samples. This pre-clinical study demonstrates that the IRs can be safely implanted in brain and leg tissues and that repeated, non-invasive, deep-tissue oxygen measurements can be obtained using in vivo EPR oximetry.

Quantitative Longitudinal Imaging Reveals that Inhibiting Hedgehog Activity Alleviates the Hypoxic Tumor Landscape

Metastases account for the majority of mortalities related to breast cancer. The onset and sustained presence of hypoxia strongly correlates with increased incidence of metastasis and unfavorable prognosis in patients with breast cancer. The Hedgehog (Hh) signaling pathway is dysregulated in breast cancer, and its abnormal activity enables tumor progression and metastasis. In addition to programming tumor cell behavior, Hh activity enables tumor cells to craft a metastasis-conducive microenvironment. Hypoxia is a prominent feature of growing tumors that impacts multiple signaling circuits that converge upon malignant progression. We investigated the role of Hh activity in crafting a hypoxic environment of breast cancer. We used radioactive tracer [¹⁸F]-fluoromisonidazole (FMISO) positron emission tomography (PET) to image tumor hypoxia. We show that tumors competent for Hh activity are able to establish a hypoxic milieu; pharmacologic inhibition of Hh signaling in a syngeneic mammary tumor model mitigates tumor hypoxia. Furthermore, in hypoxia, Hh activity is robustly activated in tumor cells and institutes increased HIF signaling in a VHL-dependent manner. The findings establish a novel perspective on Hh activity in crafting a hypoxic tumor landscape and molecularly navigating the tumor cells to adapt to hypoxic conditions. IMPLICATIONS: Importantly, we present a translational strategy of utilizing longitudinal hypoxia imaging to measure the efficacy of vismodegib in a preclinical model of triple-negative breast cancer.

Can High b Value Diffusion Be a Surrogate Marker for PET-A MR/PET Study in Neurooncology Set Up

Purpose: Hybrid whole-body magnetic resonance/positron emission tomography (MR/PET) systems are new diagnostic tools enabling the simultaneous acquisition of morphologic and multiparametric functional data, which allow for a diversified characterization of oncological diseases. This study aimed to compare the diagnostic ability of MRI with the diffusion-weighted image (DWI), and simultaneous integrated positron emission tomography MR/PET to detect malignant lesions and elucidate the utility and limitations of these imaging modalities in preoperative and postoperative follow up in cancer patients. Material and Methods: A total of 45 patients undergoing simultaneous MR/PET for CNS ICSOL in our institution between January 2016 and July 2020 were considered in this study. Post-processing was done in Siemens syngo software to generate a b2000 image. This image was then inverted to grayscale and compared with the NAC image of PET. Results: The lesion-based sensitivity, specificity, positive predictive value, and negative predictive value for DWI were 92.3, 83.3, 97.3, and 62.5%, respectively (at 95% CI and p was 0.000). The lesion-based sensitivity, specificity, positive predictive value, and negative predictive value for PET were 97.4, 71.4, 94.9, and 83.3%, respectively (at 95% CI and p was 0.000). The lesion-based sensitivity and specificity of DWI were comparable with those of PET. Conclusions: Although DWI and FDG-PET reflect different tissue properties, there is an association between the measures of both methods in CNS tumors probably because of the coupling of cellularity with tumor metabolism as seen on FDG and other PET tracers. Our study shows that DWI acts as a surrogate biomarker for FDG PET and other tracers in tumors. The method of DWI image generation is simple, radiation-free, and cost-effective in a clinical setup. The simultaneous DWI-PET study provides evidence and confirms the role of DWI in surveillance imaging of tumors.

A novel 111indium-labeled dual carbonic anhydrase 9-targeted probe as a potential SPECT imaging radiotracer for detection of hypoxic colorectal cancer cells

Tumor hypoxia is a common feature in colorectal cancer (CRC), and is associated with resistance to radiotherapy and chemotherapy. Thus, a specifically targeted probe for the detection of hypoxic CRC cells is urgently needed. Carbonic anhydrase 9 (CA9) is considered to be a specific marker for hypoxic CRC diagnosis. Here, a nuclear imaging Indium-111 (¹¹¹In)-labeled dual CA9-targeted probe was synthesized and evaluated for CA9 detection in in vitro, in vivo, and in human samples. The CA9-targeted peptide (CA9tp) and CA9 inhibitor acetazolamide (AAZ) were combined to form a dual CA9-targeted probe (AAZ-CA9tp) using an automatic microwave peptide synthesizer, which then was conjugated with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) for radioisotope (¹¹¹In) labeling (¹¹¹In-DOTA-AAZ-CA9tp). The assays for cell binding, stability, and toxicity were conducted in hypoxic CRC HCT15 cells. The analyses for imaging and biodistribution were performed in an HCT15 xenograft mouse model. The binding and distribution of ¹¹¹In-DOTA-AAZ-CA9tp were detected in human CRC samples using microautoradiography. AAZ-CA9tp possessed good CA9-targeting ability in hypoxic HCT15 cells. The dual CA9-targeted radiotracer showed high serum stability, high surface binding, and high affinity in vitro. After exposure of ¹¹¹In-DOTA-AAZ-CA9tp to the HCT15-bearing xenograft mice, the levels of ¹¹¹In-DOTA-AAZ-CA9tp were markedly and specifically increased in the hypoxic tumor tissues compared to control mice. ¹¹¹In-DOTA-AAZ-CA9tp also targeted the areas of CA9 overexpression in human colorectal tumor tissue sections. The results of this study suggest that the novel ¹¹¹In-DOTA-AAZ-CA9tp nuclear imaging agent may be a useful tool for the detection of hypoxic CRC cells in clinical practice.

