Metabolic and Physiologic Imaging Biomarkers of the Tumor Microenvironment Predict Treatment Outcome with Radiation or a Hypoxia-Activated Prodrug in Mice

Evaluations of an Early Change in Tumor Pathophysiology in Response to Radiotherapy with Oxygen Enhanced Electron Paramagnetic Resonance Imaging (OE EPRI)

PURPOSE

Electron Paramagnetic Resonance Imaging (EPRI) can image the partial pressure of oxygen (pO2) within in vivo tumor models. We sought to develop Oxygen Enhanced (OE) EPRI that measures tumor pO2 with breathing gases of 21% O2 (pO221%) and 100% O2 (pO2100%), and the differences in pO2 between breathing gases (ΔpO2). We applied OE EPRI to study the early change in tumor pathophysiology in response to radiotherapy in two tumor models of pancreatic cancer.

PROCEDURES

We developed a protocol that intraperitoneally administered OX071, a trityl radical contrast agent, and then acquired anatomical MR images to localize the tumor. Subsequently, we acquired two pO221% and two pO2100% maps using the T1 relaxation time of OX071 measured with EPRI and a R1-pO2 calibration of OX071. We studied 4T1 flank tumor model to evaluate the repeatability of OE EPRI. We then applied OE EPRI to study COLO 357 and Su.86.86 flank tumor models treated with 10 Gy radiotherapy.

RESULTS

The repeatability of mean pO2 for individual tumors was ± 2.6 Torr between successive scans when breathing 21% O2 or 100% O2, representing a precision of 9.6%. Tumor pO221% and pO2100% decreased after radiotherapy for both models, although the decreases were not significant or only moderately significant, and the effect sizes were modest. For comparison, ΔpO2 showed a large, highly significant decrease after radiotherapy, and the effect size was large. MANOVA and analyses of the HF10 hypoxia fraction provided similar results.

CONCLUSIONS

EPRI can evaluate tumor pO2 with outstanding precision relative to other imaging modalities. The change in ΔpO2 before vs. after treatment was the best parameter for measuring the early change in tumor pathophysiology in response to radiotherapy. Our studies have established ΔpO2 from OE EPRI as a new parameter, and have established that OE EPRI is a valuable new methodology for molecular imaging.

Machine learning identifies multi-parametric functional PET/MR imaging cluster to predict radiation resistance in preclinical head and neck cancer models

PURPOSE

Tumor hypoxia and other microenvironmental factors are key determinants of treatment resistance. Hypoxia positron emission tomography (PET) and functional magnetic resonance imaging (MRI) are established prognostic imaging modalities to identify radiation resistance in head-and-neck cancer (HNC). The aim of this preclinical study was to develop a multi-parametric imaging parameter specifically for focal radiotherapy (RT) dose escalation using HNC xenografts of different radiation sensitivities.

METHODS

A total of eight human HNC xenograft models were implanted into 68 immunodeficient mice. Combined PET/MRI using dynamic [18F]-fluoromisonidazole (FMISO) hypoxia PET, diffusion-weighted (DW), and dynamic contrast-enhanced MRI was carried out before and after fractionated RT (10 × 2 Gy). Imaging data were analyzed on voxel-basis using principal component (PC) analysis for dynamic data and apparent diffusion coefficients (ADCs) for DW-MRI. A data- and hypothesis-driven machine learning model was trained to identify clusters of high-risk subvolumes (HRSs) from multi-dimensional (1-5D) pre-clinical imaging data before and after RT. The stratification potential of each 1D to 5D model with respect to radiation sensitivity was evaluated using Cohen's d-score and compared to classical features such as mean/peak/maximum standardized uptake values (SUVmean/peak/max) and tumor-to-muscle-ratios (TMRpeak/max) as well as minimum/valley/maximum/mean ADC.

RESULTS

Complete 5D imaging data were available for 42 animals. The final preclinical model for HRS identification at baseline yielding the highest stratification potential was defined in 3D imaging space based on ADC and two FMISO PCs ([Formula: see text]). In 1D imaging space, only clusters of ADC revealed significant stratification potential ([Formula: see text]). Among all classical features, only ADCvalley showed significant correlation to radiation resistance ([Formula: see text]). After 2 weeks of RT, FMISO_c1 showed significant correlation to radiation resistance ([Formula: see text]).

CONCLUSION

A quantitative imaging metric was described in a preclinical study indicating that radiation-resistant subvolumes in HNC may be detected by clusters of ADC and FMISO using combined PET/MRI which are potential targets for future functional image-guided RT dose-painting approaches and require clinical validation.

MR-Nucleomics: The study of pathological cellular processes with multinuclear magnetic resonance spectroscopy and imaging in vivo

Clinical medicine has experienced a rapid development in recent decades, during which therapies targeting specific cellular signaling pathways, or specific cell surface receptors, have been increasingly adopted. While these developments in clinical medicine call for improved precision in diagnosis and treatment monitoring, modern medical imaging methods are restricted mainly to anatomical imaging, lagging behind the requirements of precision medicine. Although positron emission tomography and single photon emission computed tomography have been used clinically for studies of metabolism, their applications have been limited by the exposure risk to ionizing radiation, the subsequent limitation in repeated and longitudinal studies, and the incapability in assessing downstream metabolism. Magnetic resonance spectroscopy (MRS) or spectroscopic imaging (MRSI) are, in theory, capable of assessing molecular activities in vivo, although they are often limited by sensitivity. Here, we review some recent developments in MRS and MRSI of multiple nuclei that have potential as molecular imaging tools in the clinic.

EPR and Related Magnetic Resonance Imaging Techniques in Cancer Research

Imaging tumor microenvironments such as hypoxia, oxygenation, redox status, and/or glycolytic metabolism in tissues/cells is useful for diagnostic and prognostic purposes. New imaging modalities are under development for imaging various aspects of tumor microenvironments. Electron Paramagnetic Resonance Imaging (EPRI) though similar to NMR/MRI is unique in its ability to provide quantitative images of pO₂ in vivo. The short electron spin relaxation times have been posing formidable challenge to the technology development for clinical application. With the availability of the narrow line width trityl compounds, pulsed EPR imaging techniques were developed for pO₂ imaging. EPRI visualizes the exogenously administered spin probes/contrast agents and hence lacks the complementary morphological information. Dynamic nuclear polarization (DNP), a phenomenon that transfers the high electron spin polarization to the surrounding nuclear spins (¹H and ¹³C) opened new capabilities in molecular imaging. DNP of ¹³C nuclei is utilized in metabolic imaging of 13C-labeled compounds by imaging specific enzyme kinetics. In this article, imaging strategies mapping physiologic and metabolic aspects in vivo are reviewed within the framework of their application in cancer research, highlighting the potential and challenges of each of them.

Approaches towards Elucidating the Metabolic Program of Hematopoietic Stem/Progenitor Cells

Hematopoietic stem cells (HSCs) in bone marrow continuously supply a large number of blood cells throughout life in collaboration with hematopoietic progenitor cells (HPCs). HSCs and HPCs are thought to regulate and utilize intracellular metabolic programs to obtain metabolites, such as adenosine triphosphate (ATP), which is necessary for various cellular functions. Metabolites not only provide stem/progenitor cells with nutrients for ATP and building block generation but are also utilized for protein modification and epigenetic regulation to maintain cellular characteristics. In recent years, the metabolic programs of tissue stem/progenitor cells and their underlying molecular mechanisms have been elucidated using a variety of metabolic analysis methods. In this review, we first present the advantages and disadvantages of the current approaches applicable to the metabolic analysis of tissue stem/progenitor cells, including HSCs and HPCs. In the second half, we discuss the characteristics and regulatory mechanisms of HSC metabolism, including the decoupling of ATP production by glycolysis and mitochondria. These technologies and findings have the potential to advance stem cell biology and engineering from a metabolic perspective and to establish therapeutic approaches.

Circulating metabolites associated with tumor hypoxia and early response to treatment in bevacizumab-refractory glioblastoma after combined bevacizumab and evofosfamide

Glioblastomas (GBM) are the most common and aggressive form of primary malignant brain tumor in the adult population, and, despite modern therapies, patients often develop recurrent disease, and the disease remains incurable with median survival below 2 years. Resistance to bevacizumab is driven by hypoxia in the tumor and evofosfamide is a hypoxia-activated prodrug, which we tested in a phase 2, dual center (University of Texas Health Science Center in San Antonio and Dana Farber Cancer Institute) clinical trial after bevacizumab failure. Tumor hypoxic volume was quantified by 18F-misonidazole PET. To identify circulating metabolic biomarkers of tumor hypoxia in patients, we used a high-resolution liquid chromatography-mass spectrometry-based approach to profile blood metabolites and their specific enantiomeric forms using untargeted approaches. Moreover, to evaluate early response to treatment, we characterized changes in circulating metabolite levels during treatment with combined bevacizumab and evofosfamide in recurrent GBM after bevacizumab failure. Gamma aminobutyric acid, and glutamic acid as well as its enantiomeric form D-glutamic acid all inversely correlated with tumor hypoxia. Intermediates of the serine synthesis pathway, which is known to be modulated by hypoxia, also correlated with tumor hypoxia (phosphoserine and serine). Moreover, following treatment, lactic acid was modulated by treatment, likely in response to a hypoxia mediated modulation of oxidative vs glycolytic metabolism. In summary, although our results require further validation in larger patients’ cohorts, we have identified candidate metabolic biomarkers that could evaluate the extent of tumor hypoxia and predict the benefit of combined bevacizumab and evofosfamide treatment in GBM following bevacizumab failure.

Assessing the therapeutic response of tumors to hypoxia-targeted prodrugs with an in silico approach

Tumor hypoxia is commonly recognized as a condition stimulating the progress of the aggressive phenotype of tumor cells. Hypoxic tumor cells inhibit the delivery of cytotoxic drugs, causing hypoxic areas to receive insufficient amounts of anticancer agents, which results in adverse treatment responses. Being such an obstruction to conventional therapies for cancer, hypoxia might be considered a target to facilitate the efficacy of treatments in the resistive environment of tumor sites. In this regard, benefiting from prodrugs that selectively target hypoxic regions remains an effective approach. Additionally, combining hypoxia-activated prodrugs (HAPs) with conventional chemotherapeutic drugs has been used as a promising strategy to eradicate hypoxic cells. However, determining the appropriate sequencing and scheduling of the combination therapy is also of great importance in obtaining favorable results in anticancer therapy. Here, benefiting from a modeling approach, we study the efficacy of HAPs in combination with chemotherapeutic drugs on tumor growth and the treatment response. Different treatment schedules have been investigated to see the importance of determining the optimal schedule in combination therapy. The effectiveness of HAPs in varying hypoxic conditions has also been explored in the study. The model provides qualitative conclusions about the treatment response, as the maximal benefit is obtained from combination therapy with greater cell death for highly hypoxic tumors. It has also been observed that the antitumor effects of HAPs show a hypoxia-dependent profile.

PEGPH20, a PEGylated human hyaluronidase, induces radiosensitization by reoxygenation in pancreatic cancer xenografts. A molecular imaging study

PURPOSE

PEGylated human hyaluronidase (PEGPH20) enzymatically depletes hyaluronan, an important component of the extracellular matrix, increasing the delivery of therapeutic molecules. Combinations of chemotherapy and PEGPH20, however, have been unsuccessful in Phase III clinical trials. We hypothesize that by increasing tumor oxygenation by improving vascular patency and perfusion, PEGPH20 will also act as a radiosensitization agent.

EXPERIMENTAL DESIGN

The effect of PEGPH20 on radiation treatment was analyzed with respect to tumor growth, survival time, p02, local blood volume, and the perfusion/permeability of blood vessels in a human pancreatic adenocarcinoma BxPC3 mouse model overexpressing hyaluronan synthase 3 (HAS3).

RESULTS

Mice overexpressing HAS3 developed fast growing, radiation resistant tumors that became rapidly more hypoxic as time progressed. Treatment with PEGPH20 increased survival times when used in combination with radiation therapy, significantly more than either radiation therapy or PEGPH20 alone. In mice that overexpressed HAS3, EPR imaging showed an increase in local pO2 that could be linked to increases in perfusion/permeability and local blood volume immediately after PEGPH20 treatment. Hyperpolarized [1-13C] pyruvate suggested PEGPH20 caused a metabolic shift towards decreased glycolytic flux. These effects were confined to the mice overexpressing HAS3 - no effect of PEGPH20 on survival, radiation treatment, or pO2 was seen in wild type BxPC3 tumors.

CONCLUSIONS

PEGPH20 may be useful for radiosensitization of pancreatic cancer but only in the subset of tumors with substantial hyaluronan accumulation. The response of the treatment may potentially be monitored by non-invasive imaging of the hemodynamic and metabolic changes in the tumor microenvironment.

Open Search-Based Proteomics Reveals Widespread Tryptophan Modifications Associated with Hypoxia in Lung Cancer

The tryptophan residue has a large hydrophobic surface that plays a unique role in the folded protein conformation and functions. Tryptophan modifications are presumably to be readily detected in proteins due to the vulnerability of the indole structure to electrophilic attacks. In this study, we report a systematic identification of sequence variations at tryptophan, termed tryptophan variants, from the proteome of patients with nonsmall cell lung cancer (NSCLC). Using shotgun proteomics and a modified open search algorithm, 25 tryptophan variants on 2481 sites in over 858 proteins were identified. Among these, 6 tryptophan variants are previously identified, 15 are newly annotated, and 4 are still unknown, most of which are involved in the cascade of oxidation in the blood microparticle. Remarkably, Trp313 of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was up-oxidized whereas Trp16 and Trp38 of hemoglobin (HBB) were down-oxidized in NCSLC tissues. The results were further supported by an independent cohort of 103 lung adenocarcinoma samples, reflecting a negative feedback and potential detoxification mechanism against tumor glycolysis and hypoxia. Overall, the study reports a quick approach to explore tryptophan variants at the proteomic scale. Our findings highlight the predominant role of tryptophan oxidation in regulating the redox balance of cancer cells and its potential role as prognostic biomarker for patients with NSCLC.

Structure-guided design enables development of a hyperpolarized molecular probe for the detection of aminopeptidase N activity in vivo

Dynamic nuclear polarization (DNP) is a cutting-edge technique that markedly enhances the detection sensitivity of molecules using nuclear magnetic resonance (NMR)/magnetic resonance imaging (MRI). This methodology enables real-time imaging of dynamic metabolic status in vivo using MRI. To expand the targetable metabolic reactions, there is a demand for developing exogenous, i.e., artificially designed, DNP-NMR molecular probes; however, complying with the requirements of practical DNP-NMR molecular probes is challenging because of the lack of established design guidelines. Here, we report Ala-[1-¹³C]Gly-d₂-NMe₂ as a DNP-NMR molecular probe for in vivo detection of aminopeptidase N activity. We developed this probe rationally through precise structural investigation, calculation, biochemical assessment, and advanced molecular design to achieve rapid and detectable responses to enzyme activity in vivo. With the fabricated probe, we successfully detected enzymatic activity in vivo. This report presents a comprehensive approach for the development of artificially derived, practical DNP-NMR molecular probes through structure-guided molecular design.

pH-taxis drives aerobic bacteria in duodenum to migrate into the pancreas with tumors

As oral or intestinal bacteria have been found in pancreatic cystic fluid and tumors, understanding bacterial migration from the duodenum into the pancreas via hepato-pancreatic duct is critical. Mathematical models of migration of aerobic bacteria from the duodenum to the pancreas with tumors were developed. Additionally, the bacterial distributions under the pH gradient and those under flow were measured in double-layer flow based microfluidic device and T-shaped cylinders. Migration of aerobic bacteria from the duodenum into pancreas is counteracted by bile and pancreatic juice flow but facilitated by pH-taxis from acidic duodenum fluid toward more favorable slightly alkaline pH in pancreatic juice. Additionally, the reduced flow velocity in cancer patients, due to compressed pancreatic duct by solid tumor, facilitates migration. Moreover, measured distribution of GFP E. coli under the pH gradient in a microfluidic device validated pH-tactic behaviors. Furthermore, Pseudomonas fluorescens in hydrochloride solution, but not in bicarbonate solution, migrated upstream against bicarbonate flow of > 20 μm/s, with an advancement at approximately 50 μm/s.

Hypoxia-Activated Prodrug Evofosfamide Treatment in Pancreatic Ductal Adenocarcinoma Xenografts Alters the Tumor Redox Status to Potentiate Radiotherapy

Aims: In hypoxic tumor microenvironments, the strongly reducing redox environment reduces evofosfamide (TH-302) to release a cytotoxic bromo-isophosphoramide (Br-IPM) moiety. This drug therefore preferentially attacks hypoxic regions in tumors where other standard anticancer treatments such as chemotherapy and radiation therapy are often ineffective. Various combination therapies with evofosfamide have been proposed and tested in preclinical and clinical settings. However, the treatment effect of evofosfamide monotherapy on tumor hypoxia has not been fully understood, partly due to the lack of quantitative methods to assess tumor pO₂in vivo. Here, we use quantitative pO₂ imaging by electron paramagnetic resonance (EPR) to evaluate the change in tumor hypoxia in response to evofosfamide treatment using two pancreatic ductal adenocarcinoma xenograft models: MIA Paca-2 tumors responding to evofosfamide and Su.86.86 tumors that do not respond. Results: EPR imaging showed that oxygenation improved globally after evofosfamide treatment in hypoxic MIA Paca-2 tumors, in agreement with the ex vivo results obtained from hypoxia staining by pimonidazole and in apparent contrast to the decrease in K^trans observed in dynamic contrast-enhanced magnetic resonance imaging (DCE MRI). Innovations: The observation that evofosfamide not only kills the hypoxic region of the tumor but also improves oxygenation in the residual tumor regions provides a rationale for combination therapies using radiation and antiproliferatives post evofosfamide for improved outcomes. Conclusion: This study suggests that reoxygenation after evofosfamide treatment is due to decreased oxygen demand rather than improved perfusion. Following the change in pO₂ after treatment may therefore yield a way of monitoring treatment response. Antioxid. Redox Signal. 35, 904-915.

A single microbubble formulation carrying 5-fluorouridine, Irinotecan and oxaliplatin to enable FOLFIRINOX treatment of pancreatic and colon cancer using ultrasound targeted microbubble destruction

FOLFIRINOX and FOLFOXIRI are combination chemotherapy treatments that incorporate the same drug cocktail (folinic acid, 5-fluorouracil, oxaliplatin and irinotecan) but exploit an altered dosing regimen when used in the management of pancreatic and colorectal cancer, respectively. Both have proven effective in extending life when used to treat patients with metastatic disease but are accompanied by significant adverse effects. To facilitate improved tumour-targeting of this drug combination, an ultrasound responsive microbubble formulation loaded with 5-fluorouridine, irinotecan and oxaliplatin (FIRINOX MB) was developed and its efficacy tested, together with the non-toxic folinic acid, in preclinical murine models of pancreatic and colorectal cancer. A significant improvement in tumour growth delay was observed in both models following ultrasound targeted microbubble destruction (UTMD) mediated FIRINOX treatment with pancreatic tumours 189% and colorectal tumours 82% smaller at the conclusion of the study when compared to animals treated with a standard dose of FOLFIRINOX. Survival prospects were also improved for animals in the UTMD mediated FIRINOX treatment group with an average survival of 22.17 ± 12.19 days (pancreatic) and 44.40 ± 3.85 days (colorectal) compared to standard FOLFIRINOX treatment (15.83 ± 4.17 days(pancreatic) and 37.50 ± 7.72 days (colon)). Notably, this improved efficacy was achieved using FIRINOX MB that contained 5-fluorouricil, irinotecan and oxaliplatin loadings that were 13.44-fold, 9.19-fold and 1.53-fold lower than used for the standard FOLFIRINOX treatment. These results suggest that UTMD enhances delivery of FIRINOX chemotherapy, making it significantly more effective at a substantially lower dose. In addition, the reduced systemic levels of 5-fluorouracil, irinotecan and oxaliplatin should also make the treatment more tolerable and reduce the adverse effects often associated with this treatment.

Oxygen-Sensitive MRI: A Predictive Imaging Biomarker for Tumor Radiation Response?

PURPOSE

To develop a noninvasive prognostic imaging biomarker related to hypoxia to predict SABR tumor control.

METHODS AND MATERIALS

A total of 145 subcutaneous syngeneic Dunning prostate R3327-AT1 rat tumors were focally irradiated once using cone beam computed tomography guidance on a small animal irradiator at 225 kV. Various doses in the range of 0 to 100 Gy were administered, while rats breathed air or oxygen, and tumor control was assessed up to 200 days. Oxygen-sensitive magnetic resonance imaging (MRI) (T₁-weighted, ΔR₁, ΔR₂*) was applied to 79 of these tumors at 4.7 T to assess response to an oxygen gas breathing challenge on the day before irradiation as a probe of tumor hypoxia.

RESULTS

Increasing radiation dose in the range of 0 to 90 Gy enhanced tumor control of air-breathing rats with a TCD₅₀ estimated at 59.6 ± 1.5 Gy. Control was significantly improved at some doses when rats breathed oxygen during irradiation (eg, 40 Gy; P < .05), and overall there was a modest left shift in the control curve: TCD₅₀(oxygen) = 53.1 ± 3.1 Gy (P < .05 vs air). Oxygen-sensitive MRI showed variable response to oxygen gas breathing challenge; the magnitude of T₁-weighted signal response (%ΔSI) allowed stratification of tumors in terms of local control at 40 Gy. Tumors showing %ΔSI >0.922 with O₂-gas breathing challenge showed significantly better control at 40 Gy during irradiation while breathing oxygen (75% vs 0%, P < .01). In addition, increased radiation dose (50 Gy) substantially overcame resistance, with 50% control for poorly oxygenated tumors. Stratification of dose-response curves based on %ΔSI >0.922 revealed different survival curves, with TCD₅₀ = 36.2 ± 3.2 Gy for tumors responsive to oxygen gas breathing challenge; this was significantly less than the 54.7 ± 2.4 Gy for unresponsive tumors (P < .005), irrespective of the gas inhaled during tumor irradiation.

CONCLUSIONS

Oxygen-sensitive MRI allowed stratification of tumors in terms of local control at 40 Gy, indicating its use as a potential predictive imaging biomarker. Increasing dose to 50 Gy overcame radiation resistance attributable to hypoxia in 50% of tumors.

Real-Time insight into in vivo redox status utilizing hyperpolarized [1-13C] N-acetyl cysteine

Drastic sensitivity enhancement of dynamic nuclear polarization is becoming an increasingly critical methodology to monitor real-time metabolic and physiological information in chemistry, biochemistry, and biomedicine. However, the limited number of available hyperpolarized ¹³C probes, which can effectively interrogate crucial metabolic activities, remains one of the major bottlenecks in this growing field. Here, we demonstrate [1-¹³C] N-acetyl cysteine (NAC) as a novel probe for hyperpolarized ¹³C MRI to monitor glutathione redox chemistry, which plays a central part of metabolic chemistry and strongly influences various therapies. NAC forms a disulfide bond in the presence of reduced glutathione, which generates a spectroscopically detectable product that is separated from the main peak by a 1.5 ppm shift. In vivo hyperpolarized MRI in mice revealed that NAC was broadly distributed throughout the body including the brain. Its biochemical transformation in two human pancreatic tumor cells in vitro and as xenografts differed depending on the individual cellular biochemical profile and microenvironment in vivo. Hyperpolarized NAC can be a promising non-invasive biomarker to monitor in vivo redox status and can be potentially translatable to clinical diagnosis.

KRAS, A Prime Mediator in Pancreatic Lipid Synthesis through Extra Mitochondrial Glutamine and Citrate Metabolism

Kirsten rat sarcoma viral oncogene homolog (KRAS)-driven pancreatic cancer is very lethal, with a five-year survival rate of <9%, irrespective of therapeutic advances. Different treatment modalities including chemotherapy, radiotherapy, and immunotherapy demonstrated only marginal efficacies because of pancreatic tumor specificities. Surgery at the early stage of the disease remains the only curative option, although only in 20% of patients with early stage disease. Clinical trials targeting the main oncogenic driver, KRAS, have largely been unsuccessful. Recently, global metabolic reprogramming has been identified in patients with pancreatic cancer and oncogenic KRAS mouse models. The newly reprogrammed metabolic pathways and oncometabolites affect the tumorigenic environment. The development of methods modulating metabolic reprogramming in pancreatic cancer cells might constitute a new approach to its therapy. In this review, we describe the major metabolic pathways providing acetyl-CoA and NADPH essential to sustain lipid synthesis and cell proliferation in pancreatic cancer cells.

Multimodal Molecular Imaging Detects Early Responses to Immune Checkpoint Blockade

Immune checkpoint blockade (ICB) has become a standard therapy for several cancers, however, the response to ICB is inconsistent and a method for noninvasive assessment has not been established to date. To investigate the capability of multimodal imaging to evaluate treatment response to ICB therapy, hyperpolarized ¹³C MRI using [1-¹³C] pyruvate and [1,4-¹³C2] fumarate and dynamic contrast enhanced (DCE) MRI was evaluated to detect early changes in tumor glycolysis, necrosis, and intratumor perfusion/permeability, respectively. Mouse tumor models served as platforms for high (MC38 colon adenocarcinoma) and low (B16-F10 melanoma) sensitivity to dual ICB of PD-L1 and CTLA4. Glycolytic flux significantly decreased following treatment only in the less sensitive B16-F10 tumors. Imaging [1,4-¹³C2] fumarate conversion to [1,4-¹³C2] malate showed a significant increase in necrotic cell death following treatment in the ICB-sensitive MC38 tumors, with essentially no change in B16-F10 tumors. DCE-MRI showed significantly increased perfusion/permeability in MC38-treated tumors, whereas a similar, but statistically nonsignificant, trend was observed in B16-F10 tumors. When tumor volume was also taken into consideration, each imaging biomarker was linearly correlated with future survival in both models. These results suggest that hyperpolarized ¹³C MRI and DCE MRI may serve as useful noninvasive imaging markers to detect early response to ICB therapy. SIGNIFICANCE: Hyperpolarized ¹³C MRI and dynamic contrast enhanced MRI in murine tumor models provide useful insight into evaluating early response to immune checkpoint blockade therapy.See related commentary by Cullen and Keshari, p. 3444.

Deep-learning and MR images to target hypoxic habitats with evofosfamide in preclinical models of sarcoma

Rationale: Hypoxic regions (habitats) within tumors are heterogeneously distributed and can be widely variant. Hypoxic habitats are generally pan-therapy resistant. For this reason, hypoxia-activated prodrugs (HAPs) have been developed to target these resistant volumes. The HAP evofosfamide (TH-302) has shown promise in preclinical and early clinical trials of sarcoma. However, in a phase III clinical trial of non-resectable soft tissue sarcomas, TH-302 did not improve survival in combination with doxorubicin (Dox), possibly due to a lack of patient stratification based on hypoxic status. Therefore, we used magnetic resonance imaging (MRI) to identify hypoxic habitats and non-invasively follow therapies response in sarcoma mouse models. Methods: We developed deep-learning (DL) models to identify hypoxia, using multiparametric MRI and co-registered histology, and monitored response to TH-302 in a patient-derived xenograft (PDX) of rhabdomyosarcoma and a syngeneic model of fibrosarcoma (radiation-induced fibrosarcoma, RIF-1). Results: A DL convolutional neural network showed strong correlations (>0.76) between the true hypoxia fraction in histology and the predicted hypoxia fraction in multiparametric MRI. TH-302 monotherapy or in combination with Dox delayed tumor growth and increased survival in the hypoxic PDX model (p<0.05), but not in the RIF-1 model, which had a lower volume of hypoxic habitats. Control studies showed that RIF-1 resistance was due to hypoxia and not other causes. Notably, PDX tumors developed resistance to TH-302 under prolonged treatment that was not due to a reduction in hypoxic volumes. Conclusion: Artificial intelligence analysis of pre-therapy MR images can predict hypoxia and subsequent response to HAPs. This approach can be used to monitor therapy response and adapt schedules to forestall the emergence of resistance.

Preclinical Applications of Multi-Platform Imaging in Animal Models of Cancer

In animal models of cancer, oncologic imaging has evolved from a simple assessment of tumor location and size to sophisticated multimodality exploration of molecular, physiologic, genetic, immunologic, and biochemical events at microscopic to macroscopic levels, performed noninvasively and sometimes in real time. Here, we briefly review animal imaging technology and molecular imaging probes together with selected applications from recent literature. Fast and sensitive optical imaging is primarily used to track luciferase-expressing tumor cells, image molecular targets with fluorescence probes, and to report on metabolic and physiologic phenotypes using smart switchable luminescent probes. MicroPET/single-photon emission CT have proven to be two of the most translational modalities for molecular and metabolic imaging of cancers: immuno-PET is a promising and rapidly evolving area of imaging research. Sophisticated MRI techniques provide high-resolution images of small metastases, tumor inflammation, perfusion, oxygenation, and acidity. Disseminated tumors to the bone and lung are easily detected by microCT, while ultrasound provides real-time visualization of tumor vasculature and perfusion. Recently available photoacoustic imaging provides real-time evaluation of vascular patency, oxygenation, and nanoparticle distributions. New hybrid instruments, such as PET-MRI, promise more convenient combination of the capabilities of each modality, enabling enhanced research efficacy and throughput.

Optical and magnetic resonance imaging approaches for investigating the tumour microenvironment: state-of-the-art review and future trends

The tumour microenvironment (TME) strongly influences tumorigenesis and metastasis. Two of the most characterized properties of the TME are acidosis and hypoxia, both of which are considered hallmarks of tumours as well as critical factors in response to anticancer treatments. Currently, various imaging approaches exist to measure acidosis and hypoxia in the TME, including magnetic resonance imaging (MRI), positron emission tomography and optical imaging. In this review, we will focus on the latest fluorescent-based methods for optical sensing of cell metabolism and MRI as diagnostic imaging tools applied both in vitro and in vivo. The primary emphasis will be on describing the current and future uses of systems that can measure intra- and extra-cellular pH and oxygen changes at high spatial and temporal resolution. In addition, the suitability of these approaches for mapping tumour heterogeneity, and assessing response or failure to therapeutics will also be covered.

A Simple Aggregation-Induced Emission Nanoprobe with Deep Tumor Penetration for Hypoxia Detection and Imaging-Guided Surgery in Vivo

The pan-cancer detection and precise visualization of tiny tumors in surgery still face great challenges. As tumors grow aggressively, hypoxia is a common feature of solid tumors and has supplied a general way for detecting tumors. Herein, we report a simple aggregation-induced emission nanoprobe-TPE-4NE-O that can specifically switch on their fluorescence in the presence of cytochrome P450 reductase, a reductase which is overexpressed under hypoxia conditions. The probe can selectively light up the hypoxia cells and has shown enhanced deep tumor penetration via charge conversion both in vitro and in vivo. After being modified with FA-DSPE-PEG, higher tumor uptake can be seen and FA-DSPE/TPE-4NE-O showed specific visualization to the hypoxia cancer cells. Excitingly, much brighter fluorescence was accumulated at the tumors in the FA-DSPE/TPE-4NE-O group, even though the tumor was as small as 2.66 mm. The excellent performance of FA-DSPE/TPE-4NE-O in detecting tiny tumors has made it possible for imaging-guided tumor resection. More importantly, the probe exhibited good biocompatibility with negligible organ damage and eliminated a hemolysis risk. The simple but promising probe has supplied a new strategy for pan-cancer detection and tiny tumor visualization, which have shown great potential in clinical translation.

Imaging of Tumor Hypoxia for Radiotherapy: Current Status and Future Directions

Tumor regions that are transiently or chronically undersupplied with oxygen (hypoxia) and nutrients, and enriched with acidic waste products, are common due to an abnormal and inefficient tumor vasculature, and a deviant highly glycolytic energy metabolism. There is compelling evidence that tumor hypoxia is strongly linked to poor prognosis since oxygen-deprived cells are highly resistant to therapy including radio- and chemotherapy, and survival of such cells is a primary cause of disease relapse. Despite a general improvement in cancer survival rates, hypoxia remains a formidable challenge. Recent progress in radiation delivery systems with improved spatial accuracy that allows dose escalation to hypoxic tumors or even tumor subvolumes, and the development of hypoxia-selective drugs, including bioreductive prodrugs, holds great promise for overcoming this obstacle. However, apart from one notable exception, translation of promising preclinical therapies to the clinic have largely been disappointing. A major obstacle in clinical trials on hypoxia-targeting strategies has been the lack of reliable information on tumor hypoxia, which is crucial for patient stratification into groups of those that are likely to benefit from intervention and those who are not. Further, in many newer trials on hypoxia-selective drugs the choice of cancer disease and combination therapy has not always been ideal, especially not for clinical proof of principle trials. Clearly, there is a pending need for clinical applicable methodologies that may allow us to quantify, map and monitor hypoxia. Molecular imaging may provide the information required for narrowing the gap between potential and actual patient benefit of hypoxia-targeting strategies. The grand majority of preclinical and clinical work has focused on the usefulness of PET-based assessment of hypoxia-selective tracers. Since hypoxia PET has profound inherent weaknesses, the use of other methodologies, including more indirect methods that quantifies blood flow or oxygenation-dependent flux changes through ATP-generating pathways (eg, anaerobic glycolysis) is being extensively studied. In this review, we briefly discuss established and emerging hypoxia-targeting strategies, followed by a more thorough evaluation of strengths and weaknesses of clinical applicable imaging methodologies that may guide timely treatment intensification to overcome hypoxia-driven resistance. Historically, most evidence for the linkage between hypoxia and poor outcome is based on work in the field of radiotherapy. Therefore, main emphasis in this review is on targeting and imaging of hypoxia for improved radiotherapy.

Oxygen-Enhanced Optoacoustic Tomography Reveals the Effectiveness of Targeting Heme and Oxidative Phosphorylation at Normalizing Tumor Vascular Oxygenation

Multispectral optoacoustic tomography (MSOT) is an emerging noninvasive imaging modality that can detect real-time dynamic information about the tumor microenvironment in humans and animals. Oxygen enhanced (OE)-MSOT can monitor tumor vasculature and oxygenation during disease development or therapy. Here, we used MSOT and OE-MSOT to examine in mice the response of human non-small cell lung cancer (NSCLC) xenografts to a new class of antitumor drugs, heme-targeting agents heme-sequestering peptide 2 (HSP2) and cyclopamine tartrate (CycT). HSP2 inhibits heme uptake, while CycT inhibits heme synthesis in NSCLC cells, where heme is essential for ATP generation via oxidative phosphorylation. HSP2 and CycT can inhibit ATP generation and thereby suppress NSCLC cell tumorigenic functions. MSOT showed that treatment of NSCLC tumors with HSP2 or CycT reduced total hemoglobin, increased oxygen saturation, and enhanced the amplitude of response to oxygen gas breathing challenge. HSP2 and CycT normalized tumor vasculature and improved tumor oxygenation, where levels of several hypoxia markers in NSCLC tumors were reduced by treatment with HSP2 or CycT. Furthermore, treatment with HSP2 or CycT reduced levels of angiogenic factor VEGFA, its receptor VEGFR1, and vascular marker CD34. Together, our data show that heme-targeting drugs HSP2 and CycT elicit multiple tumor-suppressing functions, such as inhibiting angiogenic function, normalizing tumor vasculature, alleviating tumor hypoxia, and inhibiting oxygen consumption and ATP generation. SIGNIFICANCE: Heme-targeting agents HSP2 and CycT effectively normalize tumor vasculature and alleviate tumor hypoxia, raising the possibility of their combination with chemo-, radio-, and immunotherapies to improve antitumor efficacy.See related commentary by Tomaszewski, p. 3461.

A Novel Model System for Understanding Anticancer Activity of Hypoxia-Activated Prodrugs

Reports on the comprehensive factors for design considerations of hypoxia-activated prodrugs (HAPs) are rare. We introduced a new model system composed of a series of highly water-soluble HAPs, providing a platform to comprehensively understand the interaction between HAPs and hypoxic biosystems. Specifically, four kinds of new HAPs were designed and synthesized, containing the same biologically active moiety but masked by different bioreductive groups. Our results demonstrated that the activity of the prodrugs was strongly dependent on not only the molecular structure but also the hypoxic tumor microenvironment. We found the presence of a direct linear relationship between cytotoxicity of the HAPs and the reduction potential of whole molecule/oxygen concentration/reductase expression. Moreover, limited blood vasculature in hypoxic regions was also a critical barrier for effective activation of the HAPs. This study offers a comprehensive insight into understanding the design factors required for HAPs.

Imaging of glucose metabolism by 13C-MRI distinguishes pancreatic cancer subtypes in mice

Metabolic differences among and within tumors can be an important determinant in cancer treatment outcome. However, methods for determining these differences non-invasively in vivo is lacking. Using pancreatic ductal adenocarcinoma as a model, we demonstrate that tumor xenografts with a similar genetic background can be distinguished by their differing rates of the metabolism of 13C labeled glucose tracers, which can be imaged without hyperpolarization by using newly developed techniques for noise suppression. Using this method, cancer subtypes that appeared to have similar metabolic profiles based on steady state metabolic measurement can be distinguished from each other. The metabolic maps from 13C-glucose imaging localized lactate production and overall glucose metabolism to different regions of some tumors. Such tumor heterogeneity would not be not detectable in FDG-PET.

Pancreatic ductal adenocarcinoma: biological hallmarks, current status, and future perspectives of combined modality treatment approaches

Pancreatic ductal adenocarcinoma (PDAC) is a highly devastating disease with poor prognosis and rising incidence. Late detection and a particularly aggressive biology are the major challenges which determine therapeutic failure. In this review, we present the current status and the recent advances in PDAC treatment together with the biological and immunological hallmarks of this cancer entity. On this basis, we discuss new concepts combining distinct treatment modalities in order to improve therapeutic efficacy and clinical outcome - with a specific focus on protocols involving radio(chemo)therapeutic approaches.

Multiparametric MRI and Coregistered Histology Identify Tumor Habitats in Breast Cancer Mouse Models

It is well-recognized that solid tumors are genomically, anatomically, and physiologically heterogeneous. In general, more heterogeneous tumors have poorer outcomes, likely due to the increased probability of harboring therapy-resistant cells and regions. It is hypothesized that the genomic and physiologic heterogeneity are related, because physiologically distinct regions will exert variable selection pressures leading to the outgrowth of clones with variable genomic/proteomic profiles. To investigate this, methods must be in place to interrogate and define, at the microscopic scale, the cytotypes that exist within physiologically distinct subregions ("habitats") that are present at mesoscopic scales. MRI provides a noninvasive approach to interrogate physiologically distinct local environments, due to the biophysical principles that govern MRI signal generation. Here, we interrogate different physiologic parameters, such as perfusion, cell density, and edema, using multiparametric MRI (mpMRI). Signals from six different acquisition schema were combined voxel-by-voxel into four clusters identified using a Gaussian mixture model. These were compared with histologic and IHC characterizations of sections that were coregistered using MRI-guided 3D printed tumor molds. Specifically, we identified a specific set of MRI parameters to classify viable-normoxic, viable-hypoxic, nonviable-hypoxic, and nonviable-normoxic tissue types within orthotopic 4T1 and MDA-MB-231 breast tumors. This is the first coregistered study to show that mpMRI can be used to define physiologically distinct tumor habitats within breast tumor models. SIGNIFICANCE: This study demonstrates that noninvasive imaging metrics can be used to distinguish subregions within heterogeneous tumors with histopathologic correlation.

Diarylidenylpiperidones, H-4073 and HO-3867, Induce G2/M Cell-Cycle Arrest, Apoptosis and Inhibit STAT3 Phosphorylation in Human Pancreatic Cancer Cells

Pancreatic cancer has a 5-year survival rate below 10% and the treatment options are limited. Signal transducer and activator of transcription (STAT3) is a constitutively expressed protein in human pancreatic cancers and is associated with their poor prognosis. Targeting of STAT3 signaling using novel therapeutic agents is a potential strategy for pancreatic cancer treatment. Diarylidenylpiperidone (DAP) compounds, such as H-4073 and HO-3867, have been shown to be STAT3 inhibitors in several human ovarian cancers. Particularly, HO-3867 is an N-hydroxypyrroline derivative of DAP that has targeted cytotoxicity toward cancer cells without affecting healthy cells. In the present study, we evaluated the anticancer efficacy of H-4073 and HO-3867 in a human pancreatic cell line (AsPC-1). We found that both the compounds exhibited potential cytotoxicity to AsPC-1 cells by inducing G2/M cell-cycle arrest, apoptosis, and cell death, by mitochondrial damage and inhibition of STAT3 phosphorylation. In summary, H-4073 and HO-3867 are cytotoxic to AsPC-1 cells and seem to act through similar mechanisms, including STAT3 inhibition, cell-cycle arrest, and apoptosis.

Combating pancreatic cancer with PI3K pathway inhibitors in the era of personalised medicine

Pancreatic ductal adenocarcinoma (PDAC) is among the most deadly solid tumours. This is due to a generally late-stage diagnosis of a primarily treatment-refractory disease. Several large-scale sequencing and mass spectrometry approaches have identified key drivers of this disease and in doing so highlighted the vast heterogeneity of lower frequency mutations that make clinical trials of targeted agents in unselected patients increasingly futile. There is a clear need for improved biomarkers to guide effective targeted therapies, with biomarker-driven clinical trials for personalised medicine becoming increasingly common in several cancers. Interestingly, many of the aberrant signalling pathways in PDAC rely on downstream signal transduction through the mitogen-activated protein kinase and phosphoinositide 3-kinase (PI3K) pathways, which has led to the development of several approaches to target these key regulators, primarily as combination therapies. The following review discusses the trend of PDAC therapy towards molecular subtyping for biomarker-driven personalised therapies, highlighting the key pathways under investigation and their relationship to the PI3K pathway.

Imaging Metabolic Processes to Predict Radiation Responses

The aberrant vasculature in the tumor microenvironment creates hypoxic zones, poor perfusion, and high interstitial fluid pressure. Also, the tumor cell metabolic phenotype utilizes the aerobic glycolytic pathways for energy source and generation of cell mass. These physiologic and metabolic phenotypes in solid tumors are amenable for molecular imaging techniques to extract imaging biomarkers such as pO₂ and enzyme kinetics reflecting glycolysis. The imaging biomarkers have value in diagnostic and prognostic purposes. Additionally, they can be used to guide choices for tailored treatment regimens. Electron paramagnetic resonance imaging for pO₂ imaging and ¹³C magnetic resonance imaging with hyperpolarized ¹³C probes such as ¹³C-labeled pyruvate have shown significant potential in characterizing the tumor microenvironment physiologically and metabolically.