In vivo19F Magnetic Resonance Spectroscopy and Chemical Shift Imaging of Tri-Fluoro-Nitroimidazole as a Potential Hypoxia Reporter in Solid Tumors

Circulating levels of hydroxylated bradykinin function as an indicator of tissue HIF-1α expression

The critical roles of oxygen homeostasis in metabolism are indisputable and hypoxic responses are correlated with the pathogenesis of gastrointestinal, pulmonary, renal diseases and cancers. Evaluating tissue hypoxia to predict treatment outcome is challenging, however, due to the lack of rapid, accurate and non-invasive methods. Hypoxia enhances prolyl-4-hydroxylase α1 (P4HA1) expression, which can convert bradykinin (BK) to hydroxyprolyl-BK (Hyp-BK), leading us to hypothesize that circulating Hyp-BK/BK ratios may reflect tissue hypoxia and predict treatment outcomes. Direct quantification of Hyp-BK peptides in serum or plasma by conventional MALDI-TOF MS analysis is technically challenging. In our study, a nanopore-based fractionation and enrichment protocol was utilized to allow the simple workflow for circulating Hyp-BK/BK analysis. Hypoxia is linked to poor prognosis due to its role in promoting pancreatic cancer progression and metastasis. Here we show that P4HA1 expression was increased in pancreatic tumors versus adjacent tissue, associated with poor survival, and corresponded with tumor expression of the hypoxia inducible factor 1α (HIF-1α) and carbonic anhydrase 9 (CA9). Hypoxia-induced P4HA1 expression and BK conversion to Hyp-BK were found to be HIF-1α dependent, pre-treatment serum Hyp-BK/BK ratios corresponded with tissue HIF-1α and P4HA1 expression, and high Hyp-BK/BK levels corresponded with poor response to therapy. These results suggest that pre-treatment circulating Hyp-BK/BK ratios may have value as a non-invasive, surrogate indicator of tissue hypoxia and tumor responses to therapy.

Preclinical Applications of Magnetic Resonance Imaging in Oncology

The evolving possibilities of molecular imaging (MI) are fundamentally changing the way we look at cancer, with imaging paradigms now shifting away from basic morphological measures toward the longitudinal assessment of functional, metabolic, cellular, and molecular information in vivo. Recent developments of imaging methodology and probe molecules utilizing the vast number of novel animal models of human cancers have enhanced our ability to non-invasively characterize neoplastic tissue and follow anticancer treatments. While preclinical molecular imaging offers a whole palette of excellent methodology to choose from, we will focus on magnetic resonance imaging (MRI) techniques, since they provide excellent molecular imaging capabilities and bear high potential for clinical translation. Prerequisites and consequences of using animal models as surrogates of human cancers in preclinical molecular imaging are outlined. We present physical principles, values, and limitations of MRI as molecular imaging modality and comment on its high potential to non-invasively assess information on metabolism, hypoxia, angiogenesis, and cell trafficking in preclinical cancer research.

Remarkable Levels of 15N Polarization Delivered through SABRE into Unlabeled Pyridine, Pyrazine, or Metronidazole Enable Single Scan NMR Quantification at the mM Level

While many drugs and metabolites contain nitrogen, harnessing their diagnostic ¹⁵N NMR signature for their characterization is underutilized because of inherent detection difficulties. Here, we demonstrate how precise ultralow field signal amplification by reversible exchange (±0.2 mG) in conjunction parahydrogen and an iridium precatalyst of the form IrCl(COD)(NHC) with the coligand d₉-benzylamine allows the naturally abundant ¹⁵N NMR signatures of pyridine, pyrazine, metronidazole, and acetonitrile to be readily detected at 9.4 T in single NMR observations through >50% ¹⁵N polarization levels. These signals allow for rapid and precise reagent quantification via a response that varies linearly over the 2-70 mM concentration range.

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.

Quasi-Resonance Signal Amplification by Reversible Exchange

Here we present the feasibility of NMR signal amplification by reversible exchange (SABRE) using radio frequency irradiation at low magnetic field (0.05 T) in the regime where the chemical shifts of free and catalyst-bound species are similar. In SABRE, the 15N-containing substrate and parahydrogen perform simultaneous chemical exchange on an iridium hexacoordinate complex. A shaped spin-lock induced crossing (SLIC) radio frequency pulse sequence followed by a delay is applied at quasi-resonance (QUASR) conditions of 15N spins of a 15N-enriched substrate. As a result of this pulse sequence application, 15N z-magnetization is created from the spin order of parahydrogen-derived hyperpolarized hydrides. The repetition of the pulse sequence block consisting of a shaped radio frequency pulse and the delay leads to the buildup of 15N magnetization. The modulation of this effect by the irradiation frequency, pulse duration and amplitude, delay duration, and number of pumping cycles was demonstrated. Pyridine-15N, acetonitrile-15N, and metronidazole-15N2-13C2 substrates were studied representing three classes of compounds (five- and six-membered heterocycles and nitrile), showing the wide applicability of the technique. Metronidazole-15N2-13C2 is an FDA-approved antibiotic that can be injected in large quantities, promising noninvasive and accurate hypoxia sensing. The 15N hyperpolarization levels attained with QUASR-SABRE on metronidazole-15N2-13C2 were more than 2-fold greater than those with SABRE-SHEATH (SABRE in shield enables alignment transfer to heteronuclei), demonstrating that QUASR-SABRE can deliver significantly more efficient means of SABRE hyperpolarization.

Clinical imaging of hypoxia: Current status and future directions

Tissue hypoxia is a key feature of many important causes of morbidity and mortality. In pathologies such as stroke, peripheral vascular disease and ischaemic heart disease, hypoxia is largely a consequence of low blood flow induced ischaemia, hence perfusion imaging is often used as a surrogate for hypoxia to guide clinical diagnosis and treatment. Importantly, ischaemia and hypoxia are not synonymous conditions as it is not universally true that well perfused tissues are normoxic or that poorly perfused tissues are hypoxic. In pathologies such as cancer, for instance, perfusion imaging and oxygen concentration are less well correlated, and oxygen concentration is independently correlated to radiotherapy response and overall treatment outcomes. In addition, the progression of many diseases is intricately related to maladaptive responses to the hypoxia itself. Thus there is potentially great clinical and scientific utility in direct measurements of tissue oxygenation. Despite this, imaging assessment of hypoxia in patients is rarely performed in clinical settings. This review summarises some of the current methods used to clinically evaluate hypoxia, the barriers to the routine use of these methods and the newer agents and techniques being explored for the assessment of hypoxia in pathological processes.

Facile Removal of Homogeneous SABRE Catalysts for Purifying Hyperpolarized Metronidazole, a Potential Hypoxia Sensor

We report a simple and effective method to remove IrIMes homogeneous polarization transfer catalysts from solutions where NMR Signal Amplification By Reversible Exchange (SABRE) has been performed, while leaving intact the substrate's hyperpolarized state. Following microTesla SABRE hyperpolarization of ¹⁵N spins in metronidazole, addition of SiO₂ microparticles functionalized with 3-mercaptopropyl or 2-mercaptoethyl ethyl sulfide moieties provides removal of the catalyst from solution well within the hyperpolarization decay time at 0.3 T (T ₁>3 mins)-and enabling transfer to 9.4 T for detection of enhanced ¹⁵N signals in the absence of catalyst within the NMR-detection region. Successful catalyst removal from solution is supported by the inability to "re-hyperpolarize" ¹⁵N spins in subsequent attempts, as well as by ¹H NMR and ICP-MS. Record-high ¹⁵N nuclear polarization of up to ~34% was achieved, corresponding to >100,000-fold enhancement at 9.4 T, and approximately 5/6^th of the ¹⁵N hyperpolarization is retained after ~20-second-long purification procedure. Taken together, these results help pave the way for future studies involving in vivo molecular imaging using agents hyperpolarized via rapid and inexpensive parahydrogen-based methods.

Spin-Lattice Relaxation of Hyperpolarized Metronidazole in Signal Amplification by Reversible Exchange in Micro-Tesla Fields

Simultaneous reversible chemical exchange of parahydrogen and to-be-hyperpolarized substrate on metal centers enables spontaneous transfer of spin order from parahydrogen singlet to nuclear spins of the substrate. When performed at sub-micro-Tesla magnetic field, this technique of NMR Signal Amplification by Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH). SABRE-SHEATH has been shown to hyperpolarize nitrogen-15 sites of a wide range of biologically interesting molecules to a high polarization level (P > 20%) in one minute. Here, we report on a systematic study of 1H, 13C and 15N spin-lattice relaxation (T1) of metronidazole-13C2-15N2 in SABRE-SHEATH hyperpolarization process. In micro-Tesla range, we find that all 1H, 13C and 15N spins studied share approximately the same T1 values (ca. 4 s at the conditions studied) due to mixing of their Zeeman levels, which is consistent with the model of relayed SABRE-SHEATH effect. These T1 values are significantly lower than those at higher magnetic (i.e. the Earth's magnetic field and above), which exceed 3 minutes in some cases. Moreover, these relatively short T1 values observed below 1 micro-Tesla limit the polarization build-up process of SABRE-SHEATH- thereby, limiting maximum attainable 15N polarization. The relatively short nature of T1 values observed below 1 micro-Tesla is primarily caused by intermolecular interactions with quadrupolar iridium centers or dihydride protons of the employed polarization transfer catalyst, whereas intramolecular spin-spin interactions with 14N quadrupolar centers have significantly smaller contribution. The presented experimental results and their analysis will be beneficial for more rational design of SABRE-SHEATH (i) polarization transfer catalyst, and (ii) hyperpolarized molecular probes in the context of biomedical imaging and other applications.

Spin Relays Enable Efficient Long-Range Heteronuclear Signal Amplification By Reversible Exchange

A systematic experimental study is reported on the polarization transfer to distant spins, which do not directly bind to the polarization transfer complexes employed in Signal Amplification By Reversible Exchange (SABRE) experiments. Both, long-range transfer to protons and long-range transfer to heteronuclei i.e. 13C and 15N are examined. Selective destruction of hyperpolarization on 1H, 13C, and 15N sites is employed, followed by their re-hyperpolarization from neighboring spins within the molecules of interest (pyridine for 1H studies and metronidazole-15N2-13C2 for 13C and 15N studies). We conclude that long-range sites can be efficiently hyperpolarized when a network of spin-½ nuclei enables relayed polarization transfer (i.e. via short-range interactions between sites). In case of proton SABRE in the milli-Tesla regime, a relay network consisting of protons only is sufficient. However, in case 13C and 15N are targeted (i.e. via SABRE in SHield Enables Alignment Transfer to Heteronuclei or SABRE-SHEATH experiment), the presence of a heteronuclear network (e.g. consisting of 15N) enables a relay mechanism that is significantly more efficient than the direct transfer of spin order from para-H2-derived hydrides.

Theranostic Probes for Targeting Tumor Microenvironment: An Overview

Long gone is the time when tumors were thought to be insular masses of cells, residing independently at specific sites in an organ. Now, researchers gradually realize that tumors interact with the extracellular matrix (ECM), blood vessels, connective tissues, and immune cells in their environment, which is now known as the tumor microenvironment (TME). It has been found that the interactions between tumors and their surrounds promote tumor growth, invasion, and metastasis. The dynamics and diversity of TME cause the tumors to be heterogeneous and thus pose a challenge for cancer diagnosis, drug design, and therapy. As TME is significant in enhancing tumor progression, it is vital to identify the different components in the TME such as tumor vasculature, ECM, stromal cells, and the lymphatic system. This review explores how these significant factors in the TME, supply tumors with the required growth factors and signaling molecules to proliferate, invade, and metastasize. We also examine the development of TME-targeted nanotheranostics over the recent years for cancer therapy, diagnosis, and anticancer drug delivery systems. This review further discusses the limitations and future perspective of nanoparticle based theranostics when used in combination with current imaging modalities like Optical Imaging, Magnetic Resonance Imaging (MRI) and Nuclear Imaging (Positron Emission Tomography (PET) and Single Photon Emission Computer Tomography (SPECT)).

Molecular imaging of the tumor microenvironment

The tumor microenvironment plays a critical role in tumor initiation, progression, metastasis, and resistance to therapy. It is different from normal tissue in the extracellular matrix, vascular and lymphatic networks, as well as physiologic conditions. Molecular imaging of the tumor microenvironment provides a better understanding of its function in cancer biology, and thus allowing for the design of new diagnostics and therapeutics for early cancer diagnosis and treatment. The clinical translation of cancer molecular imaging is often hampered by the high cost of commercialization of targeted imaging agents as well as the limited clinical applications and small market size of some of the agents. Because many different cancer types share similar tumor microenvironment features, the ability to target these biomarkers has the potential to provide clinically translatable molecular imaging technologies for a spectrum of cancers and broad clinical applications. There has been significant progress in targeting the tumor microenvironment for cancer molecular imaging. In this review, we summarize the principles and strategies of recent advances made in molecular imaging of the tumor microenvironment, using various imaging modalities for early detection and diagnosis of cancer.

Molecular imaging of the tumor microenvironment for precision medicine and theranostics

Morbidity and mortality from cancer and their associated conditions and treatments continue to extract a heavy social and economic global burden despite the transformative advances in science and technology in the twenty-first century. In fact, cancer incidence and mortality are expected to reach pandemic proportions by 2025, and costs of managing cancer will escalate to trillions of dollars. The inability to establish effective cancer treatments arises from the complexity of conditions that exist within tumors, the plasticity and adaptability of cancer cells coupled with their ability to escape immune surveillance, and the co-opted stromal cells and microenvironment that assist cancer cells in survival. Stromal cells, although destroyed together with cancer cells, have an ever-replenishing source that can assist in resurrecting tumors from any residual cancer cells that may survive treatment. The tumor microenvironment landscape is a continually changing landscape, with spatial and temporal heterogeneities that impact and influence cancer treatment outcome. Importantly, the changing landscape of the tumor microenvironment can be exploited for precision medicine and theranostics. Molecular and functional imaging can play important roles in shaping and selecting treatments to match this landscape. Our purpose in this review is to examine the roles of molecular and functional imaging, within the context of the tumor microenvironment, and the feasibility of their applications for precision medicine and theranostics in humans.

GdDO3NI, a nitroimidazole-based T1 MRI contrast agent for imaging tumor hypoxia in vivo

Tumor hypoxia is known to affect sensitivity to radiotherapy and promote development of metastases; therefore, the ability to image tumor hypoxia in vivo could provide useful prognostic information and help tailor therapy. We previously demonstrated in vitro evidence for selective accumulation of a gadolinium tetraazacyclododecanetetraacetic acid monoamide conjugate of 2-nitroimidazole (GdDO3NI), a magnetic resonance imaging T1-shortening agent, in hypoxic cells grown in tissue culture. We now report evidence for accumulation of GdDO3NI in hypoxic tumor tissue in vivo. Our data show that GdDO3NI accumulated significantly (p < 0.05) in the central, poorly perfused regions of rat prostate adenocarcinoma AT1 tumors (threefold higher concentration than for the control agent) and showed better clearance from well-perfused regions and complete clearance from the surrounding muscle tissue. Inductively coupled plasma mass spectroscopy confirmed that more GdDO3NI than control agent was retained in the central region and that more GdDO3NI was retained in the central region than at the periphery. These results show the utility of GdDO3NI to image tumor hypoxia and highlight the potential of GdDO3NI for application to image-guided interventions for radiation therapy or hypoxia-activated chemotherapy.

MR - eyes for cancer: looking within an impenetrable disease

Probe development is a critical component in cancer imaging, and novel probes are making major inroads in several aspects of cancer detection and image-guided treatments. Intrinsic MR probes such as signals from metabolites and their chemical shifts have been used for more than a decade to understand cancer physiology and metabolism. Through the integration of technology, molecular biology, and chemistry, the last few years have witnessed an explosion of extrinsic probes for molecular and functional imaging of cancer that, together with techniques such as CEST and hyperpolarization, have significantly expanded the repertoire of MR techniques in basic and translational investigations of many different aspects of cancer. Furthermore, incorporation of MR probes into multifunctional nanoparticles and multimodality imaging platforms have opened new opportunities for MR in image-guided diagnosis and therapy of cancer. Here we have provided an overview of recent innovations that have occurred in the development of MRI probes for molecular and functional imaging of cancer. Although most of these novel probes are not clinically available, they offer significant promise for future translational applications. In this review, we have highlighted the areas of future development that are likely to have a profound impact on cancer detection and treatment.

Correlations of noninvasive BOLD and TOLD MRI with pO2 and relevance to tumor radiation response

PURPOSE

To examine the potential use of blood oxygenation level dependent (BOLD) and tissue oxygenation level dependent (TOLD) contrast MRI to assess tumor oxygenation and predict radiation response.

METHODS

BOLD and TOLD MRI were performed on Dunning R3327-AT1 rat prostate tumors during hyperoxic gas breathing challenge at 4.7 T. Animals were divided into two groups. In Group 1 (n = 9), subsequent (19) F MRI based on spin lattice relaxation of hexafluorobenzene reporter molecule provided quantitative oximetry for comparison. For Group 2 rats (n = 13) growth delay following a single dose of 30 Gy was compared with preirradiation BOLD and TOLD assessments.

RESULTS

Oxygen (100%O2 ) and carbogen (95%O2 /5%CO2 ) challenge elicited similar BOLD, TOLD and pO2 responses. Strong correlations were observed between BOLD or R2* response and quantitative (19) F pO2 measurements. TOLD response showed a general trend with weaker correlation. Irradiation caused a significant tumor growth delay and tumors with larger changes in TOLD and R1 values upon oxygen breathing exhibited significantly increased tumor growth delay.

CONCLUSION

These results provide further insight into the relationships between oxygen sensitive (BOLD/TOLD) MRI and tumor pO2 . Moreover, a larger increase in R1 response to hyperoxic gas challenge coincided with greater tumor growth delay following irradiation.

Preclinical molecular imaging using PET and MRI

Molecular imaging fundamentally changes the way we look at cancer. Imaging paradigms are now shifting away from classical morphological measures towards the assessment of functional, metabolic, cellular, and molecular information in vivo. Interdisciplinary driven developments of imaging methodology and probe molecules utilizing animal models of human cancers have enhanced our ability to non-invasively characterize neoplastic tissue and follow anti-cancer treatments. Preclinical molecular imaging offers a whole palette of excellent methodology to choose from. We will focus on positron emission tomography (PET) and magnetic resonance imaging (MRI) techniques, since they provide excellent and complementary molecular imaging capabilities and bear high potential for clinical translation. Prerequisites and consequences of using animal models as surrogates of human cancers in preclinical molecular imaging are outlined. We present physical principles, values and limitations of PET and MRI as molecular imaging modalities and comment on their high potential to non-invasively assess information on hypoxia, angiogenesis, apoptosis, gene expression, metabolism, and cell trafficking in preclinical cancer research.

In vitro and in vivo evaluation of [(18)F]F-GAZ, a novel oxygen-mimetic azomycin-glucose conjugate, for imaging hypoxic tumor

Several F-18-labeled 2-nitroimidazole (azomycin) derivatives have been proposed for imaging hypoxia using positron emission tomography (PET). Their cell penetration is based on passive diffusion, which limits their intracellular concentration maxima. The purpose of this study was to investigate the uptake of N-(2-[(18)F]fluoro-3-(6-O-glucosyl)propyl-azomycin ([(18)F]F-GAZ), a new azomycin-glucose conjugate, in vitro and in vivo. [(18)F]F-GAZ was synthesized from its tetraacetyl nosylate precursor by nucleophilic radiofluorination. [(18)F]F-GAZ was evaluated in vivo in EMT-6 tumor-bearing Balb/C mice utilizing the PET and biodistribution analysis. In vitro uptake of [(18)F]FDG by EMT-6 cells was measured in the presence of unlabeled F-GAZ, 2-FDG, and D-glucose. [(18)F]F-GAZ was rapidly cleared from all tissues, including the blood pool and kidneys, with ultimate accumulation in the urinary bladder. Uptake of tracer doses of [(18)F]F-GAZ into EMT-6 tumors was fast, reaching a standardized uptake value of 0.66±0.05 within 5-6 minutes postinjection (p.i.), and decreased to 0.24±0.04 by 60 minutes p.i. (n=6). A tumor-muscle ratio of 1.87±0.18 was observed after 60 minutes. Total uptake of [(18)F]F-GAZ in tumors (60 minutes) amounted to 1.25%±0.15% ID/g versus 0.61%±0.14% ID/g (n=4) in muscle. Similar biodistribution and excretion were observed using carrier-added (100 mg/kg) doses of F-GAZ. In vitro, D-glucose and unlabeled 2-FDG were two orders of magnitude more potent than F-GAZ as competitive inhibitors of [(18)F]FDG uptake into EMT-6 cells. Besides its interaction with glucose transporters, F-GAZ seems to be not transported in the presence of glucose. Furthermore, [(18)F]F-GAZ is unlikely to be effective as a hypoxia imaging agent. The low in vivo toxicity and substantial retention in tumor observed at high doses of F-GAZ do provide rationale for further testing as a radiosensitizer for external beam radiation therapy of radioresistant, hypoxic tumors.

MRI of metastasis-permissive microenvironments

One of the earliest documented observations of the importance of the microenvironment in metastasis was made by Stephen Paget in 1889. More than a century later, the metastatic cascade remains a major cause of mortality from cancer. Cancer meets the criterion of a successful organization that is able to survive by adapting to changing environments. In fact, the tumor microenvironment and stroma are co-opted and shaped by cancer cells to derive a survival advantage. Cohesive strategies integrating advances in molecular biology and chemistry, with noninvasive multimodality imaging, provide new insights into the role of the tumor microenvironment in promoting metastasis from primary tumors as well as insights into environments that attract and permit cancer cells to establish colonies in distant organs. This article provides an overview of molecular and functional imaging characterization of microenvironments that can promote or permit cancer cells to metastasize and the microenvironmental characteristics of distant metastases.

MRI & MRS assessment of the role of the tumour microenvironment in response to therapy

MRI and MRS techniques are being applied to the characterisation of various aspects of the tumour microenvironment and to the assessment of tumour response to therapy. For example, kinetic parameters describing tumour blood vessel flow and permeability can be derived from dynamic contrast-enhanced MRI data and have been correlated with a positive tumour response to antivascular therapies. The ongoing development and validation of noninvasive, high-resolution anatomical/molecular MR techniques will equip us with the means to detect specific tumour biomarkers early on, and then to monitor the efficacy of cancer treatments efficiently and reliably, all within a clinically relevant time frame. Reliable tumour microenvironment imaging biomarkers will provide obvious advantages by enabling tumour-specific treatment tailoring and potentially improving patient outcome. However, for routine clinical application across many disease types, such imaging biomarkers must be quantitative, robust, reproducible, sufficiently sensitive and cost-effective. These characteristics are all difficult to achieve in practice, but image biomarker development and validation have been greatly facilitated by an increasing number of pertinent preclinical in vivo cancer models. Emphasis must now be placed on discovering whether the preclinical results translate into an improvement in patient care and, therefore, overall survival.

Radiosynthesis of the tumor hypoxia marker [18F]TFMISO via O-[18F]trifluoroethylation reveals a striking difference between trifluoroethyl tosylate and iodide in regiochemical reactivity toward oxygen nucleophiles

The MRI hypoxia marker trifluoromisonidazole (TFMISO) [1-(2-nitro-1H-imidazol-1-yl)-3-(2,2,2-trifluoroethoxy)propan-2-ol] was successfully labeled with (18)F to expand its role into a bimodal PET/MRI probe. (18)F-Labeling was achieved via a three-step procedure in which 2,2,2-[(18)F]trifluoroethyl p-toluenesulfonate prepared by (18)F-(19)F exchange served as the [(18)F]trifluoroethylating agent. The O-[(18)F]trifluoroethylation reaction proceeded efficiently to give the intermediate 1,2-epoxy-3-(2,2,2-[(18)F]trifluoroethoxy)propane, with approximately 60% of (18)F incorporated from the tosylate precursor, which was condensed with 2-nitroimidazole to yield [(18)F]TFMISO. Approximately 40% of the [(18)F]trifluoroethyl tosylate precursor was converted into the final product. In stark contrast, 2,2,2-[(18)F]trifluoroethyl iodide failed to produce [(18)F]TFMISO, giving instead 1,1-[(18)F]difluoro-2-iodoethoxy and 1-[(18)F]fluoro-2-iodovinyloxy analogs of [(18)F]TFMISO. Thus, this investigation has identified 2,2,2-[(18)F]trifluoroethyl tosylate as an excellent [(18)F]trifluoroethylating agent, which can convert efficiently an alcohol into the corresponding [(18)F]trifluoroethyl ether.

MR oximetry

MR oximetry includes methods for assessing tissue oxygenation. This chapter focuses on direct measurements of oxygenation. These can be divided into three methods. The first and most common has been termed BOLD MRI and relates to the quantification of deoxyhemoglobin. The second method uses an injected fluorinated agent which has a T (1) that is sensitive to tissue oxygen levels. The third is a direct measurement of T (1) under conditions where the variation in T (1) can be limited to that caused by changes in pO(2). These conditions can be met in the vitreous of the eye or the cerebrospinal fluids. Such changes in the eye have been called the retinal oxygenation response.

Imaging tumor hypoxia by magnetic resonance methods

Tumor hypoxia results from the negative balance between the oxygen demands of the tissue and the capacity of the neovasculature to deliver sufficient oxygen. The resulting oxygen deficit has important consequences with regard to the aggressiveness and malignancy of tumors, as well as their resistance to therapy, endowing the imaging of hypoxia with vital repercussions in tumor prognosis and therapy design. The molecular and cellular events underlying hypoxia are mediated mainly through hypoxia-inducible factor, a transcription factor with pleiotropic effects over a variety of cellular processes, including oncologic transformation, invasion and metastasis. However, few methodologies have been able to monitor noninvasively the oxygen tensions in vivo. MRI and MRS are often used for this purpose. Most MRI approaches are based on the effects of the local oxygen tension on: (i) the relaxation times of (19)F or (1)H indicators, such as perfluorocarbons or their (1)H analogs; (ii) the hemodynamics and magnetic susceptibility effects of oxy- and deoxyhemoglobin; and (iii) the effects of paramagnetic oxygen on the relaxation times of tissue water. (19)F MRS approaches monitor tumor hypoxia through the selective accumulation of reduced nitroimidazole derivatives in hypoxic zones, whereas electron spin resonance methods determine the oxygen level through its influence on the linewidths of appropriate paramagnetic probes in vivo. Finally, Overhauser-enhanced MRI combines the sensitivity of EPR methodology with the resolution of MRI, providing a window into the future use of hyperpolarized oxygen probes.

Pre-clinical evaluation of a 3-nitro-1,2,4-triazole analogue of [18F]FMISO as hypoxia-selective tracer for PET

Hypoxia in solid tumours is associated with the promotion of various metabolic mechanisms and induces resistance to radio- and chemotherapy. Non-invasive positron emission tomography (PET) or single photon emission computed tomography by use of selective biomarkers has emerged as valuable tools for the detection of hypoxic areas within tumours so treatment can be modified accordingly. The aim of this investigation was to evaluate [(18)F]3-NTR, a 3-nitro-1,2,4-triazole analogue (N(1) substituted) of [(18)F]FMISO as a potential hypoxia selective tracer. 3-NTR and its (18)F-radiolabelled isotopic isomer were synthesised and compared with FMISO in vitro and in vivo. Their physicochemical properties were measured, the enzymatic reduction was evaluated, and the reactivity of their metabolites was investigated. Biodistribution and PET scans were performed on CBA mice bearing hypoxic CaNT tumour cells, using (18)F-labelled versions of the tracers. [(18)F]3-NTR uptake within hypoxic cells was lower than [(18)F]FMISO and [(18)F]3-NTR did not exhibit any better selectivity than FMISO as a PET tracer in vivo. Both (18)F-radiolabelled compounds are relatively evenly distributed within the whole body and the radioactive uptake within hypoxic tumours reaches a maximum at 30 min post injection and decreases thereafter. Xanthine oxidase exhibited a nitroreductase activity toward 3-NTR under anaerobic conditions, but reduced metabolites did not bind covalently. It is confirmed that 3-NTR is an electron acceptor. It is postulated that radiolabelled metabolites and fragments of [(18)F]3-NTR are freely diffusing due to their poor binding capacities. Thus [(18)F]3-NTR cannot be used as a hypoxia selective tracer for PET. The investigation provides insights into the importance of the propensity to form covalent adducts for such biomarkers.

A phase I study of the nitroimidazole hypoxia marker SR4554 using 19F magnetic resonance spectroscopy

Background:

SR4554 is a fluorine-containing 2-nitroimidazole, designed as a hypoxia marker detectable with ¹⁹F magnetic resonance spectroscopy (MRS). In an initial phase I study of SR4554, nausea/vomiting was found to be dose-limiting, and 1400 mg m⁻² was established as MTD. Preliminary MRS studies demonstrated some evidence of ¹⁹F retention in tumour. In this study we investigated higher doses of SR4554 and intratumoral localisation of the ¹⁹F MRS signal.

Methods:

Patients had tumours ⩾3 cm in diameter and ⩽4 cm deep. Measurements were performed using ¹H/¹⁹F surface coils and localised ¹⁹F MRS acquisition. SR4554 was administered at 1400 mg m⁻², with subsequent increase to 2600 mg m⁻² using prophylactic metoclopramide. Spectra were obtained immediately post infusion (MRS no. 1), at 16 h (MRS no. 2) and 20 h (MRS no. 3), based on the SR4554 half-life of 3.5 h determined from a previous study. ¹⁹Fluorine retention index (%) was defined as (MRS no. 2/MRS no. 1)*100.

Results:

A total of 26 patients enrolled at: 1400 (n=16), 1800 (n=1), 2200 (n=1) and 2600 mg m⁻² (n=8). SR4554 was well tolerated and toxicities were all ⩽grade 1; mean plasma elimination half-life was 3.7±0.9 h. SR4554 signal was seen on both unlocalised and localised MRS no. 1 in all patients. Localised ¹⁹F signals were detected at MRS no. 2 in 5 out of 9 patients and 4 out of 5 patients at MRS no. 3. The mean retention index in tumour was 13.6 (range 0.6–43.7) compared with 4.1 (range 0.6–7.3) for plasma samples taken at the same times (P=0.001) suggesting ¹⁹F retention in tumour and, therefore, the presence of hypoxia.

Conclusion:

We have demonstrated the feasibility of using ¹⁹F MRS with SR4554 as a potential method of detecting hypoxia. Certain patients showed evidence of ¹⁹F retention in tumour, supporting further development of this technique for detection of tumour hypoxia.

Significance of nitroimidazole compounds and hypoxia-inducible factor-1 for imaging tumor hypoxia

A tumor-specific microenvironment is characterized by hypoxia, in which oxygen tension is considerably lower than in normal tissues. The hypoxic status of various solid tumors has been attributed as an indicator of adverse prognosis due to tumor progression toward a more malignant phenotype with increased metastatic potential and resistance to treatment. Various exogenous and endogenous markers for hypoxia are currently available and studied in relation to each other, tumor architecture, and tumor microenvironment. Over the last few decades, various methods have been suggested to assess the level of oxygenation in solid tumors. Among them, nitroimidazole compounds have provided promising information on tumor hypoxia. To quantify the extent of hypoxia requires that nitroimidazole binding be primarily dependent on oxygen concentration as well as nitroreductase levels in the tumor cells. Furthermore, recent progress in molecular biology has highlighted a transcription factor, hypoxia-inducible factor (HIF)-1, whose activity is induced by hypoxia. HIF-1 plays a central role in malignant progression by inducing the expression of various genes, whose functions are strongly associated with malignant alteration of the entire tumor. The cellular changes induced by HIF-1 are extremely important therapeutic targets of cancer therapy, particularly in the therapy against refractory cancers. In this review, we will discuss the significance of pimonidazole and HIF-1 as exogenous and endogenous hypoxia markers, respectively, as well as their evaluation and imaging of tumor hypoxia.

Radiosynthesis of [(131)I]IAZGP via nucleophilic substitution and its biological evaluation as a hypoxia marker - is specific activity a factor influencing hypoxia-mapping ability of a hypoxia marker?

INTRODUCTION

The hypoxia marker IAZGP, 1-(6-deoxy-6-iodo-beta-d-galactopyranosyl)-2-nitroimidazole, has been labeled with (123)I/(124)I/(125)I/(131)I via iodine-radioiodine exchange, which gives the radiotracer in a specific activity of 10-90 MBq/micromol. We synthesized the same radiotracer possessing several hundred to thousand times higher specific activity (high-SA IAZGP) via nucleophilic substitution and compared its biological behavior with that of conventionally produced IAZGP (low-SA IAZGP) to determine if specific activity is a factor influencing cell uptake kinetics, biodistribution and intratumor microregional localization of the radiotracer.

METHODS

High-SA [(131)I]IAZGP was prepared by substitution of the tosyl functionality with [(131)I]iodide. In vitro uptake of high- and low-SA [(131)I]IAZGP by HCT8 and HT29 cells was assessed in normoxic and hypoxic conditions. Biodistribution and intratumor localization of high- and low-SA [(131)I]IAZGP were determined by injection into HT29 tumor-bearing mice.

RESULTS

The nucleophilic substitution reaction proceeded efficiently in acetonitrile at 150 degrees C, giving the final product in an average yield of 42% and an average specific activity of 30 GBq/micromol. In vitro, high-SA [(131)I]IAZGP was incorporated into the tumor cells with similar kinetics and oxygen dependence to low-SA [(131)I]IAZGP. In HT29 tumor-bearing mice, biodistributions of high- and low-SA [(131)I]IAZGP were equivalent. Ex vivo autoradiography revealed heterogeneous intratumor localization of high-SA [(131)I]IAZGP corresponding closely to distributions of other exogenous and endogenous hypoxia markers. Comparable microregional distribution patterns were observed with low-SA [(131)I]IAZGP.

CONCLUSIONS

Radiolabeled IAZGP produced via nucleophilic substitution is validated as an exogenous hypoxia marker. Specific activity does not appear to influence the in vivo hypoxia-mapping ability of the radiotracer.

Noninvasive multimodality imaging of the tumor microenvironment: registered dynamic magnetic resonance imaging and positron emission tomography studies of a preclinical tumor model of tumor hypoxia

In vivo knowledge of the spatial distribution of viable, necrotic, and hypoxic areas can provide prognostic information about the risk of developing metastases and regional radiation sensitivity and may be used potentially for localized dose escalation in radiation treatment. In this study, multimodality in vivo magnetic resonance imaging (MRI) and positron emission tomography (PET) imaging using stereotactic fiduciary markers in the Dunning R3327-AT prostate tumor were performed, focusing on the relationship between dynamic contrast-enhanced (DCE) MRI using Magnevist (Gd-DTPA) and dynamic (18)F-fluoromisonidazole ((18)F-Fmiso) PET. The noninvasive measurements were verified using tumor tissue sections stained for hematoxylin/eosin and pimonidazole. To further validate the relationship between (18)F-Fmiso and pimonidazole uptake, (18)F digital autoradiography was performed on a selected tumor and compared with the corresponding pimonidazole-stained slices. The comparison of Akep values (kep = rate constant of movement of Gd-DTPA between the interstitial space and plasma and A = amplitude in the two-compartment model (Hoffmann U, Brix G, Knopp MV, Hess T and Lorenz WJ (1995). Magn Reson Med 33, 506-514) derived from DCE-MRI studies and from early (18)F-Fmiso uptake PET studies showed that tumor vasculature is a major determinant of early (18)F-Fmiso uptake. A negative correlation between the spatial map of Akep and the slope map of late (last 1 hour of the dynamic PET scan) (18)F-Fmiso uptake was observed. The relationships between DCE-MRI and hematoxylin/eosin slices and between (18)F-Fmiso PET and pimonidazole slices confirm the validity of MRI/PET measurements to image the tumor microenvironment and to identify regions of tumor necrosis, hypoxia, and well-perfused tissue.

Molecular imaging of hypoxia

Hypoxia, a condition of insufficient O2 to support metabolism, occurs when the vascular supply is interrupted, as in stroke or myocardial infarction, or when a tumor outgrows its vascular supply. When otherwise healthy tissues lose their O2 supply acutely, the cells usually die, whereas when cells gradually become hypoxic, they adapt by up-regulating the production of numerous proteins that promote their survival. These proteins slow the rate of growth, switch the mitochondria to glycolysis, stimulate growth of new vasculature, inhibit apoptosis, and promote metastatic spread. The consequence of these changes is that patients with hypoxic tumors invariably experience poor outcome to treatment. This has led the molecular imaging community to develop assays for hypoxia in patients, including regional measurements from O2 electrodes placed under CT guidance, several nuclear medicine approaches with imaging agents that accumulate with an inverse relationship to O2, MRI methods that measure either oxygenation directly or lactate production as a consequence of hypoxia, and optical methods with NIR and bioluminescence. The advantages and disadvantages of these approaches are reviewed, along with the individual strategies for validating different imaging methods. Ultimately the proof of value is in the clinical performance to predict outcome, select an appropriate cohort of patients to benefit from a hypoxia-directed treatment, or plan radiation fields that result in better local control. Hypoxia imaging in support of molecular medicine has become an important success story over the last decade and provides a model and some important lessons for development of new molecular imaging probes or techniques.

Imaging transgene activity in vivo

The successful translation of gene therapy for clinical application will require the assessment of transgene activity as a measure of the biological function of a therapeutic transgene. Although current imaging permits the noninvasive detection of transgene expression, the critical need for quantitative imaging of the action of the expressed transgene has not been met. In vivo magnetic resonance spectroscopic imaging (MRSI) was applied to quantitatively delineate both the concentration and activity of a cytosine deaminase-uracil phosphoribosyltransferase (CD-UPRT) fusion enzyme expressed from a transgene. MRSI enabled the generation of anatomically accurate maps of the intratumoral heterogeneity in fusion enzyme activity. We observed an excellent association between the CD-UPRT concentration and activity and the percentage of CD-UPRT(+) cells. Moreover, the regional levels of UPRT activity, as measured by imaging, correlated well with the biological affect of the enzyme. This study presents a translational imaging paradigm for precise, in vivo measurements of transgene activity with potential applications in both preclinical and clinical settings.

Fluorine-19 NMR chemical shift probes molecular binding to lipid membranes

The binding of amphiphilic molecules to lipid bilayers is followed by 19F NMR using chemical shift and line shape differences between the solution and membrane-tethered states of -CF 3 and -CHF 2 groups. A chemical shift separation of 1.6 ppm combined with a high natural abundance and high sensitivity of 19F nuclei offers an advantage of using 19F NMR spectroscopy as an efficient tool for rapid time-resolved screening of pharmaceuticals for membrane binding. We illustrate the approach with molecules containing both fluorinated tails and an acrylate moiety, resolving the signals of molecules in solution from those bound to synthetic dimyristoylphosphatidylcholine bilayers both with and without magic angle sample spinning. The potential in vitro and in vivo biomedical applications are outlined. The presented method is applicable with the conventional NMR equipment, magnetic fields of several Tesla, stationary samples, and natural abundance isotopes.

Molecular imaging of reporter gene expression in prostate cancer: an overview

Prostate cancer remains an important and growing health problem. Advances in imaging of prostate cancer may help to achieve earlier and more accurate diagnosis and treatment. We review the various strategies using reporter genes for molecular imaging of prostate cancer. These approaches are emerging as valuable tools for monitoring gene expression in laboratory animals and humans. Further development of more sensitive and selective reporters, combined with improvements in detection technology, will consolidate the position of reporter gene imaging as a versatile method for understanding of intracellular biological processes and the underlying molecular basis of prostate cancer, as well as potentially establishing a future role in the clinical management of patients afflicted with this disease.

Non-invasive molecular and functional imaging of cytosine deaminase and uracil phosphoribosyltransferase fused with red fluorescence protein

INTRODUCTION

Increased expression of cytosine deaminase (CD) and uracil phosphoribosyltransferase (UPRT) may improve the antitumoral effect of 5-fluorouracil (5-FU) and 5-fluorocytosine (5-FC), and thereby enhance the potential of gene-directed enzyme prodrug therapy. For the applicability of gene-directed enzyme prodrug therapy in a clinical setting, it is essential to be able to monitor the transgene expression and function in vivo. Thus, we developed a preclinical tumor model to investigate the feasibility of using magnetic resonance spectroscopy and optical imaging to measure non-invasively CD and UPRT expression and function.

MATERIALS AND METHODS

Expression vectors of CD or CD/UPRT fused to monomeric DsRed (mDsRed) were constructed and rat prostate carcinoma (R3327-AT) cell lines stably expressing either CD/mDsRed or CD/UPRT/mDsRed were generated. The expression of the fusion proteins was evaluated by flow cytometry, fluorescence microscopy, and Western blot analysis. The function of the fusion protein was confirmed in vitro by assessing 5-FC and 5-FU cytotoxicity. In vivo fluorine-19 magnetic resonance spectroscopy ((19)F MRS) was used to monitor the conversion of 5-FC to 5-FU in mice bearing the R3327-CD/mDsRed and R3327-CD/UPRT/mDsRed tumor xenografts.

RESULTS

Sensitivity to 5-FC and 5-FU was higher in cells stably expressing the CD/UPRT/mDsRed fusion gene than in cells stably expressing CD/mDsRed alone or wild-type cells. Whole tumor (19)F MRS measurements showed rapid conversion of 5-FC to 5-FU within 20 min after 5-FC was administered intravenously in both CD/mDsRed and CD/UPRT/mDsRed tumors with subsequent anabolism to cytotoxic fluoronucleotides (FNucs). CD/UPRT/mDsRed tumor was more efficient in these processes.

CONCLUSION

This study demonstrates the utility of these tumor models stably expressing CD or CD/UPRT to non-invasively evaluate the efficacy of the transgene expression/activity by monitoring drug metabolism in vivo using MRS, with potential applications in preclinical and clinical settings.