Hypoxia in Lung Cancer Management: A Translational Approach

Simple Summary

Hypoxia is a common feature of lung cancers. Nonetheless, no guidelines have been established to integrate hypoxia-associated biomarkers in patient management. Here, we discuss the current knowledge and provide translational novel considerations regarding its clinical detection and targeting to improve the outcome of patients with non-small-cell lung carcinoma of all stages.

Abstract

Lung cancer represents the first cause of death by cancer worldwide and remains a challenging public health issue. Hypoxia, as a relevant biomarker, has raised high expectations for clinical practice. Here, we review clinical and pathological features related to hypoxic lung tumours. Secondly, we expound on the main current techniques to evaluate hypoxic status in NSCLC focusing on positive emission tomography. We present existing alternative experimental approaches such as the examination of circulating markers and highlight the interest in non-invasive markers. Finally, we evaluate the relevance of investigating hypoxia in lung cancer management as a companion biomarker at various lung cancer stages. Hypoxia could support the identification of patients with higher risks of NSCLC. Moreover, the presence of hypoxia in treated tumours could help clinicians predict a worse prognosis for patients with resected NSCLC and may help identify patients who would benefit potentially from adjuvant therapies. Globally, the large quantity of translational data incites experimental and clinical studies to implement the characterisation of hypoxia in clinical NSCLC management.

Detecting hypoxia in vitro using 18F-pretargeted IEDDA "click" chemistry in live cells

We have exemplified a pretargeted approach to interrogate hypoxia in live cells using radioactive bioorthogonal inverse electron demand Diels–Alder (IEDDA) “click” chemistry. Our novel ¹⁸F-tetrazine probe ([¹⁸F]FB-Tz) and 2-nitroimidazole-based TCO targeting molecule (8) showed statistically significant (P < 0.0001) uptake in hypoxic cells (ca. 90 %ID per mg) vs. normoxic cells (<10 %ID per mg) in a 60 min incubation of [¹⁸F]FB-Tz. This is the first time that an intracellularly targeted small-molecule for IEDDA “click” has been used in conjunction with a radioactive reporter molecule in live cells and may be a useful tool with far-reaching applicability for a variety of applications.

Bioorthogonal IEDDA “click” can interrogate intracellular hypoxia using a radioactive reporter molecule.

68Ga-labeled PET tracers for targeting tumor hypoxia: Role of bifunctional chelators on pharmacokinetics

INTRODUCTION

By virtue of their oxygen dependant accumulation in hypoxic cells, radiolabeled nitroimidazole analogues have been widely used for detecting tumor hypoxia. Present study evaluates two 2-nitroimidazole (2-NIM) based 68Ga-labeled radiotracers, [68Ga]Ga-DOTAGA-2-NIM and [68Ga]Ga-NODAGA-2-NIM, for hypoxia targeting applications.

METHODS

Bifunctional chelating agents suitable for radiolabeling with 68Ga, viz. 1,4,7,10-tetraazacyclododececane,1-(glutaric acid)-4,7,10-triacetic acid (DOTAGA) and 1,4,7-triazacyclododececane,1-(glutaric acid)-4,7-diacetic acid (NODAGA), were coupled to appropriately modified 2-nitroimidazole to obtain 2-NIM-DOTAGA and 2-NIM-NODAGA, respectively. These ligands were radiolabeled using [68Ga]GaCl3 obtained from a commercial 68Ge/68Ga-generator to obtain corresponding 68Ga-complexes. Both the radiotracers were tested for their hypoxia selectivity in CHO cells under hypoxic and normoxic conditions. Biodistribution studies in fibrosarcoma tumor bearing Swiss mice were carried out to evaluate the radiotracer in vivo.

RESULTS

The 68Ga complexes of 2-NIM-DOTAGA and 2-NIM-NODAGA could be prepared in ~82% and ~90% yield, respectively. In vitro studies of the complexes in CHO cells showed significant accumulation of [68Ga]Ga-NODAGA-2-NIM complex under hypoxic conditions with hypoxic to normoxic ratio of 2.88 ± 0.36 at 180 min post incubation. The [68Ga]Ga-DOTAGA-2-NIM complex also showed hypoxia selectivity albeit to a lesser extent. Biodistribution studies of the complexes in Swiss mice bearing fibrosarcoma tumor showed significant tumor uptake by both radiolabeled complexes. [68Ga]Ga-NODAGA-2-NIM showed a more favorable pharmacokinetics with respect to [68Ga]Ga-DOTAGA-2-NIM.

CONCLUSION

The nitroimidazole radiotracer with NODAGA chelator displayed more favorable pharmacokinetics and good hypoxia selectivity, making it a promising candidate for further investigation. The present study also provides an insight into the possible role of bifunctional chelator on overall pharmacokinetics of small molecule radiotracers.

Effect of inhaled oxygen concentration on 129 Xe chemical shift of red blood cells in rat lungs

PURPOSE

To investigate the dependence of dissolved ¹²⁹ Xe chemical shift on the fraction of inhaled oxygen, F_i O₂ , in the lungs of healthy rats.

METHODS

The chemical shifts of ¹²⁹ Xe dissolved in red blood cells, δ_RBC , and blood plasma and/or tissue, δ_Plasma , were measured using MRS in 12 Sprague Dawley rats mechanically ventilated at F_i O₂ values of 0.14, 0.19, and 0.22. Regional effects on the chemical shifts were controlled using a chemical shift saturation recovery sequence with a fixed delay time. MRS was also performed at an F_i CO₂ value of 0.085 to investigate the potential effect of the vascular response on δ_RBC and δ_Plasma .

RESULTS

δ_RBC increased with decreasing F_i O₂ (P = .0002), and δ_Plasma showed no dependence on F_i O₂ (P = .23). δ_RBC at F_i CO₂ = 0 (210.7 ppm ± 0.1) and at F_i CO₂ = 0.085 (210.6 ppm ± 0.2) were not significantly different (P = .67). δ_Plasma at F_i CO₂ = 0 (196.9 ppm ± 0.3) and at F_i CO₂ = 0.085 (197.0 ppm ± 0.1) were also not significantly different (P = .81).

CONCLUSION

Rat lung δ_RBC showed an inverse relationship to F_i O₂ , opposite to the relationship previously demonstrated for in vitro human blood. Rat lung δ_RBC did not depend on F_i CO₂ .

In vivo noninvasive preclinical tumor hypoxia imaging methods: a review

Tumors exhibit areas of decreased oxygenation due to malformed blood vessels. This low oxygen concentration decreases the effectiveness of radiation therapy, and the resulting poor perfusion can prevent drugs from reaching areas of the tumor. Tumor hypoxia is associated with poorer prognosis and disease progression, and is therefore of interest to preclinical researchers. Although there are multiple different ways to measure tumor hypoxia and related factors, there is no standard for quantifying spatial and temporal tumor hypoxia distributions in preclinical research or in the clinic. This review compares imaging methods utilized for the purpose of assessing spatio-temporal patterns of hypoxia in the preclinical setting. Imaging methods provide varying levels of spatial and temporal resolution regarding different aspects of hypoxia, and with varying advantages and disadvantages. The choice of modality requires consideration of the specific experimental model, the nature of the required characterization and the availability of complementary modalities as well as immunohistochemistry.

A NIR fluorescent smart probe for imaging tumor hypoxia

BACKGROUND

Tumor hypoxia is a characteristic of paramount importance due to low oxygenation levels in tissue negatively correlating with resistance to traditional therapies. The ability to noninvasively identify such could provide for personalized treatment(s) and enhance survival rates. Accordingly, we recently developed an NIR fluorescent hypoxia-sensitive smart probe (NO₂ -Rosol) for identifying hypoxia via selectively imaging nitroreductase (NTR) activity, which could correlate to oxygen deprivation levels in cells, thereby serving as a proxy. We demonstrated proof of concept by subjecting a glioblastoma (GBM) cell line to extreme stress by evaluating such under radiobiological hypoxic (pO₂  ≤ ~0.5%) conditions, which is a far cry from representative levels for hypoxia for brain glioma (pO₂ = ~1.7%) which fluctuate little from physiological hypoxic (pO₂ = 1.0-3.0%) conditions.

AIM

We aimed to evaluate the robustness, suitability, and feasibility of NO₂ -Rosol for imaging hypoxia in vitro and in vivo via assessing NTR activity in diverse GBM models under relevant oxygenation levels (pO₂ = 2.0%) within physiological hypoxic conditions that mimic oxygenation levels in GBM tumor tissue in the brain.

METHODS

We evaluated multiple GBM cell lines to determine their relative sensitivity to oxygenation levels via measuring carbonic anhydrase IX (CAIX) levels, which is a surrogate marker for indirectly identifying hypoxia by reporting on oxygen deprivation levels and upregulated NTR activity. We evaluated for hypoxia via measuring NTR activity when employing NO₂ -Rosol in in vitro and tumor hypoxia imaging studies in vivo.

RESULTS

The GBM39 cell line demonstrated the highest CAIX expression under hypoxic conditions representing that of GBM in the brain. NO₂ -Rosol displayed an 8-fold fluorescence enhancement when evaluated in GBM39 cells (pO₂ = 2.0%), thereby establishing its robustness and suitability for imaging hypoxia under relevant physiological conditions. We demonstrated the feasibility of NO₂ -Rosol to afford tumor hypoxia imaging in vivo via it demonstrating a tumor-to-background of 5 upon (i) diffusion throughout, (ii) bioreductive activation by NTR activity in, and (iii) retention within, GBM39 tumor tissue.

CONCLUSION

We established the robustness, suitability, and feasibility of NO₂ -Rosol for imaging hypoxia under relevant oxygenation levels in vitro and in vivo via assessing NTR activity in GBM39 models.

Greater reductions in blood flow after anti-angiogenic treatment in non-small cell lung cancer patients are associated with shorter progression-free survival

To evaluate tumor blood flow using ¹⁵O-water positron emission tomography (PET) in patients with non-small cell lung cancer (NSCLC) before and after chemotherapy with bevacizumab, and to investigate the effects of bevacizumab on tumor blood flow changes and progression-free survival (PFS). Twelve patients with NSCLC were enrolled. Six patients underwent chemotherapy with bevacizumab and the other six without bevacizumab. ¹⁵O-water dynamic PET scans were performed within 1 week before the start of chemotherapy and within 1 week after the first day of chemotherapy. Tumor blood flow was analyzed quantitatively using a single one-tissue compartment model with the correction of pulmonary circulation blood volume and arterial blood volume via an image-derived input function. In the bevacizumab group, mean tumor blood flow was statistically significantly reduced post-chemotherapy (pre-chemotherapy 0.27 ± 0.14 mL/cm³/min, post-chemotherapy 0.18 ± 0.12 mL/cm³/min). In the no bevacizumab group, there was no significant difference between mean tumor perfusion pre-chemotherapy (0.42 ± 0.42 mL/cm³/min) and post-chemotherapy (0.40 ± 0.27 mL/cm³/min). In the bevacizumab group, there was a positive correlation between the blood flow ratio (tumor blood flow post-chemotherapy/tumor blood flow pre-chemotherapy) and PFS (correlation coefficient 0.94). Mean tumor blood flow decreases after bevacizumab administration and was positively correlated with longer PFS.

Biological validation of electron paramagnetic resonance (EPR) image oxygen thresholds in tissue

Measuring molecular oxygen levels in vivo has been the cornerstone of understanding the effects of hypoxia in normal tissues and malignant tumors. Here we discuss the advances in a variety of partial pressure of oxygen ( P O 2 ) measurements and imaging techniques and relevant oxygen thresholds. A focus on electron paramagnetic resonance (EPR) imaging shows the validation of treating hypoxic tumours with a threshold of P O 2  ≤ 10 Torr, and demonstrates utility for in vivo oxygen imaging, as well as its current and future role in cancer studies.

Functional CT Contrast Nanoagents for the Tumor Microenvironment

Understanding the detailed tumor microenvironment (TME) is essential to achieve effective treatment of tumor, because TME has an extremely profound influence on the occurrence, development, invasion, and metastasis of tumor. It is of great significance to realize accurate diagnosis of the TME by using functional computed tomography (CT) contrast nanoagents (FCTNAs). Here, an overview of FCTNAs that respond to the overexpressed receptors, acidic microenvironment, overexpressed glutathione and enzymes, and hypoxia in tumor is provided, and also prospects the advance of novel spectral CT technique to detect the TME precisely. Utilizing FCTNAs is expected to achieve accurate monitoring of the TME and further provide guidance for the effective personalized tumor treatment in clinic.

In vivo hypoxia characterization using blood oxygen level dependent magnetic resonance imaging in a preclinical glioblastoma mouse model

PURPOSE

Hypoxia measurements can provide crucial information regarding tumor aggressiveness, however current preclinical approaches are limited. Blood oxygen level dependent (BOLD) Magnetic Resonance Imaging (MRI) has the potential to continuously monitor tumor pathophysiology (including hypoxia). The aim of this preliminary work was to develop and evaluate BOLD MRI followed by post-image analysis to identify regions of hypoxia in a murine glioblastoma (GBM) model.

METHODS

A murine orthotopic GBM model (GL261-luc2) was used and independent images were generated from multiple slices in four different mice. Image slices were randomized and split into training and validation cohorts. A 7 T MRI was used to acquire anatomical images using a fast-spin-echo (FSE) T2-weighted sequence. BOLD images were taken with a T2*-weighted gradient echo (GRE) and an oxygen challenge. Thirteen images were evaluated in a training cohort to develop the MRI sequence and optimize post-image analysis. An in-house MATLAB code was used to evaluate MR images and generate hypoxia maps for a range of thresholding and ΔT2* values, which were compared against respective pimonidazole sections to optimize image processing parameters. The remaining (n = 6) images were used as a validation group. Following imaging, mice were injected with pimonidazole and collected for immunohistochemistry (IHC). A test of correlation (Pearson's coefficient) and agreement (Bland-Altman plot) were conducted to evaluate the respective MRI slices and pimonidazole IHC sections.

RESULTS

For the training cohort, the optimized parameters of "thresholding" (20 ≤ T2* ≤ 35 ms) and ΔT2* (±4 ms) yielded a Pearson's correlation of 0.697. These parameters were applied to the validation cohort confirming a strong Pearson's correlation (0.749) when comparing the respective analyzed MR and pimonidazole images.

CONCLUSION

Our preliminary study supports the hypothesis that BOLD MRI is correlated with pimonidazole measurements of hypoxia in an orthotopic GBM mouse model. This technique has further potential to monitor hypoxia during tumor development and therapy.

Imaging for Response Assessment in Cancer Clinical Trials

The use of biomarkers is integral to the routine management of cancer patients, including diagnosis of disease, clinical staging and response to therapeutic intervention. Advanced imaging metrics with computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET) are used to assess response during new drug development and in cancer research for predictive metrics of response. Key components and challenges to identifying an appropriate imaging biomarker are selection of integral vs integrated biomarkers, choosing an appropriate endpoint and modality, and standardization of the imaging biomarkers for cooperative and multicenter trials. Imaging biomarkers lean on the original proposed quantified metrics derived from imaging such as tumor size or longest dimension, with the most commonly implemented metrics in clinical trials coming from the Response Evaluation Criteria in Solid Tumors (RECIST) criteria, and then adapted versions such as immune-RECIST (iRECIST) and Positron Emission Tomography Response Criteria in Solid Tumors (PERCIST) for immunotherapy response and PET imaging, respectively. There have been many widely adopted biomarkers in clinical trials derived from MRI including metrics that describe cellularity and vascularity from diffusion-weighted (DW)-MRI apparent diffusion coefficient (ADC) and Dynamic Susceptibility Contrast (DSC) or dynamic contrast enhanced (DCE)-MRI (Ktrans, relative cerebral blood volume (rCBV)), respectively. Furthermore, Fluorodexoyglucose (FDG), fluorothymidine (FLT), and fluoromisonidazole (FMISO)-PET imaging, which describe molecular markers of glucose metabolism, proliferation and hypoxia have been implemented into various cancer types to assess therapeutic response to a wide variety of targeted- and chemotherapies. Recently, there have been many functional and molecular novel imaging biomarkers that are being developed that are rapidly being integrated into clinical trials (with anticipation of being implemented into clinical workflow in the future), such as artificial intelligence (AI) and machine learning computational strategies, antibody and peptide specific molecular imaging, and advanced diffusion MRI. These include prostate-specific membrane antigen (PSMA) and trastuzumab-PET, vascular tumor burden extracted from contrast-enhanced CT, diffusion kurtosis imaging, and CD8 or Granzyme B PET imaging. Further excitement surrounds theranostic procedures such as the combination of 68Ga/111In- and 177Lu-DOTATATE to use integral biomarkers to direct care and personalize therapy. However, there are many challenges in the implementation of imaging biomarkers that remains, including understand the accuracy, repeatability and reproducibility of both acquisition and analysis of these imaging biomarkers. Despite the challenges associated with the biological and technical validation of novel imaging biomarkers, a distinct roadmap has been created that is being implemented into many clinical trials to advance the development and implementation to create specific and sensitive novel imaging biomarkers of therapeutic response to continue to transform medical oncology.

Non-invasive imaging prediction of tumor hypoxia: A novel developed and externally validated CT and FDG-PET-based radiomic signatures

BACKGROUND

Tumor hypoxia increases resistance to radiotherapy and systemic therapy. Our aim was to develop and validate a disease-agnostic and disease-specific CT (+FDG-PET) based radiomics hypoxia classification signature.

MATERIAL AND METHODS

A total of 808 patients with imaging data were included: N = 100 training/N = 183 external validation cases for a disease-agnostic CT hypoxia classification signature, N = 76 training/N = 39 validation cases for the H&N CT signature and N = 62 training/N = 36 validation cases for the Lung CT signature. The primary gross tumor volumes (GTV) were manually defined by experts on CT. In order to dichotomize between hypoxic/well-oxygenated tumors a threshold of 20% was used for the [¹⁸F]-HX4-derived hypoxic fractions (HF). A random forest (RF)-based machine-learning classifier/regressor was trained to classify patients as hypoxia-positive/ negative based on radiomic features.

RESULTS

A 11 feature "disease-agnostic CT model" reached AUC's of respectively 0.78 (95% confidence interval [CI], 0.62-0.94), 0.82 (95% CI, 0.67-0.96) and 0.78 (95% CI, 0.67-0.89) in three external validation datasets. A "disease-agnostic FDG-PET model" reached an AUC of 0.73 (0.95% CI, 0.49-0.97) in validation by combining 5 features. The highest "lung-specific CT model" reached an AUC of 0.80 (0.95% CI, 0.65-0.95) in validation with 4 CT features, while the "H&N-specific CT model" reached an AUC of 0.84 (0.95% CI, 0.64-1.00) in validation with 15 CT features. A tumor volume-alone model was unable to significantly classify patients as hypoxia-positive/ negative. A significant survival split (P = 0.037) was found between CT-classified hypoxia strata in an external H&N cohort (n = 517), while 117 significant hypoxia gene-CT signature feature associations were found in an external lung cohort (n = 80).

CONCLUSION

The disease-specific radiomics signatures perform better than the disease agnostic ones. By identifying hypoxic patients our signatures have the potential to enrich interventional hypoxia-targeting trials.

99mTc Labelling Strategies for the Development of Potential Nitroimidazolic Hypoxia Imaging Agents

Technetium-99m has a rich coordination chemistry that offers many possibilities in terms of oxidation states and donor atom sets. Modifications in the structure of the technetium complexes could be very useful for fine tuning the physicochemical and biological properties of potential 99mTc radiopharmaceuticals. However, systematic study of the influence of the labelling strategy on the “in vitro” and “in vivo” behaviour is necessary for a rational design of radiopharmaceuticals. Herein we present a review of the influence of the Tc complexes’ molecular structure on the biodistribution and the interaction with the biological target of potential nitroimidazolic hypoxia imaging radiopharmaceuticals presented in the literature from 2010 to the present. Comparison with the gold standard [18F]Fluoromisonidazole (FMISO) is also presented.

Preclinical Molecular Imaging for Precision Medicine in Breast Cancer Mouse Models

Precision and personalized medicine is gaining importance in modern clinical medicine, as it aims to improve diagnostic precision and to reduce consequent therapeutic failures. In this regard, prior to use in human trials, animal models can help evaluate novel imaging approaches and therapeutic strategies and can help discover new biomarkers. Breast cancer is the most common malignancy in women worldwide, accounting for 25% of cases of all cancers and is responsible for approximately 500,000 deaths per year. Thus, it is important to identify accurate biomarkers for precise stratification of affected patients and for early detection of responsiveness to the selected therapeutic protocol. This review aims to summarize the latest advancements in preclinical molecular imaging in breast cancer mouse models. Positron emission tomography (PET) imaging remains one of the most common preclinical techniques used to evaluate biomarker expression in vivo, whereas magnetic resonance imaging (MRI), particularly diffusion-weighted (DW) sequences, has been demonstrated as capable of distinguishing responders from nonresponders for both conventional and innovative chemo- and immune-therapies with high sensitivity and in a noninvasive manner. The ability to customize therapies is desirable, as this will enable early detection of diseases and tailoring of treatments to individual patient profiles. Animal models remain irreplaceable in the effort to understand the molecular mechanisms and patterns of oncologic diseases.

Advances in PET Diagnostics for Guiding Targeted Cancer Therapy and Studying In Vivo Cancer Biology

Purpose of the Review

We present an overview of recent advances in positron emission tomography (PET) diagnostics as applied to the study of cancer, specifically as a tool to study in vivo cancer biology and to direct targeted cancer therapy. The review is directed to translational and clinical cancer investigators who may not be familiar with these applications of PET cancer diagnostics, but whose research might benefit from these advancing tools.

Recent Findings

We highlight recent advances in 3 areas: (1) the translation of PET imaging cancer biomarkers to clinical trials; (2) methods for measuring cancer metabolism in vivo in patients; and (3) advances in PET instrumentation, including total-body PET, that enable new methodologies. We emphasize approaches that have been translated to human studies.

Summary

PET imaging methodology enables unique in vivo cancer diagnostics that go beyond cancer detection and staging, providing an improved ability to guide cancer treatment and an increased understanding of in vivo human cancer biology.

PET/CT in radiation oncology

The progressive integration of positron emission tomography/computed tomography (PET/CT) imaging in radiation therapy has its rationale in the biological intertumoral and intratumoral heterogeneity of malignant lesions that require the individual adjustment of radiation dose to obtain an effective local tumor control in cancer patients. PET/CT provides information on the biological features of tumor lesions such as metabolism, hypoxia, and proliferation that can identify radioresistant regions and be exploited to optimize treatment plans. Here, we provide an overview of the basic principles of PET-based target volume selection and definition using ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) and then we focus on the emerging strategies of dose painting and adaptive radiotherapy using different tracers. Previous studies provided consistent evidence that integration of ¹⁸F-FDG PET/CT in radiotherapy planning improves delineation of target volumes and reduces the uncertainties and variabilities of anatomical delineation of tumor sites. PET-based dose painting and adaptive radiotherapy are feasible strategies although their clinical implementation is highly demanding and requires strong technical, computational, and logistic efforts. Further prospective clinical trials evaluating local tumor control, survival, and toxicity of these emerging strategies will promote the full integration of PET/CT in radiation oncology.

Insights into Hypoxia: Non-invasive Assessment through Imaging Modalities and Its Application in Breast Cancer

Oxygen is crucial to maintain the homeostasis in aerobic cells. Hypoxia is a condition in which cells are deprived of the oxygen supply necessary for their optimum performance. Whereas oxygen deprivation may occur in normal physiological processes, hypoxia is frequently associated with pathological conditions. It has been identified as a stressor in the tumor microenvironment, acting as a key mediator of cancer development. Numerous pathways are activated in hypoxic cells that affect cell signaling and gene regulation to promote the survival of these cells by stimulating angiogenesis, switching cellular metabolism, slowing their growth rate, and preventing apoptosis. The induction of dysregulated metabolism in cancer cells by hypoxia results in aggressive tumor phenotypes that are characterized by rapid progression, treatment resistance, and poor prognosis. A non-invasive assessment of hypoxia-induced metabolic and architectural changes in tumors is advisable to fully improve breast cancer (BC) patient management, by potentially reducing the need for invasive biopsy procedures and evaluating tumor response to treatment. This review provides a comprehensive overview of the molecular changes in breast tumors secondary to hypoxia and the non-invasive imaging alternatives to evaluate oxygen deprivation, with an emphasis on their application in BC and the advantages and limitations of the currently available techniques.

Buparlisib with thoracic radiotherapy and its effect on tumour hypoxia: A phase I study in patients with advanced non-small cell lung carcinoma

BACKGROUND

Pre-clinically, phosphoinositide 3-kinase (PI3K) inhibition radiosensitises tumours by increasing intrinsic radiosensitivity and by reducing tumour hypoxia. We assessed whether buparlisib, a class 1 PI3K inhibitor, can be safely combined with radiotherapy in patients with non-small cell lung carcinoma (NSCLC) and investigated its effect on tumour hypoxia.

METHODS

This was a 3 + 3 dose escalation and dose expansion phase I trial in patients with advanced NSCLC. Buparlisib dose levels were 50 mg, 80 mg and 100 mg once daily orally for 2 weeks, with palliative thoracic radiotherapy (20 Gy in 5 fractions) delivered during week 2. Tumour hypoxic volume (HV) was measured using 18F-fluoromisonidazole positron-emission tomography-computed tomography at baseline and following 1 week of buparlisib.

RESULTS

Twenty-one patients were recruited with 9 patients evaluable for maximum tolerated dose (MTD) analysis. No dose-limiting toxicity was reported; therefore, 100 mg was declared the MTD, and 10 patients received this dose in the expansion phase. Ninety-four percent of treatment-related adverse events were ≤grade 2 with fatigue (67%), nausea (24%) and decreased appetite (19%) most common per patient. One serious adverse event (grade 3 hypoalbuminaemia) was possibly related to buparlisib. No unexpected radiotherapy toxicity was reported. Ten (67%) of 15 patients evaluable for imaging analysis were responders with 20% median reduction in HV at the MTD.

CONCLUSION

This is the first clinical trial to combine a PI3K inhibitor with radiotherapy in NSCLC and investigate the effects of PI3K inhibition on tumour hypoxia. This combination was well tolerated and PI3K inhibition reduced hypoxia, warranting investigation into whether this novel class of radiosensitisers can improve radiotherapy outcomes.

Examining correlations of oxygen sensitive MRI (BOLD/TOLD) with [18F]FMISO PET in rat prostate tumors

Hypoxia is regarded as a potential prognostic biomarker for tumor aggressiveness, progression, and response to therapy. The radiotracer ¹⁸F-fluoromisonidazole ([¹⁸F]FMISO) has been used with positron emission tomography (PET) to reveal tumor hypoxia. Meanwhile, blood oxygen level dependent (BOLD) MRI and tissue oxygen level dependent (TOLD) MRI offer insight into oxygenation based on endogenous signals without the need for radiolabels. Here, we compared BOLD and TOLD MRI with [¹⁸F]FMISO uptake using Dunning prostate R3327-AT1 tumor bearing rats. BOLD and TOLD MRI were acquired with respect to an oxygen gas breathing challenge. The following day, dynamic PET was performed up to 90 minutes following IV injection of [¹⁸F]FMISO. Tumors showed distinct heterogeneity based on each technique. Correlations were observed between magnitude of mean BOLD or TOLD MRI signal responses to oxygen-breathing challenge and initial distribution of [¹⁸F]FMISO. Correlations were observed for whole tumor as well on a regional basis with stronger correlations in the well perfused tumor periphery indicating the strong influence of perfused vasculature. After 90 minutes most correlations with signal intensity became quite weak, but correlations were observed between hypoxic fraction based on FMISO and fractions of tumor showing BOLD or TOLD response in a subset of tumors. This emphasizes the importance of considering regional heterogeneity and responsive fractions, as opposed to simple magnitudes of responses. Although the data represent a small cohort of tumors they present direct correlations between oxygen sensitive MRI and PET hypoxia reporter agents in the same tumors, indicating the potential utility of further investigations.

Evaluating cardiac hypoxia in hibernating myocardium: Comparison of 99mTc-MIBI/18F-fluorodeoxyglucose and 18F-fluoromisonidazole positron emission tomography-computed tomography in relation to normal, hibernating, and infarct myocardium

The aim of this prospective study was to explore the feasibility of ¹⁸F-fluoromisonidazole (¹⁸F-FMISO) cardiac positron emission tomography/computed tomography (PET/CT) in the detection of cardiac hypoxia in patients of ischemic heart disease (IHD) and to compare the uptake pattern with that of ^99mTc-MIBI and ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG). Twenty-six patients suffering from IHD were evaluated in this study. The patients initially underwent ^99mTc-MIBI rest/stress myocardial perfusion imaging and ¹⁸F-FDG cardiac PET/CT as a part of their routine cardiac imaging. Patients with hibernating myocardium on these scans further underwent ¹⁸F-FMISO Cardiac PET/CT. Controls were also considered in the form of patients with scarred and normal myocardium. On visual assessment, increased ¹⁸F-FMISO uptake was noted in the hibernating myocardium compared to scarred or normal myocardium. On semiquantification analysis, there was overlap in the uptake values with a range of maximum standardized uptake value (SUV_max) in hibernating, scarred, and normal myocardium being 0.8–2.2 g/dl, 0.7–1.8 g/dl, and 0.7–1.6 g/dl, respectively. On individual patient-specific comparison in subjects harboring both hibernating and scarred myocardium, it was observed that SUV_max of ¹⁸F-FMISO in hibernating myocardium was highest, followed by scarred myocardium and normal myocardium, respectively. The ratio of ¹⁸F-FMISO SUV_max of hibernating to the normal myocardium in these subjects was always more than 1, and never less than the ratio of SUV_max of scarred to normal myocardium. Thus, in this mixed population study, it was observed that on an individual patient basis, hypoxic myocardium consistently showed higher ¹⁸F-FMISO uptake than surrounding scarred and normal myocardium. The ratio of ¹⁸F-FMISO SUV_max of hibernating to normal myocardium was higher than the ratio of scarred to the normal myocardium in all patients. On overall basis, however, there was considerable overlap in the SUV values among hibernating, scarred, and normal myocardium resulting in difficulty in differentiation of these entities with FMISO cardiac PET. ¹⁸F-FDG cardiac PET/CT remains the standard and superior method to determine hibernating myocardium in patients of IHD due to its superior contrast. The limitation of FMISO is poor signal to noise ratio because of high background uptake from the blood pool. Cardiac PET/CT with superior hypoxia tracers needs to be further examined for imaging cardiac hypoxia.

2-Nitroimidazole-Furanoside Derivatives for Hypoxia Imaging-Investigation of Nucleoside Transporter Interaction, 18F-Labeling and Preclinical PET Imaging

The benefits of PET imaging of tumor hypoxia in patient management has been demonstrated in many examples and with various tracers over the last years. Although, the optimal hypoxia imaging agent has yet to be found, 2-nitroimidazole (azomycin) sugar derivatives—mimicking nucleosides—have proven their potential with [¹⁸F]FAZA ([¹⁸F]fluoro-azomycin-α-arabinoside) as a prominent representative in clinical use. Still, for all of these tracers, cellular uptake by passive diffusion is postulated with the disadvantage of slow kinetics and low tumor-to-background ratios. We recently evaluated [¹⁸F]fluoro-azomycin-β-deoxyriboside (β-[¹⁸F]FAZDR), with a structure more similar to nucleosides than [¹⁸F]FAZA and possible interaction with nucleoside transporters. For a deeper insight, we comparatively studied the interaction of FAZA, β-FAZA, α-FAZDR and β-FAZDR with nucleoside transporters (SLC29A1/2 and SLC28A1/2/3) in vitro, showing variable interactions of the compounds. The highest interactions being for β-FAZDR (IC₅₀ 124 ± 33 µM for SLC28A3), but also for FAZA with the non-nucleosidic α-configuration, the interactions were remarkable (290 ± 44 µM {SLC28A1}; 640 ± 10 µM {SLC28A2}). An improved synthesis was developed for β-FAZA. For a PET study in tumor-bearing mice, α-[¹⁸F]FAZDR was synthesized (radiochemical yield: 15.9 ± 9.0% (n = 3), max. 10.3 GBq, molar activity > 50 GBq/µmol) and compared to β-[¹⁸F]FAZDR and [¹⁸F]FMISO, the hypoxia imaging gold standard. We observed highest tumor-to-muscle ratios (TMR) for β-[¹⁸F]FAZDR already at 1 h p.i. (2.52 ± 0.94, n = 4) in comparison to [¹⁸F]FMISO (1.37 ± 0.11, n = 5) and α-[¹⁸F]FAZDR (1.93 ± 0.39, n = 4), with possible mediation by the involvement of nucleoside transporters. After 3 h p.i., TMR were not significantly different for all 3 tracers (2.5–3.0). Highest clearance from tumor tissue was observed for β-[¹⁸F]FAZDR (56.6 ± 6.8%, 2 h p.i.), followed by α-[¹⁸F]FAZDR (34.2 ± 7.5%) and [¹⁸F]FMISO (11.8 ± 6.5%). In conclusion, both isomers of [¹⁸F]FAZDR showed their potential as PET hypoxia tracers. Differences in uptake behavior may be attributed to a potential variable involvement of transport mechanisms.

Hypoxia-activated prodrugs and (lack of) clinical progress: The need for hypoxia-based biomarker patient selection in phase III clinical trials

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Hypoxia-activated prodrugs have yielded promising results up to phase II trials.

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Implementation of hypoxia-activated prodrugs in the clinic has not been successful.

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Phase III clinical trials lack patient stratification based on tumor hypoxia status.

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Stratification will decrease the number of patients needed and increase success.

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Improvements in hypoxia-activated prodrug design can also increase success rates.

Hypoxia-activated prodrugs (HAPs) are designed to specifically target the hypoxic cells of tumors, which are an important cause of treatment resistance to conventional therapies. Despite promising preclinical and clinical phase I and II results, the most important of which are described in this review, the implementation of hypoxia-activated prodrugs in the clinic has, so far, not been successful. The lack of stratification of patients based on tumor hypoxia status, which can vary widely, is sufficient to account for the failure of phase III trials. To fully exploit the potential of hypoxia-activated prodrugs, hypoxia stratification of patients is needed. Here, we propose a biomarker-stratified enriched Phase III study design in which only biomarker-positive (i.e. hypoxia-positive) patients are randomized between standard treatment and the combination of standard treatment with a hypoxia-activated prodrug. This implies the necessity of a Phase II study in which the biomarker or a combination of biomarkers will be evaluated. The total number of patients needed for both clinical studies will be far lower than in currently used randomize-all designs. In addition, we elaborate on the improvements in HAP design that are feasible to increase the treatment success rates.

The Promise of Novel Biomarkers for Head and Neck Cancer from an Imaging Perspective

It is an agreed fact that overall survival among head and neck cancer patients has increased over the last decade. Several factors however, are still held responsible for treatment failure requiring more in-depth evaluation. Among these, hypoxia and proliferation-specific parameters are the main culprits, along with the more recently researched cancer stem cells. This paper aims to present the latest developments in the field of biomarkers for hypoxia, stemness and tumour proliferation, from an imaging perspective that includes both Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) as well as functional magnetic resonance imaging (MRI). Quantitative imaging of biomarkers is a prerequisite for accurate treatment response assessment, bringing us closer to the highly needed personalised therapy